Mandatory Modulation and Coding Rate in Fixed WiMAX OFDM mode

The coding scheme of Reed-Solomon (RS) error correction code can be denoted as (n, k, T), where
n is the number of overall bytes after encoding,
k is the number of data bytes before encoding,
T is the number of data bytes which can be corrected. From the property of RS code, T=(n – k )/2.

The encoding is performed first passing the data in block format through the RS encoder and then passing it through the convolutional code (CC) encoder. Hence,
Coded block size = (the number of overall bytes after RS encoding) / CC code rate

IEEE 802.16-2004 OFDM mode require that the RS-CC overall encoding rate 1/2 should be used as the coding mode when requesting access the network (except in subchannelization modes, which use only convolution code 1/2 and bypass RS code)and in FCH burst.

For example in 16-QAM modulation using RS code (64, 48, 4), the number of overall bytes after RS encoding is 64. And then using CC code rate 2/3, the resulting coded block size is 64*(2/3) ie.96 which is also resulting the overall coding rate 1/2 (ie. 48/96).