Interleaving - Speech Transmission in GSM

Interleaving is a process of dispersing the bits of a data burst over multiple bursts in a systematic way. Benefit of this technique: when a data-burst is lost (due to burst error in the radio interface) it does not mean a 100% loss of a single burst rather a partial loss of many bursts

First level of interleaving

The channel coder provides 456 bits for every 20 ms of speech which are interleaved in eight blocks of 57 bits shown below.

In a normal burst, there is space for two of these speech blocks (Figure). Thus, if one burst transmission is lost, there is a 25% BER for the entire 20 ms of speech (2/8= 25%).

Second level of interleaving

If only one level of interleaving is used, a loss of this burst results in a total loss of 25%. This is too much for the channel decoder to correct. A second level of interleaving can be introduced to further reduce the possible BER to 12.5%.

Instead of sending two blocks of 57 bits from the same 20 ms of speech within one burst, a block from one 20 ms and a block from next sample of 20 ms are sent together. A delay is introduced in the system when the MS must wait for the next 20 ms of speech. However, the system can now afford to lose a whole burst, out of eight, as the loss is only 12.5% of the total bits from each 20ms speech frame. 12.5% is the maximum loss level that channel decoder can correct.

Speech Frame

The bits must then be sent over the air using a carrier frequency. GSM uses the GMSK modulation technique. The bits are modulated onto a carrier frequency and transmitted.

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Satya Sravan