As a modulation scheme for optical wireless communication, there is MPolSK (multilevel polarization shift keying) that modulates a state of polarization of light. MPolSK has a problem that it is severely affected by mismatched polarization axes. Although MDPolSK (multilevel differential PolSK) can overcome the problem, it is susceptible to noise, and its SER (symbol error rate) degrades as compared to MPolSK. In this paper, we propose one kind of MDPolSK that estimates the mismatched polarization axes in the receiver. We analyzed SER of the proposed scheme by computer simulations. The result shows that the proposed scheme is not affected by the mismatched polarization axes, and it provides a good SER as compared to the conventional MDPolSK. In addition, we modified the constellation used in the proposed scheme to improve SER.

The differential 4-quadrature (D4Q) coherent optical system which does not need absolute phase information is proposed. The input information is encoded in the relative position of the present symbol with respect to the reference frame constructed by the previous 3 symbols. The general theory of the system operation including encoding and decoding is presented. This system does not need to track the fluctuation of the states of polarizations which is essential for most other polarization modulation systems. As an example, the 4-symbol D4Q system is described. And the saddle point approximation is applied to estimate the bit error rate performance. The analytic approximation agrees with the simulation results very well.