Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-04-23
2004-08-17
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S140000
Reexamination Certificate
active
06778591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a mobile telephone or portable telephone system (cellular system) which uses a direct sequence code division multiple access (DS-CDMA) communication method, and more particularly to a path search circuit for a base station radio apparatus.
2. Description of the Relates Art
Mobile communications systems in which a CDMA communication method is used have been developed in recent years and include systems which are based on the IS-95 standards (TIA/EIA) and have been put into practical use already and W-CDMA (Wideband Code Division Multiple Access) systems which are third generation mobile communications systems whose standardization is being proceeded in the 3 GPP (3rd Generation Partnership Project) although they have not been put into practical use as yet.
In a system based on the IS-95 standards, a spread code obtained by multiplying a PN code having a comparatively long period of 26.6 ms (80 ms/3, 32768 chips) and a Walsh code of a code length of 64 is used as a spread code for a downlink which is a link from a base station to a mobile station. As the PN code, different codes (accurately, codes shifted by predetermined number of times from the same spread code) are used by different base stations and, even in the same base station, for different sector antennae. The Walsh code of the code length of 64 is used to distinguish a plurality of channels transmitted from one sector antenna (for the CDMA, the same carrier is shared by a plurality of channels and the channels are distinguished with the spread code). A pilot channel which is not modulated with data is transmitted with a comparatively high power for each sector. For the Walsh code used in the pilot channel, the 0th code, i.e., a code of all “0”s, is used. In other words, a signal transmitted by the pilot channel is a predetermined code sequence having a period of 26.6 ms. Accordingly, a mobile station of a system which is based on the IS-95 standards uses the pilot channel to detect a peak of a cross-correlation between a predetermined code sequence of the pilot channel and a received signal to detect a path timing. The period of the spread code is 32,768 chips and is too long to determine cross-correlation coefficients at a time. Therefore, a sliding correlator is used to successively determine correlation coefficients while the received signal and a reference signal (the predetermined spread code of the pilot channel) are successively shifted in time.
A conventional reception timing detection method (chip synchronization) is disclosed in the following reference document 1, for example:
Reference document 1: Andrew J. Viterbi, “Principle of Spread Spectrum Communication”, April 1995, Chapter 3, pp. 39-66, FIGS. 3.1, 3.2 and 3.6
According to the reception timing detection method disclosed in the reference document 1, acquisition of a timing of a signal spread with a spread code, which is a pseudo random code, is performed in two stages. In particular, the acquisition is divided into two stages of initial synchronization acquisition (search) and synchronization tracking (tracking). An initial synchronization acquisition (search) method is a method of serially searching for a timing while the reception timing is successively displaced by a ½ chip interval until the correlation power exceeds a certain threshold value as recited in Chapter 3, Paragraph 4 of the reference document 1. The synchronization tracking (tracking) is performed by a method called early-late gate or DLL (Delay Lock Loop). According to the methods, a correlation power at a timing earlier by &Dgr;t of a delay time than a timing at which the signal is to be received and another correlation power at another timing later by &Dgr;t are determined, and the timing is finely adjusted so that the difference between the correlation powers may be reduced to 0.
After a path timing is detected once, it is only required that a variation of the path timing can synchronously follow up (tracking) the variation of the propagation time between a base station and a mobile station which is caused by a movement of the mobile station and the variation of the propagation time caused by a positional relationship with a reflecting object or objects in multi-path propagation paths. Therefore, a cross-correlation coefficient (which represents a delay profile of the propagation path) may be determined within the range of several microseconds to several tens microseconds before and after the present timing. Determination of a cross-correlation coefficient (delay profile) within the range limited in this manner can be realized also by a plurality of correlators which operate simultaneously. Where it can be regarded that the path timing varies continuously in time, also a timing follow-up method in which a DLL is used as disclosed in the aforementioned reference document 1 has been realized.
Various standards for the W-CDMA are recited in the following reference document 2:
Reference document 2: 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD), 3G TS 25.213 version 3.1.0, December 1999
According to the reference document 2, the W-CDMA uses a Gold code of a period of 10 ms as a spread code and a Walsh code of a period of 1 symbol (the code length varies depending upon the symbol rate). In the downlink, different Gold codes are used in different base stations and for different sectors in the same base station. In the uplink from a mobile station to a base station, Gold codes different among different mobile stations are used, and different Walsh codes are allocated to different physical channels in the same mobile stations. In both of the uplink and the downlink, a pilot symbol modulated with a predetermined code sequence is multiplexed (code multiplexed and time multiplexed).
Different from the downlink in a system based on the IS-95 standards, a pilot symbol of the W-CDMA is not spread with the same spread code (including shifted spread codes) among all base stations or all mobile stations. However, if the spread code is known, then a code sequence of the pilot symbol can be considered as a completely known code sequence. Accordingly, in the W-CDMA, a path timing can be detected by using the pilot symbol to detect a peak of a cross-correlation between a predetermined code sequence of the pilot symbol and a received signal. As a conventional path timing detection method for the W-CDMA, a “Reception Timing Detection Circuit for a CDMA Receiver” disclosed in Japanese Patent Laid-Open No. 32523/1998, for example, is known.
In the W-CDMA, however, since the chip rate is higher than that of a system based on the IS-95 standards, a variation of the multi-path paths frequently varies the path timing discontinuously for more than one chip interval (because one chip is 60 ns, it corresponds to a propagation path difference of approximately 78 m). Accordingly, sufficient path tracking cannot be achieved by means of a DLL or a like scheme that is conventionally used for synchronization tracking (tracking) in a system based on the IS-95 standards or a like system.
Meanwhile, a path search circuit which uses the conventional path timing detection method has a problem in that, where the spread ratio of a pilot symbol is high and the code length of the pilot symbol is great, if a cross-correlation coefficient is calculated in the time domain, then a very great amount of calculation is required. Furthermore, a base station must detects path timings of received signals from a plurality of mobile stations, and the prior art has another problem in that a number of identical circuits equal to the number of mobile stations must be prepared and a great amount of calculation is required.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a path search circuit which can reduce the amount of arithmetic operation required for path search and path tracking in a mobile communications system (cellular system) that uses a DS-CDMA com
Dickstein Shapiro Morin & Oshinsky LLP.
Ghebretinsae Temesghen
NEC Corporation
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