Blind Null-Space Learning for MIMO Underlay Cognitive Radio Networks
  This paper proposes a blind technique for MIMO cognitive radio Secondary Users (SU) to transmit in the same band simultaneously with a Primary User (PU) under a maximum interference constraint. In the proposed technique, the SU is able to meet the interference constraint of the PU without explicitly estimating the interference channel matrix to the PU and without burdening the PU with any interaction with the SU.   The only condition required of the PU is that for a short time interval it uses a power control scheme such that its transmitted power is a monotonic function of the interference inflicted by the SU. During this time interval, the SU iteratively modifies the spatial orientation of its transmitted signal and measures the effect of this modification on the PU's total transmit power. The entire process is based on energy measurements which is very desirable from an implementation point of view. 
Yair Noam and Andrea Goldsmith
cs.IT math.IT