Future wireless communication systems deployment, including fourth generation (4G) cellular systems, will be based on the MIMO transmission technology. Conventional MIMO schemes usually take advantage of the many antennas available at the transmitter by simultaneously transmitting multiple data streams from all of them. Furthermore, common open-loop MIMO schemes usually assume that all transmit-antennas are simultaneously active at any time instance. By properly choosing the transmission matrices, both multiplexing and transmit-diversity gains can be obtained via space-time coding. As a consequence, higher data rates and smaller error performance are obtained at the cost of: i) increasing the signal processing complexity at the receiver, which is caused by the need to counteract the interference created by simultaneously transmitting many data streams; and ii) making more stringent the synchronization requirements among the transmit-antennas. Furthermore, more recently, with the advent of the green and sustainable information and communication era, state-of-the-art MIMO schemes are facing two additional major challenges: i) the need of multiple RF chains at the transmitter to be able to simultaneously transmit many data streams, which do not scale with Moore's law and make the transmitter very bulky; and ii) the need of independent power amplifiers for each RF chain, each one being responsible of the vast majority of the power consumed at the transmitter as well as being extremely power inefficient due to the stringent linearity requirements of state-of-the-art phase/amplitude modulations. For example, recent studies have shown that, for a fixed RF output power, the total power consumption of base stations linearly increases with the number of active RF chains. These considerations imply that a major challenge of next-generation MIMO-enabled wireless networks is the design of multi-antenna transmission schemes with a limited number of active RF chains aiming at reducing circuitry complexity, inter-antenna synchronization requirements, inter-channel interference, signal processing complexity at the receiver, as well as at improving the energy efficiency. Fueled by these considerations, SM has recently established itself as an emerging and promising transmission concept belonging to the "massive" MIMO wireless systems family but exploiting the multiple antennas in a novel way compared with state-of-the-art high-complexity and power-hungry classic MIMOs. This tutorial is intended to offer a comprehensive state-of-the-art survey on SM-MIMO, the critical appraisal of its beneficial application domains and their research challenges, the analysis of the related technological issues associated with the implementation of SM-MIMO, and, finally, the description of the world's first experimental activities in this research field.
Spatial Modulation for MIMO Wireless Systems