CN102255844A - Signal peak-to-average ratio reduction method for orthogonal frequency division multiplexing (OFDM) system - Google Patents

Abstract

The invention discloses a signal peak-to-average ratio reduction method for an orthogonal frequency division multiplexing (OFDM) system. A peak-to-average ratio is reduced by combining a constellation extension method and null sub-carriers. Regulation is performed on data sub-carriers according to constellation extension rules. Frequency domain filtering is performed on the null sub-carriers. In the method, amplitude-prelimited signal noises are processed to generate a peak cancellation vector in ways of regulating data on the data sub-carriers according to the constellation extension rules, performing a frequency domain filtering operation on the null sub-carriers, reserving noises of a null sub-carrier part and zero-setting the noises of a data sub-carrier part to completely transfer the noises to areas of the null sub-carriers and constellation extension without influencing judgment in a constellation diagram of a receiver to further reduce the peak-to-average ratio of a signal. By the method, the peak-to-average ratio of the OFDM signal can be effectively reduced, and iteration times can be reduced.

Description

Reduce the method for signal peak-to-average ratio in a kind of ofdm system

Technical field

The present invention relates to wireless communication technology field, in orthogonal frequency division multiplex OFDM (Orthogonal Frequency Division Multiplexing) system peak-to-average power ratio PAPR (Peak-to-Average Ratio Reduction) technology is fallen particularly, for reducing the method for signal peak-to-average ratio in a kind of ofdm system, be used for OFDM and fall the peak-to-average power ratio design, reduce signal peak-to-average ratio in conjunction with the gap carrier wave among planisphere development method (ACE) and the OFDM, quicken the iterative convergence speed of ACE algorithm, and more effectively reduce the signal peak-to-average power power ratio.

Background technology

Along with the continuous increase of mobile communication and wireless network demand, more and more need advanced more Radio Transmission Technology.A challenge the most direct of high-speed radiocommunication system design is to overcome the serious frequency selectivity decline that wireless channel brings.The OFDM technology can overcome the frequency selective fading of wireless channel well, because its transmission feature efficiently, OFDM has become one of core technology that realizes following high-speed radiocommunication.

One of topmost shortcoming of ofdm system is to have bigger peak-to-average force ratio, and it directly affects the operating cost and the efficient of whole system.Sometime,, thereby require power amplifier to have very wide linear amplification zone if a plurality of carrier waves will produce very big peak power when adding up with same direction.Otherwise, when signal peak drops on the nonlinear area of power amplifier, the distortion of signal will take place, thereby produce interfering with each other and out-of-band radiation between subcarrier, destroyed the orthogonality of subcarrier, reduce systematic function.For fear of this situation, power amplifier should be operated under the high-power compensating coefficient.Yet this can cause low-down power amplification efficient, and makes the cost of transmitter become very expensive.On the other hand, if the OFDM technology is applied in the mobile communication,, thereby require power amplification efficiently because the energy of portable terminal is very limited.Therefore, in OFDM multicarrier system, must adopt certain technology to reduce the peak-to-average force ratio value of signal, make that power amplifier in the transmitter can efficient operation and improve the overall performance of system.

Nearly a lot of scholars of more than ten years have done deep research to solving the peak-to-average force ratio problem, and a series of methods of effectively falling PAPR have been proposed, mainly contain compiling method, margining amplitude technique, amplitude limit adds filter method, subcarrier study in France in advance (TR), subcarrier injection method (TI), planisphere reflection method (ACE), digital pre-distortion method (DPD), selectivity is penetrated method (SLM), partial transmission sequence method (PTS) etc.Compiling method can not cause the distortion of signal but its implementation complexity increases along with sub-carrier number and sharply raises the fewer situation of only suitable sub-carrier number.Margining amplitude technique, amplitude limit adds filtering, the method for amplitude limit can make distorted signals, thereby causes in the band and out-of-band noise, amplitude limit add filter method by filtering with the out-of-band noise filtering, but can't eliminate in-band noise.Subcarrier is studied in France in advance and is used for producing the signal of offsetting peak value by the parton carrier wave of reserving, and this is a kind of undistorted transmission method, but needs repeatedly iteration.The planisphere development method makes signal change towards the method that peak-to-average force ratio reduces by the analysis of clipped noise being readjusted planisphere, and its main deficiency is to different modulation system planisphere extended method differences.SLM and PTS method are two kinds of methods that reduce signal peak-to-average ratio undistortedly, but they need be repeatedly IFFT, and complexity is than higher, and needs transmit effective side information.The DPD method is a kind of power amplifier linearization method, and by estimating power amplifier nonlinearity model adaptively, the signal to the input power amplifier carries out predistortion then, thereby realizes linearisation.

Every kind of method all has the characteristics of oneself, according to system requirements, adopts diverse ways in the design, and not having a kind of method is any system that unconditionally is adapted to.In above the whole bag of tricks, the planisphere development method is a kind of method based on amplitude limit that need not transmit side information, optimizes clipped noise by the adjustment to planisphere, finally reaches the purpose of undistorted transmission, but, need do more iteration in order to reach lower peak-to-average force ratio.

Summary of the invention

The problem to be solved in the present invention is: peak-to-average power ratio height in the ofdm system, according to system requirements, adopt diverse ways in the existing design, and be any system that unconditionally is adapted to but there is not a kind of method, the restriction that is subjected to is more.

Technical scheme of the present invention is: reduce the method for signal peak-to-average ratio in a kind of ofdm system, associating planisphere development method and gap carrier wave reduce peak-to-average force ratio in the Calais by producing the peak value offset vector mutually with original vector, may further comprise the steps:

A) set amplitude gate limit value A, the amplitude of input signal is carried out amplitude limit above the part of A, the noise c of amplitude limit introducing _ClipTransform to frequency domain as FFT and obtain C _Clip, and set the iterations i that planisphere is expanded, the initial value of i is 0;

B) with C _ClipThe data of hollow sub-carrier positions keep, and the data zero setting of data subcarrier position obtains C _Null

C) with C _ClipThe data zero setting of hollow sub-carrier positions, the rule of the data based planisphere expansion of data subcarrier position is adjusted, and promptly works as the C of corresponding data sub-carrier positions _ClipValue makes constellation point when extended area is expanded, and the value that then keeps this position is constant, as the C of corresponding data sub-carrier positions _ClipThen the value of this position is changed to zero when value makes constellation point towards the opposite area extension of planisphere expansion, obtains C _Ace

D) to C _NullAnd C _AceDo the FFT conversion, obtain peak value offset vector c _NullAnd c _Ace, based on the planisphere development method, through type x ^I+1=x ⁱ+ μ c _Ace+ β c _NullUpgrade the signal data of OFDM, reduce the peak-to-average force ratio value of signal, wherein μ and β are the step factor of peak value offset vector, and i is the pairing sequence number of iterations,

$\mathrm{\&mu;}=\underset{0\&le;k\&le;N-1}{\mathrm{<figure-callout\; id="0"\; label="min"\; filenames="HDA0000074336130000031.png,HDA0000074336130000042.png"\; state="\{\{state\}\}">min</figure-callout>}}{\mathrm{\&mu;}}_k=\underset{0\&le;k\&le;N-1}{\mathrm{<figure-callout\; id="0"\; label="min"\; filenames="HDA0000074336130000031.png,HDA0000074336130000042.png"\; state="\{\{state\}\}">min</figure-callout>}}\frac{E-\left|x_k^i\right|}{{\mathrm{Cproj}}_k-{\mathrm{Cproj}}_{n_{\max }}}$

Wherein ${\mathrm{Cproj}}_k=\frac{\mathrm{Re}\left\{x_k^i{c}_k^{*}\right\}}{\left|x_k^i\right|},$ c＝c _ace

N is the OFDM sub-carrier number, and n is the time domain sequence number of each subcarrier correspondence, n ∈ [0,1 ..., N-1], k is the frequency domain sequence number of each subcarrier correspondence, k ∈ [0,1 ..., N-1], E is

Maximum, n _MaxFor

Corresponding sequence number when getting maximum, following formula is only to Cproj _k＞0 part is operated, when there not being Cproj _k＞0 o'clock μ=1, here

N _NullBe ofdm system hollow sub-carrier number;

E) if reached the peak-to-average force ratio of expection or reached maximum iteration time, then stop and transmitting current signal, otherwise upgrade iterations i=i+1, carry out new round iteration.

Further, with the input signal amplitude | x _k| during with amplitude limit thresholding A comparison, the absolute value of usefulness real part and imaginary part and approximate to amplitude, right | x _k|＞A employing (| x _{K, R}|+| x _{K, I}|)＞A replacement, wherein x _{K, R}And x _{K, I}Be respectively the real part and the imaginary part of signal.

To step factor μ, by to many groups of OFDM symbol statistics at random, obtain the statistics of the step factor μ value of iteration for the first time earlier, carry out statistical analysis, draw the assembly average of μ, μ is set to the constant step factor.

The present invention proposes a kind of unite planisphere expansion and gap carrier wave the peak-to-average power ratio method falls, press the adjustment of planisphere propagation law on the data subcarrier, the territory filtering of the enterprising line frequency of gap carrier wave.Planisphere propagation law when Fig. 1 is QPSK and 16QAM modulation.Signal noise behind the pre-amplitude limit is handled generation peak value offset vector in the following manner, be that data on the data subcarrier are adjusted according to the planisphere propagation law, carry out the frequency domain filtering operation for the gap carrier wave, the noise of gap carrier wave part is kept, the noise of data subcarrier part is changed to zero, like this noise is all transferred to the zone of gap carrier wave and planisphere expansion, and can the judgement of receiving terminal planisphere not exerted an influence, reduced the peak-to-average power ratio of signal simultaneously again.

The gap carrier wave that the present invention unites among planisphere expansion (ACE) method and the OFDM reduces peak-to-average force ratio, the data sub-carrier positions is adjusted data according to the propagation law of planisphere, thereby the gap carrier position is carried out frequency domain filtering produce the peak value offset vector, offset the peak value of primary signal, the ofdm signal peak-to-average force ratio can be reduced effectively, and iterations can be reduced.When the ofdm signal of N=128 is modulated with QPSK, be 10 at CCDF after the iteration for the first time ^-3The time can obtain 4.02dB the PAPR performance falls, 16QAM can reduce 3dB during modulation.The method that the present invention proposes is under rayleigh fading channel, and error sign ratio performance and ideal situation are approaching when modulating with 16QAM.

Description of drawings

Fig. 1 is QPSK and 16QAM planisphere extension rule.

The flow chart of peak-to-average force ratio falls in Fig. 2 for associating planisphere expansion of the present invention and gap carrier wave.

Fig. 3 is during for amplitude limit thresholding A=6dB of the present invention, to 10 ⁶The statistical Butut of step factor μ in the individual OFDM symbol.

Fig. 4 is the PAPR after first three time iteration under the QPSK modulation for method among the present invention and SGP-ACE method.

PAPR when Fig. 5 is applied to QPSK and 16QAM for method among the present invention after first three time iteration.

Error rate when Fig. 6 modulate with 16QAM under rayleigh fading channel for method among the present invention after first three inferior iteration.

Embodiment

At first the present invention is simply introduced and then specified to the planisphere extended method below.

The planisphere development method

Planisphere development method ACE is a kind of non-bijectivity planisphere mapping algorithm, and its basic principle is to reduce peak-to-average force ratio by changing the position of putting on the planisphere.General planisphere is mapped as certain several bit is mapped to certain several fixing point on the planisphere, and the planisphere development method allows certain several constellation point is mapped to a certain specific region.Fig. 1 is the planisphere mapping mode of QPSK and 16QAM, and the point that is about to the planisphere periphery is to the direction adjustment away from other constellation point.Mathematical notation to the ACE algorithm is

Wherein

$\stackrel{\&OverBar;}{x}\left[n\right]=x\left[n\right]+c\left[n\right]$

$=\frac{1}{\sqrt{N}}\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}(X_k+C_k){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA0000074336130000031.png"\; state="\{\{state\}\}">e</figure-callout>}}^j$

Wherein N is the OFDM sub-carrier number, and n is the time domain sequence number of each subcarrier correspondence, n ∈ [0,1 ..., N-1], k is the frequency domain sequence number of each subcarrier correspondence, k ∈ [0,1 ..., N-1].C represents spread vector C _kSet,

Extendible whole vector set (the SGP-ACE[B.S.Krongold and D.L.Jones of expression ACE, " PAR reduction in OFDM via active constellation extension; " IEEE Trans.Broadcast., vol.49, no.3, pp.258-268, Sep.2003.]).The ACE basic idea is to produce the peak value offset vector by the limitation signal that is mapped to the planisphere expandable area to offset peak value, and this process repeats repeatedly peak-to-average force ratio to be dropped to threshold value, promptly

x ⁱ⁺¹＝x ⁱ+μc ⁱ

Wherein i is the iteration sequence number, and μ is the step factor of decision convergence rate.c ⁱPeak value offset vector when being the i time iteration, but the time domain form of its expanding constellation vector C when being the i time iteration are to given amplitude limit thresholding A, to the signal x of input ⁱMake soft limiting and operate the clipped noise c that obtains when thresholding is A _Clip, promptly

$c_{\mathrm{clip}}^i=\left\{\begin{array}{cc}\left(\right|x_{n-}^{i}A\left|\right)e^{j{\mathrm{\&theta;}}_n},& \left|x_n^i\right|>A\\ 0,& \left|x_n^i\right|\&le;A\end{array}\right.$

Wherein With clipped noise

Transform to frequency domain, and noise is adjusted according to the adjustment law of planisphere at the component of frequency domain, but promptly obtain expanding constellation vector C, can obtain peak value offset vector c the C transform domain ⁱ

The following describes the planisphere expansion associating gap carrier method that the present invention proposes.

The present invention proposes a kind of method that peak-to-average force ratio falls in planisphere expansion and gap carrier wave of uniting, and presses the adjustment of planisphere propagation law on the data subcarrier, the territory filtering of the enterprising line frequency of gap carrier wave.Suppose that sub-carrier number is that the ofdm system hollow sub-carrier number of N is N _Null, the arrangement set of gap carrier wave is

Wherein $0\&le;{\mathrm{<figure-callout\; id="0"\; label="i"\; filenames="HDA0000074336130000031.png,HDA0000074336130000042.png"\; state="\{\{state\}\}">i</figure-callout>}}_0<...<i_{N_{\mathrm{null}}-1}\&le;N-1,$ As shown in Figure 2, implementation process of the present invention is:

Step1: set peak value width of cloth thresholding A, to the frequency domain signal X=[X of each input ₀, X ₁... X _k... X _N-1] store the positional information I of data in planisphere in each data subcarrier.

Step2: the frequency domain data symbol X to input obtains time domain data x as IFFT, iterations i=0 is set, x ⁰=x.

Step3: to any among the time domain data x

Data slicing obtain data

${\stackrel{\&OverBar;}{x}}_k=\left\{\begin{array}{cc}{x}_k^i,& \left|x_k^i\right|\&le;A\\ {\mathrm{Ae}}^{j{\mathrm{\&theta;}}_k^i},& \left|x_k^i\right|>A\end{array}\right.$

Wherein $x_k^i=\left|x_k^i\right|e^{j{\mathrm{\&theta;}}_k^i}$

Step4: calculate the amplitude limit part

$c_{\mathrm{clip}}=x^i-\stackrel{\&OverBar;}{x}$

Step5: to c _ClipDo the FFT conversion and obtain C _Clip

Step6: to C _ClipOnly keep position on the gap carrier wave

On data, other locational data are zero, obtain gap carrier wave peak value and eliminate vectorial C _NullAccording to the positional information I in planisphere of data among the X, adjust and adjust C respectively _ClipThe real part and the imaginary part of middle data are worked as C _ClipCorresponding sub-carrier positions data when adjusting, the planisphere propagation direction are kept it, otherwise with C _ClipThe value of corresponding sub-carrier positions is made as zero, and

Last corresponding C _ClipValue also is made as zero, obtains C _AceTo C _NullAnd C _AceObtain c as IFFT respectively _NullAnd c _Ace

Step7: calculate

x ⁱ⁺¹＝x ⁱ+μc _ace+βc _null

$\mathrm{\&mu;}=\underset{0\&le;k\&le;N-1}{\mathrm{<figure-callout\; id="0"\; label="min"\; filenames="HDA0000074336130000031.png,HDA0000074336130000042.png"\; state="\{\{state\}\}">min</figure-callout>}}{\mathrm{\&mu;}}_k=\underset{0\&le;k\&le;N-1}{\mathrm{<figure-callout\; id="0"\; label="min"\; filenames="HDA0000074336130000031.png,HDA0000074336130000042.png"\; state="\{\{state\}\}">min</figure-callout>}}\frac{E-\left|x_k^i\right|}{{\mathrm{Cproj}}_k-{\mathrm{Cproj}}_{n_{\max }}}$

Wherein ${\mathrm{Cproj}}_k=\frac{\mathrm{Re}\left\{x_k^i{c}_k^{*}\right\}}{\left|x_k^i\right|},$ c＝c _ace

E is Maximum, n _MaxFor

The time domain sequence number of correspondence when getting maximum, k and n _MaxBe the label of Cproj, following formula is only to Cproj _k＞0 part is operated, when there not being Cproj _k＞0 o'clock μ=1, here $\mathrm{\&beta;}=\sqrt{N/N_{\mathrm{null}}.}$

Step8: if reached the PAPR of expection or reached maximum iteration time, then stop and transmitting x, otherwise upgrade i=i+1 and return step3.

Said method makes full use of the gap carrier wave and reduces signal peak-to-average ratio, can accelerate the convergence rate of iteration and fall the peak-to-average force ratio performance, but need to calculate gap carrier wave peak value offset vector, still have certain computation complexity, the present invention further improves, by suitable approximate realization low complex degree.

Approximate processing

Signal amplitude is approximate: the amplitude of signal and amplitude limit thresholding A need the amplitude of signal calculated when comparing | x _k|, this when realizing, need calculate respectively real part and imaginary part square and to theirs with evolution, the implementation complexity of extracting operation is than higher, and is right | x _k|＞A adopts following approximate replacement

(|x _k，R|+|x _k，I|)＞A

X wherein _{K, R}And x _{K, I}Be respectively the real part and the imaginary part of signal, this method also can apply in other approximate calculation.

Step factor μ is approximate: the calculating to step factor μ is a very complicated process, all to calculate once every group of data, and difference according to the input data, the excursion of step factor numerical value is also very big, when this realizes circuit, the figure place of data is considered to bring very large inconvenience, so this paper draws the statistical value of μ by multi-group data is carried out statistical analysis.When circuit is realized, use the constant step factor like this, simplified implementation complexity greatly.

Set amplitude gate limit value A=6dB, sub-carrier number N=128 is to 10 ⁶Group OFDM symbol statistics at random obtains Distribution Statistics such as Fig. 3 of the step factor μ value of iteration for the first time, and as can be seen from the figure μ mainly is distributed in μ＜25 parts, also has the bigger μ of some values in addition, does not show among the figure.When realizing, can choose suitable value like this, get μ=10 emulation in the emulation of the present invention according to Fig. 3.

Emulation and analysis

Use the ofdm system of sub-carrier number N=128 in the emulation, the gap carrier position be [1 ..., 6,50 ..., 80,124 ..., 128], emphasis carries out emulation to QPSK and 16QAM modulation system, to generate at random 10 ⁶Organize the OFDM symbolic analysis, and compare with the ACE-SGP algorithm.

Fig. 4 be the algorithm that proposes of SGP-ACE algorithm and the present invention at amplitude gate limit value A=6dB, the change curve that QPSK modulation decline PAPR performance increases along with iterations, two kinds of methods are fallen the PAPR effect all clearly as we can see from the figure, are 10 at CCDF ^-3The time, in the SGP-ACE algorithm for the first time iteration PAPR drop to 6.75dB by 10.57dB, the method that the present invention proposes can drop to 6.55dB, the iteration PAPR 4.02dB that on the original signal basis, descended for the first time, the method that the present invention proposes compares with the SGP-ACE method that to fall the PAPR performance during iteration in the first time more excellent, and iterative convergence speed is faster arranged, reached performance with three iteration of SGP-ACE method during iteration, reach target amplitude limit thresholding at faster speed in the second time.

The planisphere expansion is only to the some adjustment outside the planisphere, therefore, it is relevant with the modulation system of employing that the performance of peak-to-average force ratio falls in the planisphere development method, as can be seen from Figure 1, order of modulation is low more, it is big more to adjust the constellation point ratio, therefore it is also obvious more to fall the PAPR effect, the method that Fig. 5 proposes for the present invention is in the ofdm system of N=128, first three time iteration falls PAPR result when adopting QPSK and 16QAM modulation, proposition method of the present invention as seen from the figure is applied to QPSK and falls the PAPR performance and be better than 16QAM, and promptly low-order-modulated mode performance is better than high-order modulating.

Error sign ratio curve when the algorithm that Fig. 6 proposes for the present invention is modulated with 16QAM under rayleigh fading channel, under rayleigh fading channel when signal to noise ratio rises to a certain degree, the influence of decline and multipath becomes principal element, so the error sign ratio curve tends towards stability.As can be seen from the figure the method for the present invention's proposition is approaching with error sign ratio curve ideally when signal to noise ratio is lower.

Claims (3)

1. reduce the method for signal peak-to-average ratio in the ofdm system, it is characterized in that associating planisphere development method and gap carrier wave,, may further comprise the steps by producing peak value offset vector and original vector Calais's reduction peak-to-average force ratio mutually:

A) set amplitude gate limit value A, the amplitude of input signal is carried out amplitude limit above the part of A, the noise c of amplitude limit introducing _ClipTransform to frequency domain as FFT and obtain C _Clip, and set the iterations i that planisphere is expanded, the initial value of i is 0;

B) with C _ClipThe data of hollow sub-carrier positions keep, and the data zero setting of data subcarrier position obtains C _Null

C) with C _ClipThe data zero setting of hollow sub-carrier positions, the rule of the data based planisphere expansion of data subcarrier position is adjusted, and promptly works as the C of corresponding data sub-carrier positions _ClipValue makes constellation point when extended area is expanded, and the value that then keeps this position is constant, as the C of corresponding data sub-carrier positions _ClipThen the value of this position is changed to zero when value makes constellation point towards the opposite area extension of planisphere expansion, obtains C _Ace

D) to C _NullAnd C _AceDo the FFT conversion, obtain peak value offset vector c _NullAnd c _Ace, based on the planisphere development method, through type x ^I+1=x ⁱ+ μ c _Ace+ β c _NullUpgrade the signal data of OFDM, reduce the peak-to-average force ratio value of signal, wherein μ and β are the step factor of peak value offset vector, and i is the pairing sequence number of iterations,

$\mathrm{\&mu;}=\underset{0\&le;k\&le;N-1}{\min }{\mathrm{\&mu;}}_k=\underset{0\&le;k\&le;N-1}{\min }\frac{E-\left|x_k^i\right|}{{\mathrm{Cproj}}_k-{\mathrm{Cproj}}_{n_{\max }}}$

Wherein ${\mathrm{Cproj}}_k=\frac{\mathrm{Re}\left\{x_k^i{c}_k^{*}\right\}}{\left|x_k^i\right|},$ c＝c _ace

N is the OFDM sub-carrier number, and n is the time domain sequence number of each subcarrier correspondence, n ∈ [0,1 ..., N-1], k is the frequency domain sequence number of each subcarrier correspondence, k ∈ [0,1 ..., N-1], E is

Maximum, n _MaxFor Corresponding sequence number when getting maximum, following formula is only to Cproj _k＞0 part is operated, when there not being Cproj _k＞0 o'clock μ=1, here

N _NullBe ofdm system hollow sub-carrier number;

E) if reached the peak-to-average force ratio of expection or reached maximum iteration time, then stop and transmitting current signal, otherwise upgrade iterations i=i+1, carry out new round iteration.

2. reduce the method for signal peak-to-average ratio in the Frequency Division Multiplexing system according to claim 1, it is characterized in that the input signal amplitude | x _k| during with amplitude limit thresholding A comparison, the absolute value of usefulness real part and imaginary part and approximate to amplitude, right | x _k|＞A, adopt (| x _{K, R}|+| x _{K, I}|)＞A replacement, wherein x _{K, R}And x _{K, I}Be respectively the real part and the imaginary part of input signal.

3. reduce the method for signal peak-to-average ratio in the Frequency Division Multiplexing system according to claim 1 and 2, it is characterized in that step factor μ, earlier by many groups of OFDM symbols are at random added up, obtain the statistics of the step factor μ value of iteration for the first time, carry out statistical analysis, draw the assembly average of μ, μ is set to the constant step factor.

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