  Spatial multiplexing (SMX) multiple-input multiple-output (MIMO) over the
best beamspace was considered as the best solution for millimeter wave (mmWave)
communications regarding spectral efficiency (SE), referred as the best
beamspace selection (BBS) solution. The equivalent MIMO water-filling (WF-MIMO)
channel capacity was treated as an unsurpassed SE upper bound. Recently,
researchers have proposed various schemes trying to approach the benchmark and
the performance bound. But, are they the real limit of mmWave MIMO systems with
reduced radio-frequency (RF) chains? In this paper, we challenge the benchmark
and the corresponding bound by proposing a better transmission scheme that
achieves higher SE, namely the Generalized Beamspace Modulation using
Multiplexing (GBMM). Inspired by the concept of spatial modulation, besides the
selected beamspace, the selection operation is used to carry information. We
prove that GBMM is superior to BBS in terms of SE and can break through the
well known `upper bound'. That is, GBMM renews the upper bound of the SE. We
investigate SE-oriented precoder activation probability optimization,
fully-digital precoder design, optimal power allocation and hybrid precoder
design for GBMM. A gradient ascent algorithm is developed to find the optimal
solution, which is applicable in all signal-to-noise-ratio (SNR) regimes. The
best solution is derived in the high SNR regime. Additionally, we investigate
the hybrid receiver design and deduce the minimum number of receive RF chains
configured to gain from GBMM in achievable SE. We propose a coding approach to
realize the optimized precoder activation. An extension to mmWave broadband
communications is also discussed. Comparisons with the benchmark (i.e., WF-MIMO
channel capacity) are made under different system configurations to show the
superiority of GBMM.
