Methods and apparatuses to improve reception of direct detection optical signals
System and method embodiments are provided for improving reception of direct detection optical signals. In an embodiment, a method for optical transmission includes bit loading and power loading, with a digital signal processor (DSP), transmission bits of an orthogonal frequency-division multiplexing (OFDM) signal; calculating, with the DSP, a signal-signal beat interference (SSBI) component of the bit and power loaded OFDM signal by modulating each subcarrier with a symbol; and subtracting, with the DSP, the calculated SSBI component from the bit and power loaded OFDM signal.
1. A method for optical transmission, comprising:
bit loading and power loading, with a digital signal processor (DSP), transmission bits of an orthogonal frequency-division multiplexing (OFDM) signal;
calculating, with the DSP, a signal-signal beat interference (SSBI) component of the bit and power loaded OFDM signal by modulating each subcarrier with a symbol, wherein the SSBI component is determined according to a complex conjugate of a dc carrier, a complex-valued data on one or more subcarriers, and a complex conjugate of a complex-valued data on one or more of the subcarriers;
subtracting, with the DSP, the SSBI component from the bit and power loaded OFDM signal to generate an adjusted OFDM signal; and
transmitting the adjusted OFDM signal.
2. The method of claim 1 , further comprising equalizing a channel.
3. The method of claim 1 , wherein the SSBI component is calculated according to an estimated chromatic dispersion (CD) of an optical channel.
4. The method of claim 1 , wherein the SSBI component is calculated according to an estimated hardware radio-frequency (RF) response.
5. The method of claim 1 , further comprising equalizing a subcarrier phase after subtracting the SSBI component from the bit and power loaded OFDM signal.
6. The method of claim 1 , wherein calculating the SSBI component comprises determining σ k according to
σ
k
=
1
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
exp
(
j2π
nk
/
N
)
wherein σ k is an SSBI contribution to be subtracted, a* 0 is a complex conjugate of dc carrier, a m is complex-valued data on m-th subcarrier, a* m-k is complex conjugate of the complex-valued data on (m−k)-th subcarrier, j is an imaginary part of a complex number, k is a subcarrier index, n is a digital sample index in time, m is a subcarrier index, and N is a fast Fourier transform (FFT) size in a receiver DSP.
7. The method of claim 1 , wherein the calculating the SSBI component comprises determining σ k according to
σ
k
=
1
H
k
·
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
H
m
·
H
m
-
k
exp
(
j2π
nk
/
N
)
wherein σ k is the SSBI component for the k th subcarrier, H m is a complex-valued channel response from an m th subcarrier, a k is complex-value data carried on the k th subcarrier, j denoted the square root of −1, k is the OFDM subcarrier index, n is a sample index in time, and N is a fast Fourier transform (FFT) size used in an OFDM receiver.
8. A network component configured for optical transmission, the network component comprising:
a processor; and
a computer readable storage medium storing programming for execution by the processor, the programming including instructions to:
bit load and power load transmission bits of an orthogonal frequency-division multiplexing (OFDM) signal;
calculate a signal-signal beat interference (SSBI) component of the bit and power loaded OFDM signal by modulating each subcarrier with a symbol, wherein the SSBI component is determined according to a complex conjugate of a dc carrier, a complex-valued data on one or more subcarriers, and a complex conjugate of a complex-valued data on one or more of the subcarriers;
subtract the SSBI component from the bit and power loaded OFDM signal to generate an adjusted OFDM signal; and
transmit the adjusted OFDM signal.
9. The network component of claim 8 , wherein the programming further comprises instructions to perform channel equalization.
10. The network component of claim 8 , wherein the SSBI component is calculated according to an estimated chromatic dispersion (CD) of an optical channel.
11. The network component of claim 8 , wherein the SSBI component is calculated according to an estimated hardware radio-frequency (RF) response.
12. The network component of claim 8 , wherein the programming further comprises instructions to equalize a subcarrier phase after subtracting the SSBI component from the bit and power loaded OFDM signal.
13. The network component of claim 8 , wherein the instructions to calculate the SSBI component comprise instructions to determine σ k according to
σ
k
=
1
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
exp
(
j2π
nk
/
N
)
wherein σ k is an SSBI contribution to be subtracted, a* 0 is a complex conjugate of dc carrier, a m is complex-valued data on m-th subcarrier, a* m-k is complex conjugate of the complex-valued data on (m−k)-th subcarrier, j is an imaginary part of a complex number, k is a subcarrier index, n is a digital sample index in time, m is a subcarrier index, and N is a fast Fourier transform (FFT) size in a receiver DSP.
14. The network component of claim 8 , wherein the instructions to calculate the SSBI component comprise instructions to determine σ k according to
σ
k
=
1
H
k
·
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
H
m
·
H
m
-
k
exp
(
j2π
nk
/
N
)
wherein σ k is an SSBI contribution to be subtracted, H is a complex-valued channel response, a* 0 is a complex conjugate of dc carrier, a m is a complex-valued data on m-th subcarrier, a* m-k is a complex conjugate of the complex-valued data on (m−k)-th subcarrier, j is an imaginary part of a complex number, k is a subcarrier index, n is a digital sample index in time, m is a subcarrier index, and N is a fast Fourier transform (FFT) size in a receiver DSP.
15. A network component for optical transmission, comprising:
an optical transmitter;
a direct detection component to detect an optical signal from the optical transmitter and convert the optical signal into an orthogonal frequency-division multiplexing (OFDM) electrical signal; and
a digital signal processor (DSP) coupled to the direct detection component and configured to:
bit load and power load transmission bits of the OFDM electrical signal;
calculate a signal-signal beat interference (SSBI) component of the bit and power loaded OFDM electrical signal by modulating each subcarrier with a symbol, wherein the SSBI component is determined according to a complex conjugate of a dc carrier, a complex-valued data on one or more subcarriers, and a complex conjugate of a complex-valued data on one or more of the subcarriers;
subtract the SSBI component from the bit and power loaded OFDM electrical signal to generate an adjusted OFDM signal; and
transmit the adjusted OFDM signal.
16. The network component of claim 15 , wherein the DSP is further configured to perform channel equalization.
17. The network component of claim 15 , wherein the SSBI component is calculated according to an estimated chromatic dispersion (CD) of an optical channel.
18. The network component of claim 15 , wherein the SSBI component is calculated according to an estimated hardware radio-frequency (RF) response.
19. The network component of claim 15 , wherein the DSP is further configured to equalize a subcarrier phase after subtracting the SSBI component from the bit and power loaded OFDM signal.
20. The network component of claim 15 , wherein the DSP is further configured to calculate the SSBI component by determining σ k according to
σ
k
=
1
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
exp
(
j2π
nk
/
N
)
wherein σ k is an SSBI contribution to be subtracted, a* 0 is a complex conjugate of dc carrier, a m is complex-valued data on m-th subcarrier, a* m-k is complex conjugate of the complex-valued data on (m−k)-th subcarrier, j is an imaginary part of a complex number, k is a subcarrier index, n is a digital sample index in time, m is a subcarrier index, and N is a fast Fourier transform (FFT) size in a receiver DSP.
21. The network component of claim 15 , wherein the DSP is further configured to calculate the SSBI component by determining σ k according to
σ
k
=
1
H
k
·
a
0
*
∑
k
=
m
+
1
N
a
m
·
a
m
-
k
*
·
H
m
·
H
m
-
k
exp
(
j2π
nk
/
N
)
wherein σ k is an SSBI contribution to be subtracted, H is a complex-valued channel response, a* 0 is a complex conjugate of dc carrier, a m is a complex-valued data on m-th subcarrier, a* m-k is a complex conjugate of the complex-valued data on (m−k)-th subcarrier, j is an imaginary part of a complex number, k is a subcarrier index, n is a digital sample index in time, m is a subcarrier index, and N is a fast Fourier transform (FFT) size in a receiver DSP.