(Refer Slide Time: 00:04:55 min)
First we start with a brief introduction. The mobile radio channel places fundamental limitations
on the performance of wireless communication system. This is the very important point.
Wireless by definition, is a very hostile environment. We do not have the luxury of a fixed line
copper or the luxury of a large bandwidth like fiber. We have to deal with the uncertainties of the
channel. Coupled with that is multipath propagation, attenuation, scattering and host of other
problems. Now the widest transmission path may either be line of sight, if I am lucky. That is, I
have a direct line of sight from the transmitted to the receiver or I can have a non-line of sight in
which case my signal is actually obstructed either by building or foliage or hills or even cars on
the streets. In general, we deal with the non-line of sight situations in our cellular mobile
systems. We do not have the luxury to be in direct line of sight with the base station. Radio
channels are random and often time varying. This is important. Not only is it random, that is, you
can take enough measurements; come up with a statistics to model it but with time, the statistics
might change. This is a fundamental issue.
How to model a radio channel effectively and remember radio channels behave differently in
different frequencies. So for example, the model that I come up with for 900 MHz frequency
band may not be entirely applicable for 2.4GHz band or for the LMDS28GHz band. We will
realize why it is different the lambda, the wave length of transmission is going to be effected and
how it gets reflected, obstructed, absorbed or diffracted depends on the size of the wavelength.
Modeling radio channels have been one of the most difficult parts of the mobile radio designs.
This is because when we have to come up with a whole system model, I must plug in the channel
characteristics. I should have a reliable channel model so that I can simulate my system before
actually implementing it. Remember mobile systems are expensive systems. I randomly cannot
put a base station, measure the power and then decide, “Look, I didn‟t do a good job. Let me
shift the base station because I didn‟t get enough power”. So measurements are carried based on
these fundamental measurements. You try to come up with a realistic channel model. Most of the
channel models are random but as we will see, as you move to higher and higher frequencies that