As an important multi-antenna technique, beamforming can be applied to enable concurrent transmissions of Secondary Users (SUs) and Primary Users (PUs) in Cognitive Radio (CR) systems. In this way, new transmission opportunities when the channel is occupied by PUs, namely spatial or directional spectrum holes, are exploited. This paper proposes a distributed beamforming approach for directional spectrum sharing to improve the Quality-of-Service (QoS) of SUs in CR systems. In particular, an SU source broadcasts to a set of cognitive nodes that act as SU relays, which can form a distributed beamformer to forward messages in busy timeslots without causing harmful interference to PUs. By doing so, both cooperative diversity gain and power gain are achieved without consuming extra idle timeslots or temporal spectrum holes. To support various secondary traffics with different arrival patterns, we develop a series of distributed beamforming based opportunistic spectrum access schemes. Through rigorous analysis, we analyze the system performance in terms of the delay-limited outage probability, the average delay and average throughput etc. Furthermore, cross-layer optimization problems are formulated to optimize the system performance by selecting appropriate scheduling parameters. A substantial QoS gain can be obtained by distributed beamforming based directional spectrum sharing, compared to those without node cooperation or cooperative diversity schemes utilizing temporal spectrum holes only.
['Juan Liu', 'Wei Chen', 'Ying Jun Zhang', 'Zhigang Cao']
Distributed beamforming based directional spectrum sharing