CN100499622C - OFDM time and frequency synchronization method - Google Patents
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Abstract
This invention provides OFDM time frequency synchronous method, which contains making slide correlation to receiver signal and making time synchronism by circulation prefix in OFDM symbol to obtain the synchronous peak value phase information, the time synchronism can be obtained without adding leading cell in transmitter end, which raises spectrum efficiency and avoiding synchronism blooming, greatly reducing calculation and synchronism time delay, said invention has relative stable correlation time calculation quantity without influence to synchronism performance.
Description
Technical Field
The invention relates to the technical field of data transmission in the mobile communication technology, in particular to a method for Orthogonal Frequency Division Multiplexing (OFDM) time-frequency synchronization.
Background
Orthogonal Frequency Division Multiplexing (OFDM) is a mobile communication technology that utilizes parallel transmission to increase the transmission rate of communication data. The basic idea of this technique is to divide a given channel into a number of orthogonal sub-channels in the frequency domain, to modulate with one sub-carrier on each sub-channel, and to transmit the sub-carriers in parallel. Thus, although the overall channel is non-flat and frequency selective, each sub-channel is relatively flat, and narrow-band transmission is performed on each sub-channel, the signal bandwidth is less than the corresponding bandwidth of the channel, and thus interference between signal waveforms can be substantially eliminated. OFDM differs from general multicarrier transmission in that it allows partial overlap of the subcarrier spectra, and the data signals can be separated from the aliased subcarriers as long as mutual orthogonality between the subcarriers is satisfied.
The OFDM allows the spectrum of the subcarrier to be mixed and dropped, so that the spectrum efficiency is greatly improved, and meanwhile, the technology also has the advantages of multipath interference resistance, intersymbol interference resistance, easiness in realization of channel estimation and equalization, low system realization complexity and the like, so that the OFDM modulation method is an efficient modulation mode. And the technology is easily combined with various multiple access technologies, and thus is widely recognized as an indispensable core technology in fourth generation mobile communication systems. The technology has been widely applied to many data communication systems such as 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), wireless high-speed unlicensed metropolitan area network (WHUMAN), and the modulation technology will be adopted by the 802.20 mobile broadband wireless access system currently under discussion by the Institute of Electrical and Electronics Engineers (IEEE) standardization organization.
OFDM has many of the above advantages, but its requirements for frequency synchronization, especially frequency synchronization, are very high, otherwise inter-symbol interference (ISI) and inter-subcarrier interference (ICI) are easily caused. Taking IEEE802.16a wireless metropolitan area network as an example, it is required that the residual frequency offset must be less than 2% of the subcarrier interval, and therefore, carrier frequency synchronization in the OFDM system, i.e., compensating for the frequency offset between the local carrier and the transmission carrier, is an important key technology in the system.
In order to avoid inter-symbol interference (ISI) in OFDM systems, each OFDM symbol is preceded by a cyclic prefix. An OFDM symbol is composed of subcarriers in the frequency domain, and the number of subcarriers determines the number of points of a time-frequency transform (FFT). There are three types of subcarriers, which are data subcarriers, pilot subcarriers, and virtual subcarriers, respectively. Wherein, the data subcarrier is used for data transmission; the pilot subcarriers are originally used to eliminate residual phase differences, and as the technology develops, the roles of the pilot subcarriers are further expanded, and the pilot subcarriers can be used for frequency synchronization and channel estimation. The virtual sub-carrier refers to a carrier that does not transmit any data, and is introduced by the OFDM system in order to reduce interference to adjacent frequency bands.
The pilot subcarriers in an OFDM system are typically distributed equally spaced over the subcarriers using a uniform comb insertion scheme, and the values on each pilot subcarrier are generated by a known pseudo-random number generator. The frequency domain signal structure of each symbol in an OFDM system is shown in fig. 1. In the context of figure 1 of the drawings,
represents a virtual subcarrier;
represents a data subcarrier;
representing the pilot subcarriers. The spacing between each pilot subcarrier is a fixed value d.
Us patent 5,732,113 discloses a method of OFDM time and frequency synchronization. The method inserts a leader cell with a special structure at a transmitting end and utilizes the leader cell to realize time-frequency synchronization. The preamble cell is composed of two OFDM symbols (symbol) SYN _ A and SYN _ B, wherein the front part and the back part of SYN _ A are completely the same, and the receiving end utilizes the cell to complete the time synchronization and fractional frequency offset estimation of the OFDM system in the time domain. And then compensating the estimated fractional frequency offset, converting SYN _ A and SYN _ B into a frequency domain, and finishing the estimation of the integer frequency offset by utilizing the correlation of SYN _ A and SYN _ B on the frequency domain. The method can quickly realize time-frequency synchronization and has reasonable calculation complexity.
After the OFDM time-frequency synchronization method disclosed in the patent, many scholars optimize and improve the OFDM time-frequency synchronization method. Such as reducing the leading cell from two parts to one part, changing the structure of the leading cell, etc. Thus, the general flow diagram of the existing OFDM time-frequency synchronization is shown in fig. 2.
For the structure diagram of the OFDM symbol shown in fig. 1 in the frequency domain, the integer-times frequency offset formula is shown in formula (1):
wherein epsilonIWhich represents an integer multiple of the frequency offset,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+ g modulo NFFTThe latter value indicates taking the complex conjugate.
The above method has the disadvantages that: both the method described in the us patent and the improved method require a preamble cell to be added at the front end of the transmitted information, and the preamble cell, i.e. preamble sequence (preamble), is used to perform time-frequency synchronization and channel estimation, which inevitably brings extra overhead to the system, resulting in a decrease in the spectral efficiency of the system. This drawback is particularly evident when the data to be transmitted is short. Moreover, when the OFDM system is used in a mobile environment, channel estimation needs to be performed in real time, and channel estimation using only a preamble sequence cannot satisfy system requirements.
In addition, the computation of integer frequency offset estimation by using pilot frequency mobile correlation is large, and the synchronization delay is increased. Taking IEEE802.16a Orthogonal Frequency Division Multiple Access (OFDMA) mode as an example, the interval between adjacent pilots is 11 subcarriers, so that in the worst case, 11 mobile correlations are required to obtain the required peak. Meanwhile, it can be seen from equation (1) that, when the integer frequency offset is greater than the pilot interval, the method may generate synchronization ambiguity, that is, correct frequency synchronization may not be obtained, thereby causing the system to fail to operate normally.
Disclosure of Invention
In view of the above, the present invention provides a method for OFDM time-frequency synchronization, which aims to achieve time-frequency synchronization without adding a preamble cell at a transmitting end in a communication system based on an OFDM modulation technique, thereby reducing the overhead of the system and improving the spectrum efficiency; another object of the present invention is to avoid the ambiguity of frequency synchronization and reduce the amount of calculation in frequency synchronization.
In order to achieve the purpose, the technical scheme of the invention is realized as follows:
a method for orthogonal frequency division multiplexing time frequency synchronization, the method comprises the following steps:
a. carrying out time synchronization on the received signals to acquire synchronous peak value phase information of the received signals;
b. b, performing fractional frequency offset estimation according to the synchronous peak phase information in the step a, correcting the received signal according to the fractional frequency offset estimation information to ensure that only integer frequency offset exists in the corrected received signal, and then converting the received signal into a frequency domain;
c. and determining the actual virtual subcarrier starting position of the received frequency domain signal through sliding average or sliding summation, acquiring the difference between the ideal virtual subcarrier starting position of the received frequency domain signal and the actual virtual subcarrier starting position, and then carrying out integer frequency offset estimation by taking the value of the difference of the positions as the center.
Preferably, the method for performing time synchronization on the received signal in step a includes: the received signal is subjected to sliding correlation by using a time window with the length equal to the cyclic prefix length, and time synchronization is carried out by depending on the cyclic prefix in the orthogonal frequency division multiplexing OFDM symbol.
Preferably, when the length of the cyclic prefix is greater than or equal to the number of points determined by the delay spread and the signal-to-noise ratio when the system is applied, the step a adopts a time window equal to the length of the cyclic prefix; when the length of the cyclic prefix is less than the number of points determined by the delay spread and the signal-to-noise ratio when the system is applied, the step a adopts more than one time window with the length equal to the length of the cyclic prefix, and the length of an OFDM symbol is used as an interval between the time windows.
Preferably, the method for obtaining the actual virtual subcarrier start position of the received frequency domain signal in step c includes: and acquiring the actual virtual subcarrier starting position of the frequency domain signal through moving average or moving summation.
Preferably, when performing the moving average or the moving summation on the received frequency domain signal, the smoothing is performed from the maximum frequency offset position allowed by the system.
Preferably, when the pilot information between adjacent OFDM symbols is the same, the pilot position between adjacent OFDM symbols is unchanged, and the information carried on each pilot subcarrier on the adjacent OFDM symbols is the same, the performing integer frequency offset estimation is: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <msup> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo></mo> </mrow> <mo>*</mo> </msup> <mo>|</mo> </mrow> <mrow> <mo></mo> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+gMode NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
Preferably, when the pilot position is not changed between adjacent OFDM symbols, and the ratio of the value on the ith pilot subcarrier on the (n + 1) th OFDM symbol and the nth OFDM symbol is Cn(i) Then, the performing integer frequency offset estimation is: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <msup> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo></mo> </mrow> <mo>*</mo> </msup> <msub> <mi>C</mi> <mi>n</mi> </msub> <msup> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>*</mo> </msup> <mo>|</mo> </mrow> <mrow> <mo></mo> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+gMode TFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
Preferably, when the pilot information between adjacent OFDM symbols is the same and the position of the ith pilot subcarrier between adjacent OFDM symbols differs by d (i) subcarriers, the performing integer frequency offset estimation is: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <msup> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo></mo> </mrow> <mo>*</mo> </msup> <mo>|</mo> </mrow> <mrow> <mo></mo> <mo></mo> </mrow> </mrow></math>
Determining integral multiple frequency offset epsilon I; wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+gMode NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
Preferably, when the ratio of the value on the ith pilot subcarrier on the (n + 1) th OFDM symbol and the nth OFDM symbol is Cn(i) And when the position of the ith pilot frequency subcarrier between adjacent OFDM symbols differs by d (i) subcarriers, the integer frequency offset estimation is as follows: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <msup> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo></mo> </mrow> <mo>*</mo> </msup> <msub> <mi>C</mi> <mi>n</mi> </msub> <msup> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>*</mo> </msup> <mo>|</mo> </mrow> <mrow> <mo></mo> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+g]NFFTRepresents pi+ g modulo NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
The invention uses the time window with the same length as the cyclic prefix to carry out sliding correlation on the received signal, and carries out time synchronization by depending on the cyclic prefix in the OFDM symbol to obtain the synchronous peak phase information of the received signal, so that the time synchronization can be realized without adding a preamble cell at a transmitting end in a communication system based on the OFDM modulation technology, thereby reducing the extra expense of the system and improving the spectrum efficiency. Meanwhile, the invention carries out coarse estimation of integer frequency offset by combining the characteristics of the virtual subcarriers, avoids the synchronization fuzzy phenomenon during the estimation of the integer frequency offset, and greatly reduces the operation amount related to movement, thereby reducing the synchronization time delay. For different frequency offsets, the method of the invention has relatively stable correlation times and calculation amount, and the performance of the integer frequency offset estimation is finally ensured by the mobile correlation, so that the synchronization performance is not influenced.
Drawings
Fig. 1 is a schematic diagram of a frequency domain signal structure of each OFDM symbol;
FIG. 2 is a general block diagram of a conventional OFDM system for implementing time-frequency synchronization;
FIG. 3 is a general block diagram of an OFDM system implementing time-frequency synchronization according to the present invention;
FIG. 4 illustrates an example of a frequency domain signal;
fig. 5 is a diagram of the frequency domain signal of fig. 4 after being subjected to a moving average.
Detailed Description
In order to make the technical scheme of the invention clearer, the invention is further described below with reference to the accompanying drawings.
The idea of the present invention is shown in fig. 3, and fig. 3 is a general block diagram of the OFDM system of the present invention for implementing time-frequency synchronization.
And step 306, performing integer frequency offset fine estimation. And performing integer frequency offset fine estimation by using the certainty of the pilot frequency subcarrier information on the front and the back adjacent OFDM symbols to realize time-frequency synchronization.
In the OFDM time-frequency synchronization scheme proposed by the present invention, steps 302 to 304 are consistent with the conventional OFDM time-frequency synchronization method, and the following description focuses on the implementation methods of step 301, step 305, and step 306.
In step 301, the received signal is subjected to sliding correlation using a time window equal to the cyclic prefix length, and time synchronization is performed by means of the cyclic prefix within the OFDM symbol. For example, in an OFDM system with a long cyclic prefix, such as the ieee802.16aadma mode, the length of the cyclic prefix is greater than or equal to the number of points determined by the delay spread and the snr when the system is applied, and for example, when the number of points is 64, a time window equal to the length of the cyclic prefix is used to obtain a good time synchronization performance. When the cyclic prefix in the OFDM system is short, for example, the length of the cyclic prefix is smaller than the number of points determined by the delay spread and the signal-to-noise ratio when the system is applied, for example, 64 points, more than one time window with the same length as the cyclic prefix is adopted to perform sliding correlation on the received data, and the time windows take the length of an OFDM symbol as an interval, so that good time synchronization performance can be obtained.
Fig. 4 shows an example of a frequency domain signal. In the illustrated embodiment, the number of FFT points is 2048 points, the signal-to-noise ratio is 8dB, and the integer frequency offset is the frequency domain signal at 30 subcarrier intervals. The abscissa is the position of the subcarrier and the ordinate is the amplitude. Since the virtual sub-carrier is a sub-carrier with amplitude 0 that does not transmit any information in the OFDM system, its role is to reduce interference of the OFDM system to adjacent frequency bands. Except the direct current component, the rest components are distributed at two sides of the frequency band, and under the noise-free environment, after FFT is carried out on the received data, the amplitude of a section of subcarrier in the middle is zero. The signal strength on the virtual sub-carriers is still low relative to the data sub-carriers after passing through the multi-path channel and adding gaussian noise.
The method for implementing coarse estimation of integer frequency offset described in step 305 is to use the above-mentioned characteristics of the virtual subcarrier to perform a moving average on the modulus (the absolute value of the real part or the imaginary part) of the frequency domain signal, so that a minimum value will appear at the start position of the virtual carrier. The minimum value is the actual virtual sub-carrier starting position, and further the difference between the ideal virtual sub-carrier starting position and the actual virtual sub-carrier starting position, namely the position of the minimum value obtained by smoothing, is obtained. The difference between the positions is the coarse estimated value of the integer frequency offset. To further reduce the computational complexity, smoothing may be performed starting from the maximum frequency offset position allowed by the system.
Fig. 5 shows the frequency domain signal of fig. 4 after a moving average. It can be seen from the figure that after the smoothing process, a very sharp minimum appears at the start position of the virtual sub-carrier. Since the position of the minimum value is the actual virtual subcarrier starting position, the coarse estimation value of the integer frequency offset can be easily obtained.
The method for implementing the fine estimation of the integer frequency offset in step 306 comprises: and taking the integer frequency offset coarse estimation value obtained in the integer frequency offset coarse estimation as a center, applying the formula (1), calculating a maximum value by changing the value of g, and performing integer frequency offset fine estimation, thereby realizing time-frequency synchronization.
When the coarse estimation of the integer frequency offset is carried out, the frequency offset position can be obtained more accurately, and because the coarse estimation error is far smaller than the interval of the pilot frequency subcarriers, the synchronous ambiguity can not occur when the fine estimation of the integer frequency offset is carried out next time. In addition, since the frequency offset is preliminarily determined in the coarse estimation, the number of times of the moving correlation can be greatly reduced in the fine estimation of the integer frequency offset. The method adopts smoothing operation when the integer frequency offset is roughly estimated, only addition and subtraction operation is needed for each smoothing, and the complexity is very low. For different frequency offsets, the method has relatively stable correlation times and calculation amount, and the performance of the integer frequency offset estimation is finally ensured by the mobile correlation, so that the synchronization performance is not influenced.
Of course, the coarse estimation of integer frequency offset is not limited to the method using the moving average, and may be implemented by using the moving sum or other possible methods.
The above-mentioned integer frequency offset fine estimation is performed based on the formula (1), and the formula (1) is obtained on the premise that the pilot frequency information between adjacent OFDM symbols is the same, the pilot frequency position between adjacent OFDM symbols is not changed, and the information carried on each pilot frequency subcarrier on the adjacent OFDM symbols is the same.
If the pilot frequency information between the adjacent OFDM symbols is different, the certainty of the pilot frequency information on the adjacent OFDM symbols can be utilized, and the method and the device for determining the pilot frequency information on the adjacent OFDM symbols can be adoptedThe method carries out frequency synchronization by only slightly changing the formula (1). Assume that the ratio of the value on the i-th pilot subcarrier on the n +1 th OFDM symbol to the n-th OFDM symbol is Cn(i) Then, the values on the ith pilot subcarrier between adjacent OFDM symbols have the following relationship:
Xn+1(Pi)＝Xn(Pi)Cn(i) (2)
wherein, Xn(Pi) Representing the transmitted signal on the ith pilot subcarrier on the nth OFDM symbol, Xn+1(Pi) Representing the transmitted signal on the ith pilot subcarrier on the (n + 1) th OFDM symbol, equation (1) will be replaced by
Therefore, when the method of the invention is applied under the condition that the pilot frequency information between adjacent OFDM symbols is different, the calculation can be carried out by applying the formula shown in the formula (3) when the integer frequency offset fine estimation is carried out.
If the pilot position is variable between adjacent OFDM symbols, it is assumed that the position of the ith pilot subcarrier differs by d (i) subcarriers between adjacent OFDM symbols,
when the information carried by each pilot frequency subcarrier on adjacent OFDM symbols is the same, the formula (1) is changed into the following form
When the information carried on each pilot subcarrier on adjacent OFDM symbols is different, assuming that the relationship described by the formula (2) is satisfied, the formula (1) is changed into the following form
Under the condition that the pilot frequency position between adjacent OFDM symbols is variable, when the method is applied, the formula shown in the formula (4) or the formula (5) is only needed to be applied for calculation when the integer frequency offset fine estimation is carried out.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (8)
1. A method for orthogonal frequency division multiplexing time frequency synchronization is characterized in that the method comprises the following steps:
a. carrying out time synchronization on the received signals to acquire synchronous peak value phase information of the received signals;
b. b, performing fractional frequency offset estimation according to the synchronous peak phase information in the step a, correcting the received signal according to the fractional frequency offset estimation information to ensure that only integer frequency offset exists in the corrected received signal, and then converting the received signal into a frequency domain;
c. and determining the actual virtual subcarrier starting position of the received frequency domain signal through sliding average or sliding summation, acquiring the difference between the ideal virtual subcarrier starting position of the received frequency domain signal and the actual virtual subcarrier starting position, and carrying out integer frequency offset estimation by taking the value of the difference of the positions as the center.
2. The method of claim 1, wherein the step a of time synchronizing the received signals comprises: the received signal is subjected to sliding correlation by using a time window with the length equal to the cyclic prefix length, and time synchronization is carried out by depending on the cyclic prefix in the orthogonal frequency division multiplexing OFDM symbol.
3. The method of claim 2, wherein when the cyclic prefix length is greater than or equal to the number of points determined by the delay spread and the snr when the system is applied, the step a employs a time window equal to the cyclic prefix length; when the length of the cyclic prefix is less than the number of points determined by the delay spread and the signal-to-noise ratio when the system is applied, the step a adopts more than one time window with the length equal to the length of the cyclic prefix, and the length of an OFDM symbol is used as an interval between the time windows.
4. The method of claim 1, wherein the smoothing is performed starting from a maximum frequency offset position allowed by a system when performing the moving average or the moving summation on the received frequency domain signals.
5. The method according to any of claims 1 to 4, wherein when the pilot information between adjacent OFDM symbols is the same, the pilot position between adjacent OFDM symbols is unchanged, and the information carried on each pilot subcarrier on adjacent OFDM symbols is the same, the integer frequency offset estimation is performed as follows: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo>*</mo> <mo>|</mo> </mrow> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+ g modulo NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
6. Method according to any of claims 1 to 4, characterized in that when the pilot position is unchanged between adjacent OFDM symbols and the ratio of the value on the i pilot subcarrier on the (n + 1) th OFDM symbol and the n OFDM symbol is Cn(i) Then, the performing integer frequency offset estimation is: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo>*</mo> <msub> <mi>C</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>*</mo> <mo>|</mo> </mrow> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,
represents pi+ g modulo NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
7. The method of any of claims 1 to 4, wherein when the pilot information is the same between adjacent OFDM symbols and the position of the ith pilot subcarrier differs by d (i) subcarriers, the integer frequency offset estimation is performed as: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo>*</mo> <mo>|</mo> </mrow> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,represents pi+ g modulo NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
8. Method according to any of claims 1 to 4, characterized in that when the ratio of the value on the i pilot subcarrier on the (n + 1) th OFDM symbol and the n OFDM symbol is Cn(i) And when the position of the ith pilot frequency subcarrier between adjacent OFDM symbols differs by d (i) subcarriers, the integer frequency offset estimation is as follows: based on the value of the difference between said positions as a center <math> <mrow> <msub> <mi>ϵ</mi> <mi>I</mi> </msub> <mo>=</mo> <munder> <mi>max</mi> <mi>g</mi> </munder> <mrow> <mo>|</mo> <munder> <mi>Σ</mi> <mi>i</mi> </munder> <mrow> <mo>(</mo> <msub> <mi>Y</mi> <mrow> <mi>n</mi> <mo>+</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>d</mi> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <msub> <mi>Y</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <msub> <mrow> <mo>[</mo> <msub> <mi>P</mi> <mi>i</mi> </msub> <mo>+</mo> <mi>g</mi> <mo>]</mo> </mrow> <msub> <mi>N</mi> <mi>FFT</mi> </msub> </msub> <mo>)</mo> </mrow> <mo>*</mo> <msub> <mi>C</mi> <mi>n</mi> </msub> <mrow> <mo>(</mo> <mi>i</mi> <mo>)</mo> </mrow> <mo>*</mo> <mo>|</mo> </mrow> <mo></mo> </mrow> </mrow></math>
Determining integer multiple frequency offset εI(ii) a Wherein,
representing the received signal on the nth sub-carrier,
representing the received signal, P, on the (n + 1) th subcarrieriIndicates the position of the ith pilot subcarrier, g is satisfied <math> <mrow> <mo>-</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> <mo><</mo> <mi>g</mi> <mo>≤</mo> <mfrac> <mi>d</mi> <mn>2</mn> </mfrac> </mrow></math>
D denotes the spacing between pilot subcarriers, NFFTThe number of points representing the FFT is shown,represents pi+ g modulo NFFTThe value of the latter is taken as the value,*indicating taking the complex conjugate.
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| WO2007066293A2 (en) * | 2005-12-08 | 2007-06-14 | Koninklijke Philips Electronics N.V. | System, apparatus, and method for single-carrier block transmission with adaptive guard interval |
| CN101030955B (en) * | 2006-02-28 | 2013-04-24 | 世意法（北京）半导体研发有限责任公司 | Method for synchronizing multi-user signal, time and frequency synchronization method and device |
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| CN100355255C (en) * | 2006-04-29 | 2007-12-12 | 北京理工大学 | Synchronous method of large search range OFDM system carrier based on statistical uniform |
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| CN101277290B (en) * | 2007-03-26 | 2011-01-26 | 富士通株式会社 | Method and apparatus for synchronization of orthogonal frequency division multiplexing system frequency |
| CN101309248B (en) * | 2007-05-16 | 2012-03-07 | 富士通株式会社 | Frequency synchronizing method and apparatus suitable for OFDM communication system |
| CN101056300B (en) * | 2007-06-14 | 2013-10-30 | 威盛电子股份有限公司 | Relevancy interval synchronization device and method |
| US7961816B2 (en) * | 2007-11-28 | 2011-06-14 | Industrial Technology Research Institute | Device for and method of signal synchronization in a communication system |
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| CN102413079B (en) * | 2011-11-10 | 2014-09-03 | 复旦大学 | Initial fraction frequency offset estimation method used in downlink of 3 generation partnership project long term evolution (3GPP-LTE) system |
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