  The performance of downlink massive multiple-input-multiple-output networks
with co-channel device-to-device communications is investigated in this paper.
Specifically, we consider a cellular network with sufficient number of antennas
at the base station and typical hexagonal cell coverage, where the cell users
and device-to-device transmitters are randomly and uniformly distributed. To
obtain the analytical expressions of system-level performance, the asymptotic
signal-to-interference ratios for both downlink and device-to-device links are
first obtained, which depend on the pathloss and small-scale fading of the
interference channels. Since these information may not be available at the
service base station or device-to-device transmitters, there exists a chance of
packet outage. Therefore, we continue to derive the closed-form approximation
of the average goodput, which measures the average number of information bits
successfully delivered to the receiver. Hence, the system design trade-off
between downlink and co-channel device-to-device communications can be
investigated analytically. Moreover, the performance region in which the
co-channel device-to-device communications could lead to better overall
spectral efficiency can be obtained. Finally, it is shown by simulations that
the analytical results matches the actual performance very well.
