Relay-assisted Automatic Repeat reQuest (ARQ), which allows the relays instead of the source to carry out retransmissions of packets erroneously received at the destination, is an efficient cooperative transmission technique to improve link reliability with high spectrum efficiency in wireless networks. In cognitive radio (CR) systems, however, this relay-assisted ARQ could induce large latency, since a successful transmission of a packet may consume two idle timeslot or temporal spectrum holes. To overcome this limitation, an active interference cancellation (IC)-aided distributed relay-assisted ARQ method is proposed in this article to serve heterogeneous elastic traffics with diverse quality of service (QoS) demands in CR systems. Specifically, by applying our recently proposed distributed beamforming-based IC method in the physical layer, cognitive relays can exploit spatial spectrum holes (SSHs) to retransmit while keeping the interference to primary users at a tolerable level. Meanwhile, at the MAC layer, two scheduling policies, namely probabilistic and queue-length-based scheduling, are proposed to obtain an efficient allocation of temporal and SSHs to direct transmissions and retransmissions. The performance of the proposed schemes are analyzed and optimized by adjusting the scheduling parameters. As a result, the secondary users can obtain significant QoS gains, as validated by theoretical and simulation results.
['Juan Liu', 'Wei Chen', 'Zhigang Cao', 'Ying Jun Zhang']
Active interference cancellation-aided QoS-aware distributed ARQ for cognitive radios with heterogeneous traffics