Duobinary pulse shaping for optical transmission systems employing pulse amplitude modulation techniques
View Patent ↗A duobinary optical communication system is disclosed that employs pulse amplitude modulation (PAM) techniques to provide further improvements in spectral efficiency. A disclosed PAM duobinary optical transmitter converts a plurality of input bits to an N level signal using PAM techniques; adds a current N level signal to a previous N level signal to produce a 2N−1 level signal; and converts the 2N−1 level signal to an optical signal for transmission to a receiver. A disclosed PAM duobinary optical receiver detects a power level of the received optical signal (encoded using pulse amplitude modulation and duobinary encoding techniques to encode a plurality of bits) and maps the detected power level to a plurality of bits to return the transmitted information. An exemplary PAM-4 duobinary optical communication system combines PAM-4 modulation techniques with duobinary pulse shaping techniques to provide an overall factor of four improvement in spectral efficiency by reducing the bandwidth of the optical signal.
1. A method for transmitting information, said method comprising the steps of:
converting a plurality of input bits to an N level signal;
adding a current N level signal to a previous N level signal to produce a 2N−1 level signal, wherein said 2N−1 level signal is mapped to a corresponding voltage value using a reordered set of said 2N−1 levels such that following a squaring operation at a receiver a received signal appears as a modulo N operation; and
converting said 2N−1 level signal to an optical signal for transmission to said receiver.
2. The method of claim 1 , wherein said step of converting a plurality of input bits to an N level signal further comprises the step of adding a current plurality of input bits to a previous plurality of output bits to produce said N level signal.
3. The method of claim 2 , wherein said previous plurality of output bits are obtained using a delay operator.
4. The method of claim 2 , wherein said step of converting a plurality of input bits to an N level signal implements a 1/1(1+D) {mod N}operation, where D is a delay operation that delays a sequence of bits by one bit interval.
5. The method of claim 1 , wherein said optical signal is received by a receiver and wherein said receiver further comprises the steps of detecting a power level of said received optical signal and mapping said detected power level to a plurality of bits.
6. The method of claim 1 , wherein said adding step further comprises the step of mapping a seven level value to a corresponding voltage value, as follows: (0, 1, 2, 3, 6, 5, 4)→(−3V, −2V, −V, 0, V, 2V, 3V), respectively.
7. The method of claim 6 , wherein said mapping is performed by a digital to analog converter.
8. A method for receiving information, said method comprising the steps of:
receiving an optical signal, said optical signal is encoded using pulse amplitude modulation and duobinary encoding techniques to encode a plurality of bits;
detecting a power level of said received optical signal; and
mapping said detected power level to a plurality of bits, wherein a 2N−1 level signal is mapped to a corresponding voltage value using a reordered set of said 2N−1 levels such that following a squaring operation the received signal appears as a modulo N operation.
9. The method of claim 8 , wherein said detecting step further comprises the step of converting said optical signal to an electrical signal.
10. The method of claim 9 , wherein said step of converting said optical signal to an electrical signal performs a squaring operation on the optical signal.
11. The method of claim 10 , wherein said squaring operation converts a 2N−1 level signal to an N level signal.
12. A system for transmitting information, said system comprising:
a precoder for converting a plurality of input bits to an N level signal;
an adder for adding a current N level signal to a previous N level signal to produce a 2N−1 level signal, wherein said 2N−1 level signal is mapped to a corresponding voltage value using a reordered set of said 2N−1 levels such that following a squaring operation at a receiver a received signal appears as a modulo N operation; and
a digital to analog converter for converting said 2N−1 level signal to an optical signal for transmission to said receiver.
13. The system of claim 12 , wherein said precoder comprises an adder for adding a current plurality of input bits to a previous plurality of output bits to produce said N level signal.
14. The system of claim 13 , wherein said previous plurality of output bits are obtained using a delay operator.
15. The system of claim 13 , wherein said precoder implements a 1/(1+D) {mod N}operation, where D is a delay operation that delays a sequence of bits by one bit interval.
16. The system of claim 12 , wherein said optical signal is received by a receiver and wherein said receiver further comprises a power detector for detecting a power level of said received optical signal and a slicer for mapping said detected power level to a plurality of bits.
17. A system for receiving information, said system comprising:
an input port for receiving an optical signal, said optical signal is encoded using pulse amplitude modulation and duobinary encoding techniques to encode a plurality of bits;
a power detector for detecting a power level of said received optical signal; and
a slicer for mapping said detected power level to a plurality of bits, wherein a 2N−1 level signal is mapped to a corresponding voltage value using a reordered set of said 2N−1 levels such that following a squaring operation the received signal appears as a modulo N operation.
18. The system of claim 17 , wherein said power detector converts said optical signal to an electrical signal.
19. The system of claim 18 , wherein said optical to electrical conversion performs a squaring operation on the optical signal.
20. The system of claim 19 , wherein said squaring operation converts a 2N−1 level signal to an N level signal.