What Is Spread Spectrum Communication?

Spread spectrum communication is a transmission method in which the signal's energy is deliberately spread over a frequency band far wider than the minimum bandwidth required to carry the information. This expansion is controlled by a pseudo-random code that is independent of the data, giving the technique its fundamental properties: resistance to narrowband interference, low probability of interception, and the ability for multiple users to occupy the same band simultaneously. Spread spectrum was originally developed for military communications in the 1940s and later became the foundation for commercial wireless systems including CDMA-based 3G cellular networks and the IEEE 802.11 Wi-Fi family of standards.

The processing gain, defined as the ratio of the spread bandwidth to the information bandwidth, measures how much the signal is expanded. Higher processing gain improves interference rejection and makes the signal harder to detect, at the cost of requiring more radio spectrum. Channel estimation is essential in wideband spread spectrum receivers because multipath propagation and timing offsets must be corrected before the pseudo-random despreading sequence can be synchronized.

Direct Sequence Spread Spectrum

In direct sequence spread spectrum (DSSS), a high-rate pseudo-noise (PN) code is multiplied directly with the information bit stream, expanding the signal in frequency by a factor equal to the ratio of the chip rate to the bit rate. The receiver correlates the incoming signal with a locally generated copy of the same PN code, collapsing the spread signal back to its original bandwidth and rejecting out-of-band interference in the process. DSSS is the basis for code division multiple access (CDMA), where different users transmit on the same frequency simultaneously, each distinguished by a unique orthogonal spreading code. The IS-95 standard and its successor CDMA2000 deployed DSSS extensively in 3G mobile communication networks. National Instruments' overview of spread spectrum for communications describes how DSSS processing gain enables coexistence of multiple users within a shared channel. A ScienceDirect overview of direct sequence spread spectrum summarizes the chipping codes and modulation schemes used across DSSS standards.

Frequency Hopping Spread Spectrum

In frequency hopping spread spectrum (FHSS), the carrier frequency changes rapidly according to a pseudo-random sequence known to both transmitter and receiver. Because the signal occupies any given sub-channel only briefly, narrowband interference or jamming affects only a fraction of the transmitted data. FHSS is employed in the Bluetooth standard, which uses 79 one-megahertz channels in the 2.4 GHz industrial, scientific, and medical (ISM) band with a hopping rate of 1,600 hops per second. The IEEE 802.11 legacy standard also specified FHSS as one of its physical layer options, operating across 79 channels at 2.5 hops per second. FHSS is particularly valued in electronic countermeasures applications, where an adversary attempting to jam a fixed frequency cannot predict the next hop.

Multiuser Detection

When multiple DSSS users share a channel, non-orthogonal codes or asynchronous timing can create multiple access interference (MAI), where one user's signal appears as noise to another. Multiuser detection replaces the simple matched filter with receivers that jointly estimate all users' signals, suppressing MAI and improving capacity. Successive interference cancellation (SIC) and minimum mean square error (MMSE) detectors are among the practical implementations studied in the literature. Research published in IEEE Xplore on frequency hopping sequence sets illustrates how careful code design bounds cross-user interference in both FHSS and DSSS systems.

Applications

Spread spectrum communication has applications in a range of fields, including:

• 3G and 4G mobile cellular networks, through CDMA and wideband CDMA air interfaces
• Wireless local area networks, via the IEEE 802.11 family of standards
• Military and government communications, where low probability of intercept and jam resistance are required
• Global navigation satellite systems, including GPS, GLONASS, and Galileo, which use DSSS ranging codes
• Industrial and consumer wireless devices, through Bluetooth and Zigbee frequency hopping protocols