Here the FH/MFSK radio receiver merely needs to be synchronized with the intended transmitted signal. For spread-spectrum signals the notion of TDMA has a new form. Here we merely require spread-spectrum signals using the same key to use fixed time delays relative to each other where time delay slots are spaced every 4This may be implemented using a pseudorandomly time-varying surface acoustic wave (SAW) matched filter [83]-[86]. PSEUDO- RANDOMLY TIME-VARYING MATCHED FILTER Figure 2.8. Resolving time shifted spread-spectrum signals sharing common PN sequence. Summary of Multiple Access Techniques chip time interval. This is due to the fact that despreading the received radio signal at the receiver essentially filters out all signals except the one signal that is synchronized with the receivers. For many terrestrial military radio applications, however, maintaining time accuracy up to a chip time interval may be too difficult. This is especially true of DS/BPSK systems where chip times are usually much smaller than the chip (hop) time of FH/MFSK systems. Random access is like a TDMA scheme except here each radio transmits a signal whenever it has a message to send without regard for other radios in the network. The Aloha random access scheme [42] is the simplest in that there are no restrictions on when a radio can transmit. In this scheme a radio transmits any time it has a message and listens for an acknowledgment from the receiving radio. If there is no acknowledgment, it retransmits the mes- sage after a random delay. Slotted Aloha [45] is a scheme where the ran- dom transmissions are restricted to fixed time slots. This implies that all radios must maintain a time reference. In carrier sense multiple access (CSMA) [44] techniques the radio senses the channel before transmitting and delays transmission if it is already being used. There are several varia- tions on the CSMA technique [45]-[51]. The more complex random access techniques allow more efficient utilization of the channel but also require more side i