CN102377726A - Timing synchronization method of OFDM (Orthogonal Frequency Division Multiplexing) system - Google Patents

Abstract

The present invention relates to a timing synchronization method of an OFDM system. The method comprises the following steps: 1) generating a random sequence A at the sending end of the OFDM system, and outputting a record sequence B in reverse to form a symmetrical structure training sequence S=[AB]; 2) Establish the required OFDM baseband signal, use the sequence generated in step 1 as the training sequence, add it before the data to be sent, and send it into the channel; 3) The signal received by the receiving end of the OFDM system is synchronized in the time domain. The timing synchronization method can improve the accuracy and stability of timing synchronization without bringing obvious increase in calculation amount, and is suitable for complex multipath fading channels.

Description

Timing Synchronization Method for OFDM System

technical field

The invention relates to the technical field of communication, in particular to a timing synchronization method based on an OFDM system.

Background technique

Modern communication systems widely use Orthogonal Frequency Division Multiplexing (OFDM) technology as one of the physical layer standards. The basic principle of OFDM is to convert a serial high-speed data stream into a low-speed sub-data stream, and use a group of sub-carriers orthogonal to each other to transmit data. The frequency spectrum of the sub-channels is orthogonal and overlaps each other, so the OFDM spectrum utilization rate is high.

Because of converting serial data into low-speed parallel data, OFDM systems can effectively combat the multipath effects of wireless channels, but it requires more precise system synchronization than single-carrier communication. Time synchronization errors will not only distort the amplitude and phase of the signal, but also cause interference between OFDM symbols, introduce ISI (Inter-Symbol Interference, inter-symbol interference), and destroy the integrity of the symbols, thereby reducing the significant performance of the system. decline. Traditional OFDM timing synchronization usually adds a training sequence with a special structure before the data frame, and the starting point of timing synchronization is obtained by taking the peak value through the autocorrelation operation of the input signal at the receiving end. This method can achieve good results in non-multipath and multipath environments where the first path is the strongest path, but in harsh wireless channel environments where the first path is not the strongest path, it often causes timing delays. Some people propose to sum the results of correlation operations and search for the maximum value of the correlation sum as a timing measure. This method needs to accurately determine the size of the search window, otherwise it will also cause timing inaccuracy.

Contents of the invention

The purpose of the present invention is to propose a timing synchronization method for an OFDM system to improve the accuracy of OFDM system timing synchronization under a multipath channel, aiming at the problem that the existing timing synchronization technology of an OFDM system under a complex multipath channel is likely to cause a timing backward shift. sex and stability.

Technical scheme of the present invention is:

A timing synchronization method for an OFDM system, the method comprising the steps of:

1) Generate random sequence A at the sending end of the OFDM system, and record sequence B according to its reverse output to form a symmetrical structure training sequence S=[A B];

2) Establish the required OFDM baseband signal, the training sequence adopts the sequence generated in step 1, add it before the data to be sent, and send it into the channel;

3) The signal received by the receiving end of the OFDM system performs timing synchronization in the time domain.

Further, in the step 1), the specific length of the training sequence can be determined according to the length of the OFDM symbol to be used.

Further, the OFDM baseband signal in step 2) adopts a multipath channel model.

Further, the timing synchronization in the step 3) can be specifically the following steps:

Step 101: when performing timing synchronization, start from the training sequence position of a certain symbol in the received OFDM signal, and take data segments with a length of L forward and backward to perform correlation operations, and L is half of the length of the training sequence in step 1; at the same time Use the energy of the received signal to normalize the results of the correlation operation, so that a calculation metric M(t) can be obtained for each sampling point, where t is the number of sampling points;

式中，需要确定搜索窗口W，W应约为信道的最大时延；Step 102: Define the correlation sum value V(t) of M(t),

In the formula, the search window W needs to be determined, and W should be approximately the maximum delay of the channel;

Step 103, detect the first falling edge of V(t), that is, the timing position should be the first moment that satisfies the inequality V(t+1)-V(t)≥T _h , where _Th is defined as The detection threshold should be greater than the mean value of channel noise and less than the peak value of the strongest multipath signal.

The advantage of the present invention is that, in the timing synchronization of an OFDM receiving system, detecting the first falling edge of the timing measurement instead of directly taking the peak value can improve the accuracy and stability of the timing synchronization without bringing about an obvious increase in the amount of computation, and Suitable for complex multipath fading channels.

Description of drawings

Figure 1 is a general OFDM system link-level model.

Fig. 2 is a data frame structure adopted by the present invention, which is composed of a training sequence and a data stream.

Fig. 3 is a schematic diagram of specific steps of timing synchronization in the present invention.

Fig. 4 is an effect diagram of determining the size of the search window by using the cross-correlation operation.

Fig. 5 is a schematic diagram of performing a summation operation on M(t).

Fig. 6 is a comparison of the effects of the method of the present invention and the traditional method in a complex multipath environment.

Detailed ways

The implementation examples are described in detail below in conjunction with the accompanying drawings. It should be noted that the following descriptions are only exemplary, and are not intended to limit the present invention.

Figure 1 is a block diagram of a general OFDM system structure, which includes the basic modules that a complete OFDM system should have.

Step 1: The transmitter sets the number of subcarriers of the OFDM system to 1024, the cyclic prefix to 128 bits, and the mapping method to select QPSK. The training sequence consists of a 128-bit random sequence and its reverse sequence to form a symmetrical structure, denoted as T. After the training sequence is added to each frame of data, the frame structure shown in Figure 2 is formed and sent to the channel for transmission.

Step 2: After the signal passes through the multipath fading channel, it will be affected by the interference of multipath effect and Gaussian noise. The number of paths in the multipath channel is 4, and the energy of the first path is smaller than that of the strongest path. The signal received by the receiving end is the result of superposition of the 4 paths after the corresponding delay and power attenuation respectively, which is denoted as r( t).

进行自相关运算，得到相关度量M(t)，其中

L为训练序列的长度。同时利用已知的训练序列T和接收信号r(t)进行互相关运算，

计算得到的互相关系数t_s会在多径延迟分布的采样点出现峰值，可以粗略的体现出多径的能量分布。记录最后一个大于阈值T_h的t_s值得位置可以近似等于信道的最大时延，阈值的选取可以根据实际情况确定，一般定为t_s峰值的0.2倍。图4显示了使用这种方法得到的信道最大时延的效果。Step 3: The receiving end of the OFDM system receives the signal r(t), and it should be time-frequency synchronized before demodulation. The process of the specific timing synchronization method proposed by the present invention is shown in FIG. 3 . Use after receiving the signal

Carry out autocorrelation operation to get the correlation measure M(t), where

L is the length of the training sequence. At the same time, the known training sequence T and the received signal r(t) are used to perform cross-correlation operations,

The calculated cross-correlation coefficient t _s will have a peak at the sampling point of the multipath delay distribution, which can roughly reflect the energy distribution of the multipath. The position of recording the last t _s value greater than the threshold T _h can be approximately equal to the maximum time delay of the channel. The selection of the threshold can be determined according to the actual situation, and is generally set at 0.2 times the peak value of t _s . Figure 4 shows the effect on channel maximum delay obtained using this approach.

在搜索窗Set the calculation window W for M(t) to perform the summation operation as the maximum channel delay obtained in the previous step, after determining the search window W, perform the summation operation on M(t) within W, as shown in Figure 5, get

in the search window

The final timing synchronization result can be obtained by detecting the first falling edge of V(t) in port W. Timing synchronization result t _time should be

The first position that satisfies the inequality V(t _time +1)-V(t _time )≥T _h .

Experiments have proved that the timing accuracy and stability can be obviously improved by using the timing synchronization method proposed by the present invention compared with the traditional method. Under the same multipath fading environment, through the aforementioned specific implementation steps, the timing effect of the traditional method and the method of the present invention varies with the signal-to-noise ratio as shown in FIG. 6 . By comparison, it can be found that in such a complex multipath environment where the first path is not obvious, the timing results of the traditional method are unreliable at a small SNR, and as the SNR increases, the timing synchronization results gradually become stronger. The diameter is close, and the position of the first diameter cannot be accurately located. After the method of the invention meets the requirement of the signal-to-noise ratio, the first path can be accurately positioned without causing timing synchronization errors.

In terms of algorithm complexity, compared with the traditional method, the present invention adds an addition operation and a fixed finite number of subtraction operations on the basis of the traditional method, which does not bring about an obvious increase in the amount of calculation, but can obtain a good timing effect .

The above descriptions are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. It can be understood that the protection scope of the present invention is subject to the protection scope of the claims.

Claims (4)

1. The timing synchronization method of OFDM system is characterized in that, the method comprises the steps: 1) Generate random sequence A at the sending end of the OFDM system, and record sequence B according to its reverse output to form a symmetrical structure training sequence S=[A B]; 2) Establish the required OFDM baseband signal, the training sequence adopts the sequence generated in step 1, add it before the data to be sent, and send it into the channel; 3) The signal received by the receiving end of the OFDM system performs timing synchronization in the time domain. 2. the timing synchronization method of OFDM system according to claim 1, is characterized in that, in described step 1), the concrete length of training sequence can be decided according to the OFDM symbol length that needs to use. 3. the timing synchronization method of OFDM system according to claim 1, is characterized in that, OFDM baseband signal adopts multipath channel model in described step 2). 4. the timing synchronization method of OFDM system according to claim 1, is characterized in that, described step 3) in timing synchronization can be specifically the following several steps: Step 101: when performing timing synchronization, start from the training sequence position of a certain symbol in the received OFDM signal, and take data segments with a length of L forward and backward to perform correlation operations, and L is half of the length of the training sequence in step 1; at the same time Use the energy of the received signal to normalize the results of the correlation operation, so that a calculation metric M(t) can be obtained for each sampling point, where t is the number of sampling points; Step 102: Define the correlation sum value V(t) of M(t),

In the formula, the search window W needs to be determined, and W should be approximately the maximum delay of the channel; Step 103, detect the first falling edge of V(t), that is, the timing position should be the first moment that satisfies the inequality V(t+1)-V(t)≥T _h , where _Th is defined as The detection threshold should be greater than the mean value of channel noise and less than the peak value of the strongest multipath signal.

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