WiMax Operator's Manual

114 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES
over free space, ultimately limiting its useful range, the effect of absorption of energy by physical
structures such as walls or trees is to further reduce the distance over which a reliable
connection may be maintained, and often very appreciably at that. Indeed, absorption losses
can, if significantly severe, interrupt the signal entirely. A good example of this is provided by a
large park full of high trees where anyone attempting to blast through the treetops with a
microwave signal to reach buildings on the other side will be completely unable to establish an
airlink even with the most advanced NLOS equipment. Incidentally, as a good rule of thumb, a
direct signal path through dense foliage will result in a loss of signal strength of approximately
1 decibel per meter.
Reflection
The effect of reflection depends heavily on whether the main lobe or the side lobes are
reflected. If only the side lobes are reflected, multipath distortion will occur in the main lobe,
compromising signal integrity and resulting in loss of data but not interrupting the signal
entirely. If, on the other hand, the main lobe is reflected by an obstruction standing directly in
its path, then almost no energy from that lobe will appear at the receiver’s antenna. In such
instances, reflected energy, most likely from a side lobe, may reach the receiver at a sufficient
level to provide a usable signal, although the fade margin will be vastly reduced over what it
would be with a direct signal. The problem here, however, is more significant than a simple
reduction in signal level because now the receiver is operating entirely in the multipath environment,
and it is likely to be subject to not one but multiple reflections, each of which will
interfere severely with one another, further reduce fade margin, and substantially increase the
bit error rate.
To get a better idea of how multipath occurs, refer to Figure 5-5 for a schematized depiction
of multipath reflections. The severity of mulitpath will depend on the frequency of the
transmission, the distribution of reflective surfaces in the area separating the transmitter from
the receiver, and the directivity characteristics of the transmitter receiver.
Figure 5-5. Multipath reflections
Now it is entirely possible to set up a deliberate reflection in an effort to reach an
obstructed receiver, but again one is likely to be operating in a multipath environment because
the side lobes will be encountering their own obstructions, possibly the same obstruction
affecting the main lobe, and will likely be reflected into the path of the main lobe. In addition,
the direct signal will lose energy to whatever object is reflecting it.
A number of radios on the market do have the ability to function in a pure multipath environment,
but the important thing to remember is that they do not function nearly as well in