High-order QAM (Quadrature Amplitude Modulation) optical general filtering multi-carrier transmission method based on odd subcarrier auxiliary channel estimation

Abstract

The invention provides a high-order QAM (Quadrature Amplitude Modulation) optical general filtering multicarrier transmission method based on odd subcarrier auxiliary channel estimation, which comprises the following steps: a sending end maps a binary bit stream into a high-order QAM symbol, and transmits a sending signal through an optical fiber transmission channel after inverse fast Fourier transform and sub-carrier band general filtering; a receiving end performs time domain signal processing on the sending signal, separates an even-numbered subcarrier receiving signal and an odd-numbered subcarrier receiving signal through fast Fourier transform, and performs matched filtering on the even-numbered subcarrier receiving signal and the odd-numbered subcarrier receiving signal; performing channel frequency response estimation by using the matched and filtered odd subcarrier receiving signal to assist the matched and filtered even subcarrier receiving signal; and performing zero-forcing equalization on the even subcarrier receiving signal after matched filtering by using channel frequency response estimation to obtain an equalized even subcarrier sending signal, and performing de-mapping to obtain a recovered binary bit stream. According to the method, the defects of traditional LS channel estimation are compensated, and the accuracy of channel estimation is effectively improved under the condition that extra system resource overhead is not increased.

Description

Technical Field

[0001] The present invention relates to the technical field of optical universal filtering multi-carrier transmission, and in particular to a transmission method for a high-order QAM optical universal filtering multi-carrier system. Background Art

[0002] The large-scale commercialization of next-generation network architectures has led to an increasingly diverse range of communication technology service scenarios, driving a continuous increase in information transmission volume. With significant improvements in network connection speed and reliability, new services such as telemedicine, smart transportation, ultra-high-definition video, and cloud computing are experiencing explosive growth. In the future, communication networks will achieve integrated living and transportation, comprehensive coverage of space, air, and land, and universal industrial internet capabilities, evolving from connecting tens of billions of people to hundreds of billions of things. With the rapid increase in network terminals, network transmission systems face unprecedented challenges. New service demands not only require significant increases in network bandwidth but also impose stricter standards on system latency and reliability. Against this backdrop, fiber-optic communications, as the primary carrier of network traffic, urgently need to further increase capacity, enhance reliability, and reduce network latency. Due to its long transmission distance, high bandwidth, and low transmission loss, fiber-to-the-home (FTTH) deployment has been widely implemented, and fiber-to-the-room (FTTR) is gradually being implemented. Passive Optical Networks (PONs), which do not require active components during signal transmission, offer a wider range of applications and lower maintenance costs, making them a key technology in optical access networks. Multi-carrier PONs, which utilize multi-carrier modulation technology on PONs, are a hot research topic due to their low latency, high spectral efficiency, and greater flexibility.

[0003] Universal Filter Multi-Carrier (UFMC) technology is a multicarrier system that uses universal filters to divide subcarriers into different subbands and filters each subband separately. It inherits a series of advantages of traditional Orthogonal Frequency Division Multiplexing (OFDM), such as flexible spectrum configuration and strong resistance to fiber dispersion. Band filtering also uses filters to suppress out-of-band leakage of subcarriers, breaking the orthogonality between subcarriers. This makes the system more robust to time-frequency offset, eliminating the need for additional cyclic prefixes to combat time-frequency offset and improving the system's effective spectral efficiency.

[0004] Furthermore, high-order modulation formats such as 64-QAM, 256-QAM, and 1024-QAM can effectively improve signal spectral efficiency and have become a widely adopted mainstream solution in the industry, widely used in wireless communication channels such as 5G and WiFi. This technology theoretically offers significant advantages, with effective information entropy reaching 6-10 bits / symbol. However, as the modulation order increases, the signal will experience more severe linear and nonlinear impairments during transmission. Consequently, traditional channel estimation methods lack accuracy, making it difficult to effectively recover the transmitted signal. Summary of the Invention

[0005] In response to the technical problem that existing methods have insufficient channel estimation accuracy when facing high-order modulation, the present invention proposes a high-order QAM optical universal filtered multi-carrier transmission method based on odd subcarrier assisted channel estimation. In the high-order QAMUFMC transmission system, the channel characteristics of odd subcarriers are utilized to compensate for the shortcomings of traditional LS channel estimation, thereby improving the accuracy of channel estimation without increasing additional system resource overhead.

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

[0007] A high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation, comprising the following steps:

[0008] S1: The transmitter maps the binary bit stream into high-order QAM symbols, and then transmits the signal through the optical fiber transmission channel after frequency-time domain transformation and sub-carrier universal filtering.

[0009] S2: The receiving end performs time domain signal processing on the transmitted signal, separates the even subcarrier received signal and the odd subcarrier received signal through time domain-frequency domain transformation, and performs matched filtering;

[0010] S3: using the matched filtered odd subcarrier received signal to assist the matched filtered even subcarrier received signal to perform channel frequency response estimation;

[0011] S4: Using the channel frequency response estimation, zero-forcing equalization is performed on the even subcarrier received signal after matched filtering to obtain an equalized even subcarrier transmitted signal, and demapping is performed to obtain a recovered binary bit stream.

[0012] Furthermore, each of the high-order QAM symbols includes N subcarriers, of which Nc subcarriers are data subcarriers for transmitting data signals, and the remaining subcarriers are empty carriers; during the transmission of the high-order QAM symbols, the N subcarriers are divided into M independent subbands, and the number of subcarriers in each subband is dynamically configured according to the quality of service requirements; during the transmission of the high-order QAM symbols, block pilots are used: in the data frame of each complete frame, a pilot block is sent once at a certain interval, each pilot block is a high-order QAM symbol and all subcarriers in each pilot block are pilot subcarriers for transmitting pilot signals.

[0013] Furthermore, the implementation method of step S1 is:

[0014] S1.1: The transmitter maps the binary bit stream to each high-order QAM symbol, converts it from serial to parallel, and loads the parallel signal onto the corresponding subcarrier of each high-order QAM symbol to obtain the frequency domain signal U of each subband according to different needs. i (k);

[0015] S1.2: Frequency domain signal U for each sub-band i (k) Perform inverse Fourier transform to convert into the time domain signal u of each subband i (n);

[0016] S1.3: Obtain the time domain signal u of each subband i (n), use the filter bank to filter each sub-band and obtain the filtered signal x of each sub-band i (n);

[0017] S1.4: The filtered signal x of each sub-band i (n) performing superposition to obtain the transmission signal x(n) corresponding to each high-order QAM symbol, combining multiple transmission signals x(n) and pilot blocks into a data frame, and inserting a pseudo-noise sequence before the data frame as a frame synchronization identifier;

[0018] Among them, i is the subband number, k is the frequency domain index, and n is the time domain index.

[0019] Furthermore, the implementation method of step S2 is:

[0020] S2.1: At the receiving end, the signal is received through a strength detector and frame synchronization is performed according to the pseudo-noise sequence to obtain the received signal y(n);

[0021] S2.2: Pad the end of each symbol of the received signal y(n) with zeros to make the data length of each symbol 2N. Then perform a 2N-point fast Fourier transform to obtain the frequency domain signal Y(m).

[0022] S2.3: Considering the influence of noise and channel, the received signal Y(m) in the frequency domain is expressed as the received signal Y of the even subcarriers even (m) and odd subcarrier received signal Y odd (m);

[0023] S2.4: Receive signal Y for even-numbered subcarriers even (m) and odd subcarrier received signal Y odd (m) Perform matched filtering respectively.

[0024] Furthermore, the received signal Y(m) in the frequency domain is represented as an even subcarrier received signal Y even (m) and odd subcarrier received signal Y odd The calculation method of (m) is:

[0025]

[0026] Among them, m is the frequency domain index after 2N-point fast Fourier transform, H even The channel frequency response experienced by the even subcarriers is H odd is the channel frequency response experienced by the odd subcarrier transmission signal, W(m) is the additive noise; X(·) is the subcarrier signal after 2N-point FFT at the transmitting end.

[0027] Furthermore, the implementation method of step S3 is: using LS channel estimation to obtain the odd pilot channel frequency response estimation at each pilot block and even pilot channel frequency response estimation And perform odd and even pilot frequency response estimation and joint mean filtering; perform jump detection and response correction based on the even pilot channel frequency response estimation to obtain the final channel frequency response estimation H OE .

[0028] Furthermore, the odd pilot channel frequency response estimation The calculation method is:

[0029]

[0030] Among them, Y podd Receive signals for all odd subcarriers after matched filtering The pilot received signal, X podd The pilot signal of the transmitting end is the odd subcarrier pilot signal after the pilot signal has been subjected to 2N-point FFT and filter matching.

[0031] Furthermore, the method for performing odd and even pilot frequency response estimation combined with mean filtering is:

[0032] Combined even pilot channel frequency response estimation and odd pilot channel frequency response estimation Get the combined pilot channel frequency response estimate H combine , estimate the combined pilot channel frequency response H combine Perform mean filtering to obtain a smoothed channel frequency response estimate H LS ;

[0033]

[0034] H LS =mean(H combine )

[0035] in, Indicates matrix merging, and mean(·) represents the mean function.

[0036] Furthermore, the method for performing jump detection and response correction based on the even pilot channel frequency response estimation is as follows: calculating the difference between the odd and even pilot channel frequency response estimations Calculate the global average difference μ d =mean(H d ), with 3μ d is the jump threshold, the current H d >3μ d When , it is determined that the channel frequency response estimate at the odd subcarrier of the pilot block is abnormal, and the abnormal point is covered by the even pilot channel frequency response estimate to obtain the final channel frequency response estimate:

[0037]

[0038] Furthermore, the even subcarrier receiving signal after matched filtering is subjected to zero-forcing equalization to obtain the equalized even subcarrier transmitting signal: the final channel frequency response is used to estimate H OE Receive signals for all even subcarriers after matched filtering The even subcarrier data reception signal Y i,Deven (m) Perform zero-forcing equalization to obtain an even-numbered subcarrier data transmission signal after equalization:

[0039]

[0040] The beneficial effects of the present invention are:

[0041] In high-order QAM UFMC transmission systems, the channel characteristics of odd subcarriers are utilized to compensate for the shortcomings of traditional LS channel estimation and effectively improve the accuracy of channel estimation without increasing additional system resource overhead. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 This is a framework diagram of the high-order QAM optical universal filtering multi-carrier transmission system based on odd subcarrier assisted channel estimation of the present invention.

[0044] Figure 2 Figure 2 is the constellation diagram of the universal filtered multi-carrier UFMC receiver, where (a) is the even subcarrier and (b) is the odd subcarrier.

[0045] Figure 3 Schematic diagram of odd and even subcarrier transmission according to the present invention.

[0046] Figure 4 The even-odd subcarrier channel frequency response body of the present invention, wherein (a) is the even subcarrier amplitude response, (b) is the odd subcarrier amplitude response, (c) is the even subcarrier phase response, and (d) is the odd subcarrier phase response.

[0047] Figure 5 Schematic diagram of cross-estimation of UFMC 64-QAM 30km optical fiber transmission according to the present invention.

[0048] Figure 6 This is a channel amplitude response diagram of the UFMC 64-QAM 30km optical fiber transmission odd subcarrier at different subcarriers of the present invention.

[0049] Figure 7 This is a diagram of the experimental architecture of the IM / DD UFMC optical fiber transmission system of the present invention.

[0050] Figure 8 1 is a bit error rate curve diagram of the UFMC optical fiber transmission system of the present invention, wherein (a) is 64-QAM, (b) is 256-QAM, and (c) is 1024-QAM.

[0051] Figure 9 Figure 1 is the constellation diagram before and after UFMC transmission of the present invention, where (a) is before 64-QAM even subcarrier transmission, (b) is after 64-QAM even subcarrier transmission, (c) is before 64-QAM odd subcarrier transmission, (d) is after 64-QAM odd subcarrier transmission, (e) is before 1024-QAM even subcarrier transmission, (f) is after 1024-QAM even subcarrier transmission, (g) is before 1024-QAM odd subcarrier transmission, and (h) is after 1024-QAM odd subcarrier transmission. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0053] A high-order QAM optical universal filtering multi-carrier transmission method based on odd subcarrier assisted channel estimation, such as Figure 1 As shown, the steps are:

[0054] S1: The transmitter maps the binary bit stream into high-order QAM symbols, and transmits the signal through the optical fiber transmission channel after inverse fast Fourier transform and sub-carrier universal filtering.

[0055] like Figure 1 As shown, each high-order QAM symbol (i.e., UFMC symbol) contains N subcarriers (N=512 points), of which Nc subcarriers are data subcarriers for transmitting data signals, and the remaining subcarriers are empty carriers. In order to further adapt to multi-service needs, the N subcarriers are divided into M independent subbands, and the number of subcarriers in each subband is dynamically configured according to the quality of service (QoS) requirements; this flexible resource allocation mechanism significantly improves the spectrum utilization on the one hand, and on the other hand, through refined management and control at the subband level, it can provide differentiated support for scenarios such as machine-type communications (MTC) and ultra-reliable low-latency communications (URLLC), meeting the diverse needs of emerging applications. In addition, the present invention adopts block pilots: in the data frame of each complete frame, a pilot block is sent at a certain interval, each pilot block is a UFMC symbol, and all subcarriers in each pilot block are pilot subcarriers for transmitting pilot signals.

[0056] S1.1: If Figure 1 As shown, the transmitter maps the binary bit stream to each high-order QAM symbol, converts it from serial to parallel, and loads the parallel signal onto the corresponding subcarrier of each high-order QAM symbol to obtain the frequency domain signal U of each subband according to different needs. i ={U i (0),U i (1),...U i (N i -1)}.

[0057] S1.2: Frequency domain signal U for each sub-band i(k) Perform inverse Fourier transform (IFFT) to convert the signals into time domain signals of each sub-band:

[0058]

[0059] Among them, u i (n) is the time domain signal corresponding to the i-th subband, N is the total number of subcarriers, k is the frequency domain index, n is the time domain index, n∈[0,N], N i is the number of subcarriers in the ith subband, and j is an imaginary unit.

[0060] S1.3: After obtaining the time domain signals of each subband, filter banks are used to filter each subband of the signal to suppress the out-of-band leakage power of the multi-carrier signal. The filtered signals of each subband are expressed as:

[0061]

[0062] Among them, f i (n) represents the time domain impulse response function of the ith subband bandpass filter, Represents the convolution operation. The filter will produce a tail. Assuming that the length of the filter is uniformly L=43, the filtered signal x of each sub-band can be obtained. i The length of (n) is N+L-1.

[0063] S1.4: The filtered signal x of each sub-band i (n) are superimposed to obtain the transmit signal x(n) corresponding to each high-order QAM symbol. Multiple transmit signals x(n) and pilot blocks are combined to form a data frame, and a pseudo-noise sequence (PN) is inserted before the data frame as a frame synchronization marker. A complete frame used in the experiment consists of a frame header (i.e., synchronization marker) plus a data frame. The frame header length is 127, and the data frame length is H. It consists of H UFMC symbols, each of which is N+L-1 in length. In this embodiment, H = 64. Each transmit signal x(n) is a UFMC symbol, and each pilot block is a UFMC symbol.

[0064] The final transmitted signal x(n) corresponding to each high-order QAM symbol is composed of the filtered signal x i (n) is obtained by superposition:

[0065]

[0066] The final transmission signal x(n) is transmitted through the optical fiber transmission channel, where n is the time domain index.

[0067] S2: The receiving end performs time domain signal processing on the transmitted signal and separates the even subcarrier received signal and the odd subcarrier received signal through fast Fourier transform and performs matched filtering.

[0068] S2.1: At the receiving end, the signal is received through a strength detector and frame synchronization is performed. That is, the PN sequence is used to locate the starting position of the signal to obtain the received signal y(n):

[0069]

[0070] Where h(n) is the time domain response of the channel, w(n) is the channel noise, Represents element-wise multiplication.

[0071] S2.2: The length of each symbol of the received signal y(n) is N+L-1, which does not meet the Fourier transform requirement of an integer multiple of 2. Therefore, here we use the method of padding zeros at the end of each symbol of the received signal y(n) to make the data length of each symbol 2N. Then, a 2N-point fast Fourier transform is performed to obtain the received signal Y(m) in the frequency domain:

[0072]

[0073] S2.3: Considering the influence of noise and channel, the received signal Y(m) in the frequency domain can be expressed as the received signal Y of the even subcarriers even (m) and odd subcarrier received signal Y odd (m):

[0074]

[0075] Among them, m is the frequency domain index after 2N-point fast Fourier transform, H even The channel frequency response experienced by the even subcarriers is H odd is the channel frequency response experienced by the odd subcarrier transmission signal, W(m) is the additive noise. X(·) is the subcarrier signal after the 2N-point FFT at the transmitting end. Y i (m) is the frequency domain received signal of each sub-band corresponding to the frequency domain received signal Y(m), i,even (m) is the received signal of the even subcarrier of the i-th subband, Y i,odd (m) is the received signal of the odd subcarriers of the i-th subband.

[0076] S2.4: Receive signal Y for the even subcarriers of the i-th subband i,even (m) and odd subcarrier received signal Y i,odd (m) Perform matched filtering respectively:

[0077]

[0078] Among them, f i -1 (·) is the inverse filter function, is the even subcarrier received signal of the ith subband to be filtered, The odd subcarrier received signal of the ith subband is defiltered.

[0079] S3: Using the odd subcarrier received signals after matched filtering to assist the even subcarrier received signals after matched filtering to perform channel frequency response estimation.

[0080] In traditional UFMC-PON, in order to suppress inter-carrier interference, only even-numbered subcarriers are extracted for signal recovery, and odd-numbered subcarriers are completely discarded. In fact, the odd-numbered subcarriers that are usually discarded also carry the correct channel frequency response information. The symbols of the odd-numbered subcarriers are obtained by superimposing the data of the original subcarriers, so it is difficult to directly separate the original data from them. Figure 2 As shown, Figure 2 (a) and (b) are the constellation diagrams of the even subcarriers and odd subcarriers at the receiving end of the UFMC 64-QAM system, respectively, ignoring the influence of the channel. Figure 2 As can be seen, the constellation diagram of the even subcarrier is the same as the constellation of the original data Figure 1 The constellation diagram of the odd subcarrier is no longer distinguishable. odd (m) formula, this part of the data also contains the original data. In the case of ignoring the channel, the signal can be converted into two signals by 2 times FFT after zero padding before entering the channel at the transmitter. Figure 3 As shown in the figure, in actual transmission systems, it can be considered that the even and odd subcarriers are aggregated and pass through the channel together. At the receiving end, only the even subcarriers are processed, and the channel frequency response is estimated using the LS channel estimation technique, which is then applied to subsequent channel equalization. Here, odd and even subcarriers are treated equally, and the LS channel estimation technique can also be used to estimate the channel through which the odd subcarriers pass. After LS channel estimation, the channel frequency responses of the even and odd subcarriers can be obtained separately.

[0081] In optical UFMC transmission systems, signal transmission undergoes multiple steps, including digital-to-analog conversion, electro-optical conversion, optical fiber transmission, optoelectronic conversion, and analog-to-digital conversion. Imperfect transmission characteristics of any of these components can affect the signal, and these impairments can be collectively considered channel impairments. In multi-carrier systems, pilot-assisted channel estimation techniques are often used to balance channel effects.

[0082] In channel estimation, all the processes from signal sending to receiving are considered as a whole, so the sending to receiving process can be expressed as:

[0083] Y=XH+W; (1)

[0084] Formula (1) is the frequency domain representation of signal transmission, where Y represents the received signal, H is the channel frequency response, X is the transmitted signal, and W is the channel noise. Let the estimated channel frequency response be Then according to the least squares theorem, in the theoretical model close to the actual situation, the sum of squares of errors is the smallest, so the cost function for:

[0085]

[0086] Where () H It represents taking the Hermite symmetry operation, taking the partial derivative of the cost function and making the partial derivative equal to 0, and the minimum point of the cost function can be obtained:

[0087]

[0088] Solving the equation, we can get the LS channel frequency response estimate as:

[0089] H=(X H X) -1 X H Y=X -1 Y; (4)

[0090] According to equation (4), the channel frequency response estimation at the pilot block can be easily obtained:

[0091]

[0092] S3.1: Specifically, using equation (5), LS channel estimation is used to obtain the odd pilot channel frequency response estimate at each pilot block and even pilot channel frequency response estimation

[0093] The data frame in optical fiber transmission contains both pilot signals and data signals. Since the data signal is unknown but the pilot signal is known, the pilot signal needs to be used to estimate the frequency response.

[0094] The odd pilot channel frequency response estimation The calculation method is:

[0095]

[0096] Among them, Y podd Receive signals for all odd subcarriers after matched filtering The pilot received signal, X poddThe pilot signal of the transmitting end is the odd subcarrier pilot signal after the pilot signal has been subjected to 2N-point FFT and filter matching.

[0097] The even pilot channel frequency response estimation The calculation method is:

[0098]

[0099] Among them, Y peven Receive signals for all even subcarriers after matched filtering The pilot received signal in X peven The pilot signal of the transmitting end is the even subcarrier pilot signal after the pilot signal has been subjected to 2N-point FFT and filter matching.

[0100] Figure 4 (a), (b), (c) and (d) are the channel amplitude response and phase response of the odd and even subcarriers at each subcarrier when the UFMC 64-QAM signal is transmitted over 30 km of optical fiber with a received optical power of 10 dBm. Figure 4 As can be seen from the figure, the channel frequency responses of even and odd subcarriers exhibit a strong correlation in the frequency domain. It is also noted that the amplitude and phase responses on each subcarrier are not completely consistent. This indicates that the channel frequency response information carried by odd and even subcarriers is different. However, due to the influence of noise in the channel and the relatively chaotic constellation diagram of odd subcarriers, this observation alone cannot determine whether the channel frequency response extracted from odd subcarriers is suitable for signal demodulation. Therefore, further verification is required to determine whether the channel frequency response extracted from odd subcarriers carries the correct channel frequency response information.

[0101] If the channel frequency response of the odd subcarriers can also play the same role as the channel frequency response of the even subcarriers, it means that the channel frequency response of the odd subcarriers also carries the correct channel information. Therefore, the present invention adopts a cross-estimation method, using the channel estimation of the odd subcarriers to act on the equalizer of the even subcarriers. The final equalization result is as follows Figure 5 As shown. Figure 5 As can be seen from the figure, using the channel frequency response of odd subcarriers for channel estimation can also restore the constellation diagram of even subcarriers to the correct position. This shows that the channel frequency response of odd subcarriers can also correctly represent the channel frequency response of the channel. However, the figure also shows that compared to the constellation diagram before equalization, some constellation points appear discrete after using the channel frequency response of odd subcarriers. This is because the signals of odd subcarriers resemble irregular modulation, with irregular distribution in the frequency domain, making them more susceptible to channel noise, resulting in jumps in the channel frequency response. Figure 6The channel amplitude response of each odd subcarrier when the received optical power is 10dBm for UFMC 64-QAM signal transmission over 30km of optical fiber. Figure 6 It can be seen that compared with Figure 4 , the channel frequency response of odd subcarriers is more likely to have large amplitude jumps. Although amplitude jumps occur occasionally, it is difficult to recover the signal based on the information of the odd subcarriers themselves. The channel frequency response of even subcarriers H even and the channel frequency response H for odd subcarriers odd The channel noise is also included in the estimation. Since the channel noise included in the two is different, the channel frequency response H of the even subcarrier can be even and the channel frequency response H for odd subcarriers odd Mean filtering is performed together, so the present invention proposes a method for odd and even subcarrier frequency response estimation combined with mean filtering.

[0102] S3.2: Perform odd and even subcarrier frequency response estimation Joint mean filtering: Combine even pilot channel frequency response estimation and odd pilot channel frequency response estimation Get the combined pilot channel frequency response estimate H combine , estimate the combined pilot channel frequency response H combine Perform mean filtering to obtain a smoothed channel frequency response estimate H LS :

[0103]

[0104] H LS =mean(H combine );

[0105] in, Indicates matrix merging, mean(·) indicates the mean function; however, here, due to the amplitude jump problem of the odd subcarrier channel frequency response estimation, even if the mean is used, these extremely large peak-to-peak values ​​will still cause a large deviation in the channel frequency response estimation. Since the channel frequency response estimation of the even subcarrier is relatively free from this problem, the channel frequency response estimation of the even subcarrier can be used as a benchmark. When a large jump occurs, the even pilot channel frequency response estimation is directly used instead of the mean filter. Assign the value of H LS .

[0106] S3.3: Perform transition detection and response correction based on the even pilot channel frequency response estimate: Calculate the difference between the odd and even pilot channel frequency response estimates Calculate the global average difference μ d =mean(Hd ), with 3μ d is the jump threshold, the current H d >3μ d When , it is determined that the channel frequency response estimate at the odd subcarrier of the pilot block is abnormal, and the abnormal point is covered by the even pilot channel frequency response estimate to obtain the final channel frequency response estimate:

[0107]

[0108] S4: performing zero-forcing equalization on the even subcarrier signal after matched filtering using the channel frequency response estimation to obtain an equalized even subcarrier signal, and performing demapping to obtain a recovered binary bit stream.

[0109] Because the optical fiber channel is a slowly time-varying channel, the channel frequency response can generally be approximately considered to be fixed. Therefore, it is assumed here that the channel frequency response estimated by the pilot is the channel frequency response of the data. Therefore, the channel frequency response estimated by the pilot can be used to recover the data signal.

[0110] Use the final channel frequency response to estimate H OE Receive signals for all even subcarriers after matched filtering The even subcarrier data receiving signal is subjected to zero-forcing equalization to obtain the equalized even subcarrier data sending signal:

[0111]

[0112] Among them, Y i,Deven (m) is the even subcarrier received signal The even-numbered subcarrier data in the received signal.

[0113] Send signal for equalized even subcarrier data Demapping is performed to obtain the recovered binary bit stream.

[0114] Experiment: The optical fiber transmission experimental architecture of the present invention is shown in the figure. Figure 7This is the experimental architecture for an intensity modulation / direct detection universal filtered multicarrier (IM / DD UFMC) optical fiber transmission system. First, a baseband signal is generated. Here, a pseudo-random binary sequence (PRBS) is used to generate QAM symbols through QAM constellation mapping. After assigning these symbols to subcarriers, an inverse fast Fourier transform (IFFT) is used to convert the frequency-domain signal into a time-domain signal. After the time-frequency conversion, a universal filtering bank is used to suppress inter-carrier interference (ICI). Finally, a pseudo-noise (PN) sequence is inserted before each frame as a frame synchronization marker, forming a complete UFMC data frame. The upsampled UFMC signal is converted to an analog signal by an arbitrary waveform generator (AWG). This signal drives a Mach-Zehnder modulator (MZM) to intensity modulate the continuous wave light output by a continuous-wave laser with a center frequency of 1550.116 nm. The AWG sampling rate is set to 50 GSa / s, and the signal rate is 12.5 GBd. The modulated optical signal enters a 30 km standard single-mode fiber (SSMF). After fiber transmission, the signal is cascaded through a variable optical attenuation (VOA) and an erbium-doped fiber amplifier (EDFA). The noise level is dynamically adjusted by varying the optical power (ROP) entering the EDFA. Due to the sensitivity limitations of the receiving photodetector (PD), a VOA2 is used to control the optical power entering the PD. A scilloscope is used as an oscilloscope.

[0115] At the receiving end, after downsampling and synchronization, a time domain compensation algorithm can be used. A nonlinear equalizer (in this embodiment, a Voltaire equalizer) can be used to compensate the time domain signal, or no compensation algorithm can be used. The real signal is then restored to a complex signal, and after zero padding each frame of the signal, it is converted to the frequency domain through a 2N-point FFT. In the frequency domain, the influence of the transmitting filter group must first be eliminated, so the inverse function of the corresponding filter must be multiplied. LS channel estimation and zero-forcing equalization are then used to reduce the impact of the channel on the signal. The equalized signal is sent to the QAM demapping module to obtain binary bit data. By comparing the obtained bit data with the original bit data, the bit error rate (BER) of the entire system transmission process can be tested.

[0116] Experimental results and analysis: Figure 8 The bit error rate (BER) versus received optical power curves for UFMC 64-QAM, 256-QAM, and 1024-QAM signals transmitted over a 30km SSMF using odd-subcarrier-assisted channel estimation (OSCAE) were compared. The experimental results show that the 64-QAM signal achieves approximately 0.25dB improvement in receiver sensitivity near the hard decision threshold. In contrast, the performance improvement for the 256-QAM signal is relatively limited, with only a slight improvement compared to the original signal. Meanwhile, the 1024-QAM signal achieves a 1dB improvement in receiver sensitivity near the decision threshold.

[0117] In the 1024-QAM system, this technology has a significant effect, and the reason can be attributed to two key factors. First, the higher received optical power ensures a better SNR, making the channel frequency response estimation value of the odd subcarrier closer to the true value. The transmission of high-order QAM itself requires a higher SNR, and its signal requirements are very high. This is an objective requirement. For channel estimation, less noise interference will increase the accuracy of channel estimation, so the effect will be better after adding the auxiliary algorithm. Secondly, from the experimental results Figure 8 As can be seen from the figure, the improvement effect from -4dBm to 0dBm on high-order QAM is actually the same, so SNR is not the main reason. This advantage allows the influence of noise after mean filtering to be more effectively suppressed. The deeper reason lies in the unique characteristics of high-order modulation: as the modulation order increases to 1024, the Euclidean distance between each symbol point in the constellation diagram is greatly reduced, resulting in a denser distribution of constellation points in the entire signal space. More importantly, by comparison Figure 9As can be seen from (b), (d) and (f), (h), under 1024-QAM conditions, after channel transmission, the distribution patterns of even subcarriers and odd subcarriers on the constellation diagram show a higher degree of similarity. This enhanced distribution similarity provides a more reliable basis for channel feature estimation, thereby obtaining a more accurate channel estimation result. However, since the data of odd subcarriers are discrete points, the Euclidean distance between them is very small and they are more susceptible to noise. When the signal's anti-noise ability is improved, the effect of subcarrier-assisted channel estimation will also be improved. The present invention proposes an odd subcarrier-assisted channel estimation technology for optical UFMC PON systems. According to the characteristic that the UFMC system needs to use twice the number of points FFT to separate odd and even subcarriers, the odd subcarriers that are usually abandoned are used to perform channel estimation to obtain the channel frequency response of the odd subcarriers. The channel frequency response of the odd subcarriers is used to assist the even subcarriers in channel estimation, and the channel frequency response of the odd subcarriers and the channel frequency response of the even subcarriers are mean filtered to reduce the influence of channel noise ignored in LS channel estimation. The present invention was experimentally verified in a 30km SSMF transmission system, and a 1dB improvement in receiving sensitivity was achieved near the decision threshold under a 1024-QAM signal.

[0118] The above description 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 scope of protection of the present invention.

Claims

1. A high-order QAM optical universal filtering multi-carrier transmission method based on odd subcarrier assisted channel estimation, characterized in that: The steps are: S1: The transmitter maps the binary bit stream into high-order QAM symbols, and then transmits the signal through the optical fiber transmission channel after frequency-time domain transformation and sub-carrier universal filtering. S2: The receiving end performs time domain signal processing on the transmitted signal, separates the even subcarrier received signal and the odd subcarrier received signal through time domain-frequency domain transformation, and performs matched filtering; S3: using the matched filtered odd subcarrier received signal to assist the matched filtered even subcarrier received signal to perform channel frequency response estimation; S4: Using the channel frequency response estimation, zero-forcing equalization is performed on the even subcarrier received signal after matched filtering to obtain an equalized even subcarrier transmitted signal, and demapping is performed to obtain a recovered binary bit stream.

2. The high-order QAM optical universal filtering multi-carrier transmission method based on odd subcarrier assisted channel estimation according to claim 1, characterized in that: Each of the high-order QAM symbols includes N subcarriers, of which Nc subcarriers are data subcarriers for transmitting data signals, and the remaining subcarriers are null carriers; during the transmission of the high-order QAM symbols, the N subcarriers are divided into M independent subbands, and the number of subcarriers in each subband is dynamically configured according to the quality of service requirements; During the transmission of the high-order QAM symbols, block pilots are used: in each complete frame of data, a pilot block is sent at regular intervals, each pilot block is a high-order QAM symbol and all subcarriers in each pilot block are pilot subcarriers for transmitting pilot signals.

3. The high-order QAM optical universal filtering multi-carrier transmission method based on odd subcarrier assisted channel estimation according to claim 2, characterized in that: The implementation method of step S1 is: S1.1: The transmitter maps the binary bit stream to each high-order QAM symbol, converts it from serial to parallel, and loads the parallel signal onto the corresponding subcarrier of each high-order QAM symbol to obtain the frequency domain signal U of each subband according to different needs. i (k); S1.2: Frequency domain signal U for each sub-band i (k) Perform inverse Fourier transform to convert into the time domain signal u of each subband i (n); S1.3: Obtain the time domain signal u of each subband i (n), use the filter bank to filter each sub-band and obtain the filtered signal x of each sub-band i (n); S1.4: The filtered signal x of each sub-band i (n) performing superposition to obtain the transmission signal x(n) corresponding to each high-order QAM symbol, combining multiple transmission signals x(n) and pilot blocks into a data frame, and inserting a pseudo-noise sequence before the data frame as a frame synchronization identifier; Among them, i is the subband number, k is the frequency domain index, and n is the time domain index.

4. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 3, characterized in that: The implementation method of step S2 is: S2.1: At the receiving end, the signal is received through a strength detector and frame synchronization is performed according to the pseudo noise sequence to obtain the received signal y(n); S2.2: Pad the end of each symbol of the received signal y(n) with zeros to make the data length of each symbol 2N. Then perform a 2N-point fast Fourier transform to obtain the frequency domain signal Y(m). S2.3: Considering the influence of noise and channel, the received signal Y(m) in the frequency domain is expressed as the received signal Y of the even subcarriers even (m) and odd subcarrier received signal Y odd (m); S2.4: Receive signal Y for even-numbered subcarriers even (m) and odd subcarrier received signal Y odd (m) Perform matched filtering respectively.

5. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 4, characterized in that: The received signal Y(m) in the frequency domain is represented as the received signal Y of the even subcarriers. even (m) and odd subcarrier received signal Y odd The calculation method of (m) is: Among them, m is the frequency domain index after 2N-point fast Fourier transform, H even The channel frequency response experienced by the even subcarriers is H odd is the channel frequency response experienced by the odd subcarrier transmission signal, W(m) is the additive noise; X(·) is the subcarrier signal after 2N-point FFT at the transmitting end.

6. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to any one of claims 1 to 5, characterized in that: Step S3 is implemented by using LS channel estimation to obtain the frequency response estimate of the odd pilot channel at each pilot block. and even pilot channel frequency response estimation And perform odd and even pilot frequency response estimation and joint mean filtering; perform jump detection and response correction based on the even pilot channel frequency response estimation to obtain the final channel frequency response estimation H OE .

7. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 6, characterized in that: The odd pilot channel frequency response estimation The calculation method is: Among them, Y podd Receive signals for all odd subcarriers after matched filtering The pilot received signal, X podd The pilot signal of the transmitting end is the odd subcarrier pilot signal after the pilot signal has been subjected to 2N-point FFT and filter matching.

8. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 6, characterized in that: The method for performing odd and even pilot frequency response estimation combined with mean filtering is: Combined even pilot channel frequency response estimation and odd pilot channel frequency response estimation Get the combined pilot channel frequency response estimate H combine , estimate the combined pilot channel frequency response H combine Perform mean filtering to obtain a smoothed channel frequency response estimate H LS ; H LS =mean(H combine ) in, Indicates matrix merging, and mean(·) represents the mean function.

9. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 7 or 8, characterized in that: The method for performing jump detection and response correction based on the even pilot channel frequency response estimation is as follows: calculating the difference between the odd and even pilot channel frequency response estimations Calculate the global average difference μ d =mean(H d ), with 3μ d is the jump threshold, the current H d >3μ d When , it is determined that the channel frequency response estimate at the odd subcarrier of the pilot block is abnormal, and the abnormal point is covered by the even pilot channel frequency response estimate to obtain the final channel frequency response estimate:

10. The high-order QAM optical universal filtering multi-carrier transmission method based on odd-numbered subcarrier assisted channel estimation according to claim 9, characterized in that: The even subcarrier receiving signal after matched filtering is subjected to zero-forcing equalization to obtain the equalized even subcarrier transmitting signal: the final channel frequency response is used to estimate H OE Receive signals for all even subcarriers after matched filtering The even subcarrier data reception signal Y i,Deven (m) Perform zero-forcing equalization to obtain an even-numbered subcarrier data transmission signal after equalization:

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