3.1 Principles of Direct Sequence Spread Spectrum

A direct sequence spread spectrum (DSSS) signal is created by modulating a transmitted signal with a defined sequence of bits having a shorter duration than the data bits or symbols. These bandwidth spreading bits are called chips, and the sequence is called a code. Chip modulation may be any type, but generally binary phase shift keying (BPSK) is employed. A BPSK modulator shifts the phase of the RF carrier 180 according to the state of the modulating data bits or code chips "zero" or "one." The data, whose symbol period is larger than the chip period, also modulates the RF signal. Data modulation is often M-ary phase shift keying, the number of phase levels depending on the data rate to symbol rate ratio. To demodulate the symbol at the receiver, first the signal bandwidth spreading by the code chips has to be canceled out. This is done by lining up a replica of the spreading code with the chips in the received signal and multiplying the locally generated and received signals together. The despreading process reduces the bandwidth of the received signal to that normally required by the data modulation, which is a function of the symbol rate. It also lowers the level of interference from cochannel narrowband signals and jammers, as well as wide bandwidth signals with different or same but unaligned spreading codes.