A communication apparatus, a communication system, a communication method, and a non-transitory computer readable medium capable of reducing differences among receiving characteristics of a plurality of quadrature amplitude-modulated signals and optimizing the capacity of transmission signals are provided. A communication apparatus (1) according to the present disclosure includes a transmitting unit (2) capable of transmitting a plurality of types of quadrature amplitude-modulated signals, and an addition unit (3) that adds error correction codes to the quadrature amplitude-modulated signals. Further, the communication apparatus (1) includes a control unit (4) that changes the number of bits of the error correction code according to the type of the quadrature amplitude-modulated signal.

An optimization unit in an optical receiver divides a symbol region out of a plurality of symbol regions into which signal points that specifies symbol information included in an optical signal are classified, into a plurality of division regions from the symbol center coordinate of the symbol region. Moreover, the optimization unit accumulates the signal points of the symbol information for every division region in the symbol region. Furthermore, based on the accumulated number of signal points for every division region, the optimization unit controls the average length of a phase estimation unit when the phase noise of the optical signal is calculated.

The invention pertains to methods, apparatus, and systems for controlling the sub-carrier frequencies in an optical frequency division multiplex communication system by using a metric available from the sub-carrier modem's digital receive block as an indicator of cross-talk between sub-carriers and adjusting the sub-carrier frequency separation as a function of that metric.