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:

• FCC §15.247

• ISED RSS-247

• ETSI EN 300 328

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

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.