Hello,
See Mathworks Simulink or AWR Visual System Simulator (VSS) user reference documents on communication channel models such as AWGN, MULTIPATH, RUMMLER, RAYLEIGH, etc...
Also see these reference articles too...
[1] P. A. Bello, "Characterization of Randomly Time-Variant Linear Channels", Transactions on Communications Systems, CS(11), pp. 360-393, Dec. 1963.
[2] C. Komninakis, "A Fast and Accurate Rayleigh Fading Simulator", in the General Conference of IEEE Globecom 2003, San Francisco, CA, Dec. 1-5, 2003.
---manju---