Specifications

1.800.553.1170
Communication Overview
33
COM
Introduction to Spread Spectrum Technology
Spread Spectrum (SS) technology is not very new (relatively
speaking). An actress, Hedy Lamarr, invented SS in WWII with
the aid of an avant-garde composer named George Antheil.
Spread Spectrum was originally designed to hide the signals that
controlled torpedoes. The topic was classified by the U.S. military,
and only recently were the patents made public. Although the
Navy decided not to use the invention in WWII, the U.S.
government began using the technology in the '50s and '60s to
keep communications secret.
Today, there are basically two types of SS: Frequency Hopping
(FH) and Direct Sequence (DS). There is also a third classification
called Hybrid, which is a combination of DS and FH. As processor
speeds have increased, these technologies have found greater
use in commercial products. Notice that FH and DS are not
modulation techniques, but are coding techniques to disperse
the signal in the frequency domain.
Frequency Hopping
Frequency Hopping radios most closely utilize the methodology
of Lamarr and Antheil. They implemented the design with a
player piano roll with 88 “pseudo-random” (PN) notes. Current
systems use synthesized Voltage Controlled Oscillators (VCO) to
rapidly change transmit frequencies.
Bluetooth uses 79 pseudo-random hopping frequencies
set 1MHz apart, and uses Frequency Shift Keying (FSK)
modulation to send ones and zeros. (See page 34 for
more on Bluetooth.) If the transmit frequency moves up at least
115kHz, then it is a one. If it moves down at least 115kHz then
it is a zero. The biggest hurdle is synchronizing the receiver and
the transmitter to the same hopping sequence. Bluetooth can
theoretically provide about 723kbps net throughput.
(FHSS (Frequency Hopping) Spectrum)
Wireless Communication
The bus interfaces discussed in the
previous sections all require a wired
connection between PCs and
peripheral devices. While practical
for many applications, they cannot
provide the flexibility of wireless
systems. Advances in wireless
technology have been occurring at
a remarkable pace. The following
sections cover the technologies
around which Quatech is currently
developing products. The table to
the right provides a comparison of
the features available for each
choice.
Interface Speed Range
OS Support
Protocol Antenna
RF Modem
802.11b
Bluetooth
TM
external
DSSS with GMSK
modulation
Windows
3.1/95/98/Me/
2000 and DOS
500 meters
line-of-sight
57.6 kbps
up to
1Mbps
723 kbps
100 meters
line-of-sight
10 meters (with
obstacles)
Windows
98SE/Me/
2000/CE
Windows
98SE/Me/
2000/CE
DSSS (QPSK and
BPSK with CCK
modulation)
Bluetooth
(FHSS)
embedded
embedded