In this paper, we present a novel scheme that aligns the interference from the antennas of the cognitive user (CU) such that the interference to the primary user (PU) is nullified or minimized. The two-user multiple-input multiple-output (MIMO) interference channel is considered. We assume that the transmission bandwidth of the CU is wider than that of the PU, and the spectrum of the PU 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 CU. The other source is the intercarrier interference (ICI) from the un-victim band of CU to the victim band of PU. An active antenna of the cognitive user is used to align with the spatial interference from the other antennas of the cognitive user and ICI such that the interference to the PU is nullified. The simulation shows that the scheme creates a greater than -200dB notch on the transmission spectrum of the victim band. This scheme allows the CU to utilize the victim band simultaneously with the PU and leads to underlay cognitive radio transmission. We also present a successive interference cancellation scheme at the cognitive receiver 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.
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