Dynamic spatial spectrum access with opportunistic orthogonalization

Opportunistic Spatial Orthogonalization (OSO) is a new cognitive radio scheme that allows the existence of secondary users and hence increases the system throughput, even if the primary user occupies all the frequency bands all the time. Notably, this throughput advantage is obtained without sacrificing the performance of the primary user, if the interference margin is carefully chosen. The key idea is to exploit the spatial dimensions to orthogonalize users and hence minimize interference. However, unlike the time and frequency dimensions, there is no universal basis for the set of all multi-dimensional spatial channels, which motivated the development of OSO. 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. Throughput advantages are studied both analytically and numerically.