Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
2002-06-07
2003-07-15
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S183000
Reexamination Certificate
active
06592274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to optical communication systems and, more particularly, to transmission and reception of digital information bits encoded in duobinary, multilevel pulse-amplitude modulation optical signals which, for a given bit rate, have a narrow optical spectrum and low symbol rate, and enable the information bits to be recovered from the intensity of the received optical signal without potential error propagation.
2. Description of the Prior Art
It is well known that in optical communication systems conveying digital information, whether they transmit a single signal at a single carrier wavelength or transmit multiple signals at different carrier wavelengths (i.e., employ wavelength-division multiplexing), for a fixed bit rate per carrier wavelength, it is beneficial to design the transmitted signal to have a narrow optical spectrum and to use a long symbol interval. Throughout this patent, the term “optical spectrum” refers to the power spectral density of the transmitted optical electric field.
Furthermore, implementation of optical communication systems is simplified greatly if the transmitted signal is designed so that the transmitted information bits can be recovered at the receiver simply by extracting from the received optical signal an electrical signal proportional to the intensity of the received optical signal (i.e., the absolute square of the received optical electric field), and performing symbol-by-symbol decisions. Currently, almost all practical optical communication systems use direct detection, in which a photodetector generates a photocurrent proportional to the received optical signal intensity. It is also possible to extract an electrical signal proportional to the received optical signal intensity through other means, e.g., asynchronous homodyne or asynchronous heterodyne detection.
Single-sideband amplitude modulation is a traditional means to narrow the spectrum of a modulated signal by a factor of two, and involves modulation of a signal and its Hilbert transform onto quadrature carriers at the same carrier frequency. A few prior works have described single-sideband modulation of optical signals, but the single-sideband optical modulation schemes proposed to date are very difficult to implement in practice. Vestigial-sideband amplitude modulation is essentially an imperfect practical implementation of single-sideband amplitude modulation. Optical vestigial-sideband amplitude modulation can be implemented by first generating an amplitude-modulated optical signal and then filtering it with an optical filter having a sharp cutoff centered at the optical carrier frequency but, in practice, it is difficult to fabricate filters having sufficiently sharp cutoff and to match the optical carrier frequency and filter cutoff frequency with sufficient accuracy.
Multiple-subcarrier modulation (also called subcarrier multiplexing) represents a well-known approach to increasing the symbol interval. In this approach, the information bit stream is divided into multiple substreams at lower bit rates, and each substream is modulated onto an electrical subcarrier at a different subcarrier frequency. The modulated subcarriers are summed to form a frequency-division-multiplexed electrical signal, which is then modulated onto an optical carrier, usually by intensity modulation. While multiple-subcarrier modulation lengthens the interval of symbols transmitted on individual subcarriers, it does not necessarily reduce the total optical bandwidth of the transmitted signal. Multiple-subcarrier modulation offers poor average optical-power efficiency (e.g., compared to on-off keying, which is the same as 2-ary pulse-amplitude modulation), and this efficiency decreases further as the number of subcarriers is increased. Multiple-subcarrier modulation requires transmitters and receivers significantly more complicated than those required by baseband modulation techniques, such as on-off keying and M-ary pulse-amplitude modulation.
Modulation of information bits onto optical signals using M-ary phase-shift keying (for M≧3) or using M-ary quadrature-amplitude modulation (for M≧4) represent other well-known means to narrow the optical spectrum and lengthen the symbol interval of the transmitted signal. However, very complicated phase-sensitive detection techniques are required to recover the transmitted bits, such as synchronous homodyne or synchronous heterodyne detection.
It is well-known that M-ary pulse-amplitude modulation, in which information bits are encoded in one of M intensity levels during each symbol interval, where M≧3, represents a means to narrow the optical spectrum and lengthen the symbol interval as compared to on-off keying (which is equivalent to 2-ary pulse-amplitude modulation). It is well-known that for a given information bit rate, as M is increased, the spectrum narrows and the symbol interval increases. A key drawback of M-ary pulse-amplitude modulation is that for a given M, it does not offer the maximal spectral narrowing that can be achieved.
M-ary pulse-amplitude modulation with duobinary encoding is a well-known modulation technique that has been widely studied for a variety of communication media. For reasons to be described below, to date, only M=2 has been chosen in optical communication systems. In this technique, a sequence of M-ary pulse-amplitude modulation symbols, I
m
, where m is a time index of symbol intervals, is encoded to yield a duobinary symbol sequence B
m
=I
m
+I
m−1
, which is transmitted. Duobinary encoding narrows the spectrum of the transmitted signal, and choosing M>2 provides additional spectral narrowing and lengthens the symbol interval. A duobinary M-ary pulse-amplitude modulation signal takes on 2M−1 possible levels, including M−1 negative levels, M−1 positive levels, and zero. Optimal detection of duobinary M-ary pulse-amplitude modulation signals requires maximum-likelihood sequence detection, but at high bit rates, this is difficult to implement, so that symbol-by-symbol detection is typically performed, and the symbol sequence I
m
is precoded to avoid error propagation in the recovered information bits.
Numerous patents and research papers have documented the use of 2-ary pulse-amplitude modulation (which is equivalent to on-off keying) with duobinary encoding in optical communication systems. To our knowledge, all of these works have utilized preceding to permit symbol-by-symbol detection without error propagation. While these works have described many different techniques to implement precoding, duobinary encoding and modulation of the duobinary signal onto the optical carrier, all of these techniques result in transmission of equivalent optical signals, which take on one of three possible electric-field amplitude values, e.g., {−a, 0, a}. Using precoded, 2-ary pulse-amplitude modulation with duobinary encoding, it is possible to recover the transmitted information bits by performing symbol-by-symbol detection on a signal proportional to the received optical intensity, such as the photocurrent in a direct-detection receiver. 2-ary pulse-amplitude modulation with duobinary encoding offers essentially the same average optical-power efficiency as on-off keying. While this technique narrows the optical spectrum by about a factor of two (as compared to on-off keying), it does not provide the narrowing that would be possible for M>2, nor does it lengthen the symbol interval (as compared to on-off keying).
It is highly desirable to employ duobinary M-ary pulse-amplitude modulation, M>2, in optical communication systems, to achieve both a narrower optical spectrum and a longer symbol interval. However, with all previously known precoding techniques, it is not possible to recover the transmitted information bits using symbol-by-symbol detection on a signal proportional to the received optical intensity, such as the photocurrent in a direct-detection receiver, without potential er
Ho Keangpo
Kahn Joseph Mardell
Gildea David R.
Pascal Leslie
Singh Dalzid
StrataLight Communications, Inc.
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