The Interference Alignment In Cognitive Radio Systems

Cognitive radio has been recently proposed as a potential solution for efficient utilization ofthe scarce radio resources. The main idea of cognitive radio is to let a class of radio devices,called secondary users, opportunistically access certain portions of the spectrum, called whitespaces, which are left unused by the licensed system, called primary users. The concept of whitespaces can be associated with a specific frequency carrier, time slot, or spatial direction. In theabsence of those spectrum holes, secondary systems are unable to transmit without producingadditional interference on the primary systems. The interference alignment (IA) is the onesolution to this problem.IA refers to the construction of signals such that the resulting interference signal lies in asubspace orthogonal to the one spanned by the signal of interest at each receiver.In this paper, IA scheme is studied in a scenario with multiple primary users and multiplesecondary users, and the secondary users could use the same frequency band of the primary linkswithout generating any interference to the primary users. First, an outer bound is developed onthe degrees of freedom (DOF) of the secondary users. Then, in the case that the interferencebetween primary users is not considered, under power constraints, the primary user canmaximize its rate by water-filling on the singular values of its channel matrix. The secondaryusers’ pre-coding schemes are designed to align the interference produced on the primary linkswith such unused sub-channels of the primary users, thus no interference is generated to theprimary users. Two methods are presented for designing the secondary users’ post-processingschemes. One is to white the interference, the other is to eliminate the interference with IA. Last,the IA scheme is also studied in the case of considering the interference between primary users.And two situations are considered. One is that each user could only transmit one symbolsimultaneously, and there are only two primary users but multiple secondary users. The other isthat each user could transmit multiple symbols simultaneously, and there are multiple primaryusers and multiple secondary users.