Why is interleaving needed?

FEC codes are designed to correct random errors distributed across the codeword. Burst errors (e.g., from fading or impulse noise) concentrate multiple errors in consecutive bits, overwhelming the FEC. Interleaving spreads burst errors across multiple codewords so each codeword sees only a few random errors, which the FEC can correct.

Block vs convolutional interleaver?

Block interleaver: fills an R x C matrix row-by-row, reads column-by-column. Fixed latency of R x C bits. Simple but requires full block before output. Convolutional interleaver: uses shift registers of increasing length, has lower latency (R x C/2), and starts output sooner. Convolutional is used in DVB-T; block is used in many LTE channels.

How large should the interleaver be?

The interleaver depth must exceed the expected burst length. If fading dips last 5 ms and the bit rate is 1 Mbps, the burst is 5,000 bits. The interleaver needs R x C > 5,000. Deeper interleaving provides more protection but increases latency: latency = R x C / bit_rate. Balance protection vs latency requirements.

Signal Processing

Block Interleaver

A Block Interleaver rearranges coded bits by writing them into an R×C matrix row-by-row and reading them out column-by-column, spreading consecutive bits across R different positions in the output stream. A burst error of up to C consecutive bits is distributed as single errors across R different codewords, enabling the FEC decoder to correct each one independently. The latency is R×C bits.

Category: Signal Processing

Latency: R×C bits

Understanding Block Interleavers

Consider R=4 rows, C=6 columns. Bits 1-6 fill row 1, bits 7-12 fill row 2, etc. Reading column-by-column: bits 1,7,13,19,2,8,14,20,... A burst of 6 consecutive errors in the transmitted stream affects one bit in each of 6 different rows, spreading the burst across 6 codewords. If each codeword's FEC can correct 1 error, the burst is fully recoverable.

The deinterleaver at the receiver performs the inverse operation (write column-by-column, read row-by-row), restoring original order with the burst errors now scattered.

Block Interleaver Parameters

Matrix: R rows × C columns
Write: row-by-row (R codewords of C bits)
Read: column-by-column

Burst protection: up to C bits
Latency: R × C / bit_rate
Memory: R × C bits

Interleaver Type Comparison

Type	Latency	Memory	Burst Protection	Used In
Block	R×C	R×C	C bits	LTE, GSM
Convolutional	R×C/2	R×C/2	C bits	DVB-T, DAB
Random	Block length	Block	Statistical	Turbo codes
S-Random	Block length	Block	Guaranteed S	LTE Turbo

Common Questions

Frequently Asked Questions

Why interleave?

FEC corrects random errors. Bursts overwhelm FEC. Interleaving spreads bursts across codewords so each sees only random errors.

Block vs convolutional?

Block: R×C matrix, full latency. Convolutional: shift registers, half latency. Convolutional starts output sooner.

How deep?

Depth > expected burst length. 5 ms fade at 1 Mbps = 5,000 bit burst. Need R×C > 5,000. Balance protection vs latency.

Related Terms

← Block Error Rate Block Upconverter →

Coding Solutions

Request a Quote

Need FEC/interleaver IP, channel coding tools, or DSP development? Contact our team.

Get in Touch