Conventionally, in the CDMA mobile communication system, a RAKE reception method is used in which signal quality is improved by separating delayed multiple waves arrived from one base station through a plurality of transmission paths having delay times different from each other and synthesizing a signal by adjusting the delay times. Hereinafter, the transmission path may be referred to as a path.
In the RAKE reception method, a signal, which is delayed multiple waves in which radio waves arrived through different paths are overlapped with each other, is inputted into each of a plurality of fingers that include a correlator, a reverse diffuser, and an integrator in a mobile terminal. The finger receives an input of a reverse diffusion code delayed according to timing of each wave, and separates a carrier wave transmitted through each path from the delayed multiple waves. Thereafter, signal quality is improved by adjusting the timings of the waves and synthesizing the waves at appropriate rates.
When a difference occurs between a frequency of a carrier wave from a base station performing communication and a reference frequency in a mobile terminal, such as when the mobile terminal moves at high-speed, a follow-up control of the reference frequency of the mobile terminal is performed on the basis of a Doppler frequency component of the carrier wave frequency of the base station by so-called AFC (Auto Frequency Control) processing.
Such a series of processing is appropriately performed, so that high throughput communication can be maintained between the base station and the mobile station.
By the way, in a mobile communication system of non-HSPA (High Speed Packet Access) method, a terminal may receive signal transmission by connecting with cells of a plurality of base stations at the same time. Also, in a mobile communication system of HSPA method, for example, at outer edge of a cell, which is an active cell, a mobile terminal is easily affected by communication radio wave from another cell, and the mobile terminal may communicate with a plurality of cells at the same time. Also, when a handover occurs frequently among a plurality of cells, such as when the mobile terminal moves at high-speed, there is a possibility that the mobile terminal is connected to a plurality of cells at the same time.
In a conventional mobile terminal, control is performed using a result obtained by performing RAKE processing on Doppler frequency fluctuation components of all active cells including an HSPA serving cell, and AFC processing is performed so that the reference frequency of the mobile terminal comes close to the centroid of the frequencies of the carrier waves from base stations controlling the active cells. In particular, in a mobile terminal communicating with a plurality of cells, stable AFC processing is realized for all active cells by controlling the reference frequency of the mobile terminal to approach the centroid obtained by combining Doppler frequency fluctuation components of all the active cells. Such a conventional technique is disclosed in Japanese Laid-open Patent Publication No. 2006-115206.
When a mobile terminal of HSPA method is in an environment moving at high speed, the mobile terminal does not necessarily perform optimal AFC control for the serving cell among a plurality of cells. Therefore, in HSDPA (High Speed Downlink Packet Access), a decrease of CQI (Channel Quality Indicator) and a deterioration of block error rate in DSCH (Downlink Shared Channel) occur, and this may result in low throughput.
This is the same in a path search for a serving cell and a finger assignment to the path of the serving cell in an environment in which a mobile terminal is connected to a plurality of cells. For example, in the path search, if the scheduling of the path search is performed equally for a plurality of active cells including a serving cell and non-serving cells, the schedule may not be an optimal schedule for the serving cell. When fingers are assigned to paths of the active cells, if the fingers are assigned to the paths in descending order of received power, sufficient finger function may not be assigned to the path of the serving cell.
As described above, in an environment in which a mobile terminal is connected to a plurality of cells at the same time, according to the mobile terminal and the communication control method used in the conventional mobile communication system, optimal communication may not be performed for the serving cell, and it may result in low reception quality and low throughput.
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