Method and Apparatus for Transmitting and Receiving Coherent Optical Ofdm

1. An apparatus for transmitting coherent optical OFDM, comprising: a transmitted OFDM digital signal processing unit outputting an in-phase (I) component digital signal and a quadrature phase (Q) component digital signal; a digital-analog converter converting the in-phase (I)-component digital signal and the quadrature-phase (Q)-component digital signal into an in-phase (I)-component analog signal and a quadrature-phase (Q)-component analog signal, respectively; an adder adding an additional pilot tone signal to each of the in-phase (I) component analog signal and the quadrature-phase (Q)-component analog signal outputted from the digital-analog converter; an optical I/Q modulator up-converting the in-phase (I)-component analog signal added with the additional pilot tone signal and the quadrature-phase (Q)-component analog signal added with the additional pilot tone signal to an optical domain to output a coherent optical OFDM signal including the additional pilot tone signal; wherein an OFDM subcarrier band is above or below a frequency which is centered on a DC component, wherein the additional pilot tone signal is present in a low frequency or high-frequency band which is not overlapped with one or more OFDM data subcarrier bands, and wherein a phase of the additional pilot tone signal is estimated at a receiver and an OFDM data subcarrier is multiplied by a complex conjugate of the estimated phase to compensate a phase noise of the OFDM data subcarrier.

2. The apparatus of claim 1 , wherein the additional pilot tone signal may be a double side band (DSB) signal or a single side band (SSB) signal.

3. An apparatus for transmitting coherent optical OFDM, comprising: a transmitted OFDM digital signal processing unit outputting an in-phase (I) component digital signal and a quadrature phase (Q) component digital signal; a digital-analog converter converting the in-phase (I)-component digital signal and the quadrature-phase (Q)-component digital signal into an in-phase (I)-component analog signal and a quadrature-phase (Q)-component analog signal, respectively; a first optical I/Q modulator up-converting the in-phase (I)-component analog signal and the quadrature-phase (Q)-component analog signal outputted from the digital-analog converter to an optical domain to output a coherent optical OFDM signal; a second optical I/Q modulator up-converting an applied additional pilot tone signal to the optical domain to output an additional pilot tone optical signal; and an optical coupler outputting the coherent optical OFDM signal including the additional pilot tone optical signal by optically coupling the coherent optical OFDM signal and the additional pilot tone optical signal with each other; wherein the additional pilot tone signal is present in a low-frequency or high-frequency band which is not overlapped with one or more OFDM data subcarrier bands; wherein the one or more OFDM subcarrier bands are above or below a frequency which is centered on a DC component, and wherein a phase of the additional pilot tone signal is estimated at a receiver and an OFDM data subcarrier is multiplied by a complex conjugate of the estimated phase to compensate a phase noise of the OFDM data subcarrier.

4. The apparatus of claim 3 , wherein the additional pilot tone signal may be a double side band (DSB) signal or a single side band (SSB) signal.

5. An apparatus for receiving coherent optical OFDM, comprising: an optical down-converting unit outputting an in-phase (I)-component analog signal and a quadrature-phase (Q)-component analog signal by optically down converting a coherent optical OFDM signal; wherein the coherent optical OFDM signal includes OFDM data and an additional pilot tone signal added at a transmitter to each of an in-phase (I)-component analog signal and a quadrature-phase (Q)-component analog signal outputted from a digital-analog converter; an analog-digital converter converting the in-phase (I)-component analog signal and the quadrature-phase (Q)-component analog signal into an in-phase (I)-component digital signal and a quadrature-phase (Q)-component digital signal, respectively; a phase noise compensation digital signal processing unit separating the OFDM data and the additional pilot tone signal from each of the in-phase (I)-component digital signal and the quadrature-phase (Q)-component digital signal 5 outputted from the analog-digital converter; and a received OFDM data signal processing unit demodulating the OFDM signal in which phase noise is compensated; wherein the additional pilot tone signal is present in a low frequency or high-frequency band which is not overlapped with one or more OFDM data subcarrier bands, wherein the one or more OFDM data subcarrier bands are above or below a frequency which is centered on a DC component, and wherein a phase of the additional pilot tone signal is estimated at the phase noise compensation digital signal processing unit and an OFDM data subcarrier is multiplied by a complex conjugate of the estimated phase to compensate the phase noise of the OFDM data subcarrier.

6. The apparatus of claim 5 , wherein the optical down-converting unit includes: an optical hybrid outputting the coherent optical OFDM signal into an in-phase (I)-component optical signal and a quadrature-phase (Q)-component optical signal; and an optical detector optically down-converting the in-phase (I)-component optical signal and the quadrature-phase (Q)-component optical signal.

7. The apparatus of claim 5 , wherein the phase noise compensation digital signal processing unit includes a high-pass filter, a low-pass filter, a conjugate complex numberer, and a multiplier.

8. The apparatus of claim 7 , wherein the high-pass filter and the low-pass filter is a digital filter or an analog filter.

9. A method for transmitting coherent optical OFDM, comprising: converting an in-phase (I) component digital signal and a quadrature phase (Q) component digital signal into an in-phase (I) component analog signal and a quadrature phase (Q) component analog signal, respectively; adding an additional pilot tone signal to each of the in-phase (I)-component analog signal and the quadrature-phase (Q)-component analog signal; up-converting the in-phase (I)-component analog signal added with the additional pilot tone signal and the quadrature-phase (Q)-component analog signal added with the additional pilot tone signal to an optical domain to generate and transmit a coherent optical OFDM signal including the additional pilot tone signal; wherein an OFDM subcarrier band is above or below a frequency which is centered on a DC component, wherein the additional pilot tone signal is present in a low-frequency or high-frequency band which is not overlapped with the OFDM data subcarrier bands, and wherein a phase of the additional pilot tone signal is estimated at a receiver and an OFDM data subcarrier is multiplied by a complex conjugate of the estimated phase to compensate a phase noise of the OFDM data subcarrier.

10. The method of claim 9 , wherein the additional pilot tone signal may be a double side band (DSB) signal or a single side band (SSB) signal.

11. A method for receiving coherent optical OFDM, comprising: generating an in-phase (I) component analog signal and a quadrature phase (Q) component analog signal by optically down-converting a received coherent optical OFDM signal; wherein the coherent optical OFDM signal includes OFDM data and an additional pilot tone signal added at a transmitter to each of an in-phase (I)-component analog signal and a quadrature-phase (Q)-component analog signal outputted from a digital-analog converter; converting the in-phase (I)-component analog signal and the quadrature-phase (Q)-component analog signal into an in-phase (I) component digital signal and a quadrature phase (Q) component digital signal, respectively; separating the OFDM data and the additional pilot tone signal from each of the in phase (I)-component digital signal and the quadrature-phase (Q)-component digital signal and multiplying the separated OFDM data by complex conjugate of additional pilot tone signal to generate an OFDM signal in which phase noise is compensated; and demodulating the OFDM signal in which phase noise is compensated; wherein the additional pilot tone signal is present in a low frequency or high-frequency band which is not overlapped with one or more OFDM data subcarrier bands, wherein the one or more OFDM data subcarrier bands are above or below a frequency which is centered on a DC component, and wherein a phase of the additional pilot tone signal is estimated at the phase noise compensation digital signal processing unit and an OFDM data subcarrier is multiplied by a complex conjugate of the estimated phase to compensate the phase noise of the OFDM data subcarrier.