An efficient HARQ retransmission algorithm in OFDMA based wireless networks

In wireless transmission, hybrid automatic repeat request (HARQ) can improve the throughput performance by using error correction and error detection techniques when the signal quality is poor. In this paper, the effectiveness of HARQ in orthogonal frequency division multiple access (OFDMA) based wireless networks is studied. In OFDMA networks, every channel is split into a set of orthogonal sub-carriers collectively. Every user is assigned a set of sub-carriers for data transmission. When the network is overloaded, some of the packet retransmissions could be delayed. For applications with critical delay and jitter requirement, such as video and VoIP, packets with large delay because of too many retransmission attempts are usually discarded. If those retransmissions are assigned to sub-carriers with better signal quality, the transmission efficiency can be improved. In this paper, we propose an enhanced HARQ algorithm to schedule retransmissions to subcarriers to achieve higher throughput. A timestamp, determined by delay requirements, is attached to the retransmission request. The OFDMA scheduler allocates sub-carriers and time offsets to retransmission packets based on channel properties and the timestamp value. Retransmissions with smaller timestamp value than the estimated propagation time will be aborted. Therefore, the traffic load introduced by unnecessary retransmissions is reduced and the throughput can be improved.