CN1599298A - OFDM frequence synchronous method at multi-path channel - Google Patents
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Abstract
The invention provides a method of using the OFDM frequency synchronization of the PN sequence in the multi path channels. The transmitting terminal extends a PN sequence cycle to form the training sequence, carries out the point-to-point weight number overlapping with the OFDM original data sequence and sends it out after being added the protection prefix. The receiving terminal adopts one of the following three methods to carry out the frequency synchronization according to the weight number of the training sequence: A) use the receiving sequence and local PN sequence to calculate the mutuality, the difference mutuality to get and compensate the preliminary frequency deviation estimated value and make use of the circle prefix to calculate and compensate the secondary frequency deviation estimated value; B) use the receiving sequence to calculate the difference mutuality directly to get and compensate the frequency deviation estimated value; and C) on the basis of the B), use the circle prefix to calculate and compensate the secondary frequency deviation estimated value.
Description
Technical Field
The invention belongs to the field of wireless communication or wired communication, and particularly relates to an OFDM synchronization method.
Background
The OFDM technology has the advantages of high data transmission rate, strong multipath interference resistance, high spectrum efficiency, and the like, and thus is receiving more and more attention. It has been successfully used for wired, wireless communications. Such as: DAB (digital Audio broadcasting), DVB, EEE802.11a and HyperLAN/2 are also related to OFDM technology in IEEE 802.16. OFDM, a new modulation technique, is also used in new generation mobile communication systems. The OFDM technology can greatly improve the transmission data rate and the spectrum efficiency of a new generation mobile communication system, and has good multipath resistance.
One of the weaknesses of OFDM technology is that the requirements for time and frequency synchronization, in particular frequency synchronization, are much higher than for single carrier systems. Systems employing OFDM techniques are typically required to have a frequency offset at the receiving end of no more than 2% of their subcarrier spacing.
OFDM synchronization is divided into time synchronization and frequency synchronization. The location of the synchronization module is shown in module 11 in fig. 1. The purpose of OFDM frequency synchronization is to estimate and compensate for frequency offset between transceiving.
Let N be the FFT length in OFDM systems, NgObtaining a received sequence gamma n for the cyclic prefix length of an OFDM symbol through time synchronization]Starting point estimation value of middle OFDM symbolThen, there are two conventional frequency offset estimation methods:
1) using the cyclic prefix of the OFDM symbol, frequency offset estimation is performed according to equation (1) (as shown in fig. 2):
the disadvantage of this synchronization method is that the frequency offset estimation range is small, and only has positive and negative 1/2 sub-carrier intervals.
2) See the document "Time and frequency synchronization for OFDM using PN-sequence preambles" (OFDM Time-frequency synchronization method using PN preamble sequences, Tufvesson, f.edfors, o., Faulkner, m., Vehicular Technology Conference, 1999.VTC 1999-fall. ieee VTS 50th, Volume: 4,1999, Page(s): 2203-2207), the originating terminal repeatedly places the PN sequence with length of K as m [ N ] N ∈ [0, K-1] in OFDM symbol to form training sequence with length of N t [ N ] (as shown in FIG. 3). Accordingly, the receiving end performs correlation calculation on the received signal and the known PN sequence m [ n ] according to the formula (2), and obtains a frequency offset estimation value (as shown in fig. 4):
wherein,is a PN sequencem[n]In the training sequence t [ n ]]Number of full repeats. P is a parameter for adjusting the frequency synchronization accuracy and range, and is generally equal to 1. That is, the receiving end performs correlation operation on the received sequence and the local sequence, and then performs differential correlation, thereby obtaining a frequency offset estimation value.
The weaknesses of this frequency synchronization method are: in a multipath channel, a received sequence contains multipath components, but due to the correlation characteristics of the PN sequence, only the component of the strongest path can be used for frequency offset estimation, and thus the frequency synchronization accuracy is not high in a multipath fading channel.
Disclosure of Invention
The invention aims to provide an OFDM frequency synchronization method using PN sequences under a multipath channel, which can provide higher frequency synchronization precision under the multipath channel environment by using multipath information in the multipath channel.
The invention is an OFDM frequency synchronization method under multipath channel, the transmitting end extends a PN sequence period to form a training sequence, and carries on point-to-point weighted superposition with OFDM original data sequence, adds protective prefix and sends out, characterized in that after the receiving end gets OFDM time synchronization, according to the different weight of the transmitting end training sequence, it can adopt one of the following three methods to carry on frequency synchronization: A) when the sequence sent by a sending end is not a training sequence completely, a receiving sequence and a local PN sequence are used for calculating correlation, the result is calculated for differential correlation again to obtain a first frequency offset estimation value, and after the first frequency offset estimation value is compensated, the cyclic prefix is used for calculating the differential correlation to obtain and compensate a second frequency offset estimation value; B) when the sequence sent by the sending end is completely the training sequence, the receiving sequence is used for directly calculating the differential correlation (as shown in figure 5), thereby obtaining and compensating the frequency offset estimation value; C) when the sequence transmitted by the transmitting end is completely the training sequence, the receiving sequence is used to directly calculate the differential correlation (as shown in fig. 5) so as to obtain and compensate the first frequency offset estimation value, and then the cyclic prefix is used to calculate the differential correlation so as to obtain and compensate the second frequency offset estimation value.
Namely, the method of the present invention comprises the steps of:
firstly, a transmitting end:
the processing steps of the transmitting end to the transmitting signal are as follows (as shown in fig. 6):
1) let the PN sequence of length K be m [ n ]]n∈[0，K-1]， <math> <mrow> <mi>m</mi> <mo>[</mo> <mi>n</mi> <mo>]</mo> <mo>∈</mo> <mo>{</mo> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>,</mo> <mfrac> <mrow> <mo>-</mo> <mn>1</mn> </mrow> <msqrt> <mn>2</mn> </msqrt> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>}</mo> <mo>,</mo> </mrow> </math> Will be provided with
And m [ N ] is extended to FFT point number N according to the formula (3) to obtain a training sequence t [ N ]:
t[n]＝m[n mod K] n∈[0，N-1] (3)
2) and performing point-to-point weighted superposition on the training sequence t [ N ] and a normalized OFDM original data sequence d [ N ] with the length of N points according to a formula (4) to obtain a sequence d' [ N ]:
wherein, the power of the sequences t [ n ] and d [ n ] is 1, and rho represents the power ratio of the training sequence in the transmitting sequence.
3) And adding an OFDM cyclic prefix according to the formula (5) by using the sequence d' [ n ], and forming an OFDM symbol s [ n ]:
4) s [ n ] is sent out (s [ n ] is constructed as shown in FIG. 7).
Second, receiving end
The receiving end carries out time synchronization to obtain the starting point of the OFDM symbol
The invention is then characterized in that it also comprises a reception sequence r [ n ]]The frequency synchronization processing step (shown in fig. 8):
(A) when the originating sn is not completely the training sequence y n (i.e. 0 < rho < 1), the following steps are used for frequency synchronization:
1) will receive the sequence gamma n]With local PN sequence m [ n ]]Calculating differential correlation according to the formula (6) to obtain a first frequency offset estimation value
2) Using first frequency offset estimateCompensating the sequence gamma n according to formula (7)]To obtain the sequence gamma1[n]：
3) Using cyclic prefix, sequence gamma1[n]Calculating the difference correlation according to the formula (8) to obtain the second frequency offset estimation value
4) Using second frequency offset estimateCompensating the sequence gamma according to the formula (9)1[n]To obtain the sequence gamma2[n]：
5) Will sequence gamma2[n]To the back end.
(B) When the originating s [ n ] is completely the training sequence t [ n ] (i.e., ρ ═ 1), the following steps may be used for frequency synchronization:
1) will receive the sequence r [ n ]]Directly calculating differential correlation according to the formula (10) to obtain a frequency offset estimation value
2) Using frequency offset estimation
Compensating the sequence gamma n according to the formula (11)]To obtain the sequence gamma1[n]：
3) Will sequence r1[n]To the back end.
(C) When the originating s [ n ] is completely the training sequence t [ n ] (i.e., ρ ═ 1), the following steps may be used for frequency synchronization:
1) will receive the sequence gamma n]Directly calculating differential correlation according to the formula (12) to obtain a first frequency offset estimation value
2) Using first frequency offset estimateCompensating the sequence r [ n ] according to the formula (13)]To obtain the sequence gamma1[n]：
3) Using cyclic prefix, sequence gamma1[n]Calculating the differential correlation according to the formula (14) to obtain a second frequency offset estimation value
4) Using second frequency offset estimate
Compensating the sequence gamma according to the formula (15)1[n]To obtain the sequence gamma2[n]：
5) Will sequence gamma2[n]To the back end.
The innovation of the invention is that when the PN sequence is used for frequency offset estimation, the differential correlation calculation is carried out on the receiving sequence in whole or in part, thereby carrying out the frequency offset estimation. Under the multipath channel, the receiving sequence contains multipath components, and the processing method of the present invention can align the multipath components in the receiving sequence, so that the multipath information can be utilized fully, and the frequency synchronization precision is superior to that of the conventional method.
The basis of the design method is as follows:
1) when the selected PN sequence m [ n ] has excellent autocorrelation characteristics, time and frequency synchronization of OFDM is easily achieved.
2) The conventional frequency synchronization method utilizes the correlation of cyclic prefix, and the estimation range of frequency offset is smaller because the difference distance (FFT length) is larger, while the t [ n ] of the transmitting end is extended by m [ n ] period, and the difference distance of the receiving end can be the period length of PN sequence m [ n ] and is smaller than the FFT length, so the estimation range of frequency offset is larger.
3) In a multipath channel, the received sequence contains multipath components. Due to the characteristics of the PN sequence, the second conventional frequency synchronization method can only align with the strongest path component in the received sequence, and suppress other path components as interference, so that only the strongest path component can be utilized. At least part of the three frequency synchronization methods provided by the invention uses the receiving sequence to carry out differential correlation, so that the multipath components of the receiving sequence are aligned and used, and the frequency synchronization precision is greatly improved compared with the prior method.
The invention has the following characteristics:
1. the training sequence of the transmitting end is formed by a certain PN sequence period continuation.
2. The receiving end performs frequency synchronization after acquiring time synchronization.
3. When the method (A) is used for frequency synchronization, the receiving sequence is correlated with the local PN sequence, the training sequence component is extracted, then differential correlation is carried out, the first frequency offset estimation value is obtained and then compensated, and then the cyclic prefix of the OFDM symbol is used for carrying out differential correlation calculation, thereby estimating and compensating the second frequency offset estimation.
4. When the method (B) is used for frequency synchronization, the correlation is not made with the local PN sequence, but is made differentially within the received sequence, thereby obtaining and compensating the frequency offset estimation value.
5. When the method (C) is used for frequency synchronization, frequency offset estimation and compensation are performed more than the method (B) once, and the frequency estimation using the cyclic prefix has an advantage of high accuracy, so that the final frequency offset estimation has high accuracy.
The invention has the following advantages:
1. compared with the original method, the frequency synchronization method provided by the invention greatly improves the frequency estimation precision compared with the prior method because of using the multipath signals in the multipath channel;
2. the invention provides three frequency synchronization methods, which can be flexibly selected according to different conditions, thereby improving the flexibility of selecting the frequency synchronization method;
3. the frequency synchronization method (A) provided by the invention has the advantages of large frequency estimation range and high precision because the first frequency offset estimation is firstly carried out and compensated, and then the cyclic prefix is utilized to carry out the second coarse frequency offset estimation;
4. according to the frequency synchronization methods (B) and (C) provided by the invention, since the receiving sequence does not need to be firstly correlated with the local PN sequence when the frequency offset estimation is carried out, the calculation cost can be saved, and the calculation speed is increased;
5. the frequency synchronization methods (B) and (C) provided by the invention select proper PN sequence length and parameter P, and can simultaneously achieve the advantages of large frequency estimation range and high precision;
6. since the receiving end is known to the training sequence signal, it can also be used for channel estimation or other purposes.
Drawings
FIG. 1 is a block diagram of a conventional OFDM system
In the figure, 11 is a synchronization module.
FIG. 2 is a block diagram of a frequency offset estimation method in the first conventional frequency synchronization method
In the figure, γ [ n ]]For received sequences, N is the number of FFT points of the OFDM system, z-NIndicating delay N point, (.)*Representing taking the conjugate value, arg (-) representing finding the phase,representing the frequency offset estimate.
FIG. 3 is a schematic diagram of the structure of training sequence for synchronization
In the figure, the training sequence is obtained by extending the period of the PN sequence with length of K, the total length is FFT point number N, wherein the last PN sequence may not be complete, TsRepresenting the duration of each point in each sequence and T representing the OFDM symbol duration before the cyclic prefix is added.
FIG. 4 is a block diagram of a frequency offset estimation method in the second conventional frequency synchronization method
In the figure, m [0 ]]，m[1]，...，m[K-1]For PN sequence data, z-K·PRepresenting the K.P point of delay, K.P +1 the K.P +1 point of delay, it can be seen that the received sequence γ [ n ]]Firstly, carrying out correlation operation with the local sequence, and then carrying out differential correlation operation.
FIG. 5 is a block diagram of the frequency offset estimation method in the frequency synchronization methods (B) and (C) of the present patent
In the figure, it can be seen that the received sequence does not correlate with the local sequence, but directly performs the differential correlation operation itself.
FIG. 6 is a flow chart of originating signals
In the figure, ρ represents the ratio of the power occupied by the training sequence in the transmission sequence, and whether ρ is equal to 1 or not can be selected to make the transmission sequence be the training sequence completely or be the superposition of the training sequence and the OFDM original data sequence.
FIG. 7 is a diagram illustrating a configuration of a transmission sequence
In the figure, the transmission sequence is formed by point-to-point weighted superposition of a training sequence and an OFDM original data sequence, cyclic prefixes 22 and 23 are copies of 26 and 27, and T isgRepresenting the duration of the cyclic prefix.
FIG. 8 is a flow chart of the receive signal processing
In the figure, ρ represents the power ratio of the training sequence in the transmission sequence, the system performs the frequency synchronization operation after completing the time synchronization, and the frequency synchronization method can use the method (a), (B) or (C).
Detailed Description
The following provides a specific implementation method of the present patent under OFDM configuration.
Let FFT length be N4096 and cyclic prefix length be N in OFDM systemg1024. The PN sequence has a period of K127, and is selectedThe power ratio occupied by the training sequence in the transmission sequence is ρ 0.5.
Firstly, a transmitting end:
the m sequence with the period of K-127 is recorded as m [ n ]] n∈[0，126]， <math> <mrow> <mi>m</mi> <mo>[</mo> <mi>n</mi> <mo>]</mo> <mo>∈</mo> <mo>{</mo> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>,</mo> <mfrac> <mrow> <mo>-</mo> <mn>1</mn> </mrow> <msqrt> <mn>2</mn> </msqrt> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>}</mo> <mo>.</mo> </mrow> </math> M [ n ] is]Forming training sequence t [ n ] according to formula (3) cycle continuation]Then m [ n ]]In the training sequence t [ n ]]The number of repetitions in (1) is:
since ρ is 0.5, the weight corresponding to the training sequence is <math> <mrow> <msqrt> <mi>ρ</mi> </msqrt> <mo>=</mo> <msqrt> <mn>0.5</mn> </msqrt> <mo>=</mo> <mfrac> <msqrt> <mn>2</mn> </msqrt> <mn>2</mn> </mfrac> <mo>,</mo> </mrow> </math> The weight corresponding to the original OFDM data sequence is <math> <mrow> <msqrt> <mn>1</mn> <mo>-</mo> <mi>ρ</mi> </msqrt> <mo>=</mo> <msqrt> <mn>1</mn> <mo>-</mo> <mn>0.5</mn> </msqrt> <mo>=</mo> <mfrac> <msqrt> <mn>2</mn> </msqrt> <mn>2</mn> </mfrac> <mo>.</mo> </mrow> </math> According to the above weight and the formulas (4) and (5), training sequence t [ n ] is used]And normalizing the OFDM raw data sequence d [ n ]]Form OFDM symbols s [ n ]]And sending out.
Second, receiving end
The receiving end firstly acquires time synchronization and then performs frequency synchronization. Because the rho is 0.5 < 1 for the originating selectionThis frequency synchronization is performed according to method (a), specifically: firstly, receiving sequence gamma [ n ]]Performing a first frequency offset estimation according to the formula (6), performing a first frequency offset compensation according to the formula (7), performing a second frequency offset estimation according to the formula (8), performing a second frequency offset compensation according to the formula (9), and sending a frequency-synchronized sequence gamma to the back end2[n]。
Claims (1)
1. A OFDM frequency synchronization method under multipath channel, the transmitting end extends a PN sequence period to form a training sequence, and carries on point-to-point weighted superposition with OFDM original data sequence, and sends out after adding protective prefix, characterized in that after the receiving end gets OFDM time synchronization, according to the different weight of the transmitting end training sequence, it can adopt one of the following three methods to carry on frequency synchronization: A) when the sequence sent by a sending end is not a training sequence completely, a receiving sequence and a local PN sequence are used for calculating correlation, the result is calculated for differential correlation again to obtain a first frequency offset estimation value, and after the first frequency offset estimation value is compensated, the cyclic prefix is used for calculating the differential correlation to obtain and compensate a second frequency offset estimation value; B) when the sequence sent by the sending end is completely the training sequence, the receiving sequence is used for directly calculating the differential correlation, thereby obtaining and compensating the frequency offset estimation value; C) when the sequence sent by the sending end is completely the training sequence, the receiving sequence is used for directly calculating the differential correlation so as to obtain and compensate the first frequency offset estimation value, and then the cyclic prefix is used for calculating the differential correlation so as to obtain and compensate the second frequency offset estimation value.
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| CN101874392A (en) * | 2007-11-30 | 2010-10-27 | Nxp股份有限公司 | Pn phase recovery in a DMB-T system |
| CN101188447B (en) * | 2006-11-15 | 2011-08-03 | 华为技术有限公司 | A method and device for carrier frequency deviation estimation |
| CN101345549B (en) * | 2008-08-29 | 2011-11-30 | 北京天碁科技有限公司 | Frequency deviation estimation method and apparatus used for time division-synchronous code division multiple access system |
| CN101312444B (en) * | 2007-05-24 | 2012-06-20 | 佳世达科技股份有限公司 | Method and apparatus for acquiring frequency of multi-path signal |
| CN101534184B (en) * | 2008-03-11 | 2012-07-04 | 卓胜微电子（上海）有限公司 | Sampling frequency offset estimation method |
| CN103037402A (en) * | 2011-09-29 | 2013-04-10 | 鼎桥通信技术有限公司 | Time delay detection method and device of antenna calibration link |
| CN1996791B (en) * | 2006-01-06 | 2015-09-09 | 上海原动力通信科技有限公司 | A kind of downlink synchronization method of broadband time and division duplex cellular system |
| CN105516045A (en) * | 2015-12-04 | 2016-04-20 | 中国科学院上海微系统与信息技术研究所 | OFDM (Orthogonal Frequency-Division Multiplexing) training sequence construction method and synchronization method |
| CN110086738A (en) * | 2019-05-06 | 2019-08-02 | 深圳市中科汉天下电子有限公司 | A kind of carrier frequency bias estimation and system |
| WO2019205925A1 (en) * | 2018-04-28 | 2019-10-31 | 华为技术有限公司 | Communication method and communication apparatus |
| CN113746767A (en) * | 2021-08-31 | 2021-12-03 | 中国电子科技集团公司第五十四研究所 | Frequency selective fading compensation method and device in broadband data transmission |
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| CN1996791B (en) * | 2006-01-06 | 2015-09-09 | 上海原动力通信科技有限公司 | A kind of downlink synchronization method of broadband time and division duplex cellular system |
| CN101188447B (en) * | 2006-11-15 | 2011-08-03 | 华为技术有限公司 | A method and device for carrier frequency deviation estimation |
| CN101312444B (en) * | 2007-05-24 | 2012-06-20 | 佳世达科技股份有限公司 | Method and apparatus for acquiring frequency of multi-path signal |
| CN101874392A (en) * | 2007-11-30 | 2010-10-27 | Nxp股份有限公司 | Pn phase recovery in a DMB-T system |
| CN101534184B (en) * | 2008-03-11 | 2012-07-04 | 卓胜微电子（上海）有限公司 | Sampling frequency offset estimation method |
| CN101345549B (en) * | 2008-08-29 | 2011-11-30 | 北京天碁科技有限公司 | Frequency deviation estimation method and apparatus used for time division-synchronous code division multiple access system |
| CN103037402A (en) * | 2011-09-29 | 2013-04-10 | 鼎桥通信技术有限公司 | Time delay detection method and device of antenna calibration link |
| CN105516045A (en) * | 2015-12-04 | 2016-04-20 | 中国科学院上海微系统与信息技术研究所 | OFDM (Orthogonal Frequency-Division Multiplexing) training sequence construction method and synchronization method |
| CN105516045B (en) * | 2015-12-04 | 2018-07-24 | 中国科学院上海微系统与信息技术研究所 | A kind of OFDM training sequence structures and synchronous method |
| WO2019205925A1 (en) * | 2018-04-28 | 2019-10-31 | 华为技术有限公司 | Communication method and communication apparatus |
| CN110086738A (en) * | 2019-05-06 | 2019-08-02 | 深圳市中科汉天下电子有限公司 | A kind of carrier frequency bias estimation and system |
| CN110086738B (en) * | 2019-05-06 | 2021-07-27 | 深圳昂瑞微电子技术有限公司 | Carrier frequency offset estimation method and system |
| CN113746767A (en) * | 2021-08-31 | 2021-12-03 | 中国电子科技集团公司第五十四研究所 | Frequency selective fading compensation method and device in broadband data transmission |
| CN113746767B (en) * | 2021-08-31 | 2022-08-02 | 中国电子科技集团公司第五十四研究所 | Frequency selective fading compensation method and device in broadband data transmission |
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