Interleaving in Data Communication: Purpose and Advantages

This page describes the purpose of interleaving (i.e., an interleaver) as used in the physical layer of data communication systems. It outlines the benefits and advantages of interleaving techniques.

What is Interleaving?

Interleaving is used to achieve time diversity in digital data communication systems. Essentially, an interleaver disperses the sequence of bits in a bitstream to minimize the effect of burst errors during transmission.

The interleaver is always used in conjunction with some form of error-correcting code. Error-correcting codes correct lost information within their limits. However, burst errors can reduce the effectiveness of FEC (Forward Error Correction) techniques. Interleaving improves the performance of FEC codes by arranging data in a non-contiguous way.

The interleaver is used at the transmitter, while the corresponding de-interleaver is used at the receiver.

A convolutional interleaver can be used instead of a block interleaver and is ideally suited for use with convolutional codes.

The figure depicts contiguous FEC codewords without interleaver.

Using an interleaver, these codewords are arranged in a random order to achieve error correction in burst error conditions or during data fading.

Types of Interleavers

Based on various aspects, there are two main types of interleaving techniques:

1. Block Interleaver: This type takes a bit sequence as input row-wise and outputs the bit sequence column-wise. It shuffles code symbols over a span of several block lengths.

2. Convolutional Interleaver: This interleaver consists of a number of shift registers with a fixed delay. It shuffles code symbols over a span of several constraint lengths. Each new data input is fed to the next shift register, and the previous data in that register becomes part of the interleaver’s output. This interleaver has built-in memory as its operation depends on current symbols as well as previous symbols.

Block interleavers can be further categorized into the following classes:

• Matrix Interleaver
• Helical Interleaver
• Random Interleaver
• Odd-Even Interleaver

Interleaving in WiMAX Physical Layer

Figure 2 depicts a WiMAX physical layer transmitter consisting of various modules such as a scrambler, FEC encoder, interleaver, and data mapper, as per the IEEE 802.16 OFDM specification.

Interleaving is performed in two steps:

1. Rearranging Bit Order: The first step is to rearrange the order of the bits to ensure that adjacent bits are not mapped onto adjacent carriers. This helps to eliminate errors.

2. Reordering Bits: The second step reorders the bits so that the original adjacent bits are alternately mapped into more or less reliable points on the IQ data constellation, depending on the data mapping techniques used.

After interleaving, the encoded bits are mapped to the respective IQ constellation according to the modulation type (e.g., QPSK, 16QAM, 64QAM).

The block interleaver uses the above equations and interleaves all encoded data bits with a block size corresponding to NCBPS (i.e., the number of bits in a single OFDM symbol). De-interleaving performs the inverse operation as per two permutations, retrieving the original order.

Refer to the physical layer and IEEE 802.16-2004 OFDM specifications to understand the purpose of other OFDM physical layer modules, including the interleaver and de-interleaver.

Benefits or Advantages of Interleaving in Data Communication

The following are the benefits and advantages of interleaving:

• No Increase in Data Rate: Interleaving does not increase the data rate. Both input and output have a fixed width, but the output presents a different bit sequence with a different order.

• Increased Performance of Error Correction: It increases the performance of error correction techniques, such as convolutional encoders and CTC encoders.

• Protection of Speech Coders: Speech coders require several useful bits in succession. Without an interleaver, any deep fade or burst error can result in the loss or corruption of important bits. Interleaving spreads these bits in the time domain, preventing the loss of useful data bits.

• Channel Transformation: It transforms a channel with memory into a memoryless channel.

• Enables Random Error Correction: It enables random error correcting codes to function effectively in a burst noise channel.

MATLAB Code of Interleaver and Deinterleaver

Below is the MATLAB code for the interleaving equations mentioned above. This code aligns with the IEEE 802.16 WiMAX OFDM physical layer specifications.

% Interleaver PART
s=ceil(ncpc/2);
k=0:ncbps-1;

%First permutation of interleaver
m=(ncbps/12)*mod(k,12)+floor(k/12);

%Second permutation of interleaver
n=s*floor(m/s)+mod(m+ncbps-floor(12*m/ncbps),s);
interleaved_data_out(n+1)=interleaver_input(k+1)  % OUTPUT of interleaver