CN1773979B - Orthogonal Frequency Division Multiplexing Precise Time Synchronization, Frame Synchronization and Synchronization Tracking Method - Google Patents

Abstract

The invention discloses a method for precise time synchronization of orthogonal frequency division multiplexing OFDM. The method is as follows: a receiving end receives more than one OFDM symbol with different pilot sequences from a transmitting end, and performs integer frequency offset on the received signal Synchronization processing: performing frequency-time conversion on the pilot sequence corresponding to the one or more OFDM symbols, concatenating the converted time-domain signals as a time-domain reference signal, using the time-domain reference signal and integer frequency offset after synchronous processing The multi-path search is carried out on the signal to determine the arrival time of the first path and the amplitude and phase value, so as to realize precise time synchronization. At the same time, the invention also discloses a method for frame synchronization of an OFDM system and a method for synchronous tracking of an OFDM system. Applying the present invention can realize multi-path search, accurately locate multi-path arrival time and multi-path amplitude, thereby eliminating the influence of strong intersymbol interference (ISI) during fractional frequency synchronization (FFO), and improving fractional frequency offset estimation (FFO) accuracy.

Description

Orthogonal Frequency Division Multiplexing Precise Time Synchronization, Frame Synchronization and Synchronization Tracking Method

technical field

The invention relates to the technical field of data transmission in mobile communication technology, in particular to a method for precise time synchronization, frame synchronization and synchronization tracking of Orthogonal Frequency Division Multiplexing (OFDM).

Background technique

Orthogonal Frequency Division Multiplexing (OFDM, Orthogonal Frequency Division Multiplexing) is a mobile communication technology that uses parallel transmission to increase the communication data transmission rate. The basic idea of this technology is to divide a given channel into many orthogonal sub-channels in the frequency domain, use a sub-carrier for modulation on each sub-channel, and transmit each sub-carrier in parallel. In this way, although the overall channel is non-flat and has frequency selectivity, each sub-channel is relatively flat, and the narrow-band transmission is carried out on each sub-channel, and the signal bandwidth is smaller than the corresponding bandwidth of the channel, so the gap between signal waveforms can be greatly eliminated. interference. The difference between OFDM and general multi-carrier transmission is that it allows partial overlap of sub-carrier spectrum. As long as the sub-carriers are mutually orthogonal, the data signal can be separated from the aliased sub-carriers.

Because OFDM allows subcarrier spectrum aliasing, its spectral efficiency is greatly improved. At the same time, this technology also has many advantages such as anti-multipath interference and intersymbol interference, easy channel estimation and equalization, and low system implementation complexity. Therefore, it is a kind of Efficient modulation method. And this technology is easy to combine with various multiple access technologies, so it is generally considered to be an indispensable core technology in the fourth generation mobile communication system. At present, this technology has been used in digital audio broadcasting (DAB), digital video broadcasting (DVB), asymmetric digital subscriber line (ADSL), wireless local area network (WLAN), wireless metropolitan area network (WMAN), wireless personal area network (WPAN), It is widely used in many data communication systems such as wireless high-speed unlicensed metropolitan area network (WHUMAN), and OFDM technology has been determined to be used in the 802.16 protocol. The 802.20 mobile broadband wireless access currently being discussed by the Institute of Electrical and Electronics Engineers (IEEE) standardization organization The system will also use this modulation technique.

OFDM symbols are composed of sub-carriers in the frequency domain, and the number of sub-carriers determines the number of time-frequency transform (FFT) points. There are three types of subcarriers, which are data subcarriers, pilot subcarriers, and virtual subcarriers. Among them, the data subcarrier is used for data transmission; the pilot subcarrier is initially used to eliminate the residual phase difference. With the development of technology, the role of the pilot subcarrier is further expanded, and can be used for frequency synchronization and channel estimation. A virtual subcarrier refers to a carrier that does not transmit any data, and is introduced by the OFDM system to reduce interference to adjacent frequency bands.

OFDM technology has many advantages mentioned above, but it has very high requirements on time synchronization, especially precise time synchronization, so as to ensure the orthogonality between subcarriers. This is because: if you synchronize in advance, different phase differences will be introduced for different subcarriers. As the missynchronization deviation increases, the performance of the system will be affected more and more, especially for the high frequency in OFDM. The sub-carrier will introduce a large phase difference. For high-order modulation, this deviation will lead to a sharp decrease in performance and make the high-order modulation unusable. If synchronization is delayed, inter-symbol interference will be introduced, leading to a sharp drop in performance. In addition, time missynchronization will affect the performance of fractional frequency synchronization (FFO) to a considerable extent. Therefore, precise time synchronization in the OFDM system is a very important key technology in the system.

FIG. 1 is a schematic diagram of an overall process for realizing time-frequency synchronization by OFDM.

Step 101: the OFDM system performs coarse time synchronization on the received signal. The specific method is as follows: time synchronization is performed by using a preamble, that is, a preamble, that is, a rough time synchronization is obtained by correlating two pieces of the same data before and after, so as to obtain the synchronization peak phase information of the received signal. Or, use a time window equal to the length of the cyclic prefix to perform sliding correlation on the received signal, rely on the cyclic prefix in the OFDM symbol to perform time synchronization, and obtain the synchronization peak phase information of the received signal.

Step 102: Fractional frequency offset synchronization. The specific method is: perform fractional frequency offset estimation according to the synchronization peak phase information described in step 101, and correct the received signal according to the fractional frequency offset estimation information, so that only integer frequency offset exists in the corrected received signal.

Step 103: time-frequency conversion. The specific method is: transforming the signal processed in step 102 into the frequency domain through Fast Fourier Transform (FFT) or Discrete Time Fourier Transform (DFT).

Step 104: Integer frequency offset synchronization. The specific method is: firstly use the known subcarrier information in the Preamble sequence to perform differential correlation matched filtering to estimate the integer frequency offset, and then perform integer frequency offset compensation, that is, use the estimated integer frequency offset information to correct the received signal, Integer frequency offset does not exist in the corrected received signal.

Step 105: Precise time synchronization. The specific method is: use the preamble cell to perform matching filtering with the signal processed in step 104, search for and obtain multipath information, thereby accurately locate the synchronization time according to the first path, and realize precise time synchronization.

Step 106: Secondary synchronization with fractional frequency offset. That is, the fractional frequency offset estimation is performed again, and the received signal is corrected according to the fractional frequency offset estimation information, so that only integer frequency offset exists in the corrected received signal, thereby realizing precise time-frequency synchronization.

In the above method for realizing precise time-frequency synchronization, although precise time synchronization can be realized by using the preamble cell, the preamble cell consumes additional system resources. Especially in a mobile environment, precise time synchronization must be performed at intervals, otherwise the performance loss caused by time synchronization errors is unbearable for the system. In this way, the leading cell must be sent every time the time is accurately synchronized, which will inevitably lead to an increase in system overhead and a reduction in the rate of data transmission. However, there is no efficient way to achieve precise time synchronization without using preamble cells.

Moreover, the process of realizing frame synchronization and synchronization tracking in the prior art is also realized by using a leading cell. Since preamble sequences are required in the implementation process, additional overhead is also added in the current process of implementing frame synchronization and synchronization tracking.

Contents of the invention

In view of this, the object of the present invention is to provide a method for realizing fine OFDM time synchronization, so that it can realize precise time synchronization without leading cells, thereby saving system overhead and increasing data transmission rate.

The second object of the present invention is to provide a method for realizing OFDM frame synchronization, so that it can realize frame synchronization without leading cells, thereby reducing system overhead.

The third object of the present invention is to provide a method for realizing synchronous tracking, so that it can realize synchronous tracking without leading cells, thereby reducing system overhead.

In order to achieve the above object, the present invention provides a method for fine synchronization of OFDM time, which is implemented in the following way:

a. The receiving end of the OFDM system receives more than one OFDM symbol with different pilot sequences from the transmitting end, and performs integer frequency offset synchronization processing on the received signal;

b1. Perform frequency-time conversion on the pilot sequence corresponding to the one or more OFDM symbols, concatenate the converted time-domain signals and use them as a time-domain reference signal, and combine the time-domain reference signal with the integer frequency offset after processing Within the maximum error range determined by time coarse synchronization, match filtering or sliding correlation is performed on the signal to obtain the maximum peak value, and the results of matching filtering or sliding correlation are normalized;

b2. Set the amplitude threshold according to the signal quality requirements, select the first arriving path exceeding the threshold as the first path, and determine the arrival time and amplitude phase value of the first path at the same time to achieve precise time synchronization.

The time-domain reference signal in step b1 is composed of more than one OFDM symbol and its cyclic prefix.

The method further includes: setting an amplitude threshold according to the requirements for signal quality, selecting a time when the amplitude threshold exceeds the time when the multipath arrives, and the corresponding amplitude and phase value is the amplitude and phase value of the multipath.

Before performing the integer frequency offset synchronization processing in step a, it further includes:

The OFDM system performs coarse time synchronization, fractional frequency offset synchronization, and time-frequency conversion on the received signal, and the received signal in step a is the received signal after time-frequency conversion.

The time coarse synchronization is to use one or more time windows equal to the length of the cyclic prefix and spaced apart from each other by the length of the OFDM symbol plus the length of the cyclic prefix, to perform sliding correlation on the received signal, and to perform time synchronization by means of the cyclic prefix in the OFDM symbol .

The present invention provides a kind of method of OFDM synchronous tracking, and this method is realized like this:

In the OFDM system, the pilot sequence of the frame head of the signal transmitted by the transmitting end is set to be different from the pilot sequence of the subsequent OFDM symbols, and the synchronization threshold is set according to the synchronization performance requirements. The method also includes the following steps:

A1. Determine whether the ratio of the maximum absolute value obtained through matched filtering or sliding correlation to the absolute value of the average correlation value is greater than or equal to the set threshold, if yes, the synchronization is successful, and continue to maintain the synchronization tracking state; otherwise, execute the steps B1;

B1. The system enters the intermediate state, and judges whether the system is still unable to realize synchronous tracking within the preset number of times. If so, it declares that it is out of synchronous, and re-enters the synchronous search state, otherwise, execute step C1;

C1. Perform integer frequency offset synchronization processing on the received signal, match and filter the time domain signal of more than one OFDM symbol and the signal after integer frequency offset processing to complete the multipath search, determine the first path arrival time and amplitude phase value, and achieve accuracy Time synchronization to realize synchronous tracking.

According to the determined multipath precise arrival time and amplitude, select the cyclic prefix of the current OFDM symbol to remove the part that is less affected by the strong rear arrival path of the previous OFDM symbol and the part corresponding to the cyclic prefix of the current OFDM symbol Conjugate multiplication and cumulative addition, the phase of the result of the cumulative addition is the phase information of the precise synchronization peak;

The fractional frequency offset estimation is performed again according to the phase information of the precise synchronization peak, and the received signal is corrected according to the fractional frequency offset estimation information, so as to realize precise time-frequency synchronization.

The present invention provides a kind of method of OFDM frame synchronization, and this method is realized like this:

At the transmitting end, the pilot sequence of more than one OFDM symbol at the beginning of the OFDM frame transmitted by the OFDM system is set to be different from the pilot sequence of the subsequent OFDM symbol, and the method also includes the following steps:

A2. Judging the absolute value of the maximum value and the absolute value of the correlation result sequence in the correlation result sequence of the cascaded matched filtering or sliding correlation of the time domain signal of the pilot sequence identified by the frame header of the OFDM frame transmitted by the transmitting end and its cyclic prefix Whether the ratio of the mean value of is greater than a preset threshold, if yes, it is considered that frame synchronization is currently achieved, otherwise, step B2 is performed;

B2. Continue to use the concatenation of the time-domain signal of the pilot sequence and the cyclic prefix of the OFDM frame of the OFDM frame transmitted by the transmitting end to carry out matched filtering with the received signal after integer frequency offset synchronization processing to perform multipath search, and then Go to step A2.

The frame header identifier is a numerical sequence set according to variable pilots in S OFDM symbols starting from the OFDM frame.

The present invention performs integer frequency offset synchronous processing on received signals, concatenates the time-domain signals of pilot sequences of multiple OFDM symbols and their cyclic prefixes for multipath search, and determines the first-path arrival time and amplitude and phase values to realize Precise time synchronization.

Applying the method of the present invention, the OFDM receiver can perform multipath search, accurately locate the arrival time of the multipath and more accurately locate the amplitude and phase of the multipath, so that the received code space can be selected when performing the quadratic fractional frequency synchronization (FFO). Fractional frequency offset estimation is performed on the part affected by less interference (ISI), thereby improving the accuracy of fractional frequency offset estimation (FFO) and reducing system performance loss caused by synchronization errors.

Description of drawings

FIG. 1 is a schematic diagram of an overall flowchart for implementing time synchronization for OFDM;

Fig. 2 is a schematic flow sheet of the inventive method;

Fig. 3 is a schematic diagram of pilot arrangement of 6 consecutive OFDM symbols;

Fig. 4 is a schematic diagram of time windows of three OFDM symbols.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings.

The core content of the present invention is: receiving more than one OFDM symbols with different pilot sequences from the transmitting end at the receiving end, and performing integer frequency offset synchronization processing on the received signals; The concatenation of the time-domain signal and its cyclic prefix is used as a reference signal, and the time-domain reference signal is used to perform multipath search with the signal after integer frequency offset synchronization processing to determine the arrival time and amplitude phase value of the first path to achieve precise time synchronization.

Referring to Fig. 2, the specific process of realizing precise time synchronization in the present invention is as follows:

Step 201: the OFDM system performs coarse time synchronization on the received signal. That is, one or more time windows equal to the length of the cyclic prefix and spaced apart from each other by the length of the OFDM symbol plus the length of the cyclic prefix are used to perform sliding correlation on the received signal, and the time is performed by relying on the cyclic prefix in the OFDM symbol. Synchronization, to obtain the synchronous peak phase information of the received signal.

Here, the received signal includes a sequence with variable pilots. In order to improve the performance of channel estimation, variable pilots are introduced into OFDM symbols. The pilot positions are regularly spaced in several consecutive OFDM symbols, and the same pilot pattern is repeated continuously to facilitate channel estimation. In each OFDM symbol, in the same OFDM symbol, the pilot interval is N subcarriers, in adjacent OFDM symbols, the corresponding pilot interval is M subcarriers, in adjacent OFDM symbols, the pilot The data on the subcarriers are different. For example, for 6 consecutive OFDM symbols, the pilot arrangement in the case where N is 12 and M is 6 can be represented by FIG. 3 .

For example, Fig. 4 shows a schematic diagram of time windows for 3 OFDM symbols, where each OFDM symbol has 512 subcarriers, its time window is Tb, and the time window of the cyclic prefix length is Ts.

And, according to the prior art, it can be seen that the correlation measure for the time coarse synchronization of three OFDM symbols is as follows (1), and the time coarse synchronization of multiple OFDM symbols can be deduced by analogy:

(1)

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; GI</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; )</span>}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; GI</span>}}<span\; class="notranslate">\; )</span>$

The normalized measure is as formula (2)

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\left[\begin{array}{c}\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; N</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; )</span>}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GI</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; GI</span>}}^{<span\; class="notranslate">\; *</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; GI</span>}}<span\; class="notranslate">\; )</span>\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

作为为时间粗同步时刻。这里，采用多个CP来进行时间同步可以获得时间分集的优势。Calculate the power ratio of P(d) and R(d) by using some criterion $m\left(d\right)=\frac{\left|P\left(d\right)\right|}{R\left(d\right)}$ the maximum value of

Serves as the coarse synchronization moment for the time. Here, the advantage of time diversity can be obtained by using multiple CPs for time synchronization.

Step 202: Fractional frequency offset synchronization. That is, perform fractional frequency offset estimation according to the synchronization peak phase information described in step 201, and correct the received signal according to the fractional frequency offset estimation information, so that only integer frequency offsets exist in the corrected received signal.

Step 203: time-frequency conversion. The specific method is: transform the signal processed in step 202 into the frequency domain through FFT or DFT transformation.

Step 204: Integer frequency offset synchronization. The specific method is: perform integer frequency offset estimation on the frequency domain signal obtained in step 203, and then perform integer frequency offset compensation, that is, use the estimated integer frequency offset information to correct the received signal, so that the corrected received signal has There is no integer frequency offset.

Specifically, the method proposed in the Chinese patent application with the application number "200410042904.7" and the title of the invention "A Method for Orthogonal Frequency Division Multiplexing Time-Frequency Synchronization" proposed by the applicant can be used for integer frequency offset estimation; Use the method proposed in the Chinese patent application titled "Orthogonal Frequency Division Multiplexing Integer Frequency Offset Synchronization and Frame Synchronization Method" submitted by the applicant on the same day to perform integer frequency offset estimation; The large likelihood method performs integer frequency offset estimation, or other existing methods perform integer frequency offset estimation.

Step 205: IFFT transformation. According to the sequence of OFDM symbol pilot sequences determined by integer frequency offset estimation, the pilot sequences are transformed into time-domain signals through IFFT or IDFT transformation, and the cyclic prefix is added and concatenated.

Step 206: Precise time synchronization. Perform matched filtering or sliding on the time domain signal obtained in step 205 and the frequency offset corrected received signal in step 204 within the maximum error range determined by the time coarse synchronization, such as within 10 or 15 chips before and after the time coarse synchronization Correlation, finds the peak of the matched filter or sliding correlation, and normalizes the filtered (correlation) result by the absolute value of the largest peak. Set the amplitude threshold according to the signal quality requirements, such as 0.5, select the first arriving path that exceeds the threshold, this path is the first path, and the amplitude and phase value corresponding to the arrival time of the first path is the amplitude and phase value of the first path . Accurately locate the synchronization time according to the head path, and the amplitude and phase information of the precise synchronization time, so as to realize the precise time synchronization. Furthermore, set the amplitude threshold according to the requirements for signal quality, such as 0.5, and select the time when the amplitude threshold exceeds this value. .

The above-mentioned time-domain reference signal is: the time-domain signal of the pilot sequence of multiple OFDM symbols and the concatenation of the cyclic prefix thereof.

Step 207: Secondary synchronization with fractional frequency offset. The specific method is: according to the determined precise arrival time and amplitude of the multipath, select the part of the cyclic prefix of the current OFDM symbol that is less affected by the strong rear arrival path of the previous OFDM symbol to match the cyclic prefix of the current OFDM symbol. Conjugate multiplication and cumulative addition of the corresponding parts, the phase of the result of the cumulative addition is the phase information of the precise synchronization peak; according to the precise synchronization peak phase information, the fractional frequency offset estimation is performed again, and according to the fraction The frequency offset estimation information modifies the received signal to achieve precise time-frequency synchronization.

By adopting the precise time synchronization method of the present invention, the precise time synchronization performance can be obtained under both static multipath and mobile multipath conditions.

In addition, since the system must carry out synchronous tracking at intervals, based on the precise time synchronization of the present invention, it is possible to effectively implement synchronous tracking. The method for specifically realizing synchronous tracking is as follows:

Set the synchronization threshold according to the synchronization performance requirements, and the threshold is usually between 0 and 1. After each time synchronization, if the ratio of the maximum absolute value obtained by matching filtering or sliding correlation to the absolute value of the average correlation value is greater than this If the threshold has been set, it means that the synchronization is successful; otherwise, it means that the synchronization is not successful.

If the synchronization is successful, continue to maintain synchronization tracking, if the synchronization fails, enter an intermediate state, and repeat the above synchronization tracking steps after each accurate time synchronization, if the synchronization tracking still cannot be achieved within a certain number of times, then Declare out of synchronization and re-enter the synchronization search state.

Using this algorithm can completely eliminate the use of Preamble, thereby greatly reducing system overhead.

In addition, in a mobile environment, using the method of the present invention can also perform frame synchronization and mobile station identification of signals received from different cells. The specific scheme is as follows:

Judging the absolute value of the maximum value in the correlation result sequence and the absolute value of the correlation result sequence of the time domain signal of the pilot sequence of more than one OFDM symbol at the beginning of the OFDM frame transmitted by the transmitting end and its cyclic prefix concatenated matched filtering or sliding correlation Whether the ratio of the mean values of the values is greater than a preset threshold, if so, precise synchronization has been achieved and frame synchronization has been achieved.

In terms of the design of the variable pilot sequence, the variable pilot in the S consecutive OFDM symbols located at the frame header can be set as a frame identifier, that is, a specific numerical sequence is used to identify this symbol, which is different from the following data symbol, then if the ratio of the absolute value of the maximum value in the correlation result sequence of matched filtering or sliding correlation to the mean value of the absolute value of the correlation result sequence is greater than a preset threshold, it indicates that the frame synchronization is successful, thus You can locate the start of the frame. If the time coarse synchronization fails in the S OFDM symbols starting from the frame header, and the coarse synchronization succeeds in the OFDM symbols from the 2nd to S+1, the known OFDM symbols from the 2nd to S+1 can be used Matched filtering or sliding correlation is performed on the time-domain information of the pilot sequence in to carry out precise time synchronization, and frame synchronization can also be completed. If the time coarse synchronization fails in 2 to S+1 OFDM symbols from the frame header, the known time domain information of the pilot sequence in the 3rd to S+2 OFDM symbols can be used for matching Filtering or sliding correlation is used for precise time synchronization. If synchronization is successful, frame synchronization can also be completed. Going on like this, even if the coarse synchronization leak synchronization occurs in the previous S OFDM symbols, as long as the correct time coarse synchronization is completed from the S OFDM symbol to the 2S-1 OFDM symbol, the frame synchronization can still be completed by the method of the present invention, of course A precondition is that the receiver has enough memory to store information of S consecutive OFDM symbol sequences. This method can greatly increase the probability of frame synchronization.

In addition, in different cells, the base station sends out different pilot sequences, so that the receiving side, that is, the mobile station, can obtain the positioning of the signals sent by the base station in different cells and the multipath arrival time, amplitude and phase of the corresponding channel through matched filtering. Information, by judging the strength of the peak value obtained by the matched filtering result to complete soft switching, merging and other corresponding work.

The present invention uses certain information to perform matching filtering or sliding correlation with the received signal, so as to find the matching peak and determine the arrival time of the multipath in the environment of moving multipath accordingly. In this way, although the multipath often changes as the environment changes under the time-varying channel, such as adding a new arrival path or presenting different fading characteristics, etc., as long as the time of the new multipath is obtained by the method of the present invention, The amplitude and phase information can achieve the purpose of updating multipath information.

Therefore, applying the precise time synchronization method of the present invention, the OFDM receiver can perform multipath search, accurately locate the arrival time of the multipath and more accurately locate the amplitude of the multipath, thereby eliminating strong The impact of intersymbol interference (ISI) to improve the accuracy of fractional frequency offset estimation (FFO). Secondly, by using the method of the present invention, the receiver can accurately estimate the multipath time delay, and more accurately estimate the amplitude value of the power time delay spectrum. The method of the present invention can also complete accurate frame synchronization, cell search and handover merging.

In a word, the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (9)

1. a method that realizes the OFDM exact time synchronization is characterized in that, this method may further comprise the steps:

The receiving terminal of a.OFDM system receives the above OFDM symbol with different pilot frequency sequences from transmitting terminal, and carries out the integer frequency bias Synchronous Processing to the received signal;

Conversion when b1. the pilot frequency sequence of a described above OFDM symbol correspondence being carried out frequency, time-domain signal after the conversion is cascaded up and as the time domain reference signal, with described time domain reference signal and signal after integer frequency bias is handled in the thick worst error scope of determining synchronously of time, carry out matched filtering or slide relevant, obtain peak-peak, and normalizing is carried out in the matched filtering or the relevant result that slides;

B2. according to amplitude threshold is set in the requirement of signal quality, choose the footpath headed by the footpath of this threshold value that surpasses of first arrival, determine the due in and the amplitude-phase value in this head footpath simultaneously, realize exact time synchronization.

2. method according to claim 1 is characterized in that, the described time domain reference signal of step b1 is made of an above OFDM symbol and Cyclic Prefix thereof.

3. method according to claim 1, it is characterized in that this method further comprises: set amplitude threshold according to requirement, select to surpass the moment of this amplitude threshold to signal quality, as the moment of multipath arrival, and its pairing amplitude-phase value is the amplitude-phase value of multipath.

4. method according to claim 1 is characterized in that, carries out further comprising before the integer frequency bias Synchronous Processing at step a:

Ofdm system carries out thick synchronously, mark frequency deviation of time synchronously and the time-frequency conversion to received signal, and received signal described in the step a is for through the received signal after the time-frequency conversion.

5. method according to claim 4, it is characterized in that, the described time slightly is to utilize one or more equate with circulating prefix-length and space OFDM symbol lengths adds the time window of circulating prefix-length synchronously, it is relevant to slide to the received signal, relies on the Cyclic Prefix in the OFDM symbol to carry out time synchronized.

6. one kind is utilized the described method of claim 1 to realize the method for following the tracks of synchronously, it is characterized in that, the pilot frequency sequence of the frame head that transmitting terminal transmits in ofdm system is set to be different from the pilot frequency sequence of follow-up OFDM symbol, and require synchronization threshold is set according to net synchronization capability, this method is further comprising the steps of:

A1. judge ratio by matched filtering or the slide relevant maximum value that obtains and the absolute value of average correlation whether more than or equal to this preset threshold, if, then success synchronously, and continue to keep synchronous tracking mode; Otherwise execution in step B1;

B1. system enters intermediateness, judge that system is current whether in predefined number of times, still can not realize synchronous tracking, if then declaration loses synchronously, reenters the synchronous searching state, otherwise, execution in step C1;

C1. carry out the integer frequency bias Synchronous Processing to received signal, with the time-domain signal of an above OFDM symbol with handle through integer frequency bias after signal carry out matched filtering and finish Multipath searching, determine first directly arriving at constantly and the amplitude-phase value, reach exact time synchronization, realize following the tracks of synchronously.

7. method according to claim 6, it is characterized in that, according to accurate due in of fixed multipath and amplitude, choose to remove in the Cyclic Prefix of current OFDM symbol and be subjected to reaching after last OFDM symbol stronger the footpath and influence less part and multiply each other and cumulative addition with the gripping altogether of the corresponding part of Cyclic Prefix of current OFDM symbol, the phase place of the result after this cumulative addition is the phase information of precise synchronization peak value;

Phase information according to the precise synchronization peak value is carried out the mark frequency offset estimating once more, and revises according to the signal that this mark frequency offset estimating information butt joint is received, realizes accurate Time and Frequency Synchronization.

8. one kind is utilized the synchronous method of the described method achieve frame of claim 1, it is characterized in that, the pilot frequency sequence of the above OFDM symbol that the OFDM frame of launching at the transmitting terminal ofdm system begins is set to be different from the pilot frequency sequence of follow-up OFDM symbol, and this method is further comprising the steps of:

Whether the ratio of A2. judging the average of the absolute value of peaked absolute value and correlated results sequence in the matched filtering of cascade of the time-domain signal of pilot frequency sequence of frame head sign of transmitting terminal emission OFDM frame and Cyclic Prefix thereof or the relevant correlated results sequence of sliding is greater than a pre-set threshold, if, think that current achieve frame is synchronous, otherwise, execution in step B2;

B2. the cascade that continues to utilize the time-domain signal of pilot frequency sequence of frame head sign of OFDM frame of transmitting terminal emission and Cyclic Prefix thereof with carry out received signal after the integer frequency bias Synchronous Processing and carry out matched filtering and carry out Multipath searching, then then execution in step A2.

9. method according to claim 8 is characterized in that, described frame head sign is according to the set sequence of values of variable pilot tone from S the OFDM symbol that the OFDM frame begins.

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