  Two-wave with diffuse power (TWDP) is one of the most promising distribution
for description of a small-scale fading in the emerging mmWave band. However,
traditional error performance analysis in these channels faces two fundamental
issues. It is mostly based on conventional TWDP parameterization which is not
in accordance with the model's underlying physical mechanisms and which hinders
accurate observation of the impact of a model parameters on a system's
performance metrics. In addition, the existing average bit/symbol error
probability (ABEP/ASEP) expressions for most modulations and diversity schemes
are available as approximations, which are accurate only for specific
combinations of TWDP parameters. Accordingly, in this paper, the exact ASEP
expressions are derived for M-ary rectangular quadrature amplitude modulation
(RQAM) with coherent detection and for M-ary DPSK modulation, and are given in
terms of physically justified parameters. Besides, in order to relax
computational complexity of proposed exact ASEPs in high signal-to-noise ratio
(SNR) region, their asymptotic counterparts are derived as the simple
closed-form expressions, matching the exact ones for SNR>30dB. Results are
verified by Monte-Carlo simulation.
