f was made possible by improvements in technology in the late 1980s and was designed by a giant company which has a football stadium named after them in San Diego, Qualcomm. Multiple Access Schemes: Teaching Telecommunications Systems to Share The idea behind CDMA is this. Give user k a unique shape, called a signature waveform or a code, and give user j a different unique shape, also called a signature waveform or a code. Make sure that the code you give user k and the code you give user j are carefully chosen to ensure that Equation (11.5) or Equation (11.7) is satisfied. For example: 1. Let's say that we give user j the shape x(j) (t) shown in Figure 11.6(a). User j sends si )=+x(+) to say the bit sent is 1, or he sends (s) )=-(t) = to say the bit sent is 0, as shown in Figure 11.6(b). x(j)(t) (i+1)T +x(j)(t) -x(i)(t) si(j)(t)= (i + 1)T (i+1)T -0,5' Figure 11.6 (a) Code (shape) given to user j (b) Possible signals sent by user j Chapter Eleven 2. Let's also say that we give user k the shape x(k)(t) shown in Figure 11.7(a). User k sends s(k) (t) = xx (k) (t) to say the bit sent is 1, or he sends s(k) (t) -x(k)(t) to say the bit sent is 0, as shown in Figure 11.7(b). 3. Now, you can easily show = 0 for any s(k) (t) and s(6) (t) , which tells us that this choice satisfies Equation (11.5). That means that user j and user k can send these two signals and user j's signal will not appear in user k's receiver (and vice versa). x(k)(t) (i + 1)T +x(k)(t) -x(k)(t) s;(k)(t)= after (i+1)T (i + 1)T Figure 11.7 (a) Code (shape) given to user j (b) Possible signals sent by user k Multiple Access Schemes: Teaching Telecommunications Systems to Share 4. In this particular example, it is easily shown with a graphic how giving user k the code x(k) (t) and giving user j the code x(i) (t) allows us to send both user k and user j's signal and not experience any interference. Consider the example of Figure 11.8. There you see a signal sent by user j, and a signal sent by user k, and you see the combining of user k and user j's signal