Need for interleaving

Interleaving is needed because mobile fading channels cause burst errors: when the mobile is in a deep fade, many consecutive bits are lost. Most error-correcting codes are designed for random, scattered errors and cannot correct long bursts. Interleaving spreads the bits of each code word over time so that one fade damages only a few bits of each code word. It provides time diversity without adding redundant bits.

Working (block interleaver)

 Write row-wise:        Read column-wise:
  a1 a2 a3 a4           a1 b1 c1 d1 a2 b2 ...
  b1 b2 b3 b4
  c1 c2 c3 c4           Burst hits a2 b2 c2 d2
  d1 d2 d3 d4

1. Coded bits are written into an m × n array row by row (each row = one code word).
2. They are read out column by column and transmitted.
3. The receiver's de-interleaver writes column by column and reads row by row, restoring the order.
4. A burst of up to m bits in the channel becomes at most one error per code word, which the decoder corrects.

Trade-off: deeper interleaving handles longer fades but adds delay. Example: GSM interleaves each 456-bit speech block over 8 TDMA bursts.