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Global System for Mobile (GSM)

Overview  |  Reverse/Forward Process  |  Library Components


    GSM - Overview

The GSM standard has been in existence for many years and it is the most widely used system in the world. Nuntius has expertise in the Layer 1 and Layer 2 functions of GSM. Nuntius also has extensive experience in AMR as well as other vocoders. Below is a general list of some of the basic characteristics of the GSM standard. For more information, contact Nuntius Systems, Inc.

Speech coding
Full rate (RPE-LPC) codec
Half rate(VSELP) codec
Adaptive Multi Rate (AMR) vocoder

Data Rate:     9.6 kbps

Channel coding
Cyclic Redundancy encoder for error detection
Convolutional encoder of rate 1/2
Interleaving codes for error bursts
Gaussian Minimum Shift Keying GMSK (BT = 0.3)
Transmission rate = 270.833 kbps
Multiple Access

Transmit Frequency bands
Mobile station    
Base station
Reverse ch. 890 - 915 MHz
Forward ch. 935 - 960 MHz    
Duplex seperation 45 MHz
RF carrier spacing     200 kHz

Number of RF Duplex channels 124

Traffic channels per carrier     8

Cell radius:     upto 35 km

International roaming capability:     more than 16 countries

Frame Interval:     4.615 ms (8 users)
Time slot for each user:     0.577 ms

Frequency Hopping:     217 hops/s (slow)

Equalizer:     16 Ás time dispersion


    GSM - Reverse Link / Forward Link Process
Transmit power
maximum of 8W (Reverse link)
Composed of
Traffic channels
Common control channels
Dedicated control channels
Broad cast control channel (only for forward link)
Traffic channel data rate
Speech   -  22.8 kbps (Full rate), 11.4 kbps (Half rate)
Data       -  9600, 4800, 2400 bps
Speech   coder - Block of 260 bits in 20 ms (50 + 132 + 78)

Channel coder
3-bit CRC for Ist 50 bits
4-bit tail is added to 132 bits
53 bits + 136 bits are input to 1/2 rate convolutional encoder
456 bits( 378 + 78 ) are divided into 8 blocks of 57 bits
These blocks are interleaved in each TDMA frame
GMSK modulation
GMSK Demodulator
Viterbi equaliser
Viterbi decoding
Speech Decoding
    GSM - Standard Library Functions

The following is a list of the standard library functions to perform the various aspects of GSM. These functions can be implemented in a DSP or the ARM RISC architecture. The channel models are used for simulating the channel operating environment and are not a function that would be implemented in a mobile handset or base station.

Speech codec
Full rate (RPE-LPC) codec
Half rate(VSELP) codec
Adaptive Multi Rate (AMR) vocoder
Channel codec
Block coder
GSM CRC coder
Cyclic coder
Convolutional coder (1/2, 1/3 and 1/6)
Punctured convolutional coder
Fire coder
Interleaver / deinterleaver
GSM Interleaver / deinterleaver
Butterworth filters
Channel models
Rayleigh distribution
Rice distribution
Propogation models for urban and rural areas
Modulator / demodulator
GMSK modulation / demodulation
Channel decoder
Block decoder for removing tail bits
Fire decoder
Cyclic decoder
Viterbi decoder for convolutional coders
Viterbi decoder for punctured convolutional coders
Soft decoder
GSM correlator
Soft decision Viterbi equaliser
Access burst
Dummy burst
Frequency correction burst
Normal burst
Synchronization burst

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