easurement of the communication and handoffs or handovers if required. 5.4.5.6 CDMA was selected in the early 1990s to become the main digital cellular standard in the United States. The main difference between CDMA and other technologies discussed previously is that on the radio path it does not use either FDMA or TDMA. Instead, the mobiles use the wide frequency band all of the time with the help of a unique code for each user. This unique code is used to spread the signal over a wide frequency band and to detect the wanted signal at the receiving end. This American system is also referred to as narrowband CDMA (N-CDMA) or the Interim Standard-95 (IS-95) system [1]. Operating Principle of CDMA The operating principle of CDMA radio transmission is not as easy to under- stand as FDMA or TDMA. Figure 5.7 shows a very simplified diagram of a Introduction to Telecommunications Network Engineering Transmitter Receiver Regenerator generator generator Synchronization Spectrum of the wanted data at the input of Transmitted the LPF Spectrum of signal data the disturbing and code data (other users) received with the Bandwidth Frequency same code of the LPF Received data with different Figure 5.7 Operating principle of CDMA. CDMA system. In Figure 5.7 the spreading code data rate is 10 times higher than the information data. In the actual IS-95 system the code has a more than 100 times higher data rate than the user data. The exclusive-or opera- tion is performed in the transmitter with the user data and the spreading code. The exclusive-or operation gives a high state when the data and code have different states and a low state when they are equal. In our simplified example, the bits or chips of the code and data+code are then 10 times shorter than the bits of the original data. We saw in Chapter 4 that the shorter the pulses, the wider the spectrum they have. Thus the spectrum of each bit is now 10 times wider and the spectrum of the original data is spread over a 10 times wider frequency band. After modulation with the 