Research On Adaptive HARQ Schemes For OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) technology, due to its high spectrum efficiency, robustness to frequency selective fading as well as the feasibility of low-cost transceiver implementation, has become one of the promising candidates for beyond 3G mobile communication systems. On the other hand, with the rapidly increasing demand for reliable high rate wireless services in recent years, hybrid automatic repeat request (HARQ) protocol is becoming an important technology for beyond 3G mobile communication systems. By detailedly analyzing the merits of both technologies and deeply researching on key technologies for combination of the two technologies, such as adaptive interleaving, constellation rearrangement, interleaving rearrangement, maximal ratio combining, adaptive sub-carrier allocation, etc., this paper proposed two adaptive HARQ schemes for OFDM systems. The first scheme is oriented to system stability and retransmits identical bits to the initial transmission. The other one is oriented to system throughput and retransmits partly identical bits to the initial transmission.The first proposed scheme adopts constellation rearrangement, adaptive interleaving and interleaving rearrangement to guarantee system stability. And the other one adopts constellation rearrangement and adaptive sub-carrier allocation to achieve much higher thoughput. Both of them take full advantage of channel quality information to adaptively rearrange symbols at each restransmission, which leads to achieve futher frequency diversity gain besides time diversity gain. The first proposed one is a little more complex to implement than the other one. Simulation results show that: the fist proposed scheme achieves very high throughput and greatly reduces frame error rate and transmission delay, thus significantly improves system performance both in fast and slow fading environments; the other one dramatically reduces the amount of retransmitted data and the resource usage in retransmission, which intrinsically lead to higher throughput efficiency than fisrt one but longer transmission delay. Therefore, the first proposed scheme is suitable for time delay sensitive service and the other proposed scheme is more suitable for high data rate service.