CN108075872B - Auxiliary OFDM system based on pilot frequency - Google Patents
Auxiliary OFDM system based on pilot frequency Download PDFInfo
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- CN108075872B CN108075872B CN201611261187.6A CN201611261187A CN108075872B CN 108075872 B CN108075872 B CN 108075872B CN 201611261187 A CN201611261187 A CN 201611261187A CN 108075872 B CN108075872 B CN 108075872B
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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
- H04L27/2662—Symbol synchronisation
- H04L27/2665—Fine synchronisation, e.g. by positioning the FFT window
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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
- H04L27/2662—Symbol synchronisation
- H04L27/2663—Coarse synchronisation, e.g. by correlation
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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
- H04L27/2666—Acquisition of further OFDM parameters, e.g. bandwidth, subcarrier spacing, or guard interval length
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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
- H04L27/2669—Details of algorithms characterised by the domain of operation
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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
- H04L27/2673—Details of algorithms characterised by synchronisation parameters
- H04L27/2675—Pilot or known symbols
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/003—Arrangements for allocating sub-channels of the transmission path
- H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
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Abstract
The invention provides a pilot frequency-based auxiliary OFDM system, which comprises: the coarse symbol synchronization module is used for performing coarse synchronization on the received data; the guard interval removing module is used for removing the guard interval according to the symbol synchronization position; the FFT module is used for carrying out FFT operation after coarse symbol synchronization to obtain frequency domain data; the channel estimation module is used for carrying out frequency domain channel estimation by utilizing the pilot frequency to obtain frequency domain channel response; and a fine symbol synchronization module, wherein the fine symbol synchronization module comprises: the system comprises an IFFT module, a front-back path detection module and a windowing position determining module. The invention combines the channel estimation and the symbol synchronization, greatly improves the symbol synchronization precision, and the channel estimation part can be shared with other modules of the system, thereby having less hardware overhead.
Description
The invention relates to a pilot frequency auxiliary OFDM symbol synchronization method and system, which is filed by the division of the original application, wherein the application number of the original application is 201611011234.1, and the application date is 2016, 11 and 17.
Technical Field
The invention relates to the field of wireless communication data processing, in particular to a pilot frequency-based auxiliary OFDM system.
Background
Ofdm (orthogonal Frequency Division multiplexing) is called orthogonal Frequency Division multiplexing, and is an efficient transmission technique. In 1961, basic principles were proposed by Franco and Lachs, and with the rapid development and application of digital signal processing technology and large scale integrated circuits, OFDM technology is widely used in various digital communication systems, such as Digital Audio Broadcasting (DAB) system, Digital Video Broadcasting (DVB) system, Wireless Local Area Network (WLAN), fourth generation mobile communication (4G), and the like. OFDM has therefore become one of the research hotspots.
One key issue for OFDM receivers is the sensitivity to synchronization errors. Symbol synchronization leads to inter-symbol interference (ISI), carrier synchronization leads to inter-carrier interference (ICI), and both lead to degradation of system performance. Especially for the OFDM system based on the pilot, due to the equal interval insertion of the pilot, periodicity in the frequency domain will be brought, if the system cannot precisely synchronize the symbols, periodic errors will occur in the channel estimation, and the performance degradation will be more serious than ISI. Symbol synchronization is therefore one of the difficulties for pilot-based OFDM systems.
The above-mentioned pilot-based OFDM system is typically applied in DVB-T \ DVB-T2, and DVB-T will be taken as an example (not limited thereto). Fig. 1 is a structure diagram of an OFDM frame of DVB-T, wherein a pilot interval duration DX is 3 and a vertical DY is 4. Periodicity occurs in the time domain when channel estimation is performed using pilots. When symbol synchronization deviates, two phenomena occur: 1. introducing intersymbol interference (ISI); 2. confusion between the anterior and posterior paths occurs.
To solve this problem, there are several algorithms commonly used at present:
1. with cyclic prefix: the detection precision is low;
2. based on the phase information: when the large diameter is far, the large diameter is invalid, and the precision is not high.
3. Minimum Mean Square Error (MMSE): the realization is more complicated, and the precision is not high.
The above methods are all not high in precision, and when the code rate is high, the performance is difficult to meet the system requirement. In view of the defects of the above methods, a method and a system for pilot-assisted OFDM symbol synchronization are provided, which have high accuracy and are simple to implement.
Disclosure of Invention
The purpose of the invention is: the invention provides a method and a system for pilot frequency auxiliary OFDM symbol synchronization, which realize high precision and simple symbol synchronization.
The invention provides a pilot frequency auxiliary OFDM symbol synchronization method, which is characterized by comprising the following steps: after coarse symbol synchronization, removing a guard interval according to a symbol synchronization position, and performing FFT operation to obtain frequency domain data; performing frequency domain channel estimation by using the pilot frequency to obtain frequency domain channel response; performing IFFT operation on the frequency domain channel response to obtain a time domain channel response; determining the corresponding periodicity of a time domain channel based on an insertion mode of pilot frequency, performing time domain correlation identification on the multipath appearing in the first period, and distinguishing a front path or a rear path according to the position of each multipath relative to a primary windowing position; and performing correlation operation on the possible positions of the front path and the rear path, obtaining the correct time domain multipath position when the correlation value is larger, and accordingly, acquiring the windowing position again to finish fine symbol synchronization.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: wherein, in the step of detecting for distinguishing the front diameter or the rear diameter, any one of the following methods is included: performing correlation operation by using the time domain sequence of the pilot frequency as a local sequence; or the received time domain data is used as a local sequence to carry out correlation operation.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: the method comprises the following steps of performing correlation operation by using a time domain sequence of a pilot frequency as a local sequence, wherein the steps comprise: carrying out conjugate multiplication and accumulation on the received data and a local sequence which is a time domain sequence of the pilot frequency to obtain an amplitude value calculation formula; and respectively substituting the sampling delay time number when the path is judged as the front path and the sampling delay time number when the path is judged as the rear path into the amplitude value calculation formula, and correspondingly judging one of the paths as the front path.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: the method for performing correlation operation by using received time domain data as a local sequence comprises the following steps:
carrying out conjugate multiplication and accumulation on the received data and a local sequence taking the time delay corresponding to the maximum path as an amplitude value calculation formula; and respectively substituting the sampling delay time number when the path is judged as the front path and the sampling delay time number when the path is judged as the rear path into the amplitude value calculation formula, and correspondingly judging one of the paths as the front path.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: wherein, the step of coarse synchronization comprises: and obtaining a detection curve from the front and back correlations of the guard interval, and determining a position corresponding to the first position larger than the preset threshold as a coarse symbol synchronization position by using a preset threshold.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: and performing frequency domain channel estimation by using the scattered pilot frequency to obtain frequency domain channel response, wherein the value is only at the corresponding position of the scattered pilot frequency, and the rest positions are zero.
Further, the method for pilot-assisted OFDM symbol synchronization according to the present invention is characterized in that: wherein, the fine symbol synchronization step comprises: and re-sequencing the multi-paths according to the detection results of the front path and the back path, determining the symbol synchronization position according to the minimum interference criterion between codes, and re-windowing to obtain correct time domain channel response.
In addition, the invention also provides a pilot-based auxiliary OFDM system, which is characterized by comprising: the coarse symbol synchronization module is used for performing coarse synchronization on the received data; the guard interval removing module is used for removing the guard interval according to the symbol synchronization position; the FFT module is used for carrying out FFT operation after coarse symbol synchronization to obtain frequency domain data; the channel estimation module is used for carrying out frequency domain channel estimation by utilizing the pilot frequency to obtain frequency domain channel response; and a fine symbol synchronization module, wherein the fine symbol synchronization module comprises: the system comprises an IFFT module, a front-back path detection module and a windowing position determining module.
Further, in the pilot-based auxiliary OFDM system proposed in the present invention, the following features are also provided: the IFFT module is used for carrying out IFFT operation on the frequency domain channel response to obtain a time domain channel response; the front and back path detection module is used for determining the corresponding periodicity of a time domain channel based on the pilot frequency insertion mode, performing time domain correlation identification on the multi-paths appearing in the first period, and distinguishing the multi-paths into front paths or back paths according to the position of each multi-path relative to the initial windowing position; and the windowing position determining module is used for performing correlation operation on possible positions of the front path and the rear path, obtaining the position of the correct time domain multipath when the correlation value is larger, and obtaining the windowing position again to finish fine symbol synchronization.
Further, in the pilot-based auxiliary OFDM system proposed in the present invention, the following features are also provided: wherein, in the front and rear diameter detection module, the detection for distinguishing the front diameter or the rear diameter is realized by any one of the following methods: performing correlation operation by using the time domain sequence of the pilot frequency as a local sequence; or the received time domain data is used as a local sequence to carry out correlation operation.
The invention has the technical effects and effects that:
according to the pilot frequency auxiliary OFDM symbol synchronization method and the system, only time domain correlation operation is used for comparison, so that the method is simple to realize, and in addition, on the basis of time domain channel response, pilot frequency information is fully utilized, and the synchronization precision is improved.
Drawings
FIG. 1 is a schematic block diagram of symbol synchronization for a pilot-assisted OFDM-based system of the present invention;
fig. 2 is a schematic diagram of the frame structure of DVB-T in the method of the invention;
FIG. 3 is a schematic diagram of the theoretical channel and coarse symbol synchronization positions in the method of the present invention;
FIG. 4 is a diagram illustrating a time domain channel response h in the method of the present invention; and
fig. 5 is a diagram illustrating the reordered time-domain channel responses in the method of the present invention.
Detailed Description
The objects, specific structural features and advantages of the present invention will be further understood from the following description of the embodiments of the present invention taken in conjunction with the accompanying drawings.
The invention provides a method for synchronizing pilot frequency auxiliary OFDM symbols, which comprises the following steps:
1) firstly, coarse symbol synchronization is carried out, a Guard Interval (GI) is removed according to a symbol synchronization position, FFT operation is carried out to obtain frequency domain data, wherein the coarse symbol synchronization can adopt any existing coarse starting symbol synchronization method without limitation;
2) performing frequency domain channel estimation by using the pilot frequency to obtain frequency domain channel response H;
3) performing IFFT operation on the frequency domain channel response H to obtain a time domain channel response H;
4) because the insertion mode of the pilot frequency determines the corresponding periodicity of the time domain channel, the multi-path appearing in the first period is subjected to time domain correlation identification, and each multi-path is distinguished to be a front path or a rear path relative to the initial windowing position.
5) And according to the steps, obtaining the correct time domain multipath position, and accordingly, reacquiring the windowing position to complete the fine symbol synchronization.
The invention relates to the field of wireless communication data processing, and the method of the invention combines the time domain response obtained by channel estimation, which is equivalent to the fact that the possible distribution condition of a channel is predicted, and then uses time domain correlation to carry out different possible differentiation. The detection precision can be improved, and complex operation is avoided. In addition, since the frequency domain and time domain responses of the channel estimation can be shared with other modules in the system, and the time domain correlation is degraded into multiplication accumulation with fixed position due to the known channel condition, the hardware overhead is extremely small.
The frame structure of the OFDM system generally adopts a guard interval plus data form, the symbol synchronization needs to determine the start of a data segment, and because a cycle structure is adopted, as long as the FFT windowing position is in the guard interval, no intersymbol interference is introduced. Therefore, during symbol synchronization, the position of the foremost path needs to be detected, and the position of the data segment of the path is used as the FFT windowing position, so as to ensure that the system does not introduce inter-symbol interference and obtain accurate channel response. In the invention, firstly, the rough symbol synchronization is utilized, the pilot frequency is utilized to carry out channel estimation to obtain possible multi-path distribution, the time domain correlation is utilized to distinguish the front path from the rear path, and finally, the accurate symbol synchronization position is obtained. As shown in fig. 2, the data processing unit of the present invention mainly includes: coarse symbol synchronization, GI removal, FFT/IFFT operation, frequency domain channel estimation, a front and back path detection unit and a windowing position determining operation unit.
In this example, taking the 8K mode of the DVB-T system as an example, the GI mode is 1/4. Fine symbol synchronization needs to be based on the frequency domain channel response H, and obtaining H requires the following steps:
first, coarse symbol synchronization is performed. Obtaining a detection curve using a pre-post correlation of guard intervals
Taking the maximum value m as max (x (n)), and taking m-delta as a threshold, wherein the position corresponding to the first position of x (n) larger than the threshold is coarse symbol synchronizationA location; wherein, N is 8192, L is 8192 (1/4), and Delta is windowing protection margin.
And secondly, obtaining a frequency domain signal of the received data by using FFT operation. Removing a guard interval (removing GI) in a frame structure according to the obtained coarse symbol synchronization data, and performing FFT operation on a data segment to obtain a frequency domain signal R (k);
and thirdly, performing frequency domain channel estimation by using Scattered Pilots (SP) to obtain frequency domain channel response h (k) ═ r (k)/SP (k), where k is a subcarrier sequence number. The obtained H has a value only at the corresponding position of the scattered pilot, and the rest positions are 0.
A channel frequency domain response H is obtained, but the windowing position is not accurate, which is based on the coarse symbol synchronization. The real channel is assumed to consist of two multipaths, the forward path delay is-50 us and the amplitude is 20dB, the main path delay is 0us and the amplitude is 0 dB. According to the coarse symbol synchronization algorithm, the windowing position precedes the main path by 100 sample positions. See fig. 3. The ideal windowing position should be arranged at the forefront diameter of-50 us, and the coarse symbol synchronization causes larger deviation of the windowing position. The fine symbol synchronization module identifies the most forward path according to the frequency domain channel response H, and adjusts the windowing deviation to 0. The method comprises the following steps:
in the step (1), a time domain response h (n) is obtained according to the frequency domain response h (k), as shown in fig. 4. As can be seen in the figure, since the frequency domain has a value at intervals of DX of 3, channel spreading occurs for 3 cycles in the time domain. From h (n) alone, the multipath labeled X2375 cannot be distinguished as the front path or the back path.
Step (2): and detecting the front diameter and the rear diameter. From h, it can be judged that there are two possibilities for multipath with X2375: one is the back diameter, and the relative coarse windowing position is 2375 sampling delays; the other is the front diameter, and the relative windowing position is 356 sampling delays. Front-back diameter detection can take two methods: firstly, a time domain sequence of pilot frequency is used as a local sequence; second, received time domain data is used as a local sequence. The two methods are described separately below. And performing correlation operation by using the possible positions of the front path and the back path, wherein the position with the larger correlation value is the correct position.
The first front-back diameter detection method comprises the following steps: using the pilot time domain sequence as the local sequence:
it is assumed that the transmitted data is s (n), which consists of two parts, data d (n) and pilot p (n), i.e., s (n) ═ d (n) + p (n). Where, data d (n) is unknown, and pilot p (n) is known. Receive time domain data r (n) of
Namely, it is
Is the convolution operator. When p (n) is used as the local sequence at the corresponding position tau of h0Performing conjugate multiplication and accumulation to obtain
When c (τ)0) The amplitude will be equal to h (τ)0) Is in direct proportion. In this example, c (2375) and c (-356) are obtained by substituting 2375 and-356, and since the true multipath is the front path, c (-356) obtained is larger than c (2375), and it is determined as the front path.
The second front-back diameter detection method comprises the following steps: using the received time domain data as a local sequence:
receive time domain data as
If data interference of other multi-paths is ignored, each multi-path can be delayed by tauiCorresponding to the received data as l (i, n) ═ s (n) h (tau)i). Take the maximum diameter imaxCorresponding time delay taumaxFor the local sequence l (i)max,n)＝s(n)h(τmax) The other paths are multiplied by the maximum path in a conjugate manner to obtain
The resulting correlation value is proportional to the corresponding multipath strength. Therefore, when determining the front and back paths, the position where the multi-path really occurs will be larger than the other position. In this example, 2375 and-3 will be similarly applied56 corresponding to the multipath location, the path may be determined to be the front path.
Step (3): and re-ordering the multi-paths according to the detection results of the front path and the back path, and determining the symbol synchronization position according to the ISI interference minimum criterion. The correct time domain channel response is obtained after re-windowing, see fig. 5.
In summary, the present invention first obtains the frequency domain channel response H according to the coarse symbol synchronization, and the fine symbol synchronization module obtains the time domain channel response H with the periodic characteristic by using the IFFT operation unit. And (4) calculating the possible positions of each multipath according to the periodicity, and distinguishing the front path from the rear path by using time domain correlation. And re-windowing is carried out according to the obtained correct channel response, so that better system performance is obtained. The invention combines the channel estimation and the symbol synchronization, thereby greatly improving the symbol synchronization precision.
Claims (5)
1. A pilot-assisted OFDM-based system, comprising:
the coarse symbol synchronization module is used for performing coarse synchronization on the received data;
the guard interval removing module is used for removing the guard interval according to the symbol synchronization position;
the FFT module is used for carrying out FFT operation after coarse symbol synchronization to obtain frequency domain data;
the channel estimation module is used for carrying out frequency domain channel estimation by utilizing the pilot frequency to obtain frequency domain channel response; and
a fine symbol synchronization module, wherein the fine symbol synchronization module comprises: an IFFT module, a front and back path detection module and a windowing position determining module,
the front and back path detection module performs conjugate multiplication and accumulation on the received data and the received data corresponding to the time delay corresponding to the maximum path as a local sequence to obtain an amplitude value calculation formula; and
the front and rear path detection modules respectively substitute the sampling delay time when the front path is judged in advance and the sampling delay time when the rear path is judged in advance into the amplitude value calculation formula, and the multipath corresponding to the larger result obtained by the amplitude value calculation formula is judged as the front path.
2. The pilot-assisted OFDM system of claim 1, wherein,
the IFFT module performs IFFT operation on the frequency domain channel response to obtain a time domain channel response;
the front and back path detection module determines the corresponding periodicity of a time domain channel based on the insertion mode of the pilot frequency, performs time domain correlation identification on the multi-path appearing in the first period, and distinguishes the multi-path to be a front path or a back path according to the position of each multi-path relative to the initial windowing position; and
the windowing position determining module performs correlation operation on possible positions of the front path and the rear path, and if the correlation value is larger, the correct time domain multipath position is obtained, and the windowing position is obtained again, so that fine symbol synchronization is completed.
3. The pilot-assisted OFDM system of claim 1, wherein the coarse symbol synchronization module obtains a detection curve from the pre-correlation and the post-correlation of the guard interval, and determines a first position greater than the predetermined threshold as a coarse symbol synchronization position according to a predetermined threshold.
4. The pilot-based OFDM system of claim 1, wherein the channel estimation module performs the frequency-domain channel estimation using scattered pilots to obtain the frequency-domain channel response, wherein the frequency-domain channel response has values only at positions corresponding to the scattered pilots, and the remaining positions are zeros.
5. The pilot-assisted OFDM system of claim 2 wherein the windowing location determining module reorders the multipaths according to the front and back path detection results, determines the symbol synchronization location according to the minimum inter-symbol interference criterion, and obtains the correct time domain channel response after re-windowing.
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| CN108075998A (en) | 2018-05-25 |
| CN108075871A (en) | 2018-05-25 |
| CN108075871B (en) | 2020-12-29 |
| CN108075998B (en) | 2021-06-22 |
| CN108075872A (en) | 2018-05-25 |
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