Block Interleaving:

Block interleaving is a common technique used with block codes in wireless systems. A block interleaver accepts a set of symbols and rearranges them, without repeating or omitting any of the symbols in the set. The number of symbols in each set is fixed for a given interleaver. The interleaver's operation on a set of symbols is independent of its operation on all other sets of symbols.

The advantage of interleaving is that a burst error that affects a sequence of bits is spread out over a number of separate blocks at the receiver so that error correction is possible. Interleaving is accomplished by reading and writing data from memory in different orders.

In a simple and common interleaving technique the data to be transmitted are stored in a rectangular array in which each row consists of n bits, equal to the block size. Data are then read out one column at a time. The result is that the k data bits and their corresponding (n - k) check bits, which form a single n-bit block, are spread out and interspersed with bits from other blocks. At the receiver, the data are deinterleaved to recover the original order. If, during transmission, a burst of noise affects a consecutive sequence of bits, those bits belong to different blocks and hence only a fraction of the bits in error need to be corrected by anyone set of check bits. Specifically, a burst of length I = mb is broken up into m bursts of length b. Some thought should convince you of the following assertion: Suppose we have an (n, k) code that can correct all combinations of t or fewer errors, where t = l (n - k)/2J. Then if we use an interleaver of degree m, then the result is an (mn, mk) code that can correct burst errors of up to mt bits.

Fig: Block Interleaving: