OFDM signal receiving method supporting high-speed movement

Abstract

The invention discloses an OFDM (Orthogonal Frequency Division Multiplexing) signal receiving method supporting high-speed movement, and relates to the field of signal processing. According to the method, firstly, a Doppler detection method is adopted to detect whether a channel belongs to a high-speed mobile channel or not, if not, a traditional OFDM signal receiving method is adopted to receive, if yes, a traditional OFDM signal time domain processing flow is adopted to process, then, frequency domain channel estimation and time domain channel estimation are sequentially carried out, and the number and time delay of multiple paths are calculated; and constructing a time domain channel matrix according to the number of the multiple paths and the time delay, balancing time domain data according to the time domain channel matrix, and converting the time domain data into a signal after frequency domain equalization through FFT (Fast Fourier Transform), thereby completing the reception of the OFDM signal moving at a high speed. According to the method, the demodulation idea of the OTFS is utilized, but the sending signal of the OFDM is not changed, so that the receiver can realize the OFDM signal receiving process of high-speed movement.

Description

Technical Field

[0001] The invention relates to the field of signal processing, and in particular to an OFDM signal receiving method supporting high-speed movement. Background Art

[0002] Most of the current mainstream wireless communication system standards use Orthogonal Frequency Division Multiplexing (OFDM) as the basic waveform, which can greatly reduce the implementation complexity of communication transceivers in broadband channels, but has poor performance in high-speed mobility. Ultra-high mobility is manifested as fast time-varying channel response in the time domain, which puts great pressure on the real-time and accuracy of channel estimation and pilot overhead control. In addition, ultra-high mobility is manifested as a strong Doppler effect in the frequency domain, resulting in serious inter-subcarrier interference in the OFDM system, which is difficult to deal with using traditional transceiver structures. Against the above background, R. Hadani et al. proposed a new modulation technology called "Orthogonal Time-Frequency Modulation" (OTFS) in recent years. The main idea of ​​OTFS is to introduce the Delay-Doppler domain to transform the fast-varying time domain channel into a slowly varying and sparse Delay-Doppler domain channel. By estimating and equalizing this channel, the signal reception performance under high-speed mobility can be greatly improved.

[0003] However, in the OTFS modulation process, the original signal is placed in the delay-Doppler domain, which is two more FFT / IFFT processes than the OFDM system, where the original signal is placed in the frequency domain. Its receiving and demodulation processing is also more than ten to hundreds of times more complicated than OFDM. The pilot placement method of OTFS is also different from that of OFDM, making it difficult to be compatible with the OFDM system. For the receiving and demodulation processing of OTFS, please refer to the literature [1];

[0004] [1]Suvra Sekhar Das, Vivek Rangamgari, Shashank Tivari and SubhasChandra Mondal, Time Domain Channel Estimation and Equalization of CP-OTFS Under Multiple Fractional Dopplers and Residual Synchronization Errors, IEEE, 2021. Summary of the invention

[0005] In view of this, the present invention proposes a method for receiving OFDM signals supporting high-speed movement. The method utilizes the demodulation concept of OTFS but does not change the OFDM transmission signal, so that the receiver can implement the OFDM signal receiving process supporting high-speed movement.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for receiving an OFDM signal supporting high-speed movement comprises the following steps:

[0008] Step 1, using the Doppler detection method to detect whether the channel belongs to a high-speed mobile channel, if so, executing step 2, otherwise using the traditional OFDM signal receiving method for receiving;

[0009] Step 2, using the traditional OFDM signal time domain processing flow to perform time domain processing on the currently received time domain data to obtain the time domain data r after time domain processing, and then convert the time domain data r into frequency domain data Y through FFT;

[0010] Step 3, extracting a group of pilot signals of continuous symbols from the frequency domain data Y, performing frequency domain channel estimation on each pilot signal, and obtaining a frequency domain channel estimation result;

[0011] Step 4, performing IFFT processing on the frequency domain channel estimation result to obtain the corresponding time domain channel estimation result;

[0012] Step 5, calculating the number of multipaths and delay according to the time domain channel estimation result;

[0013] Step 6, after placing the time domain channel estimation result at the middle position of the time domain of the corresponding symbol, the time domain channel matrix is ​​obtained by interpolation method based on the number of multipaths and delay;

[0014] Step 7, using the time domain channel matrix to perform time domain equalization on the time domain data r;

[0015] Step 8: Convert the time-domain equalized signal into a frequency-domain equalized signal through FFT to complete the high-speed mobile OFDM signal reception.

[0016] Due to the adoption of the above technical solution, the beneficial effects of the present invention compared with the prior art are:

[0017] 1. The present invention uses the OTFS demodulation concept to receive OFDM signals passing through high-speed mobile signals without changing the OFDM transmission signal, thereby greatly improving its receiving performance. The actual receiving system can select the low-complexity traditional OFDM receiving process or the high-performance method of the present invention according to the detection of the mobile speed.

[0018] 2. The performance of the traditional OFDM receiving method is very poor under high-speed mobile channels (i.e., large Doppler), while the method of the present invention is almost unaffected by high-speed movement and has certain reference significance for engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1The figure is a flow chart of signal reception of a high-speed mobile channel in an embodiment of the present invention.

[0020] Figure 2 Schematic diagram of frequency domain signals of an OFDM system in an embodiment of the present invention.

[0021] Figure 3 The figure is a performance comparison of different receiving methods in the embodiments of the present invention under a high-speed mobile channel. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0023] A method for receiving OFDM signals supporting high-speed mobility, such as Figure 1 As shown, the following steps are included;

[0024] 1) Use the existing Doppler detection method to detect whether the channel belongs to a high-speed mobile channel. If it is not a high-speed mobile channel, the traditional OFDM receiving process is used. If it is a high-speed mobile channel, the high-speed OFDM receiving process of this embodiment is used, which is as follows:

[0025] 2) Performing existing OFDM time domain processing on the time domain data received by the RF front end, including but not limited to digital front end processing, timing synchronization, frequency offset estimation and compensation, etc., to obtain the time domain processed data r, and then obtain the frequency domain data Y through FFT;

[0026] Specifically, this embodiment receives a traditional OFDM signal, similar to but not limited to DVB-T, 5G signals, etc. Common OFDM system frequency domain signal formation methods are as follows: Figure 2 As shown, the white circle is the modulated data, and the black circle is the pilot known to the receiving end, which is used to estimate the channel. The pilots are generally placed in a comb-like manner with equal intervals, and the specific placement rules are not limited.

[0027] 3) Extract the pilot from the frequency domain data Y according to the known pilot position and perform frequency domain channel estimation

[0028] H p (l,k)=Y p (l,k) / X p (l,k)

[0029] Among them, H p (l,k) is the frequency domain channel estimation result on the pilot carrier, l is the OFDM symbol index, k is the OFDM subcarrier index, and Y p (l, k) is the value of the subcarrier where the pilot is located in the frequency domain data Y, X p (l,k) is the transmission value corresponding to the location of the pilot, which is known to the receiving end.

[0030] 4) The frequency domain channel results of the pilot estimation are compactly arranged and IFFT processed to obtain the time domain channel estimation h p (l,n), n is the delay index, and the IFFT length can be an integer power of 2 and just greater than the number of pilots in one symbol.

[0031] 5) Use the time domain channel estimation results of multiple symbols to jointly estimate the number and delay of multipaths. Taking 5G signals as an example, all pilot symbols of a time slot can be taken for joint estimation. The specific steps are as follows:

[0032] 5.a) Perform power summation on the time domain channel estimation results of multiple symbols:

[0033]

[0034] Where N l is the number of symbols estimated for the time domain channel used;

[0035] 5.b) Calculate the threshold for determining multipath:

[0036]

[0037] Where N IFFT is the number of IFFT points, K is a constant, and the appropriate value can be obtained according to the simulation, usually between 4 and 10. p (n) averaging is performed, and the results of some of the results that are determined to have no multipath are used for averaging. The method for calculating the threshold is not limited here.

[0038] According to the threshold TH in Ph p (n) Detect multipath, if Ph p (n i )≥TH, then it is considered that n i is the time delay of a path, n i The number of is the number of multipaths.

[0039] 6) Based on the detected multipath number and delay, the method in reference [1] is used to interpolate and construct the time domain channel matrix It should be noted that reference [1] uses the OTFS system, so the time domain channel estimate h p (l,n) can be directly obtained from the time domain signal, and the position before interpolation must correspond to the position of the time domain signal. p (l,n) is the frequency domain channel response estimated by the pilot signal obtained through IFFT, which can be regarded as the average effect of the time domain channel response and should be placed in the middle position of a symbol in the time domain for interpolation.

[0040] 7) Use the time domain channel matrix to perform time domain equalization on the time domain processed data r. Reference [1] has a low-complexity implementation process.

[0041] 8) The data after time domain equalization is converted to the frequency domain through FFT to complete the high-speed OFDM reception process; then the existing OFDM demodulation and decoding processing continues.

[0042] Specifically, Figure 3 As shown, it can be seen that the performance of the traditional OFDM receiving method is very poor in a high-speed mobile channel (i.e., a large Doppler), while the method of this embodiment and the method of reference [1] are almost unaffected by high-speed mobile channels and have comparable performances.

[0043] However, in the real environment, objects will not always be in a high-speed moving state. If the OTFS system is used, regardless of whether the channel is time-varying, a complex receiving and demodulation process needs to be adopted. If an OTFS / OFDM hybrid system is used, it is necessary to detect and feedback whether the channel is time-varying to switch the transmission system, and the sender and receiver shake hands to determine the current signal mode. This method is not real-time enough and also increases the transmission of interactive information. This embodiment uses the demodulation idea of ​​OTFS, but does not change the OFDM transmission signal. According to the detected moving speed, the OFDM traditional receiving process and the high-speed receiving process are switched, so that the receiver can realize the high-speed moving OFDM signal receiving process.

[0044] Those skilled in the art will appreciate that the embodiments described are intended to help readers understand the principles of the present invention, and should be understood that the scope of protection of the present invention is not limited to the embodiments described. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for receiving OFDM signals supporting high-speed mobility, characterized in that: The steps include: Step 1, using the Doppler detection method to detect whether the channel belongs to a high-speed mobile channel, if so, executing step 2, otherwise using the traditional OFDM signal receiving method for receiving; Step 2, using the traditional OFDM signal time domain processing flow to perform time domain processing on the currently received time domain data to obtain the time domain data r after time domain processing, and then convert the time domain data r into frequency domain data Y through FFT; Step 3, extracting a group of pilot signals of continuous symbols from the frequency domain data Y, performing frequency domain channel estimation on each pilot signal, and obtaining a frequency domain channel estimation result; Step 4, performing IFFT processing on the frequency domain channel estimation result to obtain the corresponding time domain channel estimation result; Step 5, calculating the number of multipaths and delay according to the time domain channel estimation result; Step 6, after placing the time domain channel estimation result at the middle position of the time domain of the corresponding symbol, the time domain channel matrix is ​​obtained by interpolation method based on the number of multipaths and delay; Step 7, using the time domain channel matrix to perform time domain equalization on the time domain data r; Step 8: Convert the time-domain equalized signal into a frequency-domain equalized signal through FFT to complete the high-speed mobile OFDM signal reception.

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