CN107317630B

CN107317630B - Multimode optical fiber transmission method based on subcarrier index modulation

Info

Publication number: CN107317630B
Application number: CN201710511740.5A
Authority: CN
Inventors: 张静; 易兴文; 黄夏涛; 周文静; 江洋; 邱昆
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2017-06-29
Filing date: 2017-06-29
Publication date: 2020-07-17
Anticipated expiration: 2037-06-29
Also published as: CN107317630A

Abstract

The invention provides a multimode fiber transmission method based on subcarrier index modulation, which is characterized in that an additional modulation dimension is added on the basis of conventional two-dimensional amplitude/phase modulation (APM), and the method adopts the main idea that the index number of a subcarrier is utilized to transmit information: and selecting a part of carriers to be silent and not to transmit data so as to avoid deep fading possibly encountered at the frequencies and achieve the aim of suppressing intersymbol interference (ISI). The SIM differs from conventional OFDM, which transmits data on all subcarriers, and transmits a part of bits in the form of subcarrier indices. The combining mathematical theory for selecting bits can correspond to different subcarrier index modes, and the part of bits are not transmitted but added in the whole system as the distribution information of the carriers. The invention can avoid the frequency selective fading of the multimode fiber at a certain probability, thereby obviously improving the performance and reducing the error rate.

Description

Multimode optical fiber transmission method based on subcarrier index modulation

Technical Field

The present technology relates to optical signal transmission, and more particularly to multimode fibre channel transmission.

Background

With the continuous growth of communication services in modern society, more perfect short-distance communication networks are more and more concerned by people, and the trend of optical fiber to the home and the trend of optical fiber to the desktop are inevitable trends in the development of optical communication. Compared with single-mode fiber SMF, the multi-mode fiber MMF has the advantages of large numerical aperture, easy alignment of connection, low cost of matched devices and the like, and becomes a preferred transmission medium for short-distance fiber communication. However, as the transmission modes in the multimode fiber are hundreds, and the propagation constant and the group velocity of each mode are different, the loss of the fiber is large, and the transmission capacity is limited due to the large influence of modal dispersion. To this end, several new techniques for increasing the transmission capacity of multimode optical fibers have been proposed. The multimode fiber communication technology based on the orthogonal frequency division multiplexing OFDM technology can effectively overcome the influence of frequency selective fading under a multimode fiber channel, and more reasonably and fully utilizes channel spectrum resources so as to greatly improve the transmission channel capacity of a multimode fiber transmission system, and is a new technology with a strong practical prospect.

Multimode optical fibers are mostly used for medium-short distance and small capacity optical fiber communication systems. Multimode fiber fading and deep fading points may occur at locations generally in the high frequency region, and the frequency selective fading is more affected by the eccentric injection than by the change in refractive index. Moreover, the modes of the multimode fiber are unstable, which causes the channel response to change with time, resulting in non-ideal transmission performance. Although, the OFDM technology can reduce the influence of frequency selective fading of the multimode fiber to some extent through adaptive bit allocation, power allocation and other algorithms. However, this comes at the cost of the application of low order modulation formats and reduced system spectrum utilization.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a multimode fiber system transmission method which can adapt to multimode fiber channel response time-varying characteristics and frequency deep fading in order to further develop the potential of the OFDM technology and improve the multimode fiber channel transmission capacity.

The technical scheme for solving the technical problems is that the multimode optical fiber transmission method based on subcarrier index modulation comprises the following steps:

1) a sending step:

1-1) grouping bits to be transmitted by a sub-carrier index modulation SIM module, wherein each group of bits is mapped to one OFDM modulation subblock, each group of bits is divided into a selection bit and an information bit, the selection bit is used for selecting part of sub-carriers in each sub-carrier in the OFDM modulation subblock to activate to transmit data, and the information bit is used for determining a transmission signal constellation diagram of the activated sub-carriers in the OFDM modulation subblock;

1-2) the OFDM modulation module maps the information bits into modulation symbols to be loaded on activated subcarriers determined according to the selection bits and the information bits input to each OFDM modulation subblock in parallel, and the inactivated subcarriers transmit 'zero' symbol data; the modulated signal output by the OFDM modulation module is sent to a multimode optical fiber;

2) a receiving step: and receiving the modulated signal through a photoelectric detector, obtaining a frequency domain symbol through Fourier transform, and finally obtaining a decoding sequence through maximum likelihood decoding.

The subcarrier index modulation SIM technique is a multi-channel transmission method. On the basis of the conventional two-dimensional amplitude/phase modulation APM, an additional modulation dimension is added, and the most important idea method is to transmit information by using the index number of a subcarrier: and selecting a part of carriers to be silent and not to transmit data so as to avoid deep fading possibly encountered at the frequencies and achieve the aim of suppressing intersymbol interference (ISI). The SIM differs from conventional OFDM, which transmits data on all subcarriers, and transmits a part of bits in the form of subcarrier indices. The combining mathematical theory for selecting bits can correspond to different subcarrier index modes, and the part of bits are not transmitted but added in the whole system as the distribution information of the carriers.

Further, in step 1), bit grouping is performed according to the channel response characteristics of the multimode optical fiber, so that the subcarriers with less selective fading are selected for bit selection and activated.

Further, after the fourier transform in step 2), the frequency domain symbols of all carriers are detected by an energy detection algorithm EDD, so as to determine activated subcarriers, and then maximum likelihood decoding is performed on the activated subcarriers.

The invention has the advantages that the frequency selective fading of the multimode fiber can be avoided at a certain probability, so the performance can be obviously improved, and the error rate is reduced. The power distribution strategy PRP is adopted to improve the channel transmitting power, and meanwhile, the noise power is not obviously improved, and the performance of a Gaussian white noise channel is also improved. Because the number of activated sub-carriers is controllable, in practical use, a trade-off can be made between complexity, spectral efficiency and transmission performance.

Drawings

FIG. 1 is a diagram of an optical OFDM system incorporating a SIM module;

FIG. 2 is a schematic diagram of a 25GHz frequency response of a multimode optical fiber.

Detailed Description

In a conventional OFDM system, signal light generated by direct modulation or external modulation is transmitted through a multimode optical fiber and received by a photodetector at a receiving end. Taking a system for direct modulation and direct detection as an example, a digital signal is directly loaded on a laser for modulation, but the system generates nonlinear distortion effects such as frequency chirp and phase modulation, which causes poor performance in high-speed transmission. In a multimode fiber channel, modal dispersion has a dominant influence on the transmission performance of a system, and is also the most main factor for limiting the transmission capability of the fiber, and the generation reason is that the multimode fiber has a plurality of transmission modes, and the optical propagation paths and speeds of the modes are different, so that the arrival time of the modes at the exit end of the fiber is inconsistent, delay difference is generated, and optical pulse broadening is caused. Under the condition that the performance of the laser and the photoelectric detector is not considered to be improved, the embodiment analyzes the channel response curve of the multimode optical fiber, correspondingly adjusts the scheme of selecting the subcarriers, and improves the transmission performance by modulating the SIM technology by using the subcarrier index.

As shown in fig. 1, a total of m information bits are sent to the SIM module into the bit distributor, which divides the m bits into g groups, each group having p bits, i.e., m ═ pg. And mapping p bits to one OFDM modulation sub-block, wherein the length of each sub-block is N, N is the total length of Fast Fourier Transform (FFT) in the OFDM modulation module, namely the total number of channels, and N is N/g. In this embodiment, for each sub-block, p is input₁The bits are converted to select k sub-carriers from n to activate to transmit data symbols. Combining mathematics is used to select bits corresponding to different subcarrier index modes. The remaining bit p₂＝k log₂M sequences are used for constellation mapping of symbols transmitted on subcarriers M-ary modulator generates symbol set S, and β th symbol sequence transmitted on active subcarrier is S_β＝{s_β,1,…,s_β,K}，s_β,r∈S，r＝1，…，K，s_β,rDenotes the r-th transmission symbol on the β -th active subcarrier because not all carriers of the present invention transmit valid information, the inactive subcarriers transmit "zero" symbol data (invalid data filled with 0), and so onTo reduce a part of the spectrum efficiency, it is necessary to consider that the index bits of the subcarriers are used to compensate for the efficiency loss.

Based on the SIM module shown in fig. 1, the SIM module divides the transmitted bit sequence into two parts, one part is a selection mode for selecting the bit sequence for mapping the subcarriers, and the part of bits makes an allocation strategy according to the channel characteristics. The rest part of data bits are mapped into modulation symbols according to the information required to be sent, and the modulation symbols are converted into signal transmission loaded on a carrier wave. The OFDM modulation module is a traditional OFDM technology, performs serial-parallel conversion and orthogonal transformation on input bits, and transmits the transformed signals to a multimode optical fiber for transmission. Fig. 2 shows a channel response curve of a 25GHz signal after passing through a multimode fiber, which can be obtained by analysis: fading and deep fading points are likely to occur at high frequency regions, and the eccentric injection has a large influence on frequency selective fading, so that the deep fading points are more likely to occur. Muting the channel at high frequency is considered as much as possible in setting the selection bit so as not to transmit data. Meanwhile, because each mode of the multimode fiber is unstable, the channel response characteristic is time-varying, corresponding to the channel response characteristic, time-varying random bits are used for reallocating transmitting subcarriers at different moments, and the transmission performance is improved by modulating the SIM technology by subcarrier indexes.

At a receiving end, a photoelectric detector is adopted to receive modulated signals from multimode fibers, frequency domain symbols are obtained through Fourier transform FFT, a maximum likelihood decoding M L with optimal performance can be adopted in a decoding method, maximum correlation degree is solved for subcarrier symbols of each group through maximum likelihood decoding, a decoding sequence is obtained through judgment, but complexity is exponentially increased along with the number of bits of each group and the number of activated subcarriers, an energy detection algorithm EDD with lower complexity is further provided, zero-forcing equalization of frequency domains is conducted on all carriers of each group, respective energy is detected, a part of subcarriers with higher energy is determined to be activated channels, a large number of impossible combination modes can be reduced, maximum likelihood decoding M L is conducted on the symbols of each activated channel, and the equivalence of the two methods can be proved.

Experiments show that due to the adoption of the subcarrier index modulation technology in the multimode fiber channel, the subcarriers of the deep fading frequency part can be kept silent with a certain probability, and the transmitted subcarriers can better adapt to the characteristics of the multimode fiber channel, so that the fading effect of multimode fibers can be avoided to a great extent, and the intersymbol interference can be remarkably reduced. Meanwhile, the multimode fiber mode is unstable, the channel response is time-varying, and the scheme of selecting the channel is also time-varying to resist various fading effects. And thirdly, because the power of the subcarrier without the transmitted signal is redistributed to the subcarrier with the transmitted data, and meanwhile, the noise power is not changed greatly, the integral signal-to-noise ratio of the system can be further improved, and the power redistribution strategy also has certain transmission performance improvement on a Gaussian white noise channel. However, since some subcarriers do not transmit data, some spectral efficiency is reduced, and some consideration needs to be given to spectral utilization when implementing the present invention.

Claims

1. The multimode optical fiber transmission method based on subcarrier index modulation is characterized by comprising the following steps of:

1) a sending step:

1-2) the OFDM modulation module maps the information bits into modulation symbols to be loaded on activated subcarriers determined according to the selection bits and the information bits input to each OFDM modulation subblock in parallel, and the inactivated subcarriers transmit 'zero' symbol data by default; the modulated signal output by the OFDM modulation module is sent to a multimode optical fiber;

2) a receiving step: receiving the modulated signal through a photoelectric detector, obtaining a frequency domain symbol through Fourier transform, carrying out zero forcing equalization on the frequency domain symbols of all carriers through an energy detection algorithm EDD, detecting respective energy, determining activated subcarriers, and carrying out maximum likelihood decoding on the activated subcarriers.

2. The method as claimed in claim 1, wherein in step 1), bits are grouped according to the channel response characteristics of the multimode fiber, so that the bits are selected to select the subcarriers with less selective fading for activation.