One of the facts that most people do not seem to know or appreciate is the huge variability in the distance over which a given wireless communication device can operate as a function of the location of the transmitter and receiver. Another little appreciated fact is that one can communicate over long distances with very little power, under favorable conditions. It is easiest to demonstrate this by an example. Consider first two radios communicating under what is called free space propagation. This corresponds to the case where the radios are located on high buildings or mountain tops and there is a clear unobstructed line of sight between them. The distance over which the radios can operate, for three different transmit power levels, is as follows
power 1 watt – distance 319 km
power 1 milliwatt – distance 10.1 km
power 1 microwatt – distance 0.319 km
Next consider the case where the two radios are in an urban environment. One radio is on a rooftop, and the other is at street level. Now the numbers look as follows
power 1 watt – distance 1.4 km
power 1 milliwatt – distance 0.3 km
power 1 microwatt – distance 0.037 km
For the technically inclined I should say that these numbers were computed using the following parameters: 900 MHz radios, combined receive/transmit antenna gains of 15dB, receiver noise figure 5 dB, Eb/N0 at the receiver 10dB. The urban example used the small city Hata model with antenna heights of 2m (mobile) and 20m (basestation).
There are two important things to observe here. The first is that small power can reach far under free space conditions. A 1 milliwatt radio can reach over 10 kilometers. The second is that there is a huge difference between the range we can get under ideal conditions (free space propagation) and the typical range we will get in a city. In fact the real range in a city will be significantly smaller than what was computed above. For the technically inclined I will mention that in reality the combined transmit/receive antenna gains can be much less than 15dB, and we will have to add a fading margin which was not done above. The point to remember is that the real range of a wireless system is very much less than the ideal free space range. So much less that the free space range figure is essentially irrelevant for almost all real deployments.
It is unfortunate that wireless companies and the media very often report the free space range, rather than the range corresponding to a typical deployment. This is very misleading to most readers who are inexperienced in the ways of the wireless world. I have talked to people who still believe that WiMax will cover an area whose radius is 30 miles. Careful analysis shows that the typical cell size of WiMax will be a fraction of a mile. And then of course there is xMax – the Florida experiment (which I mentioned in an earlier post) demonstrated communication over 18 miles using 50 milliwatts, under essentially free space conditions. As one can see from the numbers above, there is absolutely nothing remarkable about that. Furthermore, the effective range of xMax in a typical urban deployment will be a very small fraction of the free space range, same as for any other communication system. Alas, the laws of physics governing radio wave propagation, affect in the same way WiMax, xMax, 3G, and all other communication systems.