Overview
Spread spectrum communication (SSC) increases signal bandwidth to lower the signal-to-noise ratio requirement and improve interference tolerance. The transmitted signal uses a bandwidth much larger than the information bandwidth. Compared with traditional narrowband communication, spread spectrum provides advantages such as multiple access capability, low probability of interception, strong anti-jamming performance, and high time resolution, so it is widely used in wireless communication.
Representative spread spectrum techniques include two main methods: direct-sequence spread spectrum and frequency-hopping spread spectrum.
If a pseudorandom sequence is applied directly to data, the result is direct-sequence spread spectrum (DSSS). If the pseudorandom sequence controls the carrier frequency, the result is frequency-hopping spread spectrum (FHSS). The two methods can also be combined as DSSS + FHSS hybrid spread spectrum.
Direct-Sequence Spread Spectrum (DSSS)
Direct-sequence spread spectrum (DSSS) uses a high-rate spreading sequence at the transmitter to expand the signal spectrum, and the receiver uses the same spreading code to de-spread the received signal, restoring the original information.
For example, LoRa uses a form of direct-sequence-like spread modulation. In DSSS, information bits are modulated by a PN code (chip). The PN code is a pseudo-noise sequence whose chip duration is short relative to information bits. Compared with the information bits, the transmitted information occupies much more bandwidth after spreading. DSSS modulation is used in certain IEEE 802.11 WLAN-compatible systems. In DSSS systems, the entire system bandwidth is available to each user.
PRS stands for pseudo-random sequence.
DSSS characteristics:
- Fading resistance. The wide bandwidth makes the system less sensitive to frequency-selective fading that affects part of the spectrum.
- Multipath immunity. PN codes typically have good autocorrelation properties, allowing reflection paths to be separated, time- and phase-aligned, and combined to improve performance.
- Low probability of interception. After spreading, the power spectral density decreases and the signal can be buried in noise, making it difficult for receivers using different PN codes to recover the signal.
- Facilitates multiple access.
Frequency-Hopping Spread Spectrum (FHSS)
Frequency-hopping spread spectrum (FHSS) repeatedly switches the carrier frequency during radio transmission to reduce interference and avoid interception. The wide available bandwidth is divided into many possible frequency channels for use by the transmitted signal. The RF carrier hops in a pseudo-random sequence (PRS or PN sequence). Both transmitter and receiver know the PN sequence, so they can demodulate and decode the information. Within one chip duration, the RF frequency remains constant.
There are two FHSS types based on hop rate. In fast-hopping FHSS, the hop rate exceeds the message bit rate. In slow-hopping FHSS, hops occur at a rate lower than the message bit rate.
FHSS characteristics:
- Interference resistance. Communication uses discrete frequency points, so if one frequency is interfered, other frequencies can maintain communication.
- Compatibility. Large number of frequency points allows communication with different devices at different channels.
- Facilitates multiple access.
Comparison: DSSS vs FHSS
Because FHSS relies on changing RF carrier frequencies, it is prone to burst errors caused by frequency-selective fading.
Error rate: In DSSS, information bits are spread over the frequency and time plane, minimizing the effects of interference and fading. DSSS typically has a lower error rate than FHSS. FHSS tends to produce strong burst errors. When a device hops near a blocked frequency, FHSS throughput can drop. Compared with DSSS, FHSS decoding is simpler. DSSS requires specific algorithms to synchronize transmitter and receiver.
Network capacity: DSSS can support higher data rates. DSSS can provide up to 22 Mbps capacity, while FHSS supports up to about 3 Mbps.
Reliability: DSSS systems are generally more reliable than FHSS. FHSS reliability in some applications is lower, though FHSS can be more energy efficient for mobile applications.
Acquisition time: The time required to convert an analog signal to a digital signal is typically longer in DSSS systems than in FHSS systems.
Cost: DSSS implementations tend to be more costly than FHSS at low Mbps levels. At lower data rates (for example 2 Mbps), FHSS can be cheaper, but at higher data rates DSSS often becomes more cost-effective.
Typical use cases: In harsh environments with large coverage, noise, multipath, or presence of signals in Bluetooth bands, DSSS is well suited for point-to-point links. FHSS may be used for point-to-multipoint deployments where robustness across many channels is beneficial.
Time-Hopping Spread Spectrum (THSS)
Time-hopping spread spectrum (THSS) uses a pseudorandom sequence to control on-off transmission timing. It can be viewed as a linear frequency-modulated pulse technique that scans the carrier frequency linearly within a period. To date, THSS has not achieved major breakthroughs in mainstream deployments.
Applications
Many characteristics of spread spectrum communication are difficult to replicate with narrowband techniques. Spread spectrum is widely used in cellular networks, satellite communication, Bluetooth, Wi-Fi, and other fields.
