Asymmetric Differential Phase Shift Keying And Its Application In PON

The development of passive optical network (PON) demands higher transmission speed, higher spectrum efficiency, more flexibility in access method and low cost. Differential quadrature phase shift keying (DQPSK) is one of the most widely used high spectrum efficiency modulation formats in high speed optical communication system. Due to the complicated modulation/demodulation method, the cost of this format is comparably high. In order to expand the usage of DQPSK format in PON, there should be a way to decrease its cost. On the other hand, optical code division multiple access (OCDMA) is a promising candidate in next generation PON. But the generation technology of optical orthogonal address code is not mature and costs high. This technology should be modified to make the cost low.Based on the studying of DQPSK and CDMA-PON, in this paper, we proposed and experimentally demonstrated a novel modulation format and its application in PON. The main parts of our work are the following3points:(1) On the base of DQPSK format, a novel modulation format: asymmetric DQPSK format (A-DQPSK) is proposed. With Gaussian approach method, BER model of this modulation format is deduced in theory. Both of software simulation and experiment showed the validity of A-DQPSK. This format has asymmetric constellation and can be fulfilled using modifed DPMZM. It has the same frequency efficency as DQPSK, but one set of DI and PD can demodulate the new format. Thus, the demodulation cost will be drascally saved. Further study showed us that, the pre-coding algorithm of A-DQPSK is much simpler than that of DQPSK. For these reasons, it well adapts the cost-sensitive access network.(2) A carrier reusing technology basing on A-DQPSK is proposed in this paper. The theory of this scheme and simulation results are also provided. In this scheme, the down-stream (DS) data is modulated in A-DQPSK format. When re-modulate the up-stream (US) data, the US and DS data should be pre-processed together to generate a new signal which can directly phase modulated the original down-stream light. After optimization, one single PM can accomplish the A-DQPSK modulation of DS data and another PM can remodualte US data in A-DQPSK format. This method inherits the low cost and high spectrum efficiency of A-DQPSK while keeps the cost low.(3) On the study of mapping between phase difference and interfered light intensity, a differential multiple phase shift keying (DMPSK) is proposed in the paper. With this modulation format, we combined CDMA and PON to form a phase modulated CDMA-PON. BER model of this format is deduced in theory and the simulation showed the validity of this scheme. This new PON not only inherits the broadcasting ability and flexibility of add-drop users of CDMA, but also can be carrier reused because of the phase modulation feature. This scheme occupies only one wavelength so that the user number can be expanded using WDM.