  In this paper, we consider a massive multiple-input multiple-output (MIMO)
downlink system underlaid with a network of multi-antenna D2D user equipments
(UEs). Each D2D transmitter (Tx) uses all its antennas to beamform information
towards its desired D2D receiver, which uses only a single antenna for
reception. While beamforming at the D2D Tx reduces D2D interference to the
neighbouring cellular UEs (CUEs), the cellular-to-D2D interference is also
negligible due to highly directional beamforming at the massive MIMO base
station. For the above proposed system, we analyze the average per-user
spectral efficiency (SE) of CUEs ($R^{c,d}$) as a function of the D2D area
spectral efficiency (ASE). Our analysis reveals that for a fixed D2D ASE
($R_0^{(d)}$) and fixed number of D2D antennas ($N$), with increasing density
of D2D Txs ($\lambda$), $R^{c,d}$ increases (for sufficiently large $\lambda$)
and approaches a fundamental limit $R_{\infty}^{c,d}$ as $\lambda \to \infty$.
Also, $R_{\infty}^{c,d}$ depends on $R_{0}^{(d)}$ and $N$, only through the
ratio $\frac{R_{0}^{(d)}}{N-1}$, i.e, for a given fundamental limit
$R_{\infty}^{c,d}$, the D2D ASE can be approximately doubled with every
doubling in $N$.
