  In cognitive radio networks, when secondary users transmit in the null space
of their interference channel with primary user, they can avoid interference.
However, performance of this scheme depends on knowledge of channel state
information for secondary user to perform inverse waterfilling. We evaluate the
effects of imperfect channel estimation on error rates and performance
degradation of primary user and elucidate the tradeoffs, such as amount of
interference and guard distance. Results show that, based on the amount of
perturbation in channel matrices, performance of null space based technique can
degrade to that of open loop MIMO. Outcomes presented in this paper also apply
to null space based MIMO radar waveform design to avoid interference with
commercial communication systems, operating in same or adjacent bands.
