Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---8

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN
CHANNEL CODING
A detailed discussion of channel coding options in OFDMA PHY is beyond the scope of this paper; only a brief summary of the supported mandatory and optional modes are given here.
Based on terminology used in WirelessMAN OFDMA PHY, channel coding consists of Randomization, Forward Error Correction (FEC), bit interleaving, and modulation. Repetition code is used on various control messages to further enhance the error correction performance of the system. Repetition codes of 2, 4, or 6 are implemented by utilizing multiple subchannels.
Randomization is performed on both UL and DL data. The data are randomized using a PN sequence generator with a polynomial of degree 15 that is reinitialized at the beginning of each FEC block with a seed, which is a function of the OFDM symbol offset (from the start of the frame) and the starting subchannel number corresponding to the FEC block.
The OFDMA PHY supports mandatory tail-biting Convolutional Coding and three optional coding schemes: Zero Tailing Convolutional code, Convolutional Turbo code along with H-ARQ, and Block Turbo code.
The tail biting is implemented by initializing the encoders memory with the last data bits of the FEC block being encoded, and the zero tailing is implemented by appending a zero tail byte to the end of each burst.
H-ARQ mitigates the effect of impairments due to channel and external interference by effectively employing time diversity along with incremental transmission of parity codes (subpackets in this case). In the receiver, previously erroneously decoded subpackets and retransmitted subpackets are combined to correctly decode the message. The transmitter decides whether to send additional subpackets, based on ACK/NAK messages received from the receiver.
Bit interleaving is performed on encoded data at the output of FEC. The size of the interleaving block is based on the number of coded bits per encoded block size. The interleaving is performed using a two-step permutation process. The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. The second permutation ensures that adjacent coded bits are mapped alternately onto less or more significant bits of the constellation, thus avoiding long runs of lowly reliable bits.