tral node provides time synchronization² for each radio, typically through a feedback loop where each radio terminal adjusts its time reference based on feedback from the central node. Here we can expect some basic time reference available to all radios. For this case we consider the performance of various spread-spectrum multiple access schemes where several coordinated radios transmit signals to the central node. This is a multipoint-to-point system where the central node simulta- neously receives signals from many radios. A generalization of the star network is shown in Figure 2.7. Here there are several star networks with the central nodes forming a fully connected higher order network. This could represent a single processing satellite where each star network can represent a separate antenna beam, a separate frequency band, a separate spread-spectrum carrier, or any combination of these. Each network may also represent separate satellites with cross links that may or may not be fully connected. There is the possibility that any radio terminal will be able to switch from one star network to another. The star network presents the multiple access problem of how several physically separated radios simultaneously transmit spread-spectrum signals to a single central node. We shall examine various techniques for doing this and evaluate their performance. 2The central node's signal may also provide a stable frequency reference. Summary of Multiple Access Techniques 2.2 SUMMARY OF MULTIPLE ACCESS TECHNIQUES In a radio communication network where many radios communicate among themselves, there must be some means of sharing the available channel capacity. this means dividing up the overall channel into sub-channels and then assigning these to radios. Typically, there are more radios than avail- able sub-channels but only a fraction of all radios have messages to trans- mit at any given time. The assignment of sub-channels to radios can be fixed or vary in time according to some policy. Coordinating the assignment of su