Bandwidth Management

BANDWIDTH MANAGEMENT

MOBILE COMMUNICATION
OBJECTIVES
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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
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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
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FDMA
TDMA
Cellular Technology &
Frequency Re-use Scheme
Speech Coding
Modulation

FDMA
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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 

MULTIPLE ACCESS

GSM uses both FDMA & TDMA

• 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

915

25 MHz
1

0

2

890.4 890.6

200 kHz

45MHz

960

25 MHz
1

0

Mobile to Base
890.2

935

2

Base to Mobile
(MHz)

935.2

935.4

935.6

200 kHz

Channel layout and frequency bands of operation

TDMA
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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

1

F1
(Cell Rx)

2

3

4

5 6

7 8

1

F2

2

3

4

5 6

7 8

F1’
F2’
(Cell transmit)

Typical TDMA/ FDMA frame structure

Frequency

FDMA/TDMA Scheme
TIM
E
BP2
BP1
BP8
BP7
BP6

BURST
F

BP5

R

BP4

A

BP3

M

BP2
BP1

E
890.2
890.6
891.0
890.
890.4
890.8
891.2
0

FRE
Q

915.8

MHz

TIME SLOT

3 CC SMS
57 SS
T
CM
Encrypted

1
S

26

1

57

3

8.25

Training

S

Encrypted

T

GP

Normal Burst

CELLULAR
TECHNOLOGY

Cellular Technology
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Cell

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

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

Fundamental problems
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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

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

GIVEN FREQ.
RESOURCE

3

7
1

4

6
5

A CLUSTER OF CELLS

1 2 3 4 5 6 7

FREQUENCY REUSE PATTERN

4/12

7/21

CELLULAR PRINCIPLE
N=7
2
7

2

3
7

1
6

R

4
5

D

3

Given Freq Resource

1
6

4
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

D

R

Co Chl Interference is
a Function of “Q”
the re-use ratio:
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

LOWER 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 

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CLASSIFICATION OF CELLS
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MICRO CELL
MACRO CELL
UMBRELLA CELL

GSM CAPACITY CONSIDERATIONS
1
1

1

1

2

8

8 Access Channels
1--Signaling
7- Voice

With 2 % GoS

2.94 E

2.94E/25mE=120 Subs

120 Subs/Sector  3 = 360 Subscribers

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

Modulation
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GMSK = GAUSSIAN MINIMUM SHIFT
KEYING

GSM Specifications

Carrier Separation

-

200 kHz

Duplex Distance

-

45 MHz

No. of RF Carriers

-

124

Access Method

-

TDMA/FDMA

Modulation Method

-

GMSK

Transmission Rate

-

270.833 Kbps

Speech Coding

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

Speech Decoding

Channel Coding

Channel Decoding

Interleaving

De-interleaving

Burst formatting

Burst formatting

Ciphering

Deciphering

Modulation

Demodulation

200kHz BW

200kHz BW

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

2047

Superframe = 26× 51
multiframes
6.12 Seconds

0

50

0

0

1

25

26 Multiframe

51 Multiframe

120 mS

Approx 235 mS

2

24

0

1

25

0

1

TDMA
frame

2

4.615 mS
3

4

48

5

6

7

49

50

Organisation of Speech & Data
Frames 0-11 : TCH
0

1

2

3

4

Frames 12 : SACCH
5

6

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

3

Tail
bit
s

BP 1

57

Data
bits

BP 2

1

BP 3

BP 4

26

BP 5

BP 6

1

Stealin Training Stealin
sequenc
g
g
e
bit
bit

BP 7

57

Data
bits

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

8.25

Tail Guar
d
bits
bits
Normal burst
Duration 15/26
ms

GSM-- TDMA STRUCTURE
• TDMA
• Time slot duration
• Frame
• Multi 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

• Super Frame

51  4.615 = 235.365 m sec
51  Traffic Multi frames

• Hyper Frame

26  Control Multi frames
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
• IDLE MODE MODE
--

When MS is powered on (active)
without being in dedicated mode

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

For Registration
,authentication
& Handover

FCCH
SCH
BCCH
PCH
RACH
AGCH
SDCCH
SACCH
FACCH

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

ACCESS PROCDURE DEDICATED MODE

• IDLE MODE

----FCCH
---- SCH
----BCCH

• MS O/G Call

----RACH
----AGCH
----SDCCH
----TCH

• MS I/C Call

----PCH
----RACH
----AGCH
----SDCCH
----TCH