Bandwidth Management

BANDWIDTH MANAGEMENT

MOBILE COMMUNICATION
OBJECTIVES
         Any time Anywhere Mobility & Roaming High capacity & subs. density Efficient use of radio spectrum Seamless Network Architecture Low cost Flexibility Innovative Services Standard Interfaces

Public Land Mobile Network
 INDIA has adopted GSM standard for PLMN.  Digital Cellular System.  Operates at 900 MHz.  International Roaming facility.  Power class 0.8 to 20W.  Cell Radius upto 35 Kms.  Maximum mobility speed 250 Km/hr.

BANDWIDTH MANAGEMENT TECHNIQUES

 FDMA  TDMA  Cellular Technology & Frequency Re-use Scheme  Speech Coding  Modulation

FDMA
 Frequency Division Multiple Access Scheme  Uplink Frequency Band = (890 – 915) MHz  Downlink Frequency Band = (935 – 960) MHz  Absolute Radio Freq Carrier Number (ARFCN)  Bandwidth = 915 – 890 or 960 – 935 = 25 MHz

 GSM uses both FDMA & TDMA

GSM -

MULTIPLE ACCESS

• FDMA Access along Frequency axis • Each RF carrier 200khz apart

• Total 124 RF Channels available.
One or more carrier assigned to each base station 1 2 3 4 5 6 124

……...
Freq
890.2 890.4 890.6 890.8 891.0 914.8

Mhz.

GSM FDMA
890 25 MHz
0

915
1 2

935 25 MHz
0 1 Base to Mobile

960
2

Mobile to Base
890.2
890.4 890.6
(MHz)

935.2

935.4

935.6

200 kHz

45MHz

200 kHz

Channel layout and frequency bands of operation

TDMA
     Time Division Multiple Access Scheme One Radio Frequency = Eight Time Slots One TDMA Frame = Eight Time Slots One Time Slot = One Physical Channel One Time Slot Duration = 0.577 msec

GSM TDMA
Amplitude

45 MHz
7 8 5 6 2 1 3 4 2 1 3 4 5 6 7 8

Frequency

F1 (Cell Rx)

F2

F1’ F2’ (Cell transmit)

Typical TDMA/ FDMA frame structure

FDMA/TDMA Scheme
TIME

BP2 BP1 BP8 BP7 BP6 BP5 BP4 BP3 BP2 BP1 890.0 890.2 890.4 890.6 890.8 891.0 891.2

BURST

F
R A M E FREQ MHz 915.8

TIME SLOT

3 CC SMS SS 57 T CM Encrypted

1 S

26 Training

1 S

57 Encrypted

3
T

8.25 GP

Normal Burst

CELLULAR TECHNOLOGY

Cellular Technology
 Cell
 Site

CELLULAR MOBILE CONCEPTS

WHAT IS A CELL ?
• A base station (transmitter) having a number of RF

channels is called a cell • Each cell covers a a limited number of mobile subscribers within the cell boundaries ( Coverage area) • Typical Cell Radius Aprrox = 30 Km (Start up), 1 Km (Mature)

CELL
 Each Cell uses a specific set of radio frequencies  Each Base Station contains one or more TRXs

Types of Cells
OMNI CELLS SECTOR CELLS - Two Sector Cells - Three Sector Cells

Fundamental problems
 Radio range, or coverage  no. of channels, or voice circuits  Full, seamless service coverage  Large no. of subscribers in the range of millions

CELLULAR MOBILE CONCEPTS
•RADIO IN LOCAL LOOP •LIMITED AVAILABILITY OF RF SPECTRUM

•CELLULAR PRINCIPLE
•INTERFERENCE PROBLEM •INTERFERENCE AND SYSTEM CAPACITY •FREQUENCY REUSE PATTERN •CELLULAR ENVIRONMENT •CAPACITY CONSIDERATIONS •FUTURE TRENDS

Frequency Re-use Scheme

CELLULAR MOBILE CONCEPTS

CELL RADIUS (KM) 1 3 10 25

COVERAGE AREA(KM2)

No. Of MOBILE SUBS. 100 900 10,000 60,000

No. of CHANNELS REQD. 8 38 @ 360 @ 2,000

3.14 28.3 314 1960

ASSUMPTIONS

Traffic /User = 30 mE

,

GOS = 1%

CELLULAR MOBILE CONCEPTS

• 360 * 25 KHz * 2 = 18 000 KHz = 18 MHz FOR A CELL OF 10 KM RADIUS ONLY
• THIS IS IMPOSSIBLE TO BE ALLOCATED • HENCE FREQUENCY REUSE IS A MUST TO COVER THE TOTAL SERVICE AREA WITH A LIMITED AVAILABLE RF RESOURCES • HENCE THE NEED FOR A CELLULAR PRINCIPLE

Frequency Re-use Scheme
 CLUSTER

CELLULAR MOBILE CONCEPTS

2 7 3

GIVEN FREQ. RESOURCE 1 2 3 4 5 6 7

1
6 5 4

A CLUSTER OF CELLS

FREQUENCY REUSE PATTERN

4/12

7/21

CELLULAR
2 7 1 6
R

PRINCIPLE N=7
2 7

3 4 5
D

3 1 4

Given Freq Resource

6

5

12 3 456 7
Frequency Reuse Pattern “N”=7

CELLULAR MOBILE CONCEPTS

CO-CHL INTERFERENCE :Interference caused by another cell/mobile using the same frequency Co Chl Interference is a Function of “Q” the re-use ratio: R Q =D / R

Higher Q

Reduced Co-Chl Interference 

Lower Q

Increased Co-Chl Interference



Co– Channel Interference Q = D /R = 3N
N =Cluster Size R = Size (Radius of Cell) D = Distance between two Co- Channel Cells

Co– Channel Interference
Higher Q Less Interference   Higher N More Cluster Size Less RF freq/cell Less Traffic Handling Capacity of the system
Higher Interference  Increased System Handling Capacity 



LOWER Q

CLASSIFICATION OF CELLS
 MICRO CELL  MACRO CELL  UMBRELLA CELL

GSM CAPACITY CONSIDERATIONS
1
1

1

1

2

8

With 2 % GoS  2.94 E  2.94E/25mE=120 Subs 8 Access Channels 1--Signaling 7- Voice

120 Subs/Sector  3 = 360 Subscribers

Speech Coding
 Audio CODEC  RPE : LTP = Regular Pulse Excitation : Long term Prediction

Modulation
 GMSK = GAUSSIAN MINIMUM SHIFT KEYING

GSM Specifications
Carrier Separation Duplex Distance No. of RF Carriers Access Method Modulation Method Transmission Rate Speech Coding 200 kHz 45 MHz 124 TDMA/FDMA GMSK 270.833 Kbps

Full rate 13 Kbps Half rate 6.5 Kbps

GSM

Digital Voice Transmission
Speech Coding
- In GSM speech coding a block of 20 ms is encoded in one set of 260 bits.
- This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. - This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones.

GSM

Digital Voice Transmission
Channel Coding
- It uses 260 bits from speech coding as input and outputs 456 encoded bits.

Interleaving
- These 456 bits for every 20 ms of speech are interleaved forming eight blocks of 57 bits each. - In one burst one block of 57 bits from one sample and another block from another sample are sent together.

GSM

Digital Voice Transmission
Burst Formatting
- To counteract the problems encountered in radio path. - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. - One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). - GP enables MS/BTS to “ramp up” and “ ramp down”.

GSM
Speech to Radio waves
Analog
Analog

Speech Coding Channel Coding

Speech Decoding Channel Decoding

Interleaving Burst formatting

De-interleaving Burst formatting

Ciphering
Modulation
200kHz BW

Deciphering
Demodulation
200kHz BW

GSM Radio Interface - CYCLES
Hyperframe = 2048 Superframes
3 Hours 28 Minutes 53 Seconds and 760 milliseconds
0 2047

Superframe = 26× 51 multiframes
0
0

6.12 Seconds

50
25

26 Multiframe
120 mS
0 1 2 24 25

51 Multiframe
Approx 235 mS
0 1 48 49 50

TDMA frame
4.615 mS
0 1 2 3 4 5 6 7

Organisation of Speech & Data
Frames 0-11 : TCH
0 1 2 3 4 5 6

Frames 12 : SACCH
7 8 9

Frames 13-24 : TCH

Frames 25 : Unused

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

26 – frame multiframe Duration: 120 ms
BP 0 BP 1 BP 2 BP 3 BP 4 BP 5 BP 6 BP 7

TDMA frame Duration: 60/13 ms =4.615 ms
3 8.25

3

57

1

26

1

57

Tail Data bits bits

Stealing Training Stealing Data bits sequence bit bit

Tail Guard bits bits Normal burst Duration 15/26 ms

GSM-- TDMA STRUCTURE
• TDMA • Time slot duration • Frame • Multi Frame • Super Frame 8 Time Slots / RF Channel 0.577m sec or 15 / 26 m sec 8 Burst Periods ( Time Slots) = 8 15/26 = 4.615 m sec Traffic 26  4.615 = 120 msec Control 51  4.615 = 235.365 m sec 51  Traffic Multi frames 26  Control Multi frames • Hyper Frame 2048 Super Frames = 3 28 52.76

hr min sec

GSM
LOGICAL CHANNELS
• USER INFORMATION( TRAFFIC) • SIGNALLING INFORMATION (CONTROL)

GSM
CONTOL CHHANELS OVER LOGICAL CHANNELS • Intended to carry signalling and synchronisation THREE TYPES OF CONTROL CHANNELS • Broadcast control channel BCCH

• Common control channel CCCH
• Dedicated control channel DCCH

OPERATIONAL CONCEPTS

• •

Hence IDLE MODE & DEDICATED MODE DEDICATED MODE

--

When a full Bi -directional P to P CHL has been allocated during an established call When MS is powered on (active) without being in dedicated mode

• IDLE MODE MODE

--

GSM
THREE TYPES OF CONTROL CHANNELS Broadcast control channel BCCH P- MP For Freq Correction For Syncronisation Common control channel CCCH

FCCH SCH BCCH PCH RACH AGCH
SDCCH SACCH FACCH

For ACCESS Management
Dedicated control channel DCCH P- P For Registration ,authentication & Handover

OPERATIONAL CONCEPTS
IDLE MODE -When MS is powered on (active) without being in dedicated mode

• MS stays continuously in touch with BS
• Listens to transmissions from BS to intercept Paging Messages ( for incoming calls)

• Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS
• Listens to BS to avail short message broadcast service

OPERATIONAL CONCEPTS
ACCESS PROCEDURE -- Access to system ( switch over from IDLE to DEDICATED Mode)

• MS indicates to BS that it needs a connection
• BS accepts the request & indicates which traffic CHL it may use

• For above purpose specific transmission is done over “ Common Channels”

OPERATIONAL CONCEPTS
MOBILE O/G Call • MS sends access over RACH • System allocates SDCCH through AGCH • Set up information exchanged over SDCCH ( Authentication , Measurement Reports, Power Control) • Lastly TCH is assigned through SDCCH when a conversation can start

OPERATIONAL CONCEPTS
MOBILE I/C Call • Paging to MS through PCH since MS is monitoring PAGCH

• MS responds by sending a page response over RACH
• As a result system allocates SDCCH to MS over AGCH

• Set up information exchanged over SDCCH ( Authentication, Call set-up messages , Power Control)
• Lastly TCH is allocated to mobile over SDCCH . Mobile starts conversation.

OPERATIONAL CONCEPTS
IDLE MODE • IDLE MODE ACCESS PROCDURE DEDICATED MODE ----FCCH ---- SCH ----BCCH ----RACH ----AGCH ----SDCCH ----TCH ----PCH ----RACH ----AGCH ----SDCCH ----TCH

• MS O/G Call

• MS I/C Call