Abstract
In recent years, the lack of spectrum resources has becomed a widely concerned issue in the communication system. Cognitive radio is considered as an efficient solution to this spectrum utilization problem. As we know, the fundamental idea of cognitive radio is not to interfere with the authorized user. There are a lot literature have investigated on how to null the interference cause by the cognitive users but increase the decoding performance at the cognitive user’s receiver. In this thesis, we present a novel scheme that aligns the interference from the antennas of the cognitive user such that the interference to the primary user is nullified or minimized. The two-user multiple-input multiple-output(MIMO) interference channel is considered. We assume that bandwidth of primary user and the cognitive user is unequal and the transmission bandwidth of the primary user is called victim band.Because of the unequal transmission bandwidth, the sampling rates of the two users are not necessarily equal. Two sources of interferences are considered in the alignment. One source is the spatial interference from different antennas of the cognitive user. The other source is the inter-carrier interference (ICI) from the in-victim band of cognitive user to the victim band of primary user. An active antenna of the cognitive user is used to align with the spatial interference from the antennas of the cognitive user and ICI such that the interference to the primary user is nullified. The simulation shows that the scheme created a > 100dB notch on the transmission spectrum of the victim band. This scheme allows the cognitive user to utilize the victim band simultaneously with the primary user and leads to underlay cognitive radio transmission. We also present a soft successive interference cancellation with maximum likelihood(ML) detection method at the receiver of the cognitive user to detect the signal within the victim band. The error performance at the cognitive receiver of this underlay cognitive radio system is presented as well.