Beachfront Property: The Lower Microwave Frequencies

Spectrum available for high-speed data starts in the ultrahigh frequency (UHF) bands beginning
at 300MHz and extending to 3GHz. In the United States the lowest frequencies currently
available for broadband wireless transmissions reside between the 700MHz and 800MHz spectrum
formerly assigned to television. Further spectrum is available in the United States
between 902MHz and 928MHz, at 2.3GHz, at 2.4GHz, from 2.5GHz to 2.7GHz, and in several
bands from 5GHz to 6GHz. Bands located at 2.4GHz and at 5.8GHz are widely available across
the globe. Throughout most of the world, though not in the United States, a band centered at
3.5GHz is also available for public access data networks and is fairly widely used. Early in 2005
the Federal Communications Commission (FCC) approved new unlicensed spectrum for
broadband data services located between 3650MHz and 3700MHz.
The spectrum between 3GHz and 30GHz is termed super high frequency (SHF) but is not
all of a piece in regard to the characteristics of RF transmissions within this frequency range.
Transmissions occurring from 3GHz to approximately 10GHz and occupying the lower third of
the SHF region really have more in common with UHF in that they are relatively limited in
throughput, do not readily conduce to high degrees of frequency reuse, and, perhaps most
important, share a vulnerability to what is known as multipath distortion.
Multipath distortion is a condition in which the signal interferes with itself because reflections
off physical boundaries converge with the direct signal, causing the signal level at the
receiver to swell or fade depending on the phase alignments of the converging waveforms at
the moment they interact with one another. In addition, multipath results in errors in the bitstream at the receiver inasmuch as the reflected signals are delayed relative to the direct
signal, causing bits to appear out of sequence. Multipath varies enormously according to the
position of the transmitting antenna in relationship to the ground and to large obstructions,
and according to the position of the receiving antenna in relationship to the direct and
reflected signals impinging on it. If the receiving antenna is not in an area where significant
cancellations or reinforcements are taking place, reception will be fine. But move it a few
inches, and the signal may become almost unrecoverable.
In the past, multipath has been an endemic problem for networks operating in the lower
microwave region. This is a problem that could be mitigated but never entirely solved by
careful installation and by using diversity antenna systems that consisted of two or more spaced
antennas and associated smart circuitry that would choose the optimal signal—that is, the one
least afflicted with multipath. Today various new and sophisticated modulation techniques, as
well as adaptive antenna technologies, are emerging that confer a considerable degree of
immunity from multipath on broadband receivers and render placement fairly noncritical.
The greatest reductions in the effects of multipath are to be had with certain types of adaptive
antennas; however, such technology remains expensive to implement and is by no means
widely present in the marketplace. One modulation technique, known as frequency hopping,
provides almost absolute immunity to multipath; however, it is not specified in the 802.16
standard and is not particularly spectrally efficient either.
Multipath afflicts transmissions from 300MHz all the way up to about 10MHz, but as we
enter the SHF bands (3GHz–30GHz) a new problem manifests itself: increasing susceptibility
to blockage from walls. Beyond about 2.5GHz, transmitting through walls becomes increasingly
difficult at reasonable power levels and at reasonable distances. Beyond 3GHz, the
problem becomes fairly acute, and in-building antenna mounting becomes essentially
impractical for receiving outdoor transmissions in the popular 5.8GHz band.
Another problem appearing as low as 2GHz and worsening progressively with frequency
thereafter is vulnerability to signal interruption in the presence of foliage. Trees may be
regarded as vessels filled with water, and microwaves tend to give up their energy to water
when they encounter it directly—this in fact is the principle behind microwave ovens. A
customer whose terminal is blocked by trees is therefore unlikely to be able receive a signal
of adequate strength. In such cases the network operator has two choices: elevate the transmitting
antenna sufficiently that the signal clears the treetops or see whether the owner of the
trees can be prevailed upon to trim or remove them.
In spite of the vulnerability of lower microwave transmissions to physical obstructions,
spectrum in this region, especially below 4GHz, is extremely valuable, affording the user a
combination of high throughput, fairly long distances, and some ability to pass through walls.
This spectrum, whether licensed or unlicensed, is the overwhelming choice of broadband
network operators the world over and is apt to remain so for the foreseeable future.