On Power Control and Frequency Reuse in the Two User Cognitive Channel

TL;DR

This paper investigates power control and frequency reuse strategies in cognitive radio channels, revealing that frequency reuse benefits diminish at high and low SNRs unless multiple antennas are used, where simple zero-forcing outperforms complex strategies.

Contribution

It introduces optimal power allocation policies for SISO channels and demonstrates the advantages of multiple antennas and zero-forcing techniques in cognitive radio settings.

Findings

Frequency reuse gains vanish at extreme SNRs in SISO channels.

Multiple antennas enable avoiding the loss of frequency reuse benefits.

Zero-forcing outperforms DFP in MIMO secondary channels at high SNR.

Abstract

This paper considers the generalized cognitive radio channel where the secondary user is allowed to reuse the frequency during both the idle and active periods of the primary user, as long as the primary rate remains the same. In this setting, the optimal power allocation policy with single-input single-output (SISO) primary and secondary channels is explored. Interestingly, the offered gain resulting from the frequency reuse during the active periods of the spectrum is shown to disappear in both the low and high signal-to-noise ratio (SNR) regimes. We then argue that this drawback in the high SNR region can be avoided by equipping both the primary and secondary transmitters with multiple antennas. Finally, the scenario consisting of SISO primary and multi-input multi-output (MIMO) secondary channels is investigated. Here, a simple Zero-Forcing approach is shown to significantly outperform the celebrated Decoding-Forwarding-Dirty Paper Coding strategy (especially in the high SNR regime).