8.15.8 What Is Spread Spectrum and Why Was It Developed?
- What Is Spread Spectrum?
- Why Is It Called Spread Spectrum?
- Why Would Engineers Intentionally Use More Bandwidth?
- How Does Spread Spectrum Work?
- What Is a Spreading Code?
- What Is a Pseudo-Random Sequence?
- What Is Processing Gain?
- Why Does Spread Spectrum Resist Interference?
- Why Is Spread Spectrum Resistant to Jamming?
- Why Is Spread Spectrum Difficult to Intercept?
- Does Spread Spectrum Improve Multipath Performance?
- Does Spread Spectrum Increase Capacity?
- Where Is Spread Spectrum Used?
- Are There Different Types of Spread Spectrum?
- What Are the Advantages of Spread Spectrum?
- What Are the Disadvantages?
- Why Was Spread Spectrum Such an Important Development?
Description
Discover why engineers deliberately spread signals over bandwidths much larger than necessary. Learn how spread-spectrum techniques improve resistance to interference, jamming, interception, and multipath propagation while supporting multiple-access operation.
Introduction
One of the fundamental principles of communications engineering is to use spectrum efficiently. Engineers usually strive to confine a signal within the smallest possible bandwidth because radio spectrum is both limited and valuable. At first glance, therefore, the idea of deliberately transmitting a signal over a bandwidth many times larger than necessary appears completely illogical.
Yet this seemingly inefficient approach has become one of the most important techniques in modern communications.
The technique is known as spread spectrum. Rather than concentrating a signal into a narrow frequency band, spread-spectrum systems deliberately distribute the transmitted energy across a much wider range of frequencies. Although the transmitted signal occupies considerably more bandwidth, it gains several remarkable advantages. It becomes far more resistant to interference and jamming, more difficult to intercept, less susceptible to multipath propagation, and capable of supporting multiple simultaneous users.
Spread-spectrum techniques were originally developed for military communications during the Second World War and the Cold War, where secure and reliable communications were essential. As digital electronics became more sophisticated, spread spectrum found increasing use in civilian applications. Today it underpins satellite navigation systems such as GPS, wireless local-area networks, Bluetooth, military communication systems, satellite communications, and many cellular technologies.
Understanding spread spectrum provides valuable insight into why modern communication systems often appear to violate traditional engineering principles while actually achieving superior overall performance.
What Is Spread Spectrum?
Spread spectrum is a transmission technique in which a signal occupies a bandwidth much greater than is required to carry the original information.
The spreading process is performed using a deterministic code that is known to both the transmitter and the receiver.
Only a receiver possessing the correct code can reconstruct the original information efficiently.
Why Is It Called Spread Spectrum?
The transmitted signal is deliberately spread across a wide region of the radio spectrum.
Instead of concentrating energy into a narrow frequency channel, the signal is distributed over many frequencies.
The information content remains unchanged. Only the bandwidth increases.
Why Would Engineers Intentionally Use More Bandwidth?
Although spreading appears inefficient, it provides several important advantages.
These include:
- improved resistance to interference;
- protection against intentional jamming;
- greater immunity to multipath propagation;
- support for multiple simultaneous users; and
- reduced probability of interception.
In many communication systems, these benefits greatly outweigh the increased bandwidth requirement.
How Does Spread Spectrum Work?
Before transmission, the original information signal is combined with a rapidly changing digital sequence.
This sequence changes much faster than the information itself. The result is a transmitted waveform that occupies a much wider frequency range than the original signal. At the receiver, the same sequence is applied again. This process removes the spreading and reconstructs the original information.
Signals without the correct spreading sequence remain spread over the wide bandwidth and appear as low-level noise.
What Is a Spreading Code?
A spreading code is a carefully designed binary sequence that determines how the transmitted signal is spread across the spectrum.
Good spreading codes possess several desirable properties. They:
- resemble random noise;
- exhibit excellent autocorrelation;
- exhibit low cross-correlation with other codes; and
- are easily reproduced at both transmitter and receiver.
Although they appear random, they are actually deterministic.
What Is a Pseudo-Random Sequence?
Most spreading codes are pseudo-random.
They appear random to an observer but are generated by mathematical algorithms. Because both transmitter and receiver generate exactly the same sequence, synchronization can be maintained without transmitting the code itself.
Pseudo-random sequences combine the advantages of randomness with complete reproducibility.
What Is Processing Gain?
One of the principal benefits of spread spectrum is processing gain.
Processing gain is approximately equal to: Spread bandwidth ÷ Information bandwidth. The larger this ratio:
- the greater the resistance to interference;
- the better the rejection of narrowband noise;
- the greater the resistance to jamming; and
- the easier it becomes to recover weak signals.
Processing gain is one of the defining characteristics of spread-spectrum communication systems.
Why Does Spread Spectrum Resist Interference?
Suppose a narrowband interfering signal occupies only a small portion of the spread spectrum.
After de-spreading at the receiver:
- the desired signal collapses back into its original narrow bandwidth;
- the interference remains spread across the wider bandwidth; and
- the interference power becomes much less concentrated.
Consequently, the receiver experiences far less interference than would occur in a conventional narrowband system.
Why Is Spread Spectrum Resistant to Jamming?
A jammer attempting to disrupt communication must interfere with the entire spread bandwidth.
This requires substantially more transmitter power than jamming a narrowband signal. Alternatively, if the jammer transmits only over a small portion of the spectrum, much of the desired signal remains unaffected.
Spread-spectrum systems therefore provide excellent resistance to intentional interference.
Why Is Spread Spectrum Difficult to Intercept?
The transmitted signal resembles background noise.
Without knowledge of the spreading code:
- the signal may be difficult to detect;
- its information cannot easily be recovered;
- interception becomes more difficult; and
- casual monitoring is discouraged.
Although spread spectrum does not provide encryption by itself, it contributes significantly to communication security.
Does Spread Spectrum Improve Multipath Performance?
Yes.
Many spread-spectrum receivers can distinguish between signals arriving by different propagation paths. Rather than treating delayed reflections solely as interference, advanced receivers may combine energy from several propagation paths.
This improves reception in environments where multipath propagation is severe.
Does Spread Spectrum Increase Capacity?
In many cases, yes.
Different users may employ different spreading codes. Provided the codes possess suitable correlation properties:
- several users may transmit simultaneously;
- all users occupy the same frequency band;
- the receiver distinguishes them by code; and
- overall spectrum utilization improves.
This principle forms the basis of Code-Division Multiple Access.
Where Is Spread Spectrum Used?
Spread-spectrum techniques appear in numerous communication systems.
Applications include:
- GPS and other Global Navigation Satellite Systems;
- CDMA cellular networks;
- Bluetooth;
- Wi-Fi;
- military communications;
- satellite communications; and
- industrial wireless systems.
Their versatility explains their widespread adoption.
Are There Different Types of Spread Spectrum?
Yes.
The two principal forms are:
- Direct-Sequence Spread Spectrum (DSSS); and
- Frequency-Hopping Spread Spectrum (FHSS).
Both increase the transmitted bandwidth, but they achieve this objective in different ways.
The following FAQ compares these techniques in detail.
What Are the Advantages of Spread Spectrum?
Spread-spectrum communication offers numerous important benefits.
These include:
- excellent interference resistance;
- improved resistance to jamming;
- support for multiple users;
- reduced probability of interception; and
- improved communication reliability.
These characteristics explain its widespread use in both civilian and military communication systems.
What Are the Disadvantages?
Spread spectrum also involves several compromises.
These include:
- increased bandwidth requirements;
- greater receiver complexity;
- synchronization requirements;
- more sophisticated signal processing; and
- increased implementation cost.
Fortunately, modern digital electronics have made these challenges increasingly manageable.
Why Was Spread Spectrum Such an Important Development?
Spread spectrum fundamentally changed communications engineering by demonstrating that bandwidth could be traded for improvements in reliability, security, and interference resistance. Rather than viewing bandwidth solely as a scarce resource, engineers learned that intelligently using additional bandwidth could produce communication systems that were substantially more robust than conventional narrowband systems.
Many of today's wireless technologies—including satellite navigation, military radios, Bluetooth, Wi-Fi, and cellular communications—owe much of their success to these principles.
Summary
Spread spectrum deliberately transmits information over a bandwidth much wider than is required by the original signal. Although this initially appears inefficient, the resulting processing gain provides excellent resistance to interference, jamming, multipath propagation, and interception.
By combining spreading codes with sophisticated receiver processing, spread-spectrum systems support secure and reliable communication in demanding environments. These principles underpin numerous modern communication technologies and provide the foundation for Direct-Sequence Spread Spectrum and Frequency-Hopping Spread Spectrum, which are examined in the next FAQ.
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