CN1277359C - Method for estimating carrier, frequency shifts of orthogonal FDM communication system - Google Patents

Method for estimating carrier, frequency shifts of orthogonal FDM communication system Download PDF

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CN1277359C
CN1277359C CN 02136340 CN02136340A CN1277359C CN 1277359 C CN1277359 C CN 1277359C CN 02136340 CN02136340 CN 02136340 CN 02136340 A CN02136340 A CN 02136340A CN 1277359 C CN1277359 C CN 1277359C
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training sequence
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frequency shift
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周跃峰
许婵媛
陈柏南
胡亮亮
周军
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Shanghai Jiaotong University
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Abstract

The present invention relates to a method for estimating the carrier frequency shift of an orthogonal frequency division multiplex (OFDM) communication system. Two kinds of training sequences designed specifically are added to initial positions of sequence frames, cross correlation results of the A kind of the training sequences are utilized to estimate the original value of frequency shift, namely the rough estimation of the frequency shift, and then, the B kind of the training sequences carries out autocorrelation so as to obtain the accurate estimation of signal frequency deviation within a small range, namely the accurate estimation of the frequency shift; then, the two estimated values are combined so as to obtain the exact value of system frequency shift; finally, the digitally controlled oscillator is controlled to produce the rectifying value of the system frequency shift, and the rectifying value is multiplied by time domain data so as to carry out the compensation of the frequency shift. With the advantages of high accuracy, small amount of calculation and easy realization for a system, the method of the present invention can be used in an OFDM wireless local network system and can also be used in the process of realizing various OFDM systems.

Description

Method for estimating carrier, frequency shifts of orthogonal FDM communication system
Technical field:
The present invention relates to a kind of method for estimating carrier, frequency shifts of orthogonal FDM communication system, is a kind of method that carrier frequency shift is estimated and compensated, and belongs to digital communicating field.
Background technology:
OFDM (OFDM) is a kind ofly to change high-speed serial data into the low-speed parallel data, and modulates the data-modulated mode of parallel data respectively with a plurality of mutually orthogonal subcarriers.Though subcarrier spectrum has overlapping, because each subcarrier has orthogonality, do not disturb between the carrier wave, therefore can effectively improve spectrum utilization efficiency.As a kind of emerging multi-carrier modulation technology, OFDM has stronger anti-multipath decline and frequency selective fading characteristic, at ADSL (Asymmetric Digital Subscriber Line) (ADSL), digital television broadcasting (DVB), WLAN (wireless local area network) many fields such as (IEEE802.11a) have obtained extensive use.
For guaranteeing the orthogonality between each subcarrier of ofdm system, its sub-carrier frequencies must satisfy: f n=f 0+ n/T, wherein T is a symbol period.So frequency interval Δ f=1/T between each subcarrier.Because the parameter drift of system device itself and the influence of Doppler frequency shift, the carrier frequency of receiving terminal can be offset, and this can cause disturbing between serious subcarrier (ICI).If frequency offset error is the integral multiple n of sub-carrier frequencies interval delta f, the position of the signal that then receives mobile n subcarrier on frequency domain.At this time subcarrier still keeps quadrature, but will destroy the correct reception of data fully.If frequency error is the little several times of subcarrier spacing, will cause energy at other subcarrier of frequency domain sample point aliasing, also promptly cause ICI, thereby influence the BER performance of system.So must carry out carrier frequency synchronization at receiving terminal, also promptly the carrier shift error be estimated and compensated.
General Frequency Synchronization is carried out after correcting in that time synchronization error is preliminary, can be divided into two kinds of time domain Frequency Synchronization and frequency domain Frequency Synchronization according to the position of algorithm for estimating in system.The frequency domain synchronized algorithm by Ferdinard Classen and Heinrich Meyr at civilian Frequency Synchronization Algorithms for OFDM SystemsSuitable for Communication Over Frequency Selective Fading Channels (inProc.IEEE Veh.Technol.Conf., 1994, pp.1655-1659) propose in, this method mainly is to adopt repetitive sequence and in conjunction with the method for DFT estimated frequency error, it can improve the estimation range of frequency shift (FS), but the processing procedure in early stage that needs the FFT data, in fact these processes can cause extra error, produce the actual treatment time delay, also increased the realization difficulty of system simultaneously.And the time domain frequency synchronization method by Paul HMoose at civilian A Technique for Orthogonal Frequency Division MultiplexingFrequency Offset Correction (IEEETrans.Commun., vol.42, pp.2908-2914, Oct.1994) propose in, it mainly is based on the frequency offset estimation algorithm of maximum likelihood (MLE, Maximum LikelihoodEstimation), but its amount of calculation is big, precision is not high, and the ASIC that is not easy to system realizes.
Summary of the invention:
The objective of the invention is at the deficiencies in the prior art, a kind of method for estimating carrier, frequency shifts of orthogonal FDM communication system is proposed, this method have realize simple, amount of calculation is little, the characteristics that estimated accuracy is high can satisfy the requirement of communication system to the Frequency offset estimation scope.
For achieving this end, in the technical scheme of the present invention,, add the specific training sequence of design at first at the initial position of system-frame, be beneficial to carry out the estimation of frequency shift (FS); Secondly utilize the result of the cross-correlation of the part training sequence that adds to come the initial value of estimated frequency skew, promptly carry out the rough estimate of frequency shift (FS); Utilize the part training sequence that adds to carry out auto-correlation again, obtain the accurate estimation of signal frequency deviation among a small circle, promptly carry out the essence of frequency shift (FS) and estimate; Then the value of twice estimation is united the accurate scope that obtains system frequency excursion; Control the correction value that digital controlled oscillator (NCO) produces system frequency excursion at last, and it is taken advantage of mutually again with time domain data, to carry out the compensation of frequency shift (FS).Concrete operations of the present invention are carried out as follows:
1,, adds the training sequence of particular design at the initial position of system-frame.
Because of frequency offset estimating needs, before outgoing data, increase by two class training sequences.Wherein the number of a kind of training sequence in front is more, but the sampled point of single sequence is less, is called the category-A training sequence.Being right after after the category-A training sequence is that number is less, but the more another kind of training sequence of the sampling number of single sequence is called the category-B training sequence.The Cyclic Prefix of category-B training sequence (CP) is duplicating of category-B training sequence afterbody several number certificate.After A, the B training sequence, be the system-frame outgoing data.The sampled point number of wherein single category-A training sequence is the  of system symbol outgoing data sampled point number.
Can utilize the category-A training sequence of every frame to carry out the synchronous or rough estimate of frequency rough like this, utilize the Cyclic Prefix of category-B training sequence and the cycle characteristics of data to carry out the synchronously smart or smart estimation of frequency then.
2, utilize the A training sequence to carry out the rough estimate of frequency shift (FS).
Utilize last several category-A training sequences that receive fully to carry out cross-correlation, the initial value according to its result comes the estimated frequency skew promptly carries out the rough estimate of frequency shift (FS).
Frequency offset error rough estimate evaluation f δ 1Expression formula be
Wherein N is a system-frame outgoing data sampled point length, L aBe the sampled point length of single category-A training sequence, Δ f is the frequency interval between each subcarrier of system,
R A = Σ m = 0 L a - 1 z ( k AEnd - m ) · z * ( k AEnd - L a - m )
Z (k wherein AEnd) be the last time-domain sampling point of last A training sequence, here the reception data of two category-A training sequence length are correlated with, play the effect of digital average.
Because N=4L aSo, rough estimate f δ 1Scope be [2 Δ f, 2 Δ f].
3, the essence of utilizing the B training sequence to carry out frequency shift (FS) is estimated.
Utilize several category-B training sequences of reception fully and the Cyclic Prefix of category-B training sequence to carry out auto-correlation, come the exact value of estimated frequency skew, promptly carry out the essence of frequency shift (FS) and estimate according to its result.
The smart estimated value f of frequency shift (FS) δ 2Expression formula be
Can find out that little be [Δ f/4, Δ f/4] falls in the smart estimation range of estimating.Wherein,
R B = Σ m = 0 L cpb - 1 z ( k BEnd - m ) · z * ( k BEnd - L b × N b - m )
Z (k wherein BEnd) be the last time-domain sampling point of last category-B training sequence, N bBe the number of category-B training sequence, L bBe the sampled point length of single category-B training sequence, L CpbSampled point length for the Cyclic Prefix of category-B training sequence.
4, frequency rough estimation and the smart estimation of frequency are comprehensively obtained frequency offseting value.
Frequency rough is estimated to estimate to combine with smart, obtain the frequency offseting value f of system δRepresent with following formula:
f δ = n · Δf 4 + f δ 2
Wherein
Figure C0213634000056
(
Figure C0213634000057
For under round symbol), also promptly can utilize f δ 1And f δ 2Obtain the accurate estimated value of frequency shift (FS).The total scope of the Frequency offset estimation of this algorithm is [2 Δ f, 2 Δ f].
5, utilize NCO (digital controlled oscillator) to carry out the compensation of frequency shift (FS).
Calculate after the frequency offseting value, can use digital controlled oscillator (NCO) to come the correction of frequency skew, promptly time domain data be multiply by the correction value of frequency shift (FS): e -j2 π f δ nTs, T wherein sBe the systematic sampling interval.The compensation of frequency shift (FS) finish frequency smart estimate and rough estimate after and carry out comprehensively to their results after carry out, obtain correction value and control NCO generation frequency shift (FS) correction value and time-domain signal multiplies each other according to results estimated.
The present invention at first estimates to obtain the approximate range of frequency shift (FS) by frequency rough, estimates the further refinement of the scope of frequency shift (FS) through the overfrequency essence again, so the precision height of Frequency offset estimation.Implement frequency offset estimation algorithm of the present invention in WLAN (wireless local area network), the result who obtains shows that the practical frequency error ratio is less, proves that the present invention is significantly to the effect of estimating system frequency shift (FS), makes receiving terminal can recover user data preferably.Compare with the frequency domain frequency synchronization algorithm, method of the present invention does not need to wait for the processing procedure in early stage of FFT data, and system's time delay is little; Compare with the frequency offset estimation algorithm based on maximum likelihood, the ASIC that method amount of calculation of the present invention is little, precision is high, be easy to system realizes.Method of the present invention is effective to Frequency offset estimation not only, and be easy to realize, therefore, is a more satisfactory method of estimating system frequency shift (FS) based on the carrier shift method of estimation of specific training sequence.
The present invention can be used for the OFDM wireless local area network (WLAN) system, also can be applied to DVBT, and CDMA-OFDM is in the implementation procedure of ofdm systems such as OFDMA.
Description of drawings:
Fig. 1 is the ofdm system frame structure schematic diagram that inserts behind the training sequence of the present invention's design.
As shown in Figure 1, frame structure comprises A, B two class training sequence and outgoing data, and wherein category-B training sequence and outgoing data partly include Cyclic Prefix (CP).The number N of category-A training sequence aNumber N greater than the category-B training sequence b, but the sampled point number of single category-A sequence is less than the sampled point number of category-B training sequence.The Cyclic Prefix of category-B training data (CP) is duplicating of category-B training sequence several length data of afterbody.After the A that inserts, B training sequence, be the original outgoing data of system-frame.
Fig. 2 is a frequency synchronization algorithm implementation structure of the present invention.
As shown in Figure 2, method of the present invention comprises that frequency rough is estimated, the frequency essence is estimated, Frequency Estimation is comprehensive and utilize NCO to carry out several parts of frequency compensation.Wherein frequency rough is estimated the summation of multiplying each other of category-A training sequence by certain category-A training sequence and its front that obtains through time-delay, just asks result's phase angle to obtain slightly synchronous result after the cross-correlation.Frequency is smart estimates the summation of multiplying each other of Cyclic Prefix by certain category-B training sequence and the category-B training sequence of its front that obtains through time-delay, just asks result's phase angle to obtain the synchronous result of essence after the auto-correlation.The result of these two steps is comprehensively obtained frequency control word FCW, remove to control digital controlled oscillator NCO, value that digital controlled oscillator is produced and time domain data multiply each other and carry out the compensation of frequency shift (FS).
K carries out the smart estimation of frequency in order to guarantee with first value input of category-B training sequence CP among the figure.NCO is in each clock cycle, and phase accumulator increases successively with the value of frequency control word, calculates that good phase value is just being sent into, cosine generator, multiply by again with the system baseband signal and remedies frequency shift (FS).
Fig. 3 is the application schematic diagram of the present invention at OFDM WLAN (wireless local area network) emitting-receiving system.
As shown in Figure 3, at transmitting terminal, outgoing data carries out convolutional encoding earlier and interweaves, modulates, and adds A, the B two class training sequences of particular design again before outgoing data, to carry out the estimation of frequency shift (FS).After data are handled through these, carry out the Cyclic Prefix of N point IFFT, increase data division, be modulated to intermediate frequency and radio frequency.At receiving terminal, intermediate frequency, rf data are by after being demodulated to baseband signal, and advanced line time is synchronous, just carries out the estimation and the compensation work of frequency shift (FS) afterwards.Wherein frequency offset estimating is to utilize the category-A training sequence to carry out the frequency shift (FS) rough estimate earlier, utilize the category-B training sequence to carry out the smart estimation of frequency shift (FS) again, thick, smart results estimated is comprehensively obtained accurate frequency offseting value and control NCO producing the frequency shift (FS) correction value, and itself and system's baseband signal are taken advantage of the compensation of finishing frequency shift (FS) again.The estimation and the compensation work of system carrier skew have so just been finished.Data just can revert to user data through removing Cyclic Prefix, N point FFT, frequency equilibrium and demodulation, decoding then.
Embodiment:
Below by specific embodiment the solution of the present invention is further described.
Embodiment: method of the present invention is applied to the OFDM WLAN (wireless local area network).
The present invention in the application of OFDM WLAN (wireless local area network) emitting-receiving system as shown in Figure 3.Earlier through convolutional encoding and after interweaving, modulating, at first in system-frame, add the training sequence of particular design, in the transmitting terminal data to carry out the estimation of frequency shift (FS).Owing to outgoing data sampled point length in the system-frame is 64, therefore the sampled point length of each the A training sequence that adds is at this 1 4 × 64 = 16 , The category-A training sequence is 10.According to the relation of A, B training sequence length, add 2 category-B training sequences, the sampled point length of each category-B training sequence is 64, the Cyclic Prefix of category-B training sequence is 32.Improved frame is carried out 64 IFFT (modulated process of OFDM just), add the outgoing data cyclic prefix CP, be modulated to intermediate frequency and radio frequency.
At receiving terminal, data are by after being demodulated to baseband signal, and advanced line time is synchronous, just carries out the estimation and the compensation work of frequency shift (FS) afterwards.Wherein frequency offset estimating is to utilize earlier the category-A training sequence that adds to carry out the frequency shift (FS) rough estimate, gets latter two and has been carried out cross-correlation by the category-A training sequence of complete reception and carry out rough estimate, and the formula of its rough estimate is:
R A = Σ m = 0 15 z ( k AEnd - m ) · z * ( k AEnd - 16 - m )
Get R APhase place, can obtain frequency offset error rough estimate evaluation f δ 1Expression formula,
Can obtain rough estimate f δ 1Scope be [2 Δ f, 2 Δ f]
Utilize the category-B training sequence to carry out the smart estimation of frequency shift (FS) again, category-B training sequence and its Cyclic Prefix are done auto-correlation carry out the essence estimation, its smart formula of estimating is:
R B = Σ m = 0 31 z ( k BEnd - m ) · z * ( k BEnd - 128 - m )
Get R BPhase angle, can obtain the smart estimated value f of frequency shift (FS) δ 2Expression formula:
Can obtain the smart estimation range of estimating and be [Δ f/4, Δ f/4].
Thick, smart results estimated is comprehensively obtained accurate frequency offseting value and control NCO producing the frequency shift (FS) correction value, and its system's baseband signal is taken advantage of the compensation of finishing frequency shift (FS) again.Data just can obtain user data through past Cyclic Prefix, N point FFT, frequency equilibrium and demodulation, decoding after handling through this.
The present invention's enforcement under radio local network environment has obtained obvious effects, is between the 0-45dB in signal to noise ratio, and the carrier shift max value of error of actual measurement is 2.7kHZ only, is no more than 1% of WLAN (wireless local area network) sub-carriers frequency interval.In real system, consequent ICI can ignore fully.Therefore adopt the inventive method can better overcome the skew of system carrier frequency, eliminate the harmful effect of system carrier frequency skew.
Method of the present invention not only can be estimated and compensation deals the carrier shift of OFDM wireless communication system, comprises as 802.11 serial protocols etc., also can be applied to CDMA-OFDM, OFDMA, OFDM transmission systems such as DVBT.The place of being distinguished is the different of frame structure and system global structure only, is the modulation system that adopts OFDM in 802.11 agreements; And the mode of spread spectrum that adopts in the CDMA-OFDM system and OFDM combination, but it is basic identical to use method of the present invention in these systems.

Claims (2)

1, a kind of method for estimating carrier, frequency shifts of orthogonal FDM communication system is characterized in that comprising the steps:
1) two class training sequences of adding particular design before every frame data, wherein the number of category-A training sequence is more, single sequential sampling point number is less, the category-B training sequence is right after after the category-A training sequence, its number is less, single sequential sampling point number is more, and the cyclic prefix CP of category-B training sequence is duplicating of category-B training sequence several length data of afterbody;
2) utilize the summation of multiplying each other of the category-A training sequence of certain category-A training sequence and its front that receives fully through time-delay, promptly carry out cross-correlation, the initial value according to its result's phase angle comes estimated frequency to be offset promptly carries out the rough estimate of frequency shift (FS);
3) utilize the summation of multiplying each other of the Cyclic Prefix of certain category-B training sequence and the category-B training sequence of its front that receives fully through time-delay, promptly carry out auto-correlation, come the exact value of estimated frequency skew, promptly carry out the essence estimation of frequency shift (FS) according to its result's phase angle;
4) with frequency shift (FS) smart estimate and the value of frequency shift (FS) rough estimate comprehensive, obtain the accurate scope of system frequency excursion;
5) control digital controlled oscillator NCO produces the correction value of the system frequency excursion that estimates, and itself and system's baseband signal are taken advantage of again, to carry out the compensation of frequency shift (FS).
2, as the said method for estimating carrier, frequency shifts of orthogonal FDM communication system of claim 1, the sampled point number that it is characterized in that single category-A training sequence is the  of system symbol outgoing data sampled point number.
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