Signal encoding for frequency division multiplexing on transmission lines

Apparatus for communicating on a subsea transmission line carrier signals of different frequencies ranging from a lowest frequency to a highest frequency, said apparatus comprising:

• at a first location, an encoder configured to encode the carrier signals such that the amplitude of each carrier signal above the lowest frequency carrier signal encodes a data value, and the amplitude of each other carrier signal is set for transmission via said subsea transmission line in accordance with a first ratio, relative to the amplitude of a neighboring lower frequency carrier signal, computed from the amplitude of said lower frequency carrier signal in accordance with a ratio dependent on the data value to be encoded;
• and
• at a second location, a decoder configured to decode carrier signals received via said subsea transmission line by scaling each received carrier signal amplitude in accordance with a second ratio, representative of the received carrier signal amplitude and the amplitude of said neighboring lower frequency carrier signal, to recover said data value and to compensate for a frequency-dependent effect of the subsea on amplitudes of the carrier signals.

Apparatus for communicating on a subsea transmission line carrier signals of different frequencies ranging from a lowest frequency to a highest frequency, said apparatus comprising:

• at a first location, an encoder configured to encode said carrier signals for transmission onto said subsea transmission line such that the carrier signal at said lowest frequency has a predetermined amplitude which represents a known data value, and the amplitude of each other carrier signal above the lowest frequency carrier signal encodes a data value, the amplitude of each other carrier signal being selected to have a ratio, relative to said predetermined amplitude, determined by the data value to be encoded;
• and
• at a second location a decoder configured to decode a carrier signal received from said subsea transmission line by scaling up the received carrier signal amplitude using said carrier signal of the lowest frequency as a reference, whereby to recover the encoded data value and to compensate for a frequency-dependent effect of the subsea transmission line on the amplitudes of the carrier signals.

A modem for communicating on a subsea transmission line carrier signals of different frequencies ranging from a lowest frequency to a highest frequency, said modem comprising:

• an encoder configured to encode the carrier signals such that the amplitude of each carrier signal above the lowest frequency carrier signal encodes a data value, and the amplitude of each other carrier signal is set for transmission via said subsea transmission line in accordance with a first ratio, relative to the amplitude of a neighboring lower frequency carrier signal, computed from the amplitude of said lower frequency carrier signal in accordance with a ratio dependent on the data value to be encoded;
• and
• a decoder configured to decode carrier signals received by scaling each received carrier signal amplitude in accordance with a second ratio, representative of the received carrier signal amplitude and the amplitude of said neighboring lower frequency carrier signal, to recover said data value and to compensate for a frequency-dependent effect of the subsea transmission line on amplitudes of the carrier signals.

A modem for communicating on a subsea transmission line carrier signals of different frequencies ranging from a lowest frequency to a highest frequency, said modem comprising:

• an encoder configured to encode said carrier signals such that the carrier signal at said lowest frequency has a predetermined amplitude which represents a known data value, and the amplitude of each other carrier signal above the lowest frequency carrier signal encodes a data value, the amplitude of each other carrier signal being selected to have a ratio, relative to said predetermined amplitude, determined by the data value to be encoded;
• and
• a decoder configured to decode a received carrier signal by scaling up the received carrier signal amplitude using said carrier signal of the lowest frequency as a reference, whereby to recover the encoded data value and to compensate for a frequency-dependent effect of the subsea transmission line on the amplitudes of the carrier signals.