l from equation (2.17) into equation (2.22), the QPSK symbol error probability can be given by (2.23) Now, for QPSK Es = 2Eb, where Eb is the energy per bit; then making use of equation (2.C.3) we get the bit error rate probability PBER for the QPSK system as follows: (2.24) Here we found the PBER assuming that no Nyquist filtering was present. However ac- cording to Ref. [3], this PBER also holds when root-Nyquist filters are used at the trans- mitter and receiver under the assumption that the demodulator input energy Eb and the noise power density No are the same for both cases. 2.1.2.3 CDMA System Performance As noted earlier, CDMA systems tolerate more interference than typical TDMA or FDMA systems. This implies that each additional active radio user coming into the The UMTS Network and Radio Access Technology network increases the overall level of interference to the cell site receivers receiving CDMA signals from mobile station transmitters. This depends on its received power level at the cell site, its timing synchronization relative to other signals at the cell site, and its specific cross-correlation with other CDMA signals. Consequently, the number of CDMA channels in the network will depend on the level of total interference that the system can tolerate. As a result, the FDD mode behaves as an interference limited sys- tem, where technical design will play a key role in the overall quality and capacity per- formance. Thus, despite advanced techniques such as multi-user detection and adaptive antennas, a robust system will still need a good bit error probability with a higher level of interference. When we consider that at the cell site all users receive the same signal level assuming Gaussian noise as interference, the modulation method has a relationship that defines the bit error rate as a function of the Eb/No ratio. Therefore, if we know the performance of the signal processing methods and tolerance of the digitized information to errors, we can define the minimum Eb/No ratio for a balanced system 