CN101005474B  New compression extension method for reducing OFDM signal peak to average ratio  Google Patents
New compression extension method for reducing OFDM signal peak to average ratio Download PDFInfo
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 CN101005474B CN101005474B CN2006100330168A CN200610033016A CN101005474B CN 101005474 B CN101005474 B CN 101005474B CN 2006100330168 A CN2006100330168 A CN 2006100330168A CN 200610033016 A CN200610033016 A CN 200610033016A CN 101005474 B CN101005474 B CN 101005474B
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 companding
 ofdm signal
 peak
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Abstract
The method comprises: after sending end makes inverse fast Fourier transform (IFFT), it determines the amplitude value of OFDM signals. The invention provides a companding function for making transform in order to make the transformed amplitude value submits to the linear function distribution and the phase of signal keep unchanged; the receiving end makes inverse companding transform for amplitude value of the received signals, and then makes fast Fourier transform (FFT) and decode for it.
Description
Technical field
The present invention relates to a kind of compand method that reduces signal peaktoaverage ratio in the orthogonal FDM communication system, is a kind of by the ofdm signal amplitude is carried out the method that nonlinear companding suppresses peaktoaverage force ratio, belongs to communication technical field.
Background technology
OFDM (Orthogonal Frequency Division Multiplexing, OFDM) be the very high multicarrier modulation technology of a kind of channel utilization, it can overcome the intersymbol interference that multipath fading causes effectively, thereby has obtained application in wireless communication systems such as WLAN.But, because ofdm signal is the superimposed results of a plurality of subchannel signals, so in the transmitter of system, can there be bigger instantaneous peak value in composite signal, thereby produce excessive peaktoaverage power than (Peak to Average Power Ratio, PAPR are called for short peaktoaverage force ratio).These instantaneous peak values will produce nonlinear distortion during by the limited amplifier of the linear dynamic range in the transmission system, and the harmonic wave that is produced can enter relevant subchannel, causes interchannel interference, makes the performance rapid deterioration of system.Therefore, must take measures to reduce the peaktoaverage force ratio of ofdm signal.
The method that reduces the ofdm signal peaktoaverage force ratio at present mainly contains following several:
1. cut ripple
Promptly set an amplitude limit thresholding, the big amplitude signal that is higher than this thresholding is directly pruned.This is the simplest method, can be applicable to the system of the subcarrier formation of any number.But this method can cause signal distortion, causes the performance of BER decline of system and the increase of outofband radiation power.
2. encode
Utilize the different code characters that different coding produces and select the less code character of peaktoaverage force ratio to carry out data information transmission.Shortcoming is as follows: the one, be subjected to the restriction of code modulation mode; The 2nd, be subject to number of sub carrier wave, along with the subcarrier number purpose increases, computation complexity enlarges markedly; The 3rd, the data rate behind the coding decreases.
3. selectivity mapping
With the different random phase sequence of vectors of M respectively with the list entries dot product of IFFT, and then implement the IFFT computing, the output sequence of selecting wherein to have minimum peaktoaverage force ratio transmits.Shortcoming is as follows: the one, and computation complexity is big, needs to calculate extra (M1) group IFFT computing; The 2nd, need transmit relevant supplementary safely.
4. partial transmission sequence
The data symbol of input is divided into the experimental process group, and the length of each son group still equals the number of whole subcarriers, then each subcomponent not multiply by certain weight coefficient, remerges this a little group at last, thereby reaches the purpose that reduces the whole system peaktoaverage force ratio.The shortcoming of this method is similar to the selectivity mapping, and the one, computation complexity is big, and the 2nd, need transmit supplementary safely.
5. nonlinear companding
At the transmitting terminal of communication system, reduce the amplitude of big amplitude signal and the amplitude of enhancing small magnitude signal by numerical transformation; And carry out inverse transformation to recover primary signal at receiving terminal.Up to the present concrete companding method has following three kinds:
The one, adopt the mulaw companding method of voice signal in handling to handle ofdm signal, it is exaggerated small amplitude signal, and signal is constant substantially significantly, thereby is that cost reaches the purpose that reduces peaktoaverage force ratio with the average power that increase transmits.This method makes the performance number of signal more near the nonlinear change zone of power amplifier, causes the distortion of signal easily.
The 2nd, the Gaussian Profile of the real part and the imaginary part of original ofdm signal being followed by error function is transformed to even distribution respectively;
The 3rd, index companding method, the rayleigh distributed of the ofdm signal amplitude being obeyed by the companding function are transformed to even distribution (when d=1).
Method two and three all is to reduce peaktoaverage force ratio by the statistical distribution characteristic that improves signal, discover that the distorted signals that these two kinds of methods cause is excessive, though the peaktoaverage force ratio of the normalized ofdm signal of average power can be reduced to below the 4.78dB, system's bit error rate be increased to be difficult to practical degree.
Summary of the invention
Realize characteristics simple, that computation complexity does not increase with number of sub carrier wave in order to utilize nonlinear companding method, and in order to overcome the shortcoming of existing companding method, guarantee before and after the companding that the average power of signal is constant, guarantee when effectively reducing peaktoaverage force ratio, make system have lower bit error rate, the present invention proposes a kind of new companding method that is used to reduce the ofdm signal peaktoaverage force ratio.
The invention has the beneficial effects as follows and realize by following proposal:
After the transmitting terminal of communication system carries out the IFFT computing, the Distribution Statistics of ofdm signal amplitude is transformed to the form of linear function by the companding function; Before the receiving terminal of communication system carried out the FFT computing, the inverse function by abovementioned companding function carried out contrary companding transform with received signal amplitude, reduced the peaktoaverage force ratio of ofdm signal thus.Abovementioned companding function is:
The inverse function of abovementioned companding function is:
Wherein, σ
_{s}The average power of expression ofdm signal is opened the value behind the radical sign, that is
${\mathrm{\σ}}_{s}^{2}=E\left({\left{s}_{n}\right}^{2}\right),$ s
_{n}Represent original ofdm signal.
The value of ofdm signal amplitude behind companding is distributed as linear function, and its probability density function is:
T wherein
_{n}Signal behind the expression transmitting terminal companding transform.
In technical scheme of the present invention, transmitting terminal/receiving terminal all is only the amplitude of signal to be done companding/contrary companding transform, the phase invariant of signal.
Adopt the amplitude of abovementioned companding function to ofdm signal  s
_{n} after carrying out companding transform, the average power of signal remains unchanged.
In the multicarrier communications systems that the new compression extension method of the reduction ofdm signal peaktoaverage force ratio that the present invention proposes is applied to adopt OFDM technology or other to have the peaktoaverage force ratio problem.
Compared with prior art, advantage of the present invention is as follows:
1, the effect of new compression extension method reduction peaktoaverage force ratio of the present invention is suitable with the selectivity mapping method, but its amount of calculation is far smaller than the selectivity mapping method;
2, new compression extension method of the present invention to cut wave effect so obvious unlike error function companding method and index companding method, therefore, under identical signal to noise ratio condition, system's bit error rate of new compression extension method is lower than the bit error rate of other two kinds of companding methods.
Generally speaking, it is compromise preferably that the new companding method of the invention has reached between peaktoaverage force ratio and performance of BER, and do not need the computing of large amount of complex, therefore has higher utility.
Description of drawings
Fig. 1 is for adopting the ofdm system block diagram of amplitude companding transform method;
Fig. 2 is the companding function curve comparison diagram in index companding method and the companding method of the present invention;
Fig. 3 is the CCDF comparison diagram of peaktoaverage force ratio behind several different companding methods of employing;
Fig. 4 is the CCDF curve that adopts peaktoaverage force ratio behind the selectivity reflection method in the classical documents;
Fig. 5 is the bit error rate curve comparison diagram behind several different companding methods of employing.
Embodiment
Embodiment 1
As shown in Figure 1, the information source end of communication system produces 256000 bits at random, after the QPSK modulation, distributes to 256 subcarriers one by one, realizes quadrature modulation through IFFT then, obtains original ofdm signal.Average power to signal in the present embodiment has been done normalized.Obtain the amplitude of complex signal then, the new companding function that adopts the present invention to propose is done companding transform; The receiving terminal of communication system carries out before the FFT computing, and the inverse function by abovementioned companding function carries out contrary companding transform with received signal amplitude.The companding function is
The inverse function of companding function is:
The peaktoaverage force ratio of signal distributes behind the statistics companding, can obtain the simulation curve of its CCDF.After adopting the companding function of the present invention's proposition, when probability is 10
^{4}The time correspondence peaktoaverage force ratio approximate 7.8dB, reached the purpose of effective reduction ofdm signal peaktoaverage force ratio.
Embodiment 2
The basic procedure of another routine emulation experiment also as shown in Figure 1.The information source end produces 640000 bits at random, after the DQPSK modulation, distributes to 64 subcarriers one by one, realizes quadrature modulation through IFFT then, obtains original ofdm signal.Then obtain the amplitude of complex signal, the new companding function that adopts the present invention to propose is done companding transform, and the signal after will handling then is converted to serial from walking abreast, through sending into additive white Gaussian noise channel after steering D/A conversion and the upconversion.
The signal that receiving terminal receives is parallel with signal from serial conversion after downconversion, mould/transformation of variables, obtains the amplitude of complex signal then, and the contrary companding function that adopts the present invention to propose is done contrary companding transform, and then signal is carried out the FFT computing.Be serial with signal from parallel transformation subsequently, and, count the bit error rate of signal through the DQPSK demodulation.Through more as can be known, under identical signal to noise ratio condition, the bit error rate of the bit error rate ratio error function companding method of system and index companding method is much lower behind the employing new compression extension method.As seen the new compression extension method that proposes of the present invention is simple and practical, is a kind of scheme of more satisfactory reduction peaktoaverage force ratio.
As shown in Figure 2, for the normalized ofdm signal of average power, the new companding function that the present invention proposes approaches the peak swing of largesignal
, and index companding method is approached the peak swing of largesignal
The thresholding of the former amplitude limit is the latter
Doubly, so the signal distortion ratio index companding method that new compression extension method causes is little, hence one can see that adopts that the performance of BER of system is better than index companding method behind the new compression extension method.
As shown in Figure 3, adopt that the CCDF curve of the companding method that the present invention proposes is precipitous to end in the position that peaktoaverage force ratio approximates 7.8dB.With CCDF curve (the Seung Hee Han that adopts peaktoaverage force ratio behind the selectivity reflection method in the classical documents shown in Figure 4, Jae Hong Lee, An overviewof peaktoaverage power ratio reduction techniques for multicarrier transmission, Wireless Communications, IEEE Volume 12, Issue 2, April 2005 pp.5665) contrast as can be known, the effect that new companding scheme reduces peaktoaverage force ratio is suitable with the selectivity mapping method, but its amount of calculation is far smaller than the selectivity mapping method.
As shown in Figure 5, under identical signal to noise ratio condition, the bit error rate of the bit error rate ratio error function companding method of system and index companding method is much lower behind the employing new compression extension method.What " ideal situation " was corresponding among the figure is the theoretical bit error rate curve of only considering DQPSK modulation system under the additive white Gaussian noise condition.
Claims (5)
1. a companding method that reduces the ofdm signal peaktoaverage force ratio is characterized in that after the transmitting terminal of communication system carries out the IFFT computing, the statistical property of ofdm signal amplitude is transformed to the distribution of obedience linear function by the companding function; Before the receiving terminal of communication system carried out the FFT computing, the inverse function by abovementioned companding function carried out contrary companding transform to received signal amplitude, reduced the peaktoaverage force ratio of ofdm signal thus, and abovementioned companding function is:
The inverse function of abovementioned companding function is:
Wherein, σ
_{s}The average power of expression ofdm signal is opened the value behind the radical sign, that is
s
_{n}Represent original ofdm signal.
2. the companding method of reduction ofdm signal peaktoaverage force ratio according to claim 1 is characterized in that the ofdm signal amplitude is distributed as linear function in the value of transmitting terminal behind companding, and its probability density function is:
T wherein
_{n}Signal behind the expression transmitting terminal companding transform;  t
_{n} the amplitude of the signal behind the expression transmitting terminal companding transform.
3. the companding method of reduction ofdm signal peaktoaverage force ratio according to claim 1 is characterized in that transmitting terminal/receiving terminal all is only the amplitude of signal to be done companding/contrary companding transform, the phase invariant of signal.
4. the companding method of reduction ofdm signal peaktoaverage force ratio according to claim 1 is characterized in that ofdm signal amplitude its average power behind companding remains unchanged.
5. the companding method of reduction ofdm signal peaktoaverage force ratio according to claim 1 is in the multicarrier communications systems that it is characterized in that being applied to adopt OFDM technology or other to have the peaktoaverage force ratio problem.
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CN106385394A (en) *  20160923  20170208  西安电子科技大学  Method for suppressing peaktoaverage ratio of orthogonal frequency division multiplexing (OFDM) signals 
CN106385394B (en) *  20160923  20190326  西安电子科技大学  A method of inhibiting OFDM signal peaktoaverage force ratio 
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