  Future wireless communication systems are envisioned to share radio frequency
(RF) spectrum, with other services such as radars, in order to meet the growing
spectrum demands. In this paper, we consider co-channel spectrum sharing
between cellular systems and radars. We address the problem of target detection
by radars that are subject to shape its waveform in a way that it does not
cause interference to cellular systems. We consider a multiple-input
multiple-output (MIMO) radar and a MIMO cellular communication system with $\mc
K$ base stations (BS). We propose a spectrum sharing algorithm which steers
radar nulls, by projecting radar waveform onto the null space of interference
channel, towards a `selected' BS, thus, protecting it from radar interference.
This BS is selected, among $\mc K$ BSs, on the basis of guaranteeing minimum
waveform degradation. We study target detection capabilities of this null-space
projected (NSP) waveform and compare it with the orthogonal waveform. We derive
the generalized likelihood ratio test (GLRT) for target detection and derive
detector statistic for NSP and orthogonal waveform. The target detection
performance for NSP and orthogonal waveform is studied theoretically and via
Monte Carlo simulations.
