CDMA (Code Division Multiple Access) has attracted much attention as a wireless transmission process capable of greatly increasing the subscriber capacity and is expected as a wireless access process for the next-generation mobile communication cellular system. However, CDMA has a problem in that a base station receiver tends to suffer interference from another user's signal which is making simultaneous access with the same carrier, and a mobile station receiver is liable to cause interference due to a signal set to another user. One approach to remove such interference is the use of an array antenna. An array antenna has a plurality of antennas for receiving signals and combining complex-weighted values thereof to control the amplitudes and phases of the received signals for thereby forming a directivity beam to receive a desired user's signal and suppress other user's interference signals. A multi-beam process as one of control processes for such an array antenna is known from Document 1 shown below.
Document: JP 11-266228A
FIG. 1 is a diagram showing by way of example a conventional multi-beam transmitting/receiving apparatus disclosed in Document 1. It is assumed that the number of antennas is N, N being an integer of 2 or greater, the number of multipaths is L, L being an integer of 1 or greater, and the multi-beam transmitting/receiving apparatus is assigned to a kth user, k being an integer of 1 or greater).
As shown in FIG. 1, the conventional multi-beam transmitting/receiving apparatus, which is assigned to the kth user, comprises N antennas 1-1 through 1-N, duplexers 2-1 through 2-N, L signal processing means 40-1 through 40-L corresponding to L multipaths, adder 10, decision unit 11, searcher 12, reception multibeam controller 13, maximum signal power selector 14, transmission multibeam controller 15, and transmission beam former 16.
L signal processing means 40-1 through 40-L corresponding to L multipaths are identical in construction to each other. L signal processing means 40-1 through 40-L comprise delay units 3-1 through 3-L, despreading circuits 4-1-1 through 4-L-N, reception beam formers 5-1 through 5-L, channel estimating circuits 6-1 through 6-L, complex conjugate circuits 7-1 through 7-L, signal power measuring units 8-1 through 8-L, and multipliers 9-1 through 9-L.
Searcher 12 generates delay profiles for respective preset beams, using reception signals received through N antennas 1-1 through 1-N and duplexers 2-1 through 2-N. Searcher 12 detects delay times (path timing) of L multipaths from the generated delay profiles for respective preset beams, indicates the timing of the detected delay times of L multipaths to delay units 3-1 through 3-L, and indicates the beam numbers of beams, with respect to which the delay times of multipaths are detected, to reception multibeam controller 13.
A beam refers to a directivity pattern formed by assigning a preset antenna weight to each of antennas 1-1 through 1-N, and a beam number refers to a number for specifying one of the preset beams.
Delay units 3-1 through 3-N delay reception signals received by N antennas 1-1 through 1-N by certain times based on multipath delay times set by searcher 12, thereby delaying the reception signals depending on the L multipaths to differentiate the L multipaths as a first path to an Lth path. The reception signals output from delay units 3-1 through 3-N are despread by despreading circuits 4-1-1 through 4-L-N, and thereafter sent to reception beam formers 5-1 through 5-L.
Reception multibeam controller 13 selects the reception antenna weights of beams corresponding to the beam numbers indicated by searcher 12, sends the selected reception antenna weights to reception beam formers 5-1 through 5-L, and indicates the beam numbers to maximum signal power selector 14.
Reception beam formers 5-1 through 5-L weights and combines the signals which have been delayed by delay units 3-1 through 3-N and despread by despreading circuits 4-1-1 through 4-L-N, using the reception antenna weights indicated by reception multibeam controller 13.
The above operation of searcher 12 to indicate the timing of the delay times of the multipaths to delay units 3-1 through 3-L of signal processing means 40-1 through 40-L and also to indicate the beam numbers to reception multibeam controller 13 to enable them to start signal processing, is referred to as the assignment of fingers. A path detecting method disclosed in Document 2 entitled “Path detecting method, path detecting apparatus, and array antenna reception apparatus” may be used as a path detecting process performed by searcher 12.
Document 2: JP-2002-232326A
FIG. 2 is a block diagram of each of reception beam formers 5-1 through 5-L. Reception beam formers 5-1 through 5-L are identical in construction to each other, and each comprise multipliers 20-1 through 20-N, adder 21, and complex conjugate circuits 22-1 through 22-N. For the sake of brevity, signal processing means 40-1 will be described by way of example below.
Complex conjugate circuits 22-1 through 22-N of reception beam former 5-1 generate complex conjugates of reception antenna weights selected by reception multibeam controller 13 and sends the generated complex conjugates to multipliers 20-1 through 20-N. Reception signals that have been despread by despreading circuits 4-1-1 through 4-1-N are multiplied by the complex conjugates of reception antenna weights which have been generated by complex conjugate circuits 22-1 through 22-N, by multipliers 20-1 through 20-N. The multiplied reception signals are then added together by adder 21. Therefore, the reception signals are weighted and combined. The output from adder 21 is sent to channel estimating circuit 6-1, signal power measuring unit 8-1, and multiplier 9-1. Thus, reception beam former 5-1 controls the amplitudes and phases of reception signals from antennas 1-1 through 1-N to receive a reception signal with the directivity of a beam that has been formed in a certain direction.
Channel estimating circuit 6-1 estimates a channel distortion using the output from reception beam former 5-1, and sends the estimated channel distortion to complex conjugate circuit 7-1. Complex conjugate circuit 7-1 generates a complex conjugate of the channel distortion estimated by channel estimating circuit 6-1. Multiplier 9-1 multiplies the complex conjugate of the channel distortion which has been generated by complex conjugate circuit 7-1, by the output from reception beam former 5-1, thereby compensating for the channel distortion. The output from multiplier 9-1, which has been compensated for the channel distortion, is added by adder 10 for rake combination, and input to decision unit 11. Decision unit 11 outputs its output as the reception data of the kth user.
Signal power measuring units 8-1 through 8-L measure signal power levels averaged over a desired time, using the signals weighted and combined by reception beam formers 5-1 through 5-L, and sends the measured signal power levels to maximum signal power selector 14. Maximum signal power selector 14 selects the beam of a finger with respect to which the maximum signal power level has been obtained, using the signal power levels measured by signal power measuring units 8-1 through 8-L and the beam numbers indicated by reception multibeam controller 13, and indicates the selected beam to transmission multibeam controller 15. Transmission multibeam controller 15 selects the transmission antenna weight of a corresponding beam from the beam number of the finger having the maximum signal power level indicated from maximum signal power selector 14, and sends the selected transmission antenna weight to transmission beam former 16.
Transmission beam former 16 weights and combines transmission signals, using transmission antenna weights generated by transmission multibeam controller 15.
FIG. 3 is a block diagram of transmission beam former 16. Transmission beam former 16 comprises multipliers 23-1 through 23-N and complex conjugate circuits 24-1 through 24-N. Complex conjugate circuits 24-1 through 24-N of transmission beam former 16 generate complex conjugates of transmission antenna weights selected by transmission multibeam controller 15 and sends the generated complex conjugates to multipliers 23-1 through 23-N. The transmission data of the kth user is multiplied by the complex conjugates of transmission antenna weights which have been generated by complex conjugate circuits 24-1 through 24-N, by multipliers 23-1 through 23-N. The multiplied transmission data is then transmitted through duplexers 2-1 through 2-N from antennas 1-1 through 1-N.
Generally, the beams of a multibeam pattern are disposed so as to cover a given spatial area (e.g., a sector) as uniformly as possible. There are two ways of disposing the beams. According to one way, as shown in FIG. 4, the beams are disposed using an orthogonal multibeam pattern such that the peak direction of a beam is aligned with the null direction of another beam. According to the other scheme, as shown in FIG. 5, the beams are disposed using an equally spaced multibeam pattern such that a plurality of beams are arranged at equally spaced intervals. In FIGS. 4 and 5, the number of antennas is 6 and the number of beams is 6, with the horizontal axis representing angles in the given spatial area and the vertical axis beam gains. In the vicinity of a point of intersection between two adjacent beams, their beam gains are several dB lower than the peaks of the beams. Therefore, a desired signal coming from the direction of the point of intersection may be received with the two beams adjacent to the point of intersection, and the beam outputs may be combined with each other to compensate for a reception power level.
However, the conventional multi-beam transmitting/receiving apparatus suffers the following problems: Even if the beam of a finger having the maximum signal power level is selected using the signal power levels measured by signal power measuring units 8-1 through 8-L and the beam numbers indicated by reception multibeam controller 13, and a downlink transmission is performed using the selected beam, the transmission is not optimized. The reasons for the problem are as follows: If the user who is to transmit data is positioned near a point of intersection between two adjacent beams, then when one of the beams is selected and the data is transmitted with the selected beam, since the user is present in a position displaced off the peak direction of the beam, the direction of the beam for transmitting the data is not optimum, and the transmission tends to given interference to another user who is present in the peak direction of the beam. One solution to the above problem is to increase the number of beams of the multibeam pattern for increasing the resolution in the transmission direction. However, the solution is not practical as the amount of calculations required for searcher 12 to generate delay profiles is increased.