Propagation spectrum techniques can be broadly classified into four main types:
- Frequency Hopping Offset Spectrum (FHSS): FHSS involves rapid switching of carrier frequencies based on a predetermined hopping sequence. This technique divides the available frequency spectrum into multiple channels, and the transmitter and receiver synchronize their hopping patterns to maintain communication. FHSS provides resilience against interference and improves signal security by distributing transmissions over a wide bandwidth.
- Direct Sequence Deviation Spectrum (DSSS): DSSS modulates the data signal with a deviation code that spreads the signal bandwidth over a much wider frequency range than the original signal bandwidth. This deviation code is generally a pseudorandom sequence known to both the transmitter and the receiver. DSSS improves interference resistance, improves data integrity, and provides increased robustness against channel deficiencies such as multipath propagation.
- CHIRP Deviation Spectrum (CSS): CSS uses a signal modulation technique where the frequency of the transmitted signal varies continuously over time (CHIRP signal). This continuous frequency variation allows CSS to achieve wideband transmission and improves resistance to narrowband interference and noise. CSS is commonly used in radar systems, ultrasonic sensors, and some communications applications.
- Time Hop Spread Spectrum (THSS): THSS involves transmitting short duration pulses of a signal over different time slots in a pseudorandom sequence. Each pulse occupies a specific time slot and the receiver synchronizes with the transmitter to retrieve the original data. THSS offers advantages in terms of low probability of interception and detection (LPI/LPD), making it suitable for covert communication and secure data transmission applications.
The different types of spread spectrum techniques include frequency hopping deviation spectrum (FHSS), direct sequence deviation spectrum (DSSS), CHIRP deviation spectrum (CSS) and propagation spectrum time jump (THS). Each technique offers unique advantages in terms of spectral efficiency, interference resistance, data security and application suitability.
Among the options listed, frequency hopping gap spectrum (FHSS) is a type of gap spectrum technique. FHSS involves hopping between different frequencies in a predefined sequence, thereby spreading the transmitted signal over a wider bandwidth. This hopping sequence is synchronized between the transmitter and receiver to maintain reliable communication and mitigate the effects of interference.
The basic elements of the propagation spectrum include:
- Carrier frequency: frequency range over which the signal is transmitted.
- Spreading code: a pseudorandom sequence used to modulate the data signal, broadcasting it over a wider frequency band.
- Synchronization: The process of coordinating the transmitter and receiver to ensure that they use the same deviation code and synchronization sequence.
- DESSAFRAGE: The reverse process of propagation at the receiver to recover the original data signal from the spreading spectrum transmission.
- Bandwidth: The range of frequencies over which the signal is distributed, which is usually much wider than the minimum required for transmission.
These elements work together to realize the benefits of spread spectrum techniques, including resistance to interference, improved signal security, and robustness in harsh communications environments. Spread Spectrum Technology finds applications in wireless communications systems, radar systems, secure military communications and various other areas where reliable and secure data transmission is essential.