Direct Sequence Spread Spectrum (DSSS)

This is probably the most widely recognized form of spread spectrum. The DSSS process is performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital signal. First the PN code is modulated onto the information signal using one of several modulation techniques (eg. BPSK, QPSK, etc ). Then, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to be replaced with a very wide bandwidth signal with the spectral equivalent of a noise signal. The demodulation process (for the BPSK case) is then simply the mixing/multiplying of the same PN modulated carrier with the incoming RF signal. The output is a signal that is a maximum when the two signals exactly equal one another or are "correlated." The correlated signal is then filtered and sent to a BPSK demodulator.

The signals generated with this technique appear as noise in the frequency domain. The wide bandwidth provided by the PN code allows the signal power to drop below the noise threshold without loss of information. The spectral content of an SS signal is shown in Fig. 1. Note that this is just the spectrum of a BPSK signal with a (sin x / x)2 form.

Fig. 1 BPSK DSSS Spectrum

The bandwidth in DSSS systems is often taken as the null-to-null bandwidth of the main lobe of the power spectral density plot (indicated as 2Rc in Fig. 1). The half power bandwidth of this lobe is 1.2 Rc, where Rc is the chip rate. Therefore, the bandwidth of a DSSS system is a direct function of the chip rate; specifically 2Rc/RINFO. This is just an extension of the previous equation for process gain. It should be noted that the power contained in the main lobe comprises 90 percent of the total power. This allows a narrower RF bandwidth to accommodate the received signal with the effect of rounding the received pulses in the time domain.

One feature of DSSS is that QPSK may be used to increase the data rate. This increase of a factor of two bits per symbol of transmitted information over BPSK causes an equivalent reduction in the available process gain. The process gain is reduced because for a given chip rate, the bandwidth (which sets the process gain) is halved due to the two-fold increase in information transfer. The result is that systems in a spectrally quiet environment benefit from the possible increase in data transfer rate.