Simulation and Modeling of Fading Channel and Improvement of Channel estimation using Artificial Neural Network

Signal fading refers to the rapid change in received signal strength over a small travel distance or time interval. This occurs because in a multipath propagation environment, the signal received by the mobile at any point in space may consist of a large number of plane waves having randomly distributed amplitudes, phases, delays and angles of arrival. These multipath components combine vectorily at the receiver antenna. They may combine constructively or destructively at different points in space, causing the signal strength to vary with location. If the objects in a radio channel are stationary, and channel variations are considered to be only due to the motion of the mobile, then signal fading is a purely spatial phenomenon. A receiver moving at high speed may traverse through several fades in a short period of time. If the mobile moves at low speed, or is stationary, then the receiver may experience a deep fade for an extended period of time. Reliable communication can then be very difficult because of the very low signal-to-noise ratio (SNR) at points of deep fades. Extensive field measurements have previously been done by many authors to characterize the small-scale spatial distribution of the received signal amplitude in multipath propagation environments. It has been found that for many environments, the Rayleigh distribution provides a good fit to the signal amplitude measurement in environments where no line-of-sight or dominant path exists. The probability density function of the Rayleigh distribution is given by