Multiplex communications – Communication over free space – Repeater
Patent
1996-08-23
1999-06-22
Pham, Chi H.
Multiplex communications
Communication over free space
Repeater
370335, 370342, 375200, H04B 1500, H04J 1302
Patent
active
059149434
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an apparatus and method for establishing acquisition of a spreading code in a CDMA transmission system which carries out multiple access by using spread spectrum in a mobile communications system.
BACKGROUND ART
In a direct sequence (DS) CDMA transmission, information data is transmitted after it undergoes a primary modulation, followed by a secondary modulation which spreads the primary modulated signal with a high rate spreading signal. At a receiving side, the wideband received signal (spread signal) is first converted into the original narrow-band signal through a process called despreading, and then undergoes a usual demodulation. The despreading at the receiving side is carried out by detecting correlation between the received signal and a replica of the spreading code synchronized in phase with the spreading code in the received signal. Accordingly, it is necessary for recovering the information data to synchronize the spreading code replica with the spreading code in the received signal.
The synchronization roughly falls into two processes: acquisition and tracking. The acquisition captures the phase difference between the received spreading code and the spreading code replica within a range sufficiently narrower than .+-. one chip because the autocorrelation of a quadrature code used as the spreading code can usually be obtained only within .+-. one chip. Then, the tracking keeps the phase difference between the two codes within this range. Since the present invention relates only to the acquisition, a conventional acquisition method will be described below.
The acquisition of the spreading code is generally carried out as follows: First, the correlation between the received signal and the spreading code replica is taken by multiplying the two codes, and by integrating the product for one spreading code period. Next, the establishment of the acquisition is decided by square-law detecting the correlation output followed by deciding whether or not the detection output exceeds a threshold. Since the phase of the carrier is not synchronized between the transmission side and the reception side, it is difficult to know the phase of the carrier before despreading. Thus, the non coherent method is used in which the effect of the carrier phase is removed by square-law detecting the correlated and integrated signal before performing the threshold decision.
The correlation of the acquisition can usually be detected using a sliding correlator carrying out time integral, or a matched filter performing space integral. The matched filter is an FIR filter (transversal filer) having the tap number corresponding to the processing gain and using the spreading code as tap coefficients. Since the matched filter achieves the correlation detection between the spreading code in the received signal and the spreading code replica at once by using the space integral, the acquisition is achieved in a fraction of time. However, the matched filter has taps whose number corresponds to the processing gain. In addition, the correlation detection must be performed at a rate at least twice the chip rate because it is necessary for the matched filter to adjust its sampling timing at a peak position with the band-limited received signal. Thus, the matched filter requires the tap number twice the processing gain, which presents a shortage that its circuit scale becomes larger than that of the sliding correlator.
The present invention relates to the acquisition of a long code with a very long period as compared with an information symbol, and hence it is considered very difficult to realize by using the matched filter. Thus, the acquisition with the sliding correlator will be considered in connection with the present invention.
The correlation is calculated between the spreading code replica and the received signal including noise, and the correlation output undergoes an envelope detection through a square-law detector. The detected output is integrated for a dwell time .tau. in an i
REFERENCES:
patent: 5204876 (1993-04-01), Bruckert et al.
patent: 5631922 (1997-05-01), Sekine et al.
patent: 5638376 (1997-06-01), Miki et al.
patent: 5712869 (1998-01-01), Lee et al.
patent: 5717713 (1998-02-01), Natali
Adachi Fumiyuki
Higuchi Kenichi
Sawahashi Mamoru
Duong Frank
NTT Mobile Communications Network Inc.
Pham Chi H.
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