1. Field of the Invention
The present invention relates to a mobile communication system making use of CDMA (Code Division Multiple Access) as an access method and, more particularly, to a control method implemented on the side of a mobile station when it is registered with a wireless base station in a coverage area where the mobile station exists. Also, the invention relates to the mobile station utilizing this control method. The invention is especially adapted for power savings in the mobile station.
2. Description of the Related Art
In a CDMA system, a signal is modulated by multiplying a pseudo noise codes (PN codes) which are spreading codes so as to form a spread spectrum communication signal and is transmitted from a base station to a mobile station. When a CDMA mobile station receives the signal from the base station, the mobile station demodulates the original signal by multiplying the PN codes to the received signal. This demodulation is called despreading. For this purpose, synchronization is necessary to bring the phase of PN codes into agreement with the phase of PN codes of the base station.
The process of synchronization consists of two steps: acquisition of a pilot signal and holding of the synchronization. The acquisition is generally based on calculation of correlation. A replica PN code is multiplied by a received signal while shifting the replica PN code in spreading chip increments. A decision is made as to whether a value obtained by the integration is in excess of a threshold value. If the replica PN code is not synchronized to the PN code of the base station, no peak is produced on the value obtained by the integration. Therefore, the search is continued while varying the phase of the replica PN code. In a typical CDMA system, the code length of each pilot PN code is 215, i.e., 32768. Since the search is conducted within this phase space, it is required to acquire pilot signals quickly. On the other hand, the holding of the synchronization expresses a process where synchronization within chips in terms of phases is maintained after spreading chip synchronization is acquired.
Plural base stations or sectors within a mobile communication system are so selected and positioned that they are different from each other in spreading code phase. In the typical example described above, the base stations or sectors are spaced from each other such that they achieve integral multiples of 64 PN chips. After the synchronization processing, the mobile station adjusts the phases of the despread codes and examines the strengths of other pilot signals. In this way, a handoff to a better pilot signal can be performed. A handoff performed in idle state is known as idle handoff.
The mobile station that has finished the synchronization processing performs registration. This registration is carried out by exchanging message with a base station. This permits the network to call the mobile station. Since the registration involves transmission, if registration is frequently done, then traffic on the network increases. Also, the battery power in the mobile station is consumed. Accordingly, the registration needs to be carried out minimally.
When the power supply of a mobile station is turned on, it is synchronized to a pilot signal as described above. Then, the mobile station receives access messages and makes a decision as to whether the mobile station needs to be registered newly with the coverage area where the station is currently present. If the current coverage area agrees with any one of registered areas that may or may not be in the form of a list, a message that requests registration is not sent out.
When the mobile station goes out of the coverage area described above, the station is handed over to the base station of the coverage area that the mobile station enters next.
Techniques associated with the aforementioned techniques are described, for example, in U.S. Pat. Nos. 5,267,261, 5,109,390, 5,179,571, and 5,101,501 and Japanese Patent Laid-Open Nos. 30023/1993 and 107584/1996.