Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-07-21
2001-07-24
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06266361
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of direct sequence spread spectrum communication, in particular, to the correction of carrier frequency offset and spreading code timing offset in a direct sequence spread spectrum communication system.
BACKGROUND OF THE INVENTION
In direct sequence spread spectrum communication, before data detection, the carrier frequency offset and spreading code timing offset must be corrected to synchronize to the carrier and the spreading code. Sliding correlator has been used in prior art to correct the spreading code timing offset. However, such a way is time consuming and cannot be effectively down before carrier recovery because the received signal is affected by carrier offset.
Another method uses a matched filter for code despreading and then use a phase-lock loop to estimate the carrier frequency offset. However, the application of this prior method is limited to the condition that the carrier frequency offset is smaller than the reciprocal of the period of the spreading code. In fact, in direct sequence spread spectrum communication, the carrier frequency offset is generally larger than the reciprocal of the period of the spreading code, and thus the prior method is hardly applicable.
U.S. Pat. Nos. 4,601,005 and 4,998,111 provide a method of matching the fast Fourier transform (FFT) of spreading code with the FFT of received signals. The method of the two patents is designed for the global positioning system (GPS), and utilizing the way of increasing the points of FFT calculations of a matched filter to improve the precision of the estimate of carrier frequency offset. Such a method is practicable for data transmission rate of GPS signals of 50 bits/sec. However, the data transmission rate of a general direct sequence spread spectrum communication system, such as a code division multiple access (CDMA) system, is far larger than 50 bits/sec. Therefore, the above-mentioned method is not practicable in a general direct sequence spread spectrum communication system. Besides, the above-mentioned patents both use the output maximum peak of a matched filter as the starting point of a spreading code because there is only one main path in satellite communication channel and thus only one maximum peak will be generated. However, the transmission channel of a general direct sequence spread spectrum communication system usually has multiple paths, for embodiment, the communication channel of a CDMA system is a multipath fading channel, and thus the output maximum peak of a matched filter may not be the first-path of the channel.
Based on the disadvantages of the above-mentioned prior art and the problems associated therewith, an object of the present invention is to provide a method and apparatus for correcting carrier frequency offset in a general direct sequence spread spectrum communication system.
Another object of the present invention is to provide a method and apparatus for correcting both the carrier frequency offset and spreading code timing offset of a general direct sequence spread spectrum communication system.
A further object in the present invention is to provide a method and apparatus for the correction of carrier frequency offset in all direct sequence spread spectrum communication systems and timing offset of spreading code in said systems.
Again, a further object of the present invention is to provide a method and apparatus for wholly digitized correction of carrier frequency offset and spreading code timing offset in a direct sequence spread spectrum system, so as to be implemented with integrated circuit technology to achieve a small and compact design.
SUMMARY OF THE INVENTION
The present invention first provides that the correction of carrier frequency offset is divided into the correction of integer part of carrier frequency offset and the correction of fractional part of carrier frequency offset, wherein the integer part of carrier frequency offset is defined as an integral multiple of the reciprocal of the period of a spreading code and the fractional part of carrier frequency offset is smaller than the reciprocal of the period of the spreading code.
The integer part of the carrier frequency offset can be obtained quickly, according to the regularity thereof, by matching received signals with the spreading code whose central frequency is located at integer multiples of the reciprocal of the period of the spreading code and searching the one, of the matched results, with the largest amplitude.
Based on the fact that the fractional part of carrier frequency offset is smaller than the reciprocal of the period of the spreading code, the fractional part of carrier frequency offset can be calculated by using the phase-angle difference calculated from the matched results mentioned above.
Above described method of the present invention can simply and effectively correct random carrier frequency offsets, and can be practiced in wholly digitized manner; and can be used in combination with any prior technique of correcting timing offset of a spreading code.
In addition, for the direct sequence spread spectrum communication system with carrier frequency offset been corrected, the present invention first provide that the timing offset of a spreading code can be determined by using the fact that a channel is a minimum phase system which has the characteristic of minimum energy delay to search the one with minimum delay spread. The method of the present invention for correcting timing offset of a spreading code be implemented in a wholly digitized manner.
The above-mentioned method of the present invention for correcting carrier frequency offset and spreading code timing offset can be used in a direct sequence spread spectrum communication system in a multipath fading channel.
Some direct sequence spread spectrum communication systems have a pilot signal, and some do not. For a direct sequence spread spectrum communication system with a pilot signal or not, the method and apparatus of the present invention can utilize the same structure for correct carrier frequency offset and spread spectrum codes series timing offset.
The detail technical content and other technical features of the present invention are explained in the following description with the accompanying drawings.
REFERENCES:
patent: 4601005 (1986-07-01), Kilvington
patent: 4998111 (1991-03-01), Ma et al.
patent: 5640416 (1997-06-01), Chalmers
patent: 5732111 (1998-03-01), Walley
patent: 6005889 (1999-12-01), Chung et al.
patent: 6097770 (2000-08-01), Bahai et al.
Huang Chia-Chi
Jong Yuh-Maiw
Wang Shin-Yuan
Christensen O'Connor Johnson & Kindness PLLC
Chung-Shan Institute of Science and Technology
Corrielus Jean
Pham Chi
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