Opportunistic Spatial Orthogonalization and its application in fading cognitive radio networks

Opportunistic Spatial Orthogonalization (OSO), first proposed in Shen and Fitz (2009) for the single-input-multiple-output (SIMO) system, is a cognitive radio scheme that opportunistically allows the existence of secondary users even if the primary user occupies all the frequency bands all the time. On one hand, OSO can be viewed as a multi-user diversity scheme that exploits the channel randomness and independence. On the other hand, OSO can be interpreted as an opportunistic interference alignment scheme, where the interference from multiple secondary users is opportunistically aligned at the direction that is orthogonal to the primary user´s signal space. This paper extends OSO to the full multiple-input-multiple-output (MIMO) system, for which the scheme can be roughly interpreted as ¿riding the peaks¿ over the spatial eigen-channels. Most importantly, ill-conditioned MIMO channel, which is traditionally viewed as detrimental, is shown to be beneficial with respect to the sum throughput. User fairness issue is discussed, where using multiple transmit antennas to implement random beamforming is shown to be a possible solution.