Energy-Versus-Bandwidth-Efficiency Tradeoff in Spatially Modulated Massive MIMO Downlink

In this letter, we investigate the effects of pulse shaping on the bandwidth-versus-energy-efficiency tradeoff of band-limited spatial modulation (SM) massive multiple-input multiple-output (MIMO) systems. Although single-RF SM schemes have the merit of achieving high energy and bandwidth efficiencies, owing to the benefits of its antenna-activation principle, most of the previous studies assumed the use of a time-orthogonal shaping filter, which is unrealistic for a single-RF SM transmitter. To this end, we consider the use of time-limited shaping filters, such as a truncated root-raised cosine filter and a truncated Gaussian filter, for the single-RF SM transmitter. Our information-theoretic results demonstrate that, unlike in the previous studies, single-RF SM schemes do not exhibit any substantial advantages over the conventional full-RF MIMO schemes for a low number of transmit antennas, while for a large-scale antenna array, single-RF SM schemes outperform the conventional full-RF MIMO schemes.

I. Introduction

Spatial modulation (SM) [1]–[4] has been recognized as the fifth multiple-input multiple-output (MIMO) technique, in addition to the four conventional spatial multiplexing (SMX), space-time coding, beamforming, and space-division multiple access. The principle of SM is that only one of multiple antenna elements is activated in each symbol interval, where the activated antenna index conveys information bits beyond those in the classic modulation scheme. Hence only a single RF branch is needed at the SM transmitter, despite the use of multiple antenna elements. Although the peak data rate of SM is lower than that of SMX when assuming the same number of transmit antenna elements, a SM scheme’s transmission rate logarithmically increases upon linearly increasing the number of transmit antenna elements.