Discuss signal processing in GSM. OR Draw a block diagram and explain speech signal processing in GSM. OR Describe signal processing in GSM.


Describe signal processing in GSM in detail.

Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

1 Answer

Signal Processing in GSM

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Speech Coding: The coder provides 260 bits for each 20ms blocks of speech, which yield a bit rate of 13kbps.

Channel Coding:

  • The output bits of the speech coder are ordered into groups for error protection, based on their significance in contributing to, speech quality.
  • Out of the total 260 bits in a frame, the most important 50 bits called (Ia bits) have 3 parity check (CRC) bits added to them.
  • This facilitate the non correctable errors at the receiver. The next 132 bits (Type Ib) along with 53 bits are reordered and appended by providing a data block of 189 bits.
  • This block is then encoded for error using rate ½ Convolution encoder. The least important 78 bits (Type 2) are not error protected and are just joined to form 456 bits in a 20 ms frame.


The 456 bit in 20ms speech frame is broken into Eight 57 bit sub blocks. They form the consecutive TCH time slots.


It modifies the contents of the eight interleaved blocks through the use of encryption techniques known only to a particular mobile station and bas transceiver.

Burst Formatting:

Adds binary data to the ciphered blocks to help synchronization and equalization of the received signal.


It is 0.3 GMSK. It is a type of digital FM. 1‘s and 0‘s are represented by shifting the RF carrier by +- 67.708 KHz.


The appropriate TS is demodulated with the aid of synchronization data provided by the burst formatting. After demodulation, the binary info is deciphered, de-interleaved, channel decoded and speech decoded.

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