Opportunistic Beamforming Using Dumb Basis Patterns in Cognitive Multiple Access Channels

In this paper, we investigate multiuser diversity in interference-limited multiple access underlay cognitive radio systems with line-of-sight (LoS) interference between the secondary and primary networks. Assuming Rician-K interference channels and N secondary users, it is shown that the secondary network sum capacity scales like log((K
²
+ K)/W(KeK/N)), where W(.) is the Lambert-W function. Thus, LoS interference hinders the achievable multiuser diversity gain experienced in Rayleigh interference channels, where the sum capacity grows like log(N). To overcome this problem, we propose the usage of a single radio Electronically Steerable Parasitic Array Radiator antenna at each of the secondary mobile terminals. These antennas will be used to induce artificial fluctuations in the interference channels to restore the log(N) growth rate by assigning random weights to orthogonal basis patterns. This technique will be referred to as random aerial beamforming (RAB). It is also shown that by using RAB, one can actually exploit LoS interference to improve multiuser interference diversity by boosting the effective number of users with minimal hardware complexity.