What is Interleaving?

Interleaving is a technique used to enhance existing error correcting codes so that they can perform burst error corrections more effectively. It works by rearranging data symbols before transmission to distribute burst errors across multiple codewords.

Most error correcting codes (ECCs) are designed to correct random errors caused by additive noise that occurs independently. However, burst errors are errors that occur in sequences or groups, typically caused by defects in storage media or disruption in communication signals due to external factors like lightning. Interleaving modifies how data is organized after encoding by ECCs to handle these burst errors better.

How Interleaving Works

During interleaving, message symbols are rearranged across multiple code blocks by the interleaver before transmission over network channels. This redistribution ensures that long burst noise sequences are spread out among multiple blocks. When the decoder rearranges the blocks back to their original order, the errors appear as independent random errors or short burst errors that the ECC can effectively correct.

The depth of interleaving required depends on the length of noise bursts that the ECC can recover from. Deeper interleaving provides better protection against longer burst errors but increases complexity and delay.

Types of Interleaving

• Periodic Interleaving − The message is arranged in a repeating sequence pattern. The interleaver accepts data symbols in blocks and performs identical permutations on each block before transmission. For example, sequential code blocks may be written to a matrix row-wise and then read column-wise. Block interleaving is a common type of periodic interleaving.

• Pseudo-random Interleaving − This involves rearranging message blocks according to a pseudo-random sequence generated by specific algorithms. This approach provides better randomization but requires more complex implementation.

Example

Consider four codewords: 10011101, 00101100, 10100101, and 11110000. In block interleaving, these codewords are arranged in a matrix format:

Original codewords (row-wise):
1 0 0 1 1 1 0 1
0 0 1 0 1 1 0 0
1 0 1 0 0 1 0 1
1 1 1 1 0 0 0 0

Interleaved transmission (column-wise):
1011 0010 0110 1001 1100 1110 0000 1001

If a burst error affects several consecutive bits in the transmitted sequence, after de-interleaving, these errors will be distributed across different codewords, making them easier for the ECC to correct.

Conclusion

Interleaving is an essential technique that transforms burst errors into manageable random errors by rearranging data symbols before transmission. This allows existing error correcting codes to handle burst errors effectively, significantly improving communication reliability in noisy channels.