Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Patent
1996-01-22
1997-07-29
Chin, Stephen
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
375376, H04L 700
Patent
active
056527721
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and apparatus to determine synchronization parameters for time shift, phase turning and frequency drift in a broad-band digital transmission system of the OFDM type (Orthogonal Frequency Division Multiplexing). The technique which is used is that certain time slots have known synchronization frames containing tones or subcarriers in the whole frequency spectrum with known phase selected in accordance with a pseudo-random pattern (PRBS-sequences).
The receiver carries out a series of time-shifted FFT operations over the time position where the synchronization frame is calculated to he. For each FFT operation a cross-correlation is carried out in the frequency plane of the output signal with the known frequency function of the synchronization frame. The cross-correlation means that a very well-defined correlation maximum is only obtained when the FFT operation is carried out in exactly the correct time slot. The phase for the cross-correlation maximum shows clearly on the synchronization frame phase and is used for phase correction during demodulation of data. The frequency deviation which is used when regulating this can be calculated from the cross-correlation. The frequency error is obtained by reading off the position of the cross-correlation maximum in the correlation sequence.
The apparatus is provided with RAM memory, FFT processors and digital signal processors (DSP).
STATE OF THE ART
It is already known to use correlation calculations between a known and a received sequence in the frequency plane.
U.S. Pat. No. 4,577,334 shows a receiver for digital data modulated on a carrier wave signal. At the beginning of a data transmission, a training sequence with a known frequency is transmitted. The receiver contains circuits for determining certain parameters such as the phase between the transmitted training sequence and a corresponding sequence generated in the receiver. The receiver uses a DFT (Discrete Fourier Transform) filter to determine the correlation between the input signal and a locally generated frequency.
U.S. Pat. No. 4,754,449 describes a receiver which, using a Fourier transformer, receives a number of FDMA channels and transforms these into a single TDM channel. Any phase error in the incoming information is detected by allowing a symbol window to sweep forwards and backwards over the incoming symbols. An error signal is generated if a time shift is detected for an individual symbol compared to previous transmissions.
U.S. Pat. No. 4,701,939 relates to a method and equipment for obtaining synchronization between a transmitter and a receiver. Synchronization sequences are transmitted in the form of pseudo-random patterns. The received synchronization sequences are compared by an auto-correlation technique with known sequences, and in this way synchronization between the transmitter and receiver can be obtained.
U.S. Pat. No. 5,148,451 concerns a device for synchronizing a mobile unit with an incoming carrier wave. The incoming carrier wave is modulated with data and a unique sequence. This unique sequence is periodically incorporated in the data information in each frame. The unique sequence is also stored locally in the mobile units. A circuit performs a cross-correlation between the modulated unique sequence and the stored sequence. The result of the cross-correlation is used for frame and carrier wave synchronization.
It is, however, not known to perform time-shifted FFT operations and cross-correlate the result in the frequency plane and to use such synchronization methods in OFDM networks.
BRIEF DESCRIPTION OF THE DIAGRAMS
The invention will now be described in detail with reference to the attached diagrams, of which
FIG. 1 is a block diagram of the transmitter and the receiver in an OFDM system,
FIG. 2 shows the insertion of synchronization frames in accordance with the present invention,
FIG. 3 shows a diagram of the detection of a synchronization frame, and
FIG. 4 is a block diagram of the synchronization part
REFERENCES:
patent: 4095226 (1978-06-01), Kratzer
patent: 4301534 (1981-11-01), Genter
patent: 5148451 (1992-09-01), Otani et al.
patent: 5313169 (1994-05-01), Fouche et al.
patent: 5345440 (1994-09-01), Gledhill et al.
patent: 5444697 (1995-08-01), Leung et al.
patent: 5506836 (1996-04-01), Ikeda et al.
Engstrom Bo
Isaksson Mikael
Chin Stephen
Roundtree Joseph
Telia AB
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