WiMax Operator's Manual

44 CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES
is not to say that interference is not an issue; it is in fact a matter of some concern, particularly
within the increasingly crowded 2.4GHz band. Still, for a variety of reasons, it is unlikely to be
such as to cripple a broadband public network operating in the unlicensed bands.
Transmission Control Protocol/Internet Protocol (TCP/IP), the networking protocol specified
for 802.16, is best effort in its basic form, which means that the nodes will retransmit in the
presence of contention and dropped packets. If the traffic is suited to best effort, then the only
result of a moderate increase in interference will be a slowing of throughput, which may or may
not be noticeable to the user. If the particular application depends on maintaining a certain bit
or error rate, however, then interference can pose a serious problem, and that would be the
case for voice, multimedia, or real-time interactive applications. With the expected increase in
such applications, concerns over interference and crowding in the unlicensed bands are likely
to grow as well, though at the same time, new technologies for mitigating interference will be
entering the marketplace.
The worst possible state of affairs is where users of unlicensed spectrum attempt to solve
the problem by operating at maximum or above-maximum power levels in an attempt to rise
above the interference. The situation then becomes akin to a rock band where every player is
continually cranking up his amplifier in an effort to be heard above his band mates. The end
result is a deafening cacophony where no one is heard distinctly. Unfortunately, inexperienced
network operators often set their output levels at the maximum as a matter of course. The irony
is that even when other operators are not present, the single operator may actually be
degrading the signal at the receiver by increasing the level of self-interference from reflected
signals.
Network operators faced with severe and intractable interference and with the resultant
difficulty of honoring service-level agreements in its presence have a number of options. They
can utilize high-gain, highly directional antennas that focus energy at the receiving node and
establish a high carrier-to-interference ratio at that point in space. They can seek to identify
interferers and see if a means of coexistence can be established through negotiation. They
can seek injunctive relief by complaining to the regulating body if they can determine that an
interferer or interferers are exceeding power limitations. Or they can try to confine their transmissions
to less highly trafficked unlicensed bands, such as the 5GHz bands; however, by
moving up in frequency to less-crowded regions of the spectrum, they may also be giving up
effective distance and thus are somewhat restricting their potential markets.
In the future, smart radios with adaptive antennas may be able to distinguish interference
from the desired signal and cancel it out at the antenna. This may not completely solve the
problem because radio front ends may still be subject to overload if the amount of energy in
the band rises to a certain level, but the intelligent radio will certainly provide an effective
means of coping with interference in most cases. Currently, however, the cost of radios
utilizing this technology is still high, and to date these techniques have been primarily
employed in military radios and radar to thwart jamming and spoofing attempts.
In the future, you may also see mandatory coordination of all radios in an area utilizing
unlicensed frequencies. Such coordination would serve to control the power levels of individual
transceivers and maintain energy levels below certain stated limits. Power control of
this type has long been a characteristic of licensed CDMA mobile phone networks and has
been proven to work well. Extending it over a motley assemblage of unaffiliated users may pose
some policy problems, though.