1 research outputs found
Random Aerial Beamforming for Underlay Cognitive Radio with Exposed Secondary Users
In this paper, we introduce the exposed secondary users problem in underlay
cognitive radio systems, where both the secondary-to-primary and
primary-to-secondary channels have a Line-of-Sight (LoS) component. Based on a
Rician model for the LoS channels, we show, analytically and numerically, that
LoS interference hinders the achievable secondary user capacity when
interference constraints are imposed at the primary user receiver. This is
caused by the poor dynamic range of the interference channels fluctuations when
a dominant LoS component exists. In order to improve the capacity of such
system, we propose the usage of an Electronically Steerable Parasitic Array
Radiator (ESPAR) antennas at the secondary terminals. An ESPAR antenna involves
a single RF chain and has a reconfigurable radiation pattern that is controlled
by assigning arbitrary weights to M orthonormal basis radiation patterns via
altering a set of reactive loads. By viewing the orthonormal patterns as
multiple virtual dumb antennas, we randomly vary their weights over time
creating artificial channel fluctuations that can perfectly eliminate the
undesired impact of LoS interference. This scheme is termed as Random Aerial
Beamforming (RAB), and is well suited for compact and low cost mobile terminals
as it uses a single RF chain. Moreover, we investigate the exposed secondary
users problem in a multiuser setting, showing that LoS interference hinders
multiuser interference diversity and affects the growth rate of the SU capacity
as a function of the number of users. Using RAB, we show that LoS interference
can actually be exploited to improve multiuser diversity via opportunistic
nulling