deas Look at Figure 11.1, since it explains what I describe next. user k channel 5.(k) (t) source coder modulator r(t) = + n(t) s;(i)(t) source modulator coder user j signal s;(k)(t) + s:(j)(t) sent over channel (i+1)T r(t) = s;(k)(t) + si(i)(t) decision source + n(t) device decoder (i+1)T wants to si.m(t) hear user (not user j) OPTIMAL RECEIVER DECISION DEVICE FRONT END FOR USER k FOR USER k DEMODULATOR FOR USER k (from Chapter 5) Figure 11.1 (a) User k and user j send their signals over the channel (b) A receiver tries to pick up user k's signal Multiple Access Schemes: Teaching Telecommunications Systems to Share 1. In Figure 11.1(a), we have a new naming convention. We use the notation s,(k) (t) to indicate the signal sent out by the kth user during the time [iT, (i + 1) T]. Let's say that the kth user's information, s(k) (t) is one of the following signals: {s (t), S(K) (t), SIM (1)}. (Let's assume, for example, it is a signal from a QPSK signal set.) 2. In Figure 11.1(a), we assume that there are two users, user k and user j, using the communication system. That means that the signal being sent over the channel is the combination of both sent signals, 3. In Figure 11.1(b), we want to pick up the signal from user k, s(k) (t) We do not want the signal from user j, because we are not interested in what user j is saying. From Chapter 5, we know that the receiver front end to use when we want to pick up the signal s(k)(t) (from user k) is the one shown in Figure 11.1(b). 4. Let's look at the top branch of this receiver front end. 4a. In comes r (t) s(k) (t) +su )+n(t), where n (t) is channel noise. 4b. We want to make sure that the signal 5,5 (t) in r(t) does not make it out of this top branch. If this signal does not make it out of the top branch then we have effectively eliminated the presence of (t) (user )) in this part of user k's receiver. 4c. The signal coming out of the top branch is (i+1)T (11.1) a(((s()(a (11.2) (i+1)T If we make sure that SIK (t) dt = 0, then the signal Su (t) will not make (i+1)T it 