Direct Sequence Spread Spectrum (DSSS)
Direct Sequence Spread Spectrum (DSSS) is a modulation technique used in wireless communication systems to spread a signal's bandwidth over a wider frequency range. This enhances resistance to interference, reduces signal detectability, and improves coexistence in shared spectrum environments.
Scope and Application
DSSS is used in license-exempt frequency bands, particularly:
2.4 GHz ISM band (2400–2483.5 MHz)
Sub-GHz ISM bands (e.g., 433 MHz, 868 MHz in Europe)
Common technologies include:
Wi-Fi 802.11b
Zigbee
Legacy SRD systems
GNSS systems (e.g., GPS L1 C/A signal)
Military communications and satellite telemetry
Regulatory standards that allow DSSS:
Technical Principle
DSSS multiplies the original data signal by a pseudorandom spreading code (chip sequence) at a much higher rate than the data stream. The resulting signal occupies a much wider bandwidth and appears noise-like.
At the receiver, the signal is de-spread using the same chip sequence to recover the data.
→ This requires precise synchronization between transmitter and receiver. Loss of synchronization leads to increased bit error rates and degraded signal quality.
Key Parameters
Chip rate: Determines the spreading factor, which is the ratio of chip rate to data rate
Processing gain: Ratio of spread bandwidth to original data bandwidth; improves interference resilience
Typical bandwidth: ≥ 500 kHz for regulatory compliance in unlicensed bands
Benefits
Resilience to narrowband interference
Lower probability of interception
Improved coexistence in crowded spectrum environments
Spectral efficiency when combined with coding and filtering
Regulatory Context
| Region | Details |
|---|---|
| USA |
Rule/Standard: FCC §15.247 Relevance: DSSS permitted in 2.4 GHz ISM |
| Canada |
Rule/Standard: ISED RSS-247 Relevance: Equivalent to FCC §15.247 |
| EU |
Rule/Standard: EN 300 328 Relevance: DSSS allowed under RED |
| Region | Rule/Standard | Relevance |
|---|---|---|
| USA | FCC §15.247 | DSSS permitted in 2.4 GHz ISM |
| Canada | ISED RSS-247 | Equivalent to FCC §15.247 |
| EU | EN 300 328 | DSSS allowed under RED |
To obtain regulatory approval, devices using DSSS must comply with region-specific requirements. While the core principles are similar, detailed thresholds vary by standard. Key parameters include:
Minimum channel bandwidth:
≥ 500 kHz (FCC §15.247, RSS-247)
≥ 250 kHz (EN 300 328, depending on modulation type)
Maximum output power limits
Emission limits, including spurious and out-of-band emissions
Consistency between declared and measured occupied bandwidth
These parameters are verified during type testing and form part of the compliance documentation submitted for certification or CE marking.