Optical transmitter, optical receiver, and optical communication system

An optical transmitter comprising:

• a bit converter to process input bits in units of four bits, and to generate an output bit sequence having eight bits based on one of a first conversion table, a second conversion table, and a third conversion table;
• and
• a symbol mapper to allocate the output bit sequence to an X-polarization and to a Y-polarization of each of two consecutive time slots,
• wherein
• the bit converter:
• performs, at a first ratio, a process of generating the output bit sequence based on the first conversion table;
• performs, each at a second ratio lower than the first ratio, a process of generating the output bit sequence based on the second conversion table and a process of generating the output bit sequence based on the third conversion table;
• and
• performs the process of generating the output bit sequence based on the second conversion table and the process of generating the output bit sequence based on the third conversion table such that a sequence of two most significant bits and a sequence of two least significant bits of the output bit sequence are respectively associated with values that are allocated respectively to two constellation points having phases different by 180 degrees on a complex plane.