Duobinary coding and modulation technique for optical communication systems
First Claim
1. A method for encoding a binary input sequence x(n,d) to obtain a duobinary output sequence y(+d,n,-d), comprising the steps of:
- providing a first logical level "n" for a bit yk of said duobinary output sequence y(+d,n,-d) when a corresponding bit xk of said binary input sequence x(n,d) has said first logic level "n";
switching a bit yk of said duobinary output sequence y(+d,n,-d) from said first level "n" to alternatively assume one of a second "+d" and a third "-d" logical level, whenever a succession xk-1, xk of bits in said input sequence x(n,d) comprises a change from said first logical level "n" to said second logical level "d"; and
maintaining the logical level of a bit yk as one of said second +d" and said third "-d" logical level, whenever a corresponding bit xk maintains said second logical level "d" and whenever a succession xk-1, xk of bits said input sequence x(n,d) comprises a change from said second logical level "n" to said first logical level "d".
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Abstract
A method for encoding a binary input sequence x(0,1) to obtain a duobinary output sequence y(+1,0,-1) is provided. The duobinary coding technique always provides an output bit yk =0 when the corresponding bit xk =0; bits yk alternatively assume a logical level "+1" and "-1" whenever an input bit xk-1 =0 changes to xk =1, and the output bit yk maintains the logical level "+1" or "-1" whenever the corresponding bit xk maintains the logical level "1". A coding device for encoding a binary input sequence x(0,1) to a duobinary output sequence y(+1,0,-1) is also provided, comprising a D-type flip-flop for generating a binary switch signal. A first AND circuit receives the input sequence and the switch signal, and provides a first binary sequence a(0,1), while a second AND circuit receives the input sequence and the complement of the switch signal and provides a second binary sequence b(0,1). These first and second binary sequences are applied to a summer to obtain the output sequence y(+1,0,-1). A method for differentially driving a M-Z modulator using a virtual ground level is also provided, which reduces the peak-to-peak drive voltage by a factor of two.
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Citations
20 Claims
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1. A method for encoding a binary input sequence x(n,d) to obtain a duobinary output sequence y(+d,n,-d), comprising the steps of:
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providing a first logical level "n" for a bit yk of said duobinary output sequence y(+d,n,-d) when a corresponding bit xk of said binary input sequence x(n,d) has said first logic level "n"; switching a bit yk of said duobinary output sequence y(+d,n,-d) from said first level "n" to alternatively assume one of a second "+d" and a third "-d" logical level, whenever a succession xk-1, xk of bits in said input sequence x(n,d) comprises a change from said first logical level "n" to said second logical level "d"; and maintaining the logical level of a bit yk as one of said second +d" and said third "-d" logical level, whenever a corresponding bit xk maintains said second logical level "d" and whenever a succession xk-1, xk of bits said input sequence x(n,d) comprises a change from said second logical level "n" to said first logical level "d". - View Dependent Claims (2, 3)
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4. A method for encoding a binary input sequence x(0,1) to obtain a duobinary output sequence y(+1,0,-1), comprising the steps of:
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preparing from said input sequence x(0,1) a binary switch signal Q(0,1) which maintains a current logical value when the input sequence x(0,1) comprises one of;
a succession of bits xk =0, a succession of bits xk =1, and a succession of bits xk =0, xk =1, and switches to the opposite logical value when the input sequence x(0,1) comprises a succession of bits xk =1, xk+1 =0;logically adding said binary input sequence x(0,1) and said switch signal Q(0,1) to obtain a first binary sequence a(0,1); logically adding said binary input sequence x(0,1) and the complement Q(1,0) of said switch signal to obtain a second binary sequence b(0,1); and summing said first and second binary sequences for obtaining said duobinary output sequence y(+1,0,-1). - View Dependent Claims (5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A coding device for encoding a binary input sequence x(0,1) to a duobinary output sequence y(+1,0,-1), comprising:
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means for generating a binary switch signal Q(0,1); a first AND circuit for receiving said binary input sequence x(0,1) and said switch signal Q(0,1), and providing a first binary sequence a(0,1); a second AND circuit for receiving said binary input sequence x(0,1) and the complement Q(1,0) of said switch signal to obtain a second binary sequence b(0,1); and a summer for processing said first and second binary sequences to obtain said output sequence y(+1,0,-1) on an output terminal. - View Dependent Claims (15, 16, 17, 18)
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19. A Mach-Zehnder (M-Z) interferometer for modulating a continuous wave (CW) optical carrier with a duobinary encoded driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, and a control electrode, said M-Z interferometer further comprising:
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means for applying a bias voltage VBias between said first and said second travelling wave electrode; means for providing said duobinary encoded driving signal to said control electrode; means for coupling said CW optical carrier signal to said input port; and means for coupling a duobinary modulated optical signal from said output port into an optical fiber.
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20. A Mach-Zehnder (M-Z) interferometer for modulating a continuous wave (CW) optical carrier with a duobinary encoded differential driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, said M-Z interferometer further comprising:
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means for applying said modulating signal to a splitter side of said first travelling wave electrode and for applying said complementary modulating signal to a splitter side of said second travelling wave electrode; a first matched impedance for connection with a first end to a combiner side of said first travelling wave electrode and with a second end to ground; a second matched impedance for connection with a first end to a combiner side of said second travelling wave electrode and with a second end to ground; means for coupling said CW optical carrier signal to said input port; and means for coupling a duobinary modulated optical signal from said output port into an optical fiber.
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Specification