Characteristics of wireless channel

Characteristics of wireless channel

Wireless Communication

The wireless radio channel poses a severe challenge 

as a medium for reliable high-speed communication.

It is not only susceptible to noise, interference, and 

other channel impediments, but these impediments 

change over time in unpredictable ways due to user 

movement. In this chapter we will characterize the 

variation in received signal power over distance due to

path loss , shadowing and multipath. Path loss is 

caused by dissipation of the power radiated by the 

transmitter as well as effects of the propagation channel.

Path loss models generally assume that path loss

is the same at a given transmit-receive distance. 

Shadowing is caused by obstacles between the 

transmitter and receiver that attenuate signal power 

through absorption, reflection, scattering, and 

diffraction. When the attenuation is very strong, 

the signal is blocked. Variation due to path loss

occurs over very large distances (100-1000 meters),

whereas variation due to shadowing occurs over 

distances proportional to the length of the obstructing 

object (10-100 meters in outdoor environments and 

less in indoor environments). Since variations due to

path loss and shadowing occur over relatively large 

distances, this variation is sometimes referred to as

large-scale propagation effects. Variation due to 

multi-path occurs over very short distances, on the order

of the signal wavelength, so these variations are 

sometimes referred to as small-scale propagation 

effects. Multi-path is caused by various signal

components come from reflections , these replicas

can be received in phase which leads to strong 

reception or out of phase which leads to weak signal. 

Fig.1.1 illustrates the ratio of the received-to-transmit 

power in dB versus log-distance for the combined 

effects of path loss, shadowing, and multi-path.

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