  Massive spatial modulation aided multiple-input multiple-output (SM-MIMO)
systems have recently been proposed as a novel combination of spatial
modulation (SM) and of conventional massive MIMO, where the base station (BS)
is equipped with a large number of antennas and simultaneously serves multiple
user equipment (UE) that employ SM for their uplink transmission. Since the
massive SM-MIMO concept combines the benefits of both the SM and massive MIMO
techniques, it has recently attracted substantial research interest. In this
paper, we study the achievable uplink spectral efficiency (SE) of a multi-cell
massive SM-MIMO system, and derive closed-form expressions to asymptotically
lower-bound the SE yielded by two linear BS combining schemes, including
maximum ratio (MR) combining and zero forcing (ZF) combining, when a
sufficiently large number of BS antennas are equipped. The derivation takes
into account the impact of transmitter spatial correlations, of imperfect
channel estimations, of user-specific power controls and of different pilot
reuse factors. The proposed asymptotic bounds are shown to be tight, even when
the scale of BS antennas is limited. The new SE results facilitate a
system-level investigation of the optimal number of uplink transmit antennas
(TAs) $N$ with respect to SE maximization. Explicitly, we provide theoretical
insights on the SE of massive SM-MIMO systems. Furthermore, we demonstrate that
massive SM-MIMO systems are capable of outperforming the SE of conventional
massive MIMOs relying on single-TA UEs.
