 book), as an increasing number of users decreases the area covered owing to an increase in interference. In an OFDM system, the cell overlay technique is used (similar to that in GSM), thereby reducing co-channel interference. Network planning for an OFMD system is quite similar to that for GSM/GPRS. This is because frequency re-use is reintroduced (unlike in WCDMA, where the frequency re-use factor was 1, theoretically). For this reason, the power control feature in OFDM networks is KEY TECHNOLOGIES FOR FOURTH-GENERATION NETWORKS Tx antennas Rx antennas Figure 10.1 Capacity increase using MIMO antenna systems not as essential as in WDCMA networks. In WCDMA radio networks, power control and spread spectrum are required for reducing interference. In OFDM radio networks, accurate estimation of frequency offset is required. Increasing the number of transmitting and receiving antennas can increase capac- ity. Multiple-input/multiple-output (MIMO) antenna systems can be used, as shown in Figure 10.1. Network planning for OFDM networks is simpler than for CDMA networks. OFDM reduces the amount of crosstalk in signal transmissions. Thus, in a nutshell, we can see that OFDM clearly has an edge over CDMA, making it the preferred air-interface technology for future mobile networks. 10.2.2 All-IP Networks Structure The All-IP network has been tipped as the most probable technology to be synonymous with fourth-generation networks. A simplified All-IP network is shown in Figure 10.2. The most important difference between the All-IP network and existing 2G and 3G networks is in the functionality of the RNC and BSC, which is now distributed to the BTS and a set of servers and gateways. Various elements in this network are described below. IP-BTS (IP) Servers CS-Core IP-BTS External (IP) IP Networks Networks PS-Core WCDMA IP-BTS Figure 10.2 Example of an All-IP network 4G NETWORK PLANNING IP-BTS: The functionality of the IP base station in this network is more than the func- tionality of base stations seen in earlier chapters. 