Research Of Continuous Phase Modulation Scheme Based On Coherent Optical Transmission System

Signal receiving is the demodulation and recovery of a transmitted signal in the communication system. In the traditional optical communication system, the receiving mode of the optical signal is the direct detection which translating the optical signal into the electrical signal by a photodiode. The direct detection mode can not find the phase of the signal, it can only detect the on-off keying (OOK) signal whose spectrum efficiency is very low, and it can not satisfy the demand of higher communication systems. The coherent detection mode can discover the both amplitude and phase of the optical signal. It discovers the optical signal phase by the hybrid of the signal light and local light. It makes the signal processing technology applied to the optical transmission system.The signal process in optical transmission system is called the signal recovery, which has the following steps:orthogonalization to the non-ideal hybrid, channel equalization to the impairments, synchronous of the symbol, frequency and phase recovery of the signal. Symbol estimation and decoding is carried out at last to get the original symbol at the transmitter. The main modulation formats in the coherent optical system are quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), which can develop the capacity of the system but the transmission distance, will be limited. From the steps, the two key parts are the analysis of the signal impairment mechanism and the selection of a modulated signal. This dissertation focuses on these two points and processes a theory of the signal impairment in the hybrid, a relationship between the channel impairments and equalization and a method of continuous phase modulation (CPM) applied to the optical transmission system.The signal impairment mechanism in the hybrid is based on the constellation and the theory of the polarized light. The relationship between geometric parameters on the constellation and the hybrid parameters is the signal impairment process in the hybrid. The relation between the equalization complexity and channel characteristic is according to the empirical analysis on the finite impulse response (FIR) taps needed to the different channel length.The research on the CPM signal includes the binary CPM signal and quaternary CPM signal. The binary CPM signal contains the minimum shift keying (MSK) signal and CPM signal with arbitrary modulation index. The quaternary CPM signal is the shaped offset quadrature phase shift keying (SOQPSK) signal which is applied in outer space communication system. The spectrum efficiency of the CPM signal is better than the traditional quadrature phase shift keying (QPSK) signal, it can potentially improve the ability of the inhibition to the channel impairments in the transmission system.The dissertation intends to analysis the signal impairments and modulation in the optical system. We solve the relationship issues of the impairment and equalization and design an optical coherent CPM system. Simulations show that the limitation distance of the CPM signal is20kilometers longer than that of the traditional QPSK signal.