Abstract
This paper introduces novel transmit beamforming approaches for the cognitive radio (CR) Z-channel. The proposed transmission schemes exploit non-causal information about the interference at the SBS to re-design the CR beamforming optimization problem. This is done with the objective to improve the quality of service (QoS) of secondary users by taking advantage of constructive interference in the secondary link. The beamformers are designed to minimize the worst secondary user's symbol error probability (SEP) under constraints on the instantaneous total transmit power, and the power of the instantaneous interference in the primary link. The problem is formulated as a bivariate probabilistic constrained programming (BPCP) problem. We show that the BPCP problem can be transformed for practical SEPs into a convex optimization problem that can be solved, e.g., by the barrier method. A computationally efficient tight approximate approach is also developed to compute the near-optimal solutions. Simulation results and analysis show that the average computational complexity per downlink frame of the proposed approximate problem is comparable to that of the conventional CR downlink beamforming problem. In addition, both the proposed methods offer significant performance improvements as compared to the conventional CR downlink beamforming, while guaranteeing the QoS of primary users on an instantaneous basis, in contrast to the average QoS guarantees of conventional beamformers.
Highlights
Dynamic spectrum access (DSA) in cognitive radio (CR) networks has provided an effective way to increase the radio resource utilization and spectral efficiency, by allowing the utilization of the licensed spectrum by secondary links [1]–[5]
We provide a systematic treatment of constructive interference as illustrated in Fig. 2(a), where the nominal phase-shift keying (PSK) constellation point is represented by the black circle
We exploit the constructive interference in the underlay CR Z-channel by making use of channel state information (CSI) and transmit data information jointly
Summary
Dynamic spectrum access (DSA) in cognitive radio (CR) networks has provided an effective way to increase the radio resource utilization and spectral efficiency, by allowing the utilization of the licensed spectrum by secondary links [1]–[5]. With conventional CR beamforming [5], [23], which only constrains the average interference, the instantaneous interference at the PUs at individual time instants may largely exceed the predefined thresholds This can be overcome by utilizing the knowledge of both CSI and SU’s information symbols at the SBS to exploit the resulting interference in the secondary links. In this case beamformers can be designed to enhance the useful signal by steering the received signals, containing both the desired and the interfering signals, into the correct detection region instead of separately amplifying and suppressing the desired and the interfering signals, respectively [21], [27]–[34].
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