Abstract:
A receiver ( 100 ) in a communication system for detecting a data signal included in a received signal transmitted by a transmitter is disclosed. The receiver comprises a channel estimator ( 101 - 1 ) for estimating a channel and outputting a channel estimation value; an interference signal removing unit ( 101 - 2, 101 - 3 ) for generating a received signal replica of a non-data signal included in the received signal using the channel estimation value, and removing the received signal replica of the non-data signal from the received signal; and a data signal detector ( 102 ) for detecting the data signal using an output from the interference signal removing unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to a receiver, a radio communication system and a receiving method for improving communication quality in CDMA signal transmissions.  
         [0002]     In mobile communication systems, CDMA (Code Division Multiple Access) systems are employed at present. In the CDMA system, a data signal and a non-data signal (such as a pilot signal and other control signals) are encoded with spreading codes on the same time and frequency domains to transmit.  
         [0003]     Since the CDMA spreading codes are orthogonal to each other, data signals can be detected at a receiver without interference if there is no multipath.  
         [0004]     However, in multipath channels, due to lack of perfect orthogonality between the spreading codes, there exists interference between the spreading codes of the data signals and the non-data signals, resulting in degraded reception quality.  
         [0005]     In order to reduce the degradation of the reception quality, a method is proposed for using a received signal replica of a non-data signal. This method is explained below with reference to  FIG. 1 .  
         [0006]     A transmitter transmits a data signal and a pilot signal by the CDMA transmission method. These signals are received at an antenna  13  of a receiver  10  through multipath channels.  FIG. 1  shows a two multipath case. In a first stage  11  in the receiver  10 , a channel estimator  11 - 2  performs channel estimation using a pilot signal and outputs a channel estimation value. A data signal detector  11 - 1  uses the channel estimation value and detects (despreading, RAKE combination, decoding, etc.) the data signal. The data signal detected by the data signal detector  11 - 1  is output to a multi-interference replica generator  11 - 3 .  
         [0007]     The multi-interference replica generator  11 - 3  uses the detected data signal and the channel estimation value to generate a received signal replica of each path&#39;s data signal. A pilot signal replica generator  11 - 4  uses a pilot signal and the channel estimation value to generate a received signal replica of each path&#39;s pilot signal.  
         [0008]     Next, the received signal replicas of the data signal and pilot signal for the first path are phase-inverted and added to the total received signal in an adder  11 - 5  to output a second path received signal output. The received signal replicas for the second path are phase-inverted and added to the total received signal in an adder  11 - 6  to output a first path received signal output.  
         [0009]     The output first path and second path received signals are input to a second stage  12 , where signal detection is performed. These processing procedures are done over plural stages to improve the quality of demodulated signals. [Non-patent Document #1] “Characteristic evaluation of Multipath interference canceller in W-CDMA downlink super high speed packet transmission”, RCS2000-167, Nov.  17, 2000. [Non-patent Document # 2] P. W. Wolniansky, G. J. Foschini, G. D. Golden, P. W. Wolniansky, “V-BLAST: An architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channels”, ISSSE-98, Pisa Italy.  
         [0010]     However, the above explained related art examples have the following problem.  
         [0011]     In the first stage in the receiver  10 , the subtraction of the received signal replica of the pilot signal is done (at the adders  11 - 5 ,  11 - 6 ) after the data signal detection at the data signal detector  11 - 1 . Accordingly, it is possible to improve data signal detection characteristics in and after the second stage, but it is impossible to improve data signal detection characteristics in the first stage due to pilot signal interference.  
       SUMMARY OF THE INVENTION  
       [0012]     A general object of the present invention is to provide a receiver, communication system and receiving method in which data signal detecting characteristics can be improved.  
         [0013]     The above object of the present invention is achieved by a receiver ( 100 ) in a communication system for detecting a data signal included in a received signal transmitted by a transmitter, comprising: a channel estimator ( 101 - 1 ) for estimating a channel and outputting a channel estimation value; an interference signal removing unit ( 101 - 2 ,  101 - 3 ) for generating a received signal replica of a non-data signal included in the received signal using the channel estimation value, and removing the received signal replica of the non- data signal from the received signal; and a data signal detector ( 102 ) for detecting the data signal using an output from the interference signal removing unit.  
         [0014]     In the above mentioned receiver, the interference signal removing unit may generate a received signal replica of at least one of a pilot signal and a control signal, and may remove the received signal replica of said at least one of the pilot signal and the control signal.  
         [0015]     The receiver may further comprise a plurality of antennas, and the channel estimator may estimate a channel for a received signal received at each of the antennas; and the interference signal removing unit may generate a received signal replica of a non-data signal included in the received signal received at each of the antennas and remove the received signal replica of the non-data signal from the received signal received at each of the antennas.  
         [0016]     The object of the present invention is achieved also by a communication system having a transmitter and a receiver ( 100 ) for detecting a data signal included in a received signal transmitted by the transmitter, the receiver comprising: a channel estimator ( 101 - 1 ) for estimating a channel and outputting a channel estimation value; an interference signal removing unit ( 101 - 2 ,  101 - 3 ) for generating a received signal replica of a non-data signal included in the received signal using the channel estimation value, and removing the received signal replica of the non-data signal from the received signal; and a data signal detector ( 102 ) for detecting the data signal using an output from the interference signal removing unit.  
         [0017]     According to embodiments of the present invention, a receiver, a communication system and a receiving method are realized in which data signal detection characteristics are improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram of a prior receiver;  
         [0019]      FIG. 2  is a block diagram of a receiver according to a first embodiment of the present invention;  
         [0020]      FIG. 3  is a flowchart illustrating a receiving procedure in the receiver according to the first embodiment of the present invention;  
         [0021]      FIG. 4  is a block diagram of a receiver according to a second embodiment of the present invention;  
         [0022]      FIG. 5  is a flowchart illustrating a receiving procedure in the receiver according to the second embodiment of the present invention;  
         [0023]      FIG. 6  is a block diagram of a receiver according to a third embodiment of the present invention;  
         [0024]      FIG. 7  is a flowchart illustrating a receiving procedure in the receiver according to the third embodiment of the present invention; and  
         [0025]      FIG. 8  is a graph showing advantage obtained by an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     The following is a description of embodiments of the present invention, with reference to the accompanying drawings.  
         [0027]     Throughout all the figures, members and parts having the same functions are assigned the same or similar reference numerals and redundant explanations are omitted.  
         [0028]     A communication system according to embodiments of the present invention comprises a transmitter and a receiver  100  as shown in  FIGS. 2, 4  or  6 .  
         [0029]     The transmitter transmits data signals and non-data signals to the receiver  100 . Based on the received signals transmitted by the transmitter, the receiver  100  detects data signals.  
         [0000]     [First Embodiment]  
         [0030]     A structure of the receiver  100  according to a first embodiment of the present invention is described below with reference to  FIG. 2 .  
         [0031]     In the receiver  100  of the communication system according to this embodiment, received signal replicas of non-data signals (such as pilot signals) included in the received signals are removed from the total received signal before the first data signal detection.  
         [0032]     The receiver  100  of the communication system according to this embodiment comprises an antenna  103 , a channel estimator  101 - 1  connected to the antenna and receiving a pilot signal, a pilot signal replica generator  101 - 2  connected to the channel estimator  101 - 1  and receiving the pilot signal, an adder  101 - 3  connected to the pilot signal replica generator  101 - 2  and the antenna  103 , and a data signal detector  102  connected to the adder  101 - 3  and the channel estimator  101 - 1 . The channel estimator  101 - 1 , the pilot signal replica generator  101 - 2  and the adder  101 - 3  form a first stage  101 .  
         [0033]     In the receiver  100 , the channel estimator  101 - 1  performs channel estimation based on the signals (including a pilot signal) transmitted from the transmitter and a pilot signal separately supplied. The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2  and the data signal detector  102 .  
         [0034]     The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path using the input channel estimation value and the pilot signal. The generated received signal replicas are input to the adder  101 - 3 .  
         [0035]     In the adder  101 - 3 , the received signal replicas of the pilot signal for the 1st and 2nd paths are removed (subtracted) from the total received signal, in order to remove interference by the pilot signal. In another words, the received signal replicas of the pilot signal are phase-inverted and added to the total received signal. The received signal from which the received signal replicas of the pilot signal have been removed is input to the data signal detector  102 .  
         [0036]     The data signal detector  102  uses the input received signal (from which the received signal replicas of the pilot signal have been removed) and the channel estimation value to detect data signals. The data signal detection can be performed in a single stage or multiple stages.  
         [0037]     As mentioned above, in the receiver  100  of the communication system according to this embodiment of the present invention, the received signal replicas of the pilot signal are subtracted before (not after) the first data signal detection. Therefore the interference by the pilot signal can be avoided and the first data signal detection characteristics can be improved.  
         [0038]     Next, a receiving procedure in the receiver  100  of the communication system according to this embodiment is explained below with reference to  FIG. 3 .  
         [0039]     The signals transmitted by the transmitter are received at the antenna  103  and input to the channel estimator  101 - 1 . The channel estimator  101 - 1  performs channel estimation based on the received signals and a pilot signal (step S 301 ).  
         [0040]     The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2  and the data signal detector  102 . The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path using the input channel estimation value and the pilot signal (step S 302 ).  
         [0041]     Each of the generated received signal replicas is input to the adder  101 - 3  per each path. In the adder  101 - 3 , the received signal replicas of the pilot signal for the 1st and 2nd paths are removed (subtracted) from the total received signal, in order to remove interference by the pilot signal (step S 303 ).  
         [0042]     The subtracted resultant signal is input to the data signal detector  102 . The data signal detector  102  uses the input received signal (from which the received signal replicas of the pilot signal have been removed) and the channel estimation value to detect data signals (step S 304 ).  
         [0000]     [Second Embodiment] A receiver of a communication system according to a second embodiment of the present invention is described below with reference to  FIG. 4 .  
         [0043]     The receiver  100  of the communication system according to this embodiment comprises an antenna  103 , a channel estimator  101 - 1  connected to the antenna and receiving a pilot signal, a pilot signal replica generator  101 - 2  connected to the channel estimator  101 - 1  and receiving the pilot signal, an adder  101 - 3  connected to the pilot signal replica generator  101 - 2  and the antenna  103 , and a data signal detector  102  connected to the adder  101 - 3 .  
         [0044]     The receiver  100  in this embodiment further comprises a control signal detector  101 - 5  connected to the antenna  103  and the channel estimator  101 - 1 , and a control signal replica generator  101 - 6  connected to the control signal detector  101 - 5 , the channel estimator  101 - 1  and the pilot signal replica generator  101 - 2 .  
         [0045]     The channel estimator  101 - 1 , the pilot signal replica generator  101 - 2 , the adder  101 - 3 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6  form a first stage  101 .  
         [0046]     Non-data signals in the actual communication system include a pilot signal already known to the receiver and other control signals. In the receiver  100 , the channel estimator  101 - 1  performs channel estimation based on the signals (including a pilot signal) transmitted from the transmitter and a pilot signal separately supplied. The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6 .  
         [0047]     The control signal detector  101 - 5  detects a control signal (despreading, RAKE combination, decoding, etc.) by using the received signal and the channel estimation value, and outputs the detected control signal to the control signal replica generator  101 - 6 .  
         [0048]     The control signal replica generator  101 - 6  generates a received signal replica of the control signal for each path using the received control signal and the channel estimation value. Each of the generated received signal replicas of the control signal is input to the adder  101 - 3  per each path.  
         [0049]     The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path using the input channel estimation value and the pilot signal. Each of the generated received signal replicas of the pilot signal is input to the adder  101 - 3  per each path.  
         [0050]     In the adder  101 - 3 , the received signal replicas of the control signal and the pilot signal for the 1st and 2nd paths are removed (subtracted) from the total received signal, in order to remove interference by the non-data signal. The received signal from which the received signal replicas of the control signal and the pilot signal have been removed is input to the data signal detector  102 .  
         [0051]     The data signal detector  102  uses the input received signal (from which the received signal replicas of the control signal and the pilot signal have been removed) to detect data signals. The data signal detection can be performed in a single stage or multiple stages.  
         [0052]     Next, a receiving procedure in the receiver  100  of the communication system according to this embodiment is explained below with reference to  FIG. 5 .  
         [0053]     The signals transmitted by the transmitter are received at the antenna  103  and input to the channel estimator  101 - 1 . The channel estimator  101 - 1  performs channel estimation based on the received signals and a pilot signal (step S 501 ). The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6 .  
         [0054]     The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path using the input channel estimation value and the pilot signal, and the control signal replica generator  101 - 6  generates a received signal replica of the control signal for each path using the input channel estimation value and the control signal (step S 502 ).  
         [0055]     Each of the generated received signal replicas of the pilot signal and the control signal is input to the adder  101 - 3  per each path. In the adder  101 - 3 , the received signal replicas of the pilot signal and the control signal for the 1st and 2nd paths are removed (subtracted) from the total received signal, in order to remove interference by the pilot signal and the control signal (step S 503 ).  
         [0056]     The subtracted resultant signal is input to the data signal detector  102 . The data signal detector  102  uses the input received signal (from which the received signal replicas of the pilot signal and the control signal have been removed) to detect data signals (step S 504 ).  
         [0000]     [Third Embodiment]  
         [0057]     A transmitter and a receiver of a communication system according to a third embodiment of the present invention are described below with reference to  FIG. 6 .  
         [0058]     The transmitter and the receiver  100  of the communication system according to this embodiment have a plurality of antennas. An explanation is given below with respect to the transmitter and receiver having two antennas.  
         [0059]     The transmitter transmits a control signal, a first pilot signal and a first data signal via a first transmission antenna, and transmits a second pilot signal and a second data signal via a second antenna.  
         [0060]     The same spreading code a is used for the first and second data signals. A spreading code b is used for the second pilot signal, and a spreading code c is used for the first pilot signal. A spreading code d is used for the control signal. These signals are encoded and multiplied on the same time and frequency domain.  
         [0061]     The receiver of the communication system according to this embodiment of the present invention further comprises a channel estimator  101 - 1  connected to the antennas  103 - 1 ,  103 - 2  and receiving a pilot signal, a pilot signal replica generator  101 - 2  connected to the channel estimator  101 - 1  and receiving the pilot signal, two adders  101 - 3 ,  101 - 4  connected to the pilot signal replica generator  101 - 2  and control signal replica generator and the antennas  101 - 1 ,  101 - 2  respectively, and a data signal detector  102  connected to the adders  101 - 3 ,  101 - 4 .  
         [0062]     The receiver  100  in this embodiment further comprises a control signal detector  101 - 5  connected to the antennas  103 - 1 ,  103 - 2  and the channel estimator  101 - 1 , and a control signal replica generator  101 - 6  connected to the control signal detector  101 - 5 , the channel estimator  101 - 1  and the pilot signal replica generator  101 - 2 .  
         [0063]     The channel estimator  101 - 1 , the pilot signal replica generator  101 - 2 , the adders  101 - 3 ,  101 - 4 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6  form a first stage  101 .  
         [0064]     The receiver  100  receives the signal transmitted by the transmitter at the antennas  103 - 1 ,  103 - 2 . In the receiver  100 , the channel estimator  101 - 1  performs channel estimation for each antenna, based on the signals (including a pilot signal) received at the antennas  103 - 1 ,  103 - 2  and a pilot signal separately supplied. The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6 .  
         [0065]     The control signal detector  101 - 3  detects a control signal using the received signal and the channel estimation value, and outputs the detected control signal to the control signal replica generator  101 - 6 .  
         [0066]     The control signal replica generator  101 - 6  generates a received signal replica of the control signal for each path per antenna, using the detected control signal and the channel estimation value. Each of the generated received signal replicas of the control signal for the first antenna  103 - 1  is input to the first adder  101 - 3  per each path. Each of the generated received signal replicas of the control signal for the second antenna  103 - 2  is input to the second adder  101 - 4  per each path.  
         [0067]     The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path per antenna, using the input channel estimation value and the pilot signal. Each of the generated received signal replicas of the pilot signal for the first antenna  103 - 1  is input to the first adder  101 - 3  per each path. Each of the generated received signal replicas of the pilot signal for the second antenna  103 - 2  is input to the second adder  101 - 4  per each path.  
         [0068]     In the first adder  101 - 3 , the received signal replicas of the control signal and the pilot signal for the 1st and 2nd paths and the first antenna  103 - 1  are removed (subtracted) from the total signal received at the first antenna  103 - 1 , in order to remove interference by the non-data signals. The received signal of the first antenna  103 - 1  from which the received signal replicas of the control signal and the pilot signal of the first antenna have been removed includes the only data signals (Data 1 +Data 2 ) and is input to the data signal detector  102 .  
         [0069]     In the second adder  101 - 4 , the received signal replicas of the control signal and the pilot signal for the 1st and 2nd paths and the second antenna  103 - 2  are removed (subtracted) from the total signal received at the second antenna  103 - 2 , in order to remove interference by the non-data signals. The received signal of the second antenna  103 - 2  from which the received signal replicas of the control signal and the pilot signal of the second antenna have been removed includes the only data signal (Data 1 +Data 2 ) and is input to the data signal detector  102 .  
         [0070]     The data signal detector  102  uses the input received signals (from which the received signal replicas of the control signal and the pilot signal of the first and second antennas have been removed) to detect the first and second data signals. The data signal detection performed in the data signal detector  102  can use a variety of detecting techniques such as V-BLAST (Vertical-Bell Laboratories Layered Space Time, see non-patent document #2). The data signal detection can be performed in a single stage or multiple stages.  
         [0071]     Next, a receiving procedure in the receiver  100  of the communication system according to this embodiment is explained below with reference to  FIG. 7 .  
         [0072]     The signals transmitted by the transmitter are received at the antennas  103 - 1 ,  103 - 2  and input to the channel estimator  101 - 1 . The channel estimator  101 - 1  performs channel estimation based on the received signals and a pilot signal (step S 701 ). The channel estimation value that has been estimated by the channel estimator  101 - 1  is input to the pilot signal replica generator  101 - 2 , the control signal detector  101 - 5  and the control signal replica generator  101 - 6 .  
         [0073]     The pilot signal replica generator  101 - 2  generates a received signal replica of the pilot signal for each path per receiving antenna, using the input channel estimation value and the pilot signal, and the control signal replica generator  101 - 6  generates a received signal replica of the control signal for each path per receiving antenna, using the input channel estimation value and the control signal (step S 702 ).  
         [0074]     Each of the generated received signal replicas of the pilot signal for the first receiving antenna  103 - 1  is input to the first adder  101 - 3  per each path. Each of the generated received signal replicas of the pilot signal for the second receiving antenna  103 - 2  is input to the second adder  101 - 4  per each path.  
         [0075]     Each of the generated received signal replicas of the control signal for the first receiving antenna  103 - 1  is input to the first adder  101 - 3  per each path. Each of the generated received signal replicas of the control signal for the second receiving antenna  103 - 2  is input to the second adder  101 - 4  per each path.  
         [0076]     In the first adder  101 - 3 , the received signal replicas of the control signal and the pilot signal for the 1st and 2nd paths and the first antenna  103 - 1  are removed (subtracted) from the total signal received at the first antenna  103 - 1 , in order to remove interference by the non-data signals. In the second adder  101 - 4 , the received signal replicas of the control signal and the pilot signal for the 1st and 2nd paths and for the second antenna  103 - 2  are removed (subtracted) from the total signal received at the second antenna  103 - 2 , in order to remove interference by the non-data signals (step S 703 ).  
         [0077]     The received signal of the first antenna  103 - 1  from which the received signal replicas of the control signal and the pilot signal of the first antenna have been removed includes the first data only and is input to the data signal detector  102 . The received signal of the second antenna  103 - 2  from which the received signal replicas of the control signal and the pilot signal of the second antenna have been removed includes the second data only and is input to the data signal detector  102 .  
         [0078]     The data signal detector  102  uses the input received signals (from which the received signal replicas of the control signal and the pilot signal of the first and second antennas have been removed) to detect the first and second data signals (step S 704 ).  
         [0079]     In signal transmission utilizing plural antennas, received signals include more interference signals compared with single antenna transmission. According to this embodiment of the present invention, the interference by non-data signals can be avoided and reception quality can be improved especially in multiple antenna transmission.  
         [0080]     Advantage obtained by the embodiments of the present invention is explained below with reference to  FIG. 8 .  
         [0081]      FIG. 8  shows Data Throughput characteristics (Mbps) versus SNR (signal-to-noise) (dB) in a communication system comprising a transmitter and receiver each having four antennas.  
         [0082]     In order to get this graph, data signal detection in the receiver is done with the BLAST technique, the channel estimation is assumed perfect in the channel estimator, and an adaptive modulation/demodulation technique is used where the transmission rate is independently chosen for each transmitting antennas.  
         [0083]     As a result, solid dots in the graph represent characteristics where the received signal replicas of the pilot signal and the control signal are removed (cancelled). Hollow triangles in the graph represent characteristics where the received signal replicas of the pilot signal and the control signal are not removed (cancelled).  
         [0084]     According to  FIG. 8 , it is understood that Data Throughput can be improved when the received signal replicas of the pilot signal and the control signal are removed (cancelled) before the data signal detection.  
         [0085]     In the above embodiments, the receiver has one or two antennas, but receivers having more than two antennas can improve data signal reception quality, and such receivers are within the scope of the present invention.  
         [0086]     In the above embodiments, the receiver receives two path signals, but receivers receiving one path signal or more than two path signals can improve data signal reception quality, and such receivers are within the scope of the present invention.  
         [0087]     Receivers, communication systems and receiving methods according to the present invention can be utilized in CDMA communication systems.  
         [0088]     The present application is based on Japanese Priority Application No. 2004-054291 filed on Feb. 27, 2004 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.