Why Non-line of sight WLAN.
Why would you care? Soon it will become more important as internet and WLAN use grows in metropolitan areas.
In another post a big deal about line of sight propagation caused some confusion. With reference to microwave propagation there is line of sight propagation in another form called non-line of sight propagation.Really. It is true.
First of all, the euro some common expressions used. Microwave usually refers to frequencies in the range of 300 MHz to 3000 MHz. Also, when we talk about a specific frequency, for example 900 MHz, it may not mean specifically the very exact frequency of 900 MHz. The phenomenon may occur also below or above the frequency, or even over a wide band of frequencies. Also expressions such as absorb, reflect, deflect and disturb are not absolute things in this context. Some material may absorb microwave emission, but not entirely. Likewise some materials may reflect or even deflect a microwave emission, but not entirely. I'll try to keep this simple.
In the traditional conventional line of sight method, microwave repeaters are located on tower(s) high enough above the surface to avoid objects that absorb, reflect or deflect microwave radiation. That method allows the use of low power, small bandwidth and gives high performance.I t is the most desirable form of communication for a given point point task.
Having towers everywhere is not always feasible. There is the cost involved, and one end of the propagation path might be a mobile device. We are all familiar with the cell technology where towers are located throughout the metropolis and provide communication with mobile handhold devices almost anywhere in the city. Many times the communication is really not a visual point-to-point communication. This brings up the subject of non-line of sight propagation.
This also brings up the question of which frequencies to use. It is generally held that the lower frequencies have better penetration and are less subject to absorbtion from things such as tree leaves, wood buildings and snowfall. That is generally true. However, with the higher frequencies reflections and deflections become more and more common and become part of the propagation path. In some cases the loss of signal strength and signal quality of the higher frequencies is such that compensation can be made by using wider bandwidth. This concept is hard to explain to somebody. What it basically means is that if you have enough bandwidth you can overcome reflection and deflections and resulting Moiré patterns by finding specific frequencies that work well in a dynamic environment where there are multiple reflections and deflections of a signal path that is changing with movement of a mobile device. But this requires a very sophisticated digital signal processor at both ends of the path. Getting that much CPU poser in a mobile device is hard to do. Traditional analog methods are not good enough to overcome these problems. but with the use of very sophisticated Digital Signal Processing it now is possible to use UHF and Extremely High Frequencies for non-line of sight propagation quite successfully. With enough performance to be worthwhile.
Here's an article from Wikipedia.
http://en.wikipedia.org/wiki/Non-line-of-sight_propagationNon-line-of-sight propagation -Wikipedia
(There is also non-line-of-sight with regard to weapons systems.)
Why does this matter to the average person? Because the new technologies in Cell communication and Wif-Fi will use more of this technology in the future. Same companies already have desktop size super transceivers that can communicate several miles without the need of towers. One claims they did 20 miles on 900Mhz low power with useful data rate of 3Mbps.
If the above is eve n near true, the DSL people had better lower prices go out of business. Presently, DSL has to be withing one mile of a distribution station, a station with either optical fiber or other broadband service.
This also means that the present day 802.11 x devices may well become oblolete or used by those who do not live in large cities.
The FCC has recently opened up more spectrum space in UHF, SHF and EHF bands. Under certain rules not license is need. This allows for rapid deployment in a free market. Previously commercial fatssos just sat on their spectrum and did not give a start.
