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What is interleaving | Advantages of Interleaving in data communication

This page describes purpose of Interleaving (i.e. Interleaver) used in physical layer. It mentions benefits or advantages of Interleaving techniques used in data communication.

What is Interleaving ?

Interleaving is used to achieve time diversity in digital data communication system. Basically interlaver disperses sequence of bits in bit stream to minimize effect of burst errors during transmission. Interleaver is always used in conjunction with some error correcting code.

Error correcting codes correct the lost information within its limit. Due to burst errors, capability of FEC techniques to correct errors reduces. Interleaver improves performance of FEC codes by arranging data in non-contiguous way.

The Interleaver is used at the transmitter where as de-interleaver is used at the receiver. Convolutional interleaver can be used instead of block interleaver and it is ideal for use with convolutional codes.

Interleaving

The figure depicts contiguous FEC codewords without interleaver. Using interleaver, this codewords are arranged in random order to achieve error correction in burst error condition or fading of data.

Based on various aspects, there are two types of interleaver or interleaving techniques.
1. Block Interleaver : It takes bit sequence at input row-wise and gives bit sequence at output column-wise. It shuffles code symbols over span of several block lengths.
2. Convolutional Interleaver : It is an interleaver which consists of number of shift registers with fixed delay. It shuffles code symbols over span of several constraints lengths. Every new data to input of interleaver is fed to next shift register and previous data of register becomes part of interleaver output. This interleaver has built-in memory as its operation depends on current symbols as well as previous symbols.

Block interleaver is further categorized into following classes.
• Matrix Interleaver
• Helical Interleaver
• Random Interleaver
• Odd-Even Interleaver

The figure-2 depicts WiMAX physical layer transmitter consisting of various modules such as scrambler, FEC encoder, Interleaver, data mapper as per IEEE 802.16 OFDM specification.

Interleaver deinterleaver in wimax physical layer

The interleaving is performed in two steps. The first step of Interleaving is to rearrange the ordering of the bits to make certain that adjacent bits are not mapped onto adjacent carriers. This helps eliminate errors. The second step of Interleaving is to reorder the bits so that the original adjacent bits are alternately mapped into more or less reliable points on the IQ data constellation as per different data mapping techniques. After interleaving, the en-coded bits are mapped to the respective IQ constellation as per modulation type such as QPSK, 16QAM, 64QAM etc.

Interleaver equations

The block interleaver uses above equations and interleave all encoded data bits with block size corresponding to NCBPS (i.e. number of bits in a single OFDM symbol). De-interleaver performs inverse operation as per two permutations and retrieve back the original order. Refer physical layer >> and IEEE 802.16-2004 OFDM specifications to know purpose of other OFDM physical layer modules including Interleaver and Deinterleaver.

Benefits or advantages of Interleaving in data communication

Following are the benefits or advantages of Interleaving performed by interleaver:
➨It does not increase data rate. Both input and output have fixed width but output has another bit sequence with different order.
➨It increases performance of error correction techniques such as Convolutional encoder and CTC encoder.
➨Speech coders require several useful bits in succession. Without interleaver, any deep fade or error burst results into loss of important bits or corruption of bits. Interleaver spread these bits in time space and hence prevents loss of useful data bits.
➨It transforms a channel with memory to memoryless channel.
➨It enables random error correcting codes in a burst noise channel.



MATLAB code of Interleaver and Deinterleaver

Following is the MATLAB code of interleaving equations mentioned above. This is as per IEEE 802.16 WiMAX OFDM physical layer specifications. For de-interleaver code, visit MATLAB Source code >>.

% 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


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