Master station device, slave station device, and wireless communication system

1. A master station device comprising:

a processor which, in operation, outputs a subcarrier modulation signal; and

a transmitter which, in operation, maps an in-phase (I) component and a quadrature-phase (Q) component of the subcarrier modulation signal to an optical signal to be transmitted to a fronthaul, wherein:

the subcarrier modulation signal is coded in a first quadrature amplitude modulation scheme having a plurality of signal points,

the optical signal is coded in a second quadrature amplitude modulation scheme having a plurality of candidate signal points, the second quadrature amplitude modulation scheme having a higher modulation multilevel number than the first quadrature amplitude modulation scheme, and

the transmitter sets each of the plurality of signal points of the subcarrier modulation signal to a corresponding candidate signal point of the plurality of candidate signal points of the optical signal, a distance from the each of the signal points of the subcarrier modulation signal to the corresponding candidate signal point being a shortest distance in an IQ plane of the optical signal.

2. The master station device according to claim 1 , wherein the transmitter maps the I component and the Q component of the subcarrier modulation signal as an I component and a Q component of the optical signal, respectively.

3. The master station device according to claim 1 , wherein the subcarrier modulation signal is a signal mapped to a radio resource in a radio transmission segment, and is subjected to error correction encoding corresponding to the radio transmission segment between the radio transmission segment and an optical transmission segment including the fronthaul.

4. The master station device according to claim 1 , further comprising a receiver which, in operation, receives an uplink optical signal via the fronthaul,

wherein the processor identifies an uplink subcarrier modulation signal mapped to the uplink optical signal, based on an I component and a Q component of the uplink subcarrier modulation signal.

5. The master station device according to claim 1 , wherein

an error correction encoding processing for fronthaul transmission is performed in a header part of the subcarrier modulation signal, and

the error correction encoding processing for the fronthaul transmission is not performed in a payload part of the subcarrier modulation signal.

6. A slave station device comprising:

a receiver which, in operation, receives an optical signal via a fronthaul; and

a processor which, in operation, identifies a subcarrier modulation signal based on an in-phase (I) component and a quadrature-phase (Q) component of the subcarrier modulation signal mapped to the optical signal, wherein:

the subcarrier modulation signal is coded in a first quadrature amplitude modulation scheme having a plurality of signal points,

the optical signal is coded in a second quadrature amplitude modulation scheme having a plurality of candidate signal points, the second quadrature amplitude modulation scheme having a higher modulation multilevel number than the first quadrature amplitude modulation scheme, and

the processor sets each of the plurality of candidate signal points of the optical signal to a corresponding signal point of the plurality of signal points of the subcarrier modulation signal, a distance from the each of the candidate signal points of the optical signal to the corresponding signal point being a shortest distance in an IQ plane of the subcarrier modulation signal.

7. The slave station device according to claim 6 , wherein the processor outputs an uplink subcarrier modulation signal, the device comprising a transmitter which, in operation, maps an I component and a Q component of the uplink subcarrier modulation signal to an uplink optical signal to be transmitted to the fronthaul.

8. The slave station device according to claim 6 , wherein

an error correction encoding processing for fronthaul transmission is performed in a header part of the subcarrier modulation signal, and

the error correction encoding processing for the fronthaul transmission is not performed in a payload part of the subcarrier modulation signal.

9. A wireless communication system comprising:

a master station device including

a processor which, in operation, outputs a subcarrier modulation signal, and

a transmitter which, in operation, maps an in-phase (I) component and a quadrature-phase (Q) component of the subcarrier modulation signal to an optical signal to be transmitted to a fronthaul, wherein:

the subcarrier modulation signal is coded in a first quadrature amplitude modulation scheme having a plurality of signal points,

the optical signal is coded in a second quadrature amplitude modulation scheme having a plurality of candidate signal points, the second quadrature amplitude modulation scheme having a higher modulation multilevel number than the first quadrature amplitude modulation scheme, and

the transmitter sets each of the plurality of signal points of the subcarrier modulation signal to a corresponding candidate signal point of the plurality of candidate signal points of the optical signal, a distance from the each of the signal points of the subcarrier modulation signal to the corresponding candidate signal point being a shortest distance in an IQ plane of the optical signal; and

a slave station device including

a receiver which, in operation, receives the optical signal via the fronthaul, and

a processor which, in operation, identifies the subcarrier modulation signal based on the I component and the Q component of the subcarrier modulation signal mapped to the optical signal.

10. The wireless communication system according to claim 9 , wherein

an error correction encoding processing for fronthaul transmission is performed in a header part of the subcarrier modulation signal, and

the error correction encoding processing for the fronthaul transmission is not performed in a payload part of the subcarrier modulation signal.