CN100471094C - OFDM time and frequency synchronizing method capable of correcting long-range frequency deviation - Google Patents
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Abstract
The invention provides a method of OFDM time frequency synchronization for correcting wide range frequency deviation, wherein the sending end dispensing the PN sequence replication according to code segment, then the sequences are repeated totally, constructing an exercise sequence, which is transmitted together with the OFDM primary data through spot-to-spot weighted superposition, the receiving end first obtains the time synchronism, then performs related calculus of differences to the received data with difference distance to be 1, the method by the invention can proceed evaluation and compensation in the bandwidth range of 1/2 OFDM system bandwidth.
Description
Technical Field
The invention belongs to the field of wireless communication or wired communication.
Background
OFDM has the advantages of high data transmission rate, strong multipath interference resistance, high spectrum efficiency, and the like, and is receiving increasing attention. It has been successfully used for wired, wireless communications. Such as: ADSL (asymmetric Digital Subscriber line), Wireless LAN, DAB (Digital Audio Broadcasting), DVB, EEE802.11a and HyperLAN/2. In IEEE802.16, which is currently being established, OFDM technology is also heavily involved. 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, co-channel interference and impact noise resistance, see the literature: bingham, j.a.c. "Multicarrier modulation for data transmission: an idea while time has come, "IEEE communications Magazine, Volume: 28 Issue: 5, May 1990. Page(s): 5-14; and literature: yun Hee Kim; iickho Song; hong Gil Kim; taejoo Chang; hyung Yung Kim, "Performance analysis of a coded OFDM system in time-varying multipath fading channels," Vehicular Technology, IEEE Transactions on, Volume: 48 Issue: 5, Sept.1999, Page(s): 1610 and 1615.
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. Generally, the frequency offset of a system adopting the OFDM technology at a receiving end does not exceed 2% of the subcarrier interval, see document van de Beek, j.j.; sandell, m.; borjesseson, P.O., "ML estimation of time and frequency offset in OFDMsystems," Signal Processing, IEEE Transactions on, Volume: 45 Issue: 7, July 1997, Page(s): 1800 classed in 1805. 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 time synchronization is to find the boundaries of each OFDM symbol in the received serial data stream; the purpose of frequency synchronization is to find and correct the frequency offset of the receiving end relative to the transmitting end.
In a system using OFDM technology, after time synchronization is achieved, frequency offset information can be calculated by using a method of calculating differential correlation, see Moose, p.h. "a technique for orthogonal frequency division multiplexing correction," Communications, IEEE transaction son, Volume: 42 Issue: 10, Oct.1994, Page(s): 2908 + 2914.
The basic principle of differential correlation is as follows (as shown in fig. 2): firstly, a transmitting end puts training sequences with the same two ends into data, and the distance between the same data is d. Then, the receiving end estimates the frequency offset by using the differential correlation value according to the following calculation formula:
where theta represents the estimated time synchronization point,denotes the estimated frequency offset value, r k]To receive a signal, m represents the length of the training sequence and d represents the differential distance.
The selection of the differential correlation distance d has a large influence on the estimation performance of the frequency offset. The smaller the difference distance d is selected, the larger the range of frequency offset estimation. See Marti i Puig, p.; alvarez, J.S. "Coarse frequency estimation in OFDM packet oriented systems," Acoustics, Speech, and Signal Processing, 2001. processing.2001 IEEEInternational Conference on, 2001, Page(s) ": 2337-2340 vol.4.
In practice, it is often desirable to be able to estimate and correct as large a range of frequency offsets as possible. Therefore, according to the above principle, it is necessary to reduce the differential distance as much as possible. The conventional solution is to place PN sequences (pseudo-random sequences) consecutively at the originating (as shown in fig. 3), and to narrow the differential distance d by choosing as small a PN sequence period as possible. Because the PN sequence needs to provide a certain spread spectrum gain to realize the anti-interference capability, otherwise, the performance of frequency synchronization is seriously affected, the period of the PN sequence needs to be selected to be very small, and the corresponding differential distance d also needs to be very small, which limits the range of frequency offset estimation which can be performed by the conventional method.
Disclosure of Invention
The invention aims to provide an OFDM time and frequency synchronization method capable of correcting wide-range frequency offset, which comprises training sequence setting of a transmitting end and corresponding processing of a receiving end.
The task of the invention is realized by that a PN sequence is repeatedly placed according to chips at a transmitting end, then the obtained sequence is integrally repeated to form a training sequence, and finally the training sequence and OFDM original data are transmitted after point-to-point weighted superposition; the receiving end firstly obtains time synchronization, then carries out differential correlation operation with the differential distance of 1 on the received data, and finally carries out estimation and compensation on frequency offset within the range of 1/2 OFDM system bandwidths.
The innovation of the invention is that when the transmitting end is repeated for the first time, the PN sequence is repeatedly placed according to the code sheet instead of the period of the PN sequence, and then the receiving end utilizes a corresponding differential correlation method to carry out frequency offset estimation and compensation on the received data, thereby obtaining the frequency synchronization of the OFDM system. Here, a chip means a data bit, and for example, the number of FFT points of the OFDM system is the number of chips of its original data. Obviously, the method overcomes the limitation of the high-order range of frequency offset of the conventional method by the period of the PN sequence, the frequency offset estimation range of the method is independent of the period of the PN sequence, so that the differential distance d can be reduced to the theoretical minimum value 1, and the corresponding frequency offset estimation range is 1/2 of the bandwidth of the OFDM system.
The invention relates to a method for OFDM time and frequency synchronization capable of correcting large-range frequency deviation, which is characterized by comprising the following steps:
one, send end
The steps of the sending end to the received signal processing are as follows (as shown in fig. 4):
1) selecting a period of NmM of (a) m sequence m [ k ]]，k∈[0，Nm-1]At this time m [ k ]]Can take the form of a complex number, i.e. m [ k ]]E.g., {1+ j, -1-j }, or can be in a real number form, i.e., m [ k ]]∈{1，-1}；
2) First iteration (as shown in fig. 5): m [ k ] is]Repeating by chip, each chip repeating NnNext, a length N is generatedmNnPN sequence m of1[k]，k∈[0，NmNn-1]The mathematical expression is as follows:
m1[k]＝m[k/Nn]k∈[0，NmNn-1] (2)
where/represents an integer division operation.
3) Second iteration (as shown in fig. 6): the above sequence m1[k]Number of complete repetitions N in training sequencerepIs N/(N)mNn) N is the number of FFT points of the OFDM system; the PN sequence m is converted into1[k]Continuously and repeatedly placing the whole to make the total number of points be N to obtain the training sequence t [ k ]]The mathematical expression is as follows:
t[k]＝m1[k mod NmNn]k∈[0，N-1] (3)
where mod represents the modulo operation.
4) After point-to-point weighted superposition is carried out on the training sequence t [ k ] and OFDM original data (not including a cyclic prefix part) according to the following formula (as shown in figure 7), the final transmitting data s [ k ] is obtained and transmitted out:
where ρ ∈ [0, 1] is a weighted value whose physical meaning is the normalized value of the energy of the training sequence with respect to the total energy of the transmitted data.
Second, receiving end
The receiving end processes the received signal as follows (as shown in fig. 8):
1) the objective function is calculated to achieve time synchronization as follows:
the time synchronization point is the value of a that maximizes the value of gamma k, a, where a is the number of points where the PN sequence placed in the received sequence slides relative to the local PN sequence.
2) Starting from the training sequence starting point with NmNnNrepReceived sequence of points is divided into NrepSegment, noted as:
ri[k]i∈[0，Nrep-1] k∈[0，NmNn-1] (6)
3) for each ri[k]Correlating with the local m sequence according to the following formula to obtain NnA correlation value
For each set of correlation values
Two adjacent correlation values
The conjugate multiplication and accumulation are carried out to obtain each group of conjugate correlation values
The above-mentioned NrepGroup conjugate correlation values
Accumulating, solving the amplitude angle, adjusting the coefficient, and obtaining a final frequency deviation estimated value as follows:
the general calculation formula is:
wherein,
denotes the estimated frequency offset value, r k]For received signals, N is the FFT length, NmIs a PN sequence m [ k ]]Length, NnIs a PN sequence m [ k ]]Number of times of chip repetition, NrepIs a sequence m1[k]The overall number of repetitions.
4) And carrying out corresponding frequency offset compensation.
The basis of the design method is as follows:
1) since the respective PN sequences are identical, the correlation between them can represent a frequency offset;
2) because the transmitting end repeats the PN sequence according to the chip when repeating the PN sequence for the first time, and the minimum interval between the same data in the training sequence is 1, the requirement of the minimum differential distance is theoretically met, and the receiving end can estimate the frequency offset in the maximum range by processing the data according to the differential distance;
3) because the PN sequence is not repeatedly placed according to the whole length when the transmitting terminal is repeated for the first time, the differential distance is irrelevant to the length of the PN sequence, the selection of the period of the PN sequence can be more flexible, and if the PN sequence with a longer period is selected, the anti-interference capability can be improved;
4) repeating the PN sequence m according to the chip1[k]Repeatedly placing NrepThe secondary purpose is to accumulate energy and improve performance;
5) to ensure accurate synchronization, the signal power of the training sequence component in the transmitted data may be increased during the acquisition phase by increasing the value of ρ, while the value of ρ is decreased during the tracking phase. Adjusting p may achieve optimal time and frequency synchronization. When rho is 1, the transmitted data is completely a training sequence signal; when ρ is 0, the transmission data is completely OFDM original data. The choice of the value of p is also one of the techniques of this patent.
Theoretical analysis proves that the method can maximize the frequency offset estimation range of the OFDM system, namely 1/2 OFDM bandwidths, and has strong practical value.
Drawings
FIG. 1 is a block diagram of a generic OFDM system
In the figure, 11 is a synchronization module;
FIG. 2 is a schematic diagram of the basic principle of differential correlation
The training data 1-m in fig. 2 are completely the same as 17 and 18, that is, any known sequence in fig. 3, and because there is only correlation between corresponding data, the differential distance can only be selected as the distance between corresponding data bits, note that the differential distance in fig. 2 is not necessarily the number m of training data, and it also depends on the number of data points between two training sequences;
FIG. 3 shows the structure of training sequence in conventional OFDM symbol
In the figure, the training sequence is composed of a plurality of identical PN sequences, n PN sequences 19, 20, 21, 22 are identical and they are placed consecutively, i.e. there is no space between two adjacent PN sequences, therefore, the minimum distance between the same data, i.e. the minimum differential distance, is the period of the PN sequence, which indicates that the differential distance is limited by the period of the PN sequence;
FIG. 4 shows the steps for generating training sequences in originating OFDM symbols described in this patent
In the figure, it can be seen that the processing steps at the starting end include two repeated processes, the first is repeated according to chips, and the second is repeated as a whole;
FIG. 5 shows the PN sequence m obtained after the first repetition of the originating terminal described in this patent1[k]Schematic structural diagram of
In the figure, the PN sequence has a period of NmPN sequenceColumn by chip repeats NnSecondly;
FIG. 6 is a schematic diagram of the structure of the training sequence t [ k ] obtained after the second repetition of the originating terminal described in this patent
In the figure, the training sequence t [ k ]]The length of (d) is the FFT point number N of the OFDM system, the PN sequence m1[k]In the training sequence t [ k ]]In complete repetition of NrepSecondly;
FIG. 7 is a schematic diagram of the position of the training sequence in the OFDM transmission symbol described in this patent
In the figure, a training sequence and OFDM original data are superposed in a point-to-point weighted manner;
FIG. 8 is a block diagram illustrating the steps of the receiving end data processing described in this patent
Detailed Description
The implementation steps of the present patent are given below in a specific OFDM configuration. Note: the parameters in the following examples do not affect the generality of this patent.
Let the useful symbol length of OFDM be 4096 and the PN sequence selection period be <math> <mrow> <msubsup> <mi>N</mi> <mi>m</mi> <mo>′</mo> </msubsup> <mo>=</mo> <mn>127</mn> </mrow></math>
M sequence of (2), number of first repetitions N n2, the second repetition number Nrep4096/(127 × 2) ═ 16. Let ρ be 0.5, which represents the energy of the training sequence in half of the transmitted data.
The transmitting end repeats the m sequence for 2 times according to the code sheet to obtain m1[k]M is1[k]The whole is repeated 17 times, the redundant data at the tail part is cut off, and a training sequence t [ k ] with the length of 4096 is formed]The training sequence t [ k ] is expressed according to equation 5]Point-to-point weighted superposition is carried out on the OFDM original data to form an OFDM transmitting symbol to be transmitted.
The receiving end firstly obtains time synchronization according to a formula 5, namely finding the starting point of an OFDM symbol, and recording OFDM useful data as d [ i ] i ∈ [0, 4095 ]; at the same time the starting point of the training sequence is also found. The frequency offset estimation begins as follows:
(1) starting from the start of the training sequence every NmNn127, 2, 254 data are divided into one segment, and N is totalrep16 segments, denoted as r0[k]～r15[k]k∈[0，253]；
(2) Correlating with the local m-sequence according to equation 7 to obtain 32 correlation values, which are recorded as
And namely:
(3) obtaining each group of conjugate correlation values according to the formula 8Namely:
(4) performing frequency offset estimation according to equation 9:
(5) the data is frequency compensated as follows:
Claims (2)
1. An OFDM time, frequency synchronization method that can correct the large-scale frequency offset, its characteristic is that the sending end puts the PN sequence repeatedly according to the chip first, then carry on the whole repetition of the sequence got, form the training sequence, launch after this training sequence and OFDM original data point-to-point weighted stack finally; the receiving end firstly obtains time synchronization, then carries out differential correlation operation with the differential distance of 1 on the received data, and finally carries out estimation and compensation on frequency offset within the range of 1/2 OFDM system bandwidths, and the method is characterized in that:
the initiating step is as follows:
step 1, selecting a period of NmM of (a) m sequence m [ k ]]，k∈[0，Nm-1]At this time m [ k ]]Taking the form of a complex number, i.e. m [ k ]]∈{1+j，-1-j}；
Step 2 is repeated for the first time: m [ k ] is]Repeating by chip, each chip repeating NnNext, a length N is generatedmNnPN sequence m of1[k]，k∈[0，NmNn-1](ii) a The mathematical expression is as follows:
m1[k]＝m[k/Nn] k∈[0，NmNn-1]
wherein,/represents an integer division operation;
step 3 is repeated for the second time: the above sequence m1[k]Number of complete repetitions N in training sequencerepIs N/(N)mNn) N is the number of FFT points of the OFDM system; the PN sequence m is converted into1[k]Continuously and repeatedly placing the whole to make the total number of points be N to obtain the training sequence t [ k ]](ii) a The mathematical expression is as follows:
t[k]＝m1[kmodNmNn] k∈[0，N-1]
wherein mod represents a modulo operation;
and 4, carrying out point-to-point weighted superposition on the training sequence t [ k ] and OFDM original data which does not contain the cyclic prefix part according to the following formula to obtain final transmitting data s [ k ] and transmitting the final transmitting data s [ k ]:
wherein rho belongs to [0, 1] as a weighted value;
the receiving end comprises the following steps:
step 5, calculating an objective function according to the following formula to obtain time synchronization:
the time synchronization point is the value of a which enables the value of gamma [ k, a ] to be maximum, and a is the number of sliding points of a PN sequence placed in the receiving sequence relative to the local PN sequence;
step 6 starts from the starting point of the training sequence with NmNnNrepReceived sequence of points is divided into NrepSegment, noted as:
ri[k]i∈[0，Nrep-1] k∈[0，NmNn-1]
step (ii) of7For each ri k]Correlating with the local m sequence according to the following formula to obtain NnA correlation value
：
Step 8 for each set of correlation values
Two adjacent correlation values
The conjugate multiplication and accumulation are carried out to obtain each group of conjugate correlation values
Step 9 treatment of the above-mentioned NrepGroup conjugate correlation values
Accumulating, solving the amplitude angle, and adjusting the coefficient, thereby obtaining a final frequency deviation estimation value:
step 10 performs corresponding frequency offset compensation.
2. The method of claim 1, wherein the weighted value p for point-to-point weighted overlap is different for different OFDM symbols, but within the same OFDM symbol, the value of p is unchanged; this weight is adjusted to make a trade-off between the effectiveness of the transmission and the reliability of the training sequence for the synchronization estimation.
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| CN100502377C (en) * | 2005-01-07 | 2009-06-17 | 北京邮电大学 | Timed and large-deviation combined estimation method suitable to burst transmission system |
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| CN101222306B (en) * | 2007-01-08 | 2012-02-01 | 上海无线通信研究中心 | Communication system and communication method with association frequency domain repetition and mixed automatic retransmission |
| CN113543303B (en) * | 2021-06-30 | 2022-10-21 | 紫光展锐(重庆)科技有限公司 | Synchronization method, synchronization device, chip and module equipment |
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