Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Patent
1993-12-21
1997-02-11
Chin, Stephen
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
H04L 702
Patent
active
056028814
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a receiver for a digital communication system. More particularly, the invention relates to an apparatus for and a method of compensating for frequency offset of a carrier in a receiver for a digital communication system using complex correlation for establishing and supervising synchronization with a received digital signal and to a phase locked loop suitably employed in the receiver.
BACKGROUND TECHNOLOGY
A variety of communication services which have been realized conventionally by analog system are now being changed into digital systems in view of efficient utilization of frequency, miniaturization of devices and economy. In a TDMA (TIME DIVISION MULTIPLE ACCESS) type digital communication system which is now employed in various countries, communication is performed by receiving and transmitting a data frame between sending and receiving stations. The data frame comprises a plurality of time slots. The receiving station detects a time slot assigned thereto among the time slots of the data frame at the start of a communication which has been transmitted from the sending station and performs synchronization with the detected time slot.
There are two synchronization processes which are described hereinafter. One is a synchronization establishing process to capture the time slot of the received data frame at the start of communication and the other is a synchronization supervising process to supervise the position of the received time slot to thereby compensate for the synchronization after the synchronization is once established to start communication. A reference signal is previously given to the receiver (this reference signal is hereinafter referred to as a synchronous word). The receiving station is required to perform complex correlation between the received signal and the synchronous word to thereby yield a correlation value.
There is a slight difference between a carrier frequency which is transmitted by the sending station and a carrier frequency which is received by the receiving station in a practically operated system (this slight difference is hereinafter referred to as a frequency offset). The phase of the received signal is varied to increase or decrease monotonically due to the frequency offset. As a result, there is a possibility that the computed correlation value is lowered and that the synchronization can not be detected correctly in the receiving station.
Accordingly, the receiving station is required to compensate for the phase variation due to the frequency offset. The receiving station normally controls the carrier frequency of the receiver so as to conform it to the transmitted carrier frequency using an automatic frequency control circuit (AFC). An example of a similar control method without using the AFC is disclosed in U.S. Pat. No. 5,121,414.
However, in a mobile communication system represented by a cellular system, signals are interfered with by obstacles which exist between a base station and a mobile station, which causes a multi-path transmission channel (i.e., a so-called multipath fading channel). In this case, the phase of each vector term of the complex correlation is varied depending on the magnitude, phase, lag, etc. of the interference wave. Accordingly, the mobile station is required to correctly detect and compensate for the frequency offset under such circumstances. This is needed for correctly detecting synchronization and at the same time for keeping the accuracy and stability of the frequency of the carrier transmitted from the mobile station.
SUMMARY OF THE INVENTION
An object of the invention is to provide a receiver, for use in a digital communication system which is capable of detecting and establishing suitable synchronization with a TDMA frame.
Another object of the invention is to provide a receiver capable of suitably compensating for a phase variation which is caused by a frequency offset.
Still another object of the invention is to provide a receiver capable of correctly detecting the
REFERENCES:
patent: 4873683 (1989-10-01), Borth et al.
patent: 5121414 (1992-06-01), Levine et al.
patent: 5245611 (1993-09-01), Ling et al.
M. Hata, "Fundamentals and Applications of Digital PLL", Trikeppus Co., Ltd., Extra Series 11, Jul. 4, 1990, pp. 17-22 w/English abstract.
Naoi Toshimichi
Shiino Haruhiro
Yamaguchi Norio
Chin Stephen
Kim Kevin
OKI Electric Industry Co., Ltd.
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