1. Field of the Invention
The present invention relates to a CDMA cellular radio transmission system for use in digital cellular mobile communication and the like.
2. Description of the Related Art
In digital cellular mobile communication, a multiple access method means a line connection method when a plurality of stations simultaneously communicate with one another in the same band zone. CDMA, that is, Code Division Multiple Access is a technique in which multiple access is performed by means of spectrum spread communication for transmitting an information signal spectrum by spreading the spectrum over a band sufficiently wide in comparison with the original information band width and may also be called Spectrum Spread Multiple Access (SSMA). A direct spread method is a method of multiplying an information signal directly by a spread sequence code in the spread process.
Of the detection method in digital communication, a coherent detection method has excellent static characteristics in comparison with a delay detection method and is a method in which Eb/IO necessary for obtaining a certain mean bit error rate is lowest. Moreover, a coherent detection scheme with interpolation has been proposed to compensate transmission signal distortion due to fading (Seiichi Sampei "Rayleigh Fading Compensation Method for 16QAM MODEM in Digital Land Mobile Radion Systems" JECS Paper B-11, Vol. J72-B-II No. 1 pp. 7-15, 1989). According to this method, a pilot symbol 701 is, as shown in FIG. 7, inserted in an information symbol to be transmitted with each period T so as to make wave detection by estimating a channel transfer function, that is, the circuit condition. Further, a method which apply the above method to the direct spread CDMA has been proposed (Akihiro Higashi et al., "Performance of Coherent RAKE Detection using Interpolation on DS/CDMA" JECS Technical Paper RCS94-98, 1994). There is also a pilot channel as a method enabling coherent detection in the direct spread CDMA. As shown in FIG. 8, in this method, one pilot channel 801 as a reference detection signal is continuously transmitted to be independent of channels for transmitting information data.
FIG. 4 shows the structure of a base station in a conventional CDMA cellular radio transmission system using a pilot channel. In a forward-link, transmission data (2) to (m) of mobile stations m-1 (1&lt;m) are each spread by spread codes (2) to (m), and the spread data are then multiples by weight before being multiplexed. A channel for use in transmitting the transmission data from each mobile station such as information data, control data and the like is called a "communication channel". The aforementioned weight is used for transmission power control in the forward-link so as to add weight to transmission power among transmission channels. In a pilot channel 401, on the other hand, transmission data (1) 402 is spread by a spread code (1) 403 in a spread circuit 404, and the spread data is multiplied by weight W (1). The spread data in the communication channels 406 to 411 are then multiplexed in a multiplex circuit 412, and the multiplexed data is up-converted by a radio transmission unit 413 before being transmitted from an antenna 414. At this time, since no information needs to be transmitted with the transmission data (1) 402, it may be unmodulated data (totally 0 or 1).
FIG. 5 shows the structure of a mobile station in the conventional CDMA cellular radio transmission system using a pilot channel. As shown in FIG. 5, the signal received from an antenna 501 is down-converted in a radio reception unit 502. In a pilot channel 503, a phase estimation circuit 511 detects phase information 512 from the output of a correlation circuit 505, and transfers the detected result to the detection circuit 516 of a communication channel 513. In the communication channel 513, a detection circuit 516 subjects the signal inversely spread with a spread code i (i=2 to m) in a correlation circuit 515 to coherent detection on the basis of the phase information 512, and a binary value decision circuit 517 outputs reception data 518.
On the other hand, the pilot channel can be used as information other than coherent detection. In FIG. 5, reception power for each sampling (phase) is calculated in a power detection circuit 506 from the output of the correlator of the pilot channel 503, and a chip synchronous signal 508 is output by integrating (filtering) the reception power with a predetermined time constant in a chip synchronous circuit 507. This chip synchronous signal 508 is used to determine the phase of inverse spread with a spread code (i) 514 in the correlation circuit 515 of the communication channel 513. In the case of a system wherein any other cell transmits pilot channels and the same spread code (1) 504 is used with shifting the phase, it is possible to obtain other-cell signal level information 510 from the output of the power detection circuit via an other-cell monitor circuit 509. At this time, the transmission power of the pilot channel required to obtain the chip synchronous signal 508 and the other-cell signal level information 510 can be lower than the power required for coherent detection, because the output of the power detection circuit is integrated in the chip synchronous circuit 507 and the other-cell monitor circuit 509.
Moreover, Unexamined Japanese Patent Publication (kokai) No. 7-76049 discloses that the base station controls the directivity of a transmission antenna on a communication channel to make transmission so as to deal with the suppression of interference. FIG. 6 shows the situation in which the directivity is controlled. A base station 601 transmits signals to three mobile stations 602, 603, 604 while controlling the directivity of communication channels in the three directions of A, B, C. On the other hand, a pilot channel to be transmitted as a reference signal for coherent detection needs transmitting without directivity, because the pilot channel is commonly used for the communication channels. At this time, the propagating path of the pilot channel is different from that of the communication channel. Namely, as shown in FIG. 6, paths 605, 606 refer to the situation in which the pilot channel transmitted without directivity has propagated through the path different from that of the communication channel, that is, the communication channel transmitted to have the directivity A never pass through the paths 605, 606. Therefore, the phase information of the pilot channel signal differs from that of communication channel signal.
In the conventional multiple transmission of a pilot channel, it is necessary to transmit the pilot channel with relatively strong power with respect to any other communication channel which may interfere with the pilot channel in order to increase the reliability of the pilot channel and improve coherent detection performance. More specifically, with reference to FIG. 4, the weight W (1) 405 tends to become large in value (W1&gt;Max W2 . . . Wm) as compared with the communication channel. If, however, W (1) is increased, the reliability of the reference signal for coherent detection is increased on one hand but since interference with communication channels (of any other station) is also increased, there arises a problem resulting in lowering communication channel quality.
Moreover, when the base station controls the directivity of the transmission antenna on a communication channel basis, the phase information of the pilot channel is impossible to use for making the coherent detection of the communication channel since the phase information obtained from the pilot channel differs from the detection phase of the communication channel.