CHANNEL CODING

CHANNEL CODING:

The Different methods of channel coding used in GSM are,

· Block coding

· Convolutional coding

Block coding:

When the block coding is used, one or several check bits are added to the information block. The check bits only depend on the bits in that every block. A simple form of block coding is using is parity coding. Block coding is mainly used for detecting errors.

Convolutional coding:

The Convolutional coder consists of a shift register into which the information bits are shifted one by one. Convolutional coding is not only good for detecting errors, but also for correcting them.

INTERLEAVING:

Interleaving is a method spreading the potential losses, so that they can be taken care of by “channel coding” thus minimizing the harm burst.

In GSM the channel coder produces a total of 456 bits for every 20 ms segment of speech. These are in blocks of 57 bits interleaved over the burst.

EQUALISER:

The equalizer will mainly address the problems of Inter Symbol interference, desired earlier. The problem is that the air interface affects the signals in some way that causes bit error in the receiving side.

In a normal burst, used for traffic, there is a 26 bits training sequence in the middle of the burst.

TIMING ADVANCE:

The radio signals take finite period of time to travel from the mobile station to the base station. it is called propagation delay.

The system will repeatedly send “timing advance” orders to the MS. The system will simply tell the MS how many bit times earlier, or later, to send. These decisions are based on an analysis of how the bursts are received in the base station.

• BSS calculates access delay from RACH in terms of bits.
• Informs mobile to delay its in terms of bits
• Maximum Timing advance of 63 bits.