CN115173953B - Method for improving hybrid HACO-OFDM performance by using predistortion technology - Google Patents
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Abstract
The invention discloses a method for improving the performance of mixed HACO-OFDM by using a predistortion technology, which comprises the steps of firstly carrying out FFT conversion on an ACO-OFDM time domain signal after amplitude limiting before the ACO-OFDM time domain signal is overlapped with a PAM-DMT time domain signal to form the HACO-OFDM time domain signal, obtaining an ACO-OFDM frequency domain signal, extracting interference noise of an imaginary part position in an even subcarrier of the ACO-OFDM frequency domain signal, then subtracting the ACO-OFDM frequency domain interference noise from the PAM-DMT frequency domain data at the position of the even subcarrier by using the predistortion technology, obtaining PAM-DMT pre-distorted frequency domain data, carrying out IFFT conversion on the PAM-DMT pre-distorted time domain signal to obtain a PAM-DMT pre-distorted time domain signal, and finally overlapping the PAM-DMT pre-distorted time domain signal with the ACO-OFDM time domain signal to form the HACO-OFDM time domain signal, so as to eliminate the interference of ACO-OFDM modulation on the PAM-DMT. The method provided by the invention has the advantages of low complexity, strong reliability and high bit error rate performance.
Description
Technical Field
The invention belongs to the field of visible light communication disciplines, and particularly relates to a method for improving hybrid asymmetric amplitude limiting optical orthogonal frequency division multiplexing (HACO-OFDM) performance by using a predistortion technology.
Background
Hybrid HACO-OFDM is a technique combining an asymmetric limited light orthogonal frequency division multiplexing system (Asymmetrically Clipped Optical-Orthogonal Frequency Division Multiplexing, abbreviated as ACO-OFDM) and pulse amplitude modulation-discrete multitone (Pulse Amplitude Modulation-Discrete MultiTone, abbreviated as PAM-DMT), and its main idea is to utilize odd carriers for ACO-OFDM modulation and even carriers for PAM-DMT modulation, in order to balance power efficiency and spectrum utilization. However, the mutual interference between ACO-OFDM and PAM-DMT causes great influence on the system performance. To reduce this effect, the conventional scheme adopts a Serial Interference Cancellation (SIC) technique to cancel the interference of ACO-OFDM on PAM-DMT, but the SIC technique also brings about problems of high complexity, additional delay, error propagation, and the like. How to effectively solve the mutual interference between ACO-OFDM and PAM-DMT while reducing the complexity of the system is a problem to be solved by the invention.
Disclosure of Invention
The invention aims to solve the technical problems: HACO-OFDM has a great influence on system performance due to mutual interference between ACO-OFDM and PAM-DMT. In order to reduce the influence, the traditional scheme adopts SIC technology to counteract the interference of ACO-OFDM to PAM-DMT, but SIC technology also brings the problems of high complexity, extra delay, error code propagation and the like. How to effectively solve the mutual interference between ACO-OFDM and PAM-DMT while reducing the complexity of the system is a problem to be solved by the invention.
The invention adopts the technical scheme that:
a method for improving hybrid HACO-OFDM performance using predistortion techniques, comprising:
before an ACO-OFDM time domain signal and a PAM-DMT time domain signal are overlapped to form a HACO-OFDM time domain signal, firstly, carrying out fast Fourier transform (Fast Fourier transform, FFT for short) on the ACO-OFDM time domain signal after amplitude limiting to obtain an ACO-OFDM frequency domain signal, extracting interference noise of an imaginary part position in an even subcarrier of the ACO-OFDM frequency domain signal, then subtracting the ACO-OFDM frequency domain interference noise from the PAM-DMT frequency domain data at the position of the even subcarrier by using a predistortion technology to obtain PAM-DMT predistorted frequency domain data, then carrying out fast Fourier inverse transform (Inverse Fast Fourier Transform, IFFT for short) to obtain a PAM-DMT predistorted time domain signal, and finally, overlapping the PAM-DMT predistorted time domain signal with the ACO-OFDM time domain signal to form a HACO-OFDM time domain signal, thereby eliminating interference of the PAM-DMT by ACO-OFDM modulation.
Further, the method comprises the following steps:
step 1) data layering is carried out on a signal source, wherein ACO-OFDM modulation is carried out on first layer data, and the modulated data are correspondingly placed at odd subcarrier positions; PAM-DMT modulation is carried out on the second layer of data, and the modulated data are correspondingly placed at the positions of even subcarriers;
step 2) carrying out FFT conversion again on the ACO-OFDM time domain signals, subtracting the frequency domain interference noise of ACO-OFDM from the frequency domain data of PAM-DMT at the position of even subcarriers by using a predistortion technology to obtain frequency domain data after PAM-DMT predistortion, carrying out IFFT conversion on the frequency domain data after PAM-DMT predistortion to obtain time domain signals after PAM-DMT predistortion, and superposing the time domain signals after PAM-DMT predistortion with the time domain signals of ACO-OFDM to form HACO-OFDM time domain signals so as to eliminate interference of ACO-OFDM modulation on PAM-DMT;
step 3), adding Cyclic Prefix (CP) into the overlapped HACO-OFDM time domain signal, and transmitting the signal by a photoelectric intensity modulator through DA digital-to-analog conversion and Bias Tee coupling circuit;
the modulated optical signal reaches a receiving end, and the receiving end performs photoelectric conversion, filtering amplification and AD analog-to-digital conversion on the received signal to obtain a HACO-OFDM signal to be demodulated;
step 5) HACO-OFDM demodulation is performed on the HACO-OFDM to-be-demodulated signal, wherein the first layer of data is subjected to ACO-OFDM demodulation, and because interference of the first layer ACO-OFDM modulation on the second layer PAM-DMT is eliminated at the transmitting end, the second layer of data can be directly subjected to PAM-DMT demodulation, and finally binary data recovered by demodulation is sent back to the sink.
The invention has the technical characteristics and remarkable effects that:
the invention provides a method for improving the performance of mixed HACO-OFDM by using a predistortion technology for the first time, which is different from the traditional HACO-OFDM processing method in that the mutual interference of ACO-OFDM modulation on PAM-DMT is eliminated from the angle of a frequency domain at a transmitting end, and the mutual interference of ACO-OFDM modulation on PAM-DMT is counteracted by using a SIC technology from the angle of a time domain at a receiving end. The method provided by the invention has the advantages of low complexity, strong reliability and high bit error rate performance, and because the interference of ACO-OFDM modulation on PAM-DMT is eliminated at the transmitting end, the receiving end can directly conduct parallel demodulation on data without SIC processing, so that the problems of high complexity, extra delay, error code propagation and the like caused by the fact that SIC serial interference elimination is adopted in the prior art are effectively avoided, and the method is superior to the HACO-OFDM scheme adopting SIC in bit error rate performance, and can provide a certain reference value for improving the hybrid HACO-OFDM performance by using a predistortion technology.
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Fig. 1 is a flow chart of a method for improving hybrid HACO-OFDM performance using predistortion techniques in an embodiment.
Detailed Description
The embodiments described below by referring to the drawings are illustrative and intended to explain the present invention and should not be construed as limiting the invention.
Examples:
a method for improving hybrid HACO-OFDM performance using predistortion techniques, comprising:
before an ACO-OFDM time domain signal and a PAM-DMT time domain signal are overlapped to form a HACO-OFDM time domain signal, firstly, carrying out FFT conversion on the ACO-OFDM time domain signal after amplitude limiting to obtain an ACO-OFDM frequency domain signal, extracting interference noise at an imaginary part position in an even subcarrier of the ACO-OFDM frequency domain signal, then subtracting the ACO-OFDM frequency domain interference noise from the PAM-DMT frequency domain data at the position of the even subcarrier by using a predistortion technology to obtain PAM-DMT predistorted frequency domain data, carrying out IFFT conversion to the PAM-DMT frequency domain data to obtain a PAM-DMT predistorted time domain signal, and finally, overlapping the PAM-DMT predistorted time domain signal with the ACO-OFDM time domain signal to form the HACO-OFDM time domain signal so as to eliminate interference of ACO-OFDM modulation on the PAM-DMT.
As shown in fig. 1, the method comprises the steps of:
step 1) data layering is carried out on a signal source, wherein ACO-OFDM modulation is carried out on first layer data, and the modulated data are correspondingly placed at odd subcarrier positions; PAM-DMT modulation is carried out on the second layer of data, and the modulated data are correspondingly placed at the positions of even subcarriers;
step 1) comprises the steps of:
1-1) suppose that source data transmits N/2-1 data per frame, wherein sequence numbers of the data are respectively from X 1 To X N/2-1 Then (X) 1 ,X N/2-1 ) Dividing the odd-numbered data into a first layer, and performing quadrature amplitude modulation (Quadrature Amplitude Modulation, QAM for short) to obtain X ACO,k1 Where k1= (1, 2,., N/4); and (X) 1 ,X N/2-1 ) Dividing the even numbered data into a second layer, and performing pulse amplitude modulation (Pulse Amplitude Modulation, PAM for short) to obtain X PAM,k2 Where k2= (1, 2,., N/4-1);
1-2) mapping data X to QAM of the first layer ACO,k1 Performing conjugate symmetry to obtain conjugate data X * ACO,k1 Will map data X ACO,k1 And conjugate data X * ACO,k1 The mapping according to the requirement of conjugate symmetry is as follows:
X odd ＝[0,X ACO,1 ,0,X ACO,2 ,0,X ACO,3 ,...,X ACO,N/4 ,0,X * ACO,N/4 ,...,X * ACO,3 ,0,X * ACO,2 ,0,X * ACO,1 ]
1-3) data X satisfying conjugate symmetry odd Performing IFFT to generate ACO-OFDM real signal x conforming to intensity modulation Direct Detection system (Intensity Modulation/Direct Detection, IM/DD for short) transmission ACO-OFDM ：
x ACO-OFDM ＝IFFT(X odd )
1-4) PAM mapping data X to second layer PAM,k2 Performing conjugate symmetry to obtain X * PAM,k2 Then the original value X PAM,k2 And conjugate data X * PAM,k2 The mapping according to the requirement of conjugate symmetry is as follows:
X even ＝[0,0,X PAM,1 ,0,X PAM,2 ,0,X PAM ,3,...,X PAM,N/4-1 ,0,0,0,X * PAM ,N/4-1,...,X * PAM,3 ,0,X * PAM,2 ,0,X * PAM,1 ]；
step 2) FFT conversion is carried out again on ACO-OFDM time domain signals, a predistortion technology is applied to subtract the frequency domain interference noise of ACO-OFDM from the frequency domain data of PAM-DMT at the position of even subcarriers to obtain the frequency domain data after PAM-DMT predistortion, and then the frequency domain data X 'after PAM-DMT predistortion is carried out' PAM-DMT Performing IFFT to obtain a PAM-DMT pre-distorted time domain signal x' PAM-DMT For PAM-DMT pre-distorted time domain signal x' PAM-DMT Zero crossing clipping is carried out to obtain x' PAM-DMT Finally, the data x after zero-crossing amplitude limiting " PAM-DMT With ACO-OFDM time domain signal x' ACO-OFDM Superposition to form HACO-OFDM time domain signal x HACO-OFDM So as to eliminate the interference of ACO-OFDM modulation to PAM-DMT;
step 2) comprises the steps of:
2-1) transmitting ACO-OFDM real signal x to first layer ACO-OFDM Zero-crossing amplitude limiting is carried out, negative value signals in ACO-OFDM real signals are removed, and x 'is obtained' ACO-OFDM ，
2-2) followed by a pair x' ACO-OFDM FFT conversion is carried out to obtain a new ACO-OFDM frequency domain signal X' ACO-OFDM Then extracting interference noise of the imaginary part position in the even sub-carrier;
2-3) subtracting ACO-OFDM frequency domain interference noise from PAM-DMT frequency domain data at even number sub-carrier positions to obtain PAM-DMT pre-distorted frequency domain data X' PAM-DMT ，
Wherein X is PAM-DMT Frequency domain data representing PAM-DMT, 2X imag (X' ACO-OFDM ) Representing the pair signal X' ACO-OFDM Taking the data of the imaginary part and multiplying the data by 2 times, wherein i represents a complex number;
2-4) frequency domain data X 'after predistortion of PAM-DMT' PAM-DMT Performing IFFT to obtain a PAM-DMT pre-distorted time domain signal x' PAM-DMT ，
x' PAM-DMT ＝IFFT(X' PAM-DMT )＝(x' PAM-DMT,0 ,x' PAM -DMT,1,...,x' PAM -DMT,j,...,x' PAM-DMT,N-1 ) Wherein x' PAM-DMT,j J= (0, n-1) represents the j-th PAM-DMT time domain signal of the IFFT output;
2-5) time domain signal x 'after predistortion of PAM-DMT' PAM-DMT Performing zero crossing amplitude limiting operation to remove negative value part in PAM-DMT to obtain x' PAM-DMT ：
2-6) time domain signal x' after predistortion of PAM-DMT " PAM-DMT With ACO-OFDM time domain signal x' ACO-OFDM Superposition to obtain HACO-OFDM time domain signal x HACO-OFDM So as to eliminate the interference of ACO-OFDM modulation to PAM-DMT;
x HACO-OFDM ＝x” PAM-DMT +x' ACO-OFDM ；
step 3), adding a cyclic prefix into the overlapped HACO-OFDM time domain signal, and transmitting the signal by a photoelectric intensity modulator through a DA digital-to-analog conversion and Bias Tee coupling circuit;
step 3 comprises the following steps:
3-1) time domain signal x for HACO-OFDM HACO-OFDM Inserting a cyclic prefix to obtain modulated data of HACO-OFDM;
3-2) after the modulated data of HACO-OFDM is subjected to DA digital-to-analog conversion, adding the modulated data and DC direct current Bias into a Bias Tee coupling circuit together, and driving an optical transmitter to complete the conversion from an electric signal to an optical signal;
3-3) the optical signal reaches a receiving end in a visible light communication (Visible Light Communication, VLC for short) channel through direct, scattered and reflected paths, and then is sent to the next step for processing;
the modulated optical signal reaches a receiving end, and the receiving end performs photoelectric conversion, filtering amplification and AD analog-to-digital conversion on the received signal to obtain a HACO-OFDM signal to be demodulated;
step 5) carrying out HACO-OFDM demodulation on the HACO-OFDM signal to be demodulated, wherein the first layer of data is subjected to ACO-OFDM demodulation, and because the interference of the first layer of ACO-OFDM modulation on the second layer of PAM-DMT is eliminated at a transmitting end, the second layer of data can be directly subjected to PAM-DMT demodulation, and finally binary data recovered by demodulation is sent back to a message sink;
step 5 comprises the following steps:
5-1) after receiving HACO-OFDM signals to be demodulated, firstly removing Cyclic Prefix (CP) signals to obtain N-point original HACO-OFDM signals;
5-2) carrying out FFT conversion on the N-point HACO-OFDM signal to obtain complex data of the N point again;
5-3) taking out the first layer ACO-OFDM data X 'to be demodulated output by FFT' 1，ACO-OFDM And a second layer of PAM-DMT data X 'to be demodulated' 2，PAM-DMT ；
5-4) for the first layer data X 'to be demodulated' 1，ACO-OFDM And second layer data to be demodulated X' 2，PAM-DMT Demodulating, judging the minimum Euclidean distance between the received constellation point and the theoretical constellation point by adopting a maximum likelihood ratio algorithm, and recovering the demodulated binary data X of the first layer (1) And recovering the demodulated binary data X of the second layer (2) ；
5-5) demodulating the first layer of binary data X (1) And second layer demodulated binary data X (2) The position adjustment is performed and then sent back to the sink.
By the above embodiment, the method has the advantages that: step 2), the PAM-DMT pre-distorted frequency domain data is obtained by subtracting ACO-OFDM frequency domain interference noise from PAM-DMT frequency domain data, FFT conversion is carried out on the PAM-DMT pre-distorted frequency domain data again to obtain a PAM-DMT pre-distorted time domain signal, then the limited ACO-OFDM time domain signal is overlapped with the PAM-DMT time domain signal to obtain a HACO-OFDM pre-distorted time domain signal, the interference of ACO-OFDM modulation on the PAM-DMT is eliminated at a transmitting end through the steps, a receiving end can directly carry out parallel demodulation on the data without SIC processing, the problems of high complexity, extra delay, error code propagation and the like caused by the fact that SIC serial interference elimination is adopted in the prior art are effectively avoided, and the method is superior to the HACO-OFDM scheme adopting SIC in error rate. In conclusion, the invention effectively improves the system performance of the hybrid HACO-OFDM.
Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.
Claims (3)
1. A method for improving the performance of hybrid HACO-OFDM by using a predistortion technique is characterized by comprising the following steps of
Before an ACO-OFDM time domain signal and a PAM-DMT time domain signal are overlapped to form a HACO-OFDM time domain signal, firstly, carrying out Fast Fourier Transform (FFT) on the ACO-OFDM time domain signal after amplitude limiting to obtain an ACO-OFDM frequency domain signal, extracting interference noise at an imaginary part position in an even subcarrier of the ACO-OFDM frequency domain signal, then subtracting the ACO-OFDM frequency domain interference noise from the PAM-DMT frequency domain data at the position of the even subcarrier by using a predistortion technology to obtain PAM-DMT predistorted frequency domain data, then carrying out fast Fourier transform (IFFT) on the PAM-DMT frequency domain data to obtain a PAM-DMT predistorted time domain signal, and finally, overlapping the PAM-DMT predistorted time domain signal with the ACO-OFDM time domain signal to form a HACO-OFDM time domain signal, so as to eliminate interference of ACO-OFDM modulation on the PAM-DMT;
the method specifically comprises the following steps:
step 1) data layering is carried out on a signal source, wherein ACO-OFDM modulation is carried out on first layer data, and the modulated data are correspondingly placed at odd subcarrier positions; PAM-DMT modulation is carried out on the second layer of data, and the modulated data are correspondingly placed at the positions of even subcarriers;
step 2) carrying out FFT conversion again on the ACO-OFDM time domain signals, subtracting the frequency domain interference noise of ACO-OFDM from the frequency domain data of PAM-DMT at the position of even subcarriers by using a predistortion technology to obtain frequency domain data after PAM-DMT predistortion, carrying out IFFT conversion on the frequency domain data after PAM-DMT predistortion to obtain time domain signals after PAM-DMT predistortion, and superposing the time domain signals after PAM-DMT predistortion with the time domain signals of ACO-OFDM to form HACO-OFDM time domain signals so as to eliminate interference of ACO-OFDM modulation on PAM-DMT;
step 2) comprises the steps of:
2-1) transmitting the AC O-OFDM real signal x of the first layer ACO-OFDM Zero-crossing amplitude limiting is carried out, negative value signals in ACO-OFDM real signals are removed, and x 'is obtained' ACO-OFDM ，
2-2) followed by a pair x' ACO-OFDM FFT conversion is carried out to obtain a new ACO-OFDM frequency domain signal X' ACO-OFDM Then extracting interference noise of the imaginary part position in the even sub-carrier;
2-3) subtracting ACO-OFDM frequency domain interference noise from PAM-DMT frequency domain data at even number sub-carrier positions to obtain PAM-DMT pre-distorted frequency domain data X' PAM-DMT ，
Wherein X is PAM-DMT Frequency domain data representing PAM-DMT, 2X imag (X' ACO-OFDM ) Representing the pair signal X' ACO-OFDM Taking the data of the imaginary part and multiplying the data by 2 times, wherein i represents a complex number;
2-4) frequency domain data X 'after predistortion of PAM-DMT' PAM-DMT Performing IFFT to obtain a PAM-DMT pre-distorted time domain signal x' PAM-DMT ，
x' PAM-DMT ＝IFFT(X' PAM-DMT )=(x' PAM-DMT,0 ,x' PAM-DMT,1 ,...,x' PAM-DMT,j ,...,x' PAM-DMT,N-1 )
Wherein x' PAM-DMT,j J= (0, n-1) represents the j-th PAM-DMT time domain signal of the IFFT output;
2-5) time domain signal x 'after predistortion of PAM-DMT' PAM-DMT Performing zero crossing amplitude limiting operation to remove negative value part in PAM-DMT to obtain x' PAM-DMT ：
2-6) time domain signal x' after predistortion of PAM-DMT " PAM-DMT With ACO-OFDM time domain signal x' ACO-OFDM Superposition to obtain HACO-OFDM time domain signal x HACO-OFDM So as to eliminate the interference of ACO-OFDM modulation to PAM-DMT;
x HACO-OFDM ＝x” PAM-DMT +x' ACO-OFDM ；
step 3), adding a Cyclic Prefix (CP) into the overlapped HACO-OFDM time domain signal, and transmitting the signal by a photoelectric intensity modulator through a DA digital-to-analog conversion and Bias Tee coupling circuit;
the modulated optical signal reaches a receiving end, and the receiving end performs photoelectric conversion, filtering amplification and AD analog-to-digital conversion on the received signal to obtain a HACO-OFDM signal to be demodulated;
step 5) HACO-OFDM demodulation is performed on the HACO-OFDM to-be-demodulated signal, wherein the first layer of data is subjected to ACO-OFDM demodulation, and because interference of the first layer ACO-OFDM modulation on the second layer PAM-DMT is eliminated at the transmitting end, the second layer of data can be directly subjected to PAM-DMT demodulation, and finally binary data recovered by demodulation is sent back to the sink.
2. The method for improving hybrid HACO-OFDM performance using predistortion technique according to claim 1, wherein step 1) comprises the steps of:
1-1) suppose that source data transmits N/2-1 data per frame, wherein sequence numbers of the data are respectively from X 1 To X N/2-1 Then (X) 1 ,X N/2-1 ) Dividing the odd-numbered data into a first layer, and performing Quadrature Amplitude Modulation (QAM) to obtain X ACO,k1 Where k1= (1, 2,., N/4); and (X) 1 ,X N/2-1 ) Dividing the even numbered data into a second layer, performing Pulse Amplitude Modulation (PAM) to obtain X PAM,k2 Where k2= (1, 2,., N/4-1);
1-2) mapping data X to QAM of the first layer ACO,k1 Performing conjugate symmetry to obtain conjugate data X * ACO,k1 Will map data X ACO,k1 And conjugate data X * ACO,k1 The mapping according to the requirement of conjugate symmetry is as follows:
X odd ＝[0,X ACO,1 ,0,X ACO,2 ,0,X ACO,3 ,...,X ACO,N/4 ,0,X * ACO,N/4 ,...,X * ACO,3 ,0,X * ACO,2 ,0,X * ACO,1 ]
1-3) data X satisfying conjugate symmetry odd Performing IFFT transformation to generate ACO-OFDM real signal x conforming to intensity modulation direct detection system (IM/DD) transmission ACO-OFDM ：
x ACO-OFDM ＝IFFT(X odd )
1-4) PAM mapping data X to second layer PAM,k2 Performing conjugate symmetry to obtain X * PAM,k2 Then the original value X PAM,k2 And conjugate data X * PAM,k2 The mapping according to the requirement of conjugate symmetry is as follows:
X even ＝[0,0,X PAM,1 ,0,X PAM,2 ,0,X PAM,3 ,...,X PAM,N/4-1 ,0,0,0,X * PAM,N/4-1 ,...,X * PAM,3 ,0,X * PAM,2 ,0,X * PAM,1 ]。
3. the method for improving hybrid HACO-OFDM performance using predistortion technique according to claim 1, wherein step 5) comprises the steps of:
5-1) after receiving HACO-OFDM signals to be demodulated, firstly removing Cyclic Prefix (CP) signals to obtain N-point original HACO-OFDM signals;
5-2) carrying out FFT conversion on the N-point HACO-OFDM signal to obtain complex data of the N point again;
5-3) taking out the first layer ACO-OFDM data X 'to be demodulated output by FFT' 1，ACO-OFDM And a second layer of PAM-DMT data X 'to be demodulated' 2，PAM-DMT ；
5-4) for the first layer data X 'to be demodulated' 1，ACO-OFDM And second layer data to be demodulated X' 2，PAM-DMT Demodulating, judging the minimum Euclidean distance between the received constellation point and the theoretical constellation point by adopting a maximum likelihood ratio algorithm, and recovering the demodulated binary data X of the first layer (1) And recovering the demodulated binary data X of the second layer (2) ；
5-5) demodulating the first layer of binary data X (1) And second layer demodulated binary data X (2) The position adjustment is performed and then sent back to the sink.
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