A CDMA communication system

Abstract: A CDMA communication system is disclosed which comprises: N antennas, arranged for space diversity receiving, for receiving a CDMA direct conversion radio wave signal including data to respectively generate N reception CDMA signals; a searching circuit for searching a phase of PN code in one of the N reception CDMA signals; N phase tracking circuits having N correlation detectors for tracking phases of the PN codes in the N reception CDMA signals in accordance with the searched phase to generate phase information of the PN codes, respectively; N de-spreading circuits for de-spreading the N received CDMA direct conversion signals in accordance with the phase information, respectively; and a diversity combining circuit for combining outputs of the N de-spreading circuits to provide the space diversity receiving to output the combined signal. A switch may be further provided for successively selecting one of the N reception CDMA signals periodically and supplying one of the N reception CDMA direct conversion signals to the searching circuit. The phase information is generated in various ways. For example, the larger amounts of in-phase points than a reference are used.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a CDMA communication system for receiving a direct conversion CDMA radio wave signal having a plurality of antennas arranged for a space diversity receiving. 2. Description of the Prior Art A CDMA (Code Division Multiple Access) communication system for receiving a direct conversion CDMA radio wave signal having a plurality of antennas arranged for a space diversity receiving is known. A diversity synthesis circuit for spread spectrum communication is disclosed in Japanese patent application provisional publication No. 6-14008. In such a prior art diversity synthesis circuit, a spectrum spreading signal is received by an antenna and is demodulated by a plurality of demodulators and the diversity synthesis circuit synthesizes the outputs of the demodulators. Moreover, "A Space-Path Hybrid Diversity Scheme for Base-Station Reception in CDMA Mobile Radio Communication Systems" is disclosed by Yoshio KARASAWA, Hisato IWAI, and Takayasu SHIOKAWA in TECHNICAL REPORT OF IEICE. A·P93-29, SAT 93-12(1993-5) at pages 41-47 published by THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS. In that paper, a hybrid RAKE receiver for reducing multipath fading at a base station is disclosed which comprises N antennas, N correlators supplied with spreading codes, N Tapped Delay Lines with weighting functions, and a combining circuit, wherein the fading affection is reduced by the hybrid diversity (space diversity and the path diversity). Fig. 12 is a block diagram of such a prior art RAKE receiver. In Hayashi M. et al.: "CDMA/TDD Cellular Systems Utilizing a Base-Station-Based Diversity Scheme", Proceedings of the Vehicular Technology Conference, US, New York, IEEE, vol. Conf. 45, 1995, pages 799-803, a base-station-based transmission/reception scheme for CDMA/TDD systems is proposed. By using transmission space diversity at the base station for the forward link, the transmission signal of a base station intended for a mobile station will always be transmitted in the best possible path, while the transmission signals from this same base station intended for other mobile stations will not always be transmitted in this same path. SUMMARY OF THE INVENTION The object of the present invention is to provide an improved CDMA communication system. According to the present invention, this object is achieved by a CDMA communication system according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a block diagram of a synchronizing circuit of a CDMA communication system of a first embodiment; Fig. 2 is a block diagram of the CDMA communication system of this invention; Fig. 3 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of a second embodiment; Fig. 4A is a graphical drawing of a first embodiment showing correlation outputs; Fig. 4B is a graphical drawing of the first embodiment showing a finer correlation output Fig. 5A is a block diagram of a synchronizing circuit of a CDMA communication apparatus of a third embodiment; Fig. 5B is a graphical drawing of the third embodiment showing a correlation output of the phase tracking circuit; Fig. 6A is a block diagram of a synchronizing circuit of a CDMA communication apparatus of a fourth embodiment; Fig. 6B is a graphical drawing of the fourth embodiment showing a correlation output of the phase tracking circuit Fig. 7 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of a fifth embodiment; Fig. 8 is a graphical drawing of the fifth embodiment showing a correlation output of the phase tracking circuits; Fig. 9 is a block diagram of a selection circuit of a CDMA communication apparatus of a sixth embodiment; Fig. 10 is a block diagram of a selection circuit of a CDMA communication apparatus of a seventh embodiment; Fig. 11 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of eighth to tenth embodiments; and Fig. 12 is a block diagram of a prior art RAKE receiver. The same or corresponding elements or parts are designated with like references throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION A first embodiment of the present invention will be described. Fig. 1 is a block diagram of a synchronizing circuit of a CDMA communication system of the first embodiment. Fig. 2 is a block diagram of the CDMA communication system of this invention. The CDMA communication system of the first embodiment comprises first and second antennas A and B, arranged for space diversity receiving, for receiving a CDMA direct conversion signal including data to generate first and second reception CDMA signals 101 and 102, a searching circuit (phase prediction circuit) 103 for searching (predicting) a phase of PN code in the first reception CDMA signal 101, first and second phase tracking circuits 105 and 106 having correlators 105a and 106a for tracking phases of the PN codes in the first and second reception CDMA signals 101 and 102 in accordance with the searched phase 104 to generate phase information 107 and 108 of the PN codes, respectively, a de-spreading (inverse-spectrum-spreading) circuits 209 and 210 for de-spreading the first and second reception CDMA signals 101 and 102 from the first and second antennas A and B in accordance with the phase information 107 and 108 respectively, and a RAKE combining circuit 211 for combining outputs of the de-spreading circuits 209 and 210 to output the combined signal. The first and second antennas A and B are arranged for space diversity receiving. That is, the first and second antennas A and B are arranged with a space such that there is no correlation among respective antennas in the fading variation and thus, the first and second antennas A and B are spaced about a half of a wavelength. Assuming that the CDMA direct conversion signal has a band of 2 MHz, one wavelength is 15 cm, so that the distance is about 7.5 cm. The searching circuit 103 roughly searches the phase of PN code in the first reception CDMA signal 101 within a predetermined range, for example ± one chip. The first and second phase tracking circuits 105 and 106 act so as to more accurately obtain an in-phase condition. That is, the phase tracking circuit 105 measures the PN code in the reception CDMA signal with different observation time base or different phase shift amounts to confirm the in-phase condition and trace the in-phase condition. During this operation, the phase information including in-phase point of which an amount of correlation is high and the amount is provided. Fig. 4A is a graphical drawing of the first embodiment showing correlation outputs (delayed profiles) of the searching circuit 103 with respect to respective antennas, wherein there are four paths. In the correlation outputs, there is no difference in the profiles of antennas A and B, so that phases of respect paths are the same between the antennas A and B. The searching circuit 103 searches phases for four paths. The searching circuit 103 searches the phase a 401, phase b 402, phase c 403, and phase d 404. Fig. 4B is a graphical drawing of the first embodiment showing a finer correlation output. The tracking circuit 104 observes the correlation more accurately by varying a shift amount fine. The tracking circuit 104 generates the phase information of the in-phase points and the correlation amounts of the in-phase points. Each of the de-spreading circuits 209 and 210 includes a plurality of correlators. The number of the correlators is determined by the number of the in-phase points detected by the searching circuit 103. In other words, the searching circuit 103 searches the in-phase points of which the number is equal to or less than the number of the correlators. The de-spreading circuits 209 and 210 de-spread the first and second reception CDMA signals 101 and 102 from the first and second antennas A and B in accordance with the phase information 107 and 108 respectively. That is, correlators 209a-209m de-spread the first reception CDMA signal 101 from the first antenna A with respect to the in-phase points indicated by the phase information 107 respectively. The RAKE combining circuit 211 combines outputs of the de-spreading circuits 209 and 210 such that a maximum combining ratio is provided, that is, the maximum-ratio-combing is provided to provide the path diversity. The combined signal is supplied to a demodulator 212. In the maximum-ratio-combining, the outputs of the de-spreading circuits 209 and 210 is combined with weighting. To determine the weighting coefficients, a training signal periodically transmitted is received every path and the weighting coefficients are determined such that the training signal is reproduced. As mentioned, according to the present invention, the searching is performed by one antenna because there is no difference between the delayed profiles of the antennas in the space diversity receiving. The searched phase is supplied to the tracking circuits for every antenna. The tracking circuit accurately traces in-phase points. Therefore, the searching circuit is commonly used for the tracking circuits, so that the number of the searching circuits are reduced. A second embodiment will be described. Fig. 3 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of the second embodiment. The CDMA communication apparatus of the second embodiment has substantially the same structure as that of the first embodiment. The difference is that a switch 310 is further provided. The switch 310 successively supplies either of the first CDMA reception signal 101 or the second CDMA reception signal 102 periodically in response to a switching control signal 303. Either of the first CDMA reception signal 101 or the second CDMA reception signal 102 is supplied to the searching circuit 103 which searches the phase of the PN code in the first CDMA reception signal. The searched phase is supplied to the phase tracking circuits 105 and 106 to accurately track the in-phase points to generates the phase information 107 and 108. In the first embodiment, in the case of a low relative speed, there is a possibility that the phase necessary for the antenna B cannot be caught if the searching performed only using the first CDMA reception signal because the delayed profile cannot be detected fully due to an insufficient observing interval because of the low speed. On the other hand, in the synchronizing circuit of the second embodiment, the input to the searching circuit 103 is periodically switched, so that a phase prediction error due to the low speed or the insufficient observing interval can be prevented. The cycle of the switching is determined in accordance with the moving speed of the CDMA communication apparatus, a fading variation rate, and the delayed profile observing interval. A third embodiment will be described. Fig. 5A is a block diagram of a synchronizing circuit of a CDMA communication apparatus of the third embodiment. The CDMA communication apparatus of the third embodiment has substantially the same structure as that of the first embodiment. The difference is that comparators 110 and 111 and output control circuits 109 and 112 are further provided. The comparator 110 compares the correlation amount in the phase information 107 with a reference having an absolute level. The output control circuit 109 outputs the in-phase points having the correlation amount which is larger than the reference and the correlation amount as the phase information 107. The comparator 111 and the output control circuit 112 similarly operate. Fig. 5B is a graphical drawing of the third embodiment showing a correlation output of the phase tracking circuit 105. The searching circuit searches four in-phase points 502 to 505 and the phase tracking circuits 105 and 106 detects the in-phase points more accurately. The output control circuits 109 and 112 output the phase information of only in-phase points 502 to 504 and correlation amounts at the in-phase points 502 to 504. In other words, the phase tracking circuits 105 and 106 do not output the phase information of the in-phase point 505. As mentioned in the third embodiment, the in-phase point of which the correlation amount is not larger than the reference is not outputted, so that it is prevented for the phase information having a low reliability due to the fading variation or an interference from other mobile units from being used for demodulation. The CDMA communication apparatus of the third embodiment can be modified in accordance with the second embodiment. That is, the switch 310 (Fig. 3) is provided between the antennas A and B and the searching circuit 103 shown in Fig. 5A. A fourth embodiment will be described. Fig. 6A is a block diagram of a synchronizing circuit of a CDMA communication apparatus of the fourth embodiment. The CDMA communication apparatus of the fourth embodiment has substantially the same structure as that of the first embodiment. The difference is that the phase tracking circuits 620 are used instead the phase tracking circuits 105 and 106. The phase tracking circuit comprises a correlator 610 for detecting a correlation in the CDMA reception signal, an in-phase point detection circuit 611 for detecting in-phase points in an output of the correlation detector, a max detector 612 for detecting one of the in-phase points having a largest amount of the output of the correlation detector 610, a reference level generator 613 for generating a reference level REF2 from a predetermined ratio and the largest amount, a comparator 614 for comparing amounts of the output of the correlator 610 with the reference level REF2, and an output control circuit 615 for outputting the phase information including the in-phase points showing that the amounts of the outputs of the correlation detectors are larger than the reference level REF2.The reference level generator 613 generates the reference level REF2 from a relative ratio of 50% for example and the detected largest amount as a relative reference level. Fig. 6B is a graphical drawing of the fourth embodiment showing a correlation output of the phase tracking circuit 620. The output control circuit 615 outputs the phase information of in-phase points 602 to 604 having amounts larger than the relative reference level REF2 601. A fifth embodiment will be described. Fig. 7 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of the fifth embodiment. The CDMA communication apparatus of the fifth embodiment has substantially the same structure as that of the first embodiment. The difference is that the synchronizing circuit further comprises a selection circuit 711 including a comparing circuit 714 for comparing amounts of the outputs of the correlation detectors of the phase tracking circuits and an outputting circuit 713 for outputting the phase information of a predetermined number of the in-phase points showing that the amounts of the outputs of the correlation detectors are larger than those of other in-phase points. Fig. 8 is a graphical drawing of the fifth embodiment showing a correlation output of the phase tracking circuits 105, 106, 709, and 710. The output circuit 713 outputs the phase information of four in-phase points 802 to 805 having amounts larger than those of other in-phase points. As mentioned above, providing the selection circuit 711 reduces the number of the de-spreading circuits following the selection circuit 711 because the number of the de-spreading circuits is provided by a production of the number of the antennas and the number of the in-phase points to be processed. A sixth embodiment will be described. Fig. 9 is a block diagram of a selection circuit of a CDMA communication apparatus of the sixth embodiment. The CDMA communication apparatus of the sixth embodiment has substantially the same structure as that of the fifth embodiment. The difference is that the selection circuit 912 is used instead the selection circuit 712. The selection circuit 912 comprises a max detector 913 for detecting one of the in-phase points from the phase tracking circuits 105, 106, 709, and 710 having a largest amount 917 of the outputs of the correlation detectors in the phase tracking circuits, a reference level generation circuit 914 for generating a reference level 918 from a predetermined ratio and the largest amount 917, a comparator 915 for comparing the amounts of the outputs of the correlators at the in-phase points with the reference level 902 (threshold level), and an output control circuit 916 for outputting the phase information including the in-phase points showing that the amounts of the outputs of the correlation detectors are larger than the reference level 902. The phase tracking circuits 105, 106, 709, and 710 trace five in-phase points PH1 to PH5 in accordance with the searched phases from the searching circuit 103. In Fig. 9, it is assumed that a peak 903 of the output of the correlator in the phase tracking circuit 105 shows the largest amount at the in-phase points PH5. The max detector 913 detects the peak 903 and supplies the amount of the correlator of the phase tracking circuit 105. The reference level generator 914 generates a threshold level 902 of a half of the peak value (-3 dB), that is , the amount at the peak 903. The comparator 915 and the output control circuit 916 outputs the phase information of the in-phase points of peaks 903 to 911. A seventh embodiment will be described. Fig. 10 is a block diagram of a selection circuit of a CDMA communication apparatus of the seventh embodiment. The CDMA communication apparatus of the seventh embodiment has substantially the same structure as that of the fifth embodiment. The difference is that the selection circuit 1012 is used instead the selection circuit 712. The selection circuit 1012 comprises a max detector 913 for detecting one of the in-phase points from the phase tracking circuits 105, 106, 709, and 710 having a largest amount of the outputs of the correlation detectors in the phase tracking circuits, a reference level generator 914 for generating a reference level 1002 from a predetermined ratio and the largest amount; a comparator 915 for comparing amounts of the outputs at the in-phase points with the reference level, a counter 1013 for counting the events that the amounts of the outputs at the in-phase points exceed the reference level 1002, and an output control circuit 1014 for outputting the phase information of a predetermined number (the count from the counter 1013) of the in-phase points showing that the amounts of the outputs of the correlation detectors are larger than those of other in-phase points and larger than the reference level 1002. The phase tracking circuits 105, 106, 709, and 710 trace four in-phase points PH1 to PH4 in accordance with the searched phases from the searching circuit 103. In Fig. 10, it is assumed that a peak 1003 of the output of the correlator in the phase tracking circuit 105 shows the largest amount at the in-phase point PH4. The max detector 913 detects the peak 903 and supplies the amount of the correlator of the phase tracking circuit 105. The reference level generator 914 generates the threshold level 1002 of a half of the peak value (-3 dB), that is , the amount at the peak 1003. The comparator 915 and the output control circuit 1014 only outputs the phase information of the in-phase points of peaks 1003, 1007, and 1009 using the counter 1013. An eighth embodiment will be described. Fig. 11 is a block diagram of a synchronizing circuit of a CDMA communication apparatus of the eighth embodiment which is also used in ninth and tenth embodiments. The CDMA communication apparatus of the eighth embodiment has substantially the same structure as that of the fifth embodiment. The difference is that a switch 1103 is further provided. The switch 1103 successively supplies one of the CDMA reception signals 101, 102, 703, and 704 periodically in response to a switching control signal 1102. One of the CDMA reception signals is supplied to the searching circuit 103 which searches the phase of the PN code in the CDMA reception signal. The searched phase is supplied to the phase tracking circuit 105, 106, 709, and 710 to accurately track the in-phase points to generate the phase information. In the fifth embodiment, in the case of a low relative speed, there is a possibility that the phase necessary for the antennas B to D cannot be caught if the searching performed only using the first CDMA reception signal 101 because the delayed profile cannot be detected fully due to an insufficient observing interval because of the low speed. On the other hand, in the synchronizing circuit of the eighth embodiment, the input to the searching circuit 103 is periodically switched, so that a phase prediction error due to the low speed or the insufficient observing interval can be prevented. The cycle of the switching is determined in accordance with the moving speed, a fading variation rate, and the delayed profile observing interval. A ninth embodiment will be described. The CDMA communication apparatus of the ninth embodiment is also shown in Fig. 11. The CDMA communication apparatus of the ninth embodiment has substantially the same structure as that of the eighth embodiment. The difference is that the selection circuit 912 used in the sixth embodiment (Fig. 9) is used instead the selection circuit 711 of the eighth embodiment. That is, the max detector 913 shown in Fig. 9 detects the peak 903 and supplies the amount of the correlator of the phase tracking circuit 105. The reference level generator 914 generates a threshold level 902 of a half of the peak value (-3 dB), that is , the amount at the peak 903. The comparator 915 and the output control circuit 916 outputs the phase information of the in-phase points of peaks 903 to 911. A tenth embodiment will be described. The CDMA communication apparatus of the tenth embodiment is also shown in Fig. 11. The CDMA communication apparatus of the tenth embodiment has substantially the same structure as that of the eighth embodiment. The difference is that the selection circuit 1012 used in the seventh embodiment is used instead the selection circuit 711 of the eighth embodiment. That is, the max detector 913 shown in Fig. 10 detects the peak 903 and supplies the amount of the correlator of the phase tracking circuit 105. The reference level generator 914 generates a threshold level 1002 of a half of the peak value (-3 dB), that is , the amount at the peak 1003. The comparator 915 and the output control circuit 916 only outputs the phase information of the in-phase points of peaks 1003, 1007, and 1009 using the counter 1013. A CDMA communication system is disclosed which comprises: N antennas, arranged for space diversity receiving, for receiving a CDMA direct conversion radio wave signal including data to respectively generate N reception CDMA signals; a searching circuit for searching a phase of PN code in one of the N reception CDMA signals; N phase tracking circuits having N correlation detectors for tracking phases of the PN codes in the N reception CDMA signals in accordance with the searched phase to generate phase information of the PN codes, respectively; N de-spreading circuits for de-spreading the N received CDMA direct conversion signals in accordance with the phase information, respectively; and a diversity combining circuit for combining outputs of the N de-spreading circuits to provide the space diversity receiving to output the combined signal. A switch may be further provided for successively selecting one of the N reception CDMA signals periodically and supplying one of the N reception CDMA direct conversion signals to the searching circuit. The phase information is generated in various ways. For example, the larger amounts of in-phase points than a reference are used.

Claims: A CDMA communication system comprising: N antennas (A, B), arranged for space diversity receiving, for receiving a CDMA direct conversion radio wave signal including data to respectively generate N reception CDMA signals, said N being a natural number at least equal to 2 N≥2; N phase tracking means (105, 106; 620; 709, 710) having N correlation detectors for tracking phases of the PN codes in said N reception CDMA signals in accordance with a searched phase to generate phase information of said PN codes, respectively; N de-spreading means (209, 210) for de-spreading said N reception CDMA signals in accordance with said phase information, respectively; and diversity combining means (211) for combining outputs of said N de-spreading means to provide said space diversity receiving to output the combined signal; characterized by further comprising: searching means (103) for searching the phase of PN code in only one of said N reception CDMA signals to generate the searched phase. The CDMA communication system as claimed in claim 1, further comprising switching means (301) for successively selecting one of said N reception CDMA signals periodically and supplying said one of said N reception CDMA direct conversion signals to said searching means (103). The CDMA communication system as claimed in claim 1 or 2, wherein each of said N phase tracking means (105, 106; 620; 709, 710) comprises comparing means (110, 111) for comparing an output of said correlation detector thereof with a predetermined level and outputting means (109, 112) for outputting said phase information when a level of said output exceeds said predetermined level. The CDMA communication system as claimed in claim 1, wherein each of said N phase tracking means (620) comprises in-phase point detection means (611) for detecting in-phase points in an output of said correlation detector thereof, a max detector (612) for detecting one of said in-phase points having a largest amount of said output of said correlation detector thereof, reference level generation means (613) for generating a reference level from a predetermined ratio and said largest amount, comparing means (614) for comparing amounts of said output of said correlation detector thereof at said in-phase points with said reference level, and outputting means (615) for generating said phase information including said in-phase points showing that said amounts of said outputs of said correlation detectors are larger than said reference level. The CDMA communication system as claimed in claim 1 or 2, wherein each of said N phase tracking means (105, 106; 709, 710) comprises in-phase point detection means (611) for detecting in-phase points and amounts of an output of said correlation detector thereof, said CDMA communication system further comprising comparing means (714) for comparing said amounts of said outputs of said correlation detectors of said N phase tracking means (620), and outputting means (713) for outputting said phase information of a predetermined number of said in-phase points showing that said amounts of said outputs of said correlation detectors are larger than those of other in-phase points. The CDMA communication system as claimed in claim 1 or 2, wherein each of said N phase tracking means (105, 106; 709, 710) comprises in-phase point detection means (611) for detecting in-phase points and amounts of an output of said correlation detector thereof, said CDMA communication system further comprising comparing and selection means (912) including: max detection means (913) for detecting one of said in-phase points from said N phase tracking means (105, 106; 709, 710) having a largest amount of said outputs of said N correlation detectors; reference level generation means (914) for generating a reference level from a predetermined ratio and said largest amount; comparing means (915) for comparing amounts of said outputs of said N correlation detectors at said in-phase points with said reference level; and generation means (916) for generating said phase information including said in-phase points showing that said amounts of said outputs of said N correlation detectors are larger than said reference level. The CDMA communication system as claimed in claim 1 or 2, wherein each of said N phase tracking means (105, 106; 709, 710) comprises in-phase point detection means (611) for detecting in-phase points in an output of said correlation detector thereof, said CDMA communication system further comprising comparing and selection means (1012) including: max detection means (913) for detecting one of said in-phase points from said N phase tracking means (105, 106; 709, 710) having a largest amount of said outputs of said N correlation detectors; reference level generation means (914) for generating a reference level from a predetermined ratio and said largest amount; comparing means (915) for comparing amounts of said outputs of said N correlation detectors at said in-phase points with said reference level; and generation means (1014) for generating said phase information of a predetermined number of said in-phase points showing that said amounts of said outputs of said N correlation detectors are larger than those of other in-phase points and larger than said reference level.

Inventors: MIYA KAZUYUKI, MIYA, KAZUYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H04B7
IPC codes: H04L1, H04B7, H04B1, H04J13