 D-AMPS and PDC [7]. Similar to FDMA and TDMA, CDMA also enables a base station and multiple number of mobile devices to communicate with each other at the same time. The difference is that in CDMA all users communicate using the same carrier frequency and at the same time. Inter- user interference is however mitigated through usage of a unique code in spreading narrow- band signal to a wideband spectrum. A receiver then detects its desired signal in the presence of the interfering signals using that unique code. Figure 10.19 illustrates how several users are connected using CDMA technology. A practical analogy to explain CDMA is as follows. With FDMA, one can think of two pairs of users having separate conversations, each pair conducting their separate conversations in different rooms. With TDMA the two pairs of users conduct their separate conversations in the same room but at different times. With CDMA the two pairs of users conduct their separate conversations in the same room at the same time but each pair speak using a different lan- guage. As a result, although CDMA users interfere with each other's conversations, each conversation remains comprehensible while the number of users remains sufficiently low. Because the frequency reuse factor is 1 - the same frequency may be used in all cells - the signal to interference ratio calculation must take all active users into account. That is, in the downlink the interference comes from all active base stations. Similarly in the uplink the Code channel # 1 Code channel # 2 Code channel # 3 Code channel #4 Code channel # 5 Frequency Code channel #9 Mobile 1 PDA 2 Note PC 3 Note PC 9 Base station PDA 5 Mobile 4 Figure 10.19 Resource allocation in a CDMA system Broadband Telecommunications Technologies and Management Interference sources Interference sources in the downlink in the uplink Figure 10.20 Interference sources in the downlink and the uplink of a mobile telecommunications system interference comes from all active mobile devices. These are illustrated in Figu