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What Is Spread Spectrum?

Preview: Learn more about spread-spectrum communications.

Spread spectrum is a communication technique in which a signal is intentionally transmitted over a much wider bandwidth than is required to convey the information itself. Although this may initially appear wasteful, spreading the signal provides several important advantages, including improved resistance to interference, greater immunity to intentional jamming, enhanced privacy, and the ability for many users to share the same frequency band. Today, spread-spectrum techniques are used in satellite navigation systems, military communications, wireless networks, and many other modern communications technologies.

The origins of spread spectrum date back to the Second World War. In 1941, the actress and inventor Hedy Lamarr and the composer George Antheil patented an ingenious system in which both the transmitter and receiver rapidly changed radio frequencies according to a predetermined pattern. Their objective was to prevent enemy forces from jamming radio-controlled torpedoes by making it difficult to predict the transmission frequency. Although the technology of the time prevented widespread adoption of their invention, the underlying concept later became one of the foundations of modern spread-spectrum communications.

Unlike conventional radio transmissions, which concentrate most of their energy into a relatively narrow frequency band, spread-spectrum systems distribute the transmitted energy across a much wider range of frequencies. Because the same amount of energy is spread over a larger bandwidth, the signal appears much weaker at any individual frequency and is often difficult to distinguish from background noise. However, a receiver that knows the correct spreading code can reconstruct the original signal with remarkable accuracy.

Two principal forms of spread-spectrum communication are widely used. In Frequency Hopping Spread Spectrum (FHSS), the transmitter rapidly changes its operating frequency according to a predefined sequence known to both the transmitter and receiver. If interference affects one frequency, communication simply continues on the next frequency in the hopping sequence. This technique provides excellent resistance to narrowband interference and intentional jamming and is employed by technologies such as Bluetooth.

The second major technique is Direct Sequence Spread Spectrum (DSSS). Instead of changing frequencies, each transmitted bit is multiplied by a much faster pseudo-random code sequence before transmission. This process spreads the signal over a much wider bandwidth. At the receiver, the same code sequence is used to reverse the spreading process and recover the original information. Because only receivers possessing the correct code can reconstruct the transmitted signal efficiently, DSSS also provides a degree of privacy and protection against interference.

One of the greatest advantages of spread-spectrum communication is its resistance to interference. Since the transmitted energy is distributed across many frequencies, interference affecting only a small portion of the spectrum has relatively little impact on the recovered signal. Similarly, intentional jamming becomes much more difficult because an interfering signal must disrupt a large bandwidth rather than a single narrow channel.

Spread spectrum also allows multiple users to share the same frequency band efficiently. By assigning different spreading codes to different users, several transmissions can occur simultaneously with relatively little mutual interference. This principle formed the basis of Code Division Multiple Access (CDMA), which became widely used in third-generation (3G) cellular telephone systems and continues to influence many modern wireless technologies.

Perhaps the best-known application of spread spectrum is the Global Positioning System (GPS). GPS satellites continuously transmit extremely weak spread-spectrum signals that can be received simultaneously by many users throughout the world. Because each satellite employs a unique spreading code, a GPS receiver can distinguish signals from multiple satellites operating on the same radio frequencies and determine its position with remarkable accuracy.

Today, spread-spectrum techniques are found in a wide variety of communications systems. Bluetooth employs frequency hopping to improve reliability in the crowded 2.4 GHz frequency band, while GPS and other satellite navigation systems rely on direct-sequence techniques to separate signals from numerous satellites. Military communications continue to employ spread spectrum extensively because of its resistance to jamming, interception, and interference. Variants of spread-spectrum technology have also influenced Wi-Fi, satellite communications, wireless sensors, and many Internet of Things (IoT) applications.

Spread spectrum therefore represents far more than an unusual modulation technique. It transformed the way engineers think about radio communications by demonstrating that deliberately increasing bandwidth can actually improve reliability, security, and spectrum sharing. More than eighty years after the original concept was proposed, spread-spectrum communications remain one of the most important technologies underpinning modern wireless communication systems.