Improved selective mapping scheme for OFDM-NOMA PON

Abstract

The application is 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 fields of optical communication and digital signal processing. In order to solve the problem of high peak-to-average power ratio (PAPR) of downlink signals in a non-orthogonal multiple access system using OFDM modulation, aiming at the shortcomings of high computational complexity of a traditional selective mapping (C-SLM) scheme and high error rate of explicit side information (SI) requiring additional bandwidth resources, the N-SLM scheme is proposed according to the characteristics of superimposed transmission of power domain NOMA multi-user data on subcarriers. Compared with the C-SLM scheme, the scheme has low computational complexity and does not need to send explicit SI. The application mainly includes the generation of alternative OFDM-NOMA signals at the sending end and the demodulation process at the receiving end. The sending end adopts information bit cyclic shift and inserts a few repeated coded check bits in the preamble part to generate multiple alternative bit information, thereby avoiding the explicit transmission of SI. All alternative bit information is sequentially subjected to QAM modulation, power allocation and OFDM modulation to generate multiple groups of OFDM signals with different power levels, which are arranged and combined to linearly superimpose multiple alternative OFDM-NOMA signals. The PAPR values of all alternative OFDM-NOMA signals are calculated, and the alternative OFDM-NOMA signal with the minimum PAPR is selected for subsequent signal processing. The receiving end respectively performs soft demodulation and hard demodulation on the QAM symbols corresponding to the repeated coded check bits and information bits, and performs corresponding reverse cyclic shift on the received information bits according to the check bits output by the repeated decoding. After the low-power-level data is subjected to successive interference cancellation (SIC) to obtain the corresponding information, the same processing process is performed.

Description

TECHNICAL FIELD

[0001] The application is an improved selective mapping scheme (N-SLM) for orthogonal frequency division multiplexing non-orthogonal multiple access passive optical network (OFDM-NOMA PON), and relates to the technical fields of optical communication and digital signal processing, and in particular to a low-complexity, explicit SI transmission-free probability-based PAPR suppression scheme. BACKGROUND

[0002] Due to the rapid development of wired and wireless communication services and the further growth of network capacity demand, the development of high-capacity, low-latency, and low-cost PONs is accelerated. The traditional PON based on orthogonal multiple access (OMA) technology cannot further meet the capacity demand due to the limitations of dispersion and economic considerations as the demand for data volume of new services surges. Meanwhile, it is difficult to economically and effectively increase the number of optical network units (ONUs) in a single PON due to the limitation of available bandwidth. In contrast, the OFDM-NOMA scheme proposed in recent years has been proven to be one of the promising methods to achieve massive device simultaneous connection. NOMA further improves system throughput and spectral efficiency by multiplexing the same subcarriers with multiple users by allocating different powers to users at the transmitting end and using SIC technology at the receiving end. The OFDM-NOMA scheme has been widely studied as a new promising scheme in PONs due to its large capacity, strong access flexibility, low overhead, and low complexity.

[0003] Since the optical access structure is extremely sensitive to device cost, low-cost electrical / optical devices such as Mach-Zehnder modulators (MZMs) and direct detection schemes are still the preferred choice for PONs. However, when the phases of the data on each subcarrier are the same or similar, a large instantaneous power peak will occur, resulting in a high PAPR, because an OFDM symbol is composed of multiple independently modulated subcarrier signals. High PAPR signals not only make the MZM work at a linear bias point or enter a nonlinear region, causing distortion of the high-amplitude part of the signal, but also exacerbate the nonlinear effects of the optical fiber (self-phase modulation, cross-phase modulation, four-wave mixing, etc.).

[0004] Currently, there are three main peak-to-average ratio suppression techniques in OFDM systems: pre-distortion techniques, probability-based techniques, and precoding techniques. The present application mainly improves the traditional SLM scheme in the probability-based technique. In view of the high computational complexity of the traditional SLM scheme and the high error rate of explicit SI, which occupies additional bandwidth, a low-complexity SLM scheme is proposed based on the characteristics of power-domain NOMA multi-user data superimposed transmission on subcarriers. By inserting check bits into the leading part of the information bits to be transmitted, and determining the cyclic shift of the information bits of the corresponding ONU at the receiving end according to the check bits, explicit SI transmission is not required. SUMMARY

[0005] Therefore, the present application aims to provide an improved PAPR suppression scheme for OFDM-NOMA PON with low complexity, without explicit SI transmission, and without causing signal distortion, thereby suppressing the nonlinear effects of optical fiber and improving the overall performance of the system.

[0006] To achieve the above object, the present application provides the following technical solutions:

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

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

[0009] S2: The check bits correspond to the cyclic shift values, and the length is related to the number of alternative information bit groups of the ONU. After repeating encoding the check bits, they are added to the preamble part of the corresponding alternative information bits to generate alternative bit data.

[0010] S3: QAM modulation is performed on all alternative bit data and power allocation is performed, thereby generating multiple groups of alternative QAM symbol sequences.

[0011] S4: A fixed and unchanging phase sequence is allocated to each ONU, which can be randomly generated by phase factors such as [±1, ±i], or can be a fixed row vector of matrices such as Riemann, Hadamard, etc. The alternative QAM symbol sequences of all ONUs are multiplied by the fixed phase sequence symbol by symbol to generate multiple groups of alternative frequency domain symbol sequences.

[0012] S5: Hermitian conjugate symmetry processing is performed on each alternative frequency domain symbol sequence, and IFFT conversion is performed to generate alternative OFDM signals of corresponding power levels.

[0013] S6: The alternative OFDM signals of different power level ONUs are arranged and combined linearly to generate multiple alternative OFDM-NOMA signals, and this process reduces the IFFT operation of generating the corresponding number of alternative signals through addition operation.

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

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

[0016] S8: Assuming the received data is the frequency domain data after FFT and channel estimation, the number of accessed ONUs is 2, and the data demodulation process of ONU2 (high power) is taken as an example.

[0017] S9: First, the fixed phase sequence is removed, and the QAM symbol corresponding to the repetition encoding check bit is found from the received data according to the length of the repetition encoding check bit and the QAM modulation order. The symbol is subjected to QAM soft demodulation and repetition decoding, and the final decision is output to determine the check bit.

[0018] S10: The QAM symbol corresponding to the repetition encoding check bit is removed, the remaining QAM symbol is hard demodulated, and the received information bit is subjected to corresponding reverse cyclic shift according to the check bit output by the final decision.

[0019] S11: SIC needs to be used in the data demodulation of ONU1 (low power). After the data demodulation of ONU2 is completed, the data of ONU2 is reconstructed, and the data of ONU2 is subtracted from the received data to obtain the data of ONU1.

[0020] S12: The data demodulation steps of ONU1 are the same as those of ONU2, which are described in detail in S8-S10.

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

[0022] S11: The information bit data of ONU k in a single OFDM symbol is represented as 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'), and the repetition encoding rate R c , that is:

[0025] L = M · N d -l' / R c (5)

[0026] S12: The alternative information bit of ONU k after cyclic shift is represented as:

[0027] ​

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

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

[0030] S21: Assuming that V alternative OFDM-NOMA signals need to be generated, in the N-SLM scheme signal processing process, a set of (m, u) that is the sum of all the common divisors of V and the minimum one is selected, and m+u times of IFFT operation can generate V alternative OFDM-NOMA signals.

[0031] S22: The check bits correspond to the information bit cyclic shift values, and the side information has a greater impact on the system performance, so a certain encoding mode needs to be selected, and the check bits in the text use repetition encoding with a code rate of 1 / 3.

[0032] S24: The repetition encoded check bits are combined with the corresponding alternative information bits respectively, and the repetition encoded check bits are placed in the leading part of the alternative information bits using the corresponding cyclic shift values to form the alternative bit data.

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

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

[0035] S92: The QAM symbols corresponding to the repetition encoded check bits are soft demodulated, where r can be r=s+n, s is the QAM symbol corresponding to the transmitted repetition encoded check bits, and n is the noise.b i (s) is the i-th bit corresponding to the QAM symbol s corresponding to the transmitted repetition encoded check bits, and the soft information of the i-th bit is represented as:

[0036]

[0037] For each repetition encoded check bit, the transmitted symbol set S can be divided into S 0 and S 1 , which are the sets of transmitted symbols corresponding to the repetition encoded check bits being 0 and 1 respectively.

[0038] S93: Repeat decoding the LLR of the check bits, and judging and outputting, according to the check bits of the judgment output, corresponding reverse cyclic shift is performed on the information bits to obtain corresponding data.

[0039] The present application has the advantages of:

[0040] The application provides an improved selective mapping scheme for OFDM-NOMA PON, and mainly aims to reduce the PAPR value of an OFDM-NOMA signal. In view of the high calculation complexity and the need for explicit SI of a traditional SLM scheme, according to the characteristics that power-domain NOMA multi-user information is transmitted in a power multiplexing manner on the same time-frequency resource, an improved SLM scheme with low complexity and implicit SI is provided. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to make the purposes, technical schemes and advantages of the present application clearer, the present application is described below with reference to the accompanying drawings:

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

[0043] Figure 2 A baseband signal processing process of a transmitting end of the present application;

[0044] Figure 3 A signal demodulation process of a receiving end of the present application; DETAILED DESCRIPTION

[0045] The present application will be described in detail below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0046] The present application provides an improved selective mapping scheme for OFDM-NOMA PON, and mainly aims to reduce the PAPR value of an OFDM-NOMA signal. Figure 1The OFDM-NOMA PON system structure block diagram is shown. The baseband OFDM-NOMA signal adopting the N-SLM scheme is converted into an analog signal by a digital-to-analog converter (DAC), and an optical-electric conversion is performed to modulate the electrical signal into the optical domain. Subsequently, the signal is injected into a standard single-mode fiber (SSMF) and sent to different ONUs. At the ONU end, the optical-electric conversion is performed first, and then the two ONU signals enter an analog-to-digital converter (ADC) to start digital signal processing. After the serial-to-parallel conversion and removal of the cyclic prefix (CP), the received signal at the ONU2 is converted into the frequency domain by FFT, and then the channel estimation and equalization are completed, and finally the digital signal demodulation is performed. The digital signal processing operation of the ONU1 is similar to that of the ONU2, except that before the demodulation, the SIC algorithm is used to eliminate the signal of the ONU2 on each subcarrier, and then the signal belonging to the ONU1 is decoded.

[0047] The generation process of the alternative transmission signal of the N-SLM scheme is shown in Figure 2 First, the bit data of each ONU is cyclically shifted by different values to generate multiple alternative information bits, each cyclic shift value has corresponding check bits, and the check bits after repeated encoding are inserted into the leading part of the corresponding alternative information bits to form multiple groups of different alternative bit data. The QAM modulation, power allocation, and OFDM modulation are sequentially performed on all the alternative bit data to generate multiple groups of OFDM signals with different power levels, the OFDM signals with different power levels are arranged and combined to be linearly superimposed to generate alternative OFDM-NOMA signals, and the signal with the lowest PAPR is selected from the alternative OFDM-NOMA signals for subsequent processing.

[0048] The demodulation process at the ONU2 end is shown in Figure 3 After the corresponding fixed phase sequence is removed, the QAM symbols corresponding to the repeated encoding check bits and the information bits are selected from the frequency domain QAM symbols, respectively, and the QAM soft demodulation and the hard demodulation are performed on the two, respectively. The LLR value of the repeated encoding check bits output by the QAM soft demodulation is repeatedly decoded and output, and the information bits are subjected to corresponding reverse cyclic shift according to the check bits output by the judgment to obtain corresponding received data.

[0049] Finally, it is pointed out that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit, and for those skilled in the art, it can be understood that the embodiments can be changed, modified, replaced, and modified in various ways without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. An improved selective mapping method for OFDM-NOMA PAN, characterized in that The generation of the sending end alternative OFDM-NOMA signal comprises the following steps: S1: According to the required number of alternative OFDM-NOMA signals, different cyclic shifts are made to the information bit data of the access ONU to generate multiple groups of alternative information bits; S2: The check bits correspond to the cyclic shift values, and the length is related to the number of alternative information bit groups of the ONU; after repeated encoding of the check bits, the check bits are inserted into the preamble part of the corresponding alternative information bits to generate alternative bit data; S3: QAM modulation is performed on all alternative bit data, and power allocation is performed, and multiple groups of alternative QAM symbol sequences are generated for each power level; S4: A fixed and unchanged phase sequence is allocated to each ONU, which is randomly generated by [±1, ±i] phase factors or a fixed row vector of Riemann and Hadamard matrix; all alternative QAM symbol sequences of the ONU are multiplied with the fixed phase sequence symbol by symbol to generate multiple groups of alternative frequency domain symbol sequences; S5: Hermitian conjugate symmetry processing is performed on each alternative frequency domain symbol sequence, and IFFT conversion is performed to generate alternative OFDM signals of the corresponding power level; S6: The alternative OFDM signals of different power levels are arranged and combined to generate multiple alternative OFDM-NOMA signals; the process reduces the IFFT operation of generating the corresponding number of alternative signals through real addition operation; S7: The PAPR value of each alternative OFDM-NOMA signal is calculated, and the signal with the minimum PAPR is selected for subsequent processing and sending; The demodulation process of the receiving end comprises the following steps: S8: The received data is frequency domain data after FFT and channel estimation, and the number of access ONUs is 2, and the data demodulation process of ONU2 is as follows; S9: First, remove the fixed phase sequence, find the QAM symbol corresponding to the repeated encoding check bits from the received data according to the length of the repeated encoding check bits and the QAM modulation order; after QAM soft demodulation of the symbol and repeated decoding, the check bits are finally judged and output; S10: At the same time, the information bit corresponding QAM symbol adopts a hard demodulation scheme, and the received information bit is subjected to corresponding reverse cyclic shift according to the check bits finally judged and output; S11: The data demodulation of ONU1 needs to adopt SIC; after the data demodulation of ONU2 is completed, the data of ONU2 is reconstructed, and the data of ONU1 is obtained by subtracting the data of ONU2 from the received data; S12: The data demodulation steps of ONU1 are the same as those of ONU2.

2. The improved selective mapping method for OFDM-NOMA PON according to claim 1, wherein, In step S2, the specific process of adding check bits comprises: S21: The N-SLM scheme needs to generate V alternative OFDM-NOMA signals, and in the signal processing process of the N-SLM scheme, a set of common divisors of the addition and minimum of V is selected according to formula (1), and m+u times of IFFT operation can generate V alternative sending signals; S22: Check the relationship between the bit length l' and m and u as formula (2), denotes rounding up; S23: The information bit data of ONU2 is subjected to u=4 cyclic shifts, and the relationship between the check bits and the information bit cyclic shifts is as follows: S24: The repeated encoding check bits are placed in the leading part of the candidate information bits with the corresponding cyclic shift value.

3. The improved selective mapping method for OFDM-NOMA PON according to claim 1, characterized in that The specific detection process in step S9 includes: S91: After removing the fixed phase sequence, the QAM symbols corresponding to the repeated encoding check bits and information bits are selected according to the QAM modulation order and the length of the repeated encoding check bits; S92: The QAM soft demodulation is performed on the QAM symbols corresponding to the repeated encoding check bits to output the log-likelihood ratio (LLR) soft information of each check bit; at the same time, the QAM hard demodulation is performed on the QAM symbols corresponding to the information bits to output the corresponding information bits; S93: The repeated decoding is performed on the LLR values of the repeated encoding check bits, and the check bits are output by judgment; the corresponding reverse cyclic shift is performed on the information bits according to the check bits output by the judgment to obtain the corresponding data.

Images