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

• Occupied bandwidth: The applicable measurement and limit depend on the target market, equipment type and operating mode

Benefits

• Resilience to narrowband interference

• Lower probability of interception

• Improved coexistence in crowded spectrum environments

• Spectral efficiency when combined with coding and filtering

Devices using DSSS must comply with the requirements of the target market. Similar technical concepts do not make the regional rule sets identical.

• United States: Under 47 CFR §15.247(a)(2), systems using digital modulation in the specified bands must have a minimum 6 dB bandwidth of 500 kHz.

• Canada: ISED RSS-247 is a separate Canadian rule set with similar requirements; it must be assessed independently rather than described as identical to FCC §15.247.

• European Union: EN 300 328 assesses occupied channel bandwidth and other requirements according to equipment type and operating mode. Its DSSS requirements must therefore be determined from the applicable clauses; the standard does not define a general minimum DSSS channel bandwidth.

Applicable parameters are determined during conformity assessment and included in the technical documentation. For CE marking, this documentation is retained by the manufacturer and provided to market-surveillance authorities on request; it is not routinely submitted to an authority unless the selected conformity-assessment procedure involves a Notified Body.