CN101695064B - Method and device for reducing peak-to-average ratio of OFDM signal - Google Patents
Method and device for reducing peak-to-average ratio of OFDM signal Download PDFInfo
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Abstract
The invention relates to a method and a device for reducing the peak-to-average ratio of OFDM signals, in particular to extraction of sequences to be corrected. The invention aims to reduce the operation amount of signal processing for reducing the peak-to-average ratio of OFDM signals, and provides a method for reducing the peak-to-average ratio of OFDM signals.
Description
Technical Field
The invention relates to a method and a device for reducing the peak-to-average ratio of OFDM signals, in particular to extraction of sequences to be corrected.
Background
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to divide a high-speed data stream into a plurality of low-speed data streams by serial-to-parallel conversion, and then put the data streams onto corresponding sub-carriers to be transmitted simultaneously and in parallel. Within an OFDM symbol is a composite signal comprising a plurality of modulated subcarriers, each of which may be loaded with Quadrature Amplitude Modulation (QAM). In addition, Cyclic Prefix (CP for short) introduced in OFDM as a guard interval between symbols can effectively overcome delay spread of multipath channels and reduce the influence of Inter-Symbol Interference (ISI), thereby avoiding the use of a complex equalizer.
At the transmitting end of the OFDM system, the binary bit stream generated by the information source will be pre-modulated once by using M-QAM, that isMapping binary data on each parallel channel to points on a signal constellation diagram to form a data sequence X in a complex formk. The modulation schemes on the sub-channels may be the same or different. More bits are modulated on the more gradual subchannels and less bits are modulated on the more faded subchannels. Then, Inverse Fast Fourier Transform (IFFT) is used to perform Inverse Fast Fourier Transform on the complex parallel sequence XkPerforming a second modulation, i.e. performing frequency modulation on the original signal by using a set of orthogonal sine and cosine signals to obtain a modulated parallel time domain signal xnI.e. N sample values in one symbol. The OFDM modulated time domain signal can be represented by the following equation:
x is to benAfter merging the data streams of each channel into a serial data stream through parallel-to-serial conversion, inserting a Cyclic Prefix (CP) as a guard interval to avoid the influence of ISI (inter-Symbol interference) caused by multipath fading on effective Symbol data, thereby obtaining an OFDM (orthogonal frequency division multiplexing) signal of one Symbol (Symbol). The OFDM symbol is formed by combining the data streams of the sub-channels, and a Peak to Average Power Ratio (PAPR) is too high.
The simplest and most effective way to reduce PAPR is to clip the peak of the OFDM signal, i.e. to clip the peakTime domain signal xnCarrying out a limiting operation:
wherein xnTh is the original signal and Th is the clipping threshold. When the signal is smaller than the amplitude limit threshold, the signal is unchanged; and when the signal is larger than the amplitude limit threshold, limiting the mode of the signal to the threshold size, and keeping the amplitude angle same as that of the original signal.
After amplitude limiting, the peak-to-average ratio of the signal is reduced, and in-band distortion and out-of-band noise are introduced to the signal, wherein the in-band distortion can increase the bit error rate of the system, and the out-of-band noise can cause interference to adjacent channels. Out-of-band noise is generally suppressed by a time-domain filtering method after the clipping operation, which is ineffective for in-band distortion.
In order to reduce the effect of in-band distortion, it is common to perform "pre-clipping" in the time domain (which operates the same as the above-mentioned time-domain clipping), then transform the signal into the frequency domain, and perform modification within the system specification to meet the application requirements. In this way, an FFT/IFFT frequency domain filter is used to replace filtering in the time domain, and first, signals are converted into a frequency domain form by Fast Fourier Transform (FFT), and then, zero is directly set for out-of-band frequency points, and at the same time, in-band signals are modified. An Error Vector Magnitude (EVM) index is required in the frequency domain correction, and is based on an Error Vector generated at a certain point, as shown in fig. 1.
Defining EVM as the average error in the constellation diagram with respect to the farthest constellation point power:
where N is the number of subcarriers, SmaxIs the maximum amplitude of the constellation point in the constellation diagram, and the delta I and the delta Q are the in-phase distance and the quadrature distance of the distorted constellation point and the original undistorted constellation point respectively.
For original signal X in one frequency domainkDistorted signal X due to time-domain clippingk＝FFT(xn) The correction in the frequency domain is to correct the error vector Xk-XkIs limited within a certain set value E, which is equivalent to limiting the distortion degree within a certain set range, as follows:
in the time domain amplitude limiting-frequency domain correcting method, the amplitude limiting result can be obtained by obtaining a corrected sequence after time domain pre-amplitude limiting and frequency domain correction and then transforming the corrected sequence to the time domain through IFFT.
In summary, the in-band distortion and out-of-band noise caused by time-domain clipping can be improved by time-domain filtering or frequency-domain modification. The frequency domain correction can also deal with the problem of in-band distortion which cannot be improved by time domain filtering, so the effect is better and the application range is wider.
On the basis of simple amplitude limiting, a frequency domain correction improved algorithm is developed, an optimal amplitude limiting result is obtained by carrying out certain times of amplitude limiting iteration operation, and the method comprises the following steps:
step A, carrying out amplitude limiting iteration operation on the time domain signal to be iterated to obtain an amplitude limiting result of the iteration, then jumping to step B,
step B, judging an iteration ending condition, and if the iteration ending condition is not met, jumping to the step A by taking the iteration result as a time domain signal to be subjected to iteration operation in the next iteration; if the iteration result of the time accords with the iteration ending condition, ending the iteration,
the slicing iteration operation in the step A comprises the following steps which are executed in sequence:
A10. performing time domain pre-amplitude limiting, extracting a time domain sequence to be modified,
A20. performing time domain-frequency domain transformation on the time domain sequence to be corrected to obtain a frequency domain sequence to be corrected,
A30. performing frequency domain correction on the sequence to be corrected to obtain a corrected sequence,
A40. frequency domain-time domain conversion is carried out on the corrected sequence, the amplitude limiting result is extracted,
wherein,
extracting the signal after time domain pre-amplitude limiting in A10 of the iterative operation as a time domain sequence to be corrected,
the result of the frequency-domain-to-time-domain transformation of the modified sequence is extracted as the result of said slicing in a40 of the iterative operation.
The commonly used iteration end conditions include two conditions, one is that the PAPR of the clipping result has reached the system requirement, and the other is that the iteration number has reached the preset value.
This method has a significant problem: in the iterative amplitude limiting operation a20, time-frequency domain transformation is performed on the time-domain limited signal, and the fourier transform has a huge calculation amount and needs to consume a large amount of time. Due to the existence of iterative operations, in OFDM links, time consumption becomes an important factor that must be considered for system design. Particularly, in a frequency domain correction mode with a good effect, each iteration needs to perform a pair of FFT/IFFT operations, and when the number of processing points is large, the time consumed by FFT/IFFT is considerable, thereby affecting the real-time performance of the system. From another perspective, the number of iterations needs to be limited in the case of meeting the real-time performance, thereby causing poor PAPR suppression effect of the system.
Disclosure of Invention
Object of the Invention
The invention aims to reduce the calculation amount of signal processing for reducing the peak-to-average ratio of OFDM signals.
The idea of the invention is that in a common time domain signal, because the proportion of sampling points with excessively high peak values in all the sampling points of the OFDM is very small, the time domain pre-amplitude limiting operation is only effective to the small number of sampling points actually, and most of the sampling points do not change before and after amplitude limiting, a large number of zero values exist in the difference between the signals before and after the time domain pre-amplitude limiting; the frequency domain correction is to limit a modulus of a difference between an original signal (corresponding to a signal before slicing) and a distorted signal (corresponding to a signal after slicing) in a frequency domain. Therefore, the invention converts the difference between the signals before and after the time domain pre-amplitude limiting into the frequency domain for modification, and the operation amount of Fourier transform is greatly reduced because a large number of zero values exist on the time domain in the time domain-frequency domain transformation operation.
Technical scheme
A method for reducing the peak-to-average ratio of an OFDM signal is provided, which comprises
Step A, carrying out amplitude limiting iteration operation on the time domain signal to be subjected to iteration operation in the current iteration to obtain an amplitude limiting result of the current iteration, then jumping to step B,
step B, judging an iteration ending condition, and if the iteration ending condition is not met, jumping to the step A by taking the iteration result as a time domain signal to be subjected to iteration operation in the next iteration; if the iteration result of the time accords with the iteration ending condition, ending the iteration,
the slicing iteration operation in the step A comprises the following steps which are executed in sequence:
A10. performing time domain pre-amplitude limiting, extracting a time domain sequence to be modified,
A20. performing time domain-frequency domain transformation on the time domain sequence to be corrected to obtain a frequency domain sequence to be corrected,
A30. performing frequency domain correction on the sequence to be corrected to obtain a corrected sequence,
A40. frequency domain-time domain conversion is carried out on the corrected sequence, the amplitude limiting result is extracted,
it is characterized in that the utility model is characterized in that,
extracting the difference between the signals before and after the time domain pre-amplitude limiting in A10 of the iterative operation as a time domain sequence to be modified,
and extracting the difference between the time domain signal to be subjected to iterative operation in the iteration and the frequency domain-time domain transformation result of the modified sequence in A40 of the iterative operation to obtain the amplitude limiting result.
Advantageous effects
The signal before time domain pre-limiting is xnThe time domain pre-limited signal is <math>
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</math> In the above technical solution, a10 of iterative operation takes a difference x between signals before and after time domain pre-slicingCLIPnAs a time-domain sequence to be modified, i.e. xn-xn＝xCLIPnN is 0-N-1 (the subtrahend and the subtrahend of the equation can be interchanged without affecting the appearance of zero values), then <math>
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</math> And a large number of zero values exist, so that the subsequent time domain-frequency domain transformation greatly reduces the operation amount of Fourier transformation, improves the signal processing efficiency and has strong adaptability to the real-time requirement. Correspondingly, after A40 frequency domain-time domain transformation of iterative operation, the difference between the time domain signal to be iteratively operated in the current iteration and the frequency domain-time domain transformation result is taken N is more than or equal to 0 and less than or equal to N-1 and is used as the amplitude limiting result of the current iteration and is also used as the time domain signal to be iterated in the next iteration (if the above-mentioned subtracted number and the subtracted number are interchanged, x is in the placeCLIPnThe polarity of (d) is reversed).
Drawings
Fig. 1 is a diagram of an error vector in the frequency domain with respect to an original signal and a distorted signal.
FIG. 2 is a flowchart illustrating operation of an embodiment.
Detailed Description
In order to reduce the peak-to-average ratio of the OFDM signal, the steps as shown in fig. 2 are adopted. Obtaining a time domain signal x by constellation mapping and frequency domain-time domain transformation IFFTnThen a.
The iterative operation comprises the following steps performed in sequence:
A10. performing time domain pre-limiting to obtain pre-limited signal <math>
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</math> Extracting the difference between the pre-and post-time domain pre-slicing signals <math>
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</math> As a sequence to be modified in the time domain,
A20. for the time domain to be modified sequence xCLIPnPerforming time domain-frequency domain transform FFT to obtain frequency domain signal XCLIPk＝FFT(xCLIPn) Because the time domain sequence x to be modifiedCLIPnThere are a large number of zeros, so the amount of operation is much less for the time domain signal xnThe FFT transformation is performed and the FFT is performed,
A30. to XCLIPkPerforming frequency domain correctionI.e. the modulus of the error vector is limited to a certain set value E,
A40. performing frequency-to-time domain transformation
Time domain signal x to be iterated in the iterationnDifference between the frequency domain and time domain transform result of the modified sequence N is more than or equal to 0 and less than or equal to N-1, namely the amplitude limiting result.
B. Judging iteration ending conditions, and ending the iteration if the PAPR of the iteration result reaches the system requirement or the iteration times reaches a preset value; and if the time domain signals are not in accordance with the iteration result, taking the iteration result as the time domain signal to be subjected to iteration operation in the next iteration, and performing A. iteration operation again. And B, judging the iteration ending condition until the PAPR of the iteration result reaches the system requirement or the iteration times reaches a preset value.
Claims (2)
1. A method of reducing the peak-to-average ratio of an OFDM signal comprising
Step A, carrying out amplitude limiting iteration operation on the time domain signal to be subjected to iteration operation in the current iteration to obtain an amplitude limiting result of the current iteration, then jumping to step B,
step B, judging an iteration ending condition, and if the iteration ending condition is not met, jumping to the step A by taking the iteration result as a time domain signal to be subjected to iteration operation in the next iteration; if the iteration result of the time accords with the iteration ending condition, ending the iteration,
the iterative clipping operation in a comprises the following steps performed in sequence:
A10. performing time domain pre-amplitude limiting, and extracting a time domain sequence to be corrected, wherein the pre-amplitude limiting is amplitude limiting in a peak clipping mode;
A20. performing time domain-frequency domain transformation on the time domain sequence to be corrected to obtain a frequency domain sequence to be corrected,
A30. correcting the sequence to be corrected in the frequency domain to obtain a corrected sequence, wherein the correction is to limit the modulus of an error vector within a certain set value;
A40. frequency domain-time domain conversion is carried out on the corrected sequence, the amplitude limiting result is extracted,
it is characterized in that the utility model is characterized in that,
extracting a difference value of a signal before time domain pre-amplitude limiting minus a signal after pre-amplitude limiting from A10 of iteration operation as a time domain sequence to be corrected;
and in the iteration operation A40, subtracting the frequency domain-time domain transformation result of the corrected sequence according to the time domain signal before pre-amplitude limiting to obtain the amplitude limiting result.
2. Apparatus for reducing the peak-to-average ratio of an OFDM signal, comprising,
the device A is used for carrying out amplitude limiting iterative operation on the time domain signal to be iteratively operated in the current iteration to obtain an amplitude limiting result of the current iteration and transmitting the amplitude limiting result to the device B,
the device B is used for judging the iteration ending condition, and if the iteration ending condition is not met, the iteration result is used as a time domain signal to be subjected to iteration operation in the next iteration and is transmitted to the device A; if the iteration result of the time accords with the iteration ending condition, ending the iteration,
the device for realizing amplitude limiting iterative operation on the time domain signal to be subjected to iterative operation in the device A comprises the following steps:
the device A10 is a device for performing time domain pre-clipping, which is clipping in a peak clipping manner, and extracting a time domain sequence to be corrected.
A device A20, for performing time domain-frequency domain transformation on the time domain sequence to be corrected to obtain a frequency domain sequence to be corrected,
means a30 for modifying the frequency domain sequence to be modified to obtain a modified sequence, said modification being such as to limit the modulus of the error vector to within a certain set value;
means A40 for frequency-domain-time-domain transforming the modified sequence, means for extracting the clipping result,
it is characterized in that the utility model is characterized in that,
the difference value of the signal before time domain pre-amplitude limiting minus the signal after pre-amplitude limiting is extracted from the device A10 as a time domain sequence to be corrected;
in the device a40, the frequency domain-time domain conversion result of the modified sequence is subtracted from the time domain signal before pre-amplitude limiting to obtain the amplitude limiting result.
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| CN101035105A (en) * | 2007-04-06 | 2007-09-12 | 西安电子科技大学 | Method and device for reserving the sub-carrier to reduce the peak average power ratio of the OFDM system based on IFFT/FFT |
| CN101155168A (en) * | 2007-10-12 | 2008-04-02 | 西安电子科技大学 | A Method to Reduce PAPR of OFDM System |
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| CN1929469A (en) * | 2006-10-13 | 2007-03-14 | 浙江大学 | Peak average power rate control method, receiving end and transmitting end |
| CN101035105A (en) * | 2007-04-06 | 2007-09-12 | 西安电子科技大学 | Method and device for reserving the sub-carrier to reduce the peak average power ratio of the OFDM system based on IFFT/FFT |
| CN101155168A (en) * | 2007-10-12 | 2008-04-02 | 西安电子科技大学 | A Method to Reduce PAPR of OFDM System |
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