Gray codes for Spatial Modulation systems: An open research issue

Due to the high spectral efficiency of Spatial Modulation (SM) systems, an increasing attention is being paid to investigate SM as a potential candidate for future wireless transmission. Large amount of research has been dedicated for SM performance compared to the current transmission systems. However, the problem of bit to symbol mapping in SM has been rarely investigated in the literature. Unlike other systems, the applied bit to symbol mapping in SM cannot satisfy Gray code principle, which, in turn, degrades the error performance. This article sheds light on this problem as an open research issue. The article starts by exploring the problem, discussing the encountering challenges for potential solutions, and including some expected results, aiming at motivating researchers to handle the problem.