Research On Degrees Of Freedom Of Cognitive Radio Networks

The spectrum used by wireless communications is a very limited resource. Thepresent fixed spectrum allocation consumed spectrum quickly. The scarcity ofspectrum has become a bottleneck restricting further development and applications ofwireless communications technology. However, a report of the U.S. FederalCommunications Commission has indicated that the majority of licensed spectrum hasnot been fully used, spectral efficiency is very low. Therefore, in order to alleviateproblems caused by the scarcity of spectrum, cognitive radio technology came intobeing. Obtaining the capacity region of a cognitive radio network is an open problem.However, some useful insights can be found by approximate characterizations of thecapacity, for example, the degrees of freedom(DoF).Recently, interference alignment technique has broken many classic thoughtsabout wired or wireless network throughput limits.This paper studies outer bounds of DoF of cognitive radio network withcooperation and cognition. Cooperative communications are widely studied in variouscommunication networks to improve the throughput. So we introduce cooperation inour study to achieve a higher outer bound of DoF region.The concept of spectrum sharing is extended by DoF sharing. This paper focuseson a SISO cognitive radio network consisted of one primary user and one or moresecondary users. A outer bound of DoF is obtained. And with cooperation andcognition, we propose an asymptotic interference alignment scheme to prove that theouter bound is achievable. The outer bound indicates that for single secondary usercase, the secondary user can get all the DoF left by the primary user, and for multiplesecondary users, each secondary user can get:(1) half DoF when the primary userutilizes no more than half DoF, and (20) all the DoF left by the primary user when theprimary user utilize more than half DoF. However, in this scheme, the primary usergets no benefits, and is even required to do additional precoding. To encourage theprimary user to cooperate, this paper also proposes two aided interference alignment(AIA) schemes:(1) partial AIA, secondary users assist the primary user to transmit part of primary message, and (2) full AIA, secondary users assist the primary user totransmit all primary message.In this paper, we also try to extend the conclusion of DoF outer bound of SISOcognitive radio networks to MIMO cognitive radio networks. Two cases are analyzed.One is the case of only one secondary user, we prove that the secondary user canobtain all the DoF left unused by the primary user. The other is the case of multiplesecondary users when the primary user utilize no more than half the maximumaccessible DoF, we prove that each secondary user can get half the maximumaccessible DoF.Finally, the paper points out some future research issues including outer DoFbounds of MIMO cognitive radio networks in other cases and the existence ofmultiple primary users.