  This paper studies the coexistence between a downlink multiuser massive
multi-input-multi-output (MIMO) communication system and MIMO radar. The
performance of the massive MIMO system with maximum ratio ($\MR$), zero-forcing
($\ZF$), and protective $\ZF$ ($\PZF$) precoding designs is characterized in
terms of spectral efficiency (SE) and by taking the channel estimation errors
and power control into account. The idea of $\PZF$ precoding relies on the
projection of the information-bearing signal onto the null space of the radar
channel to protect the radar against communication signals. We further derive
closed-form expressions for the detection probability of the radar system for
the considered precoding designs. By leveraging the closed-form expressions for
the SE and detection probability, we formulate a power control problem at the
radar and base station (BS) to maximize the detection probability while
satisfying the per-user SE requirements. This optimization problem can be
efficiently tackled via the bisection method by solving a linear feasibility
problem. Our analysis and simulations show that the $\PZF$ design has the
highest detection probability performance among all designs, with intermediate
SE performance compared to the other two designs. Moreover, by optimally
selecting the power control coefficients at the BS and radar, the detection
probability improves significantly.
