Multilevel amplitude modulation device, multilevel amplitude demodulation device, transmission system including these, multilevel amplitude modulation method, and multilevel amplitude demodulation method

1. A multilevel amplitude modulation device for generating, from digital data, a multilevel amplitude modulation signal having four or more signal levels and outputting the generated signal, the modulation device comprising:

an average level calculator operable to select one of a plurality of preliminarily prepared different candidates for a code word building method such that an average signal level of a symbol array, obtained by adding a symbol for a code word of digital data to be transmitted to one or more already outputted symbols included in a multilevel amplitude modulation signal already outputted, is most approximate to voltage center of the four or more signal levels, and output a selection signal indicating the selected code word building method;

a signal converter operable to form a code word of the digital data in accordance with the code word building method indicated by the selection signal; and

a multilevel modulator operable to generate a multilevel amplitude modulation signal with use of the code word and output the generated multilevel amplitude modulation signal.

2. The multilevel amplitude modulation device according to claim 1 , wherein the plurality of preliminarily prepared different candidates for a code word building method include a plurality of candidates for a code word building method for generating headers having average signal levels different from one another and an identical payload when identical data is converted into a code word.

3. The multilevel amplitude modulation device according to claim 1 , wherein the plurality of preliminarily prepared different candidates for a code word building method include a plurality of code word building methods for applying different arrangement patterns of data values on multiple signal levels, and

the signal converter generates a header including information on the selected code word building method.

4. The multilevel amplitude modulation device according to claim 3 , wherein the plurality of preliminarily prepared different candidates for a code word building method include a code word building method for applying different arrangement patterns on multiple signal levels to data values stored in a plurality of divided portions of a payload in the code word.

5. The multilevel amplitude modulation device according to claim 3 , wherein the plurality of preliminarily prepared different candidates for a code word building method include a plurality of code word building methods for applying the arrangement patterns symmetrical with each other with respect to the voltage center.

6. The multilevel amplitude modulation device according to claim 3 , wherein the plurality of preliminarily prepared different candidates for a code word building method include a plurality of code word building methods for applying the arrangement patterns inverted with each other with respect to values of uppermost bits of the data values.

7. The multilevel amplitude modulation device according to claim 1 , wherein the average level calculator selects one of the plurality of candidates for a code word building method such that the average signal level of the symbol array, obtained by adding the symbol for the code word of digital data to be transmitted to a predetermined number of most recent symbols already outputted, is most approximate to the voltage center.

8. The multilevel amplitude modulation device according to claim 1 , wherein the average level calculator selects one of the plurality of candidates for a code word building method such that the average signal level of the symbol array, obtained by adding the symbol for the code word of digital data to be transmitted to all of already outputted symbols from start of output, is most approximate to the voltage center.

9. A transmission system comprising:

a transmitter including the multilevel amplitude modulation device according to claim 1 ; and

a receiver including a multilevel amplitude demodulation device for demodulating a multilevel amplitude modulation signal having four or more signal levels and outputting digital data,

wherein the demodulation device includes:

a synchronizer operable to demodulate the multilevel amplitude modulation signal and detect a header of a code word;

a signal inverter operable to identify a code word building method employed to form the code word based on a value of the header detected by the synchronizer and execute inverse conversion corresponding to the code word building method to generate and output digital data.

10. The transmission system according to claim 9 , wherein the average level calculator of the multilevel amplitude modulation device of the transmitter selects one of the plurality of candidates for a code word building method such that the average signal level of the symbol array, obtained by adding the symbol for the code word of digital data to be transmitted to a predetermined number of most recent symbols already outputted, is most approximate to the voltage center, and

the predetermined number is larger than the number of symbols transmitted from the transmitter in a period corresponding to a reciprocal of a lower cutoff frequency of a transmission line used for transmitting the multilevel amplitude modulation signal.

11. A multilevel amplitude modulation method executed by a modulation device, of generating, from digital data, a multilevel amplitude modulation signal having four or more signal levels and outputting the generated signal, the method comprising:

receiving digital data to be transmitted;

selecting one of a plurality of preliminarily prepared different candidates for a code word building method such that an average signal level of a symbol array, obtained by adding a symbol for a code word of the digital data to one or more already outputted symbols included in a multilevel amplitude modulation signal already outputted, is most approximate to voltage center of the four or more signal levels, and

generating a multilevel amplitude modulation signal with use of the code word of the data formed in accordance with the code word building method thus selected, and outputting the generated signal.