CN1741517A - Block clipping method for resolving nonlinear distortion question in OFDM system - Google Patents

Abstract

A blocked amplitude limiting method for solving nonlinear problem in OFDM system includes presetting signal peak value power being received maximally by system , deciding out amplitude limiting rate and unblocking number according to carrier number and receivable maximum signal power , calculating out threshold value , dividing OFDM symbol to be a numbers of subblocks as per unblocking number , carrying out amplitude limitation on OFDM signal at each subblock then sending them out in combination according to their original order .

Description

Solve the block clipping method of nonlinear distortion question in the ofdm system

Technical field

The invention belongs to communication technical field, relate to nonlinear distortion question in Orthodoxy Frequency Division Multiplex OFDM (the Orthogonal FrequencyDivision Multiplexing) communication system, specifically a kind of block clipping method that solves nonlinear distortion question in the ofdm system, in order to improve the performance of communication system, reduce system cost.

Background technology

Orthogonal frequency division multiplex OFDM has become the technology that attracts most attention in the later mobile communication system of three generations.Many experts and scholars have proposed relevant theoretical foundation at the application of OFDM technology at mobile communication technology.At present, ofdm system successfully has been applied in high-speed digital communication, such as: digital audio broadcasting DAB, digital video broadcasting DVB, digital high-definition television HDTV and asymmetric digital subscriber line ADSL.In addition, ofdm system also is applied in the high frequency military system, such as: KINEPLEX, ANDEFT and KATHRYN.Why ofdm system more and more is subjected to people's attention, be because OFDM is a kind of modulation system that realizes the data efficient transmission under decay, multipath channel condition, owing to can utilize fast fourier transform FFT to realize modulation and demodulation, thereby simplified the complexity that system realizes greatly, and ofdm system easily and the use that combines of other multiple cut-in methods.

The basic principle of OFDM is exactly to be divided into a plurality of mutually orthogonal subchannels with whole channel is wide, each subchannel passes through subcarrier-modulated information symbol separately separately separately, because the symbol period in each subchannel can increase relatively, therefore can alleviate the influence of the multidiameter expansion of wireless channel.The ofdm communication system theory diagram as shown in Figure 1.

At the transmitting terminal of OFDM, be input as binary bit stream or image, or random sequence a _nThen, adopt QAM or PSK to a _nCarry out premodulated one time, obtain X (n), obtain time-domain signal x (n) again behind serial to parallel conversion and inverse fast fourier transform (IFFT), it obtains actual transmission signal s (t) behind parallel serial conversion and digital-to-analogue conversion and low pass filter.At the receiving terminal of OFDM, what in fact carry out is the inverse process of above-mentioned transmitting terminal.Real domain sampled sequence { the x of the OFDM of power normalization _nAs follows: X wherein _kRepresent k the modulation symbol on the subcarrier.

$x_n=\frac{1}{\sqrt{N}}\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{X}_k\exp (\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051004313300151.png,A20051004313300152.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;kn}/N),n=\mathrm{0,1},......N-1$

Compare with single carrier, the output of OFDM multicarrier modulation system is the stack of a plurality of sub-channel signals, very big peak value will occur when therefore the phase place of a plurality of signals is consistent.This has just proposed very high requirement to the linearity of amplifier in the transmitter, the signal that this variation is bigger in ofdm system passes through non-linear components, when for example entering the amplifier nonlinearity zone, signal can produce nonlinear distortion, produce harmonic wave, cause significantly spread spectrum interference and inband signaling distortion, cause the whole system performance decrease, also can increase the complexity of mould/number and D/A simultaneously, reduce their accuracy.For avoiding nonlinear distortion, traditional method is to adopt the linear amplifier of great dynamic range, perhaps the nonlinear amplifier working point is compensated, but the efficient of power amplifier reduces greatly, and the cost of whole system increases greatly.For this reason, multiple new solution has been proposed.At present, the method for signal non-linear distortion problem mainly contains in the solution ofdm communication system that has proposed: signal predistortion technology comprises amplitude limit, companding transform technology etc.; The high spped coding technology comprises selectivity mapping and part transfer sequence; Block encoding comprises Golay complementary series and Reed-Muller coding etc.Wherein, addressing this problem the simplest method is to adopt limiting technology to reduce peak-to-average power ratio PAPR (Peak-to-Average PowerRatio), reduces the probability of occurrence of big peak power signal, thereby avoids the appearance of nonlinear distortion.

Amplitude limit is a kind of of signal predistortion technology, carries out amplitude limit before signal amplifies through non-linear components, can be so that peak signal is lower than desired maximum level value.Although amplitude limit is very simple, it also can bring relevant problem for ofdm system.At first, the OFDM symbol amplitude is distorted, can cause system and self disturb, thereby cause the error rate BER performance of system to reduce.Secondly the nonlinear distortion of ofdm signal can cause the increase of out-of-band radiation performance number, its reason is that the amplitude limit operation can be considered to the OFDM sampled signal and rectangular window function multiplies each other, if the amplitude of ofdm signal is during less than threshold value, then the amplitude of this rectangular window function is 1; If but signal amplitude be need be limited the time, then the amplitude of this rectangular window function is less than 1.Equal the principle of frequency domain convolution according to time domain multiplication, the frequency spectrum of the OFDM symbol of process amplitude limit equals the convolution of original OFDM symbol frequency spectrum and window function frequency spectrum, therefore its out of band spectrum characteristic is mainly decided by the signal that spectrum width is bigger between the two, and just the frequency spectrum by rectangular window function decides.

Adopt limiting technology that signal is carried out amplitude limit and can introduce inband distortion and out-of-band radiation, thereby reduce bit error rate performance and spectrum efficiency.Adopt band pass filter can eliminate out-of-band radiation, the over-sampling sequence can be eliminated the peak value regeneration behind the D/A switch, yet reduces inband distortion, improves the method for error rate BER performance and still need improve.

The improvement of tradition amplitude limit concentrates on the amplitude limit algorithm mostly, and lucent algorithm, cordic algorithm, Clipping-off algorithm etc. are arranged.These algorithms mainly are to change threshold value, and the definition of threshold value is had the straight line of slope from a fixed value to, and to a function curve, this curve is corresponding functional relation between threshold value and the signal of wanting amplitude limit.Here list the definition of three kinds of threshold values respectively:

First kind, will use normalized amplitude limit thresholding, i.e. amplitude limit rate CR, and amplitude limit thresholding A=CR* σ, wherein, σ is the average power of ofdm signal.This shows, is that the baseband signal average power is under the condition of ofdm signal of N at carrier number $\mathrm{\&sigma;}=\sqrt{N},$ The average power of bandpass signal is $\mathrm{\&sigma;}=\sqrt{N/2}.$ This threshold value is exactly the i.e. fixed value of straight line.

Second kind, threshold value is defined as two fixedly straight lines that slope is arranged between the amplitude limit value, its formula is as follows:

$s\left(t\right)=\left\{\begin{array}{cc}{s}_i\left(t\right),& 0\&le;s_i\left(t\right)<C\\ \left(\frac{U-C}{L-C}\right)\&CenterDot;(s_i\left(t\right)-C)+C,& C\&le;s_i\left(t\right)\&le;L\end{array}\right.$

In the formula: s _i(t) be input signal, L is the peak amplitude of inverse fourier transform output signal, and U and C are two fixing amplitude limit value, also is the starting point and the terminal point in this amplitude limit territory.This threshold value is exactly a straight line that slope is arranged.

The third is defined as the maximum power of signal behind the amplitude limit and the ratio of the average power of limitation signal not, threshold value with amplitude limit rate CR $A=\mathrm{CR}\&times;\sqrt{P_{\mathrm{mean}}}.$

$y\left(n\right)=\left\{\begin{array}{cc}\frac{A}{{\sqrt{\left|x\left(n\right)\right|}}^2}x\left(n\right),& {\left|x\left(n\right)\right|}^2\&GreaterEqual;A^2\\ x\left(n\right),& {\left|x\left(n\right)\right|}^2<A^2\end{array}\right.$

In the formula, A is the maximum amplitude of signal behind the amplitude limit, and x (n) is the output signal of IFFT, and y (n) is the output signal that adopts this amplitude limit algorithm, and this amplitude limit value is exactly a curve with the signal variable power.

Generally speaking, it is more flexible that amplitude limit value becomes ground, and amplitude limit method is variation more.Though these methods have reduced the nonlinear distortion of ofdm signal, have alleviated out-of-band radiation, but still do not improve bit error rate performance well.

The content of invention

The object of the invention is to avoid the deficiency of above-mentioned prior art, by analyzing the essential problem in the ofdm system, start with from the average power that reduces signal, proposed a kind of block clipping method that solves signal non-linear distortion problem in the ofdm communication system, to improve bit error rate performance.

The technical scheme that realizes the object of the invention is: be that an OFDM symbol is divided into some, then respectively to every amplitude limit, combination at last sends.Reduce the linear dynamic range of signal, make it satisfy the power amplifier linearity dynamic range, avoid signal non-linear distortion.The class of algorithms of this block clipping is like the third of traditional amplitude limit, and the definition of CR is more simple, and formula is as follows:

Here, amplitude limit rate CR is defined as the maximum amplitude of signal behind the amplitude limit and the ratio of the maximum amplitude of limitation signal not.X (n), y (n) are respectively the IFFT output signals before and after the amplitude limit, and A is a threshold value.

$y\left(n\right)=\left\{\begin{array}{cc}\frac{A}{{\sqrt{\left|x\left(n\right)\right|}}^2}x\left(n\right),& \left|x\left(n\right)\right|\&GreaterEqual;A\\ x\left(n\right),& \left|x\left(n\right)\right|<A\end{array}\right.$

The foundation of the block clipping method that the present invention proposes is by signal correlation, and its theory analysis is: the signal of OFDM transmitting terminal is a continuous signal s (t),

$s\left(t\right)=\frac{1}{\sqrt{N}}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{a}_n\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051004313300151.png,A20051004313300152.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{n\&pi;t}\right).$

The instantaneous power P of s (t) then _c(t)=s (t) s ^*(t) be:

$P_c\left(t\right)=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\underset{m=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{a}_n{a}_m^{*}\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051004313300151.png,A20051004313300152.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}(n-m)t\right)$

$=\frac{1}{N}\{N+2\mathrm{Re}\{\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\underset{m=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{a}_n{a}_m^{*}\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051004313300151.png,A20051004313300152.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}(n-m)t\right)\left\}\right\}$

$=1+\frac{2}{N}\mathrm{Re}\left\{\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051004313300151.png,A20051004313300152.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;t})\underset{n=0}{\overset{N-1-k}{\mathrm{\&Sigma;}}}{a}_n{a}_{n+k}^{*}\right\}$

For any plural z since exist inequality: Re (z)≤| z| and | ∑ z _n|≤∑ | z _n|, therefore

$P_c\left(t\right)\&le;1+\frac{2}{N}\underset{k=1}{\overset{N-1}{\mathrm{\&Sigma;}}}\left|\mathrm{\&rho;}\left(k\right)\right|$

Wherein $\mathrm{\&rho;}\left(k\right)=\underset{n=0}{\overset{N-1-k}{\mathrm{\&Sigma;}}}{a}_n{a}_{n+k}^{*},(k=\mathrm{0,1}...,N-1),$ Expression auto-correlation coefficient aperiodic.

From with upper inequality as can be known, if reduce autocorrelation aperiodic, obtaining the signal maximum power value will reduce.And block clipping of the present invention is to distinguish amplitude limit again by the OFDM symbol being divided into some, has upset the autocorrelation of signal, promptly reduces autocorrelation aperiodic of signal, so resultant signal maximum power value also can diminish after through the multicarrier combination.That is to say the diminishing of dynamic range of signal power, avoided signal non-linear distortion.

In block clipping of the present invention, if the block count of OFDM symbol is many more, the high peak valve signal that amplitude limit falls is just many more, average power is just low more, the signal linear dynamic range is just more little, and this just is easy to satisfy the linear dynamic range of power amplifier, has avoided signal non-linear distortion naturally.Yet on the other hand, because the signal peak amplitude limit falls too much, the error rate has just raise naturally, and the subcarrier of block count and OFDM is relevant, so block count M will decide according to each index request of ofdm system.For this reason, the present invention adopts following process to carry out block clipping, to solve nonlinear distortion question in the ofdm system:

(1), makes maximum acceptable signal peak power P in advance according to the range of linearity of power amplifier _MaxThereby, make amplitude limit rate CR;

(2) according to sub-carrier number N and the maximum acceptable signal power P of OFDM _MaxDetermine the block count M of block clipping;

(3) according to formula $A_i=\mathrm{CR}\&times;\sqrt{P_{i\max }},i=\mathrm{1,2}...M$ Calculate amplitude limit rate A _i

(4) the OFDM symbol lengths L of OFDM transmitting terminal output is divided into the sub-piece of M, amplitude limit falls each sub-piece and is higher than threshold value A respectively _iPerformance number;

(5) each the sub-piece behind the amplitude limit is made up according to original order, and send to channel.

Said method of the present invention is applied in the ofdm communication system, and this system comprises: transmitting element, block clipping unit, receiving element.Wherein:

Transmitting element is used to receive the signal behind 4-QAM modulation and the serial to parallel conversion, and through invert fast fourier transformation, adds Cyclic Prefix and parallel serial conversion, forms OFDM transmission signal, sends into the block clipping unit;

The block clipping unit at first is divided into the OFDM symbol M sub-piece, respectively each sub-piece is carried out amplitude limit then, at last the sub-piece behind the amplitude limit is sent according to original position grouping.

Receiving element is used to receive the signal through serial to parallel conversion, and by going Cyclic Prefix, fast fourier transform, outputs to the parallel serial conversion unit, through the signal that needing to obtain after the 4-QAM demodulation.

Method of the present invention mainly realizes by the block clipping unit in the described system, this block clipping unit comprises: sub-piece dispenser, at least 2 block clipping devices, combiner, sub-piece dispenser is divided into the OFDM data flow that sends the sub-piece of at least 2 equal lengths, be transferred to the block clipping device, the divided sub-piece of each piece is carried out amplitude limit, afterwards, each the sub-piece that was limited is transferred to combiner, its sub-piece is reconfigured transmission according to original order by combiner.

The present invention compares with traditional amplitude limit method, has the following advantages:

1, traditional amplitude limit method all be for the ratio that reduces peak power and average power be PAPR, and block clipping of the present invention is by reducing peak power, the final reduction average power signal of realizing, when reducing peak power, also reduced average power, to such an extent as to this has only increased the drawback of average power signal increase system average transmit power for reducing PAPR with regard to having been avoided.

2, because block clipping of the present invention is directly to propose at nonlinear distortion question in the ofdm system, can when reducing signal peak power, reduce the average power of signal, the power dynamic range of compressed signal, thereby avoided the phenomenon of nonlinear distortion in the ofdm system well, simultaneously, reduced mould/number, the complexity of D/A and improve their accuracy has reduced the cost of whole system;

3, because block clipping method of the present invention, the CR value is not needed to resemble fixed so high of traditional amplitude limit, avoid the clipped noise that brings because of amplitude limit is too high, improved efficiency of transmission, improve the bit error rate performance of traditional amplitude limit, improved the performance of whole ofdm communication system.

4,, so can adjust block count, increase the flexibility and the practicality of communication system, and amplitude limit is simple, directly perceived according to different system because the block count M among the present invention is relevant with the sub-carrier number in the ofdm communication system.

Description of drawings

Fig. 1 is the theory diagram that has general ofdm communication system now

Fig. 2 is a block clipping method flow chart of the present invention

Fig. 3 is the theory diagram that existing ofdm communication system adopts block clipping

Fig. 4 is the block clipping theory diagram that the present invention proposes

Fig. 5 is a signal power distribution schematic diagram behind traditional amplitude limit

Fig. 6 is a signal power distribution schematic diagram behind the employing block clipping of the present invention

Fig. 7 is an example error rate comparison diagram of the present invention

Fig. 8 is an example CCDF CCDF comparison diagram of the present invention

Embodiment

Followingly the present invention is described in further detail with reference to accompanying drawing.

With reference to Fig. 2, be used for block clipping method of the present invention at ofdm system, carry out according to the following procedure:

The present invention is in l-G simulation test, the input data by information source produce at random, sub-carrier number is 1024, IFFT counts is 4096, just adopted 4 times of over-samplings, promptly the length L of OFDM=4096 are 8 according to ofdm system requirement and the fixed block count M of sub-carrier number.Because over-sampling, the length of each piece is 512 behind the piecemeal, is respectively 512 sub-piece amplitude limit then to length, and being combined into length by combiner again is that 4096 OFDM symbol sends.

At first, according to the range of linearity of power amplifier, promptly peak power is less than or equal to the input power that power amplifier allows, and makes maximum acceptable signal peak power P in advance _Max, make amplitude limit rate CR then.Amplitude limit rate CR=0.65 for example of the present invention.

Second step is according to maximum acceptable signal peak power P _MaxAnd the OFDM sub-carrier number, determine the block count M of block clipping, for example the sub-carrier number of OFDM is 1024, the M value is weighed system performance index by emulation and is decided to be 8.

In the 3rd step, define according to amplitude limit rate CR: behind every amplitude limit the maximum amplitude of signal and not the ratio of the maximum amplitude of limitation signal extrapolate $A_i=\mathrm{CR}\&times;\sqrt{P_{i\max }},i=\mathrm{1,2}...M.$ First threshold value A in the emulation of the present invention _i≈ 75.5.

In the 4th step, it is M sub-piece that the OFDM symbol lengths L of ofdm system transmitting terminal output is divided into the M equal portions, and the M here is more than or equal to 2, and amplitude limit falls the amplitude that each sub-piece is higher than threshold value respectively, and the formula of this amplitude limit is;

$y\left(n\right)=\left\{\begin{array}{cc}\frac{A}{{\sqrt{\left|x\left(n\right)\right|}}^2}x\left(n\right),& \left|x\left(n\right)\right|\&GreaterEqual;A\\ x\left(n\right),& \left|x\left(n\right)\right|<A\end{array}\right.$

In the formula: A is that the maximum amplitude of signal is a threshold value behind the amplitude limit, and x (n) is the output signal of IFFT, and y (n) is the output signal that adopts this amplitude limit algorithm.First OFDM symbol x among the present invention for example ₁(n) export y behind the amplitude limit ₁(n), n=1 wherein, 2 ..., 512.Corresponding M piece x _M(n) export y behind the amplitude limit _M(n), n=3585 wherein, 3586 ... 4096.

In the 5th step, each the sub-piece behind the amplitude limit according to original sequential combination, is about to y ₁(n)～y _M(n) sequential combination by 1～M is y (n), n=1, and 2 ... 4096, send to channel at last.

With reference to Fig. 3, the applied ofdm communication system of the present invention comprises and transmitting and receiving.Wherein radiating portion mainly comprises: deserializer 3,4-QAM modulator 4, invert fast fourier transformation IFFT digital signal processor 6, parallel-to-serial converter 7, protection be inserter 9, digital to analog converter 12, low pass filter 13 at interval; block clipping device 11, this block clipping device mainly is made up of sub-piece cutting unit 26, amplitude limit unit 27, combiner 28.Receiving unit comprises: low pass filter 16, analog to digital converter 17, protection interval removal 19, deserializer 20, fast fourier transform FFT digital signal processor 21,4-QAM demodulator 23, parallel-to-serial converter 24.The course of work of whole ofdm communication system is as follows:

One, the transmitting terminal of ofdm communication system

The data symbol stream 2 of the information of carrying that random signal generator 1 produces is delivered to first deserializer 3, then the parallel signal of output is sent into modulator 4 and carries out the quadrature amplitude modulation first time, adopts the 4-QAM modulation here.Binary data stream just is mapped as binary data on each parallel channel the point on the signal constellation which after modulating through 4-QAM, promptly forms the frequency domain data sequence of plural form, and this data sequence 5 is delivered to the transmitting terminal of OFDM.At transmitting terminal, should plural number parallel data sequence carry out the modulation second time by 6 pairs of invert fast fourier transformation IFFT digital signal processors, promptly original signal is carried out frequency modulation(FM) with one group of orthogonal sine and cosine signal, obtain the parallel complex data of real domain after the modulation, this complex data is merged into serial data stream 8 by first parallel serial conversion 7 with the data flow of each passage.For eliminating the influence of intersymbol interference ISI and many interchannel interference ICI through being caused; need to insert protection at interval; from inserting protection spacer 9 output streams 10; be that the OFDM symbol is sent to after block clipping device 11 carries out piecemeal; be input to digital to analog converter 12 again and become analog signal, this analog signal by after low pass filter 13 filtering from channel dateout 14.This channel can be that wire message way also can be a wireless channel, if wireless channel needs that then data 14 are carried out upper side frequency and handles, for example adopts to add Gauss's white noise channel.

Described amplitude limiter is different from traditional amplitude limiter, and its internal structure mainly is made up of sub-piece dispenser 26, amplitude limit unit 27, combiner 28 as shown in Figure 4, and the number of this amplitude limit unit 27 is corresponding with the sub-piece number that sub-piece cutting unit 26 is cut apart.After sub-piece cutting unit 26 is divided into the M equal portions with the length L of OFDM symbol, again each sub-piece is sent into corresponding amplitude limit unit 27 and carry out amplitude limit, for example, divided first is sent to first amplitude limiter 1, second is sent to second amplitude limiter, 2, the M pieces and is sent to M amplitude limiter M; At last each the sub-piece behind the amplitude limit is sent by combiner 28 combinations.The method of this amplitude limit is identical with traditional amplitude limit method, here CR be defined as the maximum amplitude of signal behind the amplitude limit and every not the maximum amplitude of limitation signal ratio promptly $A_i=\mathrm{CR}\&times;\sqrt{P_{i\max }},i=\mathrm{1,2}...M.$

Two, the receiving terminal of ofdm communication system

Need obtain analog signal through low pass filter 16 from the data-signal of channel output, this analog signal becomes digital signal 18 after by analog to digital converter 17.Remove protection at interval through protection interval removal 19 again; be sent to second deserializer 20; undertaken obtaining signal 22 after the demodulation by 21 pairs of these data flow of fast fourier transform FFT digital signal processor then; this signal passes through the 4-QAM demodulation 23 and second parallel serial conversion 24 again; recover and raw information 2 essentially identical reception information 25, to satisfy the requirement of the stay of two nights.

Effect of the present invention can further specify by Fig. 5～Fig. 8.

For the amplitude limit principle of block clipping is described more intuitively, Fig. 5, Fig. 6 have illustrated the difference on distribute power of traditional amplitude limit and block clipping.Wherein: selecting sub-carrier number for use is 128, under the identical condition of average power, and the CR=0.5 of traditional amplitude limit, the CR=0.65 of block clipping.

By Fig. 5 and Fig. 6 as seen, each OFDM symbol threshold value of traditional amplitude limit is the same, and peak value all has been limited, and is lower than the thresholding peak value and does not change, so limit the peak value that the falls shape that is in line.And block clipping is the threshold value difference amplitude limit with each piece, has avoided the too high signal limiter of peak value to fall on the ground too much, thereby has reduced the error rate.The peak value that limit is fallen becomes stepped, and this has just upset the autocorrelation of signal, reduces autocorrelation aperiodic of signal, so resultant signal maximum power value also can diminish after through the multicarrier combination.Thereby make that the distribution of power is more concentrated, such signal just is not easy to surpass the linear dynamic range of power amplifier, easily avoids nonlinear distortion.

Though this invention does not propose at reducing PAPR,, also adopt block clipping technology systematic function afterwards in order to study for block clipping being described in the place that is different from traditional amplitude limit aspect the substantive issue that solves ofdm system.For this reason we under the 4QAM modulation condition emulation error rate BER and CCDF CCDF.

Emulation 1

Tentatively allocate equal performance number according to the ofdm system performance requirement, adjust the amplitude limit rate CR value of traditional amplitude limit and block clipping then respectively, make the two under the approximately equalised situation of average power, carry out emulation.Its bit error rate performance Fig. 7 is that BER by ofdm system in the awgn channel is to the simulation curve of signal to noise ratio snr.Under identical SNR, BER is the smaller the better.As can be seen from Figure 7, the BER minimum of the original ofdm system of amplitude limit not, but this system is because having a strong impact on of nonlinear distortion question is normally unpractical; Under the common prerequisite that can eliminate nonlinear distortion in the ofdm system, compare traditional amplitude limit, block clipping has effectively improved bit error rate performance.

Can further be clear that from following table 1: when signal to noise ratio was 12, the error rate behind the employing block clipping was 4.3545 * 10 ^-5, the error rate behind traditional amplitude limit is 3.5665 * 10 ^-4The former is than the low order of magnitude of the latter.

The contrast table of table 1 block clipping and traditional amplitude limit

Two kinds of amplitude limit methods relatively	Amplitude limit rate (CR)	Average power	The error rate (BER) signal to noise ratio snr=12dB	Peak power ratio PAPR (dB)
					The tradition amplitude limit	0.5	1.9321×10 3	3.5665×10 -4	2.8028～6.1859
Block clipping	0.65	1.9242×10 3	4.3545×10 -5	4.4145～8.3445

Owing in emulation, do not consider chnnel coding, so some height of the error rate.If the employing chnnel coding, as: convolutional encoding, RS sign indicating number, turbo coding etc., the bit error rate performance of communication system also can obtain very big improvement.

Emulation 2

Fig. 8 is the PAPR distribution of weighing ofdm system with the CCDF CCDF of PAPR.Promptly calculate PAPR and surpass a certain threshold value PAPR ₀Probability, the CCDF CCDF that obtains.Can be expressed as with the mathematics formula: Prob{PAPR＞PAPR ₀.As can see from Figure 8: the PAPR of the PAPR behind the block clipping after than traditional amplitude limit is big.When this is the peak power reduction, the cause that average power reduces, because from calculating the PAPR formula:

${\mathrm{PAPR}}_1=10\mathrm{lo}{g}_{10}\left[\underset{n}{\max }\left({\left|x_n\right|}^2\right)/\mathrm{mean}\left({\left|x_n\right|}^2\right)\right]$

As can be seen, average power has reduced, and PAPR has raise naturally, and it illustrates that in fact block clipping is reduced in PAPR between 4.4145～7.6445dB, just can satisfy the linear power scope of power amplifier.

For those skilled in the art; may be under the situation that does not deviate from technical solution of the present invention after having understood thought of the present invention; carry out various corrections and change on form and the details, by different mode specific implementations, but still within protection scope of the present invention.

Claims (3)

1, a kind of block clipping method that is used for solving the ofdm system nonlinear distortion question, its process is as follows:

According to the range of linearity of power amplifier, make maximum acceptable signal peak power P in advance _MaxThereby, make amplitude limit rate CR value;

Determine the block count M of block clipping according to the sub-carrier number of OFDM;

According to formula $A_i=\mathrm{CR}\&times;\sqrt{P_{i\max }}--i=\mathrm{1,2}...M$ , calculate the amplitude gate limit value;

The OFDM symbol lengths L of ofdm system transmitting terminal output is divided into the sub-piece of M, and amplitude limit falls the performance number that each sub-piece is higher than threshold value A respectively;

Each height piece behind the amplitude limit is made up according to original order, and send to channel.

2, realize the block clipping device of claims 1 method, comprise sub-piece dispenser, at least two amplitude limiters, combiner, after sub-piece dispenser is divided into the sub-piece of at least 2 equal lengths with the length L of the OFDM symbol of OFDM transmitting terminal output, send into corresponding amplitude limiter respectively and carry out amplitude limit, the sub-piece output of each behind the amplitude limit is carried out former bit combination by combiner according to original order and is sent.

3, method according to claim 1, wherein scheduled communication system acceptable average power and dynamic range of signals, determine according to the linear dynamic range of communication system power amplifier that mainly promptly peak power is less than or equal to the input power that power amplifier allows.

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