26-11-2012, 01:16 PM
An Introduction to Direct-Sequence Spread-Spectrum Communications
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Abstract:
A tutorial overview of spread spectrum principles. Covers both direct sequence and fast hopping
methods. Theoretical equations are given to allow performance estimates. Relation to CDMA and TDMA is
provided. Schematic of a code sequence generator is shown. Spectral plots are shown for DSSS and FHSS
methods.
Introduction
As spread spectrum techniques become increasingly popular, electrical engineers outside the field are eager for
understandable explanations of the technology. There are many books and web sites on the subject but many
are hard to understand or describe some aspects while ignoring others (the DSSS type, for instance, with
extensive focus on PN-code generation).
A Short History
Spread-spectrum (SS) communications technology was first described on paper by an actress and a musician! In
1941, Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control
torpedos and received U.S. patent #2.292.387. It was not taken seriously at that time by the U.S. Army and was
forgotten until the 1980s, when the came alive, and has become increasingly popular for applications that
involve radio links in hostile environments.
Typical applications for the resulting short-range data transceivers include satellite-positioning systems (GPS),
3G mobile telecommunications, W-LAN (IEEE802.11a, IEEE802.11b, IEE802.11g), and Bluetooth. SS techniques
also aid in the endless race between communication needs and radio-frequency availability (the radio spectrum
is limited, and is therefore an expensive resource).
Definitions
Different SS techniques are available, but all have one idea in common: the key (also called code or sequence)
attached to the communication channel. The manner of inserting this code defines precisely the SS technique in
question. The term "spread spectrum" refers to the expansion of signal bandwidth, by several orders of
magnitude in some cases, which occurs when a key is attached to the communication channel.
The formal definition of SS is more precise: Spread spectrum is an RF communications system in which the
baseband signal bandwidth is intentionally spread over a larger bandwith by injecting a higher-frequency signal.
As a direct consequence, energy used in transmitting the signal is spread over a wider bandwith, and appears as
noise. The ratio (in dB) between the spread baseband and the original signal is called processing gain. Typical SS
processing gains run from 10dB to 60dB.
To apply an SS technique, simply inject the corresponding SS code somewhere in the transmitting chain before
the antenna. (That injection is called the spreading operation.) The effect is to diffuse the information in a larger
bandwidth. Conversely, you can remove the SS code (despreading operation) at a point in the receive chain
before data retrieval. The effect of a despreading operation is to reconstitute the information in its original
bandwidth. Obviously, the same code must be known in advance at both ends of the transmission channel. (In
some circumstances, it should be known only by those two parties.)
Resistance to Interception
Resistance to interception is the second advantage provided by SS techniques. Because non-authorized listeners
do not have the key used to spread the original signal, they cannot decode it. Without the right key, the SS
signal appears as noise or as an interferer (scanning methods can break the code, however, if the key is short.)
Even better, signal levels can be below the noise floor, because the spreading operation reduces the spectral
density (total energy is the same, but it is widely spread in frequency). The message is thus made invisible, an
effect that is particularly strong with the DSSS technique. Other receivers cannot "see" the transmission; they
only register a slight increase in the overall noise level!
SS Allows CDMA
Note that SS is not a modulation scheme, and should not be confused with other types of modulation. One can,
for example, use SS techniques to transmit a signal modulated via FSK or BPSK. Thanks to the coding basis, SS
can also be used as another method for implementing multiple access (the real or apparent coexistence of
multiple and simultaneous communication links on the same physical media). So far, three main methods are
available:
FDMA: Frequency Division Multiple Access. FDMA allocates a specific carrier frequency to a communication
channel, and the number of different users is limited to the number of slices in the frequency spectrum. FDMA is
the least efficient in term of frequency-band usage. Methods of FDMA access include radio broadcasting, TV,
AMPS, and TETRAPOLE.
Frequency Hopping Spread Spectrum (FHSS)
This method does exactly what its name implies, it causes the carrier to hop from frequency to frequency over a
wide band according to a sequence defined by the PRN. The speed at which the hops are executed depends on
the data rate of the original information, but one can distinguish between Fast Frequency Hopping (FFHSS) and
Low Frequency Hopping (LFHSS). The latter method (the most common) allows several consecutive data bits to
modulate the same frequency. FFHSS, on the other hand, is characterized by several hops within each data bit.
The transmitted spectrum of a frequency hopping signal is quite different from that of a direct sequence system.
Instead of a ((sin x)/x)²-shaped envelope, the frequency hopper's output is flat over the band of frequencies
used (see below). The bandwidth of a frequency-hopping signal is simply N times the number of frequency slots
available, where N is the bandwidth of each hop channel.