through E are the new segments
allocated to specific services. Some
of those segments are paired,
meaning that two segments are used
together. For example, blocks A,
B, and C are paired in two 6 MHz
segments for uplink and downlink
cell phone service.
New public safety (PS) segments
are also allocated for a new wireless
effort that intends to encourage the
build-out of a nationwide public
safety network that all fire, police,
and other first responders can use
to communicate with one another
without all the current interoperability
problems that prevent one agency
from speaking with another because
of frequency assignment, modulation,
access method, or other differences
that are rampant. Unfortunately, the
bids on the public safety segments
did not meet the FCC’s minimum bid
requirements so it was not awarded.
The FCC plans to reauction this
segment in the near future.
PROSPECTS FOR THE
700 MHZ BAND
The 700 MHz band is highly
prized because of its radio wave
propagation characteristics. Such
UHF waves are like light waves in
that the propagation is essentially
line-of-sight (LOS). There must be a
straight path from transmitting
antenna to receiving antenna with
little or no obstruction in between.
Of course, radio waves pass right
through most obstructions like trees
and buildings but they are greatly
attenuated making the signal
considerably smaller at the receiver.
That is why high antennas are the key
to long transmission range.
Another factor is that the 700
MHz signals travel farther than higher
frequency signals. A basic radio
propagation formula known as the
Friis Equation essentially says that
the range of transmission is inversely
proportional to the signal wavelength.
Remember, the lower the frequency,
the higher the wavelength. That
means for a given transmit power,
receiver sensitivity, and antenna
gains, a 700 MHz signal will travel
farther than a 2 GHz signal. While
microwave frequencies offer greater
spectrum for a given service, their
range is naturally restricted. What this
means is that a cell phone on 700
MHz will connect to a cell site more
reliably than one on 2.1 GHz. Wide
coverage means fewer cell sites
which are very expensive. And better
signal strength means fewer dropped
calls and less customer complaints.
Currently, most cellular service takes
place in bands from about 800
to 950 MHz and in the 1,900 to
2, 100 MHz range. The cell phone
companies are dying to get their
hands on the 700 MHz spectrum.
One other factor favoring 700
MHz signals is that lower frequencies
do not have the same multipath
problems. Multipath refers to the
multiple signals created by a signal
being reflected by objects in its path,
creating multiple versions of the
signal that are somewhat delayed.
These multipath signals add together
at the receiver antenna creating
signal cancellation. If the reflected
signals come from moving objects
like cars, the multipath varies causing
the signal to fade in and out.
Multipath gets worse at the higher
frequencies and is especially bad
above about 1 GHz. It is considerably less of a problem at the lower
frequencies. So you can see why the
cellular companies covet 700 MHz.
Finally, the FCC indicated that
part of the C blocks auctioned
off and acquired by Verizon had
to be used in “open” cell phone
applications. The term open refers to
the ability of subscribers to use any
phone or accessory software they
want. Right now, most cell phone
systems are closed meaning that
subscribers must use the phones and
accessories supplied by the carriers.
Carriers are just now beginning
to consider open systems. That
should yield some interesting new
applications and products. NV
Order online at:
Development Tools for PIC® MCUs
microEngineering Labs, Inc.
Phone: (719) 520-5323
Fax: (719) 520-1867
Colorado Springs, CO 80960
With Accessories for $119.95: