 in this way individual transmissions can be extracted as described in Chapter 3. All the Cellular telecommunications 279 mobile stations should be received by the base station with approx- imately the same signal strength for the system to be able to operate satisfactorily. Stations further away obviously will be received with a lower strength if no adjustment is made, whereas those that are close to the base station will be much stronger. If this is left uncorrected the weaker stations will be masked out by the stronger ones. This problem, often known as the 'near-far-problem' is resolved by transmitting power control information to the mobile equipment 1500 times a second. In this way the signal from each mobile can be regulated to be at the same level as the others. In just the same way that cdmaOne and CDMA2000 were able to perform a soft handover, the same is true for a UMTS system. Close to a base station the mobile equipment communicates with only one base station. However, as it moves to the edge of a cell it is possible to communicate with two base stations because the channels are created by the use of different orthogonal codes when the CDMA signal is generated. No frequency change is required. It is therefore possible for communication to take place between the mobile and two base stations, and when communication is sufficiently reliable the mobile will only communicate with the second base station. If communication with the second base station worsens then the handover can be stopped. A hard handover only occurs if the mobile has to change to a different protocol or frequency band. This may occur if the mobile reaches the edge of UMTS coverage and needs to revert to a GSM standard. This scenario will occur frequently during the early days of system roll-out when UMTS coverage is limited. Another scenario where a hard handover is required may occur if the mobile needs to change from an FDD to a TDD system. This may occur when moving from a region served by one operator to another, or if a TDD system 