What Is Direct Sequence Spread Spectrum Communication?

Direct sequence spread spectrum (DSSS) communication is a radio transmission technique in which an information signal is multiplied by a high-rate pseudo-noise (PN) code sequence before modulation onto a carrier, spreading the energy of the transmitted signal across a bandwidth far wider than the original data stream. The ratio of the transmitted bandwidth to the information bandwidth, called the processing gain, typically ranges from 100 to 1000 in deployed systems. This spreading process makes the transmitted signal appear noise-like to observers who do not possess the PN code, provides strong resistance to narrowband interference and jamming, and enables multiple users to share the same frequency band by assigning each a distinct code. These properties made DSSS the basis of IS-95 CDMA cellular networks, GPS satellite ranging, and the IEEE 802.11b wireless LAN standard.

The technique draws from information theory, digital signal processing, and coding theory. The PN sequences used in DSSS are drawn from families of deterministic binary sequences, primarily maximal-length linear feedback shift register sequences (m-sequences), Gold codes, and Kasami sequences, chosen for their autocorrelation and cross-correlation properties.

Spreading and Despreading

In DSSS, each data bit is multiplied bitwise by the PN chip sequence, which runs at a chip rate that is an integer multiple L of the data bit rate. The resulting spread signal occupies L times the original data bandwidth. An introduction to spread-spectrum communications published by Analog Devices explains the spreading and despreading operations and the relationship between chip rate, bandwidth expansion, and interference rejection. At the receiver, correlation with a synchronized local replica of the same PN code collapses the desired signal back to its original bandwidth. Narrowband interference present in the channel is spread by the despreading operation, so it occupies the full chip bandwidth; after a bandpass filter matched to the data rate, only a fraction 1/L of the interference power remains, which is the interference rejection benefit of the processing gain.

Code Design and Interference Resistance

The effectiveness of DSSS depends critically on the correlation properties of the PN codes. A code with a sharp autocorrelation peak and low sidelobes allows the receiver to acquire timing precisely while rejecting delayed multipath copies of the transmitted signal. In code-division multiple access (CDMA) systems where many users share the band simultaneously, the cross-correlation between different users' codes must be low so that one user's transmission appears as small additional noise to another's receiver. Gold codes, formed by XORing two m-sequences with specific phase relationships, provide a large family of codes with bounded cross-correlation, enabling dozens to hundreds of simultaneous users in a shared band. A DTIC technical report on direct sequence spread spectrum system design surveys the link between code family selection and system capacity.

Synchronization and Acquisition

Before despreading can begin, the receiver must align its local PN code replica with the incoming signal to within a fraction of a chip period. Code acquisition is typically accomplished by a serial search in which the receiver shifts the local code one chip at a time and tests each hypothesis by measuring the correlation energy over a dwell interval. A fast acquisition technique for direct sequence spread spectrum systems described in an IEEE conference publication introduces parallel correlator architectures that reduce mean acquisition time by testing multiple code-phase hypotheses simultaneously. Once coarse acquisition is achieved, a code-tracking loop refines and maintains alignment as the channel varies.

Applications

Direct sequence spread spectrum communication has applications in a wide range of fields, including:

• CDMA cellular telephony, where each user is assigned a unique spreading code
• GPS and GNSS satellite navigation, where DSSS enables ranging from multiple satellites
• IEEE 802.11b wireless LAN, which used DSSS at 2.4 GHz before OFDM became dominant
• Military and tactical communications requiring low probability of intercept and jam resistance
• Industrial wireless control systems in electrically noisy environments