. You can see from the com- bined signal that you can still tell what user k and user j sent: (1) user k's information is 1 if the combined signal is above the x-axis, and it is 0 if the com- bined signal is below the x-axis; (2) user j's information is 1 if the combined signal slopes upward, and his information is 0 if the combined signal slopes downward. So, by giving each user a carefully chosen code, you can send signals at the same time and at the same frequency, but still have a way to separate users. user k user j user k + user j Figure 11.8 How assigning codes (shapes) lets you determine each user's signal Chapter Eleven CDMA is a great idea, because with it you have users sending information at the same time, and over the same frequencies, but you can still perfectly separate users. An analogy to CDMA could be that of being at a cocktail party filled with two humans and two aliens. The humans and aliens talk at the same time and over the same fre- quencies, but the human communication is completely undetectable to the aliens and what the aliens speak is completely unnoticeable to the humans. There are two classes of CDMA. The first class is called orthogonal CDMA. In orthogonal CDMA, user j's and user k's signals satisfy Equation (11.5)-that is = 0. The second class is pseudo-orthogonal CDMA, where user j's and user k's signals instead satisfy Equation (11.7). In this case, a little bit of user j's signal appears in user k's signal, but just a very little, not enough to significantly affect the performance of the user k's receiver. When you use pseudo-orthogonal CDMA, you can support more users than with orthogonal CDMA, TDMA, or FDMA. 11.5.2 DS-CDMA There are three different types of CDMA, distinguished by the codes given to each user. The first and most-used form of CDMA is called direct sequence CDMA, or DS- CDMA for short. In DS-CDMA, each user is given a code like the one shown in Figure 11.9. As you can see in that figure, each code consists of short pulses of duration T, and each short pulse has a