Low-PAPR multi-tone interference signal design method based on OFDM

Abstract

The invention relates to an OFDM (Orthogonal Frequency Division Multiplexing)-based low-PAPR (Peak to Average Power Ratio) multi-tone interference signal design method, a multi-tone interference signal interferes aiming at a plurality of target frequencies, spectrum peaks of the interference signal are arranged at unequal intervals, and the method relates to a multi-carrier system and an electronic warfare system, and belongs to the field of wireless communication electronic countermeasures. Wherein any P subcarrier corresponds to a required interference signal frequency spectrum, sending an interference signal, and setting other N-P subcarriers to be 0; aiming at the P subcarriers, firstly randomly generating Q groups of P signals a0, a1, a2,..., aP-1q of which the amplitude is 1 and the phase obeys uniform distribution in the interval of 0 and 2 pi, wherein q belongs to 1 and Q, respectively placing each group of a0, a1, a2,..., aP-1q on the corresponding OFDM subcarriers, performing IFFT calculation to obtain Q groups of time domain signals, respectively calculating the PAPR of the Q groups of time domain signals, selecting the time domain signal with the minimum PAPR in the Q groups, and determining the time domain signal as an initial interference signal; and then under acceptable spectrum leakage, the initial interference signal is processed through an up-down shear filter so as to further reduce the PAPR. According to the method provided by the invention, the required interference signal can be quickly generated, and the PAPR of the multi-tone interference signal is close to 1dB under the condition that the frequency spectrum leakage does not exceed 15%.

Description

Technical Field

[0001] The invention belongs to the field of communication confrontation and signal processing, and relates to a design method of a low PAPR multi-tone interference signal based on OFDM. Background Art

[0002] In modern military communications, confidentiality is particularly critical. Modern military communications mostly use frequency hopping communications, spread spectrum communications and other communication methods to achieve confidentiality. The frequency points are non-uniform. In order to accurately interfere with the multiple target frequencies of the enemy's frequency hopping radio, it is necessary to quickly generate multi-tone interference signals. There are two technical requirements: (1) Since the enemy may change its communication frequency at any time, the process of generating multi-tone interference signals needs to be as fast as possible so that the enemy's multiple target frequencies can be interfered with in time; (2) The multi-tone interference signal has a low PAPR. This is because when the PAPR of the interference signal is high, it means that the instantaneous power of the signal is much greater than the average power, resulting in the need for a larger dynamic range for components such as the power amplifier in the interference equipment, which affects the efficiency. By reducing the PAPR, the output power and efficiency of the multi-tone interference signal can be improved, while reducing the requirements for the hardware of the interference equipment. Multi-tone interference signals interfere with multiple target frequencies, and their design methods are mostly based on orthogonal frequency division multiplexing (OFDM) technology. OFDM is a multi-carrier modulation technology that can realize modulation and demodulation of OFDM systems through fast Fourier transform, thereby improving the utilization efficiency of spectrum. It distributes data to multiple mutually orthogonal subcarriers for high-speed transmission, flexibly allocates the amplitude and phase of subcarriers, and facilitates the adjustment of signal characteristics to adapt to different communication needs. Due to the superposition of multiple subcarrier signals in the OFDM system, OFDM signals have the characteristic of high PAPR. At present, the PAPR reduction methods for OFDM signals mainly include: (1) Steepest descent method. "Research on reducing the peak-to-average power ratio of frequency non-uniformly spaced comb spectrum interference signals [J]. Electronic Information Countermeasure Technology, 2015, 30(02): 41-44+49." discloses a method of using the mathematical steepest descent method to search the objective function with negative gradient, calculate a set of optimal initial phases, and then reduce the PAPR of frequency non-uniformly spaced comb spectrum interference signals. However, when the number of subcarriers N>10, the PAPR values ​​obtained by this method are all above 3dB. (2) Immune algorithm. “Research on Peak-to-Average Power Ratio Suppression of Non-Equally Interval Comb Spectrum Signal Based on Immune Algorithm[J]. Ship Electronic Countermeasures, 2013, 36(06): 61-63+77. DOI: 10.16426 / j.cnki.jcdzdk.2013.06.023.” A search algorithm simulating a biological immune system is disclosed, which is used to search for the optimal subcarrier initial phase combination in the PAPR field to reduce the PAPR of the signal. However, since multiple iterations are required to obtain the optimal solution, the speed of reducing the PAPR of non-uniformly spaced interference signals is slow. (3) Newman phase optimization method, "An improved algorithm for reducing the peak-to-average power ratio of multi-carrier interference signals [J]. Firepower and Command Control, 2015, 40(12): 159-162." discloses a method for reducing the PAPR of a signal by setting a specific initial phase for each subcarrier signal. The PAPR value obtained by this method can be as low as 2.6 dB, but this method is not effective when processing modulated signals such as MPSK (M>2) and QAM. It is necessary to cascade a peak cancellation unit on the basis of the Newman phase optimization method to further reduce the PAPR, and the modulation method is limited. In summary, the disadvantages of the PAPR reduction method for OFDM signals include: long execution cycle, insignificant reduction of PAPR value, limited modulation method and high computational complexity. Summary of the invention

[0003] In view of the above problems, the purpose of the present invention is to design a method for designing a low PAPR multi-tone interference signal based on OFDM. The technical solution of the present invention to solve the above technical problems is as follows: Step 1: At the transmitting end, determine the carrier frequency band range, subcarrier spacing and subcarrier number N, where any P subcarriers correspond to the required interference signal spectrum, and the other NP subcarriers are set to 0; Step 2: For these P subcarriers, generate a set of P signals a0, a1, a2, ..., a with amplitude of 1 and phase uniformly distributed in the interval 0, 2π. P-1 q ; Step 3: Replace a0, a1, a2, …, a P-1 q Place it on the corresponding P OFDM subcarriers, perform NN≥P≥3-point IFFT transformation, obtain a set of time domain signals, and calculate the PAPR of the signal; Step 4, repeat the whole process of step 2 to step 3 Q times, that is, obtain Q groups of time domain signals; Step 5, select the time domain signal with the smallest PAPR among the Q times as the initial interference signal; Step 6: Perform up and down shearing filter processing on the interference signal to obtain a multi-tone interference signal with a low PAPR that meets the expected acceptable spectrum leakage. Furthermore, the steps 1 and 2 specifically include the following steps: The frequency positions of P subcarriers are determined by the required interference signal frequency; P phases in the interval 0 and 2π are randomly generated and uniformly distributed and modulated onto the corresponding subcarriers to obtain P signals a0, a1, a2, …, a P-1 q . Furthermore, the steps 3, 4 and 5 specifically include the following steps: the IFFT formula is: n=0,1,2,…,N-1, where X rand k is the transmission symbol of the kth subcarrier. Through IFFT operation, a set of time domain signals is obtained. The maximum acceptable PAPR of the signal is determined to be δ. When Q∈λ1,λ2, the probability that the minimum PAPR of the interference signal is less than δ is within the interval of 95% and 99.9%, and as λ increases, the probability increases. Repeat the whole process from step 2 to step 3 Q times, that is, generate Q groups a0, a1, a2, …, a P-1 q , each group a0,a1,a2,…,a P-1 q Place them on the corresponding OFDM subcarriers for IFFT calculation to obtain Q groups of time domain signals. The minimum PAPR value in the Q groups of time domain signals is β, and the group of time domain signals corresponding to PAPR = β is determined as the initial interference signal. Among them, minimizing PAPR is to prevent large spectrum leakage during the up and down shearing process due to excessive PAPR. Furthermore, the method for performing upper and lower shearing on the IFFT transformed signal comprises the following steps: Step 6.1, determining the upper shearing threshold of the signal as ε1 and the lower shearing threshold as ε2; Step 6.2, cut the part of the signal that exceeds the upper and lower thresholds; Step 6.3: Test the PAPR value and the proportion of spectrum leakage after the signal is clipped. If the spectrum leakage exceeds or falls short of expectations, repeat steps 6.1-6.3 until a low PAPR multi-tone interference signal that meets the expected acceptable spectrum leakage is found. The beneficial effects of the present invention are: (1) The method provided by the present invention has the advantage of a simple implementation structure. The design of the multi-tone interference signal only needs to consist of a random phase generator and an IFFT module with a length of N. If it is necessary to consider improving the efficiency of the RF power amplifier, an additional signal upper and lower shearing filter module can be added. The construction and implementation process of the three modules are extremely simple. (2) The method of the present invention generates a signal on a subcarrier that is subjected to a uniform distribution of random phase modulation at the transmitting end, and performs a small amount of repeated searches to obtain the initial interference signal corresponding to the minimum PAPR. The use of the statistical search method reduces the amount of calculation, and can quickly align the target spectrum to generate a multi-tone interference signal. In addition, although the probability that the PAPR value searched is less than the expected value does not reach 100%, the probability of not less than 95% has provided sufficient statistical significance, and the error risk is acceptable. Even if an error occurs, it can still be corrected by the upper and lower shearing filters. In addition, the upper and lower shearing filters can continue to reduce the PAPR value of the initial interference signal to around 1dB. Even after the interference signal is sheared up and down, its spectrum leakage is acceptable, and the energy of the interference signal is still concentrated within the interference spectrum range. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 Shown is a schematic diagram of the method of the present invention. DETAILED DESCRIPTION

[0005] The principles and features of the present invention are described below in conjunction with specific embodiments. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention. First, if Figure 1 As shown, the present invention designs a low PAPR multi-tone interference signal design method based on OFDM to better interfere with the enemy frequency hopping radio station. The specific steps are as follows: (1) at the transmitting end, determine the number of subcarriers N, the subcarrier spacing and the carrier frequency band range, where any P subcarriers correspond to the required interference signal spectrum, and the other NP subcarriers are set to 0; Assuming that the carrier frequency band range is 0-7040KHz and the subcarrier interval is 55KHz, the number of subcarriers N=128; the present invention selects P=16, and determines the frequency positions of 16 subcarriers according to the required interference signal spectrum, and the remaining 112 subcarriers are set to 0; (2) For these P subcarriers, a set of P signals a0, a1, a2, …, a1 with amplitude 1 and phase uniformly distributed in the interval 0, 2π is generated. P-1 q , a0,a1,a2,…,a P-1 q Place it on the corresponding P OFDM subcarriers, perform NN≥P≥3-point IFFT transformation, obtain a set of time domain signals, and calculate the PAPR of the signal; The IFFT formula is: n=0,1,2,…,N1,where N=2 7 =128,X rand k is the transmission symbol of the kth subcarrier; (3) Repeat the entire process from step 2 to step 3 Q times, that is, obtain Q groups of time domain signals, and select the time domain signal with the smallest PAPR among the Q times as the initial interference signal; In this embodiment, under 16 subcarriers, the maximum acceptable signal PAPR value δ=5 is determined. When Q∈10,24, the probability that the minimum interference signal PAPR value <5 is within the interval 95%, 99.9%. Repeat the entire process from step 2 to step 3 Q times, that is, generate Q groups of signals a0, a1, a2, ..., a 14 q , each group a0,a1,a2,…,a 14 q Place them on the corresponding OFDM subcarriers for IFFT calculation to obtain Q groups of time domain signals. The minimum PAPR value among the Q groups of time domain signals is 4.02. Then, a group of time domain signals corresponding to PAPR=4.02 is determined to be the initial interference signal. (4) The above interference signal is processed by upper and lower shearing filters to obtain a multi-tone interference signal with a low PAPR that meets the expected acceptable spectrum leakage. The interference signal corresponding to PAPR=4.02 is selected, and after the interference signal passes through the upper and lower shear filters, its PAPR is about 1.1 (about 0.41 dB), and the spectrum leakage is 11.89% at this time, which is within the expected acceptable range. Preferably, in order to meet the expected efficiency of the RF power amplifier, under the premise that the spectrum leakage is acceptable, the PAPR of the multi-tone interference signal is further reduced, and the interference signal obtained after the search is passed through the upper and lower shear filters as follows: A. Determine the upper and lower signal clipping thresholds; When the signal is clipped up and down for the first time, if the acceptable spectrum leakage is expected to be no more than 15%, the upper clipping threshold of the RF signal can be temporarily set to 0.25 and the lower clipping threshold to 0; B. Cut the part of the signal that exceeds the threshold and retain the phase information of the original signal; after setting the cutting threshold, pass the interference signal through the upper and lower cutting filters to obtain the cut signal. Cutting is a nonlinear transformation of the original signal and does not change the phase information of the original signal; C. Test the PAPR value of the signal after cutting and the proportion of spectrum leakage. The PAPR value and the proportion of spectrum leakage are calculated for the signal after the first up and down shearing. If the spectrum leakage exceeds or is less than 15%, the shearing threshold can be reduced or increased to repeat process B until the minimum PAPR of the multi-tone interference signal under the expected acceptable spectrum leakage is met. In this embodiment, the minimum PAPR is 1.1 (about 0.41dB), and the spectrum leakage is 11.89%, which is within the expected acceptable range. In summary, the OFDM-based low PAPR multi-tone interference signal design method provided by the present invention generates a signal on a subcarrier that obeys a uniform distribution of random phase modulation at the transmitting end, and performs a small amount of repeated searches to obtain the initial interference signal with the minimum PAPR, and can quickly align the target spectrum to generate a multi-tone interference signal. In addition, the PAPR of the initial interference signal can be further reduced to about 1dB by using an upper and lower shearing filter, and the signal PAPR value is significantly reduced. In addition, the present invention has the advantage of a simple implementation structure. The design of the initial interference signal only needs to consist of a random phase generator and an IFFT module with a length of N. If the efficiency of the RF power amplifier needs to be considered, an additional signal upper and lower shearing filter module can be added, and the implementation complexity of the three modules is very low. The present invention can be used for the design of low PAPR multi-tone interference signals and for scenes with high requirements for RF power amplifiers. The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for designing a low PAPR multi-tone interference signal based on OFDM, characterized in that: The following steps are involved: Step 1: At the transmitting end, determine the carrier frequency band range, subcarrier spacing and subcarrier number N, where any P subcarriers correspond to the required interference signal spectrum, and the other NP subcarriers are set to 0; Step 2: For these P subcarriers, generate a set of P signals a0, a1, a2, ..., a with amplitude of 1 and phase uniformly distributed in the interval 0, 2π. P-1 q ; Step 3: Replace a0, a1, a2, …, a P-1 q Place it on the corresponding P OFDM subcarriers, perform NN≥P≥3-point IFFT transformation, obtain a set of time domain signals, and calculate the PAPR of the signal; The step 3 specifically includes the following contents: The IFFT formula is: n=0,1,2,…,N-1, where X rand k is the transmission symbol of the kth subcarrier; Step 4, repeat the whole process of step 2 to step 3 Q times, and obtain Q groups of time domain signals; The step 4 specifically includes the following contents: Determine the maximum acceptable signal PAPR as δ. When Q∈λ1,λ2, the probability that the minimum PAPR of the interference signal is less than δ is within the interval of 95%, 99.9%, and the probability increases with the increase of λ. Repeat steps 2 to 3 Q times, that is, generate Q groups a0, a1, a2, …, a P-1 q , each group a0,a1,a2,…,a P-1 q Place them on the corresponding OFDM subcarriers and perform IFFT calculation to obtain Q groups of time domain signals; Step 5: Select the time domain signal with the smallest PAPR among Q times as the initial interference signal; The step 5 specifically includes the following contents: If the minimum PAPR value among the Q groups of time domain signals is β, then a group of time domain signals corresponding to when PAPR=β is determined to be the initial interference signal; Step 6: Process the interference signal with upper and lower shear filters to obtain a multi-tone interference signal with a low PAPR that meets the expected acceptable spectrum leakage; In step 6, the method for cutting the initial interference signal includes the following steps: Step 6.1, determine the upper shearing threshold of the signal as ε1 and the lower shearing threshold as ε2; Step 6.2, cut the part of the signal that exceeds the upper and lower thresholds; Step 6.3: Test the PAPR value and the proportion of spectrum leakage after the signal is clipped. If the spectrum leakage exceeds or falls short of expectations, repeat steps 6.1-6.3 until a low PAPR multi-tone interference signal that meets the expected acceptable spectrum leakage is found.

Images