Method and apparatus for transmitting high-level QAM optical signals with binary drive signals
View Patent ↗Methods and apparatus for power-efficiently and reliably transmitting high-level quadrature amplitude modulation (QAM) optical signals using binary drive signals. Even though binary signals are used to drive a QAM modulator directly, without digital-to-analog conversion, the methods and apparatus disclosed allow the transmission of pilot symbol sequences having near optimal properties, such as a constant power profile in the time domain; a mean power that is approximately the same as the mean power of the data symbols; and roughly uniform amplitude in the frequency domain for non-zero frequency components of the pilot symbol sequence. The binary drive signals can be processed so that the modulated optical signals are selectively constrained to a subset of points of the QAM constellation to form a QAM constellation with reduced size and a mean power that is approximately the same as the mean power of the original QAM constellation.
1. An apparatus comprising:
a quadrature amplitude modulation (QAM) modulator for generating modulated optical signals by directly driving a plurality of I/Q modulators with binary drive signals, the modulated optical signals being characterized by a QAM constellation having a mean power; and
a drive signal generator including a pilot symbol sequence (PSS) generator, the drive signal generator for providing the binary drive signals to the I/Q modulators, the PSS generator for generating a PSS having pilot symbols characterized by:
a constant power profile in the time domain;
a mean power that is approximately the same as the mean power of the QAM constellation; and
a substantially uniform amplitude in the frequency domain;
wherein pilot symbols in the PSS are a subset of the QAM symbols.
2. The apparatus of claim 1 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation.
3. The apparatus of claim 1 , wherein the PSS is generated in accordance with an approximation of a Chu sequence.
4. The apparatus of claim 3 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation nearest to a point of the Chu sequence.
5. The apparatus of claim 3 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation nearest to a rotated point of the Chu sequence.
6. The apparatus of claim 5 , wherein the QAM constellation is a 16QAM constellation and the rotated point of the Chu sequence is rotated by approximately π/8 radians.
7. The apparatus of claim 5 , wherein the QAM constellation is a 64QAM constellation and the rotated point of the Chu sequence is rotated by approximately π/16 radians.
8. The apparatus of claim 1 , wherein the drive signal generator generates the binary drive signals so that the QAM modulator generates modulated optical signals that are selectively constrained to a subset of points of the QAM constellation.
9. The apparatus of claim 8 , wherein the subset of points forms a smaller QAM constellation with a mean power that is approximately the same as the mean power of the QAM constellation.
10. The apparatus of claim 1 comprising:
a further QAM modulator for generating further modulated optical signals in accordance with further binary drive signals provided by the drive signal generator; and
a polarization beam combiner (PBC) for combining the modulated optical signals and the further modulated optical signals into polarization-division-multiplexing-QAM modulated optical signals.
11. A method comprising:
generating modulated optical signals including pilot symbols by directly driving a quadrature amplitude modulation (QAM) modulator with binary drive signals, the modulated optical signals being characterized by a QAM constellation having a mean power,
wherein the pilot symbols are a subset of the QAM symbols and are arranged in a pilot symbol sequence (PSS), said pilot symbols characterized by:
a constant power profile in the time domain;
a mean power that is approximately the same as the mean power of the QAM constellation; and
a substantially uniform amplitude in the frequency domain.
12. The method of claim 11 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation.
13. The method of claim 11 , wherein the PSS is generated in accordance with an approximation of a Chu sequence.
14. The method of claim 13 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation nearest to a point of the Chu sequence.
15. The method of claim 13 , wherein each pilot symbol of the PSS corresponds to a point in the QAM constellation nearest to a rotated point of the Chu sequence.
16. The method of claim 15 , wherein the QAM constellation is a 16QAM constellation and the rotated point of the Chu sequence is rotated by approximately π/8 radians.
17. The method of claim 15 , wherein the QAM constellation is a 64QAM constellation and the rotated point of the Chu sequence is rotated by approximately π/16 radians.
18. The method of claim 11 , wherein the binary drive signals are processed so that the modulated optical signals are selectively constrained to a subset of points of the QAM constellation.
19. The method of claim 18 , wherein the subset of points forms a smaller QAM constellation with a mean power that is approximately the same as the mean power of the QAM constellation.
20. The method of claim 11 comprising:
generating further modulated optical signals in accordance with further binary drive signals; and
combining the modulated optical signals and the further modulated optical signals into polarization-division-multiplexing-QAM modulated optical signals.