This invention relates to a mobile station device used in a digital cellular radio system such as a mobile telephone system or cordless telephone system, for example, and more particularly to a system utilizing a code division multiple access (CDMA) scheme as a radio access scheme with respect to a base station.
Recently, a spread spectrum communication system which is highly resistant to interference and disturbance has received much attention as one of the communication systems applied to a mobile communication system. The spread spectrum communication system is mainly used for realizing a cellular radio system utilizing a CDMA scheme.
The cellular radio system utilizing the CDMA scheme modulates digital audio data or image data in a device on the transmission side by use of a digital modulation system such as a PSK modulation system, for example, converts the modulated transmission data into a broad-band base band signal by use of a spread code such as a pseudorandom noise code (PN code), up-converts the frequency thereof to produce a signal of radio frequency band and transmits the same. On the other hand, a device on the reception side is so constructed as to down-convert the received signal of radio frequency band to produce a signal of intermediate frequency or base band frequency, subject the signal to the reverse spreading process by use of the same spread code as that used in the device on the transmission side, and then subject the signal to digital demodulation by use of a digital demodulation scheme such as a PSK demodulation scheme to reproduce reception data.
That is, the CDMA scheme is made to ensure channel separation between radio communications by allotting different spread codes to the radio communications between a plurality of mobile station devices and base stations.
FIG. 17 is a schematic construction diagram showing one example of a CDMA cellular radio system. In FIG. 17, a plurality of base stations BS1 to BSn are arranged in a service area, the base stations BS1 to BSn are connected to a control station CS via wire lines L1 to Ln, and the control station CS is connected to a wired communication network NW. The base stations BS1 to BSn constitute radio zones Z1 to Zn which are called cells and each of mobile stations MS1 to MSm is connected to one of the base stations which lies in a cell in which the corresponding mobile station now lies via a radio path by the CDMA scheme.
In the above system, if any one of the mobile stations MS1 to MSm moves between the cells while performing communication, so-called handoff for switching the base station to which the mobile station is connected via a radio path is performed. There are two types of handoffs: soft handoff and hard handoff.
The soft handoff is a system inherent to the CDMA cellular radio system. That is, at the time of handoff operation, the mobile station simultaneously creates a radio path between itself and a base station connected before the handoff and a radio path between itself and a base station to be connected after the handoff and makes a path diversity combination of signals received via the radio paths. After this, a base station to be connected is selected by cutting off one of the paths now subjected to the path diversity combination in which the intensity of the received electric field of the pilot channel is kept smaller than a threshold value for a preset period of time or more. Thus, the soft handoff has an advantage that the radio path selection can be smoothly made without causing hit (instantaneous interruption) of voice or the like since at least one of the two paths is always connected to a corresponding base station at the time of handoff and complete cut-off of the radio paths will not occur.
However, in order to effect the handoff, it is necessary to satisfy a condition that the base station connected before the handoff and the base station to be connected after the handoff use the same radio frequency. Therefore, for example, when the mobile station MSi moves from a cell of the station group BSa to another cell of the station group BSb or BSc in a system in which different radio frequencies f1, f2, f3 are respectively assigned to a plurality of base station groups BSa, BSb, BSc as shown in FIG. 17, the soft handoff cannot be effected.
On the other hand, the hard handoff is mainly effected in a case where application frequencies of the base station connected before the handoff and the base station to be connected after the handoff are different. That is, when it is required to change the radio frequency which the base station uses at the time of handoff, a message instructing the handoff is sent from the base station to the mobile station. When receiving the message, the mobile station temporarily interrupts the reception/transmission, creates a radio path newly allotted by the base station and starts the reception/transmission again by use of the radio path after creation of the radio path. That is, at the time of hard handoff, it is necessary to temporarily interrupt the radio path in order to change the radio frequency and newly create a radio path according to a new radio frequency.
However, in order to create the radio path between the mobile station and the base station, the mobile station first acquires a pilot channel which the base station uses for transmission so as to set up PN code synchronization. Then, after set-up of the PN code synchronization, the base station transmits synchronization information and control information by use of a sync channel and paging channel to the mobile station and then the mobile station transmits a message to the base station by use of an access channel in response to the received information. By the above signal transmission/reception, an available forward traffic channel is allotted from the base station to the mobile station, and if the mobile station can receive signals of a plurality of successive frames in good condition via the forward traffic channel, the communication state is set up between the base station and the mobile station.
Therefore, a relatively long time is required for newly creating a radio path at the time of handoff, and as a result, interruption of speaking voice or noise occurs in the process of handoff so as to degrade the speech quality. Further, a problem such as interruption of a call may occur by the failure of handoff.