U.S. patent application number 10/766979 was filed with the patent office on 2005-04-21 for variable-rate communication system with optimal filtering.
Invention is credited to Boroson, Don M., Caplan, David O., Stevens, Mark L..
Application Number | 20050084270 10/766979 |
Document ID | / |
Family ID | 31190518 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050084270 |
Kind Code |
A1 |
Caplan, David O. ; et
al. |
April 21, 2005 |
Variable-rate communication system with optimal filtering
Abstract
A variable-bit-rate communication system is described. The
communication system includes a variable-bit-rate transmitter that
generates digital data at a first or a second bit rate and a
variable-bit-rate receiver that receives the digital data. The
digital data comprises a sequence of signaling waveforms having a
first or a second duty cycle, respectively, wherein each signaling
waveform has the same shape.
Inventors: |
Caplan, David O.; (Concord,
MA) ; Stevens, Mark L.; (Pepperell, MA) ;
Boroson, Don M.; (Needham, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
31190518 |
Appl. No.: |
10/766979 |
Filed: |
January 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10766979 |
Jan 29, 2004 |
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09261628 |
Mar 3, 1999 |
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6694104 |
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60076732 |
Mar 4, 1998 |
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Current U.S.
Class: |
398/141 |
Current CPC
Class: |
H04L 1/0002 20130101;
H04B 10/505 20130101 |
Class at
Publication: |
398/141 |
International
Class: |
H04B 010/12 |
Goverment Interests
[0001] This invention was made with government support under
Contract Number F19628-95-C-0005 awarded by the U.S. Air Force. The
government has certain rights in the invention.
Claims
1-30. (canceled)
31. An optical filter that generates double pass Fabry-Perot
transmission characteristics, the filter comprising: a) a
single-polarization fiber isolator that generates a polarized
optical signal beam having a single polarization; b) a polarization
beam splitter in optical communication with the polarization fiber
isolator, the polarization beam splitter passing the polarized
optical signal beam through one port; c) a Fabry-Perot filter in
optical communication with the polarized optical signal beam; and
d) a Faraday mirror in optical communication with the Fabry-Perot
filter, the Faraday mirror modifying the polarization of the signal
beam and reflecting the signal beam back to the Fabry-Perot filter
and the polarization beam splitter, wherein the beam splitter
directs the signal beam having the modified polarization through a
second port, thereby generating the double pass Fabry-Perot
transfer function.
32. (canceled)
Description
FIELD OF THE INVENTION
[0002] The invention relates generally to the field of optical
communications. In particular, the invention relates to a
variable-bit-rate optical communication system and to methods of
operating a variable-bit rate optical communication system with
nearly optimum filtering.
BACKGROUND OF THE INVENTION
[0003] Conventional communication systems are typically peak power
limited or based on peak power limited designs. In operation, these
systems maximize transmitted power by modifying the pulse width for
each particular bit rate while maintaining a constant duty
cycle.
[0004] FIG. 1 illustrates a prior art method of increasing the
energy-per-bit in a peak power limited multi-rate communication
system. In this prior art method, the pulse width changes in
proportion to the repetition rate so that the duty cycle remains at
a constant value. The maximum peak output power is limited
regardless of the average power transmitted. Consequently, it is
advantageous to use modulation formats that maximize the power
on-to-off duty cycle. In the method of FIG. 1, the duty cycle is
50%, which is typical for on-off key (OOK) communication systems.
The energy-per-bit is increased by increasing the period from 0.1
to 1.0.
[0005] It is difficult to achieve optimum performance with peak
power limited multi-rate communication systems. In order to achieve
optimum or matched performance in these prior art peak power
limited multi-rate communication systems, the receivers of these
systems must have a sinc( ) transfer function (i.e. the Fourier
transform of the transmitted rectangular pulse). Presently, optical
sinc( ) filters are not commercially available. Furthermore, the
receivers must have a different receiver filter for each bit rate
to allow the receiver to remain matched to each specific
transmitted pulse shape. Using different filters for each bit rate
would greatly increase the cost and complexity of the multi-rate
communication system.
SUMMARY OF THE INVENTION
[0006] It is therefore an object of this invention to achieve
optimum or matched performance in a variable-rate communication
system. It is another object of this invention to provide a
variable-rate communication system having bandwidth-on-demand or
fall-back modes for communications over a noisy or uncertain
channel. It is another object of this invention to provide an
optical filter that generates a double pass Fabry-Perot transfer
function.
[0007] It is another object of this invention to provide a method
of reducing a bit error rate of a digital communication system
operating in a noisy channel. It is another object of this
invention to provide a method of reducing intersymbol interference
in a communication system.
[0008] It is another object of this invention to provide a method
of optimizing variable-bit-rate communications. It is another
object of this invention to provide a method of performing optimal
communications at one or multiple data rates using spectral or
symmetric filtering. It is another object of this invention to
provide a method of optimizing a modulator extinction ratio in a
communication system.
[0009] The present invention applies to all types of communication
systems including RF, microwave, and optical systems. The present
invention applies to communication systems operating at one or
multiple data rates.
[0010] A discovery of the present invention is that multi-rate
communications can be efficiently achieved in a communication
system that employs average power limited amplifiers, such as an
Erbium-Doped Fiber Amplifier (EDFA), operating in saturation.
[0011] Another discovery of the present invention is that near
optimum (matched) filtering can be achieved in a communication
system employing an Average Power Limited (APL) amplifier operating
in saturation and employing spectral or symmetric filtering. In
such systems, the pulse shape of the communication signal can be
adjusted prior to the APL amplifier so that the transmitted signal
is matched to the receiver thereby maximizing the received
energy-per-bit, without sacrificing transmitted power.
[0012] Another discovery of the present invention is that a
variable rate communication system employing an average power
limited amplifier operating in saturation and employing PPM
signaling can provide near quantum limited performance.
[0013] In one embodiment, the present invention features a
variable-bit-rate communication system that includes a
variable-bit-rate transmitter which generates digital data having a
bit rate. The digital data comprises a sequence of signaling
waveforms having a duty cycle, where each signaling waveform has
the same shape. The sequence of signaling waveforms is transmitted
across a channel. The channel may be any communication channel such
as free space or a fiber channel.
[0014] In another embodiment, the present invention features a
variable-bit-rate communication system that includes a
variable-bit-rate transmitter which generates digital data having
at least a first and a second bit rate. The digital data comprises
a sequence of signaling waveforms having at least a first and a
second duty cycle, respectively, where each signaling waveform has
the same shape. The amplified sequence of signaling waveforms is
transmitted across a channel. The channel may be any communication
channel such as free space or a fiber channel.
[0015] In one embodiment, the transmitter is substantially average
power limited. The transmitter includes an optical average power
limited amplifier that in one embodiment is an Erbium-doped fiber
amplifier. The optical average power limited amplifier is operated
in saturation so that each signaling waveform has a maximum power
that is determined by an average power limit of the amplifier and
the duty cycle of the waveform. The amplitude of each signaling
waveform is inversely proportional to its duty cycle.
[0016] A variable-bit-rate receiver receives the digital data
generated by the transmitter and transmitted across the channel.
The receiver typically includes an optical pre-amplifier that
amplifies the digital data. The receiver also includes a receiver
filter that in various embodiments may be a Fabry-Perot,
interference, Bragg grating, or a multi-pass optical filter such as
a multi-pass Fabry-Perot optical filter. In one embodiment, the
receiver filter has a transfer function that is substantially equal
to the net transmitter transfer function (or the conjugate match of
the net transmitter transfer function) so as to spectrally and
temporally match the transmitter and the receiver. A detector
detects digital data received by the receiver.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] This invention is described with particularity in the
appended claims. The above and further advantages of this invention
may be better understood by referring to the following description
taken in conjunction with the accompanying drawings, in which:
[0018] FIG. 1 illustrates a prior art method of increasing the
energy-per-bit in a peak power limited multi-rate communication
system.
[0019] FIG. 2 illustrates a functional block diagram of one
embodiment of a communication system that includes spectral
filtering according to the present invention
[0020] FIG. 3 illustrates the relationship between peak power and
duty cycle for an average power limited amplifier.
[0021] FIG. 4 illustrates the Duty Cycle Enhanced (DCE) or Reduced
Duty Cycle (RDC) method of operating the communication system of
FIG. 2.
[0022] FIG. 5 illustrates an embodiment of a double-pass
Fabry-Perot filter that can be employed in the communication system
of FIG. 2.
[0023] FIG. 6 illustrates Pulse Position Modulation (PPM) signaling
waveforms at multiple data rates.
[0024] FIG. 7 illustrates an embodiment of a functional block
diagram of a receiver for implementing the DCE/PPM communication
method according to the present invention.
[0025] FIG. 8 illustrates a functional block diagram of a receiver
for implementing the DCE/PPM communication method and for
performing bit-timing recovery according to the present
invention.
[0026] FIG. 9 illustrates bit timing and recovery at the
fundamental data rate using the receiver of FIG. 8.
[0027] FIG. 10 illustrates bit timing and recovery below the
fundamental data rate using the receiver of FIG. 8.
[0028] FIG. 11 illustrates a functional block diagram of the clock
recovery circuit of FIG. 8 for recovering bit timing at and below
the fundamental data rate.
[0029] FIG. 12 illustrates experimental Bit Error Rate (BER)
measurements as a function of photons per bit for a multiple bit
rate optical communication system using the DCE method of the
present invention with PPM.
[0030] FIG. 13 illustrates experimental BER measurements as a
function of channel loss for a multiple bit rate optical
communication system using the DCE method of the present
invention.
[0031] FIG. 14 illustrates experimental BER measurements as a
function of receiver optical input power for a multiple bit rate
optical communication system using the DCE/PPM method of the
present invention.
[0032] FIG. 15 illustrates peak output power for an Average Power
Limited (APL) transmitter as a function of Duty Cycle (DC) and
Extinction Ratio (ER).
DETAILED DESCRIPTION
[0033] FIG. 2 illustrates a functional block diagram of one
embodiment of a communication system 10 that includes spectral
filtering according to the present invention. The communication
system 10 includes a transmitter 12, a channel 14, and a receiver
16. The transmitter 12 includes an optical source 18, such as a CW
or short pulse laser. A modulator 20 modulates the beam generated
by the optical source 18.
[0034] In one embodiment, the modulator 20 is a wide-band modulator
that generates a digital data signal comprising a sequence of
signaling waveforms having a duty cycle and a bit rate. In another
embodiment, the modulator 20 generates a digital data signal
comprising a sequence of signaling waveforms having at least a
first and a second duty cycle and at least a first and a second bit
rate, respectively. Typically, each signaling waveform has a
predetermined shape. In one embodiment, the sequence of signaling
waveforms comprise m-ary pulse position modulation data.
[0035] A transmitter filter 22 processes the modulated beam. The
transmitter filter 22 is typically a Fabry-Perot, interference,
Bragg grating, or a multi-pass optical filter such as a multi-pass
Fabry-Perot optical filter. In one embodiment, the transmitter
filter 22 is chosen to spectrally and temporally match the
transmitter 12 to the receiver 16. There are numerous methods known
in the art of matching the transmitter to the receiver. For
example, the transfer function of the transmitter filter 22 may be
chosen to be approximately equal to the net transfer function of
the receiver 16, in the case where the modulator 20 bandwidth and
channel 14 bandwidth are much greater than the bandwidth of the
transmitter filter 22. In another embodiment, the net bandwidth of
the modulator 20, transmitter filter 22 and the channel 14 together
is chosen to be approximately equal to the net bandwidth of the
receiver 16.
[0036] A transmitter amplifier 24 amplifies the filtered modulated
beam. The transmitter amplifier 24 is an average power limited
amplifier that operates in saturation. FIG. 3 illustrates the
relationship between peak power and duty cycle for an average power
limited amplifier. As illustrated in FIG. 3, as the duty cycle
decreases, the peak power increases so that the average power is
maintained at a constant value (which is unity in FIG. 3).
[0037] In one embodiment, the transmitter amplifier 24 comprises an
Erbium-doped fiber amplifier (EDFA). The optical source is chosen
to generate sufficient optical average power so that the input
power to the transmitter amplifier 24 is greater than the
saturation input power. For example, if the transmitter amplifier
24 is an EDFA, the required average power generated by the optical
source 18 is approximately a few tens of MW.
[0038] The channel 14 carries the beam generated by the transmitter
12. The channel 14 may be any communication channel such as an
optical fiber channel, waveguide channel or free space. The
receiver 16 receives the beam transmitted through the channel 14. A
receiver amplifier 26 amplifies the received beam. The gain of the
receiver preamplifier is chosen such that the noise figure of the
preamp is not degraded by post detection electronics.
[0039] A receiver filter 28 processes the received beam and a
detector 30 detects the filtered beam. The receiver filter 28 is
typically a Fabry-Perot, interference, Bragg grating, or a
multi-pass optical filter such as a multi-pass Fabry-Perot optical
filter. In one embodiment of the invention, the transfer function
of the receiver filter 28 is chosen such that the receiver 16
spectrally and temporally matches the transmitter 12. Optimum
performance can be attained from the communication system 10 by
using a transmitter 22 and receiver filter 28 that are designed to
generate symmetric waveforms in time. Any filter that generates a
symmetrical time waveform (such as a weakly reflecting Bragg
grating) can be used. However, even non-symmetric filters, such as
the commonly used Febry-Perot can be used. Currently available
filters (such as Fabry-Perot filters) do not generate perfectly
symmetric waveforms. However, as described herein, near optimum
performance can be attained by cascading a plurality of transmitter
and receiver filters that generate non-symmetric waveforms.
Repeated filtering will result in the generation of progressively
more symmetric waveforms.
[0040] FIG. 4 illustrates the Duty Cycle Enhanced (DCE) method of
operating the communication system 10 of FIG. 2 according to the
present invention. The DCE method is also defined herein as the
Reduced Duty Cycle (RDC) method. In the DCE method, the transmitter
12 operates in an average power limited state. Also, in the DCE
method, the pulse width remains constant as the duty cycle changes.
The DCE method is characterized by the fact that as the bit rate is
reduced, the amplitude increases thereby increasing the
energy-per-bit. In the examples shown in FIG. 4, the average area
under each waveform is a constant equal to 0.5 while the period is
changed from 0.1, 0.2, 0.5 to 1.0.
[0041] There are numerous advantages of the DCE method of the
present invention. Using the DCE method, all types of pulses (i.e.
square NRZ, RZ, rounded, or triangular pulses) will be transmitted
and received with the maximum energy-per-bit for a particular bit
rate. This is because the transmitter amplifier 24 operates in
saturation and, therefore, all available power will be converted to
the amplified signals. The resulting output energy- per-bit from
the transmitter amplifier 24 and arriving at the receiver 16 is
constant regardless of the input pulse shape. Thus, assuming that
the communication system 10 has perfect extinction (i.e.
transmitted power in the off state is equal to zero), all types of
pulses will be transmitted and received with the maximum
energy-per-bit for a particular bit rate.
[0042] Another advantage of the DCE method of the present invention
is that optimum receiver sensitivity for variable bit rates can be
achieved without changing the receiver filtering. This is because
the transmitted pulse width and the average power remain constant
regardless of duty cycle. A communication system employing the DCE
method, therefore, can employ aggressive filtering in the
transmitter 12 in order to achieve optimum receiver sensitivity at
one rate, and will work equally well at other rates by simply
varying the duty cycle.
[0043] Another advantage of the DCE method of the present invention
is that it can be used to reduce bit error rate in a variable-rate
communication system. A communication system according to the
present invention can be configured for providing
bandwidth-on-demand or fall-back modes for communicating over a
noisy or uncertain channel. According to the invention, the
signal-to-noise ratio is improved by lowering the bit rate while
maintaining the same pulse shape and width (i.e., by reducing the
duty cycle). Because the system is average power limited, the
transmitted energy-per-bit increases thereby lowering the bit error
rate.
[0044] For example, a method of reducing the bit error rate of a
digital communication system according to the present invention
includes providing a variable-bit-rate average power limited
transmitter operating in saturation. The transmitter generates
digital data comprising a sequence of signaling waveforms having a
first duty cycle and a first bit rate. Each signaling waveform has
a predetermined shape and an amplitude that is inversely
proportional to its duty cycle. The modulation format may be m-ary
pulse position modulation (M-PPM) as well as other formats such as
on-off keying (OOK) and differential phase shift keying (DPSK).
[0045] If it is determined that the bit error rate of the digital
communication system exceeds a predetermined value, the transmitter
can generate digital data comprising a sequence of signaling
waveforms having a second duty cycle and a second bit rate, where
the second bit rate is lower than the first bit rate. Each
signaling waveform has a predetermined shape and an amplitude that
is inversely proportional to its duty cycle.
[0046] The digital data generated by the transmitter is transmitted
through a medium such as free space or a fiber channel to a
variable-bit-rate receiver. The receiver may be spectrally and
temporally matched to the transmitter. If the bit error rate has
exceeded the predetermined value, the receiver can receive the
digital data comprising the sequence of signaling waveforms having
the second duty cycle and the second bit rate. According to the
present invention, since the second duty cycle is less than the
first duty cycle, the transmitted energy-per-bit increases and,
consequently, the bit error rate is reduced. This method may be
repeated for a third and fourth duty cycle, or as many duty cycles
as necessary, to reduce the bit error rate to an acceptable level.
This method is particularly useful for operating in a noisy or
changing channel.
[0047] Another aspect of the present invention is that nearly
optimum variable-rate optical communications can be achieved in
practice by combining the DCE method of the present invention with
symmetric filtering (SF) that spectrally and temporally matches the
receiver 16 to the transmitter 12. In order to achieve symmetric
filtering, the transfer functions of the transmitter filter 22 and
the receiver filter 28 are chosen to be conjugate matches of each
other. Due to the properties of the APL transmitter, such matching
of the transmitter to the receiver can be achieved without
sacrificing transmitted power.
[0048] Any method of symmetric filtering known in the art may be
used in a communication system of the present invention. In one
embodiment of the invention, symmetric filtering is achieved by
using a transmitter 22 and a receiver filter 28 that are temporally
symmetric (i.e. h(t)=h(-t)) and by using a symmetric time domain
communication signal. By using the same or similar symmetric
filters in the transmitter and the receiver, nearly matched
filtering is achieved. An example of temporally symmetric filters
are weak fiber Bragg grating filters. The modulator 20 may also be
used to perform pulse shaping. In one embodiment, a strong fiber
Bragg grating can be used to generate symmetric waveforms.
[0049] A method for optimizing variable-bit-rate communications in
a digital communication system by combining the DCE method with
symmetric filtering according to the present invention includes
providing a variable-bit-rate average power limited transmitter
operating in saturation. The transmitter generates digital data
comprising a sequence of signaling waveforms at a first or a second
bit rate corresponding to a first or a second duty cycle,
respectively. Each signaling waveform has a predetermined shape and
an amplitude that is inversely proportional to its duty cycle. The
modulation format may be m-ary pulse position modulation.
[0050] The digital data is received with a variable-bit-rate
receiver. The receiver comprises a receiver filter that has a
transfer function which is substantially equal to a conjugate match
of the net transfer function of the transmitter thereby spectrally
and temporally matching the transmitter to the receiver. This
allows the receiver to receive the digital data with optimum
sensitivity.
[0051] In practice, it is difficult to achieve perfect symmetric
filtering. Near optimal performance, however, can be achieved by
using commonly available nonsymmetrical filters such as Fabry-Perot
optical filters for the transmitter 22 and the receiver filter 28.
The predicted performance for a communication system using such a
filter is approximately 3 dB below the optimum performance. This
performance is comparable to state-of-the art communication
systems. In one embodiment, electrical filters are used in addition
to the nonsymmetrical optical filters to improve the
performance.
[0052] In another embodiment, nonsymmetrical optical filters, such
as Fabry-Perot filters, are cascaded together to produce a more
symmetric filter. The net transfer function of cascaded identical
nonsymmetric filters will be a convolution of the transfer
functions of the identical filters, which is a more symmetrical,
slightly broader transfer function.
[0053] For example, a double-pass Fabry-Perot filter will result in
performance that is approximately 1 dB below optimum. A triple-pass
Fabry-Perot filter will result in performance that is approximately
0.6 dB below optimum. As additional nonsymmetrical filters are
cascaded, the time domain signal becomes increasingly symmetric
and, consequently, the departure from the optimum continues to
diminish. The performance of a communication system employing this
embodiment, compares favorably with theoretical predictions for
state-of-the art communication systems using combined optical and
electrical filtering. Even where symmetrical filtering is not used
explicitly in the transmitter, commonly transmitted waveforms, such
as return-to-zero (RZ) or non-return-to-zero (NRZ) pulses, are
symmetric. Therefore, using more symmetrical receiver filters
results in better matching and improved performance.
[0054] FIG. 5 illustrates an embodiment of a double-pass
Fabry-Perot filter 100 that can be employed in the communication
system 10 of the present invention as the transmitter filter 22 and
the receiver filter 28. The filter 100 includes an input optical
fiber 102 that accepts an optical signal beam and directs it to a
single polarization fiber isolator 104. The resulting polarized
optical signal beam is directed to a polarization beam splitter
106. The beam splitter 106 transmits the polarized optical signal
beam to a Fabry-Perot filter 108. The filtered signal beam is then
directed to a Faraday mirror 110.
[0055] The Faraday mirror 110, which may be a narrow band
reflector, changes the polarization of the signal beam and reflects
it back to the Fabry-Perot filter 108. The doubly filtered signal
beam is then directed back to the polarization beam splitter 106.
The beam splitter 106 directs the doubly filtered signal beam, with
its changed polarization, to an output 112 of the filter 100.
[0056] The resulting filtered optical signal beam has a more
symmetric waveform. Applicants predict that using such a filter in
the communication system of the present invention will result in
performance that is approximately 1 dB below optimum performance.
This type of filter is particularly well suited for use in a dense
wavelength division multiplexing (WDM) communications systems
because of its narrow bandwidth and tuning capability.
[0057] The double-pass Fabry-Perot filter 100 of FIG. 5 has
numerous advantages. One advantage is that it uses only one
Fabry-Perot filter. This guarantees that the transfer functions for
the forward and backward pass of the filter are identical. Such a
design is equivalent to cascading two identical filters. Another
advantage of the filter is that is has a narrower pass-band
compared with a single pass filter and better rejection between two
adjacent WDM channels. Such a filter is particularly useful for
polarization maintaining (PM) communication systems since the
rejected out-of-band light is reflected back through the PBS and
disposed of by the isolator, and the PBS attenuates unpolarized
noise (such as amplified spontaneous emission) by 3 dB.
[0058] There are numerous other techniques for achieving symmetric
filtering. In one embodiment of the invention, symmetric filtering
is achieved by using transmission filters that comprise
interference filters. In another embodiment, symmetric filtering is
achieved by using single-mode reflectors and circulators. The
single-mode reflectors may be dielectric mirrors or fiber Bragg
gratings (FBG's). Since these reflectors also perform spectral
shaping, they can be used to generate well defined (and potentially
very narrow) spectral characteristics, as well as symmetric
waveforms in time.
[0059] The DCE/SF method of the present invention has numerous
advantages over the prior art. The DCE/SF method allows shaping of
the pulse and/or the channel dispersion profile to simultaneously
optimize both channel transmission and detection sensitivity at
variable bit rates. Thus, the DCE/SF method is particularly well
suited for high-speed dispersion managed networks.
[0060] Moreover, the DCE/SF method simplifies the design of optical
communication systems by eliminating the need for some components
such as electrical filters. The DCE/SF method of the present
invention also provides the capability for bandwidth-on-demand and
graceful degradation in the presence of a noisy channel or
deteriorating components, extending the useful lifetime of the
communication system. Thus, the DCE/SF method of the present
invention is particularly well suited for space communications
where simplicity and robust performance are essential.
[0061] The methods of the present invention can be applied to other
communication formats, such as OOK, PSK, DPSK, and M-PPM. Many
optical communication systems use simple on-off key (OOK) signaling
because the transmitter and receiver hardware are relatively simple
and because these systems typically operate at high signal-to-noise
ratios and have small dynamic range requirements. The decision
threshold for optimum performance of OOK is a variable depending on
the received signal power, the extinction ratio (power ratio of a
logical 1 compared to a 0) of the transmitter, and the details of
the filtering in the receiver.
[0062] Some optical communication systems, such as free-space
communication systems, require receivers to operate over large
dynamic ranges. OOK signaling in these systems may require more
complicated threshold setting algorithms to maintain the optimum
receiver sensitivity. Therefore, these systems typically use a
signaling format, such as antipodal or orthogonal signaling that
inherently establishes their own decision threshold.
[0063] Phase-shift key (PSK) modulation is a common antipodal
signaling scheme, which is not often used in optical communication
systems because the phase noise inherent in typical optical sources
makes the reconstruction of a reference carrier at the receiver
difficult and costly. Frequency shift key (FSK) is a common
orthogonal signaling technique in microwave communication systems,
however it is also difficult and costly to implement in an optical
communication system.
[0064] Pulse-Position Modulation (PPM) is another orthogonal
signaling technique. PPM communication systems typically use the
same transmitter and receiver hardware as OOK systems. The
signaling waveforms for binary PPM are equivalent to Manchester
encoded OOK. PPM signaling waveforms are biphase and they provide
their own threshold by comparing energy received in one time slot
to another. PPM communication systems typically have a wider
channel bandwidth and/or a reduced data rate when compared with OOK
systems. Nevertheless, most optical communication systems have more
than enough channel bandwidth to support PPM communications.
[0065] FIG. 6 illustrates PPM signaling waveforms at multiple data
rates in a preferred embodiment of the present invention. The
general case for m-ary PPM requires that the pulse width "w" be
fixed and constant for all symbols and all data rates. This feature
is advantageous because a single receiver filter can provide
optimum sensitivity at all data rates. The general case for m-ary
PPM also requires that each symbol has a fixed delay "T.sub.n" for
all data rates. This feature is advantageous because a single set
of delay lines in the receiver can be used to demodulate all data
rates.
[0066] FIG. 6 shows a preferred embodiment for 2-ary PPM where the
delay between symbols is equal to the pulse width "w." This
configuration has numerous advantages. For example, the data rate
is maximized for a given pulse width. Also, the frequency spectrum
has a line at the clock frequency at all reduced data rates that
can be used to regenerate bit timing in the receiver as described
later in connection with FIGS. 8-10. There is no requirement that
the symbol length be an integral number of pulse widths as shown in
FIG. 6. In fact, the symbol length, and therefore the data rate,
can vary in a continuous manner from "rate 1" to any lower
rate.
[0067] Although the present invention has been illustrated in
connection with PPM signaling waveforms at multiple data rates,
numerous other modulation formats are also preferred embodiments.
For example, OOK and DPSK modulation formats are preferred
embodiments.
[0068] Signaling waveforms are illustrated for high or "one" and
low or "zero" signals at four different data rates. FIG. 6
illustrates fundamental data rate signals 126, half fundamental
data rate signals 128, quarter fundamental data rate signals 130,
and eighth fundamental data rate signals 132.
[0069] Another aspect of the present invention is that the DCE
method can be used with PPM signaling to result in a DCE/PPM
communication system that can provide near quantum limited receiver
performance at multiple data rates. A DCE/PPM communication system
according to the present invention will include a receiver that is
matched to the fundamental data rate signals 126. Such a
communication system is advantageous because the receiver will also
be matched to DCE signaling waveforms at data rates lower than the
fundamental data rates.
[0070] FIGS. 7 and 8 show two embodiments of the present invention.
In FIG. 7, the delay line is implemented optically and the balanced
detector arrangement performs a subtraction. FIG. 7 illustrates a
functional block diagram of a receiver 150 for implementing the
DCE/PPM communication method according to the present invention. A
low-noise optical amplifier 152 receives the PPM communication
signal. In one embodiment, the amplifier 152 comprises an EDFA.
[0071] A receiver filter 154 processes the amplified signal. The
filter 154 may be a fiber Fabry-Perot (FFP) filter or a symmetric
filter as described earlier. The transfer function of the receiver
filter 154 is chosen to match the fundamental PPM signal. The
transfer function of the receiver filter 154 may also be chosen to
implement the spectral filtering technique described herein.
[0072] An optical splitter 156 splits the filtered optical signal
into a first 158 and a second optical path 160. A delay element 162
delays the second path 160 by a delay that is equal to the width of
the signal pulse (one-half a bit time at the fundamental data
rate). In one embodiment, the delay element 162 comprises a
differential fiber delay that delays the second path by T1.
[0073] A high-speed photodetector 164 is optically coupled to an
end 166 of the first 158 and an end 167 of the second optical path
160. In one embodiment, a high-speed photodetector 164 terminates
the end of each of a first and a second optical fiber. The
photodetectors 164 are configured to perform direct detection of
the optical signals transmitted on the first 158 and the second
optical path 160 and to subtract the electrical signals generated
by the photodetectors 164. An electrical amplifier 168 amplifies
the resulting signal generated by the photodetectors 164. A low
pass filter 170 processes the detected signal. A clock recovery
circuit 172 recovers the received data and the clock signal.
[0074] In one embodiment, an average power limited transmitter (not
shown) transmits a PPM signaling waveform of the type shown in FIG.
6. Because the transmitter is average power limited, the power of
the transmitted signaling waveform is constant regardless of its
duty cycle. The peak power of the pulses is, therefore, inversely
proportional to the duty cycle of the signaling waveform. The
transmitted signaling waveforms are received by the receiver 150.
Due to the choice of signaling waveforms, the receiver filter 154
is matched to the signaling waveforms at the fundamental data rate
and at multiple data rates below the fundamental data rate. The
receiver 150, therefore, provides near ideal performance at
variable data rates.
[0075] The receiver 150 for implementing the DCE/PPM communication
method according to the present invention has numerous advantages.
One advantage is that a single matched receiver filter 154 can be
used for communicating at variable data rates. Another advantage of
the receiver 150 is that it can operate over a wide variety of
signal levels and/or rapidly changing signal levels. Another
advantage of the receiver 150 is that it can adapt to a variety of
channel conditions by modifying the data rate for optimum
information transfer. Another advantage of the receiver 150 is that
it does not require threshold adjustment. Another advantage of the
receiver 150 is that it does not require closed-loop control of the
optical path lengths because the detected signals are combined
non-coherently.
[0076] In yet another aspect of the present invention, a DCE/PPM
receiver can be configured to provide bit-timing recovery. FIG. 8
illustrates a functional block diagram of a receiver 200 for
implementing the DCE/PPM communication method and for performing
bit-timing recovery according to the present invention.
[0077] The front end 202 of the receiver 200 is similar to a
typical OOK receiver. The front end 202 includes the optical
amplifier 152 and the receiver filter 154 of the receiver 150 of
FIG. 7. The receiver 200, however, includes only a single detector
204 following the receiver filter 154. An amplifier 206 amplifies
the signal detected by the detector 204. A low pass filter 208
processes the detected signal.
[0078] A broadband electrical power divider 210 splits the output
of the low-pass filter 208 into a first 212 and a second electrical
path 214. A differential delay is implemented electrically with a
broadband junction hybrid performing the subtraction and addition
of the two delayed inputs. A differential delay element 216 is
positioned in the first electrical path. In one embodiment, the
delay element 216 delays the signaling waveform by a delay that is
substantially equal to a pulse duration. The first 212 and second
electrical paths 214 are recombined using a broadband hybrid
junction 218 that AC couples the signal from the first 212 and the
second electrical path 214. The hybrid junction 218 generates a sum
signal (.SIGMA.) at a sum port 220 and a difference signal
(.DELTA.) at a difference port 222.
[0079] In one embodiment, the difference signal carries the
signaling waveform and the sum signal is used to derive a phase
coherent clock signal for bit timing and regeneration. The receiver
200 includes a clock recovery circuit 224 that recovers the clock
signal from the sum signal generated by the hybrid junction 218 at
the sum port 220. The receiver 200 typically includes a latch 226
having a data input 227 electrically coupled to the difference port
222 of the hybrid junction 218. An output port 228 of the latch 226
generates the received data that is synchronized to the recovered
clock signal.
[0080] In the receiver of FIG. 8, the sum port of the junction
hybrid generates a clock signal that is phase coherent with the
information carrying waveform out of the difference port.
Extraction of the clock signal at the sum port is accomplished
using standard techniques, squaring and filtering at the highest
data rate, and filtering only at all the lower data rates. The
receiver of FIG. 8 has numerous advantages over prior art. One
advantage is-that the extracted clock signal is phased correctly to
provide symbol decisions at all data rates. Prior art clock
recovery schemes often contain electrical delays that are matched
for a particular data rate, and therefore changing data rate
requires changing the delays.
[0081] Using the signaling scheme described in FIG. 6, the
receivers described in connection with FIGS. 7 and 8 allow
multi-rate PPM operation with optimum sensitivity at all data rates
with the same fixed optical and low-pass filters and a fixed delay
T1 (the delay between symbol pulses shown in FIG. 6). This feature
of the invention is advantageous because prior art receivers
require unique filters and delays for each data rate.
[0082] FIG. 9 illustrates a diagram 300 of bit timing and recovery
at the fundamental data rate (Rate 1 in FIG. 6) using the receiver
200 of FIG. 8. At the fundamental data rate, the hybrid junction
218 generates at the sum port 220 three possible output waveforms.
If the data received by the receiver 200 comprises a string of
successive "ones" or "zeros," the hybrid junction 218 generates a
constant DC voltage, but, since the hybrid junction 218 is AC
coupled, it generates at the sum port 220 a null signal.
[0083] If the data received by the receiver 200 comprises a "one"
immediately followed by a "zero," 302 the hybrid junction 218
generates a null signal for the width of the signal pulse. But
since the hybrid junction 218 is AC coupled, it generates at the
sum port 220 a waveform that is a negative pulse 304 having a width
equal to that of the signal pulse.
[0084] If the data received by the receiver 200 comprises a "zero"
followed by a "one," 306 the hybrid junction 218 receives two
simultaneous pulses. In response, the hybrid junction 218 generates
at the sum port 220 a positive pulse 308 having a width that is
equal to the width of the signal pulse.
[0085] Therefore, at the fundamental data rate, the hybrid junction
218 generates at the sum port 220 a pulse, which may be positive or
negative, whenever there is a bit transition. Thus the pulses
generated by the hybrid junction 218 at the sum port 220 produce a
signal that identifies the symbol boundaries. The absolute value of
the sum-port waveform contains a fundamental component of the bit
clock that is correctly phased with the information carrying
waveform for optimum bit decisions.
[0086] FIG. 10 illustrates a diagram 320 of bit timing and recovery
at data rates below the fundamental data rate using the receiver
200 of FIG. 8. FIG. 10 shows bit-timing recovery at half the
highest rate (Rate 1/2 in FIG. 6) and all lower rates. The
information carrying waveform is the difference signal, and the
clock signal is carried by the sum. The recovered clock signal is
precisely aligned with the peaks of the information carrying
waveform by virtue of the signaling scheme and hardware
configuration of the present invention.
[0087] The hybrid junction 218 generates two possible output
waveforms at the sum port 220 when bit timing is below the
fundamental data rate. If the data received by the receiver 200
comprises a null signal, the hybrid junction 218 generates a null
signal at the sum port 220.
[0088] If the data received by the receiver 200 comprises either a
"one" 322 or a "zero," 324 then the hybrid junction 218 generates
at the sum port 220 a positive pulse 326 that has been stretched to
twice the length of the original signal pulse. The pulses generated
for a one and a zero are offset in time with respect to each other
by the width of the signal pulse. If the pulses generated by "ones"
and "zeros" are overlaid in time, there would be a partial overlap
of the two pulses. Between the pulses the output returns to zero.
This is illustrated in FIG. 10 by the solid and dashed lines of the
sum-port waveforms. The sum-port waveform contains a fundamental
component of the bit clock that is correctly phased with the
information carrying waveform for optimum bit decisions.
[0089] One advantage of the present invention over prior art is
that one clock recovery circuit operates at all data rates with a
fixed electrical delay. That is, a communication system according
to the present invention is self-synchronizing and provides the
proper alignment between clock and the information carrying
waveform for optimum bit decisions at all data rates.
[0090] FIG. 11 illustrates a functional block diagram of the clock
recovery circuit 224 of FIG. 8 for recovering bit timing at and
below the fundamental data rate. An input 250 of the circuit 224 is
coupled to the sum port 220 of the hybrid junction 218. A rate
sensing circuit 252 determines whether the signal produced at the
sum port 220 is at or below the fundamental data rate and directs
the signal to a fundamental data rate 254 or other data rate path
256.
[0091] If the rate sensing circuit 252 determines that the signal
produced at the sum port 220 is at the fundamental data rate, it
routes the signal to an absolute value circuit 258 that takes the
absolute value of the signal at the fundamental data rate. In one
embodiment, the absolute value circuit 258 includes a broadband
frequency doubler 260 or a full-wave rectifier 262. The circuit 258
also includes a fixed delay element 259 that delays the signal to
align the phase of the recovered clock at the fundamental data rate
with the data pulses. A narrow band-pass filter 264 filters the
resulting signal.
[0092] The absolute value circuit is not required for data rates
below the fundamental rate. Therefore, if the rate sensing circuit
252 determines that the signal produced at the sum port 220 is
below the fundamental data rate, it routes the signal directly to
the narrow-band filter 264 bypassing the absolute value circuit
258.
[0093] The narrow band-pass filter 264 in one embodiment is a
phase-locked loop. The phase locked loop places the edge of the
reconstructed clock at the midpoint of the overlap between the
stretched pulses for "ones" and "zeros." This is the same point in
time where the data waveforms generated by the hybrid junction at
the difference port is at its peak value resulting in substantially
optimum phasing between the recovered clock and the data waveforms
at all data rates at and below the fundamental rate.
[0094] FIG. 12 illustrates experimental Bit Error Rate (BER)
measurements for a multiple bit rate optical communication system
using the Duty-Cycle Enhanced (DCE) method of the present invention
with Pulse Position Modulation (PPM). FIG. 12 presents BER
measurements for 5 different bit rates. The BER measurements show a
tight cluster of BER curves at the five bit rates. These
measurements indicate state-of-the-art receiver sensitivities
(approximately 2 dB from the theoretical limit) at all bit
rates.
[0095] FIG. 13 illustrates experimental BER measurements as a
function of channel loss for a multiple-bit-rate optical
communication system using the DCE/PPM method of the present
invention. FIG. 13 illustrates that as the channel transmission
deteriorates, BER performance can be maintained by lowering the
bit-rate. For example, as illustrated in FIG. 13, a 10.sup.-9 BER
can be achieved with approximately 40.5 dB of channel attenuation
at 1.244 Gbps and also with approximately 52.5 dB of channel
attenuation at 51 Mbps. Thus a DCE/PPM communication system of the
present invention can adjust the bit rate to obtain a desired
BER.
[0096] In practice, the DCE/PPM method of the present invention is
advantageous because with a simple (single) transmitter/receiver
design, it can provide the capability to adjust the bit rate for
optimal communication for particular channel properties without
impacting receiver sensitivity. Also, in practice, the DCE/PPM
method of the present invention is advantageous because the effects
of inter-symbol interference (ISI), which is dependent on both the
channel properties and the signaling waveform, are reduced at lower
duty cycles. This is because the distance between signaling
waveforms increases as the duty cycle decreases.
[0097] FIG. 14 illustrates experimental BER measurements as a
function of average receiver optical input power for a multiple bit
rate optical communication system using the DCE/PPM method of the
present invention. FIG. 14 illustrates that the sensitivity to the
average received optical power continuously improves as bit rate is
lowered. For example, as FIG. 14 illustrates, a 10.sup.-9 BER can
be attained with approximately 10 nW of average received optical
power at 1.244 Gbps and also with approximately 500 pW ({fraction
(1/24)}.sup.th the amount) at 51 Mbps. The experimental receiver
performance illustrated in FIG. 14 at both 1.244 Gbps and 51 Mbps
is approximately 2 dB from the calculated theoretical
performance.
[0098] Thus the DCE/PPM method of the present invention can
optimize the data rate to conform to properties of the channel as
well as the power limitations of the transmitter. This is an
important feature of the present invention because optical
nonlinearities, such as stimulated Brillouin (SBS) or Raman (SRS)
scattering in fiber, are power dependent. Due to its high receiver
sensitivities, the DCE/SF and DCE/PPM methods diminish the impact
of these nonlinearities because the communication link can be
operated with less transmitted power. In addition, bit rate and
transmitted power levels can be adjusted to minimize such effects
and optimize overall performance.
[0099] The DCE/PPM method of the present invention is particularly
useful for performing flexible wavelength division multiplexed
(WDM) communications. In WDM systems, multiple signals, each at a
distinct wavelength, transmit simultaneously. The power level of
each signal must be relatively low in order to keep the total power
for all of the signals below the SBS or SRS thresholds (P.sub.th).
Using the DCE/PPM method of the present invention, the transmitted
power level and bit rate on each operating wavelength can be
adjusted so that the net throughput is maximized, as the number of
operational wavelengths varies.
[0100] FIG. 15 illustrates peak output power for an Average Power
Limited (APL) transmitter as a function of Duty Cycle (DC) and
Extinction Ratio (ER). ER is defined herein as the power off-to-on
ratio. ER measurements are typically used for characterization
purposes or as a feedback parameter in order to enhance active
control for optimizing modulator performance, which directly
impacts the communication performance.
[0101] The present invention also features a method of optimizing a
modulator extinction ratio in a communication system. For systems
with finite extinction, the peak output power from the average
power limited amplifier is maximized when the extinction ratio is
minimized. This effect is exaggerated significantly at low duty
cycles, by an amount that is approximately inversely proportional
to the duty cycle since, 1 Ppeak = Paverage D C + ER ( 1 - D C )
lim D C -> 0 Paverage ER
[0102] Average power limited transmitters often employ modulators
(such as Mach-Zehnder modulators) which require active control to
minimize the ER, an action which improves receiver sensitivity by
increasing the "one" to "zero" distance. Typically ER optimization
is accomplished by minimizing the power off transmission. However,
for low duty cycle or variable rate APL systems, a more sensitive
method of minimizing the ER is to maximize the peak power
transmitted since this value becomes increasingly sensitive to ER
as the duty cycle is minimized. These properties of an APL
transmitter can also be used to accurately measure the modulator
ER.
[0103] Thus, the present invention also features a sensitive method
of measuring extinction ratio (ER) in a communication system. By
plotting the peak APL output value as a function of DC and, as
illustrated in FIG. 15, the peak value asymptotically approaches
the ER. This is clearly illustrated in FIG. 15 and by rearranging
the equation above and solving for ER as follows: 2 ER = Paverage /
Ppeak - D C 1 - D C lim D C -> 0 Paverage / Ppeak
[0104] Equivalents
[0105] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *