A method for peak-to-average ratio suppression of reserved subcarriers for OFDM communication systems

Abstract

A method for suppressing peak-to-average power ratio (PAPR) of reserved subcarriers in OFDM communication systems includes the following steps: Step 1: digital clipping using a shearing method; Step 2: acquiring clipping noise; Step 3: suppressing out-of-band noise to obtain new clipping noise; Step 4: filtering out in-band noise to obtain canceled noise; Step 5: inversely superimposing the noise into the original signal; Step 6: repeating steps 1-5 to reach the maximum number of iterations; Step 7: outputting the signal after PAPR suppression. This invention expands the range of reserved subcarriers, combining the resources of out-of-band and in-band empty subcarriers, and uses them to generate a signal that suppresses high peak values ​​and cancels noise, thereby improving PAPR suppression gain and retaining the distortion-free characteristics of the reserved subcarrier method.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and specifically relates to a method for suppressing the peak-to-average power ratio of reserved subcarriers in OFDM communication systems. Background Technology

[0002] To ensure distortion-free OFDM time-domain signal transmission, the peak-to-average power ratio (PAPR) of an OFDM system requires that the linear range of AD / DA converters and power amplifiers be greater than the range occupied by the OFDM peak and average power. Especially for the power amplifier in the RF front-end, to ensure a high linear range, one approach is to use pre-distortion, and another is to use power back-off technology. Pre-distortion is technically more difficult and costly to implement, so power back-off technology is more commonly used. If the peak-to-average power of the OFDM system is high, using power back-off technology will result in very low power amplifier efficiency. Assuming the average transmit power of the OFDM system is 20W (43dBm) and the PAPR is 16dB, then the RF front-end power amplifier must be at least 800W to ensure distortion-free signal transmission. It can be seen that using an 800W or higher power amplifier to transmit 20W of power only achieves an efficiency of 2.5%, with most of the power consumption used for heat dissipation. Similarly, assuming a PAPR of 10dB... The system only needs to use a 200W power amplifier tube, and the emission efficiency is about 10%, which can greatly reduce the difficulty of the system's thermal design.

[0003] To suppress the peak-to-average power ratio (PAPR) in OFDM systems, one commonly used method is the reserved subcarrier method. Chinese patent application CN101771652B discloses a method and system for reducing PAPR using amplitude scaling factors and reserved subcarriers. This method includes: determining the number of subcarriers N, the number of reserved subcarriers L, and the positions of the reserved subcarriers in the peak reduction sequence based on the parameter requirements of the OFDM system; providing a PAPR threshold B and an amplitude scaling factor set R; obtaining an ideal frequency-domain peak reduction sequence D based on the PAPR threshold B; obtaining values ​​on L reserved subcarriers based on the ideal frequency-domain peak reduction sequence, and assigning data 0 to the other NL subcarriers to determine the frequency-domain peak reduction sequence C; performing an inverse Fourier transform on the frequency-domain peak reduction sequence C to obtain a time-domain peak reduction sequence c(n); and subtracting x(n) from c(n) to obtain the transmitted signal z(n).

[0004] The reserved subcarrier approach involves limiting the signal and acquiring the limiting noise. After transforming the limiting noise to the frequency domain using FFT, the limiting noise at the reserved subcarrier positions is retained, while other parts are set to zero. Then, an IFFT is applied to transform the signal to the time domain, and the noise is superimposed onto the original signal to eliminate signal peaks. The reserved subcarrier method can achieve distortion-free limiting, but it often requires a high number of iterations or a sufficiently large number of reserved subcarriers to achieve a certain peak-to-average power ratio (PAPR) suppression capability. In some scenarios, it may not meet the actual PAPR suppression gain requirements. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention aims to propose a reserved subcarrier peak-to-average ratio (PAPR) suppression method for OFDM communication systems. This method uses all noise outside the channel bandwidth and the empty subcarrier portion within the band to exchange for PAPR suppression gain, and controls the leakage ratio of adjacent channels by the order of the low-pass filter. The present invention can adaptively select the threshold value of the leakage ratio of adjacent channels according to different scenario requirements to obtain different PAPR suppression gains.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for suppressing the peak-to-average power ratio of reserved subcarriers in an OFDM communication system includes the following steps:

[0008] Step 1: Perform digital clipping on the digital baseband signal using a shearing method;

[0009] Step 2: Calculate the difference between the cut digital baseband signal obtained in Step 1 and the original digital baseband signal to obtain the amplitude limiting noise;

[0010] Step 3: Based on the amplitude-limiting noise obtained in Step 2, a low-pass filter is used to suppress the portion outside the channel bandwidth to obtain new amplitude-limiting noise;

[0011] Step 4: Filter out the new amplitude-limiting noise obtained in step 3 in the effective data subcarrier positions to obtain canceled noise;

[0012] Step 5: The noise cancellation obtained in step 4 is inversely superimposed onto the original signal to suppress peak-to-average power ratio;

[0013] Step 6: Repeat steps 1-5 above until the maximum number of iterations is reached;

[0014] Step 7: Output the signal after suppressing the peak-to-average power ratio.

[0015] In step 1, the shearing method for digital limiting restricts the magnitude of signals exceeding a preset limiting threshold to that threshold value while maintaining its phase. Other signal points remain unchanged. The formula is as follows:

[0016]

[0017] Where x(n) is the original digital baseband signal, The signal is digitally limited by the shearing method, and A is the preset limiting threshold value.

[0018] The calculation method for obtaining the amplitude-limiting noise in step 2 is as follows:

[0019]

[0020] Where m(n) is the acquired clipping noise, and x(n) is the original digital baseband signal. This is the signal after digital amplitude limiting using the shearing method.

[0021] Step 3 involves passing the clipping noise through a low-pass filter to obtain new clipping noise, which is calculated as follows:

[0022] m′(n)=conv(m(n),h(n))

[0023] m′(n)=m′(L+1:NL)

[0024] Where conv represents the convolution function, h(n) represents the filter impulse response, m′(n) represents the new clipping noise, L represents the length of the filter impulse response, and N represents the total noise length.

[0025] In step 4, filtering out new clipping noise at the effective data subcarrier positions involves transforming the clipping noise to the frequency domain using an FFT, setting the noise at the effective data subcarrier positions to zero, leaving other positions unchanged, and then transforming it to the time domain using an IFFT to obtain canceled noise, denoted as...

[0026] The calculation method for inversely superimposing the noise cancellation onto the delayed original signal in step 5 is as follows:

[0027]

[0028] in, The signal after peak cancellation is given, and x(n) is the original digital baseband signal. To cancel out noise.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] (1) Based on the reserved subcarrier method to suppress peak-to-average power ratio, this invention expands the range of reserved subcarriers, combines the resources of out-of-band and in-band empty subcarriers together to generate noise cancellation and suppress high peak value signals, thereby improving the peak-to-average power ratio suppression gain and retaining the distortion-free characteristics of the reserved subcarrier method.

[0031] (2) Since the method of processing the out-of-band portion of the frequency domain limiting noise is changed from zeroing to suppression, the limiting noise after conversion to the time domain is closer to the peak value of the original time domain signal. Therefore, it has the characteristic of improving the peak-to-average ratio suppression gain after each iteration of the reverse superposition operation.

[0032] (3) Since the order of the out-of-band noise suppression filter involved in this invention is negatively correlated with the actual peak-to-average ratio suppression gain, the lower the filter order, the weaker the suppression capability of out-of-band noise, and the amplitude-limiting noise after conversion to the time domain is closer to the peak value of the original signal, and the obtained peak-to-average ratio suppression gain is greater. Therefore, it has the characteristic of adaptively selecting the appropriate filter order according to the actual system requirements to obtain different performance improvements.

[0033] (4) Since the peak-to-average power ratio (PAPR) gain obtained in each iteration of this invention is greater than that of the traditional method, the average power of the clipping noise obtained in the next iteration of this invention is lower than that of the traditional method. Furthermore, the additional out-of-band noise compared to the traditional method when processing frequency domain noise is after being suppressed by a filter. When the filter order is greater than 12, the ratio of out-of-band noise power to signal power will reach more than -40dB. Therefore, the average power of the peak cancellation noise after transformation to the time domain in this invention is lower than that of the traditional scheme. Since the peak cancellation noise and the original signal are orthogonal in the frequency domain, the greater the noise power, the greater the transmitted power occupied, and the corresponding power occupied by the signal will decrease, thereby affecting the bit error rate performance of the system. Therefore, this invention also has a certain bit error rate performance gain. Attached Figure Description

[0034] Figure 1 This is a flowchart of the method of the present invention.

[0035] Figure 2 This is a comparison diagram of the complementary cumulative probability distribution of peak-to-average power ratio between the embodiment of the present invention (TR-new) and the original reserved subcarrier technology (TR).

[0036] Figure 3 This is a comparison chart of the system bit error rate performance curves of the embodiment of the present invention (TR-new) and the original reserved subcarrier technology (TR).

[0037] Figure 4 This is a flowchart of the transceiver signal processing for bit error rate performance testing in an embodiment of the present invention. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings.

[0039] like Figure 1As shown, a method for suppressing the peak-to-average power ratio of reserved subcarriers in an OFDM communication system includes the following steps:

[0040] Step 1: Perform digital clipping on the digital baseband signal. This involves multiplying the preset clipping rate by the average signal power to obtain the clipping threshold. Signals exceeding the threshold are limited to the threshold value, while the phase remains unchanged. Signals with a magnitude below the threshold remain unchanged. The formula is as follows:

[0041]

[0042] Where x(n) is the original digital baseband signal, The signal is digitally limited by the shearing method, and A is the preset limiting threshold value.

[0043] Step 2: Subtract the cut digital baseband signal from the original digital baseband signal to obtain the clipping noise. The calculation method is as follows:

[0044]

[0045] Where m(n) is the acquired clipping noise, and x(n) is the original digital baseband signal. This is the signal after digital amplitude limiting using the shearing method.

[0046] Step 3: Pass the clipping noise through a low-pass filter to suppress the portion outside the channel bandwidth, obtaining a new clipping noise, which is calculated as follows:

[0047] m′(n)=conv(m(n),h(n))

[0048] m′(n)=m′(L+1:NL)

[0049] Where conv represents the convolution function, h(n) represents the filter impulse response, m′(n) represents the new clipping noise, L represents the length of the filter impulse response, and N represents the total noise length.

[0050] The low-pass filter is designed based on the Blackman window function. When the filter order is set to 12, the leakage ratio of adjacent channels can be guaranteed to be greater than 45dB. The choice of filter order will affect the leakage ratio of adjacent channels and the peak-to-average power ratio suppression gain. The leakage ratio of adjacent channels decreases as the filter order decreases, while the peak-to-average power ratio suppression gain increases as the filter order decreases.

[0051] Step 4: Filter out the new clipping noise at the effective data subcarrier positions to obtain canceled noise; transform the clipping noise to the frequency domain using FFT; set the noise at the effective data subcarrier positions to zero, while leaving other positions unchanged; transform the noise to the time domain using IFFT to obtain canceled noise, denoted as...

[0052] Step 5: The noise cancellation is inversely superimposed onto the delayed original signal for peak-to-average power ratio (PAPR) suppression, calculated as follows:

[0053]

[0054] in, The signal after peak cancellation is given, and x(n) is the original digital baseband signal. To cancel out noise.

[0055] Step 6: Repeat steps 1-5 above until the maximum number of iterations is reached. The maximum number of iterations is determined by the computational complexity requirements of the actual system. In each iteration, the shearing method compares the power of the time-domain OFDM signal with the threshold. This comparison requires N subtractions. Calculating the power of N complex-valued samples requires 2N real multiplications and N real additions. After that, filtering is required to eliminate out-of-band radiation.

[0056] Assuming a finite impulse response (FIR) filter of length L is used, the filtering process requires approximately NL+L. 2 Complex multiplication and addition with +L, for OFDM systems with large values ​​of N, L=N, therefore its complexity is further approximated by NL. Each iteration requires a pair of FFT / IFFT operations, and each FFT operation requires... The algorithm requires N complex multiplications and N log₂N complex additions. If we express the above complexity using real number multiplications and real number additions, then one complex multiplication is equivalent to 4 real number multiplications and 2 real number additions, and one complex addition is equivalent to 2 real number additions. Therefore, the complexity required for each iteration is 4N log₂N + 4NL + 2N real number multiplications and 6N log₂N + 4NL + 2N real number additions. Thus, the computational complexity required by the algorithm is the above complexity multiplied by the number of iterations.

[0057] Step 7: Output the signal after suppressing the peak-to-average power ratio.

[0058] See the table below for a comparison of the additional power consumed by the embodiment of the present invention (TR-new) and the original reserved subcarrier technology (TR) at different iteration numbers:

[0059]

[0060]

[0061] As can be seen from the table, when the number of iterations is 1, the power consumption of the two is almost the same. However, as the number of iterations increases, the additional power consumed by the present invention gradually becomes lower than that of the traditional solution. Therefore, it can be concluded that as the number of iterations increases, the bit error rate performance of the present invention will be better than that of the traditional solution.

[0062] Figure 2 This is a comparison chart of the complementary cumulative probability distribution of peak-to-average power ratio (PAPR) between the present invention (TR-new) and the traditional reserved subcarrier scheme (TR). Figure 2 visible:

[0063] The OFDM signal in the simulation is a 2048-point FFT, using 64QAM modulation, with a subcarrier spacing of 12.5KHz, 1536 effective subcarriers, 512 empty subcarriers, and a 1 / 3 code rate Turbo coding method. Each frame of the signal contains 8 OFDM symbols.

[0064] The filter parameters used in the simulation of the present invention are shown in the following table:

[0065] Parameter type value Filter type Low-pass filter, Blackman window bandwidth 19.8MHz Sampling rate 51.2MHz Cutoff frequency 10MHz order 12th order

[0066] The number of reserved subcarriers used in the simulation of the traditional reserved subcarrier scheme is 512.

[0067] In the simulation, the number of iterations for both schemes was set to 45. The horizontal axis represents the peak-to-average power ratio (PAPR), and the vertical axis represents the probability that the PAPR of all time-domain signal points is greater than the current PAPR. Generally, the horizontal axis value at the probability of 1e-4 is taken as the effective PAPR of the current signal. As can be seen from the figure, the PAPR suppression gain of this patented scheme is improved by about 0.5dB compared with the traditional reserved subcarrier scheme.

[0068] Figure 3 The figure shows a comparison of the bit error rate performance curves of the system using the scheme of this invention (TR-new) and the traditional reserved subcarrier scheme (TR). The horizontal axis represents the symbol signal-to-noise ratio (SNR) and the vertical axis represents the bit error rate (BER). Generally, the SNR value at 1e-6 BER is taken as the BER threshold of the system. It can be seen from the figure that the BER performance of this patented scheme is improved compared with the traditional scheme.

[0069] Figure 4 The diagram shows the signal processing flow at the transmitter and receiver in the simulation. The transmitter mainly includes scrambling, Turbo coding, digital modulation mapping, IFFT, framing, upsampling filtering, and peak-to-average power ratio suppression of the present invention. Furthermore, the receiver mainly includes downsampling by two times, matched filtering, OFDM demodulation, 64QAM soft demodulation, Turbo decoding, and descrambling. The number of simulations is 1M.

Claims

1. A method for suppressing the peak-to-average power ratio of reserved subcarriers in an OFDM communication system, characterized in that, Includes the following steps: Step 1: Perform digital clipping on the digital baseband signal using a shearing method; Step 2: Calculate the difference between the cut digital baseband signal obtained in Step 1 and the original digital baseband signal to obtain the amplitude limiting noise; Step 3: Based on the amplitude-limiting noise obtained in Step 2, a low-pass filter is used to suppress the portion outside the channel bandwidth to obtain new amplitude-limiting noise; Step 4: Filter out the new amplitude-limiting noise obtained in step 3 in the effective data subcarrier positions to obtain canceled noise; Step 5: The noise cancellation obtained in step 4 is inversely superimposed onto the original signal to suppress peak-to-average power ratio; Step 6: Repeat steps 1-5 above until the maximum number of iterations is reached; Step 7: Output the signal after suppressing the peak-to-average power ratio; In step 1, the shearing method for digital limiting restricts the magnitude of signals exceeding a preset limiting threshold to that threshold value while maintaining its phase. Other signal points remain unchanged. The formula is as follows: Where x(n) is the original digital baseband signal, The signal is digitally limited by the shearing method, and A is the preset limiting threshold value; Step 3 involves passing the clipping noise through a low-pass filter to obtain new clipping noise, which is calculated as follows: m′(n)=conv(m(n),h(n)) m′(n)=m′(L+1:NL) Where conv represents the convolution function, h(n) represents the filter impulse response, m′(n) represents the new clipping noise, L represents the length of the filter impulse response, and N represents the total noise length; In step 4, filtering out new clipping noise at the effective data subcarrier positions involves transforming the clipping noise to the frequency domain using an FFT, setting the noise at the effective data subcarrier positions to zero, leaving other positions unchanged, and then transforming it to the time domain using an IFFT to obtain canceled noise, denoted as... In step 5, the calculation method for inversely superimposing the noise cancellation onto the delayed original signal is as follows: in, The signal after peak cancellation is given, and x(n) is the original digital baseband signal. To cancel out noise.

2. The method for suppressing peak-to-average power ratio of reserved subcarriers in an OFDM communication system according to claim 1, characterized in that, The calculation method for obtaining the amplitude-limiting noise in step 2 is as follows: Where m(n) is the acquired clipping noise, and x(n) is the original digital baseband signal. This is the signal after digital amplitude limiting using the shearing method.

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