Research On The Techniques Of Direct Detection Nyquist Modulation And Impairments Inhibition In Elastic Metro Optical Network

The increase and popularity of real-time mobile broadband services and other services promote the sustained and rapid growth of communication capacity of optical fiber transmission system. The traditional optical network with fixed grid limits the flexible allocation and efficient use of resources. Thus achieving elastic allocation of network resources becomes one of the important trends of metro optical network. The traffic and species of business in the future metro network will continue to increase, so the physical layer transmission urgently needs the support of large-capacity, high-speed, flexible modulation and demodulation technology. In addition to the advantage of low cost, the intensity modulation and direct detection (IMDD) scheme based on Nyquist modulation technique can also achieve flexibly variable bandwidth and speed by changing the order of the modulation format or combining multiple subcarrier channels. So it becomes a major potentially viable technical solution under tens to one hundred kilometers short-distance transmission scenarios. In this paper, comparison of different modulation techniques that suitable for elastic metro optical network, suppression algorithms for the nonlinear distortions caused by dispersion and square-law photo-detection in IMDD system, polarization multiplexing scheme with high spectral efficiency and other key technologies are studied. The main research work and innovations of this paper are as follows.1. At present, there are two main modulation techniques, Nyquist and orthogonal frequency division multiplexing (OFDM), which can not only meet the requirements for high-rate, high-spectrum efficiency and tens to one hundred kilometers transmission, but also support adaptive bandwidth/distance transmission of physical layer in elastic metro optical network. In this paper, the advantages and disadvantages of the two techniques are compared from the aspects of DSP algorithm complexity, the effects of the nonlinear distortions caused by dispersion and square-law photo-detection and modulator nonlinearity, transmission performance and so on. The results show that the DSP algorithm complexity of Nyquist system is about twice that of OFDM system. But Nyquist system has a higher tolerance to modulator nonlinearity than OFDM. When the optical modulation indexes of the two schemes are the same, the nonlinear distortions caused by dispersion and square-law photo-detection have the same effects on both schemes. The offline experimental results of the 40Gbps vestigial sideband (VSB)-IMDD Nyquist-subcarrier-modulation (Nyquist-SCM) 16QAM system after 100km transmission show a 1.8dB receiver sensitivity improvement than OFDM 16QAM.2. For the frequency domain equalizer, feed forward equalizer and other methods used in DD single carrier system can not equalize the nonlinear distortions caused by dispersion and square-law photo-detection, this paper proposes an iterative distortion cancellation algorithm based on block process to effectively remove the nonlinear inter-symbol interference. After the linear equlization, this algorithm reconstructs and inhibits the transmission impairments of each symbol block using the detected data based on the small-signal analysis about the characteristics of the nonlinear distortions caused by dispersion, square-law photo-detection, VSB filter and modulator chirp. The offline experimental results of 40Gbps-100km VSB-IMDD Nyquist-SCM 16QAM system based on MachZehnder modulator without chirp show that 2.2dB receiver sensitivity is improved by using the iterative nonlinear impairment cancellation algorithm.3. In order to further reduce the cost of IMDD system, while maintaining high spectral efficiency and good transmission performance, a VSB-IMDD Nyquist PAM scheme based on pulse shaping in the electrical domain is proposed in this paper. This scheme achieves Nyquist pulse shaping using an electrical analog low-pass filter and VSB filtering and optical equalization using a FBG filter in the transmitter. In the receiver, a Nyquist filtering is used to remove the injury outside the Nyquist frequency band. Compared with traditional Nyquist pulse shaping scheme in the DSP, the proposed scheme eliminates the need for pulse shaping DSP algorithm and reduce the cost of DAC. In addition, Nyquist signal shows a stronger ability to resist the nonlinear distortions caused by dispersion and square-law photo-detection in DD system based on further analysis. The offline experimental results of 20Gbps-100km VSB-IMDD 4PAM system based on electroabsorption modulator show that the VSB Nyquist system with electrical Nyquist pulse shaping has a 2.4dB receiver sensitivity improvement than conventional VSB system without Nyquist pulse shaping. Meanwhile, a joint suppression algorithm for the the nonlinear distortions caused by dispersion and square-law photo-detection and modulator nonlinearity is proposed. The experimental results show that 1.2dB receiver sensitivity is improved by using this algorithm, which confirms the validity of the algorithm.4. The current polarization demultiplexing schemes need to adaptively track changes in the polarization state of the optical signal, or precisely control the related devices to achieve separation of the two polarization states. So the complexity of practical applications is high. To solve this problem, a Nyquist polarization multiplexing scheme with two orthogonal optical carriers setting from two polarizations in the transmitter and an 4×4 MIMO polarization demultiplexing algorithm in the receiver is proposed in this paper. The MIMO process is carried out in the digital domain. Thus the proposed scheme avoids the polarization tracking in the optical domain. The spectral efficiency of the proposed scheme is four times that of conventional single polarization system. On this basis, a Nyquist four dimensional modulation scheme 128-SP-QAM is designed in this paper, which offers an optional modulation scheme for the adaptive allocation of bandwidth/distance in elastic metro optical network. The transmission performance of the proposed scheme and the conventional polarization multiplexing scheme are compared by simulation. The simulation results of 40Gbps-80km Nyquist PM-16QAM system show that the proposed scheme can demultiplex the two polarizations in the digital domain. However, increasing the polarization angle between the input and output polarizaitons of fiber would degrade the system performance, and the worst case (22.5°) would cause a 2.8dB receiver sensitivity penalty. The dispersion would hardly bring power penalty. Compared with Nyquist PM-16QAM system, Nyquist 128-SP-QAM system has a 1.1 dB and 0.8dB improvements at the same baud rate and net bit rate, respectively.