let's use a little math and see if indeed this equality holds: +1x(t)-y(t) dt (E11.3) (E11.4) (E11.5) 1/2+ ( - 1/2 (E11.6) 0 = 0 (E11.7) Yes, the equality holds. The two users will be able to share the channel without interfering with one another. Figure E11.1 Signals sent by user 1 and user 2 11.3 TDMA A very common type of multiple access system, and one that satisfies Equation (11.5), is called TDMA, short for time division multiple access. You can see this idea in Figure 11.2. User k has a slot of time in which to send his information, and then user j has a different time in which to send his information. In this way, the signal sent by user j (5,5 (t)) is 0 when user k's signal is non-zero; and the signal sent by user k (s(k) (t)) is 0 when user j's signal is non-zero. This makes sure that the product in the integral of Equation (11.5) is 0, and therefore the integral is 0. Chapter Eleven S.(k) (t) user k source coder modulator align s;(j)(t) user j source modulator coder align Figure 11.2 The TDMA idea The basic principle of TDMA is also shown in Figure 11.3. One user uses the entire frequency range of the communication channel for a brief period of time, then another user uses the entire frequency range of the communication channel for a brief time. An analogy could be a cocktail party where, to avoid hearing two conversations at once, one pair of people talk at one time, then another pair talk at another time, then another pair talk the time after that, then the first pair talk again, then the second pair talk again, then the third pair talk again, and SO on. It's a polite cocktail party where people share the time domain. Generally, in TDMA, users send an entire set of data symbols in their time slot, and this entire set is called a burst. For example, in a well-accepted standard called GSM, users send a burst of 148 data symbols at one time slot. frequency user k user k Figure 11.3 The TDMA idea in the frequency-time domain Multiple Access Schemes: Teaching Telecommunications Systems to Share Example 11