  In this paper, we propose a spatial modulation (SM) scheme referred to as
complex quadrature spatial modulation (CQSM). In contrast to quadrature spatial
modulation (QSM), CQSM transmits two complex signal constellation symbols on
the real and quadrature spatial dimensions at each channel use, increasing the
spectral efficiency. To this end, signal symbols transmitted at any given time
instant are drawn from two different modulation sets. The first modulation set
is any of the conventional QAM/PSK alphabets, while the second is a rotated
version of it. The optimal rotation angle is obtained through simulations for
several modulation schemes and analytically proven for the case of QPSK, where
both results coincide. Simulation results showed that CQSM outperformed QSM and
generalized SM (GSM) by approximately 5 and 4.5 dB, respectively, for the same
transmission rate. Its performance was similar to that of QSM; however, it
achieved higher transmission rates. It was additionally shown numerically and
analytically that CQSM outperformed QSM for a relatively large number of
transmit antennas.
