Improved selective mapping scheme for OFDM-NOMA PON

Abstract

The invention discloses an improved selective mapping scheme (N-SLM) for an orthogonal frequency division multiplexing non-orthogonal multiple access passive optical network (OFDM-NOMA PON), and relates to the technical field of optical communication and digital signal processing. In order to solve the problem that the peak-to-average power ratio (PAPR) of a downlink sending signal of a non-orthogonal multiple access system modulated by adopting OFDM is relatively high, and to overcome the defects that a traditional C-SLM scheme is high in calculation complexity and extra bandwidth resources need to be occupied due to the high error rate of explicit side information (SI), the invention provides a non-orthogonal multiple access method based on the C-SLM scheme. An N-SLM scheme is provided according to the characteristic of superposition transmission of power domain NOMA multi-user data on subcarriers. Compared with a C-SLM scheme, the scheme is low in calculation complexity and does not need to send explicit SI. The method mainly comprises the generation process of alternative OFDM-NOMA signals of a sending end and the demodulation process of a receiving end. According to the invention, the sending end adopts information bit cyclic shift, and inserts very few check bits subjected to repeated coding in a leading part to generate a plurality of pieces of alternative bit information, so that explicit transmission of SI is avoided. And performing QAM modulation, power distribution and OFDM modulation on all the alternative bit information in sequence to generate a plurality of groups of OFDM signals with different power levels, and performing permutation, combination and linear superposition on the OFDM signals to synthesize a plurality of alternative OFDM-NOMA signals. And calculating the PAPR values of all the alternative OFDM-NOMA signals, and selecting the alternative OFDM-NOMA signal with the minimum PAPR to carry out subsequent signal processing. And a receiving end respectively performs soft demodulation and hard demodulation on QAM symbols corresponding to the repeatedly coded check bit and the information bit, and performs corresponding reverse cyclic shift on the received information bit according to the check bit which is judged to be output after repeated decoding. And after serial interference cancellation (SIC) is carried out on the low-power-level data to obtain corresponding information, the same processing process is carried out.

Description

Technical Field

[0001] The present invention is an improved selective mapping scheme (N-SLM) for orthogonal frequency division multiplexing non-orthogonal multiple access passive optical network (OFDM-NOMA PON), which relates to the technical field of optical communication and digital signal processing, and specifically to a low-complexity, non-explicit SI transmission probability-based PAPR suppression scheme. Background Art

[0002] Due to the rapid development of wired and wireless communication services, the demand for network capacity has further increased, which has accelerated the development of high-capacity, low-latency, and low-cost PONs. With the surge in data demand for new services, traditional PONs based on orthogonal multiple access (OMA) technology cannot further increase their capacity requirements due to dispersion and economic considerations. At the same time, due to the limitations of available bandwidth, it is difficult to economically and effectively increase the number of optical network units (ONUs) in a single PON. In contrast, the OFDM-NOMA scheme proposed in recent years has been proven to be one of the promising methods for achieving simultaneous connection of massive devices. NOMA further improves system throughput and spectrum utilization by allocating different powers to users at the transmitter and using SIC technology at the receiver, so that multiple users can multiplex the same subcarrier. The OFDM-NOMA scheme has been widely studied as a new and promising scheme in PON due to its advantages such as large capacity, strong access flexibility, low overhead and low complexity.

[0003] Since the optical access structure is extremely sensitive to equipment costs, low-cost electrical / optical devices such as Mach-Zehnder modulators (MZMs) and direct detection solutions are still the preferred choice for PON. However, OFDM symbols are composed of multiple independent modulated subcarrier signals. When the data phases on each subcarrier are the same or similar, a large instantaneous power peak will be generated, resulting in a higher PAPR. High PAPR signals will not only cause the MZM to work at a linear bias point or enter a nonlinear region, thereby distorting the high-amplitude part of the signal, but will also aggravate the nonlinear effects of the optical fiber (self-phase modulation, cross-phase modulation, four-wave mixing, etc.).

[0004] At present, there are three main peak-to-average ratio suppression technologies in OFDM systems: pre-distortion technology, probability technology and precoding technology. The present invention mainly improves the traditional SLM scheme in the probability technology. In view of the shortcomings of the traditional SLM scheme with high computational complexity and high explicit SI error rate occupying additional bandwidth, a low-complexity SLM scheme is proposed based on the characteristics of superimposed transmission of multi-user data on subcarriers in power domain NOMA. By inserting the check bit into the leading part of the information bit to be transmitted, and judging the cyclic shift adopted by the information bit of the corresponding ONU at the receiving end based on the check bit, there is no need to send explicit SI. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide an improved PAPR suppression scheme for OFDM-NOMA PON, which has low complexity, does not require explicit SI transmission, and does not cause signal distortion, thereby suppressing the nonlinear effect of optical fiber and improving the overall performance of the system.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] An improved selective mapping scheme for OFDM-NOMA PON, the specific steps mainly include the generation of candidate OFDM-NOMA signals at the transmitting end and the demodulation process at the receiving end, and the digital signal processing at the transmitting end includes the following steps:

[0008] S1: According to the required number of candidate OFDM-NOMA signals, different cyclic shifts are performed on the information bit data accessed to the ONU to generate multiple groups of candidate information bits.

[0009] S2: The check bit corresponds to the cyclic shift value, and its length is related to the number of candidate information bit groups of the ONU. After the check bit is repeatedly encoded, it is added to the leading part of the corresponding candidate information bit to generate candidate bit data.

[0010] S3: Perform QAM modulation on all candidate bit data and perform power allocation to generate multiple groups of candidate QAM symbol sequences.

[0011] S4: Assign a fixed phase sequence to each ONU. The sequence can be randomly generated by phase factors such as [±1, ±i], or a fixed row vector of a Riemann or Hadamard matrix. All candidate QAM symbol sequences of the ONU are multiplied symbol by symbol with the fixed phase sequence to generate multiple sets of candidate frequency domain symbol sequences.

[0012] S5: Perform Hermitian conjugate symmetry processing on each candidate frequency domain symbol sequence, and convert it into a candidate OFDM signal of a corresponding power level through IFFT.

[0013] S6: Arrange and combine the candidate OFDM signals of ONUs with different power levels and linearly superpose them to generate multiple candidate OFDM-NOMA signals. This process reduces the IFFT operation of generating a corresponding number of candidate signals through addition operations.

[0014] S7: Calculate the PAPR value of each candidate OFDM-NOMA signal, select the signal with the smallest PAPR for subsequent processing and send.

[0015] The demodulation at the receiving end includes the following steps:

[0016] S8: Assume that the received data is frequency domain data after FFT and channel estimation, the number of access ONUs is 2, and the ONU 2 (high power) data demodulation process as an example.

[0017] S9: First, remove the fixed phase sequence, and find the QAM symbol corresponding to the repeated coding check bit from the received data according to the length of the repeated coding check bit and the QAM modulation order. Perform QAM soft demodulation and repeated decoding on the symbol, and finally output a decision to determine the check bit.

[0018] S10: removing the QAM symbols corresponding to the repeated coded check bits, hard demodulating the remaining QAM symbols, and performing a corresponding reverse cyclic shift on the received information bits according to the check bits output by the final decision.

[0019] S11: In ONU 1 SIC is required for data demodulation (low power). 2 After the data demodulation is completed, reconstruct its data and subtract ONU from the received data. 2 The data of ONU 1 data.

[0020] S12: ONU 1 Data demodulation steps and ONU 2 The steps are the same as those in S8-S10.

[0021] Further, in step S1, the specific process of cyclic shifting of information bits includes:

[0022] S11: ONU in a single OFDM symbol k The information bit data is represented by B k ,

[0023] B k =[b k (1),b k (2), ...b k (l)…b k (L)] T (4)

[0024] L is determined by the number of subcarriers N d , the number of alternative signals (check bit length l'), the repetition coding rate R c The decision is:

[0025] L=M·N d -l' / R c (5)

[0026] S12: ONU after cyclic shift kThe candidate information bits It is expressed as:

[0027]

[0028] J k For ONU k The set of information bit cyclic shift values, where all elements are known odd numbers, C k (u) is J k The uth element in .

[0029] Further in step S2, the specific check bit adding process includes:

[0030] S21: Assume that V candidate OFDM-NOMA signals need to be generated. In the signal processing process of the N-SLM scheme, the root selects a group (m,u) with the smallest sum of all common divisors of V. m+u IFFT operations can generate V candidate OFDM-NOMA signals.

[0031] S22: The check bit corresponds to the cyclic shift value of the information bit. The side information has a greater impact on the system performance, so a certain encoding method needs to be selected. In this paper, the check bit adopts a repetitive encoding with a code rate of 1 / 3.

[0032] S24: combining the repetitively coded check bits with the corresponding candidate information bits respectively, and placing the repetitively coded check bits in the leading part of the candidate information bits with the corresponding cyclic shift value to form candidate bit data.

[0033] In step S9, the specific QAM soft demodulation process includes:

[0034] S91: After removing the fixed phase sequence, QAM symbols corresponding to the repetition coding check bits and information bits are respectively selected according to the modulation order M and the length of the check bits after repetition coding.

[0035] S92: soft demodulate the QAM symbol corresponding to the repetitively coded check bit, where r can be r=s+n, s is the QAM symbol corresponding to the sent repetitively coded check bit, and n is noise. i (s) is the i-th bit of the QAM symbol s corresponding to the transmitted repetition coding check bit. The soft information of the i-th bit is expressed as:

[0036]

[0037] For each repetition coded check bit, the transmitted symbol set S can be divided into S 0 and S 1 The two parts are respectively a set of transmitted symbols corresponding to when the repetition coding check bit is 0 and 1.

[0038] S93: Repeatedly decode the LLR of the repeated coded check bits, and make a decision output, and perform a corresponding reverse cyclic shift on the information bits according to the check bits output by the decision output to obtain corresponding data.

[0039] The beneficial effects of the present invention are:

[0040] An improved selective mapping scheme for OFDM-NOMAPON is proposed. The present invention is mainly aimed at reducing the PAPR value of OFDM-NOMA signals. In view of the shortcomings of the traditional SLM scheme, which has high computational complexity and requires explicit SI, an improved SLM scheme with low complexity implicit SI is proposed according to the characteristics of power domain NOMA multi-user information transmitting information in a power multiplexing manner on the same time-frequency resources. This scheme can achieve PAPR suppression performance similar to that of the traditional SLM scheme with extremely low computational complexity, thereby suppressing the influence of optical fiber nonlinearity on the transmission signal and further improving the overall performance of PON. At the receiving end, since the improved SLM scheme adopts a fixed phase sequence, there is no need to send the phase sequence information. The check bit detection method greatly reduces the computational complexity of the receiving end compared to the blind SLM detection scheme that does not require SI. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to make the purpose, technical solution and beneficial effects of the present invention clearer, the present invention provides the following drawings for illustration:

[0042] Figure 1 OFDM-NOMA PON system block diagram;

[0043] Figure 2 The baseband signal processing flow of the transmitting end of this solution;

[0044] Figure 3 The signal demodulation process at the receiving end of this scheme; Specific implementation plan

[0045] The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] The present invention provides an improved selective mapping scheme for OFDM-NOMA PON, mainly for reducing the PAPR value of OFDM-NOMA signals. Figure 1The figure shows the block diagram of the OFDM-NOMA PON system. The baseband OFDM-NOMA signal using the N-SLM solution is converted into an analog signal by a digital-to-analog converter (DAC), and the electrical signal is modulated into the optical domain by optoelectronic conversion. It is then injected into a standard single-mode fiber (SSMF) and sent to different ONUs. At the ONU end, optoelectronic conversion is first performed, and then the two ONU signals enter the analog-to-digital converter (ADC) to start digital signal processing. ONU 2 After the received signal is converted from serial to parallel and the cyclic prefix (CP) is removed, the signal is transformed into the frequency domain through FFT, and then channel estimation and equalization are completed, and finally digital signal demodulation is performed. 1 Digital signal processing operation and ONU 2 The difference is that before completing the demodulation, the SIC algorithm is first used to eliminate the ONU 2 signal and then decode it into its own signal.

[0047] The generation process of the alternative transmission signal of the N-SLM scheme is as follows: Figure 2 As shown, first, the bit data of each ONU is cyclically shifted with different values ​​to generate multiple candidate information bits. Each cyclic shift value has a corresponding check bit. The repeatedly encoded check bit is inserted into the leading part of the corresponding candidate information bit to form multiple groups of different candidate bit data. All candidate bit data are sequentially subjected to QAM modulation, power allocation, and OFDM modulation to generate multiple groups of OFDM signals with different power levels. OFDM signals with different power levels are arranged, combined, and linearly superimposed to generate candidate OFDM-NOMA signals and select the signal with the lowest PAPR for subsequent processing.

[0048] ONU 2 The demodulation process at the end is as follows Figure 3 As shown. After removing the corresponding fixed phase sequence, the QAM symbols corresponding to the repeated coded check bits and information bits are respectively selected from the frequency domain QAM symbols. The two are respectively demodulated by QAM soft demodulation and hard demodulation. The LLR values ​​of the repeated coded check bits output by the QAM soft demodulation are repeatedly decoded and output by decision. According to the check bits output by decision, the information bits are reversely cyclically shifted to obtain the corresponding received data.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the present invention. For those skilled in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. An improved selective mapping scheme for OFDM-NOMA PON, characterized in that The generation of the alternative OFDM-NOMA signal at the transmitting end mainly includes the following steps: S1: According to the required number of candidate OFDM-NOMA signals, different cyclic shifts are performed on the information bit data accessed to the ONU to generate multiple groups of candidate information bits. S2: The check bit corresponds to the cyclic shift value, and its length is related to the number of candidate information bit groups of the ONU. After the check bit is repeatedly encoded, it is inserted into the leading part of the corresponding candidate information bit to generate candidate bit data. S3: Perform QAM modulation on all candidate bit data and perform power allocation, and generate multiple groups of candidate QAM symbol sequences for each power level. S4: Assign a fixed phase sequence to each ONU. The sequence can be randomly generated by phase factors such as [±1, ±i], or a fixed row vector of a Riemann, Hadamard, etc. matrix. All candidate QAM symbol sequences of the ONU are multiplied symbol by symbol with the fixed phase sequence to generate multiple sets of candidate frequency domain symbol sequences. S5: Perform Hermitian conjugate symmetry processing on each candidate frequency domain symbol sequence, and convert it into a candidate OFDM signal of a corresponding power level through IFFT. S6: Linearly superimpose the candidate OFDM signals of different power levels to generate multiple candidate OFDM-NOMA signals. This process reduces the IFFT operation of generating a corresponding number of candidate signals through real number addition operations. S7: Calculate the PAPR value of each candidate OFDM-NOMA signal, select the signal with the smallest PAPR for subsequent processing and send. The demodulation process at the receiving end includes the following steps: S8: Assume that the received data is frequency domain data after FFT and channel estimation, and the number of access ONUs is 2, and take the data demodulation process of ONU2 (high power) as an example. S9: First, remove the fixed phase sequence, and find the QAM symbol corresponding to the repeated coded check bit from the received data according to the length of the check bit after repeated coding and the QAM modulation order. Perform QAM soft demodulation on the symbol and perform repeated decoding, and finally determine the output check bit. S10: At the same time, a hard demodulation scheme is adopted for the QAM symbol corresponding to the information bit, and a corresponding reverse cyclic shift is performed on the received information bit according to the check bit output by the final judgment. S11: SIC is required for data demodulation of ONU1. After the data demodulation of ONU2 is completed, its data is reconstructed and the data of ONU2 is subtracted from the received data to obtain the data of ONU1 (low power). S12: The data demodulation steps of ONU1 are the same as those of ONU2, see S8-S10 for details.

2. According to claim 1, an improved selective mapping scheme for OFDM-NOMA PON is characterized in that: In step S2, the specific process of adding the check bit includes: S21: Assume that the N-SLM scheme needs to generate V candidate OFDM-NOMA signals. In the signal processing process of the N-SLM scheme, a set of common divisors with the smallest sum of V is selected according to formula (1). m+u IFFT operations can generate V candidate transmission signals. S22: The relationship between the check bit length l' and m and u is as shown in formula (2), Indicates rounding up; S23: If the information bit data of ONU2 undergoes u=4 cyclic shifts, the relationship between the check bit and the information bit cyclic shift is: S24: Repeatedly encode the check bits and place them in the leading part of the candidate information bits using the corresponding cyclic shift value.

3. An improved selective mapping scheme for OFDM-NOMA PON according to claim 1, characterized in that In step S9, the specific detection process includes: S91: After removing the fixed phase sequence, QAM symbols corresponding to the repetition coding check bits and information bits are selected according to the QAM modulation order and the repetition coding check bit length. S92: Perform QAM soft demodulation on the QAM symbols corresponding to the repeated coded check bits, and output the log-likelihood ratio (LLR) soft information of each check bit; at the same time, perform QAM hard demodulation on the QAM symbols corresponding to the information bits, and output the corresponding information bits. S93: Repeatedly decode the LLR value of the repeated coded check bit, and determine the output check bit, and perform a corresponding reverse cyclic shift on the information bit according to the determined output check bit to obtain corresponding data.

Images