Direct Sequence Spread Spectrum (DSSS)

Direct sequence spread spectrum (DSSS) is a process by which data is transmitted using a higher bandwidth signal that is demanded by the data rate. Using DSSS allows multiple channels to occupy the same bandwidth, thus mitigating interference from other users at the expense of bandwidth expansion.

DSSS spreads each bit of signal data at the transmitter into L chips using a pseudorandom L-chip spreading code called a code word. The length L of the pseudorandom spreading code is also known as the bandwidth expansion factor because the chips are transmitted at a rate equal to L × bit rate of the data. The spreading code appears random to all receivers except the intended one, which uses the knowledge of the spreading code to demodulate and recover the transmitted information. Thus multiple channels can occupy the same portion of the frequency spectrum by using code words that have little or no correlation with one another, and little or no autocorrelation for any shift other than zero.

Mathematically, a DSSS signal is described by

where

y(T) is the transmitted DSSS signal
g(T) is the pulse-shaping signal of duration T_c
{a_n} is the n^th information bearing symbol
{c_m} is the m^th element of the L-long pseudorandom spreading code (also known as the chip sequence)
T_c is the chip period
T=L×T_c is the symbol period