  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.
