OR

Describe signal processing in GSM in detail.

Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

Signal Processing in GSM

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.

Interleaving:

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

Ciphering:

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.

Modulation:

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.

Demodulation:

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.