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Claims  |
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What is claimed is:
1. A decoder to restore a series of binary bits coded by coding the series
of binary bits into the form of unipolar pulses of constant duration, one
of said bits occurring each period of time called a bit time, the coding
being performed by providing "n" binary patterns having at least a first
bit and a last bit each, said binary patterns matching less than all
possible sequences in said series, dividing the bit time into "n" temporal
positions, establishing a correspondence between each of said binary
patterns and temporal positions, detecting one of said binary patterns in
said series, generating one of said pulses in the temporal position
corresponding to the binary pattern detected, and after the last bit of
the binary pattern detected, and after the last bit of the binary pattern
detected, resuming said detecting step, said decoder comprising means to
detect the presence of a pulse occupying a temporal position from the "n"
temporal positions, and means to produce, once the presence of a pulse is
detected in said temporal position, the binary pattern corresponding to
this temporal position.
2. Regenerator to regenerate a signal coded by coding the series of binary
bits into the form of unipolar pulses of constant duration, one of said
bits occurring each period of time called a bit time, the coding being
performed by providing "n" binary patterns having at least a first bit and
a last bit each, said binary patterns matching less than all possible
sequences in said series, dividing the bit time into "n" temporal
positions, establishing a correspondence between each of said binary
patterns and temporal positions, detecting one of said binary patterns in
said series, generating one of said pulses in the temporal position
corresponding to the binary pattern detected, and after the last bit of
the binary pattern detected, resuming said detecting step, wherein a first
signal (API) is formed by low-pass filtering of said coded signal, said
first signal having an amplitude, a first comparison test is carried out
between said amplitude of said first signal and a first threshold (SAPI),
said first threshold being adjusted so as to minimize the error
probability, a second signal (PTI) is formed by low-pass filtering of said
coded signal, said second signal having an amplitude, a second comparison
test is carried out between said amplitude of said second signal (PTI) and
a second threshold (SPTI), said second threshold being adjusted to the
amplitude of said second signal in the absence of any pulse, a decision is
made concerning the absence or presence of a pulse in a given temporal
position of said coded signal depending on the results of said first and
second comparison tests, the presence of a pulse being decided only when
the amplitude of said first signal is higher than said first threshold and
the amplitude of the second signal is lower than said second threshold at
the time the first and second tests are carried out, the regenerator
comprising a device to conduct a pulse absence/presence test for each of
"n" temporal positions, a device to carry out a pulse presence validation
test for each of "n" temporal positions, a set of "n" logical operators to
carry out the validation operation,, a device to inhibit any validation
detection of the presence of a pulse in the two temporal positions
following the temporal position in which the validated presence of a pulse
is detected, a device to produce, once the validated presence of a pulse
is detected in a temporal position, a binary pattern corresponding to this
temporal position, a device to retrieve the clock of the transmitted
signal and a time base to produce the control signals of the pulse
absence/presence and pulse presence validation tests for each of "n"
temporal positions.
3. Regenerator according to claim 2, wherein the device to carry out a
pulse absence/presence test for each of n temporal positions includes a
low-pass filter receiving the coded signal, a comparator receiving on its
non-inverter input the filtered coded signal and on its inverter input a
threshold voltage delivered by a device and feeding the input D of n
master/slave flip-flops.
4. Regenerator according to claim 2, wherein the device to carry out a
pulse presence validation test for each of n temporal positions includes a
device receiving the coded signal and producing a suitable signal for
carrying out a test for validating the presence of a pulse and which feeds
the non-inverter input of a comparator whose inverter input is connected
to the ground, and n master-slave flip-flops whose input D is fed by the
comparator.
5. Regenerator according to claim 2 in which n=2, wherein the device to
carry out a pulse presence validation test for each of the two temporal
positions includes means receiving the coded signal and producing suitable
signals for carrying out a pulse presence validation test, these means
feeding the non-inverter input of the comparators whose inverter input is
respectively connected to the ground and means delivering threshold
voltages, two master/slave flip-flops whose input D is fed by the
comparators, a gate OR with one input being connected to the output Q of
the flip-flop and the other to the output Q of the flip-flop, a gate ET
with one input being connected to the output of the gate OR and the other
input to the output Q of the flip-flop.
6. Regenerator according to either claim 4 or 5, wherein the device to
produce a suitable signal for carrying out a pulse presence validation
test includes a low-pass filter receiving the coded signal, a differential
input amplifier receiving on its inverter input a filtered coded signal
and on its non-inverter input this same signal having traversed a delay
line and delivering a signal traversing a delay line.
7. Regenerator according to claim 2, wherein the device to retrieve the
clock of the signal transmitted includes a low-pass filter receiving the
coded signal, a comparator whose non-inverter input receives the filtered
coded signal and the inverter input a threshold voltage delivered by a
device, and feeding a flip-flop delivering one pulse for each rising
transition, a gate ET receiving on one input the signal delivered by the
flip-flop and on the other input this same signal having traversed a delay
line, and a narrow band selective filtering device connected to the output
of the gate ET and followed by a pulse shaper.
8. Regenerator according to claim 2, wherein the device to retrieve the
clock of the transmitted signal includes a low-pass filter receiving the
coded signal, a comparator whose non-inverter input receives the filtered
coded signal and the inverter input, a threshold voltage delivered by a
device and feeding a first flip-flop delivering one pulse for each rising
transition, a gate ET receiving on one input the signal delivered by the
flip-flop and on the other input this same signal having traversed a delay
line, and a second flip-flop connected to the comparator and delivering
one pulse for each rising transition and each falling transition, a narrow
band selective filtering device connected to the second flip-flop, and an
"n" divider connected to the selective filtering device and associated
with a synchronization device receiving the signal delivered by the gate
ET.
9. Regenerator according to claim 2, wherein the device to retrieve the
clock of the transmitted signal includes a low-pass filter receiving the
coded signal, a comparator whose non-inverter input receives the filtered
coded signal and the inverter input a threshold voltage delivered by a
device, a flip-flop connected to the output of the comparator and
delivering one pulse for each rising transition and each falling
transition, a narrow band selective filtering device connected to the
flip-flop, an "n" divider connected to the selective filtering device and
a synchronization device associated with the divider and receiving the
signal delivered by the comparator.
10. A method of coding a series of binary bits into the form of unipolar
pulses of constant duration, one of said bits occurring each period of
time called a bit time, the method comprising:
providing "n" binary patterns having at least a first bit and a last bit
each, said binary patterns matching less than all possible sequences in
said series;
dividing the bit time into "n" temporal positions;
establishing a correspondence between each of said binary patterns and
temporal positions;
detecting one of said binary patterns in said series;
generating one of said pulses in the temporal position corresponding to the
binary pattern detected; and
after the last bit of the binary pattern detected, resuming said detecting
step.
11. A method according to claim 10, wherein each pulse has a period twice
that separating two adjacent temporal positions.
12. Method according to claim 10, wherein the binary patterns are: "10" and
"11", or "10", "110" and "111", or "11", "100" and "101", or "10", "11"
and any number of consecutive "0's", or "10", "110", "1110" and "1111", or
"100", "110", "111" and "101", or "10", "110", "111" and any number of
consecutive "0's", or "100", "11", "101" and any number of consecutive
"0's".
13. A method to regenerate an output coded signal, wherein a first signal
(API) is formed by low-pass filtering of said coded signal, said first
signal having an amplitude, a first comparison test is carried out between
said amplitude of said first signal and a first threshold (SAPI), said
first threshold being adjusted so as to minimize the error probability, a
second signal (PTI) is formed by low-pass filtering of said coded signal,
said second signal having an amplitude, a second comparison test is
carried out between said amplitude of said second signal (PTI) and a
second threshold (SPTI), said second threshold being adjusted to the
amplitude of said second signal in the absence of any pulse, a decision is
made concerning the absence or presence of a pulse in a given temporal
position of said coded signal depending on the results of said first and
second comparison tests, the presence of a pulse being decided only when
the amplitude of said first signal is higher than said first threshold and
the amplitude of the second signal is lower than said second threshold at
the time the first and second tests are carried out.
14. A method according to claim 13, wherein said second signal is obtained
by subtracting said coded signal at a first time and said coded signal at
a second time, said first and second times being spaced apart by a period
equal to a period of a pulse of said coded signal.
15. A coder for coding a series of binary bits into the form of unipolar
pulses of constant duration, one of said bits occurring each period of
time called a bit time, the coder comprising:
means for detecting any of "n" binary patterns in said series where each
said binary pattern has at least a first bit and a second bit and match
less than all possible sequences in said series;
means for generating said pulses in any of "n" temporal positions contained
in a bit time where each temporal position corresponds to a respective
binary pattern, a pulse in the temporal position corresponding to the
binary pattern detected being generated; and
means for detecting the last bit of the binary pattern detected and causing
the means for detecting to continue in response thereto. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The object of the present invention is to provide a method to code a binary
digital signal in the form of unipolar pulses of constant duration able to
assume several positions inside a time interval equal to the period of one
bit time. The object of the invention is also to provide a coder and
decoder implementing this method. Finally, the object of the invention is
to provide a regenerator and method to regenerate a coded noised signal
according to said method. This invention can in particular be
advantageously applied in digital optical fiber transmission systems using
direct detection.
BACKGROUND OF THE INVENTION
The natural and also the simplest code for the transmission of a digital
signal is the NRZ (Non-return to zero) code, also known as pure binary.
There are known to be many other codes. Those more usually distinguished
are the codes which modify the format of the digital signal and the codes
which consist of adding additional bits, which increases the thruput but
does not modify the format of the digital signal. Amongst the first codes,
one can quote as an example the RZ code (Return to Zero), the AMI code
(Alternate Mark Inversion), the CMI code (Coded Mark Inversion) and the
Miller code. The second codes are of the nBmB type, in other words at n
bits they make m bits to correspond with m>n. They may be used to embody a
precoding prior to a coding following one of the first above mentioned
codes.
These codes are used to respond to specific needs such as the facility of
retrieving the clock, the possibility of detecting transmission errors,
and compatibility of the energy spectrum of the coded signal with the
transmission support.
As regards optical fiber digital transmission systems using direct
detection, the codes procuring the best possible sensitivity or, in other
words, the codes authorizing the lowest mean optical power at the input of
the photoreceptor for a given error rate on the regenerated binary signal,
are the NRZ and RZ codes. This is why these codes are used in the
principal transmission experiments seeking sensitivity performance. These
experiments are referred to in the article by B. L. KASPER and J. C.
CAMPBELL and entitled: "Multigigabit-per-second Avalanche Photodiode
Lightwave Receivers" published in the Journal of Lightwave Technology,
vol. LT-5, no 10, Oct. 1987, p. 1361.
SUMMARY OF THE INVENTION
The object of the present invention is to transmit a digital signal by
means of an optical fiber system using direct detection with a better
sensitivity than that provided by an NRZ or RZ code. To this effect, it is
to be noted that an NRZ or RZ signal is constituted by one pulse for each
"1". In a digital signal, all the "1's" are not isolated and single binary
patterns, as for example two consecutive "1's", often repeat. In addition,
it is possible to modify the temporal position of the pulse inside the bit
time so as to obtain information supplementary to the binary information
in the presence or absence of the pulse. Thus, it is possible to replace
two consecutive "1's" by a single pulse, the additional information then
conveyed by the pulse being marked by a modification of its temporal
position inside the bit time. The reduction thus obtained of the average
number of pulses per unit of time results in a reduction of the average
optical power transmitted and accordingly an increase of sensitivity.
This sensitivity increase is only real if the error probability relating to
the decision between two adjacent pulse temporal positions is not greater
than the error probability relating to the decision between the presence
or absence of the pulse.
The invention enables this condition to be satisfied which unmarks it from
a known coding method known as DPPM (Digital Pulse Position Modulation)
which also codes the digital signal in the form of unipolar pulses of
constant duration able to assume several temporal positions within a time
interval.
In order to more fully understand the significance of the invention, it
would be proper to refer to the functioning of DPPM modulation. There
exists a detailed study of DPPM applied to optical fiber transmissions in
the article by I. GARRETT and entitled: "Pulse-position modulation for
transmission over optical fibers with direct or heterodyne detection"
published in IEEE Transactions on communications, vol. COM-31, No 4, April
1983, pp. 518-527.
FIG. 1 annexed shows a simple example of DPPM coding in which the digital
signal has been cut into two bit cells. Thus, four cells are obtained
having different binary compositions to which four temporal pulse
positions are associated, namely P1 to P4. In addition, the pulse period
is selected as being equal to the period of the bit time. It is possible
to take a shorter pulse period, but this would require a higher pass-band
for transmission of the signal. It can be clearly seen that in these
conditions, the time interval separating two adjacent pulse temporal
positions may not exceed one quarter of the bit time.
The extraction of the information of the temporal position of a pulse for
the regeneration of a DPPM signal is illustrated in FIG. 2.
This figure shows a pulse A of width T at midheight. This pulse is not
rectangular but has a particular shape given by the transfer function of
the filter preceding the decision circuit deliberately designed to
maximize the ratio between the peak amplitude of the pulse and the noise
level. The pulse A is compared to a threshold B generally situated at
midheight of its amplitude. The figure also shows a pulse C affected by a
noise component so that its peak amplitude is slightly greater than the
threshold B. Thus, the decision concerning the presence of the pulse C may
be taken without error occuring. In a known way, the information taken
into account to determine the temporal position of a pulse, in the case of
regenerating a DPPM signal, originates from the time on which the rising
front of this pulse crosses the threshold B. The error D on this date may
thus approach a value nearing +T/2 before the noise component affecting a
pulse results in an erroneous decision concerning the pulse presence or
absence.
When the time interval separating two adjacent temporal positions is equal
to T/4, as in the example of FIG. 1, the error concerning the position of
the pulse shall be less than .+-.T/8 so to avoid resulting in the presence
of errors in the regenerated binary signal. According to the mode of
regeneration of a DPPM signal illustrated on FIG. 2, the error probability
relating to the decision between two pulse positions spaced from T/4 shall
thus be greater than the error probability relating to the decision
between any pulse presence or absence, which constitutes a drawback.
The object of the present invention is to overcome this drawback.
More precisely, the method of the invention consists of detecting, on each
bit time, the possible presence of a binary pattern from any number n of
binary patterns including at least two bits, of dividing said bit time
into n equal time intervals to which n temporal positions are associated,
of establishing a correspondence between each of n binary patterns and
each of n temporal positions, of generating, if one of the n binary
patterns is present, a pulse occupying a temporal position corresponding
to said binary pattern and of then carrying out the next detection of the
possible presence of a binary pattern from the bit following the last bit
of said binary pattern whose presence is detected.
The bit time on which detection of the possible presence of a binary
pattern corresponds to the first bit or even to the last bit of this
binary pattern.
The period of the pulse may be any period, but preferably about 2T/n.
The composition of the binary patterns may also be any. Of course, it is
advantageous to select binary patterns which minimize the average number
of pulses transmitted per unit of time. When the digital signal to be
coded may contain long series of consecutive "0's", it is preferable that
one of the patterns also only contains "0's" so that the maximum period
separating two consecutive pulses is not too long. This facilitates the
retrieval of the clock and reduces the level of low frequency components
of the energy spectrum of the coded signal.
Thus, it is possible to detect the presence of the following binary
patterns, given by way of explanation but being in no way restrictive :
"10" and "11", or "10", "110" and "111", or "11", "100" and "101" or "10",
"11" and any number of consecutive "0's", or "10", "110", "1110" and
"1111", or "100", "110", "111" and "101", or "10", "110", "111" and any
number of consecutive "0's" or even "100", "11", "101" and any number of
consecutive "0's".
The object of the invention is also to provide a coder to implement this
method. It includes a device to detect, on each bit time, the possible
presence of a binary pattern from n binary patterns, a device to produce,
once the presence of a binary pattern is detected, a pulse occupying a
temporal position corresponding to said binary pattern and a device to
then carry out the next detection from the bit following the last bit of
said binary pattern whose presence is detected.
The object of the invention is also to provide a decoder to supply a binary
digital signal from a signal coded according to the method of the
invention and including a device to detect the presence of a pulse
occupying a temporal position from n temporal positions and a device to
produce, once a pulse is detected in a temporal position, a binary pattern
corresponding to this temporal position.
Another object of the invention is to provide a method to regenerate a
noised signal coded according to the coding method defined above and which
consists of carrying out a first test on a first signal so as to take an
optimal decision between the absence and presence of a pulse and at least
one second test on at least one second signal so as to take an optimal
decision between two adjacent temporal positions.
A method for obtaining a second signal consists of taking up the slope of
the coded signal, previously filtered.
Finally, the object of the invention is to provide a regenerator
implementing the regeneration method defined above and including a device
to carry out a pulse absence/presence test for each of n temporal
positions, a device to carry out a pulse presence validation test for each
of n temporal positions, a set of n logical operators to carry out the
validation operations, a device to inhibit any validated pulse presence
detection of any pulse in the two temporal positions following the
temporal position in which the validated presence of a pulse is detected,
a device to produce, once the validated presence of a pulse is detected in
a temporal position, a binary pattern corresponding to this temporal
position, a device to retrieve the clock of the signal transmitted and a
time base to produce the control signals of the pulse absence/presence
tests and pulse presence validation tests for each of n temporal positions
.
BRIEF DESCRIPTION OF THE DRAWINGS
The characteristics and advantages of the invention shall appear more
readily from a reading of the following description, given by way of
explanation and being in no way restrictive, with reference to the
accompanying figures in which :
FIG. 1, already described, gives a known coding example according to DPPM,
FIG. 2, already described, illustrates a known method for obtaining the
information of the temporal position of a pulse,
FIG. 3 illustrates the principle of the coding according to the invention,
FIG. 4 shows the method for obtaining the temporal position information of
a pulse,
FIG. 5 represents examples of signals obtained according to the coding
method of the invention,
FIG. 6 is a functional diagram of a coder for generating a signal according
to the coding method of the invention,
FIG. 7 is a functional diagram of a decoder for restoring in binary form a
signal produced by a coder conforming to the invention,
FIG. 8 is a timing diagram illustrating the regeneration method of the
invention,
FIGS. 9a and 9b illustrate the effect of a low-pass filtering on
respectively the form of a coded signal and on the ratio between the
amplitude differences relating to this coded signal on the test dates and
the level of noise,
FIG. 10 illustrates a method to produce two additional signals to improve
the error probability relating to validating the presence of a pulse in a
temporal position when the signal to be regenerated is the signal J of
FIG. 5,
FIG. 11 represents an embodiment of a regenerator for restoring in binary
form a signal produced by a coder conforming to the invention and derived
from a transmission,
FIGS. 12a and 12b represent an embodiment of a device to carry out a pulse
presence validation test for respectively each of n temporal positions and
for each of two temporal positions when the signal to be regenerated is
the signal J of FIG. 5,
FIGS. 13a and 13b represent two embodiments of a device to produce a signal
suitable for the carrying out of a pulse presence validation test,
FIGS. 14a, 14b and 14c represent embodiments of a device to retrieve the
clock of the transmitted signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The principle of the invention may be explained with reference to FIGS. 3
and 4 to be compared with FIGS. 1 and 2, already described regarding the
prior art.
FIG. 3 shows a coding example according to the invention and using a pulse
with the same period as that on FIG. 1. In this example, the binary
patterns "10" and "11" are respectively represented by a pulse in a
temporal position P1 and a pulse in a temporal position P2. The presence
of these binary patterns is not detected with a constant periodicity as in
DPPM, but with any periodicity, in other words the presence of a binary
pattern is detected immediately it appears. When the presence of none of
these two binary patterns is detected, no pulse is transmitted. In this
way, it is possible to code any binary signal. In this coding example
according to the invention, two pulse temporal positions suffice, which
makes it possible to allocate a time interval between two adjacent
temporal positions equal to half the bit time.
One advantage of the coding illustrated by FIG. 3, when compared with the
coding illustrated by FIG. 1, resides in the fact that the time interval
between two adjacent temporal positions is twice larger. This advantage is
expressed by a lower error probability relating to the decision between
two adjacent temporal positions in identical noise/signal ratio conditions
and accordingly results in an increase of sensitivity.
Another advantage concerns the average number of pulses transmitted per
unit of time. In the example shown on FIG. 1, it shall be observed that a
pulse is systematically transmitted for two bits. On the other hand, in
the example shown on FIG. 3, sometimes one pulse is transmitted for one
"1" and sometimes one pulse for two "1's". Thus on average less than one
pulse is transmitted for two bits by, however, considering that the
digital signal to be coded is balanced, that is it contains on average as
many "1's" as "0's". This lower value of the average number of pulses
transmitted per unit of time also results in an increase of sensitivity.
The invention makes it possible to regenerate a signal coded according to
the method with an error probability, relating to the decision between two
adjacent temporal positions, smaller than that obtained according to the
method for obtaining the temporal position information illustrated on FIG.
2. To this end, the information used to determine the temporal position of
a pulse is no longer the date on which the rising front of this pulse
crosses a threshold, but the amplitude of a new signal whose two
production modes are now to be illustrated with reference to FIG. 4. This
figure shows two pulses E and F with a width T at mid-height and identical
to the pulse A, and a pulse G identical to the pulse C. The pulse E is
centered in any temporal position Pm and the pulses F and G are centered
in the temporal position Pm+1 so that Pm+1-Pm=T/2. Thus, the pulses E and
F occupy two temporal positions spaced from T/2, as shown in the example
of FIG. 3. A method to obtain information of the temporal position of the
pulses E or F consists of taking the amplitude on the dates t1 and t2 so
that t1=Pm-T/4 and t2=Pm+1+T/4. In fact, it is on the dates t1 and t2 that
the difference between the amplitudes of the pulses E and F is maximal.
Thus, when the difference between the amplitude on the date t2 and the
amplitude on the date t1 is negative, it is possible to deduce from this
that a pulse is present centered in the temporal position Pm. Similarly,
when the difference between the amplitude on the date t2 and the amplitude
on the date t1 is positive, it is possible to deduce from this that a
pulse is present centered in the temporal position Pm+1.
Another method for obtaining information of the temporal position of the
pulses E or F consists of taking the slope on the date t3 centered between
the temporal positions Pm and Pm+1. In fact, it is on the date t3 that the
difference between the slopes of the pulses E and F is maximal. Thus, when
the slope on the date t3 is negative, it is possible to deduce from this
that a pulse is present centered in the temporal position Pm. Similarly,
when the slope on the date t3 is positive, it is possible to deduce from
this that a pulse is present centered in the temporal position Pm+1.
According to either of the two above-mentioned methods for obtaining
information of the temporal Position of a pulse, the decision concerning
the temporal position of the pulse G shall not be erroneous. In fact, it
is clear that firstly the difference between the amplitudes of the pulse G
on the dates t2 and t1 is positive, and secondly that the slope of the
pulse G on the date t3 is positive.
There now follows a more detailed explanation of the coding method of the
invention with reference to the signals of FIG. 5. This figure shows at H
a binary sequence with a period T including 15 bits numbered from 1 to 15.
This sequence comprises examples of binary patterns whose presence is
advantageously detected in accordance with the invention. The signal I is
a digital signal with the format NRZ and corresponding to the sequence H.
The signals J, K and L are signals coded according to the method of the
invention and in which the pulses are able to assume two temporal
positions P1 and P2 respectively corresponding to the binary patterns "10"
and "11" (signal J), three temporal positions P1, P2 and P3 respectively
corresponding to the binary patterns "10", "110" and "111" (signal K), and
four temporal positions P1, P2, P3 and P4 respectively corresponding to
the binary patterns "10", "110", "1110" and "1111" (signal L).
Thus, the first pulse of the signal J, whose rise front is in the temporal
position P1 of the bit time No 2, corresponds to the binary pattern "10"
composed of bits No 2 and 3 and whose presence is detected on the bit time
No 2. The second pulse of the signal J, whose rise front is in the
temporal position P2 of the bit time No 4, corresponds to the binary
pattern "11" composed of bits No 4 and 5 and whose presence is detected on
the bit time No 4.
Similarly, the second pulse of the signal K, whose rise front is in the
temporal position P2 of the bit time No 4, corresponds to the binary
pattern "110" composed of bits No 4, 5 and 6 and whose presence is
detected at the bit time No 4, and the last pulse of the signal L, whose
rise front is in the temporal position P4 of the bit time No 11,
corresponds to the binary pattern "1111" composed of bits No 11, 12, 13
and 14 and whose presence is detected in the bit time No 11.
According to one characteristic of the method of the invention, the time
intervals separating two adjacent temporal positions relating to the
signals J, K and L are respectively equal to T/2, T/3 and T/4.
According to one preferred characteristic of the method of the invention,
the periods of the pulses relating to the signals J, K and L are
respectively equal to T, 2T/3 and T/2.
The signals, such as the signals J, K or L, coded according to the method
of the invention may be used advantageously in optical fiber transmission
systems using direct detection. It is also possible to use them in the
form of an amplitude modulation of an optical carrier in association with
a coherent detection (heterodyne or homodyne), but being less advantageous
as in coherent detection, the amplitude modulation is less effective in
terms of sensitivity than other modulations, such as the phase modulation
of the optical carrier.
The signal J, whose pulses have a period equal to T, require for its
transmission the same band width as the NRZ type signal I. The signals L
and L require for their transmission larger band widths inversely
proportional to the period of their respective pulses.
For a given pulse amplitude, it may be observed that the average power
relating to the signal K is much weaker than the average power relating to
the signal J. In fact, the pulses of the signal K, when compared with the
pulses of the signal J, have a shorter period and their average number per
unit of time is also less. The same observations are applicable between
the signals L and K.
The elements influencing the choice between, for example, the signals J, K
and L are thus firstly the characteristics of the transmission channel
concerning the band width and the noise/signal ratio at the output of the
receiver associated with this band width, and secondly the characteristics
of the transmission source concerning the maximum power able to be
transmitted. This maximum power may be a peak power and, in this case, the
signal J, which has the smallest ratio between the peak power and the
average power, is the most suitable, or it may even be an average power
which then favors the signal L.
A functional diagram of a coder for the production a signals according to
the coding method of the invention is represented on FIG. 6. This coder
includes a device 1 receiving the binary digital signal B to be coded, as
well as its clock signal H, for detecting on each bit time the possible
presence of a binary pattern from the binary patterns Ml to Mn, a device 2
to produce, once the presence of a binary pattern is detected, a pulse
occupying a temporal position corresponding to this binary pattern, the
succession of the pulse thus produced forming the coded signal SC, and a
device 3 to carry out the next detection, once the presence of a binary
pattern is detected, from the bit following the last bit of this binary
pattern.
The coded signal produced by the coder of FIG. 6 is transmitted, for
example, to a laser source for it to be transmitted by optical fiber. It
may be advantageous to decode the signal at the output of the decoder so
as to check it is functioning properly. As these signals are properly
calibrated and are noise-free and a clock is available, it is possible to
use a decoder for this purpose, this decoder being more simple to use than
a regenerator.
A functional diagram of such a decoder is shown on FIG. 7. It includes a
device 4 receiving the coded signal SC and the clock signal H and
detecting, once the pulse is received, its temporal position P1 to Pn, and
a device 5 to produce, once a pulse is detected in a temporal position, a
binary pattern corresponding to said temporal position, the succession of
these binary patterns forming a binary digital signal 8 identical to the
one received by the coder.
The devices 1 to 5 may be embodied with the aid of conventional logical
circuits whose implementation is well-known to experts in this field.
When the coded signal produced by the coder of FIG. 6 is derived from a
transmission, its conversion into a digital signal is effected by means of
a regenerator. One significant characteristic of such a regenerator
concerns the binary error rate obtained on the regenerated digital signal
according to the noise/signal ratio on its output. In the case of
transmission by optical fiber, this characteristic directly has an effect
on sensitivity at the input of the optical receiver and accordingly on the
range of the link. The regeneration method of the invention makes it
possible to convert a noised signal coded according to the method of the
invention into a digital signal with a minimum error probability. In order
to do this, firstly an API signal is produced to take a decision between a
pulse absence and presence with a minimum error probability, and secondly
a PTI signal to take a decision between two adjacent pulse temporal
positions with a minimum error probability.
There now follows an explanation of the regeneration method of the
invention with reference to the timing diagram of FIG. 8. This figure
shows a coded signal SCa including a pulse occupying any temporal position
Pm. The figure also shows the signals API and PTI derived from the signal
SCa whose amplitudes are respectively compared with the thresholds SAPI
and SPTI.
The position of the SAPI threshold is adjusted to minimize the error
probability relating to the test on the API signal. When the level of
noise affecting the API signal has the same value in the absence and
presence of a pulse, the SAPI threshold is situated at mid-height of a
pulse of the API signal. In the case of regenerating a coded signal
derived from an optical fiber transmission, especially when an avalanche
photodiode is used to receive the optical signal, the level of noise at
the output of the optical receiver is clearly much higher in the presence
of a pulse than in the absence of a pulse. In the presence of an optical
pulse, a popcorn noise is added to the background noise, mainly thermic,
of the photoreceptor. Also in this case, the SAPI threshold minimizing the
error probability relating to the test on the API signal is no longer
situated at mid-height of a Pulse of the API signal, but is situated in
the position bringing it close to the amplitude of the API signal where a
pulse is absent.
The position of the SPTI threshold is adjusted so as to minimize the error
probability relating to the test on the PTI signal. For obvious reasons of
symmetry, the position of the SPTI threshold, independently of the nature
of the noise affecting the coded signal, still coincides with the
amplitude of the PTI signal in the absence of any pulse.
The signals Hm-2 to Hm+2 are clock fronts indicating the dates of the tests
conducted on the API and PTI signals to detect the presence of a pulse in
the respective temporal positions Pm-2 to Pm+2. The clock front Hm
coincides with firstly the date on which the amplitude of the API signal
passes through a maximum, and secondly with the date on which the
amplitude of the PTI signal is equal to the difference between the
amplitude on the date Pm+3T/4 and the amplitude on the date Pm-T/4 of the
filtered signal Sca or, according to one variant, to the slope on the date
Pm+T/4 of the filtered signal SCa.
The decision concerning the absence or presence of a pulse in a given
temporal position thus depends on two conditions. On the date of the test
corresponding to this temporal position, it is essential that the
amplitude of the API signal is higher than the SAPI threshold and that the
amplitude of the PTI signal is lower than the SPTI threshold.
Thus, in the example of FIG. 8, this first clock front authorizing the
detection of the presence of a pulse in a temporal position is Hm. This
detection then inhibits detection in the next two temporal positions
corresponding to the clock fronts Hm+1 and Hm+2.
It may be observed that on the date of the test corresponding to the clock
front Hm-2, to be subsequently illustrated with reference to FIGS. 9a and
9b, the amplitude of the API signal is lower than the SAPI threshold
without, however, being as far from it in the absence of any pulse, which
increases the probability of an erroneous decision in favor of the "pulse
presence" state. This does not constitute a drawback as the test conducted
on the same date on the PTI signal does not validate this possibly
erroneous decision.
The clock fronts H' correspond to the clock fronts H delayed by T/4. They
may be used to carry out additional tests on the PTI signal and intended
to confirm with a smaller error probability some of the tests
corresponding to the clock fronts H. Thus, in order to decide on the
presence of a pulse in the temporal position Pm, it is essential that the
PTI signal is lower than the threshold SPTI for the two tests
corresponding to Hm and H'm. In the example of FIG. 8, it shall be
observed that only the tests corresponding to H'm-2 and H'm are
advantageous as they take place when the amplitude of the PTI signal
passes through maxima. For example, if on the date of the test
corresponding to the clock front Hm-2, the API signal is affected by a
noise making its amplitude slightly higher than the threshold SAPI and if
the PTI signal is affected by a noise making its amplitude slightly lower
than the threshold SPTI, the test corresponding to the clock front H'm-2
makes it possible to avoid there being any false detection of the presence
of a pulse in the temporal position Pm-2.
The API signal is obtained by the low-pass filtering of the coded signal.
Such a filtering makes it possible to minimize the error probability with
reference to FIGS. 9a and 9b. FIG. 9a illustrates the action of a low-pass
filter on a signal M composed of two pulses each having a period T and
centered on the dates t4 and t5 so that t5-t4=2T. This figure shows the
signal M and three other signals M1, M2 and M3 obtained by the low-pass
filtering of the signal M with the respective band widths B1, B2 and B3 so
that B1>B2>B3. The amplitude Aa represents the difference between the
amplitude on the dates t4 or t5 and the amplitude on the date t6 centered
between the dates t4 and t5 of the filtered signal M. It corresponds to
the difference between the amplitudes relating to the states "0" and "1"
in the case of regenerating an NRZ signal with a clock period of T. In
fact, in this case, the tests carried out to take the decision between the
states "0" and "1" take place periodically on dates spaced from T
including t4, t6 and t5. The amplitude Ab represents the difference
between the amplitude on the dates t4 or t5 and the amplitude in the
absence of a pulse of the filtered signal M. It corresponds to the
difference between the amplitudes relating to the "pulse presence" and
"pulse absence" states in the case of regeneration according to the
invention. In fact, the tests carried out to take the decision between the
"pulse presence" and "pulse absence" states then periodically take place
on dates spaced from T/2 including t4, t7, t6, t8 and t5, but as mentioned
earlier with reference to FIG. 8, the tests on the dates t7 and t6 are
inhibited when the presence of a pulse is detected in a temporal position
centered on the date t4 and the tests on the dates t6 and t8 are not
validated by the corresponding tests conducted on the PTI signal. It may
be mentioned that, with regard to FIG. 7a where the amplitudes AaM2 and
AbM2 relating to the signal M2 have been marked, the amplitude Ab is still
higher than or equal to the amplitude Aa.
For better visual comprehension, FIG. 9b illustrates in logarithmic scales
the evolution of these amplitudes Aa and Ab, as well as the noise power N,
according to the width of the band B of the filter. Here, the noise power
N is considered as being proportional to the band B. This figure clearly
shows that there are band widths B1 and B2 for which the respective
differences D1(dB)=Aa(dB)-N(dB) and D2(dB)=Ab(dB)-N(dB) are maximal. It
may also be observed that the band width B2 is smaller than the band width
B1 and that the difference D2 is greater than the difference D1. As a
result of this last point, for a given peak amplitude and a given period
of the pulse prior to filtering and for identical noise conditions, the
error probability concerning the decision between the "pulse absence"
state and the "pulse presence" state shall be lower than the error
probability concerning the decision between the state "0" and the state
"1" in the case of regenerating an NRZ signal.
The PTI signal is produced from a signal also derived from a low-pass
filtering of the coded signal. According to one production mode, the
difference is made between the amplitudes of the previously filtered coded
signal on two dates spaced from a period equal to the period of one pulse.
According to another pro | | |
