In conventional wireless communications, transmission typically occurs at a set frequency – and if an eavesdropper can determine what that frequency is, then it’s a fairly simple task for them to intercept the data stream, or disrupt it in some way. Military engineers have long understood this principle, and the possibilities it presents for compromising vital missions and operational security.
For this reason, a type of wireless communication known as spread spectrum (or SS) was developed during World War 2, to provide security counter-measures for communications, navigation beacons, and radar systems. Here, a deliberate variation in the frequency of a transmitted signal is made over a fairly large proportion of the electromagnetic spectrum.
Spread spectrum communications occupy a significantly greater bandwidth than is strictly needed for a transmitted signal – ranging from 1,000 to 1 million times the information bandwidth, for military applications, to anywhere from ten to 100 times the transmission frequency, for commercial uses.
Frequency modulation for spread spectrum communications is achieved by either of two different approaches – one of which is known as direct sequence spread spectrum.
Direct Sequence Spread Spectrum Defined
Direct sequence spread spectrum (or DSSS) is the method of spread spectrum frequency modulation whereby the stream of information to be transmitted is split up into small pieces, each of which is allocated to a frequency channel across the electromagnetic spectrum. At the point of transmission, a data signal is combined with what’s known as a chipping code – a higher data-rate bit sequence, which divides the information according to a pre-determined spreading ratio.
The redundant chipping code assists the signal in resisting interference and allows the original data to be recovered if any of the data bits are damaged during transmission.
DSSS is also known as direct sequence code division multiple access or DS-CDMA.
Mechanics of the Direct Sequence Spread Spectrum Process
In general terms, all spread spectrum communications include a key or sequence (also known as the chipping code), which is attached to the communication channel. Signal bandwidth is increased once this key is attached, creating the “spread spectrum” that gives the method its name.
To apply a spread spectrum technique, the relevant spread spectrum code must be applied somewhere in the transmitting chain, before the antenna or receiver. This injection is known as the spreading operation.
The spreading code may be removed at a point in the receiving chain before data retrieval, in what’s known as a despreading operation. Here, the same code must be known in advance at both ends of the transmission channel – and the transmitted data may be reconstituted into its original bandwidth.
In a direct sequence spread spectrum operation, the chipping code is a redundant bit pattern associated with each bit of data transmitted. The operation is performed by multiplying a radio frequency (RF) carrier and a pseudo-noise (PN) digital signal. The PN code is modulated onto an information signal, then a doubly-balanced mixer is used to multiply the PN modulated information signal and the RF carrier.
Benefits of Direct Sequence Spread Spectrum
DSSS (and spread spectrum technology in general) provides significant resistance to transmission interference and jamming. As long as they don’t contain the defined spread spectrum key, all intentional or unintentional interference and jamming signals will be rejected. So when a despreading operation is performed, only the desired signal (which has the correct key) will be seen at the receiver end.
Narrowband or wideband interference may be effectively ignored if it doesn’t contain the key used in the despreading operation. Other spread spectrum signals which don’t share the same key may also be overlooked. This allows a number of different spread spectrum signals to be active at the same time, within the same frequency band – with each being received and interpreted correctly by their intended recipients.
In a similar fashion, since unauthorized listeners likely won’t have the key used to spread the original signal, they won’t be able to decode it. So resistance to interception is built into DSSS. It should be noted, however, that with shorter spread spectrum keys, scanning methods may enable eavesdroppers to hack the code.
Spread spectrum communications technology was first laid out on paper in 1941, by the Hollywood actress Hedy Lamarr and George Antheil, a concert pianist. Their work described a secure radio link for controlling torpedoes and was issued with an official patent (U.S. Patent #2.292.387). Largely ignored by the U.S. military of that time, their concept was unearthed in the 1980s and used as the basis for securing applications that involve radio links in hostile environments.
The technology has since been used as the foundation for a number of short-range data transceivers (transmitter-receivers), including such household names as 3G mobile telecommunications, the satellite Global Positioning System (GPS), and Bluetooth®. Spread spectrum transmission is also used in securing local area wireless and corporate networks which use W-LAN (IEEE® 802.11a, IEEE 802.11b, IEEE 802.11g).
Besides its value as a security mechanism, DSSS also assists broadcasters in areas where the radio frequency spectrum is limited (and therefore, expensive to buy into). It provides additional bandwidth to maintain a desired level of communication performance, even when the noise to signal ratio is high.
As signal levels may be lower than the ambient noise threshold, direct sequence spread spectrum may be used to effectively conceal transmissions within an “envelope” of noise. Other receivers won’t be able to detect the transmission, and the only thing they’ll register is a slight increase in the overall noise level.
The Alternative Approach
The other spread spectrum technique most widely in use is known as frequency hopping spread spectrum (FHSS), or frequency hopping code division multiple access (FH-CDMA). Here, a broad slice of the bandwidth spectrum is divided into many possible broadcast frequencies. The technique costs less and uses less power than DSSS – but DS-CDMA systems are preferred because of their greater security, stability, and reliability.
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