.2 Propose a TDMA system which allows two users to transmit data at a rate of 1 Mb/s and allows each user to send three bits at a time. Solution: What we want to do is give user 1 a time slot long enough for three bits, which is a time slot of length (E11.8) We then want to give user 2 a slot long enough for his three bits, again a slot of duration (E11.9) We'll rotate between giving user 1 a slot for her three bits and giving user 2 the slot for his three bits. In the end, we end up with a TDMA scheme as shown in Figure E11.2, where the shaded bits represent user 2's bits, and the unshaded ones represent user 1's bits. SLOT SLOT 2 SLOT 3 SLOT 4 =3.10-6 Figure E11.2 The TDMA scheme 11.4 FDMA FDMA is another way to enable multiple users to share an entire communication resource. In FDMA (short for frequency division multiple access) each user uses a different band of frequencies to communicate his or her information. An example of FDMA is shown in Figure 11.4. There, we see user k sending his information at one frequency and user j sending her information at a different frequency. If you know who you want to listen to, you tune your receiver to pick up transmissions at the desired user's frequency. This system satisfies Equation (11.6), because user k is 0 at the frequencies where user j is transmit- ting, and user j is 0 at frequencies where user k is transmitting. That makes S(k) and therefore (s) (f) S6 (f)df=0. Chapter Eleven S (k) (f) user k source frequency modulator coder shift s (k) (f) user j source modulator frequency coder shift Figure 11.4 The FDMA idea The use of FDMA is also shown in Figure 11.5. Here, we see that each user is given all the time they could ever want, but they can only communicate over a small frequency band. frequency user k Figure 11.5 The FDMA idea in the frequency-time domain 11.5 CDMA 11.5.1 Introduction CDMA is short for code division multiple access. The idea underlying CDMA was put forward by an attractive Hollywood actress, Hedy Lamarr, and composer George Antheil. CDMA itsel