GSM Cellular No. 1Seattle Pacific University

GSM Cellular Standards:A look at the world’s most common digital cellular

system

Kevin BoldingElectrical Engineering

Seattle Pacific University

GSM Cellular No. 2Seattle Pacific University

GSM

• GSM is the world’s most popular standard for cellular

• Digital, TDMA/WDMA

HBase

H

H

H

H

• Details on

• Voice data format, encoding

• Error-control coding

• Channel sharing (TDM)

• Channel allocation (FDM)

GSM Cellular No. 3Seattle Pacific University

Base Station

Mobile Handset

Wireless Channel Uplink Architecture

Voice Encoder Modulator

ChannelEncoder

DemodulatorChannelDecoder

Channel

Noise

GSM Cellular No. 4Seattle Pacific University

Voice Coding

64kbps voice signal compressed into 13kbps signal Each 260 bit sample contains 20ms of speech data

• Analog signal is sampled using PCM at 64kbps.

• The signal is broken into 20 ms samples, which contain 1280 bits eachVoice

Encoder

• A Regular Pulse Excited - Linear Predictive Coder (RPE-LPC) is used to compress the 1280 bits into 260 bits

• Compression into just over 1/5 the size

IA – 50 bits IB – 132 bits II – 78 bits

260 bits

Most critical Very Important Icing

GSM Cellular No. 5Seattle Pacific University

Channel Coding - Blocks• The 260 bit (20ms) sample is divided into class IA,

IB and II, based on how important the bits are in determining the sound quality.

ChannelEncoder

• IA uses a 3 bit CRC. If the CRC fails, the whole sample is thrown out

• IA and IB together have a 4-bit trailer. Together, these are put into a ½-rate convolutional coder that doubles the number of bits

• II bits are appended unencoded, giving an overall sample of 456 bits

IA – 50 IB – 132 bits II – 78 bits

456 bits

IB – 132 bits

Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57• The 456 bit encoded sample is divided into 8 blocks of 57 bits each (each contains

the equivalent of 2.5 ms of speech) – these are the basic units of transmission.

One sample is 20ms of speech--> 456 bits--> 8 blocks

One block is 2.5ms of speech--> 57 bits

GSM Cellular No. 6Seattle Pacific University

TDMA Bursts• Blocks are gathered together to form a TDMA burst

• 2 separate speech sample blocks are gathered together

• Interleaved to protect against burst errors

• 26-bit training sequence

• To characterize interference and filter it out

• 16.25 tail/guard bits

Block - 57 Block - 57Training - 26TG

TG

TG

T/G

156.25 bits

IA – 50 IB – 132 bits II – 78 bitsIB – 132 bits

Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57

First sample (20ms)IA – 50 IB – 132 bits II – 78 bitsIB – 132 bits

Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57

Second sample (20ms)

• Total Burst is 156.25 bits

One burst is two blocks--> Two 2.5ms samples of speechfrom same source

ChannelEncoder

GSM Cellular No. 7Seattle Pacific University

Sharing the channel – TDMA Frames• Eight bursts (from different sources) make up a TDMA

frame One TDMA frame is eight bursts--> 8 sources x (2 x 2.5ms sample of speech)

Channel

• This allows eight sources to share a channel

Block - 57 Block - 57Training - 26TG

TG

TG

T/G

Burst Burst Burst Burst Burst Burst Burst Burst

TDMA Frame - 8 bursts - 8 x 2 x 2.5ms sample of speech - 1250 bits

Each burst comes from a different source (phone)Eight phones share a channel using TDM.

GSM Cellular No. 8Seattle Pacific University

Sharing the channel

• 26 TDMA frames make up one Multi-frame

One TDMA MultiFrame is 26 Frames (24 data)--> 24 Frames x 8 Bursts/Frame x 2 Blocks/Burst = 384 Blocks per MultiFrame--> For each of the 8 senders, there are 384/8 = 48 Blocks per MultiFrame--> For each sender, 48 blocks = 120 ms of speech (60 ms from 2 samples)

Channel

• 24 are for data (speech)

Burst Burst Burst Burst Burst Burst Burst Burst

TDMA MultiFrame - 26 Frames - 24 x 8 x 2 x 2.5ms sample of speech - 32500 bits - 120ms

F F FF F F F F F F F F F F F F F F F F F F F F F F

Each TDMA MultiFrame takes 120ms to transmit and contains 120ms of speech data from 8 sources 8 times as efficient as analog transmission

• 1 is for control , 1 is unused

• 8 Bursts per TDMA frame (2 x 2.5ms sample each)

GSM Cellular No. 9Seattle Pacific University

GSM: Modulation• Each Multiframe has 32500 bits and lasts 120ms

• 270833 bpsModulator

• Transmitted using a channel 200kHz wide

• US: 890-915MHz band for cell uplink

• Divided into 124 200kHz wide channels

• Downlink from 935-960MHz• Cell towers arranged in a hexagonal grid, usually in groups of 7 – Requires 7 sets of

independent channels

• Each cell gets 124/7 = 17 channels

• Cell capacity = 17 * 8 = 136 conversations

GSM Cellular No. 10Seattle Pacific University

GSM: Modulation• GSM uses Gaussian-filtered Minimum Shift Keying (GMSK).

• MSK is a minimum-shift form of FSK

• Gaussian pre-filter reduces bandwidthModulator

• MSK gives the best spectral efficiency of any digital bandpass signal set.

• FSK only has one amplitude level, allowing for a simpler amplifier in the handset

GSM Cellular No. 11Seattle Pacific University

Discontinuous Transmission

• Discontinuous transmission (DTX) allows for the transmitter to be turned off 60% of the time.

• Saves power

• Reduces the overall ambient noise in the cell sector.

• DTX requires voice detection, so that the handset knows when to restart transmission.

• DTX also requires a synch signal, so that the receiver can differentiate between silence and a dropped connection.

GSM Cellular No. 12Seattle Pacific University

Dynamic Power Compensation

• The power between the handset and the tower can be dynamically adjusted in response to the channel BER.

• This allows the channel to start at a minimum power level, and only increase when the signal requires a greater SNR.

• For CDMA, Dynamic Power Compensation is a necessity

• All transmit on same band at the same time

• Power must be adjusted so that all signals are received at the same strength• Otherwise, one channel would overpower all others