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Interleavers and Deinterleavers are designed and used in the context of characteristics of the errors that might occur when the message bits are transmitted through a noisy channel. To understand the functions of an interleaver/deinterleaver, understanding of error characteristics is essential. Two types are errors concern communication system design engineer. They are burst error and random error
Random Errors:
Error locations are independent of each other. Error on one location will not affect the errors on other locations. Channels that introduce these types of errors are called channels without memory (since the channel has no knowledge of error locations since the error on location does not affect the error on another location).
Burst Errors:
Errors are depended on each other. For example, in channels with deep fading characteristics, errors often occur in bursts (affecting consecutive bits). That is, error in one location has a contagious effect on other bits. In general, these errors are considered to be dependent and such channels are considered to be channels with memory.
Interleaver/Deinterleaver :
One of the most popular ways to correct burst errors is to take a code that works well on random errors and interleave the bursts to “spread out” the errors so that they appear random to the decoder. There are two types of interleavers commonly in use today, block interleavers and convolutional interleavers.
The block interleaver is loaded row by row with L codewords, each of length n bits. These L codewords are then transmitted column by column until the interleaver is emptied. Then the interleaver is loaded again and the cycle repeats. At the receiver, the codewords are deinterleaved before they are decoded. A burst of length L bits or less will cause no more than 1 bit error in any one codeword. The random error decoder is much more likely to correct this single error than the entire burst.The parameter L is called the interleaver degree, or interleaver depth. The interleaver depth is chosen based on worst case channel conditions. It must be large enough so that the interleaved code can handle the longest error bursts expected on the channel. The main drawback of block interleavers is the delay introduced with each row-by-row fill of the interleaver.
Convolutional interleavers eliminate the problem except for the delay associated with the initial fill. Convolutional interleavers also reduce memory requirements over block interleavers by about one-half [1]. The big disadvantage of either type of interleaver is the interleaver delay introduced by this initial fill. The delay is a function of the interleaver depth and the data rate and for some channels it can be several seconds long. This long delay may be unacceptable for some applications. On voice circuits (as in GSM), for example, interleaver delays confuse the unfamiliar listener by introducing long pauses between speaker transitions. Even short delays of less than one second are sufficient to disrupt normal conversation. Another disadvantage of interleavers is that a smart jammer can choose the appropriate time to jam to cause maximum damage. This problem is overcome by randomizing the order in which the interleaver is emptied.
In practice, interleaving is one of the best burst-error correcting techniques. In theory, it is the worst way to handle burst errors. Why? From a strict probabilistic sense, we are converting “good” errors into “bad” errors. Burst errors have structure and that structure can be exploited. Interleavers “randomize” the errors and destroy the structure. Theory differs from reality,however. Interleaving may be the only technique available to handle burst errors successfully.
For example, Viterbi [2] showed that, for a channel impaired by a pulse jammer, exploiting the burst structure is not enough. Interleaving is still required. This does not mean that we should be careless about our choice of code and take up the slack with long interleavers. Codes designed to correct burst errors can achieve the same performance with much shorter interleavers. Until the coding theorists discover a better way, interleaving will be an essential error control coding technique for bursty channels.
References :
[1]B. Sklar, Digital Communications Fundamentals and Applications, Englewood Cliffs, New Jersey: Prentice Hall, 1988.
[2]A. J. Viterbi, “Spread Spectrum Communications – Myths and Realities,” IEEE Communications Magazine, vol. 17, No. 5, pp. 11-18, May 1979.
See also:
[1] Block Interleaver Design for Reed Solomon Codes
[2] Random Interleavers
Additional Resources:
[1] Notes on theory and construction of Reed Solomon Codes – Bernard Sklar
[2] Concatenation and Advanced Codes – Applications of interleavers- Stanford University
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