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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 45 / 107

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All the model parameters are random in nature and only a statistical characterization of them is possible, i.e. in terms of the mean and variance value. They are dependent upon terrain, tree density, antenna height and beamwidth, wind speed and time of the year.

Path loss:

Path loss is affected by several factors such as terrain contours, different environments (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, the height and location of their antennas, etc. It has only impact on the link budget [11], that is why we will not consider it in our channel modeling.

Multipath Delay Spread:

Due to the non line of sight (NLOS) propagation nature of the WirelessMAN OFDM, we have to address multipath delay spread in our channel model. It results due to the scattering nature of the environment. Delay spread is a parameter used to signify the effect of multipath propagation. It depends on the terrain, distance, antenna directivity and other factors. The rms delay spread value can span from tens of nano seconds to microseconds.

Fading characteristics:

In a multipath propagation environment, the received signal experiences fluctuation in its amplitude, phase and angle of arrival. The effect is described by the term multipath fading. Due to ixed deployment of transmit and receive antenna, we just have to address the small­scale fading in our channel model. Small­scale fading refers to the dramatic changes in signal amplitude and phase that can be experienced as a result of small changes (as small as a half wavelength) in the spatial positioning between a receiver and a transmitter.

Small­scale fading is called Rayleigh fading if there are multiple re lective paths that are large in number and there is no line­of­sight signal component; the envelope of such a received signal is statistically described by a Rayleigh pdf. When a dominant non fading

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