As an introduction, a little history lesson and a few definitions seem to be in order.
Spread spectrum (SS) dates back to World War II (see
SSS Online's Spread Spectrum History Page, with information about Hedy Lamarr's
patented "Secret Communications Technique.").
Early research and development efforts tried to provide countermeasures for
radar, navigation beacons and communications. The U. S. Military has used SS signals over
satellites for at least 25 years. An old but faithful and highly capable design like the Magnavox
USC-28 modem is an example of this kind of equipment. Housed in two or three 6' racks, it
had selectable data rates from a few hundred bits per second to about 64 kBits per second. It
transmitted a spread bandwidth of 60 MHZ. Many newer commercial satellite systems have now
converted to spread spectrum to increase channel capacity and reduce costs.
Over the last twenty years, many spread spectrum signals have appeared on the air. The easiest
way to characterize these modulations is by their frequency spectra. These SS signals occupy a
much greater bandwidth than needed by the information bandwidth of the transmitted data. To
rate being called a spread spectrum signal, two technicalities must be met:
The signal bandwidth must be much wider than the information bandwidth.
Some code or pattern, other than the data to be transmitted, determines the actual
on-the-air transmit bandwidth.
One way to look at spread spectrum is that it trades a wider signal bandwidth for better signal to
noise ratio. In today's commercial spread spectrum systems, bandwidths of 10 to 100 times the information
rates are used. Military systems have used spectrum widths from 1000 to 1 million times the
information bandwidth. There are two very common spread spectrum modulations used in commercial
applications: frequency hopping and direct sequence. At least two other types of spreading
modulations have been used on occasion, although they are nowhere near as common: time hopping and chirp.
The following paragraphs will describe each of the common techniques in a little more detail and show that
pseudo noise code techniques provide the common thread through all spread spectrum types.
Frequency hopping is the easiest spread spectrum modulation to use. Any radio with a digitally
controlled frequency synthesizer can, theoretically, be converted to a frequency hopping radio.
This conversion requires the addition of a pseudo noise (PN) code generator to select the
frequencies for transmission or reception. Most hopping systems use uniform frequency hopping
over a band of frequencies. This is not absolutely necessary, if both the transmitter and receiver
of the system know in advance what frequencies are to be skipped. Thus a frequency hopper in
the two meter band could be made that skipped over commonly used repeater frequency pairs. A
frequency hopped system can use analog or digital carrier modulation and can be designed using
conventional narrow band radio techniques. De-hopping in the receiver is done by a synchronized
pseudo noise code generator that drives the receiver's local oscillator frequency synthesizer.
Direct sequence is the most practical all-digital version. A direct sequence system uses a
locally generated pseudonoise code to encode digital data to be transmitted. The local code runs
at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added
(an EXOR operation) with the faster pseudonoise code. The composite pseudonoise and data
can be passed through a data scrambler to randomize the output spectrum (and thereby remove
discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed
carrier modulate the carrier frequency to be transmitted. The resultant DSB suppressed carrier
AM modulation can also be thought of as binary phase shift keying (BPSK). Carrier modulation
other than BPSK is possible with direct sequence; however, binary phase shift keying is the
simplest and most often used modulation technique for spread spectrum.
A spread spectrum receiver uses a locally generated replica of the pseudonoise code and a receiver correlator to
separate out the desired coded information from all possible signals. A spread spectrum correlator can be
thought of as a very special matched filter -- it responds only to signals that are encoded with a
pseudonoise code that matches its own code. Thus, an SS correlator can be "tuned" to different
codes simply by changing its local code. This correlator does not respond to manmade, natural
or artificial noise or interference. It responds only to SS signals with identical matched signal
characteristics and pseudonoise code.
The use of these special pseudonoise codes in spread spectrum communications makes
signals appear wideband and noise-like. It is this very characteristic that makes SS signals
possess the quality of Low Probability of Intercept. SS signals are hard to detect on narrow band
equipment because the signal's energy is spread over a bandwidth of maybe 100 times the
information bandwidth.
The spread of energy over a wide band, or lower spectral power density, makes SS signals less
likely to interfere with narrowband communications. Narrowband communications, conversely,
cause little to no interference to SS systems because the correlation receiver effectively integrates
over a very wide bandwidth to recover an SS signal. The correlator then "spreads" out a narrow
band interferer over the receiver's total detection bandwidth. The total integrated signal
density or SNR at the correlator's input determines whether there will be interference or not. All
SS systems have a threshold or tolerance level of interference beyond which useful
communication ceases. This tolerance or threshold is related to the SS processing gain.
Processing gain is essentially the ratio of the RF bandwidth to the information bandwidth.
Besides being hard to intercept and jam, spread spectrum signals are hard to exploit or spoof.
Signal exploitation is the ability of an enemy (or a non-network member) to listen in to a network
and use information from the network without being a valid network member or participant.
Spoofing is the act of falsely or maliciously introducing misleading or false traffic or messages to a
network. Spread spectrum signals are naturally more secure than narrowband radio communications, and
can be made to have any degree of message privacy that is desired. Messages can also
be cryptographically encoded to any level of secrecy desired, offering even greater security.
The very nature of spread spectrum allows military or intelligence levels of privacy and
security to be had in the private sector with minimal complexity. While
these characteristics may not be very important to every business and LAN (local area
network) need, they are readily available if desired.
