PRODUCT NOTE 7106
GRE A
MERICA
, Inc. 425 Harbor Blvd. Belmont, California 94002 phone (650) 591-1400 fax (650) 591-2001
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TECHNICAL INFORMATION NOTE
THEORY
Almost everyone in our “High Tech World” is familiar with radio communication in one form or
another. We listen to music on the FM car radio, or to a commentary on an AM pocket radio and we watch
pictures sent by radio waves to our TV sets. Industry uses it to dispatch vehicles, to send messages, or to
monitor processes. Law Enforcement and Public Safety would be handicapped without it, as would
Governments and their militaries. But how does it work?
RADIO
Radio (now called WIRELESS) is a form of electromagnetic energy, as is visible light. Because
wireless signals are lower on the frequency spectrum than visible light we cannot see them, but we can use
electronic equipment to detect them. Wireless signals react in similar ways to light, especially the UHF region
(902-928 MHz) where GINA resides. To communicate from “here” to “there” by wireless (e.g., without an
interconnecting cable) it is necessary to have a carrier convey information. In the beginning this consisted of
simply “keying” the carrier on and off (this was an extension of the telegraph). Later, as we learned to
transmit more complex information (voice, data, etc.) the modulation process became more complicated.
Common modulation techniques are AM and FM. Amplitude modulation, or AM, means to make the carrier
stronger or weaker in unison with the modulating information and frequency modulation. FM means to
change the frequency of the carrier higher or lower with the modulating information. [NOTE: A change in
phase of the carrier frequency also results in an FM like signal that is easier to implement, so most of what is
called FM is really phase modulation.]
SIDEBANDS
When a carrier is modulated, the frequencies of the modulating signal will add to and subtract
from the carrier frequency, setting up sidebands on either sides of the carrier. These sidebands take up space
in the spectrum and many modulation techniques to conserve space are being used. This philosophy is termed
Narrow Band Transmission.
SPREAD SPECTRUM
Recently, a modulation scheme called “Spread Spectrum” was de-classified by
the military for civilian use. SS can be a very wide band type of transmission, but it uses the band in a way
that, theoretically, lessens the total interference. There are various kinds of spread spectrum technologies in
operation, with direct sequence modulation a more common mode of transmission. Direct sequence is a phase
shift scheme, resulting in a wide band FM-like signal. The spreading is accomplished by a Pseudo-random
Noise or PN code modulated on a radio carrier. Since the PN code is not totally random, it has a definite
pattern. The idea is that several patterns can be interleaved in a wireless band of frequencies causing little
interference. The information to be conveyed is imbedded within the PN code. GINA uses this modulating
system.
DEMODULATION
After the wireless has been modulated, it must transmit over distance to a receiving
system that reverses the modulating process. First, the modulated carrier is detected and “de-spread,” the PN
code is deciphered, and then the imbedded information is validated. The GINA 6000 has an additional step.
At the transmitting end, the imbedded data is assembled into “packets” plus some additional information to
ensure accuracy, before it is transmitted. At the receiving end, after the imbedded information is recovered,
the packets are disassembled. This packetized communication is termed modified X.25 and its purpose is to
ensure accurate data transfer. The Packet Assemble and Disassemble process is referred to as “PAD.”