New HARQ Scheme Based on Decoding of Tail-Biting Convolutional Codes in IEEE 802.16e

Traditionally, a hybrid automatic repeat request (HARQ) is executed at the physical (PHY) and medium access control (MAC) layers. A cyclic redundancy check (CRC) is usually performed at the MAC layer to decide whether a packet must be retransmitted. This design causes two problems-long latency and inefficient retransmission-when a transmission error appears in a large packet. In this paper, we propose a new HARQ scheme to solve these problems based not only on CRC but on the decoding of tail-biting convolutional codes (TBCCs) at the PHY layer as well. First, the TBCC codeword that was received at the decoder is cyclically shifted according to the reliability of the received code bits. The Viterbi algorithm (VA), starting from all states, is then applied. When every survival path in the VA is close enough to its corresponding abandoned path, retransmission of the codeword is invoked. Because retransmission is restricted to a codeword and determined at the PHY layer, short latency and efficient retransmission are achieved. Simulation results show that the proposed scheme reduces the number of retransmitted codewords by up to 43% compared with the traditional HARQ in IEEE 802.16e orthogonal frequency-division multiple access (OFDMA) under a Rayleigh faded channel when the 1024-point fast Fourier transform (FFT) and 64-state quadrature amplitude modulation (64-QAM) are employed.