Opportunistic Beamforming using Dumb Basis Patterns in Multiple Access Cognitive Channels

TL;DR

This paper introduces Random Aerial Beamforming (RAB) with ESPAR antennas to mitigate LoS interference in cognitive MAC channels, restoring multiuser diversity gains otherwise hindered by Rician interference.

Contribution

The paper proposes a novel RAB technique using ESPAR antennas to induce artificial channel fluctuations, enhancing capacity in LoS interference scenarios.

Findings

RAB restores g(N) capacity growth in LoS interference

LoS interference can be exploited to improve diversity with RAB

RAB achieves this with minimal hardware complexity

Abstract

In this paper, we investigate multiuser diversity in interference-limited Multiple Access (MAC) underlay cognitive channels with Line-of-Sight interference (LoS) from the secondary to the primary network. It is shown that for $N$ secondary users, and assuming Rician interference channels, the secondary sum capacity scales like $\log\left(\frac{K^{2}+K}{\mathcal{W}\left(\frac{K e^{K}}{N}\right)}\right)$, where $K$ is the $K$-factor of the Rician channels, and $\mathcal{W}(.)$ is the Lambert W function. Thus, LoS interference hinders the achievable multiuser diversity gain experienced in Rayleigh channels, where the sum capacity grows like $\log(N)$. To overcome this problem, we propose the usage of single radio Electronically Steerable Parasitic Array Radiator (ESPAR) antennas at the secondary mobile terminals. Using ESPAR antennas, we induce artificial fluctuations in the interference channels to restore the $\log(N)$ growth rate by assigning random weights to orthogonal {\it basis patterns}. We term this technique as {\it Random Aerial Beamforming} (RAB). While LoS interference is originally a source of capacity hindrance, we show that using RAB, it can actually be exploited to improve multiuser interference diversity by boosting the {\it effective number of users} with minimal hardware complexity.