nformation in the form of time synchronization and/or channel measurements. Since spread-spectrum signals are generally difficult to detect (see Chapter 4, Part 5), CSMA schemes are not useful for most spread-spectrum carriers. Slotted Aloha requires time synchronization among radios which is often difficult to achieve. Also, with spread-spectrum signals the notion of non-overlapping time slots is not useful since these signals do not have "collisions" even when using the same spread-spectrum carriers and the same interval as long as their relative time delays are greater than the chip time. With pure Aloha random access, two spread-spectrum radio signals can cause a "collision" at a receiving radio only if they both use the same key and transmit with relative delays of less than the chip time. Otherwise, these signals interfere with each other like independent jamming interfer- ence. If pure Aloha random access is to be used, this suggests that all radios must transmit with random delays and receiving radios must be able to acquire and synchronize over the range of possible transmission delays. These delays can be small compared with data bit time intervals in DS/BPSK systems. Finally, note that there are ways to divide up a channel using antenna tech- niques. Adaptive multiple spot beam antennas are used in satellites [87]-[89], for example, not only to separate uplink and downlink signals but also to null out undesirable interference such as intentional jamming. Antennas can Signals from terminals that are in different spot beam areas can be separated. Multiple Access also use polarization division where electromagnetic fields can be polarized into separate channels. 2.3 SPREAD-SPECTRUM MULTIPLE ACCESS WITH DS/BPSK WAVEFORMS When a transmitter and receiver use a DS/BPSK waveform to communicate, all the interference in the channel can be approximated as additive white Gaussian noise. This was shown to be a good approximation in Chapter 1 of Part 2, provided the pseudorandom sequence used is statistically indepe