Research Of Digital Signal Processing For Offset-QAM Based Co-WDM

With the development of multi-media and virtual reality in the Internet, the scheme of direct detection together with wavelength division multiplexing(WDM) developed in the 90 s can no longer satisfy the ever increasing demand of bandwidth. How to further increase the transmission capacity through the limited optical gain spectrum, indicating of the spectral efficiency(SE) improvement, becomes a hot research topic in the current research of optical fiber communication.Superchannel technique, which packs optical carriers tightly on the optical domain, can increase the transmission capacity toward Tbit/s by realizing parallel transmission of multiple optical carriers. Three kinds of superchannel techniques have been widely investigated recently, including all-optical orthogonal frequency division multiplexing(AO-OFDM), Nyquist wavelength division multiplexing(N-WDM), and offset quadrature amplitude modulation(offset-QAM) based coherent wavelength division multiplexing. My thesis is focused on the third superchannel technique, and elaborate its mathematical model and orthogonal mechanism. Furthermore, we put more efforts on the carrier phase estimation(CPE) algorithm, which is one of essential modules of receiver side digital signal processing(DSP). Finally, we propose and successfully verify several CPE algorithms for offset-QAM signal.The main contributions of my thesis are summarized as follows.(1) Two measurements are defined to evaluate the inter-symbol interference(ISI) and inter channel interference(ICI) for the recovered offset-QAM signal. We theoretically prove that the CPE designed for quadrature amplitude modulation(QAM) signal is no longer suitable for offset-QAM signal, and new CPE algorithm has to be designed specifically for the offset-QAM signal.(2) We design a new CPE algorithm suitable for offset-QAM, which is defined as modified blind phase search(BPS) based on traditional BPS method. The new algorithm can greatly enhance the laser linewidth tolerance for offset-QAM signal. When the symbol rate is 28 Gbaud, the tolerable linewidths are 18.2 MHz and 3.08 MHz for offset-QPSK and offset-16 QAM, respectively.(3) Due to the high computational complexity, MBPS is not suitable for real time implementation. Therefore, another CPE algorithm based on constellation classification with low complexity is designed for offset-QAM signal. When the test angle B=24 is chosen for MBPS, the CC of the proposed CPE algorithm can be significantly reduced by a factor of 8.47(or 7.34) in the form of multipliers(or adders).