  This paper presents the development and evaluation of WiThRay, a new wireless
three-dimensional ray-tracing (RT) simulator. RT-based simulators are widely
used for generating realistic channel data by combining RT methodology to get
signal trajectories and electromagnetic (EM) equations, resulting in
generalized channel impulse responses (CIRs). This paper first provides a
comprehensive comparison on methodologies of existing RT-based simulators. We
then introduce WiThRay, which can evaluate the performance of various wireless
communication techniques such as channel estimation/tracking, beamforming, and
localization in realistic EM wave propagation. WiThRay implements its own RT
methodology, the bypassing on edge (BE) algorithm, that follows the Fermat's
principle and has low computational complexity. The scattering ray calibration
in WiThRay also provides a precise solution in the analysis of EM propagation.
Different from most of the previous RT-based simulators, WiThRay incorporates
reconfigurable intelligent surfaces (RIS), which will be a key component of
future wireless communications. We thoroughly show that the channel data from
WiThRay match sufficiently well with the fundamental theory of wireless
channels. The virtue of WiThRay lies in its feature of not making any
assumption about the channel, like being slow/fast fading or frequency
selective. A realistic wireless environment, which can be conveniently
simulated via WiThRay, naturally defines the physical properties of the
wireless channels. WiThRay is open to the public, and anyone can exploit this
versatile simulator to develop and test their communications and signal
processing techniques.
