WiMax Operator's Manual

CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 119
as in a normal broadbeam transmission and instead is delivered almost in its entirety to the
subscriber site, where it can blast through considerable obstructions and still provide a usable
signal. By concentrating energy in such tight patterns on a channel-by-channel or even packetby-packet
basis, the adaptive array can also increase frequency reuse, theoretically up to
several times within a single cell, while still adhering to regulatory limits on transmitter power.
This in turn reduces the need for sectorization.
All adaptive array antennas provide signal diversity; that is, the mere fact that several
spaced antenna elements are exposed simultaneously to the transmission practically guarantees
that signal quality will vary from element to element. The system can then select the best
signal, and in an NLOS situation, there is a far higher likelihood that an array will find a usable
signal than a single element. In some systems the adaptive-antenna array can even take multipath
reflections impinging on the various elements and phase-align them so as to construct a
single, coherent, high-strength signal.
Other systems can combine various signals and then use vector-cancellation strategies to
reduce interference. In one experimental system developed by Lucent but never marketed, the
smart antenna system could simultaneously receive several signals over the same channel,
separate them on the basis of time of arrival and multipath signature, and then reconstruct
usable signals out of all of the interfering transmissions, thus reusing spectrum at virtually the
same point in space! Figure 5-6 shows an ArrayComm adaptive array antenna.
Figure 5-6. ArrayComm adaptive array antenna, courtesy of ArrayComm