Why use OFDM for optical transmission? What are the benefits? What key technologies need to be dealt with?

1.1OFDM1.1OFDM background The concept of OFDM (orthogonal frequency division multiplexing) was put forward in 1950s and 1960s, and the patent of 1970 OFDM was published. Its basic idea is to adopt frequency division multiplexing (OFDM) which allows subchannels to overlap without affecting each other. Because of its high spectral efficiency and anti-multipath interference characteristics, this technology has attracted wide attention all over the world. 197 1 year, Weinstein and Ebert put forward the proposal of realizing all modulation and demodulation functions in OFDM system by discrete Fourier transform, which simplified the modulation and demodulation of the system and made theoretical preparations for realizing the all-digital scheme of OFDM. After 1980s, OFDM modulation technology has become a research hotspot again. For example, in the research of wired channel, Hirosaki used the OFDM modulation technology completed by discrete Fourier transform (DFT) in 198 1, and successfully tested the telephone line modem with 16QAM multiplexing and 19.2kbit/s parallel transmission. With the maturity of technology and the reduction of cost, OFDM has been widely used in terrestrial digital audio and video broadcasting (DAB, DVB-T) and asymmetric data subscriber loop (asymmetric DSL). In recent years, with the development of optical communication systems in the direction of long distance and large capacity, many scientific research institutions and universities have begun to turn their attention to coherent optical communication systems. Due to the high detection sensitivity of coherent light detection technology, the transmission distance of the system is long. In addition, coherent optical communication system can completely compensate many linear distortions in theory. Coupled with the high spectral efficiency and anti-dispersion characteristics of OFDM technology, it is proposed to apply orthogonal frequency division multiplexing technology to coherent detection optical communication systems. Many research institutions and universities in the world have carried out research on optical OFDM technology. Optical orthogonal frequency division multiplexing has become the research focus of optical communication in the world. The main foreign research groups include Arizona University, Bangor University, Lucent-Bell Laboratory, KDDI Laboratory, Monas University, etc. These research groups have explored the OOFDM system, including nonlinear problems, performance evaluation, spectrum efficiency and so on. In China, the University of Electronic Science and Technology of China, Jilin University and other units have carried out simulation research on OOFDM implementation under multimode fiber. The basic idea of 1. 1.2 OFDM is actually a special multicarrier transmission technology, which can be regarded as both a modulation technology and a multiplexing technology. The basic principle of OFDM is similar to traditional frequency division multiplexing (FDM), that is, high-speed data streams are distributed to several frequency subchannels with relatively low rates for transmission through serial-parallel transformation. The difference is that OFDM technology makes better use of the control mode and improves the spectrum utilization rate. The biggest feature of OFDM technology is that subcarriers are orthogonal to each other. Orthogonality of OFDM carrier This structure of OFDM is not completely different from the aforementioned frequency division multiplexing. Frequency division multiplexing uses different frequencies to transmit signals. The frequency spectrum of each modulation subcarrier should not overlap, and a guard interval should be added between subcarriers, so as to demodulate correctly at the receiving end. In OFDM technology, by using the orthogonality between subcarriers, the frequency spectrum of each modulation subcarrier is overlapped, and of course there is no guard band in the middle. By using this orthogonality, the original signal can be demodulated at the receiving end, although the spectrum overlaps. Orthogonality between subcarriers can be discussed in time domain and frequency domain. From the time domain, each subcarrier contains an integer multiple period in one OFDM symbol period, and the difference between adjacent subcarriers is one period. From the frequency domain, that is, in the spectrum diagram of each subcarrier in the OFDM signal, at the maximum frequency of each subcarrier, the spectrum values of all other subchannels are exactly 0. Because it is necessary to calculate the maximum frequency of each subcarrier corresponding to these points in the process of demodulating OFDM symbols, the symbols of each subchannel can be extracted from multiple overlapping subchannel symbols without interference from other subchannels. Advantages and disadvantages analysis of 1. 1.3 OFDM system Advantages of OFDM (1) The serial-parallel conversion of high-speed data streams makes the duration of data symbols of subcarriers relatively increase, thus effectively reducing the inter-symbol interference and further reducing the complexity of equalization; (2) Because the subcarriers are orthogonal to each other, the frequency spectra of subchannels are allowed to overlap with each other, so compared with the traditional frequency division multiplexing system, the frequency spectrum utilization rate is very high; (3) The orthogonal modulation and demodulation of each sub-channel can be realized by IDFT and DFT respectively, and can be realized by IFFT and FFT in a system with a large number of sub-carriers; (4) different transmission rates of uplink and downlink are realized by using different numbers of subchannels, thus realizing asymmetric transmission of services; (5) Easy to combine with other access methods. The disadvantage of OFDM (1) is that it is easily affected by frequency offset. (2) Higher peak to average power ratio. 1.1.4 key technologies of OFDM system (1) Time domain synchronization and frequency domain synchronization OFDM system is sensitive to timing and frequency offset, especially when it is combined with FDMA, TDMA and CDMA in practical applications. (2) Channel Estimation In OFDM system, there are two main problems in the design of channel estimator: one is the selection of pilot information, and the other is the design of channel estimator with low complexity and good pilot tracking ability. In practical design, the selection of pilot information and the design of the best estimator are usually interrelated, because the performance of the estimator is related to the transmission mode of pilot information. (3) Channel coding and interleaving In order to improve the performance of digital communication systems, channel coding and interleaving are commonly used methods. For random errors in fading channels, channel coding can be used; For burst errors in fading channels, weaving technology can be used. In practical applications, channel coding and interleaving are usually used simultaneously to further improve the performance of the whole system. (4) Reducing the Peak-to-Average Power Ratio Because the OFDM signal appears as the superposition of N orthogonal subcarrier signals in the time domain, when all these N signals just appear as peaks, the OFDM signal will also produce a maximum peak, and the peak power is n times the average power. Although the probability of peak power is very low, in order to transmit these OFDM signals with high PAPR without distortion, the transmitter requires high linearity of high power amplifier, which leads to extremely low transmission efficiency, and the receiver requires high linearity of front-end amplifier and A/D converter. Therefore, high PAPR greatly reduces the performance of OFDM system, and even directly affects the practical application. In order to solve this problem, people put forward methods based on signal distortion technology, signal scrambling technology and signal space expansion to reduce the PAPR of OFDM system. (5) Equalization In general fading environment, equalization in OFDM system is not an effective method to improve system performance, because the essence of equalization is to compensate the inter-symbol interference caused by multipath channels, and OFDM technology itself has already utilized the diversity characteristics of multipath channels, so equalization is not needed. In a highly dispersed channel, the memory length of the channel is very long, and the length of the cyclic prefix CP must be very long, so as to avoid ISI as much as possible. However, too long CP length will inevitably lead to a lot of energy loss, especially for systems with fewer subcarriers. At this time, we can consider adding an equalizer and appropriately reducing the length of CP, that is, increasing the complexity of the system in exchange for improving the utilization rate of the system frequency band. The basic idea of 1.2.OOFDM The main idea of1.2.o OFDM technology is to divide a given channel into many orthogonal subchannels in frequency domain, use a subcarrier for modulation on each subchannel, and transmit each subcarrier in parallel. Because the square of dispersion tolerance is inversely proportional to the fiber bandwidth, the smaller the channel bandwidth, the greater the dispersion tolerance and the stronger the dispersion tolerance. OOFDM technology divides the optical fiber frequency band into many orthogonal sub-bands as sub-channels for transmitting information, thus making the dispersion tolerance higher. The application of OOFDM technology can realize high-speed optical fiber transmission without dispersion compensation, and at the same time reduce the requirements for optical amplifiers, which can not only save a lot of device costs, but also retain transmission quality. In OOOFDM system, the receiver can use coherent detection or direct detection to directly detect relative coherent detection, which is simple to realize and easy to realize dispersion compensation. Its simple structure makes it easier for OOOFDM system to upgrade to 100Gb/s/s ... Therefore, OOOFDM system based on DD-OOOFDM has certain development potential. The basic principle of 1.2.2 OOFDM The basic principle of OOOFDM is similar to that of OFDM. The only difference is that signal transmission has changed from wireless channel in electric domain to optical channel transmission. The schematic diagram is as follows: firstly, user data is converted into n channels by serial-parallel conversion, where n is the number of subcarriers in OFDM system. These data modulate their respective subcarriers, and the modulation methods may be the same or different. Then, the multi-channel signals are OFDM modulated by IFFT, and the multi-channel signals after OFDM modulation are converted into modulation current signals of direct modulation (internal modulation) lasers through parallel-serial conversion and digital-to-analog conversion. At the receiving end, the optical OFDM signal transmitted through the optical fiber channel is first converted into an electrical signal through photoelectric conversion, and then enters the FFT to complete OFDM demodulation after analog-to-digital conversion, and the modulation signal of each subcarrier is recovered, and then the transmitted data is recovered through corresponding demodulation. Finally, after the parallel-serial conversion, the data stream from the initiator is restored. PON 2. 1 PON Introduction According to whether there are active devices between OLT (optical line terminal) and ONU (optical network unit), optical access networks can be divided into PON (passive optical network) and AON (active optical network). PON (passive optical network) means that ODN (optical distribution network) does not contain any electronic equipment, and ODN is all composed of passive equipment such as optical splitters, without expensive active electronic equipment. The outstanding advantage of PON network is that outdoor active equipment is cancelled, and all signal processing functions are completed in switches and equipment in users' homes. Moreover, the initial investment of this access method is small, and most of the funds can only be invested when the user really accesses. Compared with the active fiber access system, the transmission distance is shorter and the coverage is smaller, but the cost is low, there is no need to set up another computer room, and it is easy to maintain. Therefore, this structure can serve home users economically. The complexity of PON lies in signal processing technology. In the downlink direction, the signal sent by the switch is broadcast to all users. In the uplink direction, each ONU must adopt a certain multiple access protocol, such as TDMA (Time Division Multiple Access) protocol, in order to complete the access to transmission channel information. The basic components of PON include OLT (optical line terminal), ODN (optical distribution network) and ONU (optical network unit), in which OLT has the function of interfacing with switches, completing the conversion from downstream electricity to light and upstream light to electricity, and distributing and controlling them respectively. The function of ODN is to establish optical transmission channel between OLT and ONU, and complete power distribution and wavelength multiplexing of optical signals. , and it is completely composed of optical fiber passive devices. ONU provides optical interface with ODN to realize user interface function. The basic structure diagram of PON is as follows: So much for the network structure diagram of PON, and I hope to adopt it.

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