11.5 Spread spectrum techniques
11 min read•august 20, 2024
Spread spectrum techniques are a game-changer in wireless communications. They spread signals over wider frequency bands, making them resistant to interference and harder to intercept. This approach offers improved security and allows multiple users to share the same frequency band.
These techniques include direct sequence, frequency hopping, time hopping, and chirp spread spectrum. Each method has unique advantages, and they can be combined for even better performance. Synchronization is crucial for successful communication in spread spectrum systems.
Definition of spread spectrum
- Spread spectrum is a technique used in wireless communications to spread a signal over a wider frequency band than the minimum required for the information being transmitted
- Involves using a code that is independent of the data to modulate the carrier signal, resulting in a wideband signal that appears as noise to uninformed receivers
- Offers several advantages compared to traditional narrowband communication systems, including improved resistance to interference, enhanced security, and the ability to share the same frequency band among multiple users
Key characteristics
Top images from around the web for Key characteristics
digital communications - spectral mask and power spectral density - Signal Processing Stack Exchange View original
Is this image relevant?
digital communications - spectral mask and power spectral density - Signal Processing Stack Exchange View original
Is this image relevant?
1 of 1
Top images from around the web for Key characteristics
digital communications - spectral mask and power spectral density - Signal Processing Stack Exchange View original
Is this image relevant?
digital communications - spectral mask and power spectral density - Signal Processing Stack Exchange View original
Is this image relevant?
1 of 1
- Wideband signal: The transmitted signal occupies a much larger than the minimum necessary to send the information
- Code-based modulation: A code that is independent of the data is used to modulate the carrier signal, spreading the signal energy over a wider frequency band
- Low power spectral density: The signal energy is spread over a large bandwidth, resulting in a low power spectral density that can be below the noise floor of narrowband receivers
- Pseudorandom noise (PN) codes: PN codes, also known as spreading codes, are used to modulate the carrier signal and spread the signal energy across the frequency spectrum
Advantages vs narrowband
- Interference resistance: Spread spectrum signals are more resistant to narrowband interference, as the interference affects only a small portion of the wideband signal
- Security: The use of PN codes makes it difficult for unauthorized users to intercept and demodulate the signal without knowledge of the specific code used
- Multiple access: Spread spectrum techniques allow multiple users to share the same frequency band by assigning different PN codes to each user (code division multiple access)
- Low probability of interception (LPI): The wideband, noise-like nature of spread spectrum signals makes them difficult to detect and intercept by hostile receivers
Types of spread spectrum
- : The data signal is directly multiplied by a high-rate PN code to spread the signal energy over a wide bandwidth
- : The carrier frequency is rapidly switched among a set of frequencies according to a PN code, spreading the signal energy over a wide bandwidth
- : The transmission time of each data symbol is varied according to a PN code, spreading the signal energy in the time domain
- : A chirp signal, which is a sinusoidal signal with linearly increasing or decreasing frequency over time, is used to modulate the data signal
Direct sequence spread spectrum
- DSSS is a spread spectrum technique in which the data signal is directly multiplied by a high-rate PN code to spread the signal energy over a wide bandwidth
- The resulting wideband signal has a low power spectral density and appears as noise to narrowband receivers not synchronized with the PN code
Spreading process
- The data signal is multiplied by the PN code, which has a much higher rate than the data signal (typically 10 to 100 times higher)
- The multiplication of the data signal and the PN code results in a wideband signal with a bandwidth approximately equal to the PN code rate
- The spread signal is then modulated onto a carrier frequency and transmitted
Despreading process
- At the receiver, the incoming signal is multiplied by a locally generated PN code that is synchronized with the PN code used at the transmitter
- The multiplication of the received signal and the synchronized PN code "despreads" the signal, recovering the original narrowband data signal
- Narrowband interference and signals from other users not synchronized with the PN code are spread by the despreading process, reducing their impact on the desired signal
Processing gain
- is a measure of the improvement in achieved by the spreading and despreading process in DSSS
- Defined as the ratio of the spread signal bandwidth to the original data signal bandwidth
- A higher processing gain results in better interference suppression and improved SNR at the receiver
- Processing gain (dB) = 10 log10(Spread signal bandwidth / Data signal bandwidth)
Code division multiple access
- Code division multiple access () is a multiple access technique based on DSSS that allows multiple users to share the same frequency band
- Each user is assigned a unique PN code, which is used to spread their data signal
- The spread signals from multiple users are transmitted simultaneously in the same frequency band
- At the receiver, the desired user's signal is despread using the corresponding PN code, while signals from other users remain spread and appear as low-level noise
Frequency hopping spread spectrum
- FHSS is a spread spectrum technique in which the carrier frequency is rapidly switched among a set of frequencies according to a PN code
- The signal energy is spread over a wide bandwidth by hopping the carrier frequency, making it resistant to narrowband interference and difficult to intercept
Slow vs fast hopping
- Slow frequency hopping: The carrier frequency is changed at a rate slower than the data symbol rate, meaning that one or more data symbols are transmitted on each frequency hop
- Fast frequency hopping: The carrier frequency is changed at a rate faster than the data symbol rate, meaning that each data symbol is transmitted over multiple frequency hops
- Fast frequency hopping provides better resistance to narrowband interference and interception compared to slow frequency hopping
Hopping pattern generation
- The hopping pattern is generated using a PN code, which determines the sequence of frequencies to be used
- The transmitter and receiver must be synchronized and use the same PN code to ensure that they hop to the same frequencies at the same time
- The PN code is designed to produce a pseudorandom hopping pattern that appears random to unauthorized users
Synchronization challenges
- Maintaining synchronization between the transmitter and receiver is crucial for successful communication in FHSS systems
- The receiver must accurately track the hopping pattern and synchronize its frequency hops with the transmitter
- Synchronization can be challenging in the presence of interference, jamming, or signal fading
- Techniques such as time and frequency synchronization, acquisition and tracking loops, and error correction codes are used to maintain synchronization
Jamming resistance
- FHSS systems are inherently resistant to narrowband jamming, as the signal energy is spread over a wide bandwidth and the jammer would need to jam all possible hopping frequencies simultaneously
- If a particular frequency is jammed, the FHSS system will simply hop to another frequency, minimizing the impact of the jamming
- The pseudorandom nature of the hopping pattern makes it difficult for a jammer to predict and follow the hopping sequence
Time hopping spread spectrum
- THSS is a spread spectrum technique in which the transmission time of each data symbol is varied according to a PN code
- The signal energy is spread in the time domain by introducing pseudorandom delays between the transmission of each data symbol
Pulse position modulation
- In THSS, pulse position modulation (PPM) is often used to encode the data symbols
- PPM represents each data symbol by the position of a pulse within a fixed time frame
- The PN code determines the pseudorandom delay applied to each pulse, spreading the signal energy in the time domain
Multiple access capability
- THSS can be used for multiple access by assigning different PN codes to each user
- The pseudorandom delays introduced by the PN codes allow multiple users to share the same time frames without significant interference
- The receiver can recover the desired user's data by correlating the received signal with the corresponding PN code
Coexistence with DSSS and FHSS
- THSS can coexist with other spread spectrum techniques, such as DSSS and FHSS, in the same frequency band
- The time-domain spreading of THSS minimizes interference with other spread spectrum signals
- Hybrid systems combining THSS with DSSS or FHSS can provide enhanced performance and flexibility
Chirp spread spectrum
- CSS is a spread spectrum technique that uses a chirp signal to modulate the data signal
- A chirp signal is a sinusoidal signal with linearly increasing or decreasing frequency over time
- CSS spreads the signal energy over a wide bandwidth, providing resistance to interference and multipath fading
Linear frequency modulation
- In CSS, linear frequency modulation (LFM) is used to generate the chirp signal
- The instantaneous frequency of the chirp signal varies linearly with time, either increasing (up-chirp) or decreasing (down-chirp)
- The bandwidth of the chirp signal determines the spreading of the signal energy in the frequency domain
Pulse compression
- CSS employs pulse compression techniques to achieve high range resolution and signal-to-noise ratio improvement
- Pulse compression involves transmitting a long-duration, wideband chirp signal and compressing it into a short-duration, high-amplitude pulse at the receiver
- The compression is achieved by matched filtering, which correlates the received signal with a replica of the transmitted chirp signal
Radar applications
- CSS is widely used in radar systems due to its pulse compression capabilities and resistance to interference
- In radar applications, CSS enables high-resolution ranging, velocity estimation, and target discrimination
- The wideband nature of CSS signals also provides improved clutter suppression and resistance to electronic countermeasures
Hybrid spread spectrum techniques
- Hybrid spread spectrum techniques combine two or more spread spectrum methods to achieve enhanced performance and flexibility
- By leveraging the strengths of different spread spectrum techniques, hybrid systems can provide improved resistance to interference, increased security, and better multiple access capabilities
Direct sequence/frequency hopping
- A hybrid system combining DSSS and FHSS can be used to improve resistance to both narrowband and wideband interference
- The data signal is first spread using a PN code (DSSS), and then the spread signal is transmitted using frequency hopping (FHSS)
- This combination provides the benefits of both DSSS (resistance to narrowband interference) and FHSS (resistance to wideband interference and interception)
Time hopping/direct sequence
- A hybrid system combining THSS and DSSS can be used to improve multiple access capabilities and resistance to multipath fading
- The data signal is first spread using a PN code (DSSS), and then the transmission time of each spread symbol is varied according to another PN code (THSS)
- This combination allows for multiple users to share the same frequency band and time frames, while also providing resistance to multipath fading through time diversity
Advantages of hybrid approaches
- Improved interference resistance: Hybrid systems can provide better resistance to both narrowband and wideband interference by combining the strengths of different spread spectrum techniques
- Enhanced security: The use of multiple spreading techniques makes it more difficult for unauthorized users to intercept and demodulate the signal
- Increased multiple access capabilities: Hybrid systems can support a larger number of users by leveraging the multiple access capabilities of different spread spectrum methods
- Flexibility in system design: Hybrid approaches allow for customization of the system to meet specific requirements, such as interference resistance, security, or multiple access needs
Synchronization in spread spectrum
- Synchronization is a critical aspect of spread spectrum systems, as the receiver must accurately align its PN code with the PN code used at the transmitter to despread the signal and recover the original data
- Synchronization involves both code acquisition (initial synchronization) and code tracking (maintaining synchronization)
Code acquisition
- Code acquisition is the process of initially synchronizing the receiver's PN code with the transmitter's PN code
- The receiver searches for the correct code phase by shifting its local PN code and correlating it with the received signal
- Acquisition techniques include serial search, parallel search, and matched filter methods
- The acquisition time depends on factors such as the code length, search algorithm, and signal-to-noise ratio
Code tracking
- Once the initial code synchronization is achieved, code tracking maintains the synchronization between the receiver's PN code and the transmitter's PN code
- Code tracking accounts for any drift or misalignment between the codes due to factors such as clock differences or Doppler effects
- Common code tracking techniques include delay-locked loops (DLL) and tau-dither loops
- These tracking loops continuously adjust the receiver's code phase to minimize the synchronization error
Carrier synchronization
- In addition to code synchronization, carrier synchronization is necessary to coherently demodulate the received signal
- Carrier synchronization involves estimating and tracking the carrier frequency and phase of the received signal
- Techniques such as phase-locked loops (PLL) and Costas loops are used for carrier synchronization
- Carrier synchronization is particularly important in coherent spread spectrum systems, where the phase information is used for demodulation
Interference and jamming resistance
- One of the key advantages of spread spectrum techniques is their inherent resistance to interference and jamming
- The wideband nature of spread spectrum signals and the use of PN codes make them more resilient to various types of interference
Narrowband interference suppression
- Spread spectrum systems can effectively suppress narrowband interference by spreading the interference energy over a wide bandwidth
- After despreading, the narrowband interference appears as low-level noise, while the desired signal is concentrated into its original narrowband
- The processing gain of the spread spectrum system determines the amount of narrowband interference suppression achieved
Wideband jamming resistance
- Spread spectrum techniques also provide resistance to wideband jamming, where the jammer attempts to disrupt communications over a wide frequency range
- The low power spectral density of spread spectrum signals makes them difficult to detect and jam effectively
- Wideband jamming would require a significant amount of power to be effective, making it impractical in many scenarios
Interception and eavesdropping prevention
- The use of PN codes in spread spectrum systems makes it difficult for unauthorized users to intercept and demodulate the signal
- Without knowledge of the specific PN code used, an eavesdropper would face significant challenges in detecting and decoding the spread spectrum signal
- The pseudorandom nature of the PN codes and the low power spectral density of the signals further enhance the security against interception and eavesdropping
Applications of spread spectrum
- Spread spectrum techniques have found widespread applications in various domains, leveraging their benefits in terms of interference resistance, security, and multiple access capabilities
Wireless communications
- Spread spectrum is extensively used in wireless communication systems, such as cellular networks (3G, 4G, 5G), wireless local area networks (WLAN), and personal area networks (PAN)
- CDMA-based cellular systems (IS-95, WCDMA) employ DSSS for multiple access and interference suppression
- IEEE 802.11 (Wi-Fi) and Bluetooth standards use FHSS or DSSS for robust and secure wireless connectivity
GPS and navigation
- The Global Positioning System () relies on spread spectrum techniques for precise positioning and timing information
- GPS satellites transmit spread spectrum signals using DSSS, allowing receivers to determine their location by measuring the time of arrival of signals from multiple satellites
- The use of spread spectrum ensures the robustness and security of GPS signals against interference and jamming
Military and secure communications
- Spread spectrum techniques are widely employed in military and secure communication systems due to their low probability of interception (LPI) and anti-jamming capabilities
- Military radios, satellite communications, and tactical data links often incorporate spread spectrum methods to protect sensitive information and maintain reliable communications in hostile environments
- Spread spectrum enables secure voice and data transmission, ensuring the confidentiality and integrity of military communications
Key Terms to Review (20)
Bandwidth: Bandwidth refers to the range of frequencies within a given band that can transmit a signal without significant loss. It determines how much data can be transferred over a communication channel in a specific amount of time, influencing the speed and capacity of signal transmission. In the context of spread spectrum techniques, bandwidth plays a critical role in ensuring robust communication by spreading the signal across a wider range of frequencies, which helps in resisting interference and improving security.
Bit Error Rate (BER): Bit Error Rate (BER) is a measure of the number of bit errors divided by the total number of transferred bits during a specific time interval. It quantifies the reliability and performance of digital communication systems, particularly in noisy environments. A low BER indicates a high-quality signal and efficient error correction mechanisms, while a high BER signifies potential issues in data transmission, making it a critical parameter in evaluating spread spectrum techniques.
Cdma: CDMA, or Code Division Multiple Access, is a digital communication technique that allows multiple users to share the same frequency channel by assigning unique codes to each user. This method enhances the efficiency of communication systems by enabling simultaneous transmission of multiple signals over a single channel, making it a key player in spread spectrum techniques.
Chip rate: Chip rate refers to the speed at which individual chips are transmitted in a spread spectrum communication system. It is a crucial factor in determining how information is encoded and transmitted over the channel, influencing the overall bandwidth and data capacity of the system. A higher chip rate allows for better resolution in distinguishing between multiple signals, which can enhance communication robustness and performance.
Chirp spread spectrum (css): Chirp spread spectrum (CSS) is a communication technique that spreads the signal over a wide bandwidth by varying the frequency of the signal in a linear fashion, also known as a chirp. This method enhances the robustness of signals against interference and improves the signal-to-noise ratio, making it particularly effective for applications like radar and wireless communication systems.
Claude Shannon: Claude Shannon was an American mathematician and electrical engineer known as the 'father of information theory.' His groundbreaking work laid the foundation for modern digital communication and signal processing, particularly through his concept of measuring information and the development of techniques like error correction and data compression. Shannon's principles are integral to understanding how spread spectrum techniques enhance communication reliability and security.
Direct sequence spread spectrum (dsss): Direct sequence spread spectrum (DSSS) is a modulation technique used in telecommunications where the signal is spread over a wider bandwidth than the minimum necessary. This technique enhances the robustness of the signal against interference and allows multiple users to share the same frequency band by assigning unique spreading codes, which makes it suitable for wireless communication systems.
Frequency hopping spread spectrum (fhss): Frequency hopping spread spectrum (FHSS) is a method used in wireless communication to transmit data by rapidly switching the carrier frequency among many frequency channels. This technique helps minimize interference, enhances security, and increases resistance to jamming. FHSS is particularly effective in environments with high noise levels and allows multiple users to share the same frequency band without causing significant interference.
GPS: GPS, or Global Positioning System, is a satellite-based navigation system that allows users to determine their exact location (latitude, longitude, and altitude) anywhere on Earth. This technology utilizes a network of satellites that send signals to GPS receivers, enabling precise location tracking and navigation. GPS plays a critical role in various applications, including transportation, mapping, and emergency services.
Interference rejection: Interference rejection refers to the techniques used to minimize or eliminate unwanted signals that can distort or degrade the quality of desired signals in various communication systems. This process is crucial for maintaining signal integrity, especially when dealing with noise and interference from other sources. Effective interference rejection allows for clearer signal processing and enhanced performance in systems like beamforming and spread spectrum techniques.
Jamming resistance: Jamming resistance refers to the ability of a communication system to maintain its performance and integrity in the presence of intentional interference or jamming signals. This characteristic is crucial for ensuring reliable data transmission, especially in environments where adversaries may attempt to disrupt communications. Techniques that enhance jamming resistance often involve spreading the signal over a wider bandwidth or using specific coding strategies that make it difficult for jammers to effectively block or interfere with the transmitted information.
Multi-user detection: Multi-user detection refers to the process of identifying and separating signals from multiple users in a communication system, often used to enhance performance in scenarios with overlapping transmissions. By leveraging knowledge of the spreading codes in spread spectrum systems, multi-user detection aims to mitigate interference and improve the overall quality of signal reception. This technique is particularly important in environments where many users access the same medium simultaneously.
Processing Gain: Processing gain refers to the improvement in signal-to-noise ratio (SNR) achieved by a receiver when utilizing specific techniques to process a spread spectrum signal. This concept is crucial in understanding how systems can effectively enhance the quality of communication by mitigating the effects of noise and interference. A higher processing gain allows a system to better distinguish between the desired signal and background noise, leading to more reliable data transmission and reception.
Pseudo-random sequences: Pseudo-random sequences are deterministic sequences of numbers that exhibit properties similar to random sequences, but are generated by a predictable process. These sequences are crucial in various applications, particularly in spread spectrum techniques, where they are used for spreading the signal across a wider bandwidth, providing resistance to interference and eavesdropping. The repeatability of pseudo-random sequences makes them suitable for cryptography and communication systems, as they can be reproduced exactly when the initial conditions are known.
Signal-to-noise ratio (SNR): Signal-to-noise ratio (SNR) is a measure used to quantify how much a signal stands out from the background noise, often expressed in decibels (dB). A higher SNR indicates a clearer signal, making it easier to detect and analyze relevant information, while a lower SNR means that noise can obscure or interfere with the signal. This concept is essential in various applications, including improving data quality, enhancing communication systems, and processing biomedical signals.
Simon Haykin: Simon Haykin is a renowned figure in the field of electrical engineering, particularly known for his contributions to signal processing and adaptive systems. His work has significantly advanced our understanding of various concepts such as adaptive noise cancellation and spread spectrum techniques, both of which play crucial roles in modern communication systems. Haykin's research emphasizes the importance of adaptive algorithms and their practical applications in enhancing signal quality and reliability.
Spread spectrum modulation theory: Spread spectrum modulation theory is a method of transmitting signals over a wide frequency band by spreading the signal across multiple frequencies. This technique enhances the robustness and security of communication systems, making them less susceptible to interference and eavesdropping. By using codes to modulate the signal, it enables multiple users to share the same frequency band without significant interference.
Spreading Gain: Spreading gain refers to the increase in signal power achieved when a signal is spread over a wider bandwidth, particularly in spread spectrum communication techniques. This process enhances the signal's robustness against interference and jamming while also improving its ability to share the same frequency band with other signals. The spreading gain is a crucial parameter that quantifies the effectiveness of the spreading process, impacting system performance and capacity.
Time Hopping Spread Spectrum (THSS): Time Hopping Spread Spectrum (THSS) is a spread spectrum technique that modulates the signal by rapidly switching the transmission time among a set of predetermined time slots. This method enhances communication security and resistance to interference by spreading the signal over time, making it difficult for unauthorized listeners to intercept or jam the signal. THSS is particularly useful in military and secure communications, as it adds an additional layer of complexity to signal detection.
Walsh codes: Walsh codes are a set of orthogonal binary sequences used in spread spectrum communication systems to enable multiple users to share the same frequency channel without interference. These codes are essential in applications such as CDMA (Code Division Multiple Access), where they help differentiate between different users' signals by assigning unique sequences to each user. Their orthogonality ensures that the cross-correlation between different codes is zero, which minimizes interference and enhances signal clarity.