Method and apparatus for synchronizing a carrier frequency...

Multiplex communications – Generalized orthogonal or special mathematical techniques

Reexamination Certificate

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Details

C370S481000, C370S517000, C375S326000

Reexamination Certificate

active

06219333

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synchronization of a carrier frequency in an orthogonal frequency division multiplexing (OFDM) transmission apparatus, and more particularly, to a system capable of synchronizing a carrier frequency efficiently even if a carrier frequency offset is above the frequency bandwidth of one subchannel. This synchronization is accomplished by incorporating both a coarse mode for synchronizing an integer part of the carrier frequency offset and a fine mode for synchronizing a prime part of the carrier frequency offset when performing the synchronization of a carrier frequency in the OFDM transmission apparatus. The present application is based upon Korean Application No. 5786/1997, which is incorporated herein by reference.
2. Description of the Related Art
Digital televisions generally adopt a frequency division multiplexing (FDM) transmission system which uses one of the multiple carrier modulation methods. Such an FDM system occupies almost the same frequency bandwidth required for transmitting data on a single carrier because the data has a longer symbol period when transmitted in parallel on several subchannels. The FDM signal is generated by an inverse fast Fourier transform (IFFT) and are modulated in parallel by carriers which are orthogonal to each other.
Synchronization of the carrier frequency should be achieved between a transmitter and a receiver before performing a fast Fourier transform at the receiver. When the carrier frequency between a transmitter and a receiver is offset, the carrier frequency offset is divided into an integer part and a prime part through a normalization of the frequency bandwidth of one subchannel.
The integer part of the offset causes the signals generated by the fast Fourier transform (FFT) to be circularly shifted at a receiver, and the prime part of the offset causes the power and phase of the signals to be disturbed by an interference with the subchannels.
As described above, when the carrier frequency is offset between a transmitter and a receiver, the offset causes interference between the carriers of subchannels, and orthogonality between the subchannels cannot be maintained, thereby increasing the error rate of transmitted data.
In the OFDM transmission systems using methods which can be used in a terrestrial high definition television, an 8K discrete Fourier transform (DFT) and a 2K DFT are standard. The recovery of the carrier has been developed as the main technology of the transmission.
The existing methods for synchronization used in the OFDM transmission system synchronize the carrier frequency by considering that its phase is rotated uniformly when the carrier frequency is not synchronized.
The conventional methods for synchronization in the OFDM transmission apparatus work well when the carrier frequency offset is within the bandwidth of one subchannel. When the carrier frequency offset is, however, above the bandwidth of one subchannel, it is difficult to measure the rotational amount of the phase, and synchronization fails.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for synchronizing a carrier frequency of an orthogonal frequency division multiplexing (OFDM) transmission apparatus, wherein the method is capable of successfully synchronizing the carrier frequency even when the offset of the carrier frequency is above the bandwidth of one subchannel.
It is another object of the present invention to synchronize the carrier frequency by regularly alternating between a coarse mode for compensating an integer part of the carrier frequency offset and a fine mode for compensating a prime part of the carrier frequency offset.
According to one aspect of the present invention, a method for synchronizing a carrier frequency of the OFDM transmission system first selects one of a coarse mode for correcting an integer part of a carrier frequency offset and a fine mode for correcting a prime part of the carrier frequency offset. When the coarse mode is selected, a strength of a carrier which is received in each subchannel is calculated, the calculated strength of the carrier and a strength of a previous carrier delayed for a duration of one symbol for a predetermined period until the accumulated value reaches a predetermined threshold value is accumulated, and the integer part of the carrier frequency offset is corrected using the accumulated value. When the fine mode is selected, a pilot signal is extracted from the carrier, a phase difference between the extracted pilot signal and a previously extracted pilot signal which is delayed for a duration of one symbol is calculated, and the prime part of the carrier frequency offset within a predetermined frequency is corrected by controlling a gain of the calculated phase difference.
Preferably, the threshold value is set to be half of the strength of the received carrier.
Preferably, the integer part of the carrier frequency offset can be corrected within half of the frequency bandwidth of one subchannel.
Preferably, the phase difference is obtained by dividing an averaged value of the phase difference between each pilot signal and its previous pilot signal within one symbol duration by a predetermined value.
According to another aspect of the present invention, an apparatus for synchronizing the carrier frequency of the OFDM transmission system includes a data demodulation unit, a first mode switch, a symbol accumulation unit, a hold value detection unit, an offset detection unit, a loop filter unit, a second mode switch, and a voltage controlling oscillation unit.
The data demodulation unit uses an oscillating voltage to sample input carrier data. The first mode switch switches the demodulated carrier data between a coarse mode and a fine mode for respectively correcting an integer part and a prime part of a carrier frequency offset. The symbol accumulation unit accumulates a strength value of the carrier frequency offset of the carrier data received from the first mode switch and a strength value of the carrier frequency offset which is delayed for a duration of one symbol. The hold value detection unit detects a hold value by comparing the strength value of the accumulated carrier frequency offset until the strength value reaches a threshold value. The offset detection unit detects the frequency offset of the demodulated carrier received from the first mode switch, and obtains a phase difference between the carrier frequency offset of the present symbol and a carrier frequency offset of a previous symbol. The loop filter unit controls a gain of the phase difference. The second mode switch selects and outputs one value of the filtered phase difference and the hold value obtained by the hold value detection unit. The voltage controlling oscillation unit, which is locked to the hold value and the phase difference value selected by the second mode switch, respectively corrects the integer part and the prime part of the carrier frequency offset of the data demodulation unit, and provides the oscillating voltage to the data demodulation unit.
The data demodulation unit includes a serial-to-parallel conversion unit, a mixer, a fast Fourier transform unit and a parallel-to-serial conversion unit. The serial-to-parallel conversion unit converts the input carrier data into parallel data. The mixer mixes the parallel data and the oscillating voltage which is fed back from the voltage controlling oscillation unit. The fast Fourier transform unit extracts sample data by converting the mixed carrier data to orthogonality. The parallel-to-serial conversion unit converts the sample carrier data into serial data, and outputs the serial sample carrier data.
Preferably, the symbol accumulation unit includes an absolute value calculation unit, a first symbol delay unit and an adder. The absolute value calculation unit calculates the strength value of the carrier frequency offset of the carrier data received from the first mode switch. The first symbol delay unit delays the strength

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