RF carrier, a wide frequency band is occupied with this CDMA radio signal. In the receiver the received signal is first demodulated and then the same code is used to detect the wanted signal. The same exclusive-or opera- tion is performed in the receiver and original 10 times longer data pulses are reproduced. The resulting data at the input of the lowpass filter in the receiver is the original low-rate data. We may imagine that the receiver, using the right code, has collected the signal energy from the wide frequency band to the baseband. Mobile Communications The other signals (of other users) on the channel were generated with different codes and they are received as a random high-rate signal with a wide spectrum, as shown in Figure 5.7. Most of these disturbing signals are fil- tered out in the lowpass filter (LPF) of the receiver, whereas most of the desired low-rate data gets through the LPF. At the output of the LPF other signals are seen as noise on top of the desired data. The regenerator detects the original data and this detection is error free if noise is not too high. For proper operation the receiver has to be accurately synchronized with the transmitter and the simultaneously used codes have to be selected to minimize interference. The CDMA also requires accurate and frequent adjustment of the transmission power levels because the power of the users influences the S/N and error rate of the other simultaneous users. The CDMA principle provides many advantages compared with FDMA or TDMA systems. It utilizes radio resources more efficiently and it is not sensi- tive to multipath fading and narrowband radio disturbances. It transmits continuously with low transmission power SO the safety risk for the users of handheld phones is reduced. IS-95 CDMA System The CDMA system supports dual-mode operation just like one of the other American systems, NADC. The CDMA resources exist in the same fre- quency band with the traditional AMPS system and it occupies 41 AMPS channels (1.23 MHz). The CDMA users use thei