a mobile station in a spread spectrum communications system includes a matched filter that can be divided into segments . on initial acquisition , when a frequency deviation between the expected receiving frequency of the mobile station and the transmitting frequency of the base station is expected to be relatively large , the device can operate in a first synchronisation mode , in which the filter is used divided into segments . on searching for alternative cells , when the frequency deviation is expected to be smaller , the device can operate in a second synchronisation mode , in which the filter is used undivided . thus , in the first mode , a reduced filter length avoids the difficulties caused by frequency deviation , while , in the second mode , an increased filter length allows faster acquisition .