Date Added: Dec 2009
The authors describe a non-cooperative Interference Alignment (IA) technique which allows an opportunistic Multiple Input Multiple Output (MIMO) link (secondary) to harmlessly coexist with another MIMO link (primary) in the same frequency band. Assuming perfect channel knowledge at the primary receiver and transmitter, capacity is achieved by transmitting along the Spatial Directions (SD) associated with the singular values of its channel matrix using a water-filling Power Allocation (PA) scheme. Often, power limitations lead the primary transmitter to leave some of its SD unused. Here, it is shown that the opportunistic link can transmit its own data if it is possible to align the interference produced on the primary link with such unused SDs.