CN103929395A - OFDM system frequency offset synchronization method based on constant envelope modulation - Google Patents

Abstract

The invention discloses an OFDM system frequency offset synchronization method based on constant envelope modulation, and belongs to the technical field of wireless communication. According to the OFDM system frequency offset synchronization method based on constant envelope modulation, a transmitter carries out constant envelope transformation on a signal processed through constellation diagram mapping again, so that data have the constant modulus characteristic before being processed through IFFT, and channels obey quasi-static fading; a receiver synchronizes frequency offset through the energy difference between a minimized receipt signal and a known channel frequency response. The OFDM system frequency offset synchronization method has the advantages of being applicable to frequency offset blind synchronization of an MIMO-OFDM system of any constellation diagram, and applicable to the situation that null carrier waves exist in the OFDM system.

Description

Ofdm system frequency deviation synchronous method based on constant envelope modulation

Technical field

The invention belongs to wireless communication technology field, relate to a kind of ofdm system frequency deviation synchronous method based on constant envelope modulation.

Background technology

Along with the continuous increase of multimedia service, future broadband wireless communication systems is had higher requirement to transmission rate, the error rate and spectrum efficiency etc., and traditional single carrier narrow band transmission technology cannot satisfy the demands.How effectively to improve the availability of frequency spectrum, message transmission rate and communication quality and become one of current hot research problem.OFDM (Orthogonal Frequency Division Multiplexing, OFDM) be a kind of orthogonal multiple carrier modulation technique, it is by expanding symbol period, wideband frequency Selective Fading Channel is converted to a series of arrowbands flat fading channel, therefore there is very strong anti-multipath fading and impulse disturbances ability, be particularly suitable for high-speed radio transmission.Compare with traditional frequency division multiplexing, in ofdm system, different sub carrier frequency spectrum is overlapped, and its availability of frequency spectrum is higher.At present, OFDM has become the physical layer transmission schemes of many international communication standards, has boundless application prospect.

But with respect to single carrier transmitting system, ofdm system is very responsive to carrier wave frequency deviation.Carrier wave frequency deviation is by not mating and causing between the time variation of wireless channel and transmitter carrier frequency and receiver local oscillator.Because the frequency spectrum of ofdm system sub-carriers is overlapped, very little frequency deviation will be destroyed the orthogonality between subcarrier, produces inter-carrier interference.Research shows, only 1% frequency deviation just can make the receiving terminal signal to noise ratio about 30dB that declines.On the other hand, multiple-input and multiple-output (Multiple-Input Multiple-Output, MIMO) technology obtains extensive concern and the research of industrial quarters and academia in recent years.By arranging that at receiving terminal and transmitting terminal many dwi hastasanas become aerial array, MIMO technology can improve exponentially transmission rate or reduce bit error rate (Bit Error Rate, BER) in the situation that not increasing transmitted power.The MIMO-OFDM that MIMO and OFDM are combined has become one of core technology of the 4th third-generation mobile communication, and MIMO-OFDM is equally very responsive to carrier wave frequency deviation.Therefore study carrier wave frequency deviation synchronized algorithm and there is very important academic significance and practical value for inhibition and elimination ofdm system inter-carrier interference, raising systematic function.

The synchronous research of ofdm system is the hot issue of academia always.Typical synchronized algorithm is data auxiliary types, need to be characterized in quick and precisely by means of the pilot tone (Pilot) periodically transmitting, and algorithm is simply effective.The most classical data auxiliary type algorithm is proposed by Schimdl, and its basic thought is the identical training symbol of two parts before and after sending, and receiving terminal utilizes the phase difference estimation frequency deviation of same section.Owing to sending pilot tone, need occupying system resources, reduced the availability of frequency spectrum, these class methods are generally applicable to the communication system of burst type transmission.Blind synchronized algorithm utilizes the synchronous frequency deviation of some special constructions in OFDM symbol, do not need pilot tone, so the availability of frequency spectrum is higher.

The problem that the current Blind Frequency Offset track algorithm for SISO-and MIMO-OFDM system exists mainly contains following 2 points: 1. algorithm adopts permanent mould planisphere, and Modern Communication System adopts the QAM modulation that order of modulation is higher etc. mostly, and often in different sub carrier, adopt various constellations figure mapping, so algorithm application is limited; 2. only considered the situation that system is fully loaded, all subcarriers are all for transfer of data, and while considering unloaded ripple in ofdm system, the applicability problem of algorithm, does not still have distance with practical application.

Summary of the invention

In view of this, the object of the present invention is to provide a kind of ofdm system frequency deviation synchronous method based on constant envelope modulation, the method can realize the ofdm system Blind Frequency Offset of any planisphere and follow the tracks of, and is applicable to exist in ofdm system the situation of unloaded ripple.

For achieving the above object, the invention provides following technical scheme:

An ofdm system frequency deviation synchronous method based on constant envelope modulation, in the method, transmitter, by the constant envelope conversion that tries again of the signal through planisphere mapping, make data before IFFT, have constant modulus property, and channel is obeyed quasistatic decline; Receiver receives signal and synchronizes frequency deviation with the energy difference of known channel frequency response by minimizing.

Further, in carrying out constant envelope conversion process, in the following manner the complex signal through planisphere mapping is converted to real signal one to one: use IFFT that complex signal is converted to real signal, be that transmitter comprises continuous IFFT twice, receiving terminal recycles twice Fourier transform (fast Fourier transform, FFT) reduction primary signal.

Further, in carrying out constant envelope conversion process, in the following manner the complex signal through planisphere mapping is converted to real signal one to one: the real part of signal and imaginary part are done respectively to constant envelope modulation, at receiving terminal, demodulate two parts of signals and be combined into original transmitted signal.

Further, in the method, ofdm system adopts the identical FFT counting and IFFT computing, and the cost function of algorithm is sine curve; Do not having in noisy situation, by the known cost function that minimizes of the character of SIN function, can obtain unique correct frequency deviation and estimate, and whether exist irrelevant with unloaded ripple.

Beneficial effect of the present invention is: it is synchronous that the ofdm system frequency deviation synchronous method based on constant envelope modulation that the present invention proposes can be applied to the Blind Frequency Offset of MIMO-OFDM system of any planisphere, and is applicable to exist in ofdm system the situation of unloaded ripple.

Accompanying drawing explanation

In order to make object of the present invention, technical scheme and beneficial effect clearer, the invention provides following accompanying drawing and describe:

Fig. 1 is ofdm system block diagram of the present invention;

Fig. 2 is the BER simulation result figure of this method and Yao algorithm under different modulating mode in embodiment;

Fig. 3 is the BER simulation result of this method in the situation of different unloaded wave numbers in embodiment.

Embodiment

Below in conjunction with accompanying drawing, the preferred embodiments of the present invention are described in detail.

Fig. 1 is ofdm system block diagram of the present invention, due to the complexity of ofdm system, it is carried out to complete description very difficult, and the model of setting up is also difficult to analyze.Therefore in modeling process, some condition of system can be simplified or ignore: 1. suppose that channel frequency response has been estimated by training symbol and remained unchanged in a frame, i.e. quasistatic fading channel (Quasi-StaticFadingChannel); 2. at receiving terminal, conventionally there is symbol timing error and carrier wave frequency deviation.Owing to comprising Cyclic Prefix in OFDM symbol, so the impact of symbol timing error is far smaller than carrier wave frequency deviation, and conventional letter synchronized algorithm is eliminated timing error here, only considers the impact of frequency deviation; 3. in theory, every a pair of dual-mode antenna in MIMO-OFDM system has different carrier wave frequency deviations, but these difference often can be ignored, the one, because complete synchronous ratio between these distributed oscillators, be easier to, the 2nd, because in many application, all dual-mode antennas between Doppler frequency shift be identical, therefore suppose that all reception antennas all experience identical frequency deviation.

In frequency deviation synchronous method of the present invention, transmitter, by the constant envelope conversion that tries again of the signal through planisphere mapping, make data before IFFT, have constant modulus property, and channel is obeyed quasistatic decline; Receiver receives signal and synchronizes frequency deviation with the energy difference of known channel frequency response by minimizing, and cost function can be written as:

$\begin{array}{c}{J}_{\mathrm{SISO}}=\left(\widetilde{\mathrm{\ϵ}}\right)=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{({\left|y_{m,n}\left(\widetilde{\mathrm{\ϵ}}\right)\right|}^2-{\left|H_{m,n}\right|}^2)}^2\\ =\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}({\left|y_{m,n}\left(\widetilde{\mathrm{\ϵ}}\right)\right|}^4+{\left|H_{m,n}\right|}^4-2{\left|y_{m,n}\left(\widetilde{\mathrm{\ϵ}}\right)\right|}^2\·{\left|H_{m,n}\right|}^2)\end{array}---\left(1\right)$

In formula: H _{m, n}for channel in transmission during m OFDM symbol, the frequency response values at the frequency sequence number subcarrier centre frequency place that is n.Above formula only has works as time obtain minimum value.

As shown in Figure 1, the key of the Blind Frequency Offset synchronized algorithm of the ofdm system that is applicable to any planisphere provided by the invention is constant envelope modulation module, be transmitter by the constant envelope conversion that tries again of the signal through planisphere mapping, make data before IFFT, just there is constant modulus property, can estimate frequency deviation by preceding method.Can the complex signal through planisphere mapping be converted to real signal one to one by following steps in the present embodiment, because only have real signal could realize constant envelope modulation.Comprise: 1. use IFFT that complex signal is converted to real signal (transmitter comprises continuous IFFT twice), receiving terminal recycles twice Fourier transform (fast Fourier transform, FFT) reduction primary signal; 2. the real part of signal and imaginary part are done respectively to constant envelope modulation, at receiving terminal, demodulate two parts of signals and be combined into original transmitted signal; 3. above two kinds of methods unification is processed, etc.

Identifiability analysis, reduced complexity and the performance evaluation of the method for the invention:

For identifiability problem, first launch every in cost function, utilize cost function to obtain at correct frequency deviation place the character that minimum value be take and derivative is zero, analysis show that cost function is a sine curve, because SIN function has and only have a minimum value in one-period, thereby proved identifiability problem.Utilize the character of SIN function, calculate cost function in the value of several particular points, then by curve, just can calculate frequency deviation, thereby simplify complexity.Table 1 has provided the complexity contrast of infinite search and two kinds of methods of curve, and the complexity has here been defined as a frequency deviation and has estimated needed comparison, real addition and real multiplications, and wherein step represents the step-length of infinite search.

The complexity contrast of the infinite search of table 1 and curve

The performance evaluation more complicated of algorithm, utilizes empirical equation to simplify analytic process in the present embodiment.In the situation that signal to noise ratio is higher, average and mean square deviation that algorithm is estimated can be approximated to be:

$E\left\{\widehat{\mathrm{\ϵ}}\right\}\≅\mathrm{\ϵ}-\frac{E\left\{J^{\text{'}}\left(\mathrm{\ϵ}\right)\right\}}{E\left\{J^{\text{'}\text{'}}\left(\mathrm{\ϵ}\right)\right\}},\mathrm{MSE}\left\{\widehat{\mathrm{\ϵ}}\right\}\≅\frac{E\left\{{\left[J^{\text{'}}\left(\mathrm{\ϵ}\right)\right]}^2\right\}}{{\left[E\right\{J^{\text{'}\text{'}}\left(\mathrm{\ϵ}\right)\left\}\right]}^2}---\left(2\right)$

In formula: represent cost function;

J'(ε) and J''(ε) represent that respectively cost function exists to the first derivative of Nonlinear Transformation in Frequency Offset Estimation and second dervative the value at place.

After utilizing above formula to calculate average and mean square deviation, just can analyze affecting the various parameters (comprising sub-carrier number N, channel frequency response and signal to noise ratio etc.) of algorithm performance.For the impact of channel estimation errors, first estimated value is modeled as to actual channel frequency response and channel estimation errors sum, that is:

${\stackrel{\‾}{H}}_{m,n}=H_{m,n}+\mathrm{\Δ}{H}_{m,n}---\left(3\right)$

In formula: represent the channel frequency response of estimating, H _{m, n}for actual channel frequency response;

Δ H _{m, n}represent channel estimation errors, average is zero, and and H _{m, n}separate.

Then bring above formula into cost function, utilize empirical equation analysis to exist algorithm in the situation of channel errors to estimate the situation of change of average and mean square deviation.

This method can be applied to the Blind Frequency Offset synchronized algorithm of the MIMO-OFDM system of any planisphere:

Without loss of generality, the MIMO-OFDM system of considering in the present embodiment comprises 2 transmitting antennas, and the situation that comprises any root transmitting antenna can be analyzed by similar approach.For mimo system, due to every antenna reception to signal be the stack of all transmitted signals, can not simply adopt the method in similar SISO system.Adopt in the present embodiment quadrature Space Time Coding, transmitter carries out constant envelope modulation to the complex signal of Space Time Coding module output, guarantees that signal has constant modulus property before IFFT, and when receiver is in harmonious proportion sky by constant envelope solution, decoding recovers primary signal.The cost function of algorithm in MIMO-OFDM system can be expressed as:

$J_{\mathrm{MIMO}}\left(\widetilde{\mathrm{\ϵ}}\right)=\underset{r=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{({\left|y_{r,m,n}\right|}^2+{y_{r,m+1,n}|}^2-2{\left|H_{1,r,m,n}\right|}^2-2{\left|H_{2,r,m,n}\right|}^2)}^2---\left(4\right)$

In formula: M _rthe number that represents reception antenna;

H _{t, r, m, n}be illustrated in while sending m OFDM symbol, between t root transmitting antenna and r root reception antenna

The frequency response of channel on n subcarrier;

Y _{r, m, n}represent the data on n subcarrier in m the OFDM symbol that r root reception antenna receives.

The applicability of algorithm when system exists unloaded ripple:

In the present embodiment, suppose N above _dindividual subcarrier is used for transmission information, after N _v=N-N _dindividually be elected to be unloaded ripple.First derive in the situation that has unloaded ripple and frequency deviation, receive the expression formula of signal and algorithm cost function, utilize time frequency analysis to launch every in cost function.Utilize equally cost function to obtain at correct frequency deviation place the character that minimum value be take and derivative is zero, analyze and find that the cost function of algorithm is exactly sine curve as long as ofdm system adopts the identical FFT counting and IFFT computing (this is apparent); Do not having in noisy situation, by the known cost function that minimizes of the character of SIN function, can obtain unique correct frequency deviation and estimate, and with unloaded ripple whether exist irrelevant, thereby there is the applicability of this method in the situation of unloaded ripple in proof.

Beneficial effect for checking the method for the invention, carries out BER emulation to it in the present embodiment.The NC-OFDM system adopting comprises 64 subcarriers altogether, and circulating prefix-length is 16; Wireless channel adopts the SUI2 model that meets 802.16d standard.All results are all to obtain through the Monte Carlo simulation of 50000 times.

Fig. 2 has provided this algorithm and Yao algorithm (Y.YaoandG.B.Giannakis under different modulating mode, " Blind carrier frequency offset estimation in SISO; MIMO; and multiuser OFDM systems, " IEEE Trans.Commun., vol.53, no.1, pp.173-183, Jan.2005.) BER performance comparison, frequency deviation is estimated to utilize two continuous receiving symbols.When signal to noise ratio (SNR) is lower, the performance of two kinds of algorithms is close as we can see from the figure; When signal to noise ratio further increases, algorithm BER is more excellent herein.Notice that Yao algorithm can introduce floor effect (ErrorFloor),, along with signal to noise ratio improves, BER is tending towards a saturation value simultaneously; If signal to noise ratio further improves, performance does not but promote.Be appreciated that its origin cause of formation, can think that Yao algorithm exists internal interference, this internal interference is (could obtain accurately estimation because Yao algorithm need to utilize a lot of symbols to average) being caused by limited NC-OFDM symbol.For system BER, when signal to noise ratio is lower, what play a decisive role is outside additive noise; And along with signal to noise ratio improves, internal interference just becomes principal element.Because internal interference and signal to noise ratio are irrelevant, so there is floor effect.And for this algorithm, as long as channel remains unchanged in the time at two continuous symbols, just can not produce internal interference, therefore along with the raising of signal to noise ratio, the BER of system can Continuous optimization.

Fig. 3 has provided the BER performance comparison of this algorithm in the situation that adopts different unloaded wave numbers.In each emulation, unloaded ripple is to be all randomly dispersed in whole frequency spectrum, uses the randomness of frequency spectrum to simulate cognitive user.As we can see from the figure, by closed portion subcarrier, NC-OFDM system can obtain the improvement of BER: for example, when signal to noise ratio is 25dB, and unloaded ripple number from 24 to 56, the error rate is by 6 * 10 ^-4be reduced to 4 * 10 ^-5.This is mainly because NC-OFDM system is much closing in the subcarrier of seizure condition, and NC-OFDM symbol is normalized into identical power after IDFT, this has been equivalent to increase the average transmitting power on data subcarrier, therefore at receiving terminal, has obtained performance gain.

Finally explanation is, above preferred embodiment is only unrestricted in order to technical scheme of the present invention to be described, although the present invention is described in detail by above preferred embodiment, but those skilled in the art are to be understood that, can to it, make various changes in the form and details, and not depart from the claims in the present invention book limited range.

Claims (4)

1. the ofdm system frequency deviation synchronous method based on constant envelope modulation, is characterized in that: transmitter, by the constant envelope conversion that tries again of the signal through planisphere mapping, make data before IFFT, have constant modulus property, and channel is obeyed quasistatic decline; Receiver receives signal and synchronizes frequency deviation with the energy difference of known channel frequency response by minimizing.

2. the ofdm system frequency deviation synchronous method based on constant envelope modulation according to claim 1, it is characterized in that: in carrying out constant envelope conversion process, in the following manner the complex signal through planisphere mapping is converted to real signal one to one: use IFFT that complex signal is converted to real signal, be that transmitter comprises continuous IFFT twice, receiving terminal recycles Fourier transform reduction primary signal twice.

3. the ofdm system frequency deviation synchronous method based on constant envelope modulation according to claim 1, it is characterized in that: in carrying out constant envelope conversion process, in the following manner the complex signal through planisphere mapping is converted to real signal one to one: the real part of signal and imaginary part are done respectively to constant envelope modulation, at receiving terminal, demodulate two parts of signals and be combined into original transmitted signal.

4. the ofdm system frequency deviation synchronous method based on constant envelope modulation according to claim 1, is characterized in that: in the method, ofdm system adopts the identical FFT counting and IFFT computing, and the cost function of algorithm is sine curve; Do not having in noisy situation, by the known cost function that minimizes of the character of SIN function, can obtain unique correct frequency deviation and estimate, and whether exist irrelevant with unloaded ripple.