Journal of Emerging Technologies in Web Intelligence Contents

126 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010VII. FORWARD ERROR CORRECTIONForward error correction [FEC] is a system of errorcontrol for data transmission, whereby the sender addsredundant data to its messages, which allows the receiverto detect and correct errors (within some bound) withoutthe need to ask the sender for additional data. Theadvantage of forward error correction is thatretransmission of data can often be avoided, at the cost ofhigher bandwidth requirements on average, and istherefore applied in situations where retransmissions arerelatively costly or impossible.Such a typical situation where the receiver is not ableto make a request for the re-transmission of message isFM reception. FEC devices are often located close to thereceiver of an analog signal, in the first stage of digitalprocessing after a signal has been received. That is, FECcircuits are often an integral part of the analog-to-digitalconversion process.Forward Error Correction Implemented:The Forward error correction algorithm consists oftwo parts namely Encoding and Decoding. The Encodingis implemented using a Convolution algorithm and theDecoding implemented is the Viterbi algorithm. Theconvolution encoder and the corresponding Viterbidecoder are implemented in C programming language.Motivation for Forward Error CorrectionThe situation in which the receiver is not able to makea request to the sender for re-transmission of messageforms the basis for all forward error correctiontechniques. A typical radio receiver is a classic examplein which the receiver has no provision to communicatewith the sender. Under such circumstances, if an erroroccurred in the signal transmitted, there will be no wayfor the receiver to detect the error unless some errorcontrol mechanisms are incorporated into the signalbefore transmission itself. This process of adding errorcontrol to the signal before transmission is calledForward Error Correction.A typical communication channel in which the datato be transmitted is encoded using some encodingalgorithm by adding some redundant bits (Figure. 6).Thus the output of the encoder is called ChannelSymbols. These channel symbols are then transmittedover the wireless channel.Thus it can be seen that unless the error correctionmechanisms are introduced there is no way for thereceiver to recover the original information correctly.Hence Forward Error Correction mechanisms areessential if the receiver is to recover the originalinformation. There are numerous forward error correctionmechanisms each applicable to particular type ofcommunication. The factors which govern which errorcorrection algorithm is used for a particularcommunication type are Bandwidth of the spectrumDepth of error correction requiredRate of encodingProcessing capacity of Sender/ReceiverIntroduction to Convolution EncodingConvolution Encoding typically involves encoding ofstream of data bits by using previous bits to perform alogical operation, the output of which is the encodedchannel symbols. Thus depending upon the type ofconvolution encoding, a single data bit can have itsinfluence on two or more adjacent bits thereby providingthe required redundancy to the data. This influence whicha bit has on other bits is what enables the receiver toidentify any bit errors that occurred during the datatransmission.A convolution encoder is called so because it performsa convolution of the input stream with encoder's impulseresponses. It can be represented mathematically aswhere x is an input sequence, y j is a sequence from outputj and h j is an impulse response for output j. A convolutionencoder is a discrete linear time-invariant system. Everyoutput of an encoder can be described by its own transferfunction, which is closely related to a generatorpolynomial.Figure 6. Typical wireless communication channel© 2010 ACADEMY PUBLISHER