A. Speech coding:
GSM speech coder is RELP (Residually Excited Predictive Coder), which is enhanced by including a Long Term Predictor (LTP).
The coder provides 260 bits for 20ms blocks of speech, which yields a bit rate of 13kbps.
GSM system operates in Discontinuous Transmission mode (DTX) by incorporating a Voice Activity Detector (VAD) in speech coder. This mode provides a longer battery life and reduces instantaneous radio interference since GSM transmitter is not active during silent periods.
A Comfort Noise Subsystem (CNS) is used at receiver which adds background acoustic noise to compensate for the annoying switched muting which occurs due to DTX.
B. Channel coding:
The outputs of the speech coder are ordered into for error protection, based upon their significance in contributing groups to speech quality.
Out of 260 bits in a frame, the most important 50 bits called type Ia bits, have 3 parity check (CRC) bits added to them to detect non-correctable errors at the receiver.
The next 132 bits with first 53 are appended by 4 trailing zero bits, thus providing a data block of 189 bits. This block is then encoded for error protection using a rate ½ convolution encoder with constant length K=5, thus providing a sequence of 378 bits.
The least important 78 bits do not have error protection and concatenated to existing sequence to form a block of 456 bits in 20ms frame, data rate of speech signal becomes 22.8kbps.
C. Interleaving:
To minimize the effect of sudden fades on the received data, the total of 456 encoded bits within each 20ms speech frame or control message frame are broken into eight 57 bits sub blocks and they are numbered even odd according to block number. These eight consecutive blocks are spread over eight consecutive TCH time slot.
If a burst is lost due to interference or fading, channel coding ensures that enough bits will still be received correctly to allow the error correction to work.
Each TCH time slot carries two 57 bits blocks of data from two different 20ms speech blocks. Time slot of first 4 frames contains even data blocks of present speech frame and odd data block of previous speech frame. Time slot of next 4 frames contains odd blocks of present speech frame and even data block of next speech frame.
D. Burst formatting:
Burst formatting adds binary data to the data block to help synchronization and equalization of the received signal.
E. Ciphering:
Ciphering modifies the contents of the eight interleaved blocks by encryption techniques known only to the particular mobile station and base transceiver station.
The A3 ciphering algorithm is used to authenticate each mobile by verifying the user password within the SIM with the cryptographic key at the MSC.
The A5 ciphering algorithm is used for encryption. It provides scrambling for 114 coded bits sent in each TS.
The A8 is used for ciphering key.
F. Modulation:
The modulation scheme used by GSM system is 0.3GMSK where 0.3 describes 3db bandwidth of the Gaussian pulse shaping filter.
The channel data rate of GSM is 270.833 kbps which is four times the RF frequency shift. This minimizes bandwidth of the modulation spectrum and hence improves channel capacity.
MSK modulated signal is then passed through Gaussian filter to smooth the rapid frequency transitions which would otherwise spread energy in adjacent channels.
G. Demodulation:
The portion of the transmitted forward channel signal which is of interest to a particular user is determined by the assigned TS and ARFCN. The appropriate TS is demodulated with aid of synchronization data provided by the burst formatting.
After demodulation the binary information is deciphered, de-interleaved, channel decoded and speech decoded.
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