CN1992691B - A Channel Synchronization Method Based on OFDM Technology - Google Patents

Abstract

The present invention provides a channel synchronization method based on OFDM technology, which is applied in the LTE system. The subframe structure of the LTE system includes a synchronization symbol and several reference symbols. The synchronization method includes the following steps: Placed in the first symbol of each subframe, the synchronization symbol is placed in the second symbol of the first subframe, the reference symbol and the synchronization symbol are both located in the first subframe, and the reference symbol and the synchronization symbol are located in adjacent positions. The intersection of the reference symbol and the sync symbol forms a continuous repeating structure in the time domain. Through this method, the ability to capture synchronization and frequency offset is enhanced, so that the starting position of the subframe can be easily found. In addition, the synchronization process is directly assisted by using the reference symbols, and it is no longer necessary to add additional synchronization symbols to achieve good performance and reduce the overhead of the synchronization channel.

Description

A Channel Synchronization Method Based on OFDM Technology

【Technical field】

The invention relates to the field of information transmission of a communication system, in particular to a method for realizing time synchronization and carrier offset channel synchronization of an LTE system by using OFDM technology.

【Background technique】

At present, as a multi-carrier transmission mode, Orthogonal Frequency Division Multiplexing (OFDM) converts a high-speed transmission data stream into a set of low-speed parallel transmission data streams, so that the system can detect multipath fading channel frequency Selective sensitivity is greatly reduced. The introduction of the cyclic prefix (CP) further enhances the ability of the system to resist inter-symbol interference (ISI). In addition, the characteristics of high bandwidth utilization and simple implementation make OFDM more and more widely used in wireless communication domains.

OFDM technology has been successfully applied in many communication systems, for example, digital broadcasting (DAB) and digital television (DVB) formulated by European Telecommunications Standardization Organization ETSI use OFDM technology as the wireless transmission standard of the air interface, and the wireless local area network standard IEEE802. 11 and the wireless metropolitan area network standard IEEE802.16 also adopt OFDM technology. The effective use of OFDM technology requires strict time synchronization and carrier synchronization on both sides of the transceiver.

The OFDM symbol is composed of multiple subcarrier signals superimposed, and each subcarrier is distinguished by orthogonality. Therefore, ensuring this orthogonality is crucial for the OFDM system, so the requirement for synchronization of the OFDM system is relatively strict. Synchronous position detection and frequency offset detection are very important steps in the OFDM system. Only when the correct synchronous position is found can the data be received correctly and the next step of processing can be performed.

Please refer to Figure 1 to Figure 4. In the existing OFDM system, there is a synchronization method, which is based on adding a symbol with repetition characteristics to the data stream at the transmitting end, and detecting the position conforming to the repetition characteristic at the receiving end as the symbol timing position. The frequency deviation between the transmitting and receiving ends is also determined by using the correlation characteristics of the transmitted code known in the frequency domain of the symbol. This method generally uses the format shown in Figure 1 to construct a symbol. First, all subcarriers are divided into two types: odd subcarriers and even subcarriers according to the odd-even sequence, and only one of them is used, such as all odd subcarriers or all even subcarriers. The modulation data is placed on the carrier, while the other sub-carrier is idle and does not transmit any data, as shown in Figure 2 in the time domain. The upper part of Fig. 2 is a structure generated by using even subcarriers, and there are two identical subparts A. The lower part of Fig. 2 is a structure generated by using odd subcarriers. The latter part AI is the opposite value of the previous part A. .

The following time-domain processing is performed at the receiving end:

${\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; r</span>}}^{<span\; class="notranslate">\; *</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; m</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

r represents the received time-domain symbol sample value, and D is half the length of the OFDM symbol not including the cyclic prefix (CP) part. Utilizing the correlation between symbols, the starting position of packet data can be judged according to the magnitude of mn.

Since only the OFDM symbol is idle and the even (odd) carrier is useless, that is, only odd subcarriers or only even subcarriers are used, and other data symbols generally occupy all available subcarriers, so only this symbol in the time domain A special repetitive structure is formed at the place (as shown in Figure 2), and the formed decision value Mn will form a platform with the highest value, corresponding to the non-multipath interference part in the CP part of the symbol, and the synchronous position of the decision should be located in this area Inside.

When the synchronization position is determined, the frequency error of the transceiver can be preliminarily estimated by using the following formula:

Φ＝angle(c _n ) When even subcarriers are used

Φ＝angle(c _n )-π When odd subcarriers are used

at the same time

Φ＝πTΔf+2nπ

so

$\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;T</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}$

Therefore, by detecting the phase of Cn, the first part of the frequency error in the above formula can be detected and compensated, which is generally called fractional frequency offset compensation.

However, since n may have different values, it is impossible to determine whether the range of frequency error exceeds the case of n=0, so the OFDM symbol will be extracted from the data stream from the synchronization position by using the symbol, and the CP part will be removed to perform FFT transformation to solve The symbol is shifted and correlated with the known transmission sequence to find the corresponding maximum correlation peak value. At this time, the corresponding shift value is the corresponding n value, thereby completing integer frequency offset compensation.

"3GPP, TR25.814 v0.41 "Physical Layer Aspects for Evolved UTRA (release 7)" provides various technical solutions for the long-term evolution (Long Term Evolution, LTE) after W-CDMA. Among them, the technical solutions based on OFDM The most eye-catching. Its frame structure is shown in Figure 3. Each frame is 10ms, and one frame contains 20 subframes with a length of 0.5ms. According to the length of the CP, each subframe includes 6 long CPs or 7 short CPs symbols. The first or second symbol of each subframe is the position where the reference symbol is placed. The function of the reference symbol is to perform channel quality measurement, channel estimation and can assist cell search and synchronization. Figure 4 provides a reference symbol example of.

Generally, a dedicated synchronization symbol needs to be added in the system to complete cell search, time synchronization and frequency synchronization. How to apply the existing synchronization methods to the LTE system, so as to achieve good synchronization and frequency offset capture performance while ensuring low system overhead is a topic that needs to be studied.

【Content of invention】

The purpose of the present invention is to provide a channel synchronization method that adopts OFDM technology to realize time synchronization and carrier offset of LTE system, and the synchronization method can be used to ensure synchronization at the same time, and the frequency offset capture has good performance and low overhead.

The purpose of the present invention is achieved through the following technical solutions: a channel synchronization method based on OFDM technology, applied in the LTE system, the subframe structure of the LTE system includes a synchronization symbol and several reference symbols, the synchronization The method includes the following steps: placing the reference symbol in the first symbol of each subframe, placing the synchronization symbol in the second symbol of the first subframe, both the reference symbol and the synchronization symbol are located in the first subframe, the reference symbol and the synchronization symbol Symbols are located in adjacent positions, wherein, all subcarriers of synchronization symbols and reference symbols are divided into two types: odd subcarriers and even subcarriers according to the odd-even sequence, and only one type of all subcarriers is used to place modulation data, while the other type The sub-carriers are idle and do not transmit any data, so as to form a continuous repetition structure in the time domain at the intersection of the reference symbol and the synchronization symbol.

Wherein, the reference symbol and the synchronization symbol only modulate data at odd subcarrier positions, and the remaining subcarriers are idle and do not transmit any data.

Wherein, the reference symbol and the synchronization symbol only modulate data at even subcarrier positions, and the remaining subcarriers are idle and do not transmit any data.

Wherein, the contents of the reference symbols conveyed by the reference symbols forming a repeated continuous structure may be the same as other reference symbols.

Wherein, the frequency of synchronous symbol transmission is set according to the specific requirements of the system.

Compared with the previous technology, the present invention has the following advantages: the reference symbol and the synchronization symbol are placed together for transmission and have the characteristics of time domain repetition at the same time, other data symbols occupy all available subcarriers, and only in the time domain A special continuous repetition structure is formed. Since the two symbols meet the repetition structure in the time domain at the same time, the ability to capture synchronization and frequency offset is enhanced, so that the starting position of the subframe can be easily found. In addition, reference symbols always need to be transmitted, directly using reference symbols to assist the synchronization process, no need to add additional synchronization symbols to achieve good performance, and reduce the overhead of synchronization channels.

【Description of drawings】

Fig. 1 is a symbol structure adopted by a traditional synchronization scheme.

FIG. 2 is a schematic diagram of a representation structure in the time domain using the symbol structure described in FIG. 1 .

Fig. 3 is a schematic diagram of an OFDM frame structure in an LTE system.

FIG. 4 is a schematic diagram of reference symbols in a subframe in an LTE system.

FIG. 5 is a synchronous design method of a synchronous channel in the present invention.

【Detailed ways】

The core idea of the present invention is to combine the design of reference symbols and synchronization symbols in the LTE system to capture synchronization and frequency offset. Wherein, the function of the synchronization symbol is to perform initial time synchronization and frequency synchronization. Put the synchronization symbol at the adjacent position of a reference symbol for transmission. At this time, all the subcarriers of the synchronization symbol and the reference symbol will be divided into two types: odd subcarrier and even subcarrier according to the odd-even sequence, and only one of them is used. For example, all odd sub-carriers or all even sub-carriers are used to place modulated data, while the other type of sub-carriers are idle and do not transmit any data. Wherein, the content of the reference symbol conveyed by the reference symbol at this position may be the same as that of other reference symbols without being affected. It just needs to empty the location originally used to transfer data, so the increased overhead is not large. The frequency of synchronous symbol transmission is set according to the specific requirements of the system. Two kinds of symbols are placed together for transmission and have the characteristics of time-domain repetition. Other data symbols generally occupy all available subcarriers, so only a special continuous repetition structure is formed at this place in the time domain. Since the two symbols satisfy the repetitive structure in the time domain at the same time, the ability to capture synchronization and frequency offset is enhanced, so that the starting position of the subframe can be easily found. In addition, the reference symbols always need to be transmitted, directly using the reference symbols to assist the synchronization process, no need to add additional synchronization symbols to achieve good performance, and reduce the overhead of the synchronization channel.

The technical content of the present invention will be further described in detail below in conjunction with the embodiments.

Please refer to FIG. 5 , which shows the synchronization design method of the synchronization channel of the present invention. In the LTE system, in the frame 100 of 10 ms, there are 20 subframes of 0.5 ms, which are respectively the first subframe 1, the second subframe 2, ..., the nineteenth subframe 19, the Twenty subframes 20, and each subframe includes several symbols.

According to the channel synchronization method of the present invention, in the LTE system, all subcarriers of synchronization symbols and reference symbols are divided into two types: odd subcarriers and even subcarriers according to the odd-even sequence. The reference symbol 31 is located in the first symbol of each subframe, and the synchronization symbol 40 is located in the second symbol of the first subframe 1, so that the reference symbol 31 and the synchronization symbol 40 are both in the first subframe 1, and the reference symbol 31 and The synchronization symbol 40 is located in an adjacent position, and the reference symbol 31 and the synchronization symbol 40 in the first subframe 1 only modulate data at the odd subcarrier position, or only modulate data at the even subcarrier position, and the remaining subcarriers are idle. pass any data. In this way, at the beginning of each frame, the reference symbol 31 and the synchronization symbol 40 have a repeating structure as shown in FIG. 2 in the time domain. Timing synchronization and fractional frequency offset can be obtained according to this repeating structure, and the reliability of the decision result is increased due to the existence of two repeating structures. At the same time, the positions of the reference symbol 31 and the synchronization symbol 40 are at the beginning of the frame, so this structure can also obtain frame timing and subframe timing at the same time. Since the reference symbol 31 and the synchronization symbol 40 satisfy the repetition structure in the time domain at the same time, the ability to capture synchronization and frequency offset is enhanced, so that the start position of the subframe can be easily found. In addition, reference symbols always need to be transmitted, directly using reference symbols to assist the synchronization process, no need to add additional synchronization symbols to achieve good performance, and reduce the overhead of synchronization channels.

The above are only the best examples of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention within.

Claims (5)

1. channel synchronization method based on the OFDM technology, be applied in the LTE system, comprise synchronous symbol and some reference symbols in the subframe structure of described LTE system, described method for synchronous may further comprise the steps: first symbol that reference symbol is placed first subframe, synchronizing symbol places second symbol of first subframe, reference symbol and synchronizing symbol all are arranged in first subframe, reference symbol and synchronizing symbol are positioned at position adjacent, wherein, whole subcarriers of synchronizing symbol and reference symbol are divided into strange subcarrier and even subcarrier two classes by the odd even preface, only utilize the whole subcarriers of a class wherein to place modulating data, and the another kind of subcarrier free time does not pass any data, is formed on continuous repetitive structure on the time domain with the intersection at reference symbol and synchronizing symbol.

2. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, only at strange sub-carrier positions modulating data, the remaining subcarrier free time does not pass any data for reference symbol and synchronizing symbol.

3. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, reference symbol and synchronizing symbol are only at even sub-carrier positions modulating data, and the remaining subcarrier free time does not pass any data.

4. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, it is identical with other reference symbols to form the reference symbol content that the reference symbol of continuous repetitive structure transmits.

5. the channel synchronization method based on the OFDM technology as claimed in claim 1, wherein, the frequency of synchronizing symbol transmission is set according to the specific requirement of system.

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