CN1992691B - Channel synchronization method based on OFDM technology - Google Patents

Abstract

This invention provides a OFDM-based channel synchronization method, used in the LTE system, and the sub-frame structure of the said LTE system includes one synchronous symbol and a number of reference symbols, the said synchronization method including following steps: placing the reference symbol at the first symbol of each sub-frame, and the synchronization symbol at the second symbol of the first sub-frame, and the reference symbol and synchronization symbol are all situated at the first sub-frame, the reference symbol and synchronization symbol with adjacent positions, and in the junction location of the reference symbol and the synchronization symbol, forming the continuous repetitive structure in the time domain. Through this method, it enhances capture capabilities of synchronization and frequency offset, thus it is very easy to find starting position of the sub-frames. In addition, directly using the reference symbol to assist the synchronization process, it no longer needs to add additional synchronization symbol so as to achieve good performance, and reduce the overhead of the synchronization channel.

Description

A kind of channel synchronization method based on the OFDM technology

[technical field]

The present invention relates to a kind of field of information transmission of communication system, the particularly a kind of time synchronized of OFDM technology realization LTE system, channel synchronization method of carrier shift of adopting.

[background technology]

At present, as a kind of multi-carrier transmission pattern, OFDM (OrthogonalFrequency Division Multiplexing, OFDM) be converted to one group of low-speed parallel data flow transmitted by data flow, system is reduced greatly to the susceptibility of multidiameter fading channel frequency selectivity a high-speed transfer.The introducing of Cyclic Prefix (CP) has further strengthened the ability of the anti-intersymbol interference of system (ISI) again.In addition bandwidth availability ratio height, realize that characteristics such as simple make OFDM more and more wider in the application in radio communication basin.

The OFDM technology successfully has been applied in many communication systems, for example, digital broadcasting (DAB) and Digital Television (DVB) the employing OFDM technology formulated by european telecommunications standards institute ETSI are the wireless transmission standards of air interface, and WLAN standard IEEE802.11 and wireless metropolitan area network standard IEEE802.1 6 have also adopted the OFDM technology in addition.Effective use of OFDM technology needs the synchronous and carrier synchronization of receiving-transmitting sides energy strict time.

The OFDM symbol is made of a plurality of sub-carrier signal stacks, utilizes orthogonality to distinguish between each subcarrier, therefore guarantees that this orthogonality is vital for ofdm system, so ofdm system is also just strict relatively to synchronous requirement.It is a very important step in the ofdm system that sync bit inspection side and frequency deviation detect, and has only found correct sync bit to carry out correct reception to data, thereby has carried out next step processing.

Please refer to Fig. 1 to Fig. 4.In prior OFDM system, a method for synchronous is arranged, just be based on symbol of adding on the transmitting terminal data flow with repeat property, the position that meets this repeated characteristic in the receiving terminal detection is as symbol position regularly.Also utilize the correlation properties of transmission sign indicating number known on this symbol frequency domain to determine frequency departure between the sending and receiving end.This method is general to be adopted as symbol of Fig. 1 form structure, at first whole subcarriers are divided into strange subcarrier and even subcarrier two classes two classes by the odd even preface, only utilize a class wherein, place modulating data as all strange subcarrier or whole even subcarrier, and the another kind of subcarrier free time does not pass any data, and its time domain shows as Fig. 2.Fig. 2 upper part is a structure of utilizing even subcarrier to produce, and two identical subdivision A are arranged, and Fig. 2 lower part is a structure of utilizing strange subcarrier to produce, a back inverse value that part A I is previous part A.。

Carrying out following time domain at receiving terminal handles:

$c_n=\underset{k=0}{\overset{L-1}{\mathrm{\Σ}}}{r}_{n+k}{r^{*}}_{n+k+D}---\left(1\right)$

$p_n=\underset{k=0}{\overset{L-1}{\mathrm{\Σ}}}{\left|r_{n+k+D}\right|}^2---\left(2\right)$

$m_n=\frac{{\left|c_n\right|}^2}{{\left(p_n\right)}^2}---\left(3\right)$

The time-domain symbol sampled value that r representative receives, D is half of OFDM symbol lengths that does not comprise Cyclic Prefix (CP) part.Utilize the correlation between the symbol, can judge the original position of grouped data according to the amplitude size of mn.

Since have only this OFDM symbol free time the idol (very) carrier wave useless, promptly only utilize strange subcarrier or only utilize even subcarrier, and other data symbols generally all are to have occupied all available subcarriers, so on time domain, have only this symbol place to form special repetitive structure (as Fig. 2), will form the platform of a peak for the decision value Mn that forms, corresponding to the no multipath interference sections in the CP part of symbol, the sync bit of judgement should be positioned at this zone.

After having determined sync bit, utilize the frequency error that following formula can the preresearch estimates transceiver:

Φ=angle (c _n) when adopting even subcarrier

Φ=angle (c _n)-π is when adopting strange subcarrier

Have simultaneously

Φ＝πTΔf+2nπ

So

$\mathrm{\Δf}=\frac{\mathrm{\Φ}}{\mathrm{\πT}}+\frac{2n}{T}$

Therefore by detecting the phase place of Cn, first's frequency error of following formula can be detected, and compensates, and is commonly referred to as the decimal frequency bias compensation.

But because n has different values, can't determine whether the scope of frequency error exceeds the situation of n=0, therefore will utilize symbol from sync bit from data flow, extract the OFDM symbol and CP partly removed after carry out the FFT conversion, solve symbol, be shifted relevant with known transmission sequence, find the correlation peak of corresponding maximum, this moment, corresponding shift value was exactly corresponding n value, thereby finished the integer frequency bias compensation.

" 3GPP, TR25.814 v0.41 " Physical Layer Aspects for Evolved UTRA (release 7) " have provided W-CDMA Long Term Evolution (Long Term Evolution, various technical schemes LTE) later on.Wherein, the most noticeable based on the OFDM technical scheme.Its frame structure as shown in Figure 3, every frame is 10ms, a frame comprises the long subframe of 20 0.5ms.According to the difference of CP length, each subframe comprises 6 long CP or 7 short CP symbols.First of each subframe or second symbol are the positions of placing reference symbol, and the effect of reference symbol is to carry out channel quality measurement, and channel estimating also can assisted cell search and synchronous.Fig. 4 has provided the example of a reference symbol.

Usually, need in system, add proprietary synchronizing symbol and finish Cell searching, time synchronized and Frequency Synchronization.How existing method for synchronous is applied in the LTE system, thus can realize synchronously, the capturing frequency deviation performance can guarantee that overhead is the problem that needs research for a short time simultaneously again.

[summary of the invention]

The object of the present invention is to provide a kind of OFDM of employing technology to realize the time synchronized of LTE system, the channel synchronization method of carrier shift, utilize this method for synchronous can guarantee simultaneously synchronously, it is little that capturing frequency deviation has good performance and expense.

The objective of the invention is to be achieved through the following technical solutions: a kind of channel synchronization method based on the OFDM technology, be applied in the LTE system, comprise synchronous symbol and some reference symbols in the subframe structure of described LTE system, described method for synchronous may further comprise the steps: first symbol that reference symbol is placed each subframe, synchronizing symbol places second symbol of first subframe, reference symbol and synchronizing symbol all are arranged in first subframe, reference symbol and synchronizing symbol are positioned at position adjacent, wherein, whole subcarriers of synchronizing symbol and reference symbol are divided into strange subcarrier and even subcarrier two classes by the odd even preface, only utilize the whole subcarriers of a class wherein to place modulating data, and the another kind of subcarrier free time does not pass any data, is formed on continuous repetitive structure on the time domain with the intersection at reference symbol and synchronizing symbol.

Wherein, only at strange sub-carrier positions modulating data, the remaining subcarrier free time does not pass any data for reference symbol and synchronizing symbol.

Wherein, reference symbol and synchronizing symbol are only at even sub-carrier positions modulating data, and the remaining subcarrier free time does not pass any data.

Wherein, the reference symbol content of the reference symbol transmission of formation repetition continuous structure can be identical with other reference symbols.

Wherein, the frequency of synchronizing symbol transmission is set according to the specific requirement of system.

The present invention compared with former technology, have the following advantages: reference symbol and synchronizing symbol are put together transmission and have the characteristic that time domain repeats simultaneously, other data symbols have occupied all available subcarriers, on time domain, only form special continuous repetitive structure at this place, because two symbols satisfy the repetitive structure of time domain simultaneously, strengthen the capture ability of synchronous and frequency deviation, thereby can find the original position of subframe easily.In addition, reference symbol always needs to transmit, and directly utilizes reference symbol to assist synchronous process, reaches good performance thereby no longer need to add extra synchronizing symbol, has reduced the expense of synchronizing channel.

[description of drawings]

Fig. 1 is the symbolic construction that a kind of traditional synchronization scenario adopts.

Fig. 2 adopts the performance structure schematic diagram of the described symbolic construction of Fig. 1 in time domain.

Fig. 3 is the schematic diagram of OFDM frames structure in the LTE system.

Fig. 4 is the schematic diagram of the reference symbol in subframe in the LTE system.

Fig. 5 is the synchronous design method of synchronizing channel of the present invention.

[embodiment]

Core thinking of the present invention is exactly the design of the reference symbol in the LTE system and synchronizing symbol to be combined carry out synchronously and the catching of frequency deviation.Wherein, the effect of synchronizing symbol is to carry out initial time synchronized and Frequency Synchronization.Transmit the adjacent position that synchronizing symbol is placed on certain reference symbol, this moment, whole subcarriers of synchronizing symbol and reference symbol can be divided into strange subcarrier and even subcarrier two classes by the odd even preface, only utilize a class wherein, place modulating data as all strange subcarrier or whole even subcarrier, and the another kind of subcarrier free time does not pass any data.Wherein, the reference symbol content of transmitting in the reference symbol of this position can be identical with other reference symbols, and is unaffected.Just needing to be used for originally the position of Data transmission puts sky, and the expense of Zeng Jiaing is little like this.The frequency of synchronizing symbol transmission is then set according to the specific requirement of system.Two kinds of symbols are put together transmission and have the characteristic that time domain repeats simultaneously, and other data symbols generally all are to have occupied all available subcarriers, so only formed special continuous repetitive structure at this place on time domain.Because two symbols satisfy the repetitive structure of time domain simultaneously, strengthened the capture ability of synchronous and frequency deviation, thereby can find the original position of subframe easily.In addition, reference symbol always needs to transmit, and directly utilizes reference symbol to assist synchronous process, reaches good performance thereby no longer need to add extra synchronizing symbol, has reduced the expense of synchronizing channel.

Below in conjunction with embodiment technology contents of the present invention is described in further detail.

Please refer to Fig. 5, this figure is the synchronous design method of synchronizing channel of the present invention.In the LTE system, in the frame 100 of 10ms, comprise the subframe of 20 0.5ms, be respectively first subframe 1, second subframe 2 ..., the 19 subframe the 19, the 20 subframe 20, each subframe comprises some symbols again.

According to channel synchronization method of the present invention, in the LTE system, whole subcarriers of synchronizing symbol and reference symbol are divided into strange subcarrier and even subcarrier two classes by the odd even preface.Reference symbol 31 is positioned at first symbol of each subframe, synchronizing symbol 40 is positioned at second symbol of first subframe 1, like this, reference symbol 31 and synchronizing symbol 40 are all in first subframe 1, reference symbol 31 is positioned at position adjacent with synchronizing symbol 40, and only at strange sub-carrier positions modulating data, perhaps only at even sub-carrier positions modulating data, the remaining subcarrier free time does not pass any data for reference symbol 31 in first subframe 1 and synchronizing symbol 40.Like this at the section start of every frame, reference symbol 31 and synchronizing symbol 40 have as shown in Figure 2 repetitive structure in time domain.Can obtain regularly synchronous and decimal frequency bias according to this repetitive structure, and owing to exist two repetitive structures can make the result reliability of judgement increase.Simultaneously, the position of reference symbol 31 and synchronizing symbol 40 is in the section start of frame, so this structure also can obtain frame timing and subframe timing simultaneously.Because reference symbol 31 and synchronizing symbol 40 satisfy the repetitive structure of time domain simultaneously, strengthened the capture ability of synchronous and frequency deviation, thereby can find the original position of subframe easily.In addition, reference symbol always needs to transmit, and directly utilizes reference symbol to assist synchronous process, reaches good performance thereby no longer need to add extra synchronizing symbol, has reduced the expense of synchronizing channel.

The above only is a preferred example of the present invention, and in order to restriction the present invention, within the spirit and principles in the present invention not all, institute changes, is equal to replacement, improvement etc., all should be included within protection scope of the present invention.

Claims (5)

1. channel synchronization method based on the OFDM technology, be applied in the LTE system, comprise synchronous symbol and some reference symbols in the subframe structure of described LTE system, described method for synchronous may further comprise the steps: first symbol that reference symbol is placed first subframe, synchronizing symbol places second symbol of first subframe, reference symbol and synchronizing symbol all are arranged in first subframe, reference symbol and synchronizing symbol are positioned at position adjacent, wherein, whole subcarriers of synchronizing symbol and reference symbol are divided into strange subcarrier and even subcarrier two classes by the odd even preface, only utilize the whole subcarriers of a class wherein to place modulating data, and the another kind of subcarrier free time does not pass any data, is formed on continuous repetitive structure on the time domain with the intersection at reference symbol and synchronizing symbol.

2. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, only at strange sub-carrier positions modulating data, the remaining subcarrier free time does not pass any data for reference symbol and synchronizing symbol.

3. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, reference symbol and synchronizing symbol are only at even sub-carrier positions modulating data, and the remaining subcarrier free time does not pass any data.

4. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, it is identical with other reference symbols to form the reference symbol content that the reference symbol of continuous repetitive structure transmits.

5. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, the frequency of synchronizing symbol transmission is set according to the specific requirement of system.

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