A means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the required bandwidth of the data modulation signal. Spread spectrum signals are typically transmitted by electromagnetic waves in free space, with usage in both nonmilitary and military systems.
Motivation for using spread spectrum signals is based on the following:
Spread spectrum systems have the ability to reject hostile as well as unintentional jamming by interfering signals.
Spread spectrum signals have a low probability of being intercepted or detected since the power in the transmitted wave is “spread” over a large bandwidth or frequency extent.
Since these signals cannot be readily demodulated without knowing the code and its precise timing, a level of message privacy is obtained.
The wide bandwidth of the spread spectrum signals provides tolerance to multipath (reflected waves that typically take longer to arrive at the receiver than the direct desired signal so that the two can be distinguished).
A high degree of precision in ranging (distance measuring) can be obtained by using one type of spread spectrum signaling called direct sequence, with applications to navigation.
Multiple access, or the ability to send many independent signals over the same frequency band, is possible in spread spectrum signaling.
There are four generic types of spread spectrum signals: direct sequence (DS) or pseudonoise (PN), frequency hopping (FH), linear frequency modulation (chirp), and time hopping (TH). The first two methods are much more commonly used today than the other two.
Direct sequence modulation is characterized by phase-modulating a sine wave by an unending string of pseudonoise code chips (symbols of much smaller time duration than a bit). This unending string is typically based on a pseudonoise code that generates an apparently random sequence of code chips that repeats only after the pseudonoise code period. Digital data representing the information to be transmitted are binary phase-shift keyed onto the carrier. Then the pseudonoise code generator also binary phase-shift keys the carrier, and the composite signal is transmitted. See also Phase modulation.
In a direct-sequence system, the phase of the carrier changes pseudorandomly with the pseudonoise code. In a frequency-hopping system, the frequency of the carrier changes according to a pseudonoise code with a consecutive group of pseudonoise code chips defining a particular frequency. Typically either multiple frequency-shift keying (MFSK) or differential phase-shift keying (DPSK) is used as the data modulation. Multiple frequency-shift keying is a modulation scheme in which one of a number of tones (2, 4, 8, and so forth) is transmitted at a given time according to a group of consecutive data bits (n bits produce 2n tones). At each hop frequency one of the 2n tones is selected according to the n bits, and one of 2n corresponding frequencies, centered about the hop frequency, is transmitted. In conjunction with frequency hopping, multiple frequency-shift keying would imply, at each instant of time, a given carrier frequency that depends on the hop-pseudonoise code sequence and the consecutive group of the most recent n data bits. Differential phase-shift keying is similar to phase-shift keying except that only the differences of the phases (not the actual phases) are encoded and noncoherent techniques (not requiring a carrier loop) can be employed at the receiver.
A device called a frequency synthesizer achieves the actual frequency selection. For example, a 12-bit segment of the pseudonoise code may correspond to one of 212 different frequencies, so that one of approximately 4000 (212) frequencies is selected each hop time. Frequency synthesizers are used in both the transmitter and the receiver. The transmitter modulates the data by typically using either multiple frequency-shift keying or differential phase-shift keying modulation, which in turn is frequency-hopped by the frequency synthesizer. At the receiver, an acquisition process is utilized to synchronize the receiver frequency synthesizer with the received hopping signal, and then a tracking system maintains synchronism. Finally a bit synchronizer provides timing for the data demodulator which demodulates the original, transmitted data bits.
An important aspect of spread spectrum communications is multiple access. Code-division multiple access (CDMA) is a method by which spread spectrum signals are utilized to allow the use of multiple signals over the same frequency band. The two common types are direct-sequence CDMA (DS/CDMA) systems and frequency-hopped CDMA (FH/CDMA) systems. See also Multiplexing and multiple access.
Although the early evolution of spread spectrum systems was motivated primarily by military interests, nonmilitary applications have enjoyed considerable development. One important example that has both military and nonmilitary users is the Global Positioning Systems (GPS), which is a direct-sequence, CDMA, spread spectrum system for transmitting the satellite ranging codes. See also Satellite navigation systems.
The space shuttle utilizes a direct-sequence spread spectrum communication system on its forward link. It relays data through the geostationary Tracking and Data Relay Satellites (TDRS). Another example in the military arena is the Milstar system, which utilizes frequency-hopping spread spectrum communication over a very large bandwidth to achieve considerable immunity from unfriendly jamming signals. See also Military satellites; Space communications; Space shuttle.
Globalstar is a commercial satellite system that utilizes a CDMA signal structure along with a bent-pipe transponder to provide communications much like a cell phone, except that Globalstar is satellite-based. Currently the handsets are considerably more expensive than cell phones. However, these phones have the advantage that they can be reached beyond the range of cell phones. See also Communications satellite.
The main reception source of news stories for radio and newspapers is small spread spectrum ground stations. Another application of code-division multiple access is transmission directly by satellite from a bank's automatic teller machine to that bank's computer facility. A home security system using spread spectrum techniques imposed on the ac power line has been used in Japan. Many cordless telephones utilize direct-sequence spread spectrum techniques.
