Explain a.) Equalisation b.) Interleaving c.) Speech coding d.) Channel coding

Explain the following: a.) Equalisation b.) Interleaving c.) Speech coding d.) Channel coding

a.) Equalisation:

- An adaptive equalizer is employed by a GSM receiver for overcoming the impact of non-ideal channel characteristics which are caused by multipath propagation.
- Adaptive equalizer is required because the channel characteristics are often changing fast.
- Tuning the for each time slot is done by the equalizer.

b.) Interleaving:

- Time diversity in a signal communication system is obtained by using Interleaving.
- The possibility of losing whole bursts will be decreased by interleaving.
- Total 456 bits from convolution encoder, including 20ms of speech, is subdivided into eight blocks. Each block consists of 57 bits.
- All these blocks are transmitted in consecutive time slots.
- Enough information is available in 7 blocks, in case one of the blocks is lost due to burst errors. This enables whole segment recovery by using error correction.

c.) Speech coding:

- Speech coding is all about turning voice into digital form.
- Speech is inherently analog, as GSM is a digital system.
- The digitization is employed by ISDN, and the current telephone systems that are used for multiplexing voice lines, with high speed trunks, optical fiber lines is done by Pulse Code Modulation(PCM).
- The PCM output is 64 kbps, which is too high over a radio link in feasibility.
- The 64kbps signal is redundant.
- The algorithm used in conventional cellular is Vector Sum Excited Linear Predictive speech compression.

d.) Channel coding

- The data rate for the radio channel is 270 kbps.
- The data rate is split into 8 full rate or 16 half rate traffic channels, along with signaling channels
- In order to have the maximum chance for detecting and correcting errors, the code is complex in a typical propagation path
- Forward Error Correction is applied in order to get the speech coder encryption, coding and interleaving in a sophisticated way
- The data is sent as bursts in 577 mus time slots. Each contains 116 encrypted bits
- Every TDMA frame consists of 8 or 16 time slots
- Transmit time slots are staggered, so that at some instant the mobile station will not receive the same instants while transmitting, which enables the simplifying the filtering requirements
- At least one spare slot between transmit and receive is available with this scheme.

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