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

Modulation

Gaussian Minimum Shift Keying GMSK (BT = 0.3)
Transmission rate = 270.833 kbps

Multiple Access

TDMA/FDMA

Transmit Frequency bands

Mobile station     Base station

Uplink Downlink

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 µs time dispersion

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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

 

Transmission

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

Reception

GMSK Demodulator
Viterbi equaliser
Deinterleaver
Viterbi decoding
Speech Decoding

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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

 

Filters

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

 

Receiver

GSM correlator
Soft decision Viterbi equaliser

 

Processing

Access burst
Dummy burst
Frequency correction burst
Normal burst
Synchronization burst

 

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