Interleaver in ofdm system

[Varshni]

What is the purpose of an interleaver in ofdm system ? Is interleaving n convolutional coding necessary before doing modulation to the random binary data ?:? Because without doing interleaving if i do qam modulation am gettin the constellation right but with the interleaving n coding done my constellation diagram is not fine !:-(

 

[ckshivaram]

http://faculty.kfupm.edu.sa/COE/ashraf/RichFilesTeaching/COE082_543/chap2_OFDM_basics.pdf

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**broken link removed**

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OFDM is invariably used in conjunction with channel coding (forward error correction), and almost always uses frequency and/or time interleaving.

Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions such as fading. For example, when a part of the channel bandwidth fades, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated. Similarly, time interleaving ensures that bits that are originally close together in the bit-stream are transmitted far apart in time, thus mitigating against severe fading as would happen when travelling at high speed.

However, time interleaving is of little benefit in slowly fading channels, such as for stationary reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer from flat-fading (where the whole channel bandwidth fades at the same time).

The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact disc (CD) playback robust.

A classical type of error correction coding used with OFDM-based systems is convolutional coding, often concatenated with Reed-Solomon coding. Usually, additional interleaving (on top of the time and frequency interleaving mentioned above) in between the two layers of coding is implemented. The choice for Reed-Solomon coding as the outer error correction code is based on the observation that the Viterbi decoder used for inner convolutional decoding produces short errors bursts when there is a high concentration of errors, and Reed-Solomon codes are inherently well-suited to correcting bursts of errors.

Newer systems, however, usually now adopt near-optimal types of error correction codes that use the turbo decoding principle, where the decoder iterates towards the desired solution. Examples of such error correction coding types include turbo codes and LDPC codes, which perform close to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel. Some systems that have implemented these codes have concatenated them with either Reed-Solomon (for example on the MediaFLO system) or BCH codes (on the DVB-S2 system) to improve upon an error floor inherent to these codes at high signal-to-noise ratios.

 

[Girly_girl]

If we have random bits 0 10100101 coming in to the transmitter ( OFDM system)...and then by applying interleaving.. how will the bits look like?

 

[Varshni]

Deargurly_gurl,
U first encode the binary data of urs using convolutional encoding,then once it is encoded....give the encoded output to a matrix interleaver...for convolutional encoding,use poly2trellis code,u wud get the codes frm matlab help...then once it isencoded,interleave it.

dont use sms kind of typing language or short hand scripts while replying.. all your further replies or threads will be deleted if continued.

 

[Girly_girl]

Many thanks for your help..
I always though that the interleaving comes before the encoding..?

Why do we do the encoding first?