Signal transmission apparatus and optical modem modulation method

A signal transmission apparatus comprising:

• a light source to emit an output light;
• an optical interface to provide a signal so that the light source emits the output light;
• an optical splitter to change an optical path of light reflected by an optical link when the output light emitted from the light source is reflected by the optical link;
• a photo detector to receive the light reflected by the optical link, transmitted from the optical splitter and to determine characteristics of the optical link;
• and
• an optical modem to determine a power level and a modulation method for each OFDM subcarrier based on the characteristics of the optical link.

A modulation method of an optical modem included in an optical line terminal (OLT) or an optical network unit (ONU) consisted of an orthogonal frequency division multiple access-passive optical network (OFDMA-PON), the modulation method comprising:

• emitting, by a light source, an output light;
• determining, by a photo detector, a length and a loss characteristic of an optical fiber included in an optical link through which the output light is transmitted;
• and
• controlling, by an optical modem, a power level and a bit allocated to a subcarrier for an optical OFDM signal based on the length and the loss characteristic of the optical fiber.