Multipath fading and motion induced fading are the two most severe performance limiting phenomena in wireless environments. In a wireless communication channel, there can be more than one path in which the signal can travel between the transmitter and receiver. The presence of multipath components may be due to atmospheric reflection or refraction, or due to reflections from other Interfering Objects (IOs). In a more general case of multipath propagation, which involves a radio channel with several IOs and a moving receiver, a deterministic description of the radio channel is not efficient, and we have to resort to statistical methods. The statistical description of the radio channels is essential for wireless communication applications. Though Rayleigh fading is an excellent approximation in a large number of practical scenarios, there are scenarios where it is not valid. The Rician model can be shown to exhibit a smaller number of deep fades; the stronger the Line-of-Sight (LOS) component, the rarer the occurrence of deep fades. In this paper, the performances of both the Rayleigh and Rician fading channels are analyzed under frequency-selective fading, in terms of error probability, and it is shown that due to the presence of an LOS component, the Rician fading exhibits less error probability than the Rayleigh fading.

In any wireless communication channel, the signal can travel from the transmitter to the receiver in more than one path. The presence of multipath may be due to the following reasons (Andreas, 2005): (1) Reflections of the propagating waves from a large, smooth surface (water or large metallic surfaces); (2) Diffractions that takes place when there are obstructions in the radio path between transmitter and receiver, causing secondary waves to form behind the obstructions. This is called shadowing and this phenomenon accounts for the radio waves reaching the receiver antenna even though there is no direct path; and (3) The scattering that results from rough surfaces, whose dimensions are of the order of wavelength, which causes the reflected energy to scatter in all directions.

where a_n(t) and t_n(t) represent the attenuation and the propagation delays associated with the n^th component, respectively. Here, the attenuations and delays are shown as functions of time to represent that as the receiver moves, attenuations, delays and multipath components vary as functions of time. In the above equation, the additional Multipath Components (MPCs) are considered to be caused by the reflections from the surrounding Interfering Objects (IOs). Furthermore, each MPC or ray may be subjected to local scattering in the vicinity of the mobile unit due to the presence of the objects such as round surfaces and trees. The total signal that arrives at the receiver is made up of the sum of the large number of the scattered components. The components add vectorially with the random phases, and hence, the resulting complex envelop can be modeled as a complex Gaussian process by virtue of central limit theorem. Movements over small distances of the order of l/2 (about 15 cm at 1 GHz) can result in significant phase changes in the scattered components and cause the components to be added constructively and destructively. This results in rapid fluctuations in the received signal amplitude and power, and the phenomenon is called small-scale fading or fast fading.

Electrical and Electronics Engineering Journal, Rayleigh and Rician Fading Channels, Multipath Fading, Rayleigh Fading, Wireless Communication Channel, Multipath Components, Inter Symbol Interference, Indoor Wireless Communication Systems, Personal Communication Systems, Scattered Components, Rayleigh Fading Models.