  Generalized spatial modulation (GSM) uses $n_t$ transmit antenna elements but
fewer transmit radio frequency (RF) chains, $n_{rf}$. Spatial modulation (SM)
and spatial multiplexing are special cases of GSM with $n_{rf}=1$ and
$n_{rf}=n_t$, respectively. In GSM, in addition to conveying information bits
through $n_{rf}$ conventional modulation symbols (for example, QAM), the
indices of the $n_{rf}$ active transmit antennas also convey information bits.
In this paper, we investigate {\em GSM for large-scale multiuser MIMO
communications on the uplink}. Our contributions in this paper include: ($i$)
an average bit error probability (ABEP) analysis for maximum-likelihood
detection in multiuser GSM-MIMO on the uplink, where we derive an upper bound
on the ABEP, and ($ii$) low-complexity algorithms for GSM-MIMO signal detection
and channel estimation at the base station receiver based on message passing.
The analytical upper bounds on the ABEP are found to be tight at moderate to
high signal-to-noise ratios (SNR). The proposed receiver algorithms are found
to scale very well in complexity while achieving near-optimal performance in
large dimensions. Simulation results show that, for the same spectral
efficiency, multiuser GSM-MIMO can outperform multiuser SM-MIMO as well as
conventional multiuser MIMO, by about 2 to 9 dB at a bit error rate of
$10^{-3}$. Such SNR gains in GSM-MIMO compared to SM-MIMO and conventional MIMO
can be attributed to the fact that, because of a larger number of spatial index
bits, GSM-MIMO can use a lower-order QAM alphabet which is more power
efficient.
