Channel coding is a process where one or more control and user data signals are combined with error protected or error correction information. The WiMAX system channel coding processes include error correction coding, interleaving and randomization.
Error Correction Coding
Error correcting codes are additional information elements that are sent along with a data signal that can be used to detect and possibly correct errors that occur during transmission and storage of the media. Error correction codes conform to specific rules or formulas to create the code from the data that is being sent. Error correction codes require an increase in the number of signal elements that are transmitted which increases the required data transmission rate. The WiMAX system can use a variety of error coding methods including Reed Soloman coding, convolutional coding (optional) and block turbo coding (optional).
Interleaving
Interleaving is the reordering of data that is to be transmitted so that consecutive bytes of data are distributed over a larger sequence of data to reduce the effect of burst errors. The use of interleaving greatly increases the ability of error protection codes to correct for burst errors. Many of the error protection coding processes can correct for small numbers of errors, but cannot correct for errors that occur in groups. The WiMAX system uses interleaving to map data onto non-adjacent sub-carriers to help overcome the effects of frequency selective (e.g. multi-path) distortion.
Randomization
Randomization is a process that rearranges data components in a serial bit sequence to statistically approximate a random sequence. For communication systems, randomization involves using a known randomization code or process in the transmitter and using the same code to decode the randomized sequence at the receiver.
The WiMAX system uses a pseudo-random binary sequence (PRBS) randomization process that ensures that there are no long sequences of bits that would cause the modulator to produce a high peak to average power ratio (PAPR) signal. PAPR is a comparison of the peak power detected over a period of sample time to the average power level that occurs over the same time period. A high PAPR would require the use of a more linear RF amplifier assembly increasing cost and decreasing power conversion efficiency (e.g. shorter battery life).
