RECEPTION OF SPREAD-SPECTRUM SIGNALS BASIC INFORMATION AND TUTORIALS

The type of receiver required for spread-spectrum reception depends on how the signal is generated. For frequency-hopped transmissions, what is needed is a relatively conventional narrowband receiver that hops in the same way as and is synchronized with the transmitter.

This requires that the receiver be given the frequency-hopping sequence, and there be some form of synchronizing signal (such as the signal usually sent at the start of a data frame in digital communication) to keep the transmitter and receiver synchronized.

Some means must also be provided to allow the receiver to detect the start of a transmission, since, if this is left to chance, the transmitter and receiver will most likely be on different frequencies when a transmission begins.

One way to synchronize the transmitter and receiver is to have the transmitter send a tone on a prearranged channel at the start of each transmission, before it begins hopping. The receiver can synchronize by detecting the end of the tone and then begin hopping according to the prearranged PN sequence.

Of course, this method fails if there happens to be an interfering signal on the designated synchronizing channel at the time synchronization is attempted.

A more reliable method of synchronizing frequency-hopping systems is for the transmitter to visit several channels in a prearranged order before beginning a normal transmission. The receiver can monitor all of these channels sequentially, and once it detects the transmission, it can sample the next channel in the sequence for verification and synchronization.

Direct-sequence spread-spectrum transmissions require different reception techniques. Narrowband receivers will not work with these signals, which occupy a wide bandwidth on a continuous basis. A wideband receiver is required, but a conventional wideband receiver would output only noise.

In order to distinguish the desired signal from noise and interfering signals, which over the bandwidth of the receiver are much stronger than the desired signal, a technique called autocorrelation is used. Essentially this involves multiplying the received signal by a signal generated at the receiver from the PN code.

When the input signal corresponds to the PN code, the output from the autocorrelator will be large; at other times this output will be very small. Of course, once again the transmitter and receiver will probably not be synchronized at the start of a transmission, so the transmitter sends a preamble signal, which is a prearranged sequence of ones and zeros, to let the receiver synchronize with the transmitter.