[Excerpt from the ARRL Spread Spectrum Sourcebook]

What can spread spectrum offer the world of packet radio?

Well, all packet radio nodes transmit and receive on the same frequency. Each node tries to get its packets out as quickly as possible and then listen for for packets addressed to it. As you might expect, sometimes two or more nodes try to send packets at the same time. The result is a collision. If a collison happens, both nodes will try again later. Some experiments have revealed that when these nodes wildly try to transmit their packets, very few packets get through. So a number of plans have been cooked up to rule the channel and give order to an unordered universe of nodes....

Now spread spectrum can help this collision problem. Spread spectrum allows a station to lock out interference from all sources except the one with the proper code sequence. Suppose that we assign a different code sequence to each packet node. In this case, a node would scan the list of sequences assigned to the other nodes and if it heard a transmission, it would quickly find the sequence of the node transmitting to it. Then it would lock onto that sequence and exclude any other node trying to interfere. There would be no collisions except when two stations started to send almost at the very same time.

Besides providing this anti-collision property, spread spectrum also provides privacy. As you might suspect, this technique has a four letter acronym SSMA (spread spectrum multiple accesss)....