Impact of Modulation Constellation Type on the Finite Signal-to-Noise Ratio Diversity Gain in the Presence of a Multipath Fading Channel.

The evolution of wireless communication in the presence of multipath fading and shadowing is generally strained by the worsening of propagation conditions. Thus, in order to estimate the achievable improvements of the link quality (due to channel enhancement), such a parameter as diversity gain is usually used. The diversity gain is classically defined as a limit for infinitely increasing signal-to-noise ratio (SNR) at the receiver input, theoretically attained only in asymptotics, and in practice attained at very high SNR values (e.g., 50–70 dB), which are unrealistic from the practical perspectives for wireless fading channels. In this case, a modified version of the same metric (i.e., the finite-SNR diversity gain; FSDG) was proposed. The presented research performs a comparative analysis of the FSDG for the case of a multipath channel with fading induced by second order scattering and fluctuating line-of-sight (LoS) component (i.e., fluctuating double-Rayleigh with Line-of-Sight; fdRLoS). The signal model included the hexagonal quadrature amplitude modulations (QAM) with regular and irregular constellations. For the considered problem, an exact expression for the FSDG was derived and analyzed as a function of channel parameters (responsible for fading severity and shadowing intensity) and modulation formats. It has been found that the FSDG can exceed the classical (asymptotic) diversity gain by the order of magnitude, and scenarios have been identified where this can occur (for example, the maximum is achieved under conditions of light shadowing and strong line-of-sight component). A nonmonotonic dependence of FSDG on the QAM constellation size (for any type of constellation) was found out: for low-to-moderate average SNR, for extremely weak and extremely strong LoS, the highest FSDG is achieved with smaller constellations, whereas for moderate LoS components, with larger constellations. [ABSTRACT FROM AUTHOR]