ABSTRACT :
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.
To achieve most of the claims made for the spread spectrum it is necessary that the bandwidth over which the message is spread be very much greater than the bandwidth of the message itself. PREAD Spectrum (SS) has been defined as a means of transmission in which the signal occupies bandwidth much in excess of the minimum necessary to send the information. The band spread is accomplished by utilizing a “code” which is independent of the data and a synchronized reception with the code at the receiver is used for de-spreading and data recovery.