Spread spectrum concepts are gaining increasing importance in the protection of communication links from jamming and detection by unfriendly forces. The most common prior art technique comprises a direct modulation of pseudorandom sequences. A binary code is generated at an information rate which is much greater than that of the data being transmitted. The data is modulo-two added to the binary code, and the resultant sequence is modulated on a suitable carrier frequency, and then it is transmitted. At the receiver, the signal is heterodyned down to baseband to remove the high frequency carrier. A high rate binary sequence which is identical to the transmitter sequence is synchronized to the incoming sequence and modulo-two added to thereby reproduce the lower rate data information back into its original form.
The above technique has several disadvantages. First, in the heterodyne procedure, a local oscillator signal of proper frequency is mixed with the incoming signal. To obtain the proper signal at baseband, the phase of the local oscillator must be identical to that of the received signal, thereby necessitating the use of elaborate techniques such as phased lock loops and in-phase and quadrature channels. Second, assuming that the signal has been properly heterodyned, to recover the data, the high-rate digital sequence generated at the receiver must be precisely synchronized with the corresponding sequence in the received waveform. As is known to those skilled in the art, achieving the requisite synchronization is a complex problem.