8.15.9 What Are Direct-Sequence Spread Spectrum (DSSS) and Frequency-Hopping Spread Spectrum (FHSS)?
- What Is Direct-Sequence Spread Spectrum?
- How Does DSSS Work?
- What Is a Chip?
- What Is Frequency-Hopping Spread Spectrum?
- How Does FHSS Work?
- Why Are the Hopping Patterns Pseudo-Random?
- How Does DSSS Resist Interference?
- How Does FHSS Resist Interference?
- Which Technique Is Better Against Jamming?
- Which Technique Is More Resistant to Interception?
- Does DSSS Support Multiple Users?
- Does FHSS Support Multiple Users?
- Which Is More Complex?
- Where Is DSSS Used?
- Where Is FHSS Used?
- Can DSSS and FHSS Be Combined?
- What Are the Advantages of DSSS?
- What Are the Advantages of FHSS?
- Why Are Both Techniques Important?
Description
Compare the two principal spread-spectrum techniques. Learn how DSSS spreads information using pseudo-random codes, how FHSS rapidly changes carrier frequency, and where each technique is commonly used.
Introduction
The previous FAQ introduced the concept of spread spectrum and explained why engineers deliberately transmit signals over bandwidths much wider than the information itself requires. By doing so, communication systems gain improved resistance to interference, jamming, interception, and multipath propagation while also supporting multiple simultaneous users.
There are several ways of spreading a signal across a wide bandwidth, but two techniques have become particularly important. The first is Direct-Sequence Spread Spectrum (DSSS), in which the information is multiplied by a high-speed pseudo-random code before transmission. The second is Frequency-Hopping Spread Spectrum (FHSS), in which the carrier rapidly changes frequency according to a predetermined hopping sequence.
Although both techniques achieve the same overall objective—spreading the transmitted energy across a much wider bandwidth—they do so in very different ways. Each has its own advantages, disadvantages, and areas of application.
Understanding these two techniques provides valuable insight into many modern communication systems, including Wi-Fi, Bluetooth, satellite navigation, military communications, and early cellular networks.
What Is Direct-Sequence Spread Spectrum?
Direct-Sequence Spread Spectrum (DSSS) spreads a signal by multiplying every information bit by a much faster pseudo-random binary sequence.
Each information bit is therefore represented by many shorter binary elements called chips.
The resulting signal occupies a much wider bandwidth than the original information stream.
How Does DSSS Work?
Before transmission:
- the information bits are generated;
- the spreading code is produced;
- every information bit is combined with the spreading code; and
- the resulting chip sequence modulates the carrier.
At the receiver, the same spreading code is generated.
Applying the correct code restores the original information while signals using different codes remain spread across the wider bandwidth.
What Is a Chip?
A chip is one element of the spreading sequence. It is not the same as a data bit. For example, if one information bit is represented by 64 chips, the chip rate is sixty-four times higher than the information bit rate.
The higher the chip rate, the greater the spread bandwidth and the larger the processing gain.
What Is Frequency-Hopping Spread Spectrum?
Frequency-Hopping Spread Spectrum (FHSS) spreads the transmitted signal by rapidly changing the carrier frequency.
Instead of remaining on one frequency, the transmitter repeatedly hops between many different frequencies according to a predetermined hopping pattern.
The receiver performs exactly the same hopping sequence in synchronism with the transmitter.
How Does FHSS Work?
Suppose a system has access to one hundred frequency channels.
Instead of transmitting continuously on one frequency:
- the transmitter uses one channel briefly;
- hops to another channel;
- continues transmitting;
- hops again; and
- repeats the process throughout the communication.
Provided the receiver follows exactly the same hopping pattern, communication proceeds normally.
To other receivers, however, the transmission appears to jump randomly around the spectrum.
Why Are the Hopping Patterns Pseudo-Random?
The hopping sequence must satisfy two important requirements.
It should:
- appear unpredictable to outsiders; and
- be completely reproducible by the receiver.
Pseudo-random sequences satisfy both requirements.
Although they appear random, they are generated deterministically, allowing transmitter and receiver to remain synchronized.
How Does DSSS Resist Interference?
Suppose narrowband interference affects part of the spread signal.
After de-spreading:
- the desired signal collapses into its original bandwidth;
- the interference remains spread over the wider bandwidth; and
- the interference power is greatly reduced.
This allows the receiver to recover information even when interference is relatively strong.
How Does FHSS Resist Interference?
Frequency hopping employs a different strategy.
If interference affects one frequency channel:
- communication is disrupted only briefly;
- the transmitter soon hops elsewhere;
- subsequent transmissions avoid the interference; and
- communication continues.
The effect of narrowband interference is therefore greatly reduced.
Which Technique Is Better Against Jamming?
Both techniques offer excellent resistance to jamming, but in different ways.
DSSS reduces the effect of interference through processing gain. FHSS avoids interference by continually changing frequency. The preferred technique depends upon:
- the type of jammer;
- available bandwidth;
- equipment complexity; and
- operational requirements.
Military systems frequently employ both approaches.
Which Technique Is More Resistant to Interception?
Both techniques make communication more difficult to detect.
Without knowledge of the spreading code or hopping sequence:
- the signal appears noise-like;
- interception becomes more difficult;
- unauthorized receivers cannot easily recover the information; and
- communication security is improved.
Although neither technique provides encryption by itself, both contribute to secure communications.
Does DSSS Support Multiple Users?
Yes.
Different users may employ different spreading codes.
Provided the codes exhibit suitable correlation properties:
- all users occupy the same frequency band;
- all users transmit simultaneously;
- the receiver separates them by code; and
- multiple-access communication becomes possible.
This principle forms the basis of Code-Division Multiple Access.
Does FHSS Support Multiple Users?
Yes, although the approach differs.
Different users may:
- employ different hopping patterns;
- begin hopping at different times;
- occupy different hopping sets; or
- use combinations of these techniques.
Careful system design minimizes collisions between hopping users.
Which Is More Complex?
Both techniques require sophisticated digital processing.
Generally:
- DSSS requires high-speed code generation and correlation; while
- FHSS requires extremely accurate frequency synthesis and synchronization.
Modern integrated circuits have made both approaches practical and economical.
Where Is DSSS Used?
Direct-Sequence Spread Spectrum has been employed in numerous communication systems.
Applications include:
- GPS;
- IS-95 cellular systems;
- CDMA2000;
- early IEEE 802.11 Wi-Fi standards; and
- military communication systems.
Its excellent processing gain makes it particularly attractive where weak-signal reception is important.
Where Is FHSS Used?
Frequency-Hopping Spread Spectrum also has many practical applications.
These include:
- Bluetooth;
- military tactical radios;
- industrial wireless systems;
- cordless telephones; and
- telemetry systems.
FHSS performs particularly well where narrowband interference is common.
Can DSSS and FHSS Be Combined?
Yes.
Some communication systems combine both techniques to obtain the advantages of each.
Hybrid systems may employ:
- direct-sequence spreading;
- frequency hopping;
- adaptive coding; and
- forward error correction.
Although more complex, these systems can provide exceptional reliability in difficult communication environments.
What Are the Advantages of DSSS?
Direct-Sequence Spread Spectrum offers several benefits.
These include:
- high processing gain;
- excellent interference rejection;
- support for CDMA;
- effective weak-signal reception; and
- good multipath performance.
These characteristics explain its widespread use in satellite navigation and cellular communications.
What Are the Advantages of FHSS?
Frequency-Hopping Spread Spectrum provides different advantages.
These include:
- excellent resistance to narrowband interference;
- reduced probability of interception;
- robustness in congested spectrum; and
- relatively simple receiver correlation.
Its flexibility has made it popular in many short-range wireless systems.
Why Are Both Techniques Important?
Although DSSS and FHSS operate differently, they share the same fundamental objective: improving communication reliability by spreading transmitted energy over a much wider bandwidth than the information itself requires. Each represents a different engineering solution to the same problem, and together they transformed both military and civilian communications.
Many modern wireless technologies continue to employ concepts derived from these two pioneering spread-spectrum techniques.
Summary
Direct-Sequence Spread Spectrum spreads information by multiplying each data bit with a high-speed pseudo-random code, while Frequency-Hopping Spread Spectrum spreads the signal by rapidly changing the carrier frequency according to a synchronized hopping sequence. Both techniques provide excellent resistance to interference, jamming, interception, and multipath propagation.
Although their methods differ, DSSS and FHSS remain two of the most influential spread-spectrum technologies ever developed. Their principles continue to underpin applications ranging from GPS and CDMA cellular systems to Bluetooth, military communications, and numerous wireless communication networks.
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