Radio waves propagate from a transmitting antenna, and travel through free space undergoing absorption, reflection, refraction, diffraction, and scattering. They are greatly affected by the ground terrain, the atmosphere, and the objects in their path, like buildings, bridges, hills, trees, etc. These multiple physical phenomena are responsible for most of the characteristic features of the received signal.
At the receiver, these multipath waves with randomly distributed amplitudes and phases combine to give a resultant signal that fluctuates in time and space. Therefore, a receiver at one location may have a signal that is much different from the signal at another location, only a short distance away, because of the change in the phase relationship among the incoming radio waves. This causes significant fluctuations in the signal amplitude. This phenomenon of random fluctuations in the received signal level is termed as fading.
The short-term fluctuation in the signal amplitude caused by the local multipath is called small-scale fading, and is observed over distances of about half a wavelength.
On the other hand, long-term variation in the mean signal level is called large-scale fading. The latter effect is a result of movement over distances large enough to cause gross variations in the overall path between the transmitter and the receiver.
gain and a linear phase response over a bandwidth larger than the bandwidth of the transmitted signal.
Under these conditions, the received signal has amplitude fluctuations due to the
variations in the channel gain over time caused by multipath. However, the spectral
characteristics of the transmitted signal remain intact at the receiver.
On the other hand, if the mobile radio channel has a constant gain and linear phase response over a bandwidth smaller than that of the transmitted signal, the transmitted signal is said to undergo frequency selective fading. In this case, the received signal is distorted and dispersed, because it consists of multiple versions of the transmitted signal, attenuated and delayed in time. This leads to time dispersion of the transmitted symbols within the channel arising from these different time delays resulting in inter symbol interference (ISI).When there is relative motion between the transmitter and the receiver, Doppler spread is Introduced in the received signal spectrum, causing frequency dispersion. If the Doppler spread is significant relative to the bandwidth of the transmitted signal, the received signal is said to undergo fast fading. This form of fading typically occurs for very low data rates. On the other hand, if the Doppler spread of the channel is much less than the bandwidth of the baseband signal, the signal is said to undergo slow fading.
The work reported here will be confined to flat fading. Results on shadowing or
log normal fading are also presented because of the existence of some general approaches, which incorporate short term and long term fading resulting in a single model.
In most of the mobile or cellular systems, the height of the mobile antenna may be smaller than the surrounding structures. Thus, the existence of a direct or line-of-sight path between the transmitter and the receiver is highly unlikely. In such a case, propagation is mainly due to reflection and scattering from the buildings and by diffraction over and/or around them. So, in practice, the transmitted signal arrives at the receiver via several paths with different time delays creating a multipath situation.
At the receiver, these multipath waves with randomly distributed amplitudes and phases combine to give a resultant signal that fluctuates in time and space. Therefore, a receiver at one location may have a signal that is much different from the signal at another location, only a short distance away, because of the change in the phase relationship among the incoming radio waves. This causes significant fluctuations in the signal amplitude. This phenomenon of random fluctuations in the received signal level is termed as fading.
The short-term fluctuation in the signal amplitude caused by the local multipath is called small-scale fading, and is observed over distances of about half a wavelength.
On the other hand, long-term variation in the mean signal level is called large-scale fading. The latter effect is a result of movement over distances large enough to cause gross variations in the overall path between the transmitter and the receiver.
Large-scale fading is also known as shadowing, because these variations in the mean signal level are caused by the mobile unit moving into the shadow of surrounding objects like buildings and hills.
Due to the effect of multipath, a moving receiver can experience several fades in a very short duration, or in a more serious case, the vehicle may stop at a location where the signal is in deep fade. In such a situation, maintaining good communication becomes an issue of great concern.
FADING:
Small-scale fading can be further classified as flat or frequency selective, and slow or fast.
A received signal is said to undergo flat fading, if the mobile radio channel has a constantgain and a linear phase response over a bandwidth larger than the bandwidth of the transmitted signal.
Under these conditions, the received signal has amplitude fluctuations due to the
variations in the channel gain over time caused by multipath. However, the spectral
characteristics of the transmitted signal remain intact at the receiver.
On the other hand, if the mobile radio channel has a constant gain and linear phase response over a bandwidth smaller than that of the transmitted signal, the transmitted signal is said to undergo frequency selective fading. In this case, the received signal is distorted and dispersed, because it consists of multiple versions of the transmitted signal, attenuated and delayed in time. This leads to time dispersion of the transmitted symbols within the channel arising from these different time delays resulting in inter symbol interference (ISI).When there is relative motion between the transmitter and the receiver, Doppler spread is Introduced in the received signal spectrum, causing frequency dispersion. If the Doppler spread is significant relative to the bandwidth of the transmitted signal, the received signal is said to undergo fast fading. This form of fading typically occurs for very low data rates. On the other hand, if the Doppler spread of the channel is much less than the bandwidth of the baseband signal, the signal is said to undergo slow fading.
The work reported here will be confined to flat fading. Results on shadowing or
log normal fading are also presented because of the existence of some general approaches, which incorporate short term and long term fading resulting in a single model.
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