Effect Of ADC Resolution On The Performance Of Carrier Phase Estimation In Optical Coherent Receivers

With the sharp increasing in communication business, such as video sharing, high-definition television and cloud computing, it is imperative to improve the data transmission capacity of backbone network. There are two main methods to increasing the bit rate within the physical layer: increasing symbol rate or increasing the number of bit per symbol, i.e., modulation order. While higher symbol rate needs higher ADC sampling rate to restore the signal with little distortion, and higher modulation order needs higher ADC resolution to distinguish signal accurately. However, there is a tradeoff between high speed and high resolution in ADC with current device technology. Since ADC with high sampling rate is easy to satisfy higher symbol rate and conduct oversampling so as to reduce the distortion on signal, it is necessary to find how ADC resolution affects the performance of the phase estimation receiver and to realize the reasonable ADC resolution.This thesis introduces the basic theory of coherent optical communication systems, including the basic structure, development and advantage of coherent optical communication systems. Then two phase estimation methods, M-th power and DAML, are introduced based on the characteristic of laser phase noise. Simulation results show that the tolerance of laser phase noise decreases with the increasing in modulation order.Secondly, the model of ADC resolution limited receiver is established. ADC quantization model is built and the difference between quantization noises and other additive noises is analyzed. Then based on the quantization model, continuous PDF(Probability Distribution Function) of signal is transformed into discrete probability distribution. Meanwhile, the BER of MPSK system is derived with and without DAML phase estimation method.Finally, simulations are performed to analyze the BER performance of M-th power and DAML receiver with various modulation methods and different ADC resolutions. The results show that N+2 bit resolution is acceptable and N+3 bit is sufficient for both M-th power and DAML receiver in MPSK(N bits per symbol) modulated systems, while for 16 QAM system, 6 bit resolution is acceptable and reasonable. Quantization range is also taken into consideration. Simulation and theory both indicate that ADC quantization range must be not less than the average amplitude of received symbols which carry the information symbols with the highest energy on the constellation, otherwise the phase estimation error will increase rapidly.