Cross-Layer Design for Combining Adaptive Modulation and Coding with Hybrid ARQ to Enhance Spectral Efficiency

To enhance spectral efficiency and channel utilization in future wireless communication systems, adaptive modulation and coding (AMC) has been advocated at the physical layer. However, since the constellation size and coding rate are chosen based on the corresponding carrier-to-noise ratio (CNR) region, AMC can not guarantee to achieve the maximum spectral efficiency, especially when the number of available AMC modes are limited and the current CNR is low. In this paper, we present a scheme for spectral efficiency enhancement under fading channels. The proposed scheme combines AMC at the physical layer with hybrid automatic repeat request (HARQ) at the data link layer in a cross-layer fashion, where the upper layer HARQ provides an effective means to fine-tune the performance of the lower layer AMC. Both type-I and type-II HARQ are considered in our design. We have analyzed the system PER performance and the spectral efficiency of the proposed cross- layer design scheme. Numerical results show that our scheme can achieve maximum spectral efficiency while satisfying QoS requirements. The cross-layer design with AMC and type-II HARQ provides a large spectral efficiency improvement over the design with AMC and type-I HARQ.