Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1999-11-17
2001-06-26
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
C375S355000
Reexamination Certificate
active
06252908
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and a device for recovery of sampling rate and symbol rate in multi-carrier modulated communication systems, preferably using copper wires as a transmission medium.
STATE OF THE ART
Multi-carrier modulation is a known method for transmitting broadband information over copper wire or radio connections. The information may be, for example, video, Internet or telephony. Very briefly explained, for example, the bits of a digitally encoded video signal that are to be transmitted, are encoded as complex numbers in a transmitter, before an Inverse Fast Fourier Transform (IFFT) is carried out.
The IFFT gives, in the modulation, a sum of orthogonal carriers or tones, the amplitudes and phase displacement of which are determined by the values and phases of the complex numbers. These carriers are then transmitted in time slots at constant time intervals and are called symbols. In a receiver a Fast Fourier Transform (FFT) is carried out instead. In this way, the original bits are retrieved. Attenuation and phase displacement may be easily compensated for, by multiplication by a complex number for each carrier.
Two similar methods in the above mentioned technology are Orthogonal Frequency Division Multiplex (OFDM), used in radio applications, and Discrete Multitone (DMT), which is used in copper wires.
In both cases the receiver must be able to adjust the correct sampling rate and to determine the beginning and the end of the transmitted symbols.
In WO 95/03656 OFDM is used. To adjust the symbol rate, a transmitter transmits synchronization frames at known intervals, that is, synchronization symbols having a pseudo random sequence of known frequencies and phase displacements, and also known time intervals in special time slots. The receiver carries out a number of FFT calculations over the position in time in which the synchronization frame is presumed to be found. For each FFT calculation a cross correlation calculation is made in the frequency plane, using the known frequency function of the synchronization frame. The correlation maximum is detected, which determines the time slot containing the synchronization frame.
SUMMARY OF THE INVENTION
The problem associated with transmitting synchronization symbols at known intervals is that it takes up time in which data could have been transmitted. Also, a complex procedure of cross correlation calculations is required to detect and analyze the synchronization symbols.
The object of the present invention is to solve the above problem by transmitting training symbols before the start of a data transmission. Each training symbol comprises at least a period of a pilot tone and is transmitted using 180° phase jumps between the symbols. The use of this simple training symbol makes it easy to detect the beginning and the end of the symbol. An FFT calculated over the length of a symbol gives the value zero at a maximally erroneous position, that is, with the phase jump in the middle of the calculation, and a maximum/minimum at the ideal position, that is, half way between two phase jumps. The simplest method is probably to look for the position in which the result of the FFT calculation is zero and then move a distance of half a symbol.
An advantage of the present invention is that the symbol rate may be restored in a fast and simple way even before the beginning of a data transmission. During the transmission it may then be sufficient to use a method known in the art for retrieving the sampling rate, because if something locks the sampling rate, the symbol rate is automatically kept constant. Another advantage is that the inventive method is simple and inexpensive to implement.
The invention will be described in more detail in the following, by means of preferred embodiments and with reference to the enclosed drawings.
REFERENCES:
patent: 3815034 (1974-06-01), Kato
patent: 4849989 (1989-07-01), Kamerman
patent: 5206886 (1993-04-01), Bingham
patent: 5901180 (1999-05-01), Aslanis et al.
patent: 0683576A1 (1995-11-01), None
patent: 2 153 186 (1985-08-01), None
patent: 2-76430 (1990-03-01), None
patent: 95/03656 (1995-02-01), None
patent: 95/19671 (1995-07-01), None
Burns Doane Swecker & Mathis L.L.P.
Pham Chi
Phu Phuong
Telefonaktiebolaget LM Ericsson (publ)
LandOfFree
Method and device in a communication system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device in a communication system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device in a communication system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2469190