Mobile and Cellular Communication Systems
Content
CONTENTS
CONTENTS
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CHAPTER 1:INTRODUCTION CHAPTER 2:CELLULAR SYSTEM BASICS 2.1:WHY CELLULAR MOBILE TELEPHONE 2.2:TRUNKING EFFECIENCY 2.3:A BASIC CELLULAR SYSTEM 2.4:UNIQUENESS OF MOBILE RADIO ENVIRONMENT CHAPTER 3:CELLULAR SYSTEM AND PCS 3.1:INTRODUCTION 3.2:HISTORICAL OVERVIEW 3.3:FIRST-GENERATION CELLULAR SYSTEMS 3.4:SECOND-GENERATION CELLULAR SYSTEMS 3.5:EUROPEAN AND JAPANESE CELLULAR SYSTEM 3.6:NORHT AMERICA CELLULAR SYSTEM 3.7:SECOND-GENERATION-PLUS PCSSYSTEMS 3.8:VISION OF THE THIRD-GENERATION SYSTEMS REFERENCES
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Chapter One
INTRODUCTION
Over the last decade, deployment of wireless communication in North America and Europe has been phenomenal. Wireless communications technology has evolved along a logical path, from simple first-generation analogue products designed for business use to second- generation digital wireless telecommunications systems for residential and business environments. As the industry plans and implements the second-generation digital networks in the mid-1990s, a vision of a next-generation wireless information network is emerging. Complete Personal Communication Services (PCS) will enable all users to economically transfer any form of information between any desired locations. PCS is likely to explode worldwide in the mid to late 1990s with very high global revenues for services and handsets. As the market expands, it will draw in a growing number of low-end residential users and will derive basic voice services up from half the PCS market in the mid-1990s to about three-fourth in 1999. PCS use both existing and future wireline and wireless networks. Three key elements of PCS will be an easy-to-use, high-functionality handset, and a single, personal number that can reach the subscriber anywhere, and finally, an individualized feature profile that follows the user and provides a customized set of services at any location. PCS will not be a single, all-encompassing wireless solution, but it will be a combination of standards, and products that meet a range of user requirements at a reasonable price with high level of support. PCS will fill gaps left by other modes of wireless and wireline telephony. As we see, the PCS is not a simple concept, which leads us to study every single part of PCS alone. In Chapter 2 we study the concept of cellular system, which acts as a backbone of PCS. In Chapter 3 we study the evolution of cellular system from first-generation, second-generation, to the vision of the third-generation cellular system.
Chapter Two
Cellular System Basics
2.1 Why Cellular Mobile Telephone Systems?
2.1.1 Limitations of conventional mobile telephone systems One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization. Limited service capability. A conventional mobile telephone system is usually designed by selecting one or more channels from a specific frequency allocation for use in autonomous geographic zones, as shown in Figure 2.1. The communications coverage area of each zone is normally planned to be as large as possible, which means that the transmitted power should be as high as the federal specification allows. The user who starts a call in one zone has to reinitiate the call when moving into a new zone (see Figure 2.1) because the call will be dropped. This is an undesirable radio telephone system since there is no guarantee that a call can be completed without a handoff capability.
Figure 2.1: Conventional mobile system The handoff is a process of automatically changing frequencies as the mobile unit moves into a different frequency zone so that the conversation can be continued in a new frequency zone without redialing. Another disadvantage of the conventional system is that the number of
active users is limited to the number of channels assigned to a particular frequency zone. Poor service performance. In the past, a total of 33 channels were allocated to three mobile telephone systems. The large number of subscribers created a high blocking probability during busy hours. Although service performance was undesirable, the demand was still great. A high-capacity system for mobile telephones was needed. Inefficient frequency spectrum utilization. In a conventional mobile telephone system, the frequency utilization measurement is defined as the maximum number of customers that could be served by one channel at the busy hour. As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area. A new cellular system that measures the frequency spectrum utilization and proves to be efficient is discussed in Section 2.1.2. 2.1.2 Spectrum efficiency considerations A major problem facing the radio communication industry is the limitation of the available radio frequency spectrum. In setting allocation policy, the Federal Communications Commission (FCC) seeks systems which need minimal bandwidth but provide high usage and consumer satisfaction. The ideal mobile telephone system would operate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are 1. Single-sideband (SSB), which divides the allocated frequency band into maximum numbers of channels 2. Cellular, which reuses the allocated frequency band in different geographic locations 3. Spread spectrum or frequency-hopped, which generates many codes over a wide frequency band In 1971, the cellular approach was shown to be a spectrally efficient system. 2.1.3 Technology, feasibility, and service affordability In 1971, the computer industry entered a new era. Microprocessors and minicomputers are now used for controlling many complicated features and functions with less power and size than was previously possible. Large-scale integrated (LSI) circuit technology reduced the size of mobile transceivers so that they easily fit into the standard automobile. These achievements were a few of the requirements for developing
advanced mobile phone systems and encouraging engineers to pursue this direction. Another factor was the price reduction of the mobile telephone unit. LSI technology and mass production contribute to reduced cost so that in the near future an average-income family should be able to afford a mobile telephone unit.
2.2 Trunking Efficiency
Figure 2.2 shows ηe by comparing one carrier per market with more than one carrier per market situations with different blocking probability conditions. The degradation of trunking efficiency decreases as the blocking probability increases. As the number of carriers per market increases the degradation increases. However, when a high percentage of blocking probability, say more than 20 percent, occurs, the performance of one carrier per market is already so poor that further degradation becomes insignificant, as Fig. 2.2 shows.
Figure 2.2: Degradation of trunking efficiency- comparing one carrier/market and other-than-one-carrier/market For a 2 percent blocking probability, the trunking efficiency of one carrier per market does show a greater advantage when compared to other scenarios.
2.3 A Basic Cellular System
As we mentioned before, most commercial radio and television systems are designed to cover as much area as possible. These systems typically operate at the maximum power and with highest antennas
allowed. The frequency used by the transmitter cannot be reused until there is enough geographical separation so that one station does not interfere significantly with another station assigned to that frequency. The cellular system takes the opposite approach. It seeks to make an efficient use of available channel by using low-power transmitters to allow frequency reuse at much smaller distances as in Figure 2.3. Maximizing the number of times each channel may be reused in a given geographic area is the key to an efficient cellular system design.
2 7 1 6 2 7 1 6 5 4 6 5 3 7 1 4 6 5 5 2 3 7 1 4 4 6 5 2 3 3 7 1 4 2 3
Figure 2.3: Frequency reuses. Cellular systems are designed to operate with groups of low-power radios spread out over the geographical service area. Each group of radios serve mobile unit presently located near them. The area served by each group of radios is called a “cell”. Each cell has an appropriate number of low-power radios for communication within itself. The power transmitted is chosen to be large enough to communicate with mobile units located near the edges of the cell. The radius of each cell may be chosen to perhaps 26km in start-up system with relatively few subscribers, down to less than 2km for mature system requiring considerable frequency reuse. As the traffic grows, new calls and channels are added to the system. If an irregular cell pattern were selected, it would lead to an inefficient of the spectrum due to its inability to reuse frequencies on account of cochannel interference. In addition, it would also result in an uneconomical deployment of equipment, requiring relocation from one cell site to another. Therefore, a great deal of engineering effort would be required to readjust the transmission, switching, and control resources every time the system goes through its development phase. All these difficulties lead to the use of a regular cell pattern in a cellular system design. In reality, the cell coverage is an irregularly shaped circle. The exact coverage of the cell will depend on the terrain and other factors. For design convenience and as a first-order approximation, we assume that the coverage areas regular polygons. For example, for an omnidirectional
antenna with constant signal power, each cell-site coverage area would be circular. To achieve full coverage without dead spots, a series of regular polygons for cell sites are required. Any regular polygon, such as an equilateral triangle, a square, or a hexagon, can be used for cell design. The hexagon is used for two reasons: first, a hexagonal layout requires fewer cells and therefore, fewer transmitter sites and second, a hexagonal cell layout is less expensive compared to square and triangular cells. In practice, after the polygons are drawn on a map of the coverage area, redial lines are drawn and the SNR ratio calculated for various directions. A basic cellular system consists of three parts: a mobile unit, a cell site, and a mobile telephone switching office (MTSO), as Figure 2.4 shows, with connections to link the three subsystems.
Figure 2.4: Cellular system 1. Mobile units. A mobile telephone unit contains a control unit, a transceiver, and an antenna system. 2. Cell site. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. 3. MTSO. The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, and handles billing activities. 4. Connections. The radio and high-speed data links connect the three subsystems.
The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and cellular administration.
2.4 Uniqueness of Mobile Radio Environment
2.4.1 Description of mobile radio transmission medium The propagation attenuation. In general, the propagation path loss increases not only with frequency but also with distance. Normally the incident angles of both the direct wave and the reflected wave are very small, as Figure 2.5 shows.
Figure 2.5: Mobile radio transmission model The incident angle of the direct wave is θ1, and the incident angle of the reflected wave is θ2. θ1 is also called the elevation angle. The propagation path loss would be 40 dB/dec., where “dec.” is an abbreviation of decade. This means that the mobile unit will observe a 40-dB loss at a single receiver as it moves from 1 to 10 km. Therefore Pr ∝ D-4 Pr = α D-4 Where Pr = received carrier power D = distance measured from the transmitter to, the receiver α = constant The difference in power reception at two different distances D1 and D2 will result in
Pr 1 D 2 = Pr 2 D1
−4
In the decibel expression ∆Pr(dB) = 10 log Pr2/P r1 = 40 log D1/D2 When D2 =2D1, ∆Pr = -12 dB; when D2 = 10D1, ∆Pr= 40 dB. This 40 dB/dec. is the general rule for the mobile radio environment and is easy to remember. It is also easy to compare to the free-space
propagation rule of 20 dB/dec. The linear and decibel scale expressions are Pr ∝ D-2 (free space) And ∆Pr = 20 log D1/D2 (free space) In a real mobile radio environment, the propagation path-loss slope varies as Pr ∝ D-γ Pr = α D-γ γ usually lies between 2 and 5 depending on the actual conditions. Of course γ cannot be lower than 2, which is the free-space condition. The decibel expression is Pr(dB) = 10 log α - 10γ log D Severe fading. Since the antenna height of the mobile unit is lower than its typical surroundings, and the carrier frequency wavelength is much less than the sizes of the surrounding structures, multipath waves are generated. At the mobile unit, the sum of the multipath waves causes a signal-fading phenomenon. The signal fluctuates in a range of about 40 dB (10 dB above and 30 dB below the average signal). We can visualize the nulls of the fluctuation at the baseband at about every half wavelength in space, but all nulls do not occur at the same level, as Figure 2.6 shows. If the mobile unit moves fast, the rate of fluctuation is fast.
Figure 2.6: A typical fading signal received while the mobile unit is moving.
2.4.2 Model of transmission medium A mobile radio signal r(t), illustrated in Figure 2.7 can be artificially characterized by two components m(t) and ro(t) based on natural physical phenomena. r(t)= m(t) ro(t) The component m(t) is called local mean, long-term fading, or lognormal fading and its variation is due to the terrain contour between the base station and the mobile unit. The factor ro is called multipath fading, short-term fading, or Rayleigh fading and its variation is due to the waves reflected from the surrounding buildings and other structures.
Figure 2.7: A mobile radio signal fading representation. (a) A mobile signal fading. (b) A short-term signal fading 2.4.3 Mobile fading characteristics Rayleigh fading is also called multipath fading in the mobile radio environment. When these multipath waves bounces back and forth due to the buildings and houses, they form many standing-wave pairs in space, as shown in Figure 2.8. Those standing-wave pairs are summed together and become an irregular wave-fading structure. When a mobile unit is standing still, its receiver only receives signal strength at that spot, so a constant signal is observed. When the mobile unit is moving, the fading
structure of the wave in the space is received. It is a multipath fading. The recorded fading becomes fast as the vehicle moves faster.
Figure 2.8: A mobile radio environment-two parts. (1) Propagation loss; (2) multipath fading The radius of the active scatterer region. The mobile radio multipath fading shown in Fig. 1.7 explains the fading mechanism. The radius is roughly 100 wavelengths. The active scatterer region always moves with the mobile unit as its center. It means that some houses were inactive scatterers and became active as the mobile unit approached them; some houses were active scatterers and became inactive as the mobile unit drove away from them. Delay spread and coherence bandwidth Delay spread. In the mobile radio environment, as a result of the multipath reflection phenomenon, the signal transmitted from a cell site and arriving at a mobile unit will be from different paths, and since each path has a different path length, the time of arrival for each path is different. For an impulse transmitted at the cell site, by the time this impulse is received at the mobile unit it is no longer an impulse but rather a pulse with a spread width that we call the delay spread. The measured data indicate that the mean delay spreads are different in different kinds of environment.
Type of environment Inside the building Open area Suburban area Urban area
Delay spread ∆, µs <0.1 <0.2 0.5 3
2.4.4 Direct wave path, line-of-sight path, and obstructive path A direct wave path is a path clear from the terrain contour. The lineof-sight path is a path clear from buildings. In the mobile radio environment, we do not always have a line-of-sight condition. When a line-of-sight condition occurs, the average received signal at the mobile unit at a 1 -mi intercept is higher, although the 40-dB/dec. path-loss slope remains the same. In this case the short-term fading is observed to be a rician fading. It results from a strong line-of-sight path and a ground-reflected wave combined, plus many weak buildingreflected waves. When an out-of-sight condition is reached, the 4O-dB/dec path-loss slope still remains. However, all reflected waves, including ground reflected waves and building-reflected waves, become dominant. The short-term received signal at the mobile unit observes a Rayleigh fading. The Rayleigh fading is the most severe fading. When the terrain contour blocks the direct wave path, we call it the obstructive path . In this situation, the shadow loss from the signal reception can be found.
Chapter Three
Cellular System and PCS
3.1 Introduction
Personal Communication Services (PCS) is a new network that has many features. One of these features is that it will be a future common global standard. PCS will not be a single, all-encompassing wireless solution, but it will be a combination of standards, and products that meet a range of users. In this chapter we briefly discuss the current and preceding cellular systems tell we reach the next generation of cellular system which expected to offered the PCS requirements. We first need to have a historical overview of mobile telephone services. We then, discuss the first- and second-generation cellular systems. We outline the problem associated with the second-generation-plus PCS system and provide a vision of third-generation system.
3.2 Historical Overview
The first mobile telephone service was introduced in the United States 1946 by AT&T. It was used to interconnect mobile users to the public telephone land-line network, thus allowing telephone calls between fixed station and m obile users. This mobile telephone system was based on Frequency Modulation (FM) transmission. Demand for mobile telephone service grew quickly and stayed ahead of the available capacity in many urban cities. The usefulness of the mobile telephone decreased as user found that blocking often prevented them from getting a circuit during the peak periods. User and the telephone companies, alike, realized that a handful of channel would not be enough for mobile telephone service to develop. Large blocks of spectrum would be needed to satisfy the demand on the urban areas. In the mid-1960s the Bell System introduced the improved Mobile Telephone Services (IMTS) with enhanced features. In the late 1960s and the early 1970s work began on the first cellular telephone systems. It should be recognized that the first-generation analogue cellular radio was not so much a new technology as it was a new idea for organizing existing IMTS technology on large scale. While the voice communication used the same analogue FM that had been used, two major technological improvements made the cellular concept a reality. In the early 1970s
microprocessor was invented. The second improvement was in the use of a digital control link between the mobile telephone and the base station In the late 1980s interest emerged in a digital cellular system, where both the voice and the control were digital. By 1991 digital cellular services began to emerge to reduce the cost of wireless communications and improve the call-handling capacity of an analog cellular system. In 1993, a digital system was placed in service.
3.3 First-Generation Cellular Systems
As the United States was planning its cellular network in the 1970s, England, Japan, Germany, and the Scandinavian countries were also planning their systems. Each system used a different frequency band and different protocols for signaling between mobile units and base stations. They all used analog FM (with different deviations and channel spacings) for their voice communications. Table 3.1 gives a comparison of various operational aspects five cellular systems used in those countries. Table 2.1: First-Generation Cellular Systems
System Parameter North America (AMPS) 870-890 825-845 45 30 666/832 2-25 FM ±12 FSK ±8 10 U.K. (TACS) Scandinavian (NMT) West Germany (C450) Japan (NTT)
Transmission frequency (MHz) *Base station *Mobile station Spacing between transmitter & receiver frequency (MHz) Spacing between channels (kHz) Number of channels Coverage radius by one base station (km) Audio signal *Modulation *Maximum frequency deviation (kHz) Control signals *Modulation *Frequency deviation Data transmission (kbs)
935-960 890-915 45 25 1000 2-20 FM ±9.5 FSK ±6.4 8
463-467.5 453-457.5 10 25 180 1.8-40 FM ±5 FSK ±3.5 1.2
461.3-465.74 870-885 451.3-455.74 925-940 10 55 20 222 5-30 FM ±4 FSK ±2.5 5.28 25 600 5-10 FM ±5 FSK ±4.5 0.3
The cellular approach promised virtually unlimited capacity through cell splitting. As the popularity of wireless escalated in the 1980s, the cellular industry faced practical limitations. For a fixed allocation of spectrum, a large increase in capacity implies corresponding reduction in cell size. As the cell get smaller, it becomes increasingly difficult to place base stations at the locations that offer necessary radio coverage. Also, reduction in cell size demand increased signaling activity as more rapid handoffs occur; in addition, base stations are required to handle more access requests and registrations from mobile stations. The problem becomes particularly difficult in large urban areas where capacity
requirements are most pressing. In addition to the capacity bottleneck, the utility of first-generation analog systems was diminished by proliferation of incompatible standards in Europe. The same mobile telephone frequencies cannot be used in different European countries. The limitations of first-generation analog system provided motivations to the second-generation systems. The principal goals of the second-generation systems were: higher capacity and hence lower cost, and, in Europe, a continental system with full international roaming and handoff capabilities. In Europe, these goals are served by new spectrum allocations and by formulation of a Pan-European Cellular Standard GSM. In North America, where one standard (United States, Canada, and Mexico) existed and covered a region as large as Europe, the push for a new system was not as strong. In the new digital systems, higher capacity is derived from applications of advanced transmission techniques including efficient speech coding, error correcting channel codes, and bandwidth-efficient modulation techniques. In Europe, the approach was to open new frequency bands for a panEuropean system and not to have compatibility with existing cellular systems. In the United States, the same frequency bands were shared with the new digital systems, and the standards supported dual-mode telephones that could be used in both analog and digital systems.
3.4 Second-Generation Cellular Systems
First-generation cellular systems were designed to satisfy the needs of business customers and some residential customers. With the increased demand of cellular telephones in Europe, several manufacturers began to look for new technologies that could overcome the problems of poor signals and battery performance. Poor signals resulted in poor performance for the user and a high frequency of false handoffs for the system operator. Better battery performance was needed to reduce size and cost of self- contained handheld units. Research efforts were directed toward wireless technologies to provide high-quality, interference-free speech and decent battery performance. The size of handset and better battery performance led to low-power designs and performance targets possible only with fully digital technologies. Digital cellular system based on the GSM (TDMA) standard have emerged in Europe, while system based on IS-54 (TDMA) and IS-95 (CDMA) are being developed in the United States. Table 3.2 provides a summary of the cellular and cordless systems.
Table 3.2: Second-Generation Cellular and cordless Systems
System Country Access technology Primary use Frequency band *Base Station (MHz) *Mobile Station (MHz) Duplexing RF channel spacing (kHz) Modulation Power, maximum/average mW Frequency assignment Power control *Base station *Mobile Station Speech coding Speech rate (kbs) Speech channel per RF channel Channel bit rate (kbs) Frame duration (ms) IS-54 U.S. TDMA/ FDMA Cellular GSM Europe TDMA/ FDMA Cellular CT-3 DECT DCT-900 U.S. Europe, Asia Sweden Europe CDMA/ FDMA TDMA/ TDMA/ (DS) DMA FDMA FDMA Cellular Cordless Cordless Cordless/ cellular 864-868 TDD 100 GFSK 10/5 Dynamic Y Y QCELP 8(variable rate) 20 N N ADPCM 32 1 72 2 862-866 1800-1900 TDD 1000 GFSK 80/5 Dynamic N N ADPCM 32 8 640 16 TDD 1728 GFSK 250/10 Dynamic N N ADPCM 32 12 1152 10 IS-95 CT-2
869-894 824-849 FDD 30 π/4 DQPSK 600/200 Fixed Y Y VSELP 7.95 3 48.6 40
935-960 869-894 890-915 824-849 FDD FDD 200 1.250 GMSK BPSK/QPSK 1000/125 600 Dynamic Y Y PRE-LTP 13 8 270.833 4.615
3.5 European and Japanese Cellular Systems
In this section we present an overview of the Global System for Mobile communications (GSM) as described in the European Telecommunication Standards Institute’s (ETSI’s) Recommendations. A brief description of the Japanese Digital Cellular (JDC) system is also given. 3.5.1 GSM Public Land Mobile Network (PLMN) GSM offers users good voice quality, call privacy, and network security. Subscriber Identity Module (SIM) cards provide the security mechanism for GSM. Of major importance is GSM’s potential enhanced services requiring multimedia communication: voice, image, and data. The key to delivering enhanced services is Signaling System Number 7 (SS7), a robust set of protocol layers designed to provide fast, efficient, reliable transfer and delivery of signaling information across the signaling network and to support both switched-voice and nonvoice application. 3.5.2 GSM Architecture
The basic subsystems of the GSM architecture are: Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operational Subsystem (OSS). The BSS provides and manages transmission paths between the mobile stations (MSs) and the NSS. This includes management of the radio interface between MSs and rest of the GSM system. The NSS has responsibility of managing communications and connecting MSs to the relevant networks or other MSs. The MS, BSS, and NSS form the operational part of the GSM system. The OSS provides the means for a service provider to control them. Figure 3.1 shows the model for the GSM system.
Figure 3.1: Model of the GSM System[1] In the GSM, interaction between the subsystems can be groped into two main parts: Operational part: external networks<->NSS<->BSS<->user Control part: OSS<->service provider The operational part provides transmission paths and establishes them. The control part interacts with the traffic-handling activity of the operational part by monitoring and modifying it to maintain or improve its functions. 3.5.2.1 GSM Subsystems Entities Figure 3.2 shows the functional entities of the GSM and their logical interconnection. 3.5.2.1.1 Mobile Station The MS consists of the physical equipment used by the subscriber to access a PLMN for offered telecommunication services. MSs come in five power classes that define the maximum PF
power level that the unit can transmit. Table1 provides the details of maximum RF power for various classes. Table 2.3: Maximum RF Power for Mobile Stations Class Max. RF Power Used For (W) 1 20 Vehicular Unit 2 8 Portable Unit 3 5 Handheld Unit 4 2 Handheld Unit 5 0.8 Handheld Unit
Figure 3.2: GSM Reference Model[1] Basically, an MS can be divided into two parts. The first part contains the hardware and software to support radio and man-machine interface functions. The second part contains terminal/user-specific data in the form of a smart card (SIM card), which can effectively be considered a sort of logical terminal. An MS has a number of identities, including the International Mobile Equipment Identity (IMEI), the International Mobile Subscriber Identity (IMSI), and the ISDN number. SIM is basically a smart card that contains all the subscriber-related information stored on the user’s side of the radio interface. 3.5.2.1.2 Base Station System The BSS is the physical equipment that provides radio coverage to prescribed geographical area, known as the cells. It contains equipment required to communicate with the MS. Functionally, a BSS consists of: a control function carried out by the Base Station Controller (BSC) and a transmitting function performed by the Base Transceiver System (BTS). The BTS is the radio transmission
equipment and covers each cell. A BSS can serve several cells because it can have multiple BTSs. 3.5.2.2 Network and Switching Subsystem The NSS includes the main switching functions of the GSM, databases required for the subscribers, and mobility management. Its main role is to manage the communications between the GSM and other network users. Within the NSS, the switching functions are performed by the MSC. Subscriber information relevant to provisioning of services is kept in the Home Location Register (HLR). The other database in the NSS is the Visitor Location Register (VLR). The Mobile Service Switching Center performs the necessary switching functions required for the MSs located in an associated geographical area, called an MSC area. The MSC monitors the mobility of its subscribers and manages necessary resources required to handle and update the location registration procedures and to carry out the handoff functions. The MSC is involved in the interworking functions to communicate with other networks such as PSTN and ISDN. The call routing and control and echo control functions are performed by the MSC. The Home Location Register has two types of information stored in it: subscriber information and part of the mobile information to allow incoming calls to be routed to the MSC for the particular mobile. The Visitor Location Register is the functional unit that dynamically stores subscriber information, such as location area, when the subscriber is located in the area covered by the VLR. 3.5.3 GSM Channel and Frame Structure The bandwidth in the GSM is 25 MHz. The frequency band for uplink is 890 to 915 MHz, whereas for the downlink is 935 to 960 MHz. The GSM has 124 channels, each with a bandwidth of 200 kHz. When the MS is assigned to an information channel, a radio channel and a timeslot are also assigned. Radio channels are assigned in frequency pairs (one for the uplink and the other for the downlink). Each pair of radio channels supports up to 8 simultaneous calls (see figure 3.3). Thus, the GSM can support up to 922 simultaneous users with the full-rate speech coder. This number will be doubled to 1984 users with the half-rate speech coder.
Figure 3.3: GSM FDMA/TDMA Structure[1] 3.5.3.1 Logical Channels In the GSM, there are three types of logical channels: Traffic Channel (TCH), control Channel (CCH), and Cell Broadcast Channel (CBCH). The TCHs are used to transmit user information (speech or data). CCHs are used to transmit control and signaling information. The CBCH is used to broadcast user information form a service center to the MS listening in a given cell area. The TCH can be TCH/Full or TCH/half. The TCH/F allows the transmission of 13 kbs of the speech or data at 12, 6, or 3.6 kbs. The TCH/H allows speech coded at a rate around 7 kbs or data at 6 or 3.6 kbs. The Control Channels (CCHs) consists of : 1-Broadcast Channel (BCH) 2-Common Control Channel (CCCH) 3-Dedicated Control Channel (DCCH) The BCHs consists of: 1-Broadcast Control Channel (BCCH) 2-Frequency Correction Channel (FCCH) 3-Synchronization Channel (SCH) The CCCHs include 1-Paging Channel (PCH) 2-Access Grant Channel (AGCH) 3-Random Access Channel (RACH) 3.5.3.2 GSM FrameThe GSM multiframe is 120 ms. It consists of 26 frames of 8 time slots. The structure of a GSM hyperframe, superframe, multiframe, frame, and time slot is shown in figure 3.4. A time slot carries 156.25 bits. The same format is used for the uplink and downlink transmission with various types as shown in Figure 3.5.
Figure 3.4: Physical Structure for GSM Hyperframe, Superframe, Multiframe, Frame, and Time Slot[1]
Figure 3.5: GSM Time Slot Structure[1] 3.5.4 GSM Speech Processing Two major tasks are involved in transmitting and receiving information over a digital radio link: information processing and modulation processing. Information processing deals with the preparation of the basic information signals so that they are protected and converted into a form that the radio link can handle. Information processing includes transcoding, channel coding, encrypting, and multiplexing. Modulation processing involves the physical preparation of the signal to carry information on an RF carrier (see Figure 3.6). In the GSM, the analog speech from the mobile station is passed through a low-pass filter to remove the high-frequency contents from the speech. The speech is sampled at the rate of 8000 samples per second, uniformly quantized to 213 (8192) levels and coded using 13 bits per sample. This results in a digital information stream at a rate of 104 kbs. At the base station, the speech signal is digital (64 kbs), which is first
transcoded from the A-law 8-bit samples into 13-bit samples corresponding to a linear representation of the amplitudes. This results in a digital information stream at a rate of 104 kbs (see Figure 3.7). 3.5.4.1Handoff Basically there are two levels of handoffs: internal and external. If the serving and target BTSs are located within the same BBS, the BSC for the BSS can perform a handoff without the involvement of the MSC. This type of handoff is referred to intra-BSS handoff. However, if the serving and target BTSs do not reside within the same BSS, an external handoff is performed. In this type of handoff the MSC coordinates the handoff and performs the switching tasks between the serving and target BTSs.
Figure 3.6: Digital Radio Link Process[1]
Figure 3.7: GSM Speech Processing[1] 3.5.5 Japanese Digital Cellular (JDC) System The JDC system uses three-channel TDMA. Two frequency bands are reserved: 800-MHz band with 130-MHz of duplex separation and 1.5GHz band with 48 MHz of duplex separation. The 800-MHz band is used first. The 1.5-GHz band will be used later on. The modulation scheme is π/4-QPSK, and the interleaved carrier spacing is 25 kHz.
In the 800-MHz band, the uplink transmission (from the MS to the BS) frequency is 940 to 956 MHz and the downlink transmission (from the BS to the MS) frequency is 810 to 826 MHz. Since a 25-kHz channel bandwidth is used, this provides 640 carriers and 3 channels per carrier. Thus, a total of 1920 channels are available. The number of channels will be doubled as the half-rate speech coders are introduced. 3.5.5.1JDC Channel and Frame Structure The logical channel structure is shown in Figure 3.8. The channels are divided into the Traffic Channel (TCH) and Control Channel (CCH). The TCH is used to transmit user information and the CCH for control information. There are two types of CCH: Common Access Channel (CAC) shared by many users and USER Specific Channel (USC) dedicated to a user. The CAC is future divided into three types. The Broadcast Control Channel (BCCH) provides the MS with system information that contains the MS related data such as maximum transmission power of the MS, location identity code to register the user’s location, and the information related to CCH structure, such as the number of CCHs available in the cell. The Common Control Channel (CCCH) and the User Packet Channel (UPCH)
Figure 3.8: Logical Channel Structure for JDC[1] The JDC frame structure is given in Figures 3.9 and 3.10. The frame is 20 ms long and has three time slots. Each frame carries 840 bits; this corresponds to a data rate of 42 kbs.
Figure 3.9: JDC Frame Structure—Traffic Frame[1]
Figure 3.10: JDC Frame Structure—Control Frame[1] Figures 3.11 and 3.12 show the JDC physical channel for traffic and LDC physical channel for control, respectively. A comparison between the GSM and LDC system is given in Table 3.3.
Figure 3.11 JDC Physical Channel for Traffic[1]
Figure 3.12 JDC Physical Channel for Control[1]
Table 3.3 GSM and JDC System Comparison GSM Access Method
Frequency range (MHz)
890-915 uplink 935-960 downlink
Channel bandwidth (kHz) Modulation Bit rate (kbs) Voice channel coding Voice frame (ms) Interleaving (ms) Slot/frame No. Of channels Associate control channel
200 GMSK 270.83 RPELTP/Convolutional 13 kbs 4.6 40 8 124 Extra frame
JDC TDMA/FDMA 940-956 uplink 810-826 downlink 1447-1489 uplink 1492-1441 downlink 1501-1513 uplink 1453-1465 downlink 25 π/4-DQPSK 42.0 VSELP/Convolutional 11.2 kbs 20 26.667 3 640 Same frame
3.6 North American Cellular System
The North American cellular systems have envolved to two standards IS-136 using TDMA and IS-95 using CDMA. The cellular versions of the standards support both analog AMPS and the digital protocols. Thus, there are three types of phones in general use for cellular in North America: - an analog-only AMPS phone - a dual-mode AMPS and TDMA phone - a dual-mode AMPS and CDMA phone 3.6.1 PCS Reference Models: There are two types have a reference model TR-46 and T1P1, but each model can be converted into the other one. The main difference between the two reference models is how mobility is managed. Mobility is the capability for users to place and receive calls in systems other than their home system. 3.6.1.1 TR-46 Reference Model: The main elements of the TR-46 reference model as in the Figure 3.13:
The Personal Station (PS) The Radio System (RS) The Personal Communications Switching Center (PCSC) The Home Location Register (HLR) Data Message Handler (DMH) The Visited Location Register (VLR) The Authentication Center (AC) The Equipment Identity Register (EIR) Operations System (OS) Interworking Function (IWF)
Figure 3.13: TR-Reference Model 3.6.1.2 T1P1 PCS Reference Architecture: The T1P1 architecture in the Figure 3.14 below is similar to the TR46 model but has some differences.
Figure 3.14: T1P1 Reference Model 3.6.2 Framing of Digital Signal: 3.6.2.1 Framing of Analog Cellular: The analog cellular system uses a digital format at 10 kbs sent on separate control channel. The basic frame is 463 bits long. Synchronization of the data receiver is established via 10-bit 1010101010 (dotting) pattern and 11-bit 11100010010 framing pattern. Messages are then sent on an A of B frame and repeated five times. Each message uses a (40,28,5) BCH code. The combination of the BCH code and the five repetitions of the message provide the error detection and correction in the data receiver. After the data is encoded. It undergoes a further encoding, called Manchester Encoding, using two bits per baud. Every 10 bits in the frame, a busy/idle bit is sending an idle status. Figures 3.15, 3.16, 3.17 show the frame information of the reverse control channel and the forward and reverse blank and burst channel. Different length of the dotting pattern. On the reverse channel a (48,36,5) BCH code is used.
Figure 3.15: Reverse Control Channel Framing
Figure 3.16: Forward Voice Channel Framing
Figure 3.17: Reverse Voice Channel The analog control channel was designed in the 1970s When most data receivers were implemented in hardware, and therefore it uses inefficient coding systems. The overall throughput of the various channels (forward and reverse) is 300 to 600 bits per second even though the signaling is done at 20 kilobaud per second. The digital cellular and PCS protocols have taken advantage of improvements in microprocessor technology and coding algorithms to implement more efficient coders and decoders. The analog cellular systems are based on a frequency reuse factor (N) of 7. Nearby cells on the same frequency are coded with a different Digital Color Code (DCC) on the control channels and a different Supervisory Audio Tone (SAT) on the voice channels. 3.6.2.2 Farming of TDMA In the TDMA system, as supported at both 900 MHz for cellular and at 1,800 MHz for PCS, the overall 30 kHz RF channel structure from analog cellular is maintained. Each RF channel has six time slots and
support up to six PSc. As in analog cellular is maintained. Each Rf channel has six time slots and supports three PSs. In the future with half – rate channels, an RF channel will be able to support up to six PSc. As in analog cellular, the channels are designated as control channels of traffic channels. Figure 3.18, show is the frame structure of the digital TDMA channel frame. The frame length is 40 ms (i.e., 25 frames is segmented into six frame being 1,944 bits (972 symbols) long. The frame is segmented in six equally sized time slots (1-6) with 162 symbols (or 324 bits). Each full rate traffic channel utilizes two equally spaced time slots of the frame (i.e. 1 and 4, 2 and 5, or 3 and 6), whereas each half –rate channel uses only one time slot of the frame. Both control and traffic channels use a common frame with a slot format that is slightly different depending on whether the transmission is on a control channel or a traffic channel. The slot format on the traffic channel is defined in Figures 3.19 and 3.20. The fields in the RS to PS slot are defined as: • SYNC: 28 bits of synchronization • SYNC: 28 bits of synchronization • SACCH: 12 bits for the Slow Associated Control Channel. • DATA: two 130 bits fields of data; the data can be digitized voice or data.
Figure 3.18: TDMA frame structure
Figure 3.19: Time slot format RS to PS Traffic channel
Figure 3.20: Time slot format PS to RS Traffic channel
CDVCC: 12 bits of a Coded Digital Verification Color (similar to the SAT tone for analog cellular) • RSVD : 1 bit a reserved field. • CDL: 11 bits for a coded Digital Control Channel Locator. On the reverse traffic channel (from PS to RS), the slot format (figure 6) has the following field definitions: • G: equivalent of 6 bits guard time ( the PS does not transmit during this time) • R: equivalent of 6 bits of ramp time (the PS is powering up) • SYNC: 28 bits of synchronization. • DATA: one field of 16 bits plus two fields of 122 bits; the data can be digitized voice or data. • SACCH: 12 bits for the slow associated control channel. • CDVCC : 12 bits of a coded digital verification color code (similar to the SAT tone for analog cellular ) On forward digital control channel (RS to PS), the fields in each slot are (see Figure 3.21)
•
Figure 3.21 • • SYNC: 28 bits of synchronization. SCF: two fields of 12 bits of Shared Channel Feedback (i.e., status of reverse channel) • DATA: two 130-bit fields of data. • CFSP: 12 bits of Coded Super Frame Phase. • RSVD: 2 bits of a reserved field. On the reverse digital control channel (from PS to RS), the PS can transmit with a normal format (Figure 3.22) or an abbreviated format (Figure 3.23). For these slots, the field definitions are:
Figure 3.22: Time slot Format: PS to RS on Digital Control Channel
Figure 3.23: Abbreviated Time Slot Format: PS to RS on Digital Control Channel • G: equivalent of 6 bits guard time (the PS does not transmit during this time) • R: equivalent of 6 bits of ramp time (the PS is powering up) • PREAM: 16 bits of a preamble. • SYNC: 28 bits of synchronization. • DATA: two fields of 122 bits of eata for a normal slot or one field of 122 bits and one field of 78 bits for an abbreviated slot: the data can be traffic data (e.g., voice) or control data. • SYNC+: additional synchronization. • R: equivalent of 4 bits of ramp time (the PS is powering down) • AG: guard time for abbreviated burst.
3.7 Second-Generation- Plus PCS Systems
Although many people describe PCS as a third-generation system, the U.S. implementation uses modified cellular protocols. The opening of the 2-GHz band by the FCC has generated a flurry of activity to develop new systems. Unfortunately, in the race to deploy systems, most work has been to up band the existing cellular systems to the new 2 -GHz band. Whether the protocol is GSM, IS54, or IS-95, each proponent wants to make minimum changes in its protocol to win the PCS race. It may not be until later in the 1990s before true third-generation systems offering wireless multimedia access emerge. The initial offering may be tailored to the environment and the need for rapid entry into the market place.
Basic needs for PCS include standardized low-power technology to provide voice and moderate-rate data to small, lightweight, economical, pocket-size personal handset that can be used for tens of hours without attention to batteries and to be able to provide such communication economically over wide area, including in homes and other buildings, outdoors for pedestrians in neighborhoods and urban areas, and anywhere there are reasonable densities of people. The DCS1800 is a standard for PCN that has been developed by ETSL. It is a derivative of the GSM900 MHz cellular standard. In Europe DCS has been allocated frequencies from 1710 to 1785 MHz and 1805 to 1880 MHz to provide a maximum theoretical capacity of 375 radio carrier, each with 8 or 16 voice/data channels. In DCS1800 there are provisions fore national roaming between operators with overlapping coverage. These modification have enabled the GSM cellular standard to be enhanced to provide a high-capacity, quality PCN system that can be optimized for handheld operation. The 1800-MHz operating band in the DCS results in a small cell structure that is compatible with the PCN concept. The 1800-MHz band is occupied by fixed radio links for which alternative technologies exist, and clearance of the band can be more readily effected than attempting to manage coexistence and transition between the first- and second-generation systems at 800/900 MHz. The initial implementation of Europe PCN is based on the provision of a high-quality small cell network. Radio coverage and system parameter are optimized for low-power handsets, and emphasis is placed on providing a significantly higher statistical call success and quality level for the handheld portable than current cellular networks provide. The future evolution of DCS1800 may include microcell structure for coverage and capacity enhancement into buildings such as airport terminals, railway stations, and shopping centers, where large numbers of people gather. A further development would then be in private cells within offices to proved business communications. Ubiquitous deployment of microcells in a PCN environment will require a very fast handoff processing capability that is not currently available on DCS1800.
3.8 Vision of The Third-Generation Systems
First-generation analog and second-generation digital systems are designed to support voice communication with limited data communication capabilities. Third-generation systems are targeted to offer a wide variety of services listed in Table 2.. Most of the services are wireless extensions of Integrated Services Digital Network (ISDN), whereas services such as navigation and location information are mobile specific. Wireless network uses will expect a quality of service similar to
that provided by the wireline networks such as ISDN. Service providers will require higher- complexity protocols in physical link layer because of the unpredictable nature of the radio propagation environment and the inherent terminal mobility in a wireless network. These protocols will powerful forward error correction and digital speech interpolation techniques to match the quality of services of the fixed network. Because of the multitude of teleservices offered in different operating scenarios, the teletraffic density generated will depend on the environment, the mix of terminal types, and the terminal density. Teletraffic density will vary substantially for high-bit-rate services provided in business areas, whereas basic services such as speech and video telephony will be offered in all other environments. The third-generation network will concentrate on the service quality, system capacity, and personal and terminal mobility issues. The system capacity will be improved by using smaller cells and the reuse of frequency channels in a geographically ordered fashion. A thirdgeneration network will use different cell structure according to the operational environment. Cell structures will range from conventional macrocells to indoor picocells. In particular, microcells with low transmission power will be widely deployed in urban areas, while other cell structure will be used according to the environment to provide ubiquitous coverage. It expected that the cost of base station equipment for microcells will be significantly because of the elimination of costly high-power amplifiers and the economies of scale in microcell base station manufacturing. Nevertheless, the system’s cost will still play a dominant role in the design of the network infrastructure because more microcellular base station will be required to provide adequate radius less then 1000m will be used extensively to proved a coverage in metropolitan areas. Microcell stations will be mounted on lamp posts or on buildings where electric supply is readily available. For high-user-density areas such as airport terminals, picocells with coverage of tens of meters will be used. To facilitate efficient handoff when the vehicle-based user crosses microcells at high speed, theses calls will be handled by umbrella cells whose coverage areas will contain several to tens of microcells. The planning of third-generation system will be more complicated than the design of present speech- oriented, macrocell-based mobile systems and will require a more advanced and intelligent network planning tool.
REFERANCES
[1] V.K. Garg and J. E. Wilkes, Wireless and Personal Communications Systems , Prentice Hall PTR, Upper Saddle Rive, NJ 07458, 1996. [2] W. Lee, Mobile cellular telecommunications: analog and digital systems , 2nd ed., McGraw-Hill, NY, 1995. [3] S. Faruque, Cellular Mobile Systems Engineering , Artech House, Norwood, MA, 1996. [4] W. Lee, Mobile Communications Engineering: Theory and applications, 2nd Ed., McGraw-Hill, 1997.
CONTENTS
I
CHAPTER 1:INTRODUCTION CHAPTER 2:CELLULAR SYSTEM BASICS 2.1:WHY CELLULAR MOBILE TELEPHONE 2.2:TRUNKING EFFECIENCY 2.3:A BASIC CELLULAR SYSTEM 2.4:UNIQUENESS OF MOBILE RADIO ENVIRONMENT CHAPTER 3:CELLULAR SYSTEM AND PCS 3.1:INTRODUCTION 3.2:HISTORICAL OVERVIEW 3.3:FIRST-GENERATION CELLULAR SYSTEMS 3.4:SECOND-GENERATION CELLULAR SYSTEMS 3.5:EUROPEAN AND JAPANESE CELLULAR SYSTEM 3.6:NORHT AMERICA CELLULAR SYSTEM 3.7:SECOND-GENERATION-PLUS PCSSYSTEMS 3.8:VISION OF THE THIRD-GENERATION SYSTEMS REFERENCES
1
2 4 4 7
12 12 13 14 15 24 30 31 33
Chapter One
INTRODUCTION
Over the last decade, deployment of wireless communication in North America and Europe has been phenomenal. Wireless communications technology has evolved along a logical path, from simple first-generation analogue products designed for business use to second- generation digital wireless telecommunications systems for residential and business environments. As the industry plans and implements the second-generation digital networks in the mid-1990s, a vision of a next-generation wireless information network is emerging. Complete Personal Communication Services (PCS) will enable all users to economically transfer any form of information between any desired locations. PCS is likely to explode worldwide in the mid to late 1990s with very high global revenues for services and handsets. As the market expands, it will draw in a growing number of low-end residential users and will derive basic voice services up from half the PCS market in the mid-1990s to about three-fourth in 1999. PCS use both existing and future wireline and wireless networks. Three key elements of PCS will be an easy-to-use, high-functionality handset, and a single, personal number that can reach the subscriber anywhere, and finally, an individualized feature profile that follows the user and provides a customized set of services at any location. PCS will not be a single, all-encompassing wireless solution, but it will be a combination of standards, and products that meet a range of user requirements at a reasonable price with high level of support. PCS will fill gaps left by other modes of wireless and wireline telephony. As we see, the PCS is not a simple concept, which leads us to study every single part of PCS alone. In Chapter 2 we study the concept of cellular system, which acts as a backbone of PCS. In Chapter 3 we study the evolution of cellular system from first-generation, second-generation, to the vision of the third-generation cellular system.
Chapter Two
Cellular System Basics
2.1 Why Cellular Mobile Telephone Systems?
2.1.1 Limitations of conventional mobile telephone systems One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization. Limited service capability. A conventional mobile telephone system is usually designed by selecting one or more channels from a specific frequency allocation for use in autonomous geographic zones, as shown in Figure 2.1. The communications coverage area of each zone is normally planned to be as large as possible, which means that the transmitted power should be as high as the federal specification allows. The user who starts a call in one zone has to reinitiate the call when moving into a new zone (see Figure 2.1) because the call will be dropped. This is an undesirable radio telephone system since there is no guarantee that a call can be completed without a handoff capability.
Figure 2.1: Conventional mobile system The handoff is a process of automatically changing frequencies as the mobile unit moves into a different frequency zone so that the conversation can be continued in a new frequency zone without redialing. Another disadvantage of the conventional system is that the number of
active users is limited to the number of channels assigned to a particular frequency zone. Poor service performance. In the past, a total of 33 channels were allocated to three mobile telephone systems. The large number of subscribers created a high blocking probability during busy hours. Although service performance was undesirable, the demand was still great. A high-capacity system for mobile telephones was needed. Inefficient frequency spectrum utilization. In a conventional mobile telephone system, the frequency utilization measurement is defined as the maximum number of customers that could be served by one channel at the busy hour. As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area. A new cellular system that measures the frequency spectrum utilization and proves to be efficient is discussed in Section 2.1.2. 2.1.2 Spectrum efficiency considerations A major problem facing the radio communication industry is the limitation of the available radio frequency spectrum. In setting allocation policy, the Federal Communications Commission (FCC) seeks systems which need minimal bandwidth but provide high usage and consumer satisfaction. The ideal mobile telephone system would operate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are 1. Single-sideband (SSB), which divides the allocated frequency band into maximum numbers of channels 2. Cellular, which reuses the allocated frequency band in different geographic locations 3. Spread spectrum or frequency-hopped, which generates many codes over a wide frequency band In 1971, the cellular approach was shown to be a spectrally efficient system. 2.1.3 Technology, feasibility, and service affordability In 1971, the computer industry entered a new era. Microprocessors and minicomputers are now used for controlling many complicated features and functions with less power and size than was previously possible. Large-scale integrated (LSI) circuit technology reduced the size of mobile transceivers so that they easily fit into the standard automobile. These achievements were a few of the requirements for developing
advanced mobile phone systems and encouraging engineers to pursue this direction. Another factor was the price reduction of the mobile telephone unit. LSI technology and mass production contribute to reduced cost so that in the near future an average-income family should be able to afford a mobile telephone unit.
2.2 Trunking Efficiency
Figure 2.2 shows ηe by comparing one carrier per market with more than one carrier per market situations with different blocking probability conditions. The degradation of trunking efficiency decreases as the blocking probability increases. As the number of carriers per market increases the degradation increases. However, when a high percentage of blocking probability, say more than 20 percent, occurs, the performance of one carrier per market is already so poor that further degradation becomes insignificant, as Fig. 2.2 shows.
Figure 2.2: Degradation of trunking efficiency- comparing one carrier/market and other-than-one-carrier/market For a 2 percent blocking probability, the trunking efficiency of one carrier per market does show a greater advantage when compared to other scenarios.
2.3 A Basic Cellular System
As we mentioned before, most commercial radio and television systems are designed to cover as much area as possible. These systems typically operate at the maximum power and with highest antennas
allowed. The frequency used by the transmitter cannot be reused until there is enough geographical separation so that one station does not interfere significantly with another station assigned to that frequency. The cellular system takes the opposite approach. It seeks to make an efficient use of available channel by using low-power transmitters to allow frequency reuse at much smaller distances as in Figure 2.3. Maximizing the number of times each channel may be reused in a given geographic area is the key to an efficient cellular system design.
2 7 1 6 2 7 1 6 5 4 6 5 3 7 1 4 6 5 5 2 3 7 1 4 4 6 5 2 3 3 7 1 4 2 3
Figure 2.3: Frequency reuses. Cellular systems are designed to operate with groups of low-power radios spread out over the geographical service area. Each group of radios serve mobile unit presently located near them. The area served by each group of radios is called a “cell”. Each cell has an appropriate number of low-power radios for communication within itself. The power transmitted is chosen to be large enough to communicate with mobile units located near the edges of the cell. The radius of each cell may be chosen to perhaps 26km in start-up system with relatively few subscribers, down to less than 2km for mature system requiring considerable frequency reuse. As the traffic grows, new calls and channels are added to the system. If an irregular cell pattern were selected, it would lead to an inefficient of the spectrum due to its inability to reuse frequencies on account of cochannel interference. In addition, it would also result in an uneconomical deployment of equipment, requiring relocation from one cell site to another. Therefore, a great deal of engineering effort would be required to readjust the transmission, switching, and control resources every time the system goes through its development phase. All these difficulties lead to the use of a regular cell pattern in a cellular system design. In reality, the cell coverage is an irregularly shaped circle. The exact coverage of the cell will depend on the terrain and other factors. For design convenience and as a first-order approximation, we assume that the coverage areas regular polygons. For example, for an omnidirectional
antenna with constant signal power, each cell-site coverage area would be circular. To achieve full coverage without dead spots, a series of regular polygons for cell sites are required. Any regular polygon, such as an equilateral triangle, a square, or a hexagon, can be used for cell design. The hexagon is used for two reasons: first, a hexagonal layout requires fewer cells and therefore, fewer transmitter sites and second, a hexagonal cell layout is less expensive compared to square and triangular cells. In practice, after the polygons are drawn on a map of the coverage area, redial lines are drawn and the SNR ratio calculated for various directions. A basic cellular system consists of three parts: a mobile unit, a cell site, and a mobile telephone switching office (MTSO), as Figure 2.4 shows, with connections to link the three subsystems.
Figure 2.4: Cellular system 1. Mobile units. A mobile telephone unit contains a control unit, a transceiver, and an antenna system. 2. Cell site. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. 3. MTSO. The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, and handles billing activities. 4. Connections. The radio and high-speed data links connect the three subsystems.
The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and cellular administration.
2.4 Uniqueness of Mobile Radio Environment
2.4.1 Description of mobile radio transmission medium The propagation attenuation. In general, the propagation path loss increases not only with frequency but also with distance. Normally the incident angles of both the direct wave and the reflected wave are very small, as Figure 2.5 shows.
Figure 2.5: Mobile radio transmission model The incident angle of the direct wave is θ1, and the incident angle of the reflected wave is θ2. θ1 is also called the elevation angle. The propagation path loss would be 40 dB/dec., where “dec.” is an abbreviation of decade. This means that the mobile unit will observe a 40-dB loss at a single receiver as it moves from 1 to 10 km. Therefore Pr ∝ D-4 Pr = α D-4 Where Pr = received carrier power D = distance measured from the transmitter to, the receiver α = constant The difference in power reception at two different distances D1 and D2 will result in
Pr 1 D 2 = Pr 2 D1
−4
In the decibel expression ∆Pr(dB) = 10 log Pr2/P r1 = 40 log D1/D2 When D2 =2D1, ∆Pr = -12 dB; when D2 = 10D1, ∆Pr= 40 dB. This 40 dB/dec. is the general rule for the mobile radio environment and is easy to remember. It is also easy to compare to the free-space
propagation rule of 20 dB/dec. The linear and decibel scale expressions are Pr ∝ D-2 (free space) And ∆Pr = 20 log D1/D2 (free space) In a real mobile radio environment, the propagation path-loss slope varies as Pr ∝ D-γ Pr = α D-γ γ usually lies between 2 and 5 depending on the actual conditions. Of course γ cannot be lower than 2, which is the free-space condition. The decibel expression is Pr(dB) = 10 log α - 10γ log D Severe fading. Since the antenna height of the mobile unit is lower than its typical surroundings, and the carrier frequency wavelength is much less than the sizes of the surrounding structures, multipath waves are generated. At the mobile unit, the sum of the multipath waves causes a signal-fading phenomenon. The signal fluctuates in a range of about 40 dB (10 dB above and 30 dB below the average signal). We can visualize the nulls of the fluctuation at the baseband at about every half wavelength in space, but all nulls do not occur at the same level, as Figure 2.6 shows. If the mobile unit moves fast, the rate of fluctuation is fast.
Figure 2.6: A typical fading signal received while the mobile unit is moving.
2.4.2 Model of transmission medium A mobile radio signal r(t), illustrated in Figure 2.7 can be artificially characterized by two components m(t) and ro(t) based on natural physical phenomena. r(t)= m(t) ro(t) The component m(t) is called local mean, long-term fading, or lognormal fading and its variation is due to the terrain contour between the base station and the mobile unit. The factor ro is called multipath fading, short-term fading, or Rayleigh fading and its variation is due to the waves reflected from the surrounding buildings and other structures.
Figure 2.7: A mobile radio signal fading representation. (a) A mobile signal fading. (b) A short-term signal fading 2.4.3 Mobile fading characteristics Rayleigh fading is also called multipath fading in the mobile radio environment. When these multipath waves bounces back and forth due to the buildings and houses, they form many standing-wave pairs in space, as shown in Figure 2.8. Those standing-wave pairs are summed together and become an irregular wave-fading structure. When a mobile unit is standing still, its receiver only receives signal strength at that spot, so a constant signal is observed. When the mobile unit is moving, the fading
structure of the wave in the space is received. It is a multipath fading. The recorded fading becomes fast as the vehicle moves faster.
Figure 2.8: A mobile radio environment-two parts. (1) Propagation loss; (2) multipath fading The radius of the active scatterer region. The mobile radio multipath fading shown in Fig. 1.7 explains the fading mechanism. The radius is roughly 100 wavelengths. The active scatterer region always moves with the mobile unit as its center. It means that some houses were inactive scatterers and became active as the mobile unit approached them; some houses were active scatterers and became inactive as the mobile unit drove away from them. Delay spread and coherence bandwidth Delay spread. In the mobile radio environment, as a result of the multipath reflection phenomenon, the signal transmitted from a cell site and arriving at a mobile unit will be from different paths, and since each path has a different path length, the time of arrival for each path is different. For an impulse transmitted at the cell site, by the time this impulse is received at the mobile unit it is no longer an impulse but rather a pulse with a spread width that we call the delay spread. The measured data indicate that the mean delay spreads are different in different kinds of environment.
Type of environment Inside the building Open area Suburban area Urban area
Delay spread ∆, µs <0.1 <0.2 0.5 3
2.4.4 Direct wave path, line-of-sight path, and obstructive path A direct wave path is a path clear from the terrain contour. The lineof-sight path is a path clear from buildings. In the mobile radio environment, we do not always have a line-of-sight condition. When a line-of-sight condition occurs, the average received signal at the mobile unit at a 1 -mi intercept is higher, although the 40-dB/dec. path-loss slope remains the same. In this case the short-term fading is observed to be a rician fading. It results from a strong line-of-sight path and a ground-reflected wave combined, plus many weak buildingreflected waves. When an out-of-sight condition is reached, the 4O-dB/dec path-loss slope still remains. However, all reflected waves, including ground reflected waves and building-reflected waves, become dominant. The short-term received signal at the mobile unit observes a Rayleigh fading. The Rayleigh fading is the most severe fading. When the terrain contour blocks the direct wave path, we call it the obstructive path . In this situation, the shadow loss from the signal reception can be found.
Chapter Three
Cellular System and PCS
3.1 Introduction
Personal Communication Services (PCS) is a new network that has many features. One of these features is that it will be a future common global standard. PCS will not be a single, all-encompassing wireless solution, but it will be a combination of standards, and products that meet a range of users. In this chapter we briefly discuss the current and preceding cellular systems tell we reach the next generation of cellular system which expected to offered the PCS requirements. We first need to have a historical overview of mobile telephone services. We then, discuss the first- and second-generation cellular systems. We outline the problem associated with the second-generation-plus PCS system and provide a vision of third-generation system.
3.2 Historical Overview
The first mobile telephone service was introduced in the United States 1946 by AT&T. It was used to interconnect mobile users to the public telephone land-line network, thus allowing telephone calls between fixed station and m obile users. This mobile telephone system was based on Frequency Modulation (FM) transmission. Demand for mobile telephone service grew quickly and stayed ahead of the available capacity in many urban cities. The usefulness of the mobile telephone decreased as user found that blocking often prevented them from getting a circuit during the peak periods. User and the telephone companies, alike, realized that a handful of channel would not be enough for mobile telephone service to develop. Large blocks of spectrum would be needed to satisfy the demand on the urban areas. In the mid-1960s the Bell System introduced the improved Mobile Telephone Services (IMTS) with enhanced features. In the late 1960s and the early 1970s work began on the first cellular telephone systems. It should be recognized that the first-generation analogue cellular radio was not so much a new technology as it was a new idea for organizing existing IMTS technology on large scale. While the voice communication used the same analogue FM that had been used, two major technological improvements made the cellular concept a reality. In the early 1970s
microprocessor was invented. The second improvement was in the use of a digital control link between the mobile telephone and the base station In the late 1980s interest emerged in a digital cellular system, where both the voice and the control were digital. By 1991 digital cellular services began to emerge to reduce the cost of wireless communications and improve the call-handling capacity of an analog cellular system. In 1993, a digital system was placed in service.
3.3 First-Generation Cellular Systems
As the United States was planning its cellular network in the 1970s, England, Japan, Germany, and the Scandinavian countries were also planning their systems. Each system used a different frequency band and different protocols for signaling between mobile units and base stations. They all used analog FM (with different deviations and channel spacings) for their voice communications. Table 3.1 gives a comparison of various operational aspects five cellular systems used in those countries. Table 2.1: First-Generation Cellular Systems
System Parameter North America (AMPS) 870-890 825-845 45 30 666/832 2-25 FM ±12 FSK ±8 10 U.K. (TACS) Scandinavian (NMT) West Germany (C450) Japan (NTT)
Transmission frequency (MHz) *Base station *Mobile station Spacing between transmitter & receiver frequency (MHz) Spacing between channels (kHz) Number of channels Coverage radius by one base station (km) Audio signal *Modulation *Maximum frequency deviation (kHz) Control signals *Modulation *Frequency deviation Data transmission (kbs)
935-960 890-915 45 25 1000 2-20 FM ±9.5 FSK ±6.4 8
463-467.5 453-457.5 10 25 180 1.8-40 FM ±5 FSK ±3.5 1.2
461.3-465.74 870-885 451.3-455.74 925-940 10 55 20 222 5-30 FM ±4 FSK ±2.5 5.28 25 600 5-10 FM ±5 FSK ±4.5 0.3
The cellular approach promised virtually unlimited capacity through cell splitting. As the popularity of wireless escalated in the 1980s, the cellular industry faced practical limitations. For a fixed allocation of spectrum, a large increase in capacity implies corresponding reduction in cell size. As the cell get smaller, it becomes increasingly difficult to place base stations at the locations that offer necessary radio coverage. Also, reduction in cell size demand increased signaling activity as more rapid handoffs occur; in addition, base stations are required to handle more access requests and registrations from mobile stations. The problem becomes particularly difficult in large urban areas where capacity
requirements are most pressing. In addition to the capacity bottleneck, the utility of first-generation analog systems was diminished by proliferation of incompatible standards in Europe. The same mobile telephone frequencies cannot be used in different European countries. The limitations of first-generation analog system provided motivations to the second-generation systems. The principal goals of the second-generation systems were: higher capacity and hence lower cost, and, in Europe, a continental system with full international roaming and handoff capabilities. In Europe, these goals are served by new spectrum allocations and by formulation of a Pan-European Cellular Standard GSM. In North America, where one standard (United States, Canada, and Mexico) existed and covered a region as large as Europe, the push for a new system was not as strong. In the new digital systems, higher capacity is derived from applications of advanced transmission techniques including efficient speech coding, error correcting channel codes, and bandwidth-efficient modulation techniques. In Europe, the approach was to open new frequency bands for a panEuropean system and not to have compatibility with existing cellular systems. In the United States, the same frequency bands were shared with the new digital systems, and the standards supported dual-mode telephones that could be used in both analog and digital systems.
3.4 Second-Generation Cellular Systems
First-generation cellular systems were designed to satisfy the needs of business customers and some residential customers. With the increased demand of cellular telephones in Europe, several manufacturers began to look for new technologies that could overcome the problems of poor signals and battery performance. Poor signals resulted in poor performance for the user and a high frequency of false handoffs for the system operator. Better battery performance was needed to reduce size and cost of self- contained handheld units. Research efforts were directed toward wireless technologies to provide high-quality, interference-free speech and decent battery performance. The size of handset and better battery performance led to low-power designs and performance targets possible only with fully digital technologies. Digital cellular system based on the GSM (TDMA) standard have emerged in Europe, while system based on IS-54 (TDMA) and IS-95 (CDMA) are being developed in the United States. Table 3.2 provides a summary of the cellular and cordless systems.
Table 3.2: Second-Generation Cellular and cordless Systems
System Country Access technology Primary use Frequency band *Base Station (MHz) *Mobile Station (MHz) Duplexing RF channel spacing (kHz) Modulation Power, maximum/average mW Frequency assignment Power control *Base station *Mobile Station Speech coding Speech rate (kbs) Speech channel per RF channel Channel bit rate (kbs) Frame duration (ms) IS-54 U.S. TDMA/ FDMA Cellular GSM Europe TDMA/ FDMA Cellular CT-3 DECT DCT-900 U.S. Europe, Asia Sweden Europe CDMA/ FDMA TDMA/ TDMA/ (DS) DMA FDMA FDMA Cellular Cordless Cordless Cordless/ cellular 864-868 TDD 100 GFSK 10/5 Dynamic Y Y QCELP 8(variable rate) 20 N N ADPCM 32 1 72 2 862-866 1800-1900 TDD 1000 GFSK 80/5 Dynamic N N ADPCM 32 8 640 16 TDD 1728 GFSK 250/10 Dynamic N N ADPCM 32 12 1152 10 IS-95 CT-2
869-894 824-849 FDD 30 π/4 DQPSK 600/200 Fixed Y Y VSELP 7.95 3 48.6 40
935-960 869-894 890-915 824-849 FDD FDD 200 1.250 GMSK BPSK/QPSK 1000/125 600 Dynamic Y Y PRE-LTP 13 8 270.833 4.615
3.5 European and Japanese Cellular Systems
In this section we present an overview of the Global System for Mobile communications (GSM) as described in the European Telecommunication Standards Institute’s (ETSI’s) Recommendations. A brief description of the Japanese Digital Cellular (JDC) system is also given. 3.5.1 GSM Public Land Mobile Network (PLMN) GSM offers users good voice quality, call privacy, and network security. Subscriber Identity Module (SIM) cards provide the security mechanism for GSM. Of major importance is GSM’s potential enhanced services requiring multimedia communication: voice, image, and data. The key to delivering enhanced services is Signaling System Number 7 (SS7), a robust set of protocol layers designed to provide fast, efficient, reliable transfer and delivery of signaling information across the signaling network and to support both switched-voice and nonvoice application. 3.5.2 GSM Architecture
The basic subsystems of the GSM architecture are: Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operational Subsystem (OSS). The BSS provides and manages transmission paths between the mobile stations (MSs) and the NSS. This includes management of the radio interface between MSs and rest of the GSM system. The NSS has responsibility of managing communications and connecting MSs to the relevant networks or other MSs. The MS, BSS, and NSS form the operational part of the GSM system. The OSS provides the means for a service provider to control them. Figure 3.1 shows the model for the GSM system.
Figure 3.1: Model of the GSM System[1] In the GSM, interaction between the subsystems can be groped into two main parts: Operational part: external networks<->NSS<->BSS<->user Control part: OSS<->service provider The operational part provides transmission paths and establishes them. The control part interacts with the traffic-handling activity of the operational part by monitoring and modifying it to maintain or improve its functions. 3.5.2.1 GSM Subsystems Entities Figure 3.2 shows the functional entities of the GSM and their logical interconnection. 3.5.2.1.1 Mobile Station The MS consists of the physical equipment used by the subscriber to access a PLMN for offered telecommunication services. MSs come in five power classes that define the maximum PF
power level that the unit can transmit. Table1 provides the details of maximum RF power for various classes. Table 2.3: Maximum RF Power for Mobile Stations Class Max. RF Power Used For (W) 1 20 Vehicular Unit 2 8 Portable Unit 3 5 Handheld Unit 4 2 Handheld Unit 5 0.8 Handheld Unit
Figure 3.2: GSM Reference Model[1] Basically, an MS can be divided into two parts. The first part contains the hardware and software to support radio and man-machine interface functions. The second part contains terminal/user-specific data in the form of a smart card (SIM card), which can effectively be considered a sort of logical terminal. An MS has a number of identities, including the International Mobile Equipment Identity (IMEI), the International Mobile Subscriber Identity (IMSI), and the ISDN number. SIM is basically a smart card that contains all the subscriber-related information stored on the user’s side of the radio interface. 3.5.2.1.2 Base Station System The BSS is the physical equipment that provides radio coverage to prescribed geographical area, known as the cells. It contains equipment required to communicate with the MS. Functionally, a BSS consists of: a control function carried out by the Base Station Controller (BSC) and a transmitting function performed by the Base Transceiver System (BTS). The BTS is the radio transmission
equipment and covers each cell. A BSS can serve several cells because it can have multiple BTSs. 3.5.2.2 Network and Switching Subsystem The NSS includes the main switching functions of the GSM, databases required for the subscribers, and mobility management. Its main role is to manage the communications between the GSM and other network users. Within the NSS, the switching functions are performed by the MSC. Subscriber information relevant to provisioning of services is kept in the Home Location Register (HLR). The other database in the NSS is the Visitor Location Register (VLR). The Mobile Service Switching Center performs the necessary switching functions required for the MSs located in an associated geographical area, called an MSC area. The MSC monitors the mobility of its subscribers and manages necessary resources required to handle and update the location registration procedures and to carry out the handoff functions. The MSC is involved in the interworking functions to communicate with other networks such as PSTN and ISDN. The call routing and control and echo control functions are performed by the MSC. The Home Location Register has two types of information stored in it: subscriber information and part of the mobile information to allow incoming calls to be routed to the MSC for the particular mobile. The Visitor Location Register is the functional unit that dynamically stores subscriber information, such as location area, when the subscriber is located in the area covered by the VLR. 3.5.3 GSM Channel and Frame Structure The bandwidth in the GSM is 25 MHz. The frequency band for uplink is 890 to 915 MHz, whereas for the downlink is 935 to 960 MHz. The GSM has 124 channels, each with a bandwidth of 200 kHz. When the MS is assigned to an information channel, a radio channel and a timeslot are also assigned. Radio channels are assigned in frequency pairs (one for the uplink and the other for the downlink). Each pair of radio channels supports up to 8 simultaneous calls (see figure 3.3). Thus, the GSM can support up to 922 simultaneous users with the full-rate speech coder. This number will be doubled to 1984 users with the half-rate speech coder.
Figure 3.3: GSM FDMA/TDMA Structure[1] 3.5.3.1 Logical Channels In the GSM, there are three types of logical channels: Traffic Channel (TCH), control Channel (CCH), and Cell Broadcast Channel (CBCH). The TCHs are used to transmit user information (speech or data). CCHs are used to transmit control and signaling information. The CBCH is used to broadcast user information form a service center to the MS listening in a given cell area. The TCH can be TCH/Full or TCH/half. The TCH/F allows the transmission of 13 kbs of the speech or data at 12, 6, or 3.6 kbs. The TCH/H allows speech coded at a rate around 7 kbs or data at 6 or 3.6 kbs. The Control Channels (CCHs) consists of : 1-Broadcast Channel (BCH) 2-Common Control Channel (CCCH) 3-Dedicated Control Channel (DCCH) The BCHs consists of: 1-Broadcast Control Channel (BCCH) 2-Frequency Correction Channel (FCCH) 3-Synchronization Channel (SCH) The CCCHs include 1-Paging Channel (PCH) 2-Access Grant Channel (AGCH) 3-Random Access Channel (RACH) 3.5.3.2 GSM FrameThe GSM multiframe is 120 ms. It consists of 26 frames of 8 time slots. The structure of a GSM hyperframe, superframe, multiframe, frame, and time slot is shown in figure 3.4. A time slot carries 156.25 bits. The same format is used for the uplink and downlink transmission with various types as shown in Figure 3.5.
Figure 3.4: Physical Structure for GSM Hyperframe, Superframe, Multiframe, Frame, and Time Slot[1]
Figure 3.5: GSM Time Slot Structure[1] 3.5.4 GSM Speech Processing Two major tasks are involved in transmitting and receiving information over a digital radio link: information processing and modulation processing. Information processing deals with the preparation of the basic information signals so that they are protected and converted into a form that the radio link can handle. Information processing includes transcoding, channel coding, encrypting, and multiplexing. Modulation processing involves the physical preparation of the signal to carry information on an RF carrier (see Figure 3.6). In the GSM, the analog speech from the mobile station is passed through a low-pass filter to remove the high-frequency contents from the speech. The speech is sampled at the rate of 8000 samples per second, uniformly quantized to 213 (8192) levels and coded using 13 bits per sample. This results in a digital information stream at a rate of 104 kbs. At the base station, the speech signal is digital (64 kbs), which is first
transcoded from the A-law 8-bit samples into 13-bit samples corresponding to a linear representation of the amplitudes. This results in a digital information stream at a rate of 104 kbs (see Figure 3.7). 3.5.4.1Handoff Basically there are two levels of handoffs: internal and external. If the serving and target BTSs are located within the same BBS, the BSC for the BSS can perform a handoff without the involvement of the MSC. This type of handoff is referred to intra-BSS handoff. However, if the serving and target BTSs do not reside within the same BSS, an external handoff is performed. In this type of handoff the MSC coordinates the handoff and performs the switching tasks between the serving and target BTSs.
Figure 3.6: Digital Radio Link Process[1]
Figure 3.7: GSM Speech Processing[1] 3.5.5 Japanese Digital Cellular (JDC) System The JDC system uses three-channel TDMA. Two frequency bands are reserved: 800-MHz band with 130-MHz of duplex separation and 1.5GHz band with 48 MHz of duplex separation. The 800-MHz band is used first. The 1.5-GHz band will be used later on. The modulation scheme is π/4-QPSK, and the interleaved carrier spacing is 25 kHz.
In the 800-MHz band, the uplink transmission (from the MS to the BS) frequency is 940 to 956 MHz and the downlink transmission (from the BS to the MS) frequency is 810 to 826 MHz. Since a 25-kHz channel bandwidth is used, this provides 640 carriers and 3 channels per carrier. Thus, a total of 1920 channels are available. The number of channels will be doubled as the half-rate speech coders are introduced. 3.5.5.1JDC Channel and Frame Structure The logical channel structure is shown in Figure 3.8. The channels are divided into the Traffic Channel (TCH) and Control Channel (CCH). The TCH is used to transmit user information and the CCH for control information. There are two types of CCH: Common Access Channel (CAC) shared by many users and USER Specific Channel (USC) dedicated to a user. The CAC is future divided into three types. The Broadcast Control Channel (BCCH) provides the MS with system information that contains the MS related data such as maximum transmission power of the MS, location identity code to register the user’s location, and the information related to CCH structure, such as the number of CCHs available in the cell. The Common Control Channel (CCCH) and the User Packet Channel (UPCH)
Figure 3.8: Logical Channel Structure for JDC[1] The JDC frame structure is given in Figures 3.9 and 3.10. The frame is 20 ms long and has three time slots. Each frame carries 840 bits; this corresponds to a data rate of 42 kbs.
Figure 3.9: JDC Frame Structure—Traffic Frame[1]
Figure 3.10: JDC Frame Structure—Control Frame[1] Figures 3.11 and 3.12 show the JDC physical channel for traffic and LDC physical channel for control, respectively. A comparison between the GSM and LDC system is given in Table 3.3.
Figure 3.11 JDC Physical Channel for Traffic[1]
Figure 3.12 JDC Physical Channel for Control[1]
Table 3.3 GSM and JDC System Comparison GSM Access Method
Frequency range (MHz)
890-915 uplink 935-960 downlink
Channel bandwidth (kHz) Modulation Bit rate (kbs) Voice channel coding Voice frame (ms) Interleaving (ms) Slot/frame No. Of channels Associate control channel
200 GMSK 270.83 RPELTP/Convolutional 13 kbs 4.6 40 8 124 Extra frame
JDC TDMA/FDMA 940-956 uplink 810-826 downlink 1447-1489 uplink 1492-1441 downlink 1501-1513 uplink 1453-1465 downlink 25 π/4-DQPSK 42.0 VSELP/Convolutional 11.2 kbs 20 26.667 3 640 Same frame
3.6 North American Cellular System
The North American cellular systems have envolved to two standards IS-136 using TDMA and IS-95 using CDMA. The cellular versions of the standards support both analog AMPS and the digital protocols. Thus, there are three types of phones in general use for cellular in North America: - an analog-only AMPS phone - a dual-mode AMPS and TDMA phone - a dual-mode AMPS and CDMA phone 3.6.1 PCS Reference Models: There are two types have a reference model TR-46 and T1P1, but each model can be converted into the other one. The main difference between the two reference models is how mobility is managed. Mobility is the capability for users to place and receive calls in systems other than their home system. 3.6.1.1 TR-46 Reference Model: The main elements of the TR-46 reference model as in the Figure 3.13:
The Personal Station (PS) The Radio System (RS) The Personal Communications Switching Center (PCSC) The Home Location Register (HLR) Data Message Handler (DMH) The Visited Location Register (VLR) The Authentication Center (AC) The Equipment Identity Register (EIR) Operations System (OS) Interworking Function (IWF)
Figure 3.13: TR-Reference Model 3.6.1.2 T1P1 PCS Reference Architecture: The T1P1 architecture in the Figure 3.14 below is similar to the TR46 model but has some differences.
Figure 3.14: T1P1 Reference Model 3.6.2 Framing of Digital Signal: 3.6.2.1 Framing of Analog Cellular: The analog cellular system uses a digital format at 10 kbs sent on separate control channel. The basic frame is 463 bits long. Synchronization of the data receiver is established via 10-bit 1010101010 (dotting) pattern and 11-bit 11100010010 framing pattern. Messages are then sent on an A of B frame and repeated five times. Each message uses a (40,28,5) BCH code. The combination of the BCH code and the five repetitions of the message provide the error detection and correction in the data receiver. After the data is encoded. It undergoes a further encoding, called Manchester Encoding, using two bits per baud. Every 10 bits in the frame, a busy/idle bit is sending an idle status. Figures 3.15, 3.16, 3.17 show the frame information of the reverse control channel and the forward and reverse blank and burst channel. Different length of the dotting pattern. On the reverse channel a (48,36,5) BCH code is used.
Figure 3.15: Reverse Control Channel Framing
Figure 3.16: Forward Voice Channel Framing
Figure 3.17: Reverse Voice Channel The analog control channel was designed in the 1970s When most data receivers were implemented in hardware, and therefore it uses inefficient coding systems. The overall throughput of the various channels (forward and reverse) is 300 to 600 bits per second even though the signaling is done at 20 kilobaud per second. The digital cellular and PCS protocols have taken advantage of improvements in microprocessor technology and coding algorithms to implement more efficient coders and decoders. The analog cellular systems are based on a frequency reuse factor (N) of 7. Nearby cells on the same frequency are coded with a different Digital Color Code (DCC) on the control channels and a different Supervisory Audio Tone (SAT) on the voice channels. 3.6.2.2 Farming of TDMA In the TDMA system, as supported at both 900 MHz for cellular and at 1,800 MHz for PCS, the overall 30 kHz RF channel structure from analog cellular is maintained. Each RF channel has six time slots and
support up to six PSc. As in analog cellular is maintained. Each Rf channel has six time slots and supports three PSs. In the future with half – rate channels, an RF channel will be able to support up to six PSc. As in analog cellular, the channels are designated as control channels of traffic channels. Figure 3.18, show is the frame structure of the digital TDMA channel frame. The frame length is 40 ms (i.e., 25 frames is segmented into six frame being 1,944 bits (972 symbols) long. The frame is segmented in six equally sized time slots (1-6) with 162 symbols (or 324 bits). Each full rate traffic channel utilizes two equally spaced time slots of the frame (i.e. 1 and 4, 2 and 5, or 3 and 6), whereas each half –rate channel uses only one time slot of the frame. Both control and traffic channels use a common frame with a slot format that is slightly different depending on whether the transmission is on a control channel or a traffic channel. The slot format on the traffic channel is defined in Figures 3.19 and 3.20. The fields in the RS to PS slot are defined as: • SYNC: 28 bits of synchronization • SYNC: 28 bits of synchronization • SACCH: 12 bits for the Slow Associated Control Channel. • DATA: two 130 bits fields of data; the data can be digitized voice or data.
Figure 3.18: TDMA frame structure
Figure 3.19: Time slot format RS to PS Traffic channel
Figure 3.20: Time slot format PS to RS Traffic channel
CDVCC: 12 bits of a Coded Digital Verification Color (similar to the SAT tone for analog cellular) • RSVD : 1 bit a reserved field. • CDL: 11 bits for a coded Digital Control Channel Locator. On the reverse traffic channel (from PS to RS), the slot format (figure 6) has the following field definitions: • G: equivalent of 6 bits guard time ( the PS does not transmit during this time) • R: equivalent of 6 bits of ramp time (the PS is powering up) • SYNC: 28 bits of synchronization. • DATA: one field of 16 bits plus two fields of 122 bits; the data can be digitized voice or data. • SACCH: 12 bits for the slow associated control channel. • CDVCC : 12 bits of a coded digital verification color code (similar to the SAT tone for analog cellular ) On forward digital control channel (RS to PS), the fields in each slot are (see Figure 3.21)
•
Figure 3.21 • • SYNC: 28 bits of synchronization. SCF: two fields of 12 bits of Shared Channel Feedback (i.e., status of reverse channel) • DATA: two 130-bit fields of data. • CFSP: 12 bits of Coded Super Frame Phase. • RSVD: 2 bits of a reserved field. On the reverse digital control channel (from PS to RS), the PS can transmit with a normal format (Figure 3.22) or an abbreviated format (Figure 3.23). For these slots, the field definitions are:
Figure 3.22: Time slot Format: PS to RS on Digital Control Channel
Figure 3.23: Abbreviated Time Slot Format: PS to RS on Digital Control Channel • G: equivalent of 6 bits guard time (the PS does not transmit during this time) • R: equivalent of 6 bits of ramp time (the PS is powering up) • PREAM: 16 bits of a preamble. • SYNC: 28 bits of synchronization. • DATA: two fields of 122 bits of eata for a normal slot or one field of 122 bits and one field of 78 bits for an abbreviated slot: the data can be traffic data (e.g., voice) or control data. • SYNC+: additional synchronization. • R: equivalent of 4 bits of ramp time (the PS is powering down) • AG: guard time for abbreviated burst.
3.7 Second-Generation- Plus PCS Systems
Although many people describe PCS as a third-generation system, the U.S. implementation uses modified cellular protocols. The opening of the 2-GHz band by the FCC has generated a flurry of activity to develop new systems. Unfortunately, in the race to deploy systems, most work has been to up band the existing cellular systems to the new 2 -GHz band. Whether the protocol is GSM, IS54, or IS-95, each proponent wants to make minimum changes in its protocol to win the PCS race. It may not be until later in the 1990s before true third-generation systems offering wireless multimedia access emerge. The initial offering may be tailored to the environment and the need for rapid entry into the market place.
Basic needs for PCS include standardized low-power technology to provide voice and moderate-rate data to small, lightweight, economical, pocket-size personal handset that can be used for tens of hours without attention to batteries and to be able to provide such communication economically over wide area, including in homes and other buildings, outdoors for pedestrians in neighborhoods and urban areas, and anywhere there are reasonable densities of people. The DCS1800 is a standard for PCN that has been developed by ETSL. It is a derivative of the GSM900 MHz cellular standard. In Europe DCS has been allocated frequencies from 1710 to 1785 MHz and 1805 to 1880 MHz to provide a maximum theoretical capacity of 375 radio carrier, each with 8 or 16 voice/data channels. In DCS1800 there are provisions fore national roaming between operators with overlapping coverage. These modification have enabled the GSM cellular standard to be enhanced to provide a high-capacity, quality PCN system that can be optimized for handheld operation. The 1800-MHz operating band in the DCS results in a small cell structure that is compatible with the PCN concept. The 1800-MHz band is occupied by fixed radio links for which alternative technologies exist, and clearance of the band can be more readily effected than attempting to manage coexistence and transition between the first- and second-generation systems at 800/900 MHz. The initial implementation of Europe PCN is based on the provision of a high-quality small cell network. Radio coverage and system parameter are optimized for low-power handsets, and emphasis is placed on providing a significantly higher statistical call success and quality level for the handheld portable than current cellular networks provide. The future evolution of DCS1800 may include microcell structure for coverage and capacity enhancement into buildings such as airport terminals, railway stations, and shopping centers, where large numbers of people gather. A further development would then be in private cells within offices to proved business communications. Ubiquitous deployment of microcells in a PCN environment will require a very fast handoff processing capability that is not currently available on DCS1800.
3.8 Vision of The Third-Generation Systems
First-generation analog and second-generation digital systems are designed to support voice communication with limited data communication capabilities. Third-generation systems are targeted to offer a wide variety of services listed in Table 2.. Most of the services are wireless extensions of Integrated Services Digital Network (ISDN), whereas services such as navigation and location information are mobile specific. Wireless network uses will expect a quality of service similar to
that provided by the wireline networks such as ISDN. Service providers will require higher- complexity protocols in physical link layer because of the unpredictable nature of the radio propagation environment and the inherent terminal mobility in a wireless network. These protocols will powerful forward error correction and digital speech interpolation techniques to match the quality of services of the fixed network. Because of the multitude of teleservices offered in different operating scenarios, the teletraffic density generated will depend on the environment, the mix of terminal types, and the terminal density. Teletraffic density will vary substantially for high-bit-rate services provided in business areas, whereas basic services such as speech and video telephony will be offered in all other environments. The third-generation network will concentrate on the service quality, system capacity, and personal and terminal mobility issues. The system capacity will be improved by using smaller cells and the reuse of frequency channels in a geographically ordered fashion. A thirdgeneration network will use different cell structure according to the operational environment. Cell structures will range from conventional macrocells to indoor picocells. In particular, microcells with low transmission power will be widely deployed in urban areas, while other cell structure will be used according to the environment to provide ubiquitous coverage. It expected that the cost of base station equipment for microcells will be significantly because of the elimination of costly high-power amplifiers and the economies of scale in microcell base station manufacturing. Nevertheless, the system’s cost will still play a dominant role in the design of the network infrastructure because more microcellular base station will be required to provide adequate radius less then 1000m will be used extensively to proved a coverage in metropolitan areas. Microcell stations will be mounted on lamp posts or on buildings where electric supply is readily available. For high-user-density areas such as airport terminals, picocells with coverage of tens of meters will be used. To facilitate efficient handoff when the vehicle-based user crosses microcells at high speed, theses calls will be handled by umbrella cells whose coverage areas will contain several to tens of microcells. The planning of third-generation system will be more complicated than the design of present speech- oriented, macrocell-based mobile systems and will require a more advanced and intelligent network planning tool.
REFERANCES
[1] V.K. Garg and J. E. Wilkes, Wireless and Personal Communications Systems , Prentice Hall PTR, Upper Saddle Rive, NJ 07458, 1996. [2] W. Lee, Mobile cellular telecommunications: analog and digital systems , 2nd ed., McGraw-Hill, NY, 1995. [3] S. Faruque, Cellular Mobile Systems Engineering , Artech House, Norwood, MA, 1996. [4] W. Lee, Mobile Communications Engineering: Theory and applications, 2nd Ed., McGraw-Hill, 1997.
