CN100505595C

CN100505595C - OFDM time synchronous method based on training sequence

Info

Publication number: CN100505595C
Application number: CNB03117969XA
Authority: CN
Inventors: 赵宏志; 唐友喜; 宋丽英; 李少谦
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2003-05-30
Filing date: 2003-05-30
Publication date: 2009-06-24
Anticipated expiration: 2023-05-30
Also published as: CN1553603A

Abstract

This invention discloses a new high performance OFDM time clock method based on training sequence. Comparing with previous time clock method, its frame structure supports a packet mode for OFDM length time, namely as transferring the packet mode ,only one symbol of OFDM can complete the task of the data communication and time clock, no need to overlap other symbols of training sequence. Meantime, after the receiving end completes the first time time clock , separates the data from training sequence, then utilizes the separated training sequence to make the second time time clock, there by to get more excellent time clock effect. This invention can effectively reduce the interference of OFDM data to the training sequence to get the more excellent time clock effects, meanwhile greatly increase the efficiency of the data emitting energy.

Description

A kind of new OFDM method for synchronizing time based on training sequence

Technical field

The invention belongs to communication technical field, particularly in the communication technology based on the method for synchronizing time of training sequence.

Background technology

OFDM is owing to have the message transmission rate height, and the anti-multipath interference performance is strong, and the spectrum efficiency advantages of higher more and more comes into one's own.It has been successfully applied to wired, radio communication.As: ADSL (Asymmetric Digital Subscriber Line), Wireless LAN is among DAB (Digital Audio Broadcasting), DVB, EEE802.11a and the HyperLAN/2.In the IEEE802.16 that is formulating at present, also related to the OFDM technology in a large number.This new modulation technique of OFDM also can be applicable in the third generation mobile communication system.Use the OFDM technology will improve the transmission data rate and the spectrum efficiency of the third generation mobile communication system widely, and have good anti-multipath, cochannel interference and impact noise ability and see document: Bingham, J.A.C., " Multicarrier modulation for data transmission:an idea whose time has come; " IEEE CommunicationsMagazine, Volume:28 Issue:5, May 1990, Page (s): 5-14; And document: Yun Hee Kim; Iickho Song; Hong Gil Kim; Taejoo Chang; Hyung Myung Kim, " Performance analysis of a coded OFDM system in time-varying multipathRayleigh fading channels; " Vehicular Technology, IEEE Transactions on, Volume:48 Issue:5, Sept.1999, Page (s): 1610-1615 is described.

In ofdm system, time synchronized is the technology of a key, and the purpose of time synchronized is to find out the border of each OFDM symbol in the serial data stream of receiving.The module 1 among Fig. 1 is seen in the position of synchronization module.One of weakness of OFDM technology is more much higher than single-carrier system to synchronous requirement.With regard to present OFDM simultaneous techniques, the starting point that accurately finds the OFDM symbol is the comparison difficulty.Referring to document Donghoon Lee, Kyungwhoon Cheun, " Coarse symbolsynchronization algorithms for OFDM systems in multipath channels; " IEEE Commun.Letters, vol.10, pp.446-448, Oct.2002; . and document Negi, R., Cioffi, J.M., " Blind OFDM symbol synchronization in ISI channels, " IEEE Trans.Commun.vol.50, pp.1525～1534, Sep 2002.And, might address this problem, because be generally less than a code element (chip) width based on the timing tracking accuracy of training sequence based on the method for synchronizing time of training sequence.

The OFDM method for synchronizing time of use training sequence commonly used has:

1. adopt first and second OFDM symbols to be the method for synchronous of training sequence entirely.Referring to document P.Moose, " Atechnique for orthogonal frequency division multiplexing frequency offset correction, " IEEE Trans.Commun., vol.42, pp.2908-2914, Oct.1994.

2. only adopt first OFDM symbol to be the method for synchronous of training sequence entirely.Referring to document Hyoung-Kyu Song, Young-Hwan You, Jong-HoPaik, Yong-SooCho, " Frequency-offset synchronization and channelestimation for OFDM-based transmission, " IEEE Commun.Letters, vol.4, pp.95-97, Mar.2000.

3. adopt the method for synchronous of the training sequence shorter, it is characterized in that the back of training sequence at Cyclic Prefix than an OFDM symbol time length.Referring to document M.Speth, D.Daecke, and H.Meyr, " Minimum overhead burstsynchronization for OFDM based broadband transmission, " Proc.Globecom ' 98, pp.3227-3232.

4. adopt the method for synchronous of the training sequence shorter, it is characterized in that the front of training sequence at Cyclic Prefix than an OFDM symbol time length.Referring to document Zhou Shidong, " Introduction on B3G activities in Tsinghua Univisity, " Proc.of The International Forum on Future Mobile Telecommunications ﹠amp; China-EU Post Conference onBeyond 3G, pp.106-110, Nov.2001.

The above-mentioned four kinds of time frequency synchronizing methods of OFDM during transmitted in packets, possess a common characteristic: within a certain period of time, training sequence occupies all transmitting powers.This method of monopolizing total transmitting power in the regular hour except the efficient of carrying valid data is hanged down, also lacks the flexibility on time, the power.

5. a kind of reasonable method for synchronous based on training sequence of OFDM is: training sequence is superimposed upon on the OFDM useful data making a start.The correlation that the training sequence of receiving end utilization emission and receiving terminal have the training sequence signal is carried out time synchronized.Referring to document Tufvesson, F.; Edfors, O.; Faulkner, M., " Time and frequencysynchronization for OFDM using PN-sequence preambles; " Vehicular Technology Conference, 1999.VTC1999-Fall.IEEE VTS 50th, Volume:4,1999, Page (s): 2203-2207.

Adopting this kind method to carry out time synchronized has many good qualities: at first, as can be seen from Figure 2, training sequence is not monopolized the gross power of arbitrary section time transmitter emission in this kind method; Secondly, this method provides the ability of flexible design air interface, and promptly under the prerequisite of certain error rate and identical total transmitting power, by the size of configuration PN sequence power, can change an OFDM symbol can the data carried by data amount.Yet existing, this method for synchronous can not avoid shortcoming: the mutual serious interference of training sequence and data, be reflected on the time synchronized, and because training sequence is subjected to the influence of OFDM data, synchronization accuracy is low; And the energy efficiency of this kind method emission data is lower, during transmitted in packets, grouped data in the air interface has only several sometimes even has only an OFDM data symbol, in this case, in order to finish normal communication, will superpose in transmitting one even several training sequence symbols, obviously efficient is lower.

Summary of the invention

Task of the present invention provides a kind of high performance OFDM method for synchronizing time based on training sequence, promptly adopt method of the present invention, can reduce of the interference of OFDM data effectively to training sequence, thereby can obtain the better time synchronized effect of more traditional method for synchronizing time by the good correlation properties of training sequence, greatly increase the efficient of emission data energy simultaneously.

Innovation part of the present invention is: 1. compare with method for synchronizing time in the past, the frame structure of this patent is supported the packet mode of an OFDM length time, promptly when transmitted in packets, an OFDM symbol just can be finished the task of data communication and time synchronized, and other training sequence symbols that need in transmitting, not superpose.2. after receiving terminal is finished the time synchronized first time, the OFDM data are separated with training sequence, and then utilize training sequence separated to carry out the time synchronized second time, thereby obtain more excellent time synchronized effect.

According to a kind of new OFDM method for synchronizing time of the present invention, it is characterized in that transmitting terminal comprises the following step based on training sequence:

Step 1 as shown in Figure 2, c represents that a length is the training sequence symbols of N, d represents the OFDM data symbol (comprising guidance information) that a length is N, training sequence and OFDM data-signal are carried out the power normalization processing respectively, make the power of training sequence and OFDM data be respectively ρ and 1-ρ.

Step 2 then with this training sequence symbols c therewith OFDM symbol d superpose at transmitting terminal, be transmitted into channel after adding the protection prefix.

According to a kind of new OFDM method for synchronizing time based on training sequence of the present invention, receiving terminal comprises:

Step 3 time synchronized: shown in the module 7 of Fig. 3,, therefore can call time synchronized for the first time to this time synchronized operation because need to carry out twice time synchronized operation in this patent receiver.Its function is to find receiver to do the initial moment of FFT operation so that carry out time adjustment to received signal because the training sequence in the received signal has been subjected to the interference of ofdm signal, thus this subsynchronous can only "ball-park" estimate time synchronized point.

It is characterized in that receiver also comprises the following steps:

Step 4 suppresses the interference of OFDM data to training sequence: as shown in Figure 3, to carry out FFT (quick Fourier transformation) computing through the received signal r ' after removal protection prefix 2, time synchronized 7 operations for the first time, promptly finish the demodulation function of multi-carrier modulation, obtain sequence R; Carry out OFDM data and training sequence lock out operation then, promptly reconstruct earlier and receive the OFDM data-signal

, from total received signal r, deduct the OFDM data-signal that reconstructs then

, just obtained the training sequence in the received signal, through after the aforesaid operations, the OFDM data-signal of receiving terminal and training sequence is separated comes, OFDM are counted signal the interference of training sequence have been reduced widely.

Step 5 time synchronized: shown in Fig. 3 module 7, the training sequence that utilizes step to obtain in 2. carries out the time synchronized operation second time.Being used for synchronous training sequence signal this moment, to be subjected to the interference of OFDM data-signal less, so timing tracking accuracy is higher.

From above-mentioned steps as can be seen, adopt method for synchronizing time of the present invention, the interference of user data to training sequence can be reduced, make under the situation that system is abominable in channel variation and training sequence power strengthens and still obtain higher timing tracking accuracy.Simultaneously, when transmitted in packets,, also can normally finish communication task even have only an OFDM symbol in the air interface.

Description of drawings

Fig. 1 is traditional operation of receiver schematic diagram based on training sequence

Wherein, the 1st, lock unit, the 2nd, remove protection prefix unit, the 3rd, FFT unit, the 4th, channel estimating unit, the 5th, channel fading compensating unit, the 6th, data demodulation unit.

Fig. 2 is the stacked system figure of training sequence and OFDM data in the transmitter of this patent.

Wherein, c represents a training sequence symbols, and d represents an OFDM data symbol, Be the power normalization factor of training sequence, The power normalization factor for the OFDM data-signal.

Fig. 3 is the receiver structure block diagram in this patent.

Wherein, the 7th, the time synchronized unit, the 8th, rebuild and receive the OFDM data cell.R represents received signal, and the received signal after the protection prefix is got rid of in r ' expression, and R represents the received signal after the demodulation of process multicarrier,

The reception OFDM data-signal that expression reconstructs.

Fig. 4 is the time synchronized structured flowchart that adopts in the embodiment.

Wherein, the 9th, received signal and local training sequence relevant treatment unit, the 10th, difference relevant treatment unit, the 11st, ask form unit, τ is a signal time delay, and K is a relevant treatment length, and Ω represents the difference distance, Γ is a difference processing length, and (n τ) is difference relevant treatment result to ξ.

Fig. 5 is the model that the reconstruction adopted in the embodiment receives the OFDM data-signal.

Wherein, the 12nd, the CAZAC sequence generator, the 13rd, the IFFT unit, the 14th, the time delay unit that the first time, time synchronized obtained, λ is the part interference cancellation factor by the decision of channel estimating confidence level.

Fig. 6 is the implementation step block diagram of this patent.

Embodiment:

Provide a concrete OFDM configuration below down, the performing step of this patent.Need to prove: the parameter in following does not influence the generality of this patent.

If the useful symbol lengths of OFDM is N=4096, the protection gap length is G=1024.Training sequence selects to have the CAZAC sequence of good period correlation properties and normal amplitude characteristic.

At transmitting terminal: the length training sequence symbols that is N and length OFDM data symbol (comprising guidance information) that is N is carried out power normalization respectively handle, make the power of training sequence and OFDM data be respectively ρ and 1-ρ; Then with this training sequence symbols therewith the OFDM symbol superpose at transmitting terminal, be transmitted into channel after adding the protection prefix.

At receiving terminal: 1. as shown in Figure 3, at first carry out the time synchronized operation first time 7, its concrete steps are referring to Fig. 4, received signal r (n) carries out relevant treatment operations through unit 9 and local training sequence, the relevant treatment length K is less than N, and then carry out the difference relevant treatment that spacing is Ω through unit 10, relevant treatment length is Γ, after above two steps operation, (n τ), asks modulo operation 11 to this decision signal to obtain decision signal ξ, the result is compared with deciding thresholding, just can determine the time synchronized point roughly

2. then as shown in Figure 3,, received signal is gone to protect the prefix operation through unit 2, finish the demodulation function of multi-carrier modulation again through FFT converter unit 3 according to the result of the time synchronized first time.3. once more, rebuild the operation of receiving terminal OFDM data-signal 8, concrete steps are referring to Fig. 5: extract guidance information and 4 carry out channel estimating in the unit from sequence R; Carry out the reconstruction of training sequence received signal simultaneously, promptly produce the CAZAC sequence of making a start used by training sequence generator unit 12 earlier, the sequence that produces be multiply by amplitude factor

Training sequence after will rebuilding then and channel estimation results and part interference cancellation factor lambda multiply each other, from sequence R, deduct this result of product again, promptly finish the function of part interference cancellation, reduce the interference of training sequence, just obtained than direct demodulation data message more accurately to data-signal; The output result of this data message and channel estimation module gripped altogether multiply each other, just remove the suffered channel fading influence of data message, the signal after the compensation is exported by data demodulates and decision unit 6; The result that will export then multiply by amplitude factor again through IFFT converter unit 13 And channel estimation results, the while is according to the result of the time synchronized first time

Signal is carried out time delay operation 14, rebuild the signal that receives

Finish the reconstruction of receiving terminal OFDM data-signal.4. from total received signal, deduct the OFDM data-signal that reconstructs, just obtained the training sequence signal of receiving terminal; Carrying out the time synchronized second time with this training sequence of having eliminated the interference of OFDM data-signal at last, synchronizing process is identical with unit 7, just obtains more precise time synchronized result

Claims

1, a kind of new OFDM method for synchronizing time based on training sequence is characterized in that comprising following step:

Transmitting terminal comprises:

Step 1 a couple training sequence symbols c and an OFDM data symbol d carry out power normalization respectively to be handled, make the power of training sequence and OFDM data be respectively ρ and 1-ρ, c represents that a length is the training sequence symbols of N, and d represents the OFDM data symbol that a length is N;

Step 2 then with this training sequence symbols c therewith OFDM symbol d superpose at transmitting terminal, be transmitted into channel after adding the protection prefix;

Receiving terminal comprises:

Step 3 is time synchronized for the first time: time synchronized is the correlation that unit (9) utilizes receiving terminal received signal and local training sequence for the first time, find receiver to do the initial moment of FFT operation, so that carry out time adjustment to received signal, because the training sequence in the received signal has been subjected to the interference of ofdm signal, thus this subsynchronous can only "ball-park" estimate time synchronized point;

Step 4 suppresses the influence of OFDM data to training sequence: to carrying out quick Fourier transformation FFT operation through the received signal after time synchronized and the time adjustment for the first time, obtain sequence R; Carry out OFDM data and training sequence lock out operation then, promptly reconstruct by unit (11) earlier and receive the OFDM data-signal

, just having obtained the training sequence in the received signal, OFDM counts signal the interference of training sequence has been reduced widely in this sequence;

Step 5 is time synchronized for the second time: unit (9) utilize the training sequence that obtains in the step 4 to carry out the time synchronized operation second time.