MULTIPLE-ACCESS INTERFERENCE
In a multiuser or a multiple-access communications system, different users or devices send information independently and concurrently over a shared transmission medium (such as the air interface in wireless communications). At the receiving end, one or more receivers should be able to separate users and detect information from the user of interest. Interference from other users with the user of interest is called multiple-access interference (MAI), which is a limiting factor to channel capacity and the
performance of such receivers. The addition of MAI to the unavoidable channel noise and narrowband interference discussed earlier can significantly degrade the low-powered UWB pulses and make the detection process very difficult. Figure 1–9 represents a UWB multiple-access channel.

As shown in Figure 1–9, separating each user’s information from the combination of heavily distorted and low-powered UWB signals from all users is a very challenging task. A comprehensive study of multiple-access techniques in UWB systems appears in Chapter 3. Table 1–2 summarizes the challenges and problems that narrow pulses can bring to UWB communications systems.