CN107508780B - Timing synchronization method of OFDM system based on IEEE 802.11ac - Google Patents
Timing synchronization method of OFDM system based on IEEE 802.11ac Download PDFInfo
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- CN107508780B CN107508780B CN201710882853.6A CN201710882853A CN107508780B CN 107508780 B CN107508780 B CN 107508780B CN 201710882853 A CN201710882853 A CN 201710882853A CN 107508780 B CN107508780 B CN 107508780B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2656—Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2668—Details of algorithms
- H04L27/2681—Details of algorithms characterised by constraints
- H04L27/2684—Complexity
Abstract
The invention discloses a timing synchronization method of an OFDM (Orthogonal Frequency Division Multiplexing) system based on IEEE 802.11ac, which relates to a mobile communication technology and realizes that the initial position of a frame is correctly detected under the condition of transmitting multi-frame data. The method comprises the following main steps: firstly sampling a received signal, detecting whether the data volume is larger than the data volume of a minimum frame, then expanding a local sequence in a non-very high throughput, then performing inverse Fourier transform and expansion, and finally, correlating the sampled data with the processed local sequence to form a cost function so as to correctly detect the initial position of a frame of data. The invention combines the characteristics of the received signal, the characteristics of an IEEE 802.11ac frame structure and the characteristics of a training sequence with very high throughput, reduces the realization complexity of a timing synchronization algorithm, and provides a simple and effective solution for frame timing synchronization.
Description
Technical Field
The invention belongs to the technical field of mobile communication, and particularly relates to a timing synchronization method of an OFDM system based on IEEE 802.11 ac.
Background
OFDM is a multi-carrier high-speed spread spectrum transmission technology, and is widely used in WLAN (Wireless Local Area Networks) transmission, such as IEEE 802.11ac protocol. Although the carriers of each sub-channel in the OFDM system are orthogonal to each other and the frequency spectrums are overlapped with each other, not only mutual interference between the sub-carriers is reduced, but also the frequency spectrum utilization rate is improved, but also the corresponding synchronization requirement on the receiver is particularly high, so that timing synchronization is particularly important as the first step of receiver processing. If the timing is not accurate enough, it can result in a dramatic drop in system performance.
For data transmission of the OFDM system of IEEE 802.11ac, a preamble sequence is generally used to achieve synchronization. Coarse and fine synchronization of frames using short and long training sequences, respectively, with very low throughput, and algorithms based on this are proposed in many documents, such as the document "timing and frequency synchronization for IEEE 802.11n MIMO-OFDM wireless local area network systems". Although the document can effectively detect the starting position of the frame, the algorithm has huge calculation amount and high complexity, weak correlation exists between the short training sequence and the long training sequence in very high throughput, and the scheme is likely to have the condition that the frame starting point is misjudged under the condition of low signal-to-noise ratio.
In addition, the research of simple and effective timing synchronization technology has great significance for manufacturers of WLAN equipment. By testing performance indexes such as constellation diagram vector mean square error, IQ (In-phase and Quadrature-phase) offset, carrier frequency error and the like of the WLAN product, equipment produced by manufacturers is ensured to meet the standard formulated by IEEE, and the development of the technology for manufacturing WLAN equipment In China is promoted.
Disclosure of Invention
The present invention is directed to solving the above problems of the prior art. The timing synchronization method of the OFDM system based on the IEEE 802.11ac has better detection performance under the new fall of multipath fading and lower complexity compared with the common timing synchronization method. The technical scheme of the invention is as follows:
a timing synchronization method of an OFDM system based on IEEE 802.11ac comprises the following steps:
A. at the transmitting end, the radio frequency signal is transmitted by the signal source, at the receiving end, the received signal is extracted, and the received signal data y (k) is converted into the signal with the sampling rate FSBase band digital signal y1(k) Wherein k is the position of the signal, and judging whether the length of the intercepted data segment meets (meets the requirement that the length of the index data segment is greater than or equal to) the length L of the minimum frame;
B. short training sequence S in very high throughput in IEEE 802.11ac frame structureL-STFCarrying out head and tail zero padding expansion and carrying out IFFT inverse Fourier transform to obtain a time domain signal short (t) of a local sequence;
C. b, carrying out periodic expansion on the time domain signal short (t) of the local sequence in the step B;
D. and constructing a cost function M (n) by utilizing the correlation between the received data and the short training sequence in the non-very high throughput, wherein n is the position of the cost function, and adopting a sliding correlation method to find the timing synchronization position of the frame.
2. The method for timing synchronization of an IEEE 802.11 ac-based OFDM system as claimed in claim 1, wherein the length L of the minimum frame in step A is determined according to a modulation scheme of a protocol.
Further, the sampling in the step a obtains a baseband digital signal y1(k) The preprocessing step enables the received signal to be rapidly demodulated at the receiving end, judges whether the section of data exceeds the data L of one frame or not, does not execute the subsequent steps if the section of data does not exceed the data L of one frame, and displays that the complete signal of one frame cannot be detected.
Further, the short training sequence S with not very high throughput in step BL-STFThe number of carriers is not 2NAnd if the number of the (N-2, 4,6L) does not meet the rule of the standard IFFT, the IFFT condition is met by a zero padding expansion method, and then the frequency domain signals are converted into time domain data.
Further, the step C is according to: the very high throughput short training sequence portion of the IEEE 802.11ac signal frame is made up of 10 identical training sequences, thus extending to as many as it is.
Further, the method utilizes the correlation between the received data and the short training sequence in the very high throughput to construct a cost function M (n), where n is the position of the cost function, and the cost functionsshort(k) Representing data after the periodic expansion of S (k), and then k + +, wherein k + + represents that one is automatically added, the cost function is fed back again, the traversal is continued until the traversal times are finished, and the number of times of the common sliding is N ═ length (y)1(k) 320, length () represents the length of the data string in parentheses; traversing the whole cost function again to find the peak value V of the functionpeak(m) and corresponding position Ppeak(m) and recording the number N of peaks, the positions P corresponding to said peakspeak(m) is the start position of the frame.
Further, when the IEEE 802.11ac signal is multi-frame, the value of the maximum position of the peak of the cost function and the data of one OFDM symbol length around the maximum position are cleared first, under the condition of a preset threshold value tau, and the data are stored in the storage unitRepeatedly traversing the whole cost function to search a peak value, finding out the maximum peak value of the cost function, and if the module value of the difference between the peak value and the first detected peak value exceeds a threshold value, indicating that the initial position of the frame is completely detected, and completing the timing synchronization of the frame signal; if the module value of the detected peak value difference is smaller than the threshold value, the peak value is continuously searched until the cycle is jumped out and the initial positions d of all the detection framesstart。
The invention has the following advantages and beneficial effects:
the invention provides a short training sequence S using only a very high throughputL-STFAnd realizing the frame timing synchronization of the received signals y (k). The conventional algorithm is mostly based on a short training sequence S of not very high throughput in the received signalL-STFBlind estimation is carried out on self characteristics (consisting of a plurality of repeated training sequences), coarse synchronization of frames is completed, fine synchronization is carried out by utilizing a maximum likelihood algorithm of a long training sequence with non-very high throughput, and the method carries out blind estimation on short training sequence S with non-very high throughputL-STFThe preprocessing (filling zero at the head and the tail of the sequence, then performing IFFT, and finally performing period expansion) is performed, so that the maximum cross-correlation maximum likelihood estimation of the received signals can be realized to the maximum extent, the estimated frame starting position is more accurate when determined, and the fine synchronization step is not needed, thereby reducing the complexity of the algorithm. In addition, under the multipath fading channel, the cyclic prefix of the OFDM signal is interfered, and compared with the cyclic prefix synchronous timing based on the OFDM signal, the method has higher accuracy.
Drawings
FIG. 1 is a block diagram of the demodulation of a received signal in accordance with the preferred embodiment of the present invention;
FIG. 2 is a block flow diagram of the timing synchronization of an 802.11ac signal in accordance with a preferred embodiment of the present invention;
FIG. 3 is an expanded block diagram of the 802.11ac signal frame structure according to the preferred embodiment of the present invention;
fig. 4 is a block diagram of the start position of an 802.11ac signal frame in accordance with a preferred embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be described in detail and clearly with reference to the accompanying drawings. The described embodiments are only some of the embodiments of the present invention.
The technical scheme for solving the technical problems is as follows:
the method is applied to the timing synchronization estimation of the OFDM system of IEEE 802.11ac based on the implementation example of the invention. The specific implementation steps are given below:
1. a transmitting end modulates a desired signal from a source, wherein the bandwidth BW40Hz and MCS (Modulation Coding Scheme) are 4, NssTaking IEEE 802.11ac signal with Spatial Streams (Spatial Streams) of 1 as an example, through a transmitting device, a data segment with a sampling frequency F is extracted at a receiving end through a channel, taking waveform data with F of 80MHz as an example, timing synchronization is performed on the data segment, and padding is provided for a subsequent demodulation part, as shown in fig. 1.
2. Sampling and preprocessing a received signal, as shown in a dotted line block diagram in fig. 2, specifically including the following steps:
step 201, receiving a signal with an ideal sampling rate FS40Hz, the received signal is sampled first, so that the number of samples D corresponding to the very high throughput short midamble in its frame structure is 320,
in step 202, during the process of continuing to process data after sampling, several parameters are defined, such as a sliding window WT-SHORTLength of minimum frame of 320Wherein N isL-STF、NL-LTF、NL-SIG、NSIG-A、NH-STF、NH-LTF、NSIG-B、DDATAAnd NofdmThe number of initialization peaks N is 0 and a threshold τ is set to 0.01, wherein the number of initialization peaks N is 0, and the number of initialization peaks N is 0.01.
Step 203, a decision is made on the data following step 22, if y1(k) If the length of the data segment is smaller than the minimum length L of the frame, the detection is finished, and it is impossible to detect data of one frame in the data segment, so the number of the sampling points is about 10 data of the minimum frame length, which prevents interference of a silence region between frames and causes insufficient data amount, and so, take as an example extracting 40000 data in the receiving end.
3. Extracting a very high throughput short training sequence S corresponding to a 40MHz bandwidth according to an IEEE 802.11ac protocolL-STFSimilarly, the data is processed:
step 204, short training sequence S in non-very high throughput due to extractionL-STFDoes not satisfy 2 when performing a transform from the frequency domain to the time domainNSo that the zero padding of the training sequence is s (z), where z is 1,2, L,128, and the formula is adopted in transforming it from the frequency domain to the time domainWhere Δ f is the subcarrier spacing and k is the position of the time domain signal.
Step 205, periodically expanding the local sequence of the time domain to make it satisfy that the 40MHz bandwidth is equal to the length of the short training sequence with the very high throughput of the corresponding frame (refer to fig. 3), s (k) data s after periodic expansionshort(k) The number is D. And then the sequence is conjugated, so that a cost function can be conveniently constructed later.
4. Data after sampling is y1(k) Sliding window WT-SHORTFor 320, a cost function is constructed by starting traversal when a window is slid and k is equal to 1And then k + +, wherein k + + represents that one is automatically added, the cost function is fed back again, the traversal is continued until the traversal number is finished, and the number of the total sliding N is length (y)1(k) 320, length () represents the length of the data string in parentheses.
5. The entire cost function is traversed again and,find the peak value V of the functionpeak(m) and corresponding position Ppeak(m) and recording the number N of peaks, the positions P corresponding to said peakspeak(m) is the start position of the frame.
6. In the case of multiple frames, it is necessary to add a processing step to clear the value of the maximum position of the peak of the cost function and the data of one OFDM symbol length around the maximum position, so as to prevent the peak proximity from interfering with the detection of the timing synchronization of other frames. Under the condition of a preset threshold value tau, repeating the step 5, finding the maximum peak value of the cost function, and if the module value of the difference between the peak value and the first detected peak value exceeds the threshold value, indicating that the initial position of the frame is completely detected, and completing the timing synchronization of the frame signal; if the module value of the detected peak value difference is smaller than the threshold value, the step 5 is continued until the cycle is jumped out and the initial positions d of all the detection framesstartAs shown in fig. 4.
In summary, the present invention provides a timing synchronization method for an OFDM system based on IEEE 802.11 ac. Compared with the traditional method, because the high-throughput short training sequence and the non-very high-throughput long training sequence in the IEEE 802.11ac frame structure bring errors to the traditional timing synchronization method, the traditional method can further judge by using the non-very high-throughput long training sequence on the basis. The method provides a simple and effective solution, which is suitable for receiving end baseband signal processing.
The above examples are to be construed as merely illustrative and not limitative of the remainder of the disclosure. After reading the description of the invention, the skilled person can make various changes or modifications to the invention, and these equivalent changes and modifications also fall into the scope of the invention defined by the claims.
Claims (7)
1. A timing synchronization method of an OFDM system based on IEEE 802.11ac is characterized by comprising the following steps:
A. at the transmitting end, the signal source is used to transmit radio frequency signal, at the receiving end, the received signal is extracted and receivedIs converted into signal data y (k) with a sampling rate FSBase band digital signal y1(k) Wherein k is the position of the signal, and whether the length of the intercepted data segment meets the requirement that the length L of the data segment is greater than or equal to the length L of the minimum frame is judged; the minimum frame refers to a signal frame not containing a data portion;
B. short training sequence S in very high throughput in IEEE 802.11ac frame structureL-STFCarrying out head and tail zero padding expansion and carrying out IFFT inverse Fourier transform to obtain a time domain signal short (t) of a local sequence; due to the extracted short training sequence S in the non-very high throughputL-STFDoes not satisfy 2 when performing a transform from the frequency domain to the time domainNSo that the zero padding of the training sequence is s (z), where z is 1,2, …,128, and the formula is used to transform it from the frequency domain to the time domainWherein, Δ f is the subcarrier interval, and k is the position of the time domain signal;
C. b, carrying out periodic expansion on the time domain signal short (t) of the local sequence in the step B to ensure that the bandwidth of 40MHz is equal to the length of the short training sequence with the non-very high throughput of the corresponding frame, and S (k) carrying out data s after the periodic expansionshort(k) The number is D, and the sequence is conjugated, so that a cost function can be constructed conveniently;
D. constructing a cost function M (n) by utilizing the correlation between the received data and the short training sequence in the non-very high throughput, wherein n is the position of the cost function, and adopting a sliding correlation method to find the timing synchronization position of the frame; data after sampling is y1(k) Sliding window WT-SHORTFor 320, a cost function is constructed by starting traversal when a window is slid and k is equal to 1And then k + +, wherein k + + represents adding one, calculating the cost function again, and continuing the traversal until the traversal number is finished, and the number of co-sliding N is length (y)1(k) 320, length () represents the length of the data string in parentheses;
traversing the whole cost function again to find the peak value V of the functionpeak(m) and corresponding position Ppeak(m) and recording the number N of peaks, the positions P corresponding to said peakspeak(m) is the start position of the frame; for the condition of multiple frames, a processing step must be added, firstly, the value of the maximum position of the peak value of the cost function and data of one OFDM symbol length at the left and right sides of the maximum position are cleared, the detection of other frame timing synchronization is prevented from being interfered by the proximity point of the peak value, under the condition of a preset threshold value tau, the step is repeated, the maximum peak value of the cost function is found, if the module value of the difference between the peak value and the first detected peak value exceeds the threshold value, the starting position of the frame is completely detected, and the timing synchronization of the frame signal is completed; if the module value of the detected peak value difference is smaller than the threshold value, the steps are continued until the cycle is jumped out and the initial positions d of all the detection framesstart。
2. The method for timing synchronization of an IEEE 802.11 ac-based OFDM system as claimed in claim 1, wherein the length L of the minimum frame in step A is determined according to a modulation scheme of a protocol.
3. The method for timing synchronization of an IEEE 802.11 ac-based OFDM system as claimed in claim 1 or 2, wherein the sampling in step A is performed to obtain a baseband digital signal y1(k) The preprocessing step enables the received signal to be rapidly demodulated at the receiving end, judges whether the section of data exceeds the data L of one frame or not, does not execute the subsequent steps if the section of data does not exceed the data L of one frame, and displays that the complete signal of one frame cannot be detected.
4. The method of claim 1, wherein the step B is a short training sequence S with very high throughputL-STFThe number of carriers is not 2N(2, 4,6 …), not satisfying the rule of standard IFFT transformation, then satisfying the IFFT condition by zero filling extension method, then transforming the frequency domain signal into time domain signalDomain data.
5. The method for timing synchronization of an IEEE 802.11 ac-based OFDM system according to claim 1, wherein the step C is based on: the very high throughput short training sequence portion of the IEEE 802.11ac signal frame is made up of 10 identical training sequences, thus extending to as many as it is.
6. The method of claim 1, wherein the cost function M (n) is constructed by using correlation between the received data and the short training sequence in the very high throughput, where n is the position of the cost function, and the cost function is the position of the cost functionsshort(k) Representing data after the periodic expansion of S (k), wherein y () is a receiving end time domain signal, then k + +, wherein k + + represents adding one, calculating the cost function again, continuing the traversal until the traversal number is finished, and the number of times of the common sliding is Num ═ length (y +++, where1(k) 320, length () represents the length of the data string in parentheses; traversing the whole cost function again to find the peak value V of the functionpeak(m) and corresponding position Ppeak(m) and recording the number N of peaks, the positions P corresponding to said peakspeak(m) is the start position of the frame.
7. The timing synchronization method of the OFDM system based on IEEE 802.11ac as claimed in claim 6, wherein when the IEEE 802.11ac signal is multi-frame, firstly clearing the value of the maximum position of the peak of the cost function and the data of the length of each OFDM symbol around the maximum position, under the condition of the pre-established threshold τ, repeatedly traversing the whole cost function to find the peak, finding the maximum peak of the cost function, if the module value of the difference between the peak and the first detected peak exceeds the threshold, it is indicated that the start position of the frame has been completely detected, and the timing synchronization of the frame signal is completed; if the modulus of the detected peak difference is smaller than the threshold value, thenContinuing to search for peak until jumping out of loop, detecting start position d of all detection framesstart。
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