Patent ID: 8655193

Claim:
An optical multilevel transmission system, comprising: at least one optical multilevel transmitter for transmitting an optical multilevel signal obtained by modulating both a phase and an amplitude on a complex optical field with an information signal; and an optical multilevel receiver for receiving the optical multilevel signal and demodulating the received optical multilevel signal on a complex plane, the received optical multilevel signal having a larger noise in an angular direction than in a radial direction, wherein the at least one optical multilevel transmitter performs phase pre-integration signal processing of integrating phase components of the optical multilevel signal at every time T in advance on the optical multilevel signal, wherein the optical multilevel signal is an optical multilevel QAM signal which is processed by the phase pre-integration signal processing, wherein the optical multilevel receiver includes a light intensity receiver and at least two optical delay detection receivers with a delay time, wherein the optical multilevel receiver directly detects and receives a light intensity of the optical multilevel signal by using the light intensity receiver and a phase difference between the times T by using the optical delay detection receivers, wherein the optical multilevel receiver sets, in a symbol decision of the directly detected and received optical multilevel signal demodulated on the complex plane, for positions of all or some of ideal signal points, a width in the angular direction of a decision area, to which each of the ideal signal points belongs and which is measured along a circumference of a circle centered at an origin and passing through the center of the each of the ideal signal points, larger than a width in the angular direction of a decision area defined based on a Euclidean distance, and wherein the optical multilevel receiver calculates a non-Euclidean distance by weighting the received optical multilevel signal so that the complex optical field has a distance in the radial direction that is larger than a distance in the angular direction.