m. Multiple Access Schemes: Teaching Telecommunications Systems to Share Problems 1. Consider the signals shown in Figure Q11.1(a), (b), (c), (d). Two users want to use the system. User 1 will send +S1 (t) for bit 1 and -s(t) for bit 0. User 2 will send +S2 (t) for bit 1 and -S2(t) for bit 0. (a) If the two users want to experience no interference with one another, which of the four signal sets should they use? Explain. (b) If the two users want to share the same channel but can tolerate a little bit of interference, which of the four signal sets would you recommend? Explain. (c) Which signal sets would you recommend they avoid? Explain. S1 (t) s2(t) Figure Q11.1 s1(t) s2(t) Four signal sets s1(t) s2(t) S (f) S2(f) A2 fm A1 = A2 Chapter Eleven 2. Propose a TDMA system which allows four users to send 148 bits at a time at a data rate of 9.6 kb/s. Explain what each user sends, when they send it, and explain why. 3. Propose an FDMA system which allows seven users to send data at a rate of 9.6 kb/s. Explain what each user sends, in what frequency bands, and explain 4. You are asked to build a DS-CDMA system to support (a) two orthogonal users and (b) four orthogonal users. Using trial and error (or any other method), determine the codes (see equation (11.10)) assigned to each user. 5. Consider the three-user CDMA system with codes as shown in Figure Q11.2. Determine if these codes are orthogonal or pseudo-orthogonal. Explain. 6. The CIMA user code is generated using Figure 11.16. Show, without skipping any lines of math, that the output signal corresponds to equation (11.18). X (1) (t) Figure Q11.2 CDMA codes x(2) (t) x(3) (t) Chapter Analog Communications ow, the last chapter, which is about something that came first. Before there were digital communications, before there were source coding and channel coding and digital modulation and equalization, there were analog communications. I will not go into the details of analog communications in this book, because in most cases it is being quickly replaced by powerful di