FEC allows the WiMAX MAC layer to detect errors introduced during the transmissions of frames over the air link. There are three methods of FEC specified in the WiMAX system; Reed-Solomon concatenated with convolutional code (RS-CC), block turbo code (BTC), and convolutional turbo code (CTC). RS-CC is mandatory, while BTC and CTC are made optional due to their complexity, eventhough they provide 2–3 dB better coding gain than RS-CC. For the 802.16e, a hybrid ARQ (H-ARQ) has been included as an optional feature. There are three types of H-ARQ, classified based on the manner in which they handle the retransmissions.
Type I H-ARQ retransmits lost or unacknowledged blocks using chase combining in which the old erroneous block is stored at the receiver and compared with the retransmitted copy. This helps to increase the probability of successful decoding at the FEC block during the retransmission attempts. Type II/III H-ARQ uses incremental coding rate to ensure successful decoding at the FEC block during the retransmission attempts. Rate adaptation works hand-in-hand with the FEC block in the WiMAX system. When a user experiences good channel condition, it is desirable to exploit these peaks in the channel gain to increase throughput.
This is achieved by having the SS increase the coding rate, e.g., from rate 1/2 code to rate 1/4 code, so that more information bits can be transmitted per channel use while still keeping to the target bit error rate (BER). When the channel degrades, the rate is reduced back to the next minimum to ensure that the target BER is met. This dynamic process is carried out on a frame-by-frame basis in the WiMAX system, using the flexibility provided by MAP signaling to adaptively adjust the UL/DL rates.
