In order to reduce feedback while maintaining user performance in multiuser orthogonal frequency division multiplexing (OFDM) relay system, feedback mechanism based on user traffic rate is proposed. From the aspects of feedback outage probability, user weighted channel gain related to user traffic rate is compared with the system threshold. The higher user traffic rate is, the bigger the user traffic weight becomes. So the high traffic rate user has greater probability to be fed back, which can support different traffic rate. Simulation results show that the proposed feedback mechanism can bring more feedback sub-carriers to the user with high traffic rate.

In order to guarantee the QoS requirement of secondary users and not to affect the outage probability of primary user in cognitive relay networks, we propose two optimal power allocation models: (1) maximizing the transmission rate of secondary users; (2) minimizing the total power consumption. Theory analysis shows that two optimal power allocation models conflict between spectrum efficiency and power consumption. Furthermore, an optimal power allocation model which joints the transmission rate and the total power consumption in cognitive relay networks is proposed. By using the Lagrangian method, the optimization algorithm for this model is designed. The proposed algorithm can achieve the trade-off between the transmission rate and the total power consumption by varying the weight. Simulation results show that the proposed algorithm can effectively adjust the transmission rate and the total power consumption of secondary users.

We propose a cross-layer design combining adaptive modulation and coding (AMC) at the physical layer with a hybrid automatic repeat request (HARQ) protocol at the data link layer in multi-relay networks with imperfect channel state information (CSI). Based on a simple and distributed relay selection strategy, we derive the closed-form expressions for performance metrics such as average packet loss rate (PER) and spectral efficiency; and further formulate an optimization model to maximize the spectral efficiency under the target PER constraint. The numerical results show that the average PER is lower and the spectral efficiency is higher with the number of the relay increasing.