CN115378783A - OFDM system time offset calculation method based on cyclic prefix - Google Patents
OFDM system time offset calculation method based on cyclic prefix Download PDFInfo
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Abstract
The invention relates to the technical field of signal processing, in particular to a method for calculating time offset of an OFDM system based on cyclic prefix, which comprises the following steps: acquiring a time domain signal, wherein the time domain signal comprises M +1 continuous OFDM symbols, and each OFDM symbol is provided with a plurality of sampling points; presetting two sliding windows, respectively screening sampling points in the two sliding windows, spacing Ns sampling points between the two screened sampling points, and sequentially performing absolute value, difference and square operation on the sampling values corresponding to the two screened sampling points to generate difference value search data of an OFDM symbol; and sequentially generating difference value search data of M OFDM symbols, carrying out mean value operation on the M difference value search data, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve. By adopting the scheme, the technical problem that the time synchronization precision of the OFDM system is low under the conditions of large frequency deviation and low signal-to-noise ratio in the prior art can be solved.
Description
Technical Field
The invention relates to the technical field of signal processing, in particular to a method for calculating time offset of an OFDM system based on cyclic prefix.
Background
OFDM (Orthogonal Frequency Division Multiplexing), which is an Orthogonal Frequency Division Multiplexing technique, is one of multicarrier modulation schemes. In an OFDM system, inter-symbol interference may cause a high error rate, which degrades system performance, and to reduce the effect of inter-symbol interference, a guard interval is usually inserted into an OFDM symbol. The guard interval may not contain any signal but this introduces inter-carrier interference, destroying the orthogonality between sub-carriers, and is therefore usually constituted by a cyclic extension of the signal, i.e. the introduction of a cyclic prefix. CP (Cyclic Prefix), i.e., cyclic Prefix, is configured by copying a signal at the end of an OFDM symbol to the head.
Because the OFDM system is very sensitive to the synchronization error, in order to reduce the time synchronization error, the time offset estimation is carried out based on the cyclic prefix, thereby determining the time synchronization position. In the prior art, the cyclic prefix-based time offset estimation comprises a correlation method and a difference method, and when the correlation method is adopted for time offset estimation, the time offset estimation is sensitive to frequency offset, so that the time offset estimation is easy to generate large errors; when the time offset estimation is carried out by adopting a difference method, the frequency offset resistance is good, but under the condition of low signal-to-noise ratio, the time offset estimation error is increased. Therefore, a time offset calculation method for improving the time synchronization accuracy of the OFDM system under the conditions of large frequency offset and low signal-to-noise ratio is needed.
Disclosure of Invention
The invention aims to provide a method for calculating time offset of an OFDM system based on cyclic prefix, which aims to solve the technical problem that the time synchronization precision of the OFDM system is low under the conditions of large frequency offset and low signal-to-noise ratio in the prior art.
The present invention provides the following basic scheme:
a time offset calculation method of an OFDM system based on cyclic prefix comprises the following steps:
acquiring a time domain signal, wherein the time domain signal comprises M +1 continuous OFDM symbols, and each OFDM symbol is provided with a plurality of sampling points;
presetting two sliding windows, respectively screening sampling points in the two sliding windows, spacing Ns sampling points between the two screened sampling points, and sequentially performing absolute value, difference and square operation on the sampling values corresponding to the two screened sampling points to generate difference value search data of an OFDM symbol;
and sequentially generating difference value search data of M OFDM symbols, carrying out mean value operation on the M difference value search data, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve.
The beneficial effects of the basic scheme are as follows:
according to the scheme, sampling points are selected based on a sliding window to obtain sampling values, operation is carried out based on the two selected sampling values to obtain difference value search data of each OFDM symbol, then mean value operation is carried out on the difference value search data, and corresponding sampling points are screened out from a mean value curve based on a mean value operation result, namely the sampling points are time synchronization positions.
Compared with the prior art, the scheme is based on the characteristics of the OFDM symbol, does not need additional data symbols, signal-to-noise ratio and other channel information, and effectively utilizes channel resources. Meanwhile, the scheme generates difference value search data according to absolute values, difference values and square operations, reduces the influence of frequency deviation on time deviation estimation precision, and improves the frequency deviation resistance. The scheme also carries out operation positioning based on a plurality of difference value search data, increases the anti-noise performance of time offset estimation, and improves the precision of time offset estimation under the condition of low signal to noise ratio.
Further, a time domain signal is obtained, which includes the following:
and acquiring a receiving signal of a receiving end, and randomly intercepting the receiving signal according to a preset data length to acquire a time domain signal.
Has the advantages that: the time domain signal is obtained by adopting a random interception mode, and the sampling mode is simple and visual.
Further, the OFDM symbol includes a cyclic prefix, and the data length of the sliding window is equal to the data length of the cyclic prefix.
Has the advantages that: when the cyclic prefix can completely fall in the sliding window, part of effective data in the OFDM symbol falls in the other sliding window, and the calculation of the time offset is realized through the data falling in the two sliding windows.
Further, sampling values corresponding to the two screened sampling points are sequentially subjected to absolute value, difference value and square operation according to the following formula, and difference value search data of the OFDM symbol is generated:
where Ng is the number of sampling points of the cyclic prefix, n and i are the unit of count, y m [n+i]For the n + i sample signal on the m received OFDM symbol, corresponding to y m [n+Ns+i]And delta is a timing offset value for the n + Ns + i sampled signal on the mth OFDM symbol received.
Has the advantages that: thereby realizing the generation of difference search data.
Further, carrying out mean operation on the M difference value search data according to the following formula, generating a mean curve according to a mean operation result, and screening out a sampling point corresponding to the lowest point of the mean curve:
in the formula (I), the compound is shown in the specification,
is an optimal estimate of the timing deviation value delta.
Has the beneficial effects that: thereby realizing the screening of the final sampling point.
Furthermore, the number of sampling points of each OFDM symbol is Nc, where the effective data length is Ns sampling points, and Ns sampling points are spaced between two sliding windows.
Has the beneficial effects that: the number of the sampling points at the interval of the two sliding windows is the same as that of the two selected sampling points, so that each sampling point in the two sliding windows corresponds to one.
Drawings
FIG. 1 is a flowchart illustrating a method for calculating a time offset of an OFDM system based on cyclic prefix according to an embodiment of the present invention;
FIG. 2 is a schematic timing signal diagram of an embodiment of a method for calculating a time offset of an OFDM system based on cyclic prefix according to the present invention;
fig. 3 is a simulation graph of the timing offset STO =5 and the carrier frequency offset CFO = 0;
fig. 4 is a simulation graph of the timing deviation STO = -3 and the carrier frequency offset CFO = 0;
fig. 5 is a simulation graph of the timing offset STO =5 and the carrier frequency offset CFO = 0.5;
fig. 6 is a simulation graph of timing deviation STO = -3 and carrier frequency offset CFO = 0.5;
fig. 7 is a simulation graph of the timing deviation STO =5 and the carrier frequency offset CFO = -0.3;
fig. 8 is a simulation graph of timing deviation STO = -3 and carrier frequency deviation CFO = -0.3.
Detailed Description
The following is further detailed by way of specific embodiments:
examples
A method for calculating time offset of an OFDM system based on cyclic prefix, as shown in fig. 1, includes the following steps:
s1: acquiring a time domain signal, wherein the time domain signal comprises M +1 continuous OFDM symbols, each OFDM symbol has a plurality of sampling points, and specifically, the number of the sampling points of each OFDM symbol is Nc. As shown in fig. 2, the OFDM symbol includes a cyclic prefix and effective data, where the number of sampling points of the cyclic prefix is Ng, and the number of sampling points of the effective data is Ns, that is, the effective data length is Ns sampling points.
Acquiring a time domain signal, comprising: and acquiring a receiving signal of a receiving end, and randomly intercepting the receiving signal according to a preset data length to acquire a time domain signal. E.g. one slot of time domain signal data in a 5G NR system.
S2: presetting two sliding windows W1 and W2, respectively screening sampling points in the two sliding windows, and spacing Ns sampling points between the two screened sampling points. The data length of the sliding window is equal to the data length of the cyclic prefix, namely the number of sampling points of the sliding window is Ng, ns sampling points are spaced between the two sliding windows, and specifically Ns sampling points are spaced between the left ends of the two sliding windows.
S3: and sequentially carrying out absolute value, difference and square operation on the sampling values corresponding to the two screened sampling points to generate difference value search data of the OFDM symbols.
Specifically, the absolute value, the difference value and the square operation are sequentially performed on the sampling values corresponding to the two screened sampling points according to the following formula, and difference value search data of the OFDM symbol is generated:
in the formula (1), ng is the number of sampling points of the cyclic prefix, n and i are counting units, y m [n+i]For the n + i sample signal on the m-th OFDM symbol received, corresponding to y m [n+Ns+i]And delta is a timing offset value for the n + Ns + i sampled signal on the mth received OFDM symbol.
S4: and sequentially generating difference value search data of M OFDM symbols, carrying out mean value operation on the M difference value search data, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve.
Sequentially generating difference value search data of M OFDM symbols according to the steps S2 and S3, carrying out mean value operation on the M difference value search data according to the following formula, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve:
in the formula (2), the reaction mixture is,
for optimal estimation of the timing deviation value δ, arg min is a function of the corresponding sampling point of the lowest point of the calculated mean curve.
Compared with the prior art, the method and the device have the advantages that the difference value search data is generated according to the absolute value, the difference value and the square operation, the influence of frequency deviation on the time deviation estimation precision is reduced, and the frequency deviation resistance is improved. Meanwhile, operation positioning is carried out on the basis of a plurality of difference value search data, so that the anti-noise performance of time bias estimation is improved, and the precision of time bias estimation under the condition of low signal-to-noise ratio is improved.
As shown in fig. 3-8, the graphs are simulation curves of time offset estimation using the difference method of the present embodiment and the prior art, and the abscissa represents the signal-to-noise ratio (SNB) and the ordinate represents the Root Mean Square Error (RMSE) of the corresponding time offset estimation value. According to the simulation results shown in fig. 3-8, compared with the prior art, the time offset estimation error is smaller and the relative time offset estimation accuracy is higher under the condition of low signal-to-noise ratio, and along with the increase of the signal-to-noise ratio, the time offset estimation error of the scheme tends to 0, and the time offset estimation accuracy is higher than that of the prior art. Meanwhile, simulation results show that the time offset estimation error is smaller and even tends to 0 under different carrier frequency offsets, and the frequency offset resistance is improved.
The foregoing is merely an example of the present invention, and common general knowledge in the field of known specific structures and characteristics is not described herein in any greater extent than that known in the art at the filing date or prior to the priority date of the application, so that those skilled in the art can now appreciate that all of the above-described techniques in this field and have the ability to apply routine experimentation before this date can be combined with one or more of the present teachings to complete and implement the present invention, and that certain typical known structures or known methods do not pose any impediments to the implementation of the present invention by those skilled in the art. It should be noted that, for those skilled in the art, without departing from the structure of the present invention, several changes and modifications can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicability of the patent. The scope of the claims of the present application shall be determined by the contents of the claims, and the description of the embodiments and the like in the specification shall be used to explain the contents of the claims.
Claims (6)
1. A method for calculating time offset of an OFDM system based on cyclic prefix is characterized by comprising the following steps:
acquiring a time domain signal, wherein the time domain signal comprises M +1 continuous OFDM symbols, and each OFDM symbol is provided with a plurality of sampling points;
presetting two sliding windows, respectively screening sampling points in the two sliding windows, spacing Ns sampling points between the two screened sampling points, and sequentially performing absolute value, difference and square operation on the sampling values corresponding to the two screened sampling points to generate difference value search data of an OFDM symbol;
and sequentially generating difference value search data of M OFDM symbols, carrying out mean value operation on the M difference value search data, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve.
2. The method according to claim 1, wherein the method for calculating the time offset of the OFDM system based on cyclic prefix comprises: acquiring a time domain signal, comprising:
and acquiring a receiving signal of a receiving end, and randomly intercepting the receiving signal according to a preset data length to acquire a time domain signal.
3. The method according to claim 1, wherein the method for calculating the time offset of the OFDM system based on cyclic prefix comprises: the OFDM symbol includes a cyclic prefix, and the data length of the sliding window is equal to the data length of the cyclic prefix.
4. The method according to claim 1, wherein the method for calculating the time offset of the OFDM system based on cyclic prefix comprises: and sequentially carrying out absolute value, difference value and square operation on the sampling values corresponding to the two screened sampling points according to the following formula to generate difference value search data of the OFDM symbol:
where Ng is the number of sampling points of the cyclic prefix, n and i are the unit of count, y m [n+i]For the n + i sample signal on the m received OFDM symbol, corresponding to y m [n+Ns+i]And delta is a timing offset value for the n + Ns + i sampled signal on the mth received OFDM symbol.
5. The method according to claim 1, wherein the method for calculating the time offset of the OFDM system based on cyclic prefix comprises: carrying out mean value operation on the M difference value search data according to the following formula, generating a mean value curve according to a mean value operation result, and screening out a sampling point corresponding to the lowest point of the mean value curve:
in the formula (I), the compound is shown in the specification,
is an optimal estimate of the timing deviation value delta.
6. The method according to claim 1, wherein the method for calculating the time offset of the OFDM system based on cyclic prefix comprises: the number of sampling points of each OFDM symbol is Nc, wherein the effective data length is Ns sampling points, and two sliding windows are spaced by Ns sampling points.
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