gnal is then spread using a Walsh code to bring the intermediate data rate up to 307 kbps. This does not provide sufficient spreading and it would not be able to counteract the levels of interference encountered. As a result the signal is further spread using another sequence known as a PN code. This multiplication further spreads the intermediate data by a factor of 4, resulting in a data rate of 1.2288 Mbps, the same as the rate transmitted by the base station. The reason that the uplink and downlink are treated differently is that Walsh codes need to be synchronized if they are to remain orthogonal. As the signals transmitted from the mobile stations travel over different distances because of the variety of locations of the mobiles they will all arrive at slightly different times, and hence they will not be synchronized to one another. The addition of the PN code overcomes this problem, even though Walsh codes have been used for part of the spreading process in the mobile. One of the advantages of CDMA systems is associated with handover. As a result of the fact that the same frequency is used for adjacent cells, mobiles are able to receive signals from more than one base station at the same time. Normally they would reject all but the signal from the wanted base station as part of the decoding process. However, the ability to receive more than one signal is advantageous when handing over from one cell to the next. During the handover process the two adjacent cells both send the same data signal to the mobile. During the handover 272 Newnes Guide to Radio and Communications Technology process the mobile listens to both signals, and when the handover is complete the mobile returns to listening to just the one base station. This approach significantly reduces the possibility of a mobile being lost or dropping reception during handover, and it also ensures optimum voice quality when a mobile is on the edge of a cell. This form of handover is known as a 'soft' handover or handoff. While soft handover appears to be