Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Reexamination Certificate
1998-04-02
2001-11-06
Olms, Douglas (Department: 2661)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
C375S344000
Reexamination Certificate
active
06314083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency control device and method for frequency synchronization with a multiplex signal using OFDM (Orthogonal Frequency Division Multiplexing), a receiving device, and a communication device, and in particular to a frequency control device and method, receiving device, and communication device which are suitable for frequency synchronization with a multiplex signal having deviation not less than half a sub-carrier interval.
2. Description of Related Art
As a modulation system in digital communications, the OFDM system has found extensive practical use.
As an example of systems employing OFDM, there can be listed the EUREKA-147 SYSTEM, which is generally called DAB (Digital Audio Broadcasting), the EURELA-147 DAB System, etc. In the following, we will use the “EUREKA-147 DAB System”. In November, 1994, ITU-R (International Telecommunication Union-Radio communication Sector) admitted EUREKA-147 DAB System as System-A, and it has become an international standard. This standard is issued as “ETS 300401”.
In OFDM systems, data are multiplexed by dividing them onto a plurality of sub-carriers which are quadrate to one another. A baseband frequency of each sub-carrier is selected to be an integer multiple of a certain fundamental frequency. Assuming that one cycle of the fundamental frequency is a significant symbol duration, a product of sub-carriers that are different from each other is 0 when integrated in the significant symbol interval. In that case, it is said that these sub-carriers are quadrate.
In OFDM systems, when differences arise in frequency between sending and receiving sides, orthogonality to the other sub-carriers cannot be maintained at the time of demodulating. Owing to interference, this becomes the cause of errors in the demodulated data. As causes of the above-described frequency difference, there may be listed, for example, error and variation of an oscillation frequency of a reference oscillator on each of the sending and receiving sides, Doppler shift due to relative movement between the sending and receiving sides, and the like.
To obtain demodulated data with a smaller number of errors even when frequency difference is produced, frequency synchronization systems have been studied. For example, such a frequency synchronization system is described in “A New Frequency Synchronization Technique for OFDM Demodulators Using Guard Intervals”, (ITEJ(Institute of Television Engineers of Japan) Technical Report, Vol. 19, No. 38, pp. 13-18). This frequency synchronization system utilizes the guard interval in which a signal waveform in the significant symbol duration is repeated cyclically.
Namely, a received signal is translated into the baseband through a quadrature detection circuit, and each carrier component is then demodulated by a FFT (Fast Fourier Transform) circuit. In
FIG. 14
, (
1
) illustrates in-phase axis output of the quadrature detector. Here, an n-th OFDM symbol consists of a guard part Gn (sample number: Ng) and a significant symbol part Sn (sample number: Ns), and signal Gn in the guard interval is a copy from a part Gn′ of the significant symbol. In
FIG. 14
(
2
), the signal shown in
FIG. 14
(
1
) is delayed by the significant symbol duration.
FIG. 14
(
3
) is a result of obtaining correlation between the two signals (
1
) and (
2
), by multiplying the signals (
1
) and (
2
), and then calculating a moving average in the width of the guard interval (Ng samples). As shown in
FIG. 14
(
3
), since Gn and Gn′ have the same signal waveform, the correlation output has peaks at boundaries of symbols.
Assuming that the peak value for the correlation between the in-phase axis data and the in-phase axis data delayed by the significant symbol duration is Sii, and that a peak value for correlation between the in-phase axis data and quadrature axis data delayed by the significant symbol duration is Siq, frequency error &dgr; is obtained by the following equation:
δ
=
arctan
(
Siq
Sii
)
(
1
)
SUMMARY OF THE INVENTION
According to the above-described method of obtaining frequency error using the arc tangent function, a relation between frequency offset normalized with the sub-carrier interval and the frequency error &dgr;
0
obtained as described above is so periodic that one interval is equal to the sub-carrier interval, as shown in FIG.
15
. Accordingly, it is not possible to differentiate an interval to which a frequency offset corresponding to a frequency error &dgr; belongs. For example, as shown in
FIG. 15
, when a frequency error &dgr;
1
is obtained, the point B and point C, in addition to the point A, correspond to the frequency error &dgr;
1
. Thus, even if the true normalized frequency offset is O
A
, it cannot be differentiated from the offsets O
B
, O
C
, etc. Thus, when there exists a deviation of more than or equal to half the sub-carrier interval, it is difficult to specify its frequency offset, and it may erroneously reverse the direction of the offset.
In OFDM, however, an interval between sub-carriers is set to be narrow, and it is difficult to hold down a frequency difference to less than half of the sub-carrier interval.
For example, Mode I of EUREKA-147 DAB System defines the central frequency as 230 MHz and the sub-carrier interval as 1 kHz. Namely, the frequency corresponding to half the sub-carrier interval is 500 Hz, which means a frequency accuracy of about 2.2 PPM. However, it is not easy to hold down a frequency difference between a transmitting frequency and a receiving frequency to less than 2.2 PPM. Attaining this accuracy is inevitably accompanied by increased cost of an oscillator, and is not practical.
Further, when relative distance between transmitting and receiving sides is changed as in the case of, for example, receiving a broadcast using a receiver mounted on a moving object, frequency deviation (Doppler shift) arises due to the Doppler effect. Thus, unfavorably, relative speed between the transmitting and receiving sides is so restricted that the above-described Doppler shift is within half the sub-carrier interval.
Further, the above-described method can be applied only to an OFDM signal which includes guard intervals. When a guard interval does not exist, it is difficult to detect the frequency deviation.
Thus, a first object of the present invention is to provide a frequency control device which can identify an extent of frequency deviation of an OFDM signal and can perform frequency synchronization, even when the frequency deviation is more than or equal to half a sub-carrier interval.
A second object of the present invention is to provide a frequency control device which can detect frequency deviation of an OFDM signal and can perform frequency synchronization, even when the OFDM signal does not include a guard interval.
Further, a third object of the present invention is to provide a receiving device and a communication device which are suitable in the case where relative distance between transmitting and receiving sides is changed and Doppler shift arises that is more than half a sub-carrier interval.
To attain the above-described first and second objects, a first mode of the present invention provides a frequency control device for frequency synchronization with a multiplex signal produced by orthogonal frequency division multiplexing into a plurality of sub-carriers, comprising:
quadrature detection means for performing quadrature detection on said multiplex signal to obtain a first detection axis signal and a second detection axis signal, which are quadrate to each other;
discrete Fourier transform means for obtaining a plurality of metrics distributed in a frequency domain, by sampling respective time axis waveforms of the two detection axis signals at a predetermined sampling frequency, and by performing discrete Fourier transform on thus-sampled data;
operation control means for generating frequency change instructions in accordance with difference between distribution of the metrics
Kishimoto Takurou
Sakamoto Tadahiko
Dickstein , Shapiro, Morin & Oshinsky, LLP
Nippon Columbia Co. Ltd.
Olms Douglas
Sam Phirin
LandOfFree
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