CN100550875C - Method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology - Google Patents

Abstract

Method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology of the present invention, the pilot tone/synchronizing symbol of structure when framing that comprises the steps: to make a start, described pilot tone/synchronizing symbol comprises Cyclic Prefix and PN sequence, is launched by antenna after the framing; Receiving terminal carries out thick Synchronous Processing; The output result and the coupling system thresholding of the thick synchronization module of described code element are adjudicated; The synchronous points set of described thick synchronization module output is input to the smart synchronization module of described code element, and carries out related operation, find synchronous points in the position, meeting point; After finding synchronous points, utilize the estimation of the output acquisition frequency deviation of FFT largest passages.Method for synchronous of the present invention is because employing time in ofdm communication system of the present invention is slightly synchronous and the scheme of smart synchronous estimation and large-scale frequency offset estimating, when having improved synchronization accuracy greatly, do not expend too much system resource, and operand is very little, is beneficial to very much Project Realization.

Description

Method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology

Technical field

The present invention relates to the method for time synchronized and Frequency Synchronization in a kind of orthogonal frequency division multiplexi OFDM wireless receiving system.

Background technology

Along with the development of Digital Signal Processing and high speed device, orthogonal frequency division multiplexi (OFDM) has obtained successful application in systems such as DVB, DSL and WLAN.OFDM is divided into the subchannel of several quadratures to frequency spectrum at frequency domain, and the carrier wave of each subchannel is overlapped, has improved the availability of frequency spectrum.Because the bandwidth relative narrower of each subchannel, therefore the frequency-selective channel to whole transmitted bandwidth signal but is a flat fading for each sub-channel signal, and equilibrium just can be carried out respectively each subcarrier, has simplified receiver structure greatly.Because OFDM has availability of frequency spectrum height, balanced simple advantage, is very suitable for wired and wireless transmission at a high speed, has therefore obtained broad research.

Compare with single-carrier system, OFDM also has the shortcoming of self when having above advantage: very responsive to frequency shift (FS).In order to adopt the OFDM technology, carrier deviation is compared with subcarrier spacing, must be very little, otherwise the demodulation performance of OFDM will be received very big influence.Yet because transceiver frequency stability and user's influences such as high-speed mobile, this requirement is difficult to be met, and must adopt advanced person's signal processing technology to estimate and compensates this frequency deviation.Simultaneously, the symbol timing of ofdm system must drop in the scope that Cyclic Prefix (CP) allows, otherwise at this moment the FFT demodulation window has comprised the information of non-current code element, will cause the interference between code element.Therefore, effectively regularly also quite important to OFDM synchronously.Utilize known information to analyze synchronously, as CP information, be current popular processing method, the article ML Estimation of Time and Frequency Offset in OFDM System that delivered 97 years the 7th phases of IEEE Trans.SP as van de Beek J J etc.Equally also there is the document introduction to utilize additional information to carry out synchronous method, equal the U. S. application number of on June 20th, 1996 application as Schmidl: 666237, publication number: US 5732113 patent Timingand frequency synchronization of OFDM signals, this patent has proposed to utilize two synchronizing pilot symbols to carry out symbol, the method of Frequency Synchronization, comprise two identical parts (frequency domain character is that the carrier wave of even number is 0 entirely) in first synchronizing symbol, use relevant method can obtain symbol synchronization information, utilize the phase information of synchronization point output that frequency deviation is proofreaied and correct, but frequency offset estimation range is less than a subcarrier spacing.Utilize the frequency deviation that estimates for the first time that two frequency pilot signs are carried out frequency compensation and carry out the FFT computing then, utilize former and later two frequency pilot signs integral multiple subcarrier frequency deviation to be estimated in the feature of frequency domain, the frequency deviation addition of twice estimation is promptly obtained final frequency offset estimation result, realized the synchronous of ofdm system.

But the relevant peaks during this method symbol synchronization is difficult to judge best symbol synchronization constantly than broad, and synchronization overhead will take two symbols simultaneously, and system resource expends too big.

Utilize the method for synchronous of CP, the ML method of van de Beek J J as mentioned above can not need the extra resource of system to realize that synchronously, amount of calculation is also little simultaneously.But it is comparatively smooth that its shortcoming is a relevant peaks, is unfavorable for judgement, and simultaneously frequency offset estimation range is less than half of subcarrier spacing.The number of repetition that suitably is increased in identical information in the symbol can obviously increase frequency offset estimation range, yet the decline of the related gain of this moment causes synchronous error to strengthen, even comparatively lost efficacy under the complicated situation at low signal-to-noise ratio or environment.

The Canadian application number that the Lai Nabo top grade was applied on January 29th, 2002: 02806607.3, publication number: the frequency of CA 1531808A patent OFDM receiver and the method for time synchronizedization have proposed to search for to obtain the method for synchronous of time and frequency in time, frequency bidimensional window.This method is at first determined the window of time synchronized and Frequency Synchronization, but this will utilize some prior informations, and such as the maximum frequency deviation scope, maximum delay scope etc. are carried out demodulation, equilibrium, judgement to received signal with each possible time and group of frequencies are incompatible then.When not having error or error very little synchronously, the pilot tone error minimum in the restituted signal.This method has avoided using too much symbol to carry out having improved resource utilization ratio synchronously, yet this is to get in return with the huge operand of bidimensional search, and this method can not get the closed solutions of frequency deviation simultaneously.

Therefore, there is defective in prior art, and awaits improving and development.

Summary of the invention

The purpose of this invention is to provide the method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology, promptly provide time in a kind of ofdm communication system synchronously thick and smartly estimate synchronously and the scheme of frequency offset estimating on a large scale, thereby make the OFDM receiving system under with the cost of small system resources, realize symbol synchronization, and the frequency deviation in can estimating in a big way by low complexity algorithm.

Technical scheme of the present invention comprises:

Method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology, the device of this method comprises: one based on the relevant thick synchronization module of code element of cyclic prefix CP; One based on the smart synchronization module of the code element of pilot configuration; An and frequency deviation estimating modules on a large scale; Described method comprises the steps:

A), the pilot tone/synchronizing symbol of structure of making a start when framing, described pilot tone/synchronizing symbol comprises Cyclic Prefix and PN sequence, is launched by antenna after the framing;

B), receiving terminal utilizes Cyclic Prefix in the orthogonal frequency division multiplexi symbol to carry out the relevant of significant character length, and correlated results carries out normalized to the correlated series energy, is thick Synchronous Processing, corresponding module is the thick synchronization module of described code element;

C), the output result and the coupling system thresholding of the thick synchronization module of described code element are adjudicated;

D), the synchronous points set of described thick synchronization module output is input to the smart synchronization module of described code element, and the synchronous points position in described synchronous set carries out related operation, finds synchronous points;

E), find synchronous points after, signal section with the half symbols length of being separated by of correspondence carries out point-to-point multiplying each other respectively, being that the part of conjugation is point-to-point each other in the synchronizing symbol multiplies each other, to newly-generated length is that two vector sequences of efficient orthogonal frequency division multiplexing technical symbol length 1/4th carry out the FFT computing, utilizes the output of FFT largest passages to obtain the estimation of frequency deviation.

Described method for synchronous, wherein, described pilot tone/synchronizing symbol comprises 5 parts, first is described Cyclic Prefix, second and third, four, five parts are made of the identical PN sequence of length, its length all is 1/4th of efficient orthogonal frequency division multiplexing technical data length, the time domain sequences of third part is the opposite sequence of second portion time domain sequences, and the 4th part is the conjugation of second portion, and the 5th part is the conjugation of third part.

Described method for synchronous wherein, also comprises and utilizes the CP related synchronization, and its output synchronously obtains a set that comprises best synchronous points, and relevant extreme value output back half choose synchronous points when promptly not having orthogonal frequency division multiplexi symbol inter-block-interference.

Method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology provided by the present invention, because employing time in ofdm communication system of the present invention is slightly synchronous and the scheme of smart synchronous estimation and large-scale frequency offset estimating, when having improved synchronization accuracy greatly, do not expend too much system resource, and operand is very little, is beneficial to very much Project Realization.

Description of drawings

Fig. 1 is the implementing procedure figure of the method for synchronous in the described radio system of orthogonal frequency division multiplex technology of the present invention;

Fig. 2 is thick synchronizer figure of time in the method for synchronous in the radio system of orthogonal frequency division multiplex technology of the present invention;

Fig. 3 is smart synchronizer figure of time in the method for synchronous in the radio system of orthogonal frequency division multiplex technology of the present invention.

Embodiment

Below in conjunction with accompanying drawing, will carry out comparatively detailed explanation to specific embodiments of the invention.

Method for synchronous in the radio system of orthogonal frequency division multiplex technology of the present invention, can be simultaneously by synchronously thick based on the relevant time of CP and realize that based on smart synchronizer of the time of pilot configuration this invention is made up of three modules: the thick synchronization module of code element of being correlated with based on CP; Based on the smart synchronization module of the code element of pilot configuration; Frequency deviation estimating modules on a large scale.Three modules connect each other, and thick synchronization module provides possible synchronous points scope for smart synchronization module, and smart synchronization module provides temporal information for frequency deviation estimating modules.

The inventive method provides the slightly synchronous and smart method of frequency offset estimating on a large scale that reaches synchronously of time in a kind of wireless communication system, as shown in Figure 1, mainly may further comprise the steps:

1) makes a start when framing and to consider pilot tone/synchronizing symbol of structure, synchronizing symbol is made up of 5 parts, first is common Cyclic Prefix (CP), second and third, four, five parts are made of the identical PN sequence of length, its length all is 1/4th of effective OFDM data length, and third part length is the time domain opposite sequence of second portion, and the 4th part is the conjugation of second portion, the 5th part is the conjugation of third part, is launched by antenna after this sequence and other the data message framing.

2) receiving terminal utilizes CP in the OFDM symbol to carry out the relevant of significant character length, and correlated results carries out normalized to the correlated series energy, and this is thick Synchronous Processing, and corresponding module is the thick synchronization module of code element.

3) the output result and the coupling system thresholding of the thick synchronization module of code element are adjudicated.Utilize the CP related synchronization, it exports broad synchronously, be difficult to obtain best synchronous points, so this part is output as a set among the present invention, and best synchronous points just is included in wherein.For fear of directly utilizing the relevant very difficult problem of determining synchronous points of method for synchronous of carrying out of CP, the length of generally choosing CP is very long, such as greater than 2 times of maximum multipath time delay etc., do not choose synchronous points relevant extreme value output back half (having OFDM symbol inter-block-interference).Do to obtain good time synchronized effect really like this, yet long CP has reduced system resource utilization efficient, because CP is present in each symbol after all.

4) the synchronous points set with thick synchronization module output is input to thin synchronization module, and carry out related operation in the position, meeting point, mainly utilize the special shape of synchronizing symbol/frequency pilot sign, can judge the synchronous points position by this output and coupling system thresholding, solved the problem that is difficult to judgement when utilizing CP relevant, and the synchronous error of this method is very little.

5) find synchronous points after, signal section with the half symbols length of being separated by of correspondence carries out point-to-point multiplying each other respectively, being that the part of conjugation is point-to-point each other in the synchronizing symbol multiplies each other, to newly-generated length is that two vector sequences of effective OFDM symbol lengths 1/4th carry out the FFT computing, utilizes the output of FFT largest passages can obtain the estimation of frequency deviation.

Conventional OFDM emission system by chnnel coding, serial to parallel conversion, IFFT, add CP and pass through antenna at last through digital-to-analogue conversion (D/A) and up-conversion and launch.Different with the single carrier mode is the IFFT part, and it is considered as frequency domain symbol with input information, through behind the IFFT with each symbol-modulated to different subcarriers, and subcarrier is mutually orthogonal.Relatively enrich and the bigger channel of multidiameter delay for multipath like this, what each subcarrier information experienced may be flat fading, is beneficial to equilibrium, yet this has also brought other problem, such as when skew takes place carrier wave, the orthogonality between subcarrier is damaged, the demodulation failure.Therefore compare with single-carrier system, multicarrier system is more responsive to frequency deviation etc., must carry out processing such as Frequency Synchronization.Receiver is the inverse process of emission, comprise down-conversion, analog-to-digital conversion (ADC), time/code element and Frequency Synchronization, channel estimation balancing, FFT demodulation, and string change and channel-decoding.Be applied to code element, frequency deviation sync section in the OFDM receiver in the inventive method, it is the key technology in the OFDM receiver, utilizing in the synchronous method of CP of prior art, in order to simplify the synchronization decisions difficulty, generally require CP length 2 times greater than the channel maximum delay, these all can reduce resource utilization ratio to a certain extent, and the inventive method does not have this constraint, only requires CP to get final product greater than the channel maximum delay.

Below describe explanation preferred embodiment of the present invention in detail:

The k time sampling supposing received signal is output as r (k), is made of signal and receiver noise through channel.Sampled signal is cushioned, and buffer length is carried out relevant treatment to obtain thick synchronizing information of time with symbol lengths of buffering signals time-delay then greater than an OFDM symbol, and this module is thick synchronization module.Thick synchronization module is by buffer, delayer, but the correlator that iteration realizes, and decision device is formed.

At first, as shown in Figure 2, buffering signals and be delayed the signal that N orders and sent into correlator synchronously, be an effective OFDM symbol time time of delay, if corresponding sample frequency is the N gall nut carrier spacing, the delay sampling number is N so, otherwise will change, the present invention only discusses the former, but and be without loss of generality; Correlator carries out following operation:

$b\left(k\right)=\underset{m=0}{\overset{P-1}{\mathrm{\Σ}}}r(m+k)r^{*}(m+k+N)---\left(1\right)$

Wherein, P represents CP length, O ^*The expression conjugation.In k dropped on the CP scope constantly, this relevant output was very big, otherwise output is very little, and relevant peaks length is about CP.Above-mentioned relevant output can realize by following formula (2) iteration:

b(k+1)＝b(k)-r(k)r ^*(k+N)+r(P+k)r ^*(P+k+N)(2)

Thereby reduced the related operation amount.

The output of described correlator is to the signal power normalization in the correlation time length, utilize normalization output and realize the time synchronized judgement via checkout gear, detection threshold is provided by system, yet as previously discussed, this synchronous error is bigger, particularly under the Complex Channel situation, and because correlation length is limited, can not make full use of the power of whole symbol, therefore relevant affected by noise bigger.

This module is not used for realizing synchronization decisions among the present invention, but possible synchronous points (point that relevant peaks is bigger) position is designated as { k ₁, k ₂..., k _nAs output, and via the smart synchronizer of slip associated pilot sequence finish final synchronously.

The possible synchronous points that is input as of smart synchronizer is gathered { k as shown in Figure 3 ₁, k ₂..., k _nAnd the buffering sampled data r (k), be output as synchronous points position accurately.This synchronizer makes full use of the structural design of pilot tone, makes that the synchronous output in this device is quite sharp-pointed, helps synchronization decisions.Simultaneously, owing to utilized the signal energy of whole symbol, it is good therefore to work under little state of signal-to-noise.Correlation module in the described smart synchronizer module carries out following operation

$c\left(k\right)=\underset{m=0}{\overset{N/2-1}{\mathrm{\Σ}}}r(k-m)r(k+m+1),k=k_1,k_2\·\·\·,k_n---\left(3\right)$

Obviously, the correlated process of formula (3) can not realize with iteration, but its computing is only for gathering { k in thick synchronous points ₁, k ₂..., k _nCarry out in the scope, so its operand is not very big.Simultaneously, because the time sampling sequence of received signal has experienced identical decline, therefore described smart synchronization module is when accurate synchronous points, and formula (3) homophase addition obtains maximum, and relatively more sharp-pointed, can directly adjudicate according to formula (4)

$\left(4\right),k_m=\underset{k\∈\{k,k_2,\·\·\·,k_{n1}\}}{\arg \max }\frac{c\left(k\right)}{\underset{m=0}{\overset{N/2-1}{\mathrm{\Σ}}}{\left|r(k-m)\right|}^2}$

The method of described smart synchronization module is not subjected to the influence of frequency deviation, yet, because the influence of complex channel directly can not be obtained the estimation of frequency deviation by smart synchronization module.Promptly directly by c (k _m) phase information can not obtain the estimation of frequency deviation.

After described smart synchronization module output synchronizing information, with the sampled data r (k) and the synchronous points information k of buffering _mBe input to described frequency deviation estimating modules, described frequency deviation estimating modules just can realize the estimation of frequency deviation on a large scale according to Processing Algorithm under input and the combination, constructs two vectors respectively:

$X=\left[\begin{array}{c}r(k_m-N/2)r\left(k_m\right)\\ r(k_m-N/2+1)r(k_m+1)\\ \·\\ \·\\ \·\\ r(k_m-N/4+1)r(k_m+N/4-1)\end{array}\right]---\left(5\right)$

$\left(6\right),Y=\left[\begin{array}{c}r(k_m-N/4)r(k_m+N/4)\\ r(k_m-N/4+1)r(k_m+N/4+1)\\ \·\\ \·\\ \·\\ r(k_m+1)r(k_m+N/2-1)\end{array}\right]$

Wherein, [x] _m=r (k _m-N/2+m) r (k _m+ m), [Y] _m=r (k _m-N/4+m) r (k _m+ N/4+m), and m=0,1 ..., N/4-1.

Can obtain the estimation of frequency deviation on a large scale by processing, and amount of calculation is quite little to two constructed variables.For the estimation of frequency deviation, the present invention considers following two kinds of situations:

First kind of situation: in real system, when the frequency stability of transceiver very high, and consider that the frequency deviation of this moment only was (to being a decimal after the subcarrier spacing normalization) of little several times when Doppler influenced the back frequency deviation value still less than the ofdm system subcarrier spacing.Frequency deviation estimating modules is carried out according to following step:

The first step: calculate [z] _m=[y] _m/ [x] _m

Second step: the vector to first step output is averaged;

The 3rd step: find the solution the phase place of second step output average, and then obtain frequency offset estimating.

Use top three steps can obtain the estimation of system frequency deviation, estimation range is a subcarrier spacing.

Second kind of situation: in real system, when the frequency stability of transceiver not high, and consider that Doppler influences its value of back during much larger than the ofdm system subcarrier spacing, in fact frequency deviation not only comprises little several times, but also comprises several integral multiples (being the integral multiple of subcarrier spacing).At this moment, frequency deviation estimating modules is carried out following operation:

The first step: respectively to vectorial X, Y is N point FFT, finds two output results' maximum then, is designated as Varya, and Varyb also writes down its coordinate figure indexa, and indexb analyzes and knows indexa=indexb, therefore looks for extreme point only to carry out once getting final product;

Second step: calculate Varya and Varyb ^*Result of product;

The 3rd step: find the solution the phase angle of second step product output, and revise this phase angle;

The 4th step: the output in the 3rd step is added that indexa is frequency offset estimation result.

Wherein, the FFT of the first step can handle by the FFT in the receiver and examine now.

The maximum Doppler of the frequency stability of system's transceiver, system's support and subcarrier spacing are known, therefore be easy to learn that according to different system parameters designs frequency offset estimating is first kind of situation or second kind, carry out different processing according to different situations.

The inventive method can be divided into following step with whole OFDM synchronizing process: 1. construct the emission of pilot tone and framing; 2. slightly synchronous in conjunction with the time of CP; 3. utilize the time of the pilot tone of constructing synchronously smart; 4. Frequency Synchronization.Thick synchronization module utilizes the CP information of OFDM to provide possible synchronous points scope for smart synchronization module, the thresholding that smart synchronization module carries out relevant treatment and coupling system and provides at designed pilot tone carries out synchronization decisions, obtain synchronizing information accurately, can obtain large-scale frequency offset estimating in conjunction with synchronous points and according to top description, the time synchronized and the Frequency Synchronization value of last output OFDM.Come ratio with existent method, method for synchronous of the present invention has following characteristics: 1. time synchronization error is little; 2. the time synchronized operand is less; 3. the Frequency Synchronization amount of calculation is little; 4. frequency lock-in range is big.Be fit to very much handle in real time.

In a word, the method for synchronous of a kind of wireless OFDM disclosed in this invention system, described method uniqueness, novelty, with traditional based on CP synchronously, compare based on the method for a plurality of frequency pilot signs and multi-dimensional search and to have following characteristics:

1. only need a frequency pilot sign can realize the time of OFDM and frequency synchronously, for the system resource that expends synchronously smaller;

2. avoided utilizing in the method for CP related synchronization requiring CP length will be far longer than the constraint of maximum multipath time-delay, further improved resource utilization ratio;

3. realize that synchronization accuracy is higher, its relevant peaks in smart synchronization module output is quite sharp-pointed, and the output around maximum is all very little, be easy to adjudicate, and because thick synchronization module estimates that to the synchronous points scope therefore smart operand is little synchronously;

4. can be only realize in a big way frequency offset estimating with a frequency pilot sign, this is based on, and method such as CP can't accomplish, and the Frequency Synchronization operand is very little.

In a word, OFDM method for synchronous proposed by the invention do not expend too much system resource, and operand is very little when improving synchronization accuracy greatly, be beneficial to very much Project Realization.

But should be understood that above-mentioned description at preferred embodiment of the present invention is comparatively concrete, can not therefore think the restriction to scope of patent protection of the present invention, scope of patent protection of the present invention should be as the criterion with claims.

Claims (3)

1, the method for synchronous in a kind of radio system of orthogonal frequency division multiplex technology, the device of this method comprises: one based on the relevant thick synchronization module of code element of cyclic prefix CP; One based on the smart synchronization module of the code element of pilot configuration; An and frequency deviation estimating modules on a large scale; Described method comprises the steps:

A), the pilot tone/synchronizing symbol of structure of making a start when framing, described pilot tone/synchronizing symbol comprises Cyclic Prefix and PN sequence, is launched by antenna after the framing;

B), receiving terminal utilizes Cyclic Prefix in the orthogonal frequency division multiplexi symbol to carry out the relevant of significant character length, and correlated results carries out normalized to the correlated series energy, is thick Synchronous Processing, corresponding module is the thick synchronization module of described code element;

C), the output result and the coupling system thresholding of the thick synchronization module of described code element are adjudicated;

D), the synchronous points set of described thick synchronization module output is input to the smart synchronization module of described code element, and the synchronous points position in described synchronous points set carries out related operation, finds synchronous points;

E), find synchronous points after, signal section with the half symbols length of being separated by of correspondence carries out point-to-point multiplying each other respectively, being that the part of conjugation is point-to-point each other in the synchronizing symbol multiplies each other, to newly-generated length is that two vector sequences of efficient orthogonal frequency division multiplexing technical symbol length 1/4th carry out the FFT computing, utilizes the output of FFT largest passages to obtain the estimation of frequency deviation.

2, method for synchronous according to claim 1, it is characterized in that, described pilot tone/synchronizing symbol comprises 5 parts, first is described Cyclic Prefix, second and third, four, five parts are made of the identical PN sequence of length, its length all is 1/4th of efficient orthogonal frequency division multiplexing technical data length, the time domain sequences of third part is the opposite sequence of the time domain sequences of second portion, and the 4th part is the conjugation of second portion, and the 5th part is the conjugation of third part.

3, method for synchronous according to claim 2, it is characterized in that, described method also comprises utilizes the CP related synchronization, its output synchronously obtains a set that comprises best synchronous points, and relevant extreme value output back half choose synchronous points when promptly not having orthogonal frequency division multiplexi symbol inter-block-interference.

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