t signals being received by the base station have to be at approximately the same level. For this reason power level information is sent by the base station to each individual mobile to enable it to set its power output satisfactorily. A further difference between CDMA systems and FDMA or TDMA systems is that base stations can operate on the same frequency. In other words the system can have a cluster size of 1. It is recognized that transmissions from neighbouring cells will cause interference and this will reduce the capacity of the system somewhat. Nevertheless there is still an improvement in performance. 270 Newnes Guide to Radio and Communications Technology (a) Frequency reuse with FDMA system (b) Frequency reuse with CDMA system Figure 10.9 Cellular system using CDMA to provide a cluster size of 1 As the number of users on a given channel increases, the level of interference naturally increases and as a result there is a limit to the number of users that can be successfully supported on a given channel. Accordingly using different channels on different frequencies provides further capacity. In the case of cdmaOne, the channels are spaced 1.25 MHz apart, and in this sense there is some element of frequency division multiplexing. To understand the operation of the system it is necessary to look at the transmissions that are made by the system. First, look at the downlink transmission, i.e. the transmission from the base station to the mobile. This consists of three common channels and the traffic channels. These are created by using different orthogonal codes within the main overall channel to or from the base station. The three common channels consist of the pilot channel used to give the mobile an estimate of the path loss between it and the base station. In this way it can set its power level accordingly. It also carries synchroniza- tion information, as well as frequency offset information to enable the mobile to correct the receiver frequency. The second control channel is the synchronization channel. 