Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Reexamination Certificate
1998-08-24
2001-10-16
Chin, Wellington (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
C370S203000, C370S480000, C370S484000, C375S230000, C375S260000, C375S298000
Reexamination Certificate
active
06304545
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and a circuit arrangement for the correction of phase and/or frequency errors in digital multicarrier signals which have been transmitted, in particular, using the OFDM method.
The so-called OFDM (Orthogonal Frequency Division Multiplexing) modulation method is used for transmitting terrestrial digital broadcast radio and television signals, for example in accordance with the DAB or DVB-T Standard. This method makes use of a large number of modulated carriers, which are transmitted using frequency-division multiplexers. This results in various advantages, such as improved bandwidth utilization or a reduction in interference effects from multipath reception. However, one disadvantage of the OFDM method in comparison with single-carrier modulation methods is the necessity to reproduce exactly the frequency and phase of the originally transmitted carrier signals. This causes increased sensitivity to frequency errors and phase noise, and thus places more stringent requirements on the frequency and phase purity of the oscillators used for frequency conversion in the receiver.
The phase and frequency errors can be split into two components. The so-called self-noise component in this case represents the noise component of the respective carrier, which is caused by said carrier itself and is mapped onto itself. The so-called external-noise component is, in contrast, caused by adjacent carrier interference from the so-called leakage effect of the Fast-Fourier Transformation (FFT) used in the receiver for demodulation.
A method by means of which the self-noise component of the error can be estimated and corrected has been disclosed in Robertson P., Kaiser S.: “Analysis of the Effects of Phase-Noise in Orthogonal Frequency Division Multiplex (OFDM) Systems”, ICC 1995, pages 1652-1657. In this case, a common rotation phase &psgr;
e
is determined in the receiver, after the FFT, by averaging all the carriers. All the carriers are then shifted back through this error amount, which is also called Common Phase Error (CPE), by multiplication by the factor e
−j&psgr;e
. This completely, or at least partially, corrects the self-noise component. However, the external-noise component cannot be corrected either by this method or by other known methods, even though this is highly desirable in order to improve the signal-to-noise ratio.
SUMMARY OF THE INVENTION
The invention is based on the object of specifying a method for the correction of phase and/or frequency errors in digital multicarrier signals, which method makes it possible to estimate and correct the total phase error which results from the self-noise and external-noise components. This object is achieved by the method specified in Claim
1
.
The invention is based on the further object of specifying a circuit arrangement for use of the method according to the invention. This object is achieved by the circuit arrangement specified in Claim
10
.
The correction of the CPE can be expressed by:
X
OFDM.CPE
(
n
)=
FFT{x
OFDM.CPE
(
k
)}=
X
OFDM
(
n
)·
e
−j&psgr;
e
(1)
where
X(n): carrier
x(k): sample
Subject to the precondition that the angle &psgr;
e
is small, it can be stated as an approximation that:
X
OFDM
.
CPE
(
n
)
=
X
OFDM
(
n
)
·
[
cos
(
ϕ
e
)
-
j
sin
(
ϕ
e
)
]
≈
X
OFDM
(
n
)
-
j
ϕ
e
·
X
OFDM
(
n
)
≈
X
OFDM
(
n
)
-
jFFT
{
ϕ
e
·
X
OFDM
(
k
)
}
(
2
)
It can be seen from equation (2) that a correction of the CPE that is carried out after the FFT (frequency domain, running variable n) corresponds approximately to a correction before the FFT (time domain, running variable k). For this reason, the CPE correction can be carried out after the FFT, as described by Robertson et al.
If the intention is to correct the mean frequency error rather than the CPE &psgr;
e
, then this can be determined from the integration of &psgr;
e
. This correction according to the invention is called “Common Frequency Error” in the following text, or CFE correction for short. The CFE correction is:
X
OFDM
.
CFE
(
n
)
=
FFT
{
X
OFDM
.
CFE
(
k
)
}
=
FFT
{
X
OFDM
(
k
)
·
ⅇ
-
j2
(
k
/
N
)
·
ϕe
}
(
3
)
As can be seen from equation (3), this must be carried out before the FFT. The phase error is then at least theoretically completely suppressed in the CFE correction, so that the received signal is ideally compensated.
In principle, the method according to the invention for the correction of phase and/or frequency errors in digital multicarrier signals comprises the use of a further Fourier transformation to estimate the self-noise component, and the digital signals being corrected depending on the estimated self-noise component.
This correction results in additional computation complexity or labour effort, since an FFT must be carried out twice. Nevertheless, this additional effort is acceptable since the reduction in the phase noise according to the invention makes it possible to reproduce OFDM-transmitted signals even using receivers which have a comparatively poor phase-noise response, such as PAL television receivers.
The Fourier transformation for error processing is preferably carried out before the Fourier transformation for demodulation. This allows particularly good reproduction of the carrier orthogonality, and thus a particularly clear improvement in the crosstalk response.
It is advantageous for the length of the Fourier transformation for error processing to be shorter than the length of the Fourier transformation for demodulation, since this is adequate to estimate the self-noise component, and it reduces the additional computation complexity.
It is furthermore advantageous for the length of the Fourier transformation for demodulation to be N, and for the length of the Fourier transformation for error processing to be N/M, in which case M may be a power of two.
It is particularly advantageous in this case for the complexity for the Fourier transformation for error processing to be reduced by using a 2M radix algorithm.
Furthermore, the digital signals are preferably delayed before they are fed to the Fourier transformation for demodulation, in order to compensate for the calculation time of the Fourier transformation for estimating the self-noise component.
It is furthermore advantageous for either the total phase error or only the self-noise component to be corrected, depending on the estimated self-noise component.
It may likewise be advantageous for a further correction of the self-noise component to be carried out in the Fourier transformation for demodulation.
Finally, it is particularly advantageous for the samples to be multiplied by the complex vector e
−j[(2&pgr;f
err
kT
a
)+&psgr;e
] for correction of the self-noise component, or by e
−j[(2&pgr;f
err
kT
a
)+2k&psgr;
e
/N
] for correction of the total phase error,
f
err
being the frequency error between the oscillator and the ideal nominal frequency,
N being the number of OFDM carriers,
k being a discrete time variable where k=1, 2, 3, . . . , N and
T
a
being the sampling period.
In principle, the circuit arrangement according to the invention comprises, for a method for the correction of phase and/or frequency errors in digital multicarrier signals
means for estimating the self-noise component, which means supply a correction signal for the digital multicarrier signals;
a delay stage to which samples of the digital multicarrier signals are fed in order to delay these signals for compensation of the calculation time of the means for estimating the self-noise component;
a frequency mixing unit by means of which adapted phase or frequency signals are produced from the delayed samples with the aid of the correction signal;
transformation means to which the samples of
Armbruster Veit
Muschallik Claus
Chin Wellington
Deutsche Thomson-Brandt GmbH
Kurdyla Ronald H.
Laks Joseph J.
Phan M.
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