 +1 or -1 is applied to each frequency. User k sends his information bit of 1 by sending the code for user k multiplied by +1, and sends the information bit 0 by sending the code for user k multiplied by -1. THESE +1 and -1 values are different for different users cos(Wct) user k cos((wc+4co)t) x(k)(t) os((W) + (N-1)-w)t) Figure 11.14 The creation of user k's code x(k)(t) User j sends his information in exactly the same way-the only difference is that his code uses different +1 and -1 values to multiply each frequency. For example, user j may send his signal as +1 or -1 multiplied by the code generated as shown in Figure 11.15. If you compare Figure 11.15, which generates the code for user j's signal, to Figure 11.14, which generates the code for user k's signal, you can immediately see that the only difference is in the +1 and -1's at each frequency of the code. By carefully choosing the +1 and -1 values for each user's code, we can make sure that their codes, and therefore their transmitted signals, are orthogonal or pseudo-orthogonal (that is, they satisfy Equation (11.5) or Equation (11.7)). cos(Wct) user j cos((@c+4w)t) x(j)(t) cos ((wc + (N-1)-w)t) Figure 11.15 The creation of user j's code (t) Multiple Access Schemes: Teaching Telecommunications Systems to Share The MC-CDMA code can be expressed mathematically according to the equation (11.12) where c(k) is either 0 or 1, meaning that (-1) c is either +1 or -1; this tells us that each frequency component is multiplied by either +1 or -1. The p(t) represents the pulse of duration T sent at each of the carrier frequencies. 11.6 CIMA CIMA, short for carrier interferometry multiple access, is a novel set of multiple access techniques under development by friends Steve and Arnold at Idris Communi- cations (which also holds the patent). I've spent the last year researching it, and my graduate students and I think it is a revolutionary multiple access scheme, SO I de- cided to include it here. The easiest way to understand CIMA is to consider one simple example, 