CN102664858A - Combined method for reducing peak-to-average ratio of OFDM (orthogonal frequency division multiplexing) system and tracking carrier frequency - Google Patents

Abstract

The invention provides a combined method for reducing peak-to-average ratio of an OFDM (orthogonal frequency division multiplexing) system and tracking carrier frequency. The combined method is special for a wireless communication system with the OFDM technology as core and includes that an emitter selects information data of a subcarrier fixing position in a frequency domain, solves complex conjugate of the subcarrier fixing position, fills conjugate coding data into subcarriers at other fixing positions, and modulates pilot frequency subcarriers by conjugate of frequency domain data so as to offset phases and reduce peak power of the system; meanwhile, a receiver extracts information data and conjugate coded data of the corresponding subcarrier position of each character in the frequency domain, estimates character phase rotation caused by residual carrier frequency offset on the basis of conjugate relation of the subcarrier position, and carries out frequency offset compensation on the received data in the time domain or frequency domain. The combined method can track carrier frequency offset precisely, and can reduce the peak-to-average ratio of the OFDM system.

Description

Ofdm system reduces peak-to-average force ratio and carrier frequency tracking integrated processes

Technical field

The present invention relates to a kind of ofdm system and reduce peak-to-average force ratio and carrier frequency tracking integrated processes; Be prone to ruined problem to the ofdm system peak-to-average power than high and responsive, orthogonality, propose that subcarrier is carried out the conjugation Methods for Coding and make system overcome two big defectives simultaneously frequency deviation.It is in the wireless communication system of core that this invention can be widely used in the OFDM technology.

Background technology

OFDM (OFDM) system is because its band efficiency is high, anti-intersymbol string (ISI) is disturbed plurality of advantages such as strong, the anti-frequency selective fading ability of ability is strong; Be described as the core of next generation wireless communication technology, but also have the very responsive and high deficiency of peak-to-average force ratio (PAPR) simultaneously carrier wave frequency deviation.Carrier frequency shift causes the orthogonality between subcarrier to be destroyed; Thereby disturb between the generation subchannel (ICI); Big peak-to-average force ratio causes signal non-linear distortion or the follow-up amplifying circuit linearity is required high and be difficult to realize that these all make systematic function descend greatly.

Two stages are caught and followed the tracks of to the Frequency Synchronization branch, and net synchronization capability quality key is to follow the tracks of.Existing repeating transmission symbol tracking method, better performances but take additional band.Cyclic Prefix tracing converges faster but its performance is influenced by circulating prefix-length need be than long loop prefix.Difference decision-feedback method and relevant decision-feedback method, operand is bigger, and complexity is higher.Aspect the reduction peak-to-average force ratio, classic methods has the partial transmission sequence method and selects reflection method.

OFDM is a kind of special multi-carrier communication scheme, and its basic thought is that the data code flow with high speed serialization changes the parallel low speed data code stream in N road into, and linear modulation is transmitted to an equifrequent N at interval mutually orthogonal subcarrier simultaneously then.For the ofdm system that comprises the N number of sub-carrier, the power normalization complex radical belt signal that obtains through invert fast fourier transformation (IFFT) is:

$x\left(n\right)=\frac{1}{\sqrt{N}}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}s\left(k\right)\exp \left(\frac{2\mathrm{\πjnk}}{N}\right)$

N=0,1,2 ..., N-1, wherein s (k) representes the modulation signal on the k number of sub-carrier, it is the mapping value of QAM for the MQAM-OFDM system.Compare with single-carrier system; Because the OFDM symbol is independently formed through the sub-carrier signal addition of ovennodulation by a plurality of, such composite signal just might produce bigger peak power, can bring bigger peak-to-average power ratio thus; Be called for short peak-to-average force ratio, be defined as:

$\mathrm{PAPR}\left(\mathrm{dB}\right)=10\lg \frac{\max \left\{{\left|x\left(n\right)\right|}^2\right\}}{E\left\{{\left|x\left(n\right)\right|}^2\right\}}$

Wherein x (n) expression is through resulting output signal after the IFFT computing.For the ofdm system that comprises the N subchannel, when the N sub-signals was sued for peace with identical phase place, the peak power of resulting signal will be N a times of average power, thereby the maximum peak-to-average force ratio of baseband signal can be 10lgN.

Make a start behind the IFFT,

$x_N\left(m\right)=\frac{1}{\sqrt{N}}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}s\left(k\right)e^{j(2\mathrm{\π}/N)\mathrm{mk}},$ m＝0，1，...，N-1

After adding Cyclic Prefix, X=[X _LX _N], through behind the channel,

y(l)＝x(l)e ^j(2π/N)/ε+n(l)，l＝0，1，...，N+L-1

Wherein ε is the normalization frequency deviation.After receiving end removal CP,

y _N(m)=x _N(m) e ^{J ((m+L) ε of 2 π/N)}+ n (m), m=0,1 ..., behind the N-1FFT,

$r\left(i\right)=\frac{1}{\sqrt{N}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_N\left(m\right)e^{-j(2\mathrm{\π}/N)\mathrm{mi}},i=\mathrm{0,1},...,N-1$

$=\frac{1}{N}{e}^{j(2\mathrm{\π}/N)\mathrm{L\ϵ}}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{e}^{j(2\mathrm{\π}/N)m(k-i+\mathrm{\ϵ})}+\frac{1}{\sqrt{N}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}n\left(m\right)e^{-j(2\mathrm{\π}/N)\mathrm{mi}}$

$=\frac{1}{N}{e}^{j(2\mathrm{\π}/N)\mathrm{L\ϵ}}s\left(i\right)\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{e}^{j(2\mathrm{\π}/N)\mathrm{m\ϵ}}+\frac{1}{N}{e}^{j(2\mathrm{\π}/N)\mathrm{L\ϵ}}\underset{\underset{k\≠i}{k=0}}{\overset{N-1}{\mathrm{\Σ}}}s\left(k\right)\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{e}^{j(2\mathrm{\π}/N)m(k-i+\mathrm{\ϵ})}+\frac{1}{\sqrt{N}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}n\left(m\right)e^{-j(2\mathrm{\π}/N)\mathrm{mi}}$

$=s\left(i\right)\frac{\sin \left(\mathrm{\π\ϵ}\right)e^{\mathrm{j\π\ϵ}(N+2L-1)/N}}{N\sin (\mathrm{\π\ϵ}/N)}+\underset{\underset{k\≠i}{k=0}}{\overset{N-1}{\mathrm{\Σ}}}s\left(k\right)\frac{\sin \left(\mathrm{\π}(\mathrm{\ϵ}+k-i)\right)e^{\mathrm{j\π}[2\mathrm{L\ϵ}+(N-1)(\mathrm{\ϵ}+k-i)]/N}}{N\sin (\mathrm{\π}(\mathrm{\ϵ}+k-i)/N)}+\frac{1}{\sqrt{N}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}n\left(m\right)e^{-j(2\mathrm{\π}/N)\mathrm{mi}}$

First of following formula is the useful signal of i number of sub-carrier, and second is the interference of other subcarriers, i.e. ICI, and the 3rd is that white Gaussian noise disturbs.

When frequency deviation was very little, amplitude fading was approximately 1, the signal interference ratio approximates infinity, and can ignore amplitude fading and ICI this moment; When frequency deviation was big, amplitude fading and ICI were comparatively serious, needed it is compensated.

Summary of the invention

The peak-to-average force ratio height that the present invention seeks to exist to ofdm system reaches the responsive two big defectives of frequency deviation; Proposition reduces peak-to-average force ratio based on conjugate code and carrier wave frequency deviation is followed the tracks of integrated processes; It is big not enough to remedy system two simultaneously with a kind of technology, reduces the system peak-to-average ratio thereby reduce system peak power, and tiny carrier frequency offset of compensatory tracking stage; Overcome the asynchronous of transceiver oscillator, applicable to all wireless communication systems based on the OFDM technology.

Technical scheme of the present invention:

The present invention proposes a kind of frequency tracking method based on conjugate code that is applicable to ofdm system.The MQAM mapping value is got conjugation and insert at frequency domain and wherein form a symbol and carry out IFFT, effectively reduce system peak power through phase cancellation and reduce peak-to-average force ratio.Utilize the symbol conjugate relation to estimate that thereby phase deviation obtains the normalization frequency deviation and in time domain or frequency domain frequency deviation followed the tracks of simultaneously.

Beneficial effect of the present invention:

The present invention has designed a kind of ofdm system and has reduced peak-to-average force ratio and carrier frequency tracking integrated processes.This method can effectively be followed the tracks of carrier frequency offset, makes system have lower peak-to-average force ratio again, thereby has compensated two big defectives of ofdm system simultaneously with a kind of technology.In addition, this integrated processes has the characteristics that operand is little, complexity is low simultaneously.

Description of drawings

Fig. 1 is the simple model of MQAM-OFDM baseband system

Fig. 2 is the N number of sub-carrier data that add conjugate code

Fig. 3 (a) is the time-domain signal energy diagram of traditional navigation system

Fig. 3 (b) is the time-domain signal energy diagram of conjugation coded system

Fig. 4 is the CCDF curve of reflection system peak-to-average ratio

Fig. 5 is based on the carrier frequency tracking ring block diagram of conjugation coding

Fig. 6 (a) is the system receiving terminal planisphere that does not add frequency-tracking

Fig. 6 (b) is the system receiving terminal planisphere behind the adding frequency-tracking

Fig. 7 is a track loop convergence jitter conditions

Fig. 8 is a mean square error curve of following the tracks of the back residual frequency deviation

Embodiment

Be described further below in conjunction with accompanying drawing with through the embodiment specific embodiments of the invention:

The present invention has designed a kind of ofdm system and has reduced peak-to-average force ratio and carrier frequency tracking integrated processes, and it is characterized in that: this programme may further comprise the steps:

A. transmitter is chosen the information data of stator carrier position at frequency domain, asks its complex conjugate, and gained conjugation coded data is inserted in the subcarrier of other fixed position;

B. receiver is estimated the symbol phase deviation that carrier wave frequency deviation causes in the information data and the conjugation coded data of each symbol respective sub position of frequency domain extraction based on its conjugate relation, and gives compensate of frequency deviation at frequency domain or time domain to receiving data.

The described information data of step a is the data after the QAM modulation.

The number of the described information data of step a equals the number of conjugation coded data, and half the less than the system subcarrier number.

The position of the position of the described information data of step a and conjugation coded data all can be selected arbitrarily in all sub-carrier positions, and its effect that reduces the system peak-to-average ratio is identical.

The estimation formulas of the said symbol phase deviation of step b does

$\mathrm{\Δ\θ}=\frac{1}{2}\mathrm{angle}\left(\underset{i=0}{\overset{M-1}{\mathrm{\Σ}}}r\left(i\right)q\left(i\right)\right)$

Wherein r (i), q (i) are respectively i the information data and i the corresponding conjugation coded data of each symbol, and M is the conjugation coding number in the symbol, and N is the system subcarrier number.

To phase deviation of each sign estimation, estimated value is sent into loop filter carry out the smooth noise processing among the step b, its result divides the data in cos and the sin two-way compensation forward path.

Embodiment

If the ofdm system sub-carrier number is N, the data of transmitting on the N number of sub-carrier be S=[s (0), s (1) ..., s (N-1)], P wherein be a conjugate of symbol, P＜N/2, N-P is the useful data after QAM shines upon in addition.Also getting conjugation respectively promptly forms P conjugate code from N-P useful data, to select P.P choosing with the position of its conjugate code in subcarrier of useful QAM value can be designed as required in the method, and it is the same for falling the peak-to-average force ratio effect.Shown in Figure 2 for all being added in conjugate code a kind of formation situation at symbol end, wherein

$s\left(i\right)=\left\{\begin{array}{cc}s\left(i\right),& 0\≤i\≤N-P-1\\ \mathrm{conj}\left(s(i-N+P)\right),& N-P\≤i\≤N-1\end{array}\right.$

The capturing frequency deviation stage is controlled at frequency deviation in the very little scope, can be similar to during tracking to think do not have to disturb between amplitude fading and subcarrier or assign them as noise processed.Traditional tracking often utilizes the pilot sub-carrier that carries known constant information, uses the conjugate code that is designed to carry out frequency-tracking as pilot tone here, thereby reaches the purpose of not only falling peak-to-average force ratio but also effectively carrying out frequency-tracking.Shown in Figure 5 is the time-frequency track loop of encoding based on conjugation.P in the symbol carries out phase estimation to conjugate code,

Utilize following formula to phase deviation of each sign estimation, get into loop filter then successively and carry out smooth noise and handle, its result divides cos and sin two-way to constitute exp (j Δ θ) to multiply each other with the reception data at frequency domain it is fed back correction.Frequency domain compensation is only applicable to the very little situation of normalization frequency deviation.If the normalization frequency deviation is bigger, time domain compensation is reasonable method, and can only correcting perhaps in time domain, time domain adds frequency-domain combined correction.

Be the performance of check algorithm, design an orthogonal frequency-division multiplex base band system: 100 useful information subcarriers, 12 pilot sub-carriers, 32QAM and 128 FFT, chnnel coding is RS (31, a 27) sign indicating number.One frame is included as 64 OFDM symbols, and leading symbol is made up of a null symbol and a PN sequence, and circulating prefix-length is 37.Shown in Figure 4 is the ofdm signal power integral distribution curve (CCDF) that 12 pilot tones of native system are selected unknown conjugate code and conventionally known fixed data for use, and it has reflected the probability of signal peak-to-average ratio greater than certain thresholding.From figure, can find out and use the conjugation subcarrier to reducing the system peak-to-average ratio certain effect to be arranged, be 10 greater than certain thresholding probability ^-4The time, can peak-to-average power ratio be reduced about 3dB.

For reducing system complexity, adopt here only to correct and follow the tracks of in time domain, the loop filter gain factor suitably is set during emulation.Fig. 6 (a) (b) be depicted as under 25dB, normalization frequency deviation 0.02 condition follow the tracks of before and after planisphere.When as can be seen from the figure not adopting frequency tracking algorithm; The subcarrier of OFDM symbol receives the ICI influence, and constellation point produces disperse, and all symbols receive frequency deviation to influence its phase deviation with the linear increase of symbol sequence number; Thereby the constellation point of disperse is constantly rotated, and forms annulus.Through frequency-tracking, eliminated the influence of frequency deviation, the annulus of disperse reverts to correct 32QAM constellation point.Shown in Figure 7 is normalization frequency deviation 0.02, and under the different signal to noise ratios, this tracing convergence jitter conditions relatively.Can see that under the low signal-to-noise ratio of 5dB, it is still smaller to restrain shake after the loop stability, tracking stability is better.Shown in Figure 8 is under the different normalization frequency deviations, follows the tracks of the mean square error curve of back inherent spurious frequency deviation.Can find out no matter frequency deviation is less, tracking accuracy is all than higher.During 20dB frequency deviation 0.02, MSE can reach 10 ^-7Magnitude.

Claims (6)

1.OFDM system reduces peak-to-average force ratio and carrier frequency tracking integrated processes, it is characterized in that: this programme may further comprise the steps:

A. transmitter is chosen the information data of stator carrier position at frequency domain, asks its complex conjugate, and gained conjugation coded data is inserted in the subcarrier of other fixed position;

B. receiver is estimated the symbol phase deviation that carrier wave frequency deviation causes in the information data and the conjugation coded data of each symbol respective sub position of frequency domain extraction based on its conjugate relation, and gives compensate of frequency deviation at frequency domain or time domain to receiving data.

2. ofdm system according to claim 1 reduces peak-to-average force ratio and carrier frequency tracking integrated processes, it is characterized in that: the described information data of step a is the data after the QAM modulation.

3. ofdm system according to claim 1 reduces peak-to-average force ratio and carrier frequency tracking integrated processes, and it is characterized in that: the number of the described information data of step a equals the number of conjugation coded data, and less than half of system subcarrier number.

4. ofdm system according to claim 1 reduces peak-to-average force ratio and carrier frequency tracking integrated processes; It is characterized in that: the position of the position of the described information data of step a and conjugation coded data all can be selected arbitrarily in all sub-carrier positions, and its effect that reduces the system peak-to-average ratio is identical.

5. ofdm system according to claim 1 reduces peak-to-average force ratio and carrier frequency tracking integrated processes, and it is characterized in that: the estimation formulas of the said symbol phase deviation of step b does

$\mathrm{\Δ\θ}=\frac{1}{2}\mathrm{angle}\left(\underset{i=0}{\overset{M-1}{\mathrm{\Σ}}}r\left(i\right)q\left(i\right)\right)$

Wherein r (i), q (i) are respectively i the information data and i the corresponding conjugation coded data of each symbol, and M is the conjugation coding number in the symbol, and N is the system subcarrier number.

6. ofdm system according to claim 1 reduces peak-to-average force ratio and carrier frequency tracking integrated processes; It is characterized in that: among the step b to phase deviation of each sign estimation; Estimated value is sent into loop filter carry out the smooth noise processing, its result divides the data in cos and the sin two-way compensation forward path.

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