ooks like a signal is sent at one frequency, then suddenly it jumps (hops) to another frequency, and then it suddenly hops to a different frequency. Hence the name frequency hopping. To keep users' signals from interfering with one another, you make sure that the frequency jumps that one user takes never collide with the frequency jumps that the other users take. Basically, the system must make sure that two users are never using the same frequency at the same time. x(k)(t) cos(W1t) cos(W2t) cos(W3t) cos(W4t) Figure 11.11 Code (shape) assigned to user k Multiple Access Schemes: Teaching Telecommunications Systems to Share In FH-CDMA, the code used by a user can be described mathematically by the equation (11.11) Here, cos represents a cosine waveform which exists over the very short period of time [0,T]]. The value w,(k) indicates the frequency of this cosine wave- form and is typically chosen from a finite set of possible frequencies. 11.5.4 MC-CDMA The final member of the exclusive three-member CDMA club is a newcomer that announced its presence in 1993, and has been slowly growing in popularity since its late arrival. Its name is multicarrier CDMA, or MC-CDMA for short. In MC-CDMA, each user is given a code that is best understood in the frequency domain. I'll explain this code in two parts. 1. The code for user k is generated by the block diagram of Figure 11.12. There you see a pulse of duration Tis put at many different frequencies. Figure 11.13 shows the code for user k in the frequency domain. Each bump in frequency represents a pulse of duration T in time. cos(Wct) cos((@c+4w)t) x(k)(t) cos(W + (N-1)-w)t) Figure 11.12 The creation of user k's code x(k)(t) for MC-CDMA (main idea) x(k)(f) fc fc+4 If fc + (N-1)^f Figure 11.13 The frequency make-up of user k's code x(k)(t) Chapter Eleven 2. Actually, the code is a little different than that explained in part 1. Figure 11.14 shows the actual code given to user k. The code for user k is not only a pulse sent over the same frequencies over and over again, but a