DirectSequence

Direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique in which a data signal is multiplied by a high-rate pseudo-random sequence, or chip sequence, to produce a signal that occupies a much wider bandwidth than the original. The data, typically modulated by BPSK or QPSK, is spread with a PN code at the chip rate, increasing the signal bandwidth by the spreading factor SF = chip_rate / data_rate. At the receiver, a correlator synchronized to the same PN code despreads the signal, recovering the original data.

DSSS provides several key benefits. The wide bandwidth makes the transmitted signal more resistant to narrowband interference and jamming, and can improve robustness in multipath environments. The approach also enables multiple users to share the same frequency band by using different spreading codes with low cross-correlation, a concept central to code-division multiple access (CDMA). The choice of spreading codes, such as Gold or Kasami sequences, affects multiple-access performance through their autocorrelation and cross-correlation properties.

Challenges and considerations include the need for precise time and code synchronization, effective power control to mitigate the near-far problem in CDMA systems, and increased transmitter and receiver complexity. Regulatory limits on spectral emissions and the requirement to maintain acceptable spectral efficiency influence system design.

Applications range from global positioning systems (where the C/A code uses DSSS) to various wireless communication standards. DSSS was widely used in early commercial WLANs (such as 802.11b) and remains a foundational technique in many CDMA-based cellular and military communications.