Chase Combining Hybrid ARQ Type II Cross-Layer Optimization for Underlay Cognitive Radio

A novel cross-layer optimization (CLO) scheme to significantly enhance the spectral efficiency (SE) of a secondary user (SU) transmission in an underlay cognitive radio system (UCRS) is proposed. The scheme jointly optimizes the physical layer (PHY) and data link control layer (DLC) of the SU transmission. At the PHY, a joint bit and power loading (JBPL) with coding adaptation (CA) using rate-matched low-density-parity-check (LDPC) coding is employed. A truncated hybrid automatic repeat request (THARQ) using chase combining (CC), known as ARQ type II, is implemented at the DLC. An analytic lower bound on the SE for coded transmission is derived and maximized subject to different quality-of-service (QoS) and stochastic chance-based constraints as well as third-party colored Gaussian interference at the SU receiver. The system performance is compared in terms of the number of required retransmissions and the achievable SU SE for different code rates. A significant improvement in system performance is observed by using CC for the SU transmission.