 height of either +1 or -1. In Figure 11.9, four short pulses comprise the user's code. In general, there are N short pulses that make up a user's code. User k takes her code, say the one in Figure 11.9, called x(k) (t), and, in the simplest case, sends s(k) (t) to indicate the information bit is 1, and sends s(k) (t) indicate that the information bit is 0. x(k) (t) Figure 11.9 Code (shape) assigned to user k in a DS-CDMA system Multiple Access Schemes: Teaching Telecommunications Systems to Share Each user is given a unique code. For example, Figure 11.10 shows four codes, each one of which is given to a different user. User 1 uses the top code x(1)(t), , user 2 uses the second code x(2)(t), and SO on. For the codes in Figure 11.10, it is easy to show that for all k, j(k#jj) (11.8) Withs(t)x()(t),s(t)x(t), and SO on, this guarantees that (11.9) x(1)(t) x(2)(t) Figure 11.10 Code for user 1, 2, 3, and 4 x(3)( (t) Chapter Eleven Hence, the example of Figure 11.10 represents an example of orthogonal CDMA. In general, in DS-CDMA, the code for user k can represented as (11.10) Here, the c(k) is either 0 or 1, and hence the (-1)c(4) is either +1 or -1. This term tells us if the pulses that make up the code have an amplitude of -1 or +1. The PT (t) represents the basic pulse shape that the code is made up of-in all the examples I've been drawing, this pulse shape is a simple rectangular shape. In general, the basic pulse can take on slightly different shapes, but all of these are, approximately speak- ing, rectangular. 11.5.3 FH-CDMA Another type of CDMA, not nearly as common today as DS-CDMA but still used, is called frequency-hopping CDMA, or FH-CDMA for short. In FH-CDMA, the codes are not made up of little pulses, as in DS-CDMA, but instead they are made up of little cosine waveforms, as shown in Figure 11.11. What happens in FH-CDMA is this. The user k, using the code x(k) (t) in Figure 11.11, sends either s(k) (t) = +x(k) (t) to represent the bit 1 or s(k) to represent the bit 0. To the communication channel, it l