In an aspect, an apparatus for distribution of frequency reference to a receiving end over a transmission medium comprises a first mixer adapted to mix a frequency reference signal having a reference frequency with a local oscillator signal having a local oscillator frequency to provide a forward frequency reference signal, a communication section adapted to transmit the forward frequency reference signal and receive a first backward frequency reference signal, a second mixer adapted to mix the first backward frequency reference signal with the local oscillator signal to provide a second backward frequency reference signal and a phase comparator and control circuit adapted to adjust the local oscillator frequency based on a phase shift of the second backward frequency reference signal so as to compensate for a phase shift of the forward frequency reference signal.

In an aspect, an apparatus for distribution of frequency reference to a receiving end over a transmission medium comprises a first mixer adapted to mix a frequency reference signal having a reference frequency with a local oscillator signal having a local oscillator frequency to provide a forward frequency reference signal, a communication section adapted to transmit the forward frequency reference signal and receive a first backward frequency reference signal, a second mixer adapted to mix the first backward frequency reference signal with the local oscillator signal to provide a second backward frequency reference signal and a phase comparator and control circuit adapted to adjust the local oscillator frequency based on a phase shift of the second backward frequency reference signal so as to compensate for a phase shift of the forward frequency reference signal.

In a time synchronization system, a network controller sets transmission distance information on a transmission distance through which an optical signal is transmitted between transmission and receiving servers via a queueless network to a receiving server, based on network topology information. The transmitting server transmits a transmitting side current time synchronized with a reference time to the receiving server 12 via the queueless network. The receiving server calculates a transmission delay time between the transmission and receiving servers by dividing the transmission distance, which is based on the set transmission distance information, by light speed in the queueless network. A receiving side current time is calculated by adding the transmission delay time to the transmitting side current time.

A forward optical intensity modulation signal, generated by optical intensity-modulating a laser signal using a forward microwave phase modulation signal, is transmitted from a base to a remote station. A backward microwave phase modulation signal, in which frequency of the forward microwave phase modulation signal is changed by demodulating the forward optical intensity modulation signal, is generated, and a backward optical intensity modulation signal, generated by optical intensity-modulating the laser signal using the backward microwave phase modulation signal, is transmitted from the remote station to the base. The backward microwave phase modulation signal is extracted by photoelectric converting the backward optical intensity modulation signal, a round trip timing is extracted by demodulating the backward microwave phase modulation signal, and transmission delay is determined from a difference between the timing and the round trip timing.

An in-vehicle communication system includes: an optical coupler; a first in-vehicle device group composed of a plurality of in-vehicle devices connected to a first end of the optical coupler; and a second in-vehicle device group composed of a plurality of in-vehicle devices connected to a second end of the optical coupler. The in-vehicle devices in the first in-vehicle device group are communicable with the in-vehicle devices in the second in-vehicle device group via a common transmission path in the optical coupler. The in-vehicle devices in the second in-vehicle device group are communicable with the in-vehicle devices in the first in-vehicle device group via a common transmission path in the optical coupler.