Channelization

• Frequency band has several application segments

• Certain blocks of the Band are reserved for certain applications by regulating authorities

• Technologies have decided their frequency bands

• E.g. AMPS/DAMPS: 824-894 MHz

Channelization methods

Channelization can be done primarily by three methods:– FDMA (Frequency Division Multiple Access)– TDMA (Time Division Multiple Access)– CDMA (Code Division Multiple Access)

CDMA

• Frequency channel is divided into code channels

• E.g. in IS-95 CDMA, 1.25 MHz channel is divided into 64 Code Channels

• Each user has a particular code

• Codes are orthogonal to each other, do not interfere with each other

Duplex Access Methods

F1 F2 Frequency

Amplitude

Time

Tx Rx

• Frequency Division Duplex (FDD)

• Transmit on one frequency and receive on another frequency

Time Division Duplex

• Time division duplex

• Tx and Rx is on the same frequency but on different times

F1 Frequency

Amplitude

Time

Tx

Rx

GSM Air Interface

• Separate Bands for Uplink and Downlink– Downlink: 935-960Mhz (EGSM: 925-960MHz)– Uplink: 890-915 MHz (EGSM: 880-915 MHz)

• TDMA and TDMA Multiplex– 124 Frequency Channels (ARFCN) for GSM900– 1 to 124 fro current band– 975 to 1023 for E-GSM – 200kHz Channels– 8 Mobiles share ARFCN by TDMA

The GSM Burst

3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

Speech Coder

• RPE/LTP coder (Regular Pulse excitation/Long term Prediction)

• Converts 64 kbps speech to 13 kbps

• At the end we get 13kbps speech i.e. 260 bits in 20 ms

20 ms blocks

Speech Coder

Bits Ordered

50 very important bits

132 important bits

78 other bits

Error Correction

Type 1a 50 3(CRC)Type 1b 132 Type II 78

Reordering

25 66366 25 4 Type II 78

Type 1a

Type 1b Type 1b

Type 1a

Tail

Half rate convolutional code

378 Type II 78

456 bits from 20 ms of speech

Diagonal Interleaving

456 bits from 20ms of speech 456 bits from 20ms of speech

57 57575757575757 57 57575757575757

57 57 57 5757 5757 5757 5757 5757 5757 57

• Traffic channel (TCH) bursts carry two 57 bit blocks (114)

• Each 120 ms of speech = 456*6 = 2736 bits2736/114 = 24 bursts i.3. 24 frames

Multiframe has 26 frames in 120ms.There are 2 spare frames .. 1 SACCH, 1 Idle

Convolutional Coding and Interleaving

• Bits to be Tx ed: HELLO

• Convolutionally encoded: HHEELLLLOO

• Interleaved: EE HH LL LL OO

• Bits Rx ed: EE HH LL LL OO

• De-Interleaved: HHEELLLLOO

• Viterbi Decoded: HELLO

Speech Coding Process

20 ms

Speech Coder

260 bits 13 kbps

50 1a 132 1b 78 II

Channel Coder

456 bits 22.8 kbps

Transceiver (BTS)

Transcoder Handler

260 bits

456 bits

16 kbps

TRAU frame

260 + 60 = 320 bits

Abis

13 kbps

TRAU frame

• 260 bits info + 60 TRAU bits = 320 bits/20ms = TRAU frame

• 60 bits contain frame Information data which indicates speech, data, O&M, full rate/half rate

• 60 bits = 35 synchronization + 21 control + 4 timing

Midamble or Training Bits

3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

• 8 midamble patterns (Colour codes) of 26 bits (BSIC)• RACH and SCH have longer 41 and 64 bit Midambles• Equalizer estimates channel impulse response from

midamble• Mathematically construct inverse filter• Uses inverse to decode bits

Downlink and Uplink

• Uplink lags downlink by 3 timeslots

• Uplink and downlink use same timeslot number

• Uplink and downlink use same channel number (ARFCN)

• Uplink and downlink use different bands (45 MHz apart for GSM 900)

Mobile Power ControlPath Loss

Power Command

• Mobile is commanded to change its Transmit Power

• Change in Power is proportionate to the Path Loss

• Change in Power is done in steps of 2 dbs

Timing Advance

• TDMA approach requires signals to arrive at BTS at the correct time

• A mobile at 30 km will be late by 100micro seconds

• Timing advance is in the range of 0-62

• One unit is 550m

• So maximum cell size is 63*0.55 = ~35 kms

Frames and Multiframes

0 654321 7

3 Data 1Midamble1 Data 3 8.25 bits

156.25 bits 576.92 micro sec

4.615 ms

Time

Slot

Frame

0 50 0 25

Control Channel Multiframe

Traffic Channel Multiframe

Channels in GSM Air interface

• Logical Channels : ARFCN –x, Timeslot – y, Frame Number - N

Logical Channels

Control Channels Traffic Channels

Control Channels

Control Channels

BCH – Broadcast Control Channel

CCCH – Common control Channels

DCCH – Dedicated control Channels

FCH

BCCH

SCH

PCH

RACH

AGCH

SDCCH

FACCH

SACCH

Traffic Channels

Traffic Channels

TCH - F

TCH - H

Speech Coding Process

20 ms

Speech Coder

260 bits 13 kbps

50 1a 132 1b 78 II

Channel Coder

456 bits 22.8 kbps

Transceiver (BTS)

Transcoder Handler

260 bits

456 bits

16 kbps

TRAU frame

260 + 60 = 320 bits

Abis

13 kbps

BCH (Broadcast Channel)

• One ARFCN, On all the time, in every cell

• Uses Timeslot 0 on a channel, in Downlink

• FCH – Frequency correction channl (allows mobile to tune to BTS frequency. This channel carries a 142 bit zero sequence and repeats once in every 10 frames on BCH)

• SCH – Synchronization channel (carries the Frame number and BSIC). A midamble of 64 bit helps mobiles to synchronize. It also repeats once in 10 Frames

• BCCH – Broadcast Control Channel. It has information encoded on it which identifies the network (LAC). It also carries list of the channels in use in the cell (CA tables)

BCH (Broadcast Channel)

Common Control Channel• CCCH shares Timeslot 0 with BCH on a

Multiframe

• CCCH consists of PCH, RACH and AGCH

• PCH – Paging channel is used to alert mobiles on incoming calls. PCH carries IMSI to page for Mobiles in the cell. PCH is Downlink channel.

• RACH – Random Access Channel is a short burst sent by mobile to BTS, to initiate a call request. RACH uses timeslot 0 on reverse BCH channel on uplink

RACH – Random Access Channel

8 start bits

41 Synchronization

bits

36 Encrypted

bits

3stop bits

68.25Extended

Guard Period

88 bits

• Used by mobile to get attention from Base Station

• Several mobiles might originate RACH simultaneously

• Mobile doesn’t know path delay– So RACH has to be a special short burst– Mobile sends normal burst after getting timing

advance on downlink SACCH

DCCH – Dedicated Control Channels

• SDCCH – Standalone Dedicated Common Control Channel. It is used as a interim channel before final assignment of TCH. SDCCH is used for signaling and Authentication message transfers.

It can be called the stepping stone between BSC and TCH

The SDCCH, by using less of the cells resource of physical channels, improves efficiency, and provides a useful holding channel for the mobile until speech data needs to be exchanged.

Dedicated Control Channels (cont..)

• FACCH – Fast Associated Control Channel

Interrupts TCH on uplink and downlink

Rapid message exchange for handovers

Control bits on either side of midamble – 0 indicates TCH and 1 indicates FACCH

Bothways – uplink and downlink

SACCH – Slow associated Control Channel

• Downlink (BTS – MS)

– Mobile Tx Power Commands

– Mobile Timing Advance

– Cell’s Channel Configuration• Uplink (MS - BTS)

– Received signal quality report (RXQual)– Received signal level report (RXLev)– Adjacent BCH power measurement

Common Channel Downlink

F AAAABBBBS PPPPPPPPSF DDDDSF

D DDDDSFDDD HHHHSFDDDD IHHHHH

Time Slots

01

32

01

32

Frame Numbers0 321 25

26 50

F – FCH S – SCH B – BCCH A – AGCH

P – PCH D – SDCCH H – SACCH

Common Channel Uplink

D AAAARRDDD RRRRRRAAAA RRRRRR

R RRRRRRRRR RDDDDDDDDR IDDDDR

Time Slots

01

32

01

32

Frame Numbers0 321 25

26 50

R – RACH D- SDCCH A- SACCH

TCH – Traffic Channel

• Traffic Channel carries the voice data

• Two blocks of 57 bits contain voice data

• One TCH is allocated for every active call. While call is in progress if there is degradation in quality of current channel, BTS may shift the communication to another TCH on a different Carrier and/or Timeslot

• A full rate TCH carries 13 kbps voice data, and Half rate TCH carries a 6.5 kbps voice data

3 Data 1Midamble1 Data 3 8.25 bits

TCH Multiframe

T TTTTTTTTT TTTTTTTATT -TTTTTFrame Numbers0 321 25

• 26 Frames – 120 ms

• 24 carry speech, 1 idle, 1 SACCH

Framing Structure

• 1 Frame = 8 bursts = 8*0.57692=4.616 ms

• 1 Control Ch. Multiframe = 51 TF = 234.6 ms

• 1 TCH Multiframe = 26 TF = 120 ms

• 1 Superframe = 51 * 26 CCH MF = 1326 TF = 6.12 sec

• 1 Superframe = 26 * 51 TCH MF = 1326 TF = 6.12 sec

• 1 Hyperframe = 2048 superframes = 2715648 TDMA frames = 3 hrs. 28 mins. 760 ms.

Framing Structure

• Multiframe is used for distribution of logical channels

• Superframe is used for Mobile synchronization

• Hyperframe is used for Signalling procedures and Ciphering