Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR EQUALIZING WDM SYSTEMS
BACKGROUND OF THE INVENTION
Field of the Invention
The invention is directed to mufti-channel communication systems
and in particular to methods for equalizing WDM systems.
Background Art
High capacity optical transmission networks, such as those defined
by the SONET/SDH standards,. can use wavelength-division multiplexing
(WDM) to increase the information carrying capacity of the optical fiber.
In optical WDM systems, a plurality of optical signals, each at a different
wavelength, are transmitted over a single optical fiber. The transmitter
terminal consists of a like plurality of optical transmitters, typically
semiconductor lasers, and an optical wavelength multiplexer, which
combines all optical signals into a mufti-channel signal before it is
launched over the optical fiber. Each transmitter operates at a different
wavelengths and is modulated with a different data signal, either by
directly modulating the laser or by external optical modulation.
At the receiver terminal, an optical wavelength demultiplexer
separates the light received over the fiber according to the wavelength.
The signal transmitted on each wavelength is then detected by a respective
optical receiver.
The WDM system reach, or the distance between the transmitter
and receiver sites, is limited by the attenuation or dispersion of the signal
along the optical fiber. The reach can be increased by placing optical
amplifiers at intermediate points between the terminals. Examples of
optical amplifiers are semiconductor optical amplifiers, and rare earth
doped fiber amplifiers. Optical amplifiers simultaneously amplify all
optical signals passing through it, i.e. the mufti-channel signal, by
amplifying the optical power by a gain.
Unfortunately, optical amplifiers exhibit a wavelength-dependent
gain profile, noise profile, and saturation characteristics. Hence, each
optical signal experiences a different gain along the transmission path.
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The amplifiers also add noise to the signal, typically in the form of
amplified spontaneous emission (ASE), so that the optical signal-to-noise
ratio (OSNR) decreases at each amplifier site. The OSNR is defined as the
ratio of the signal power to the noise power in a reference optical
bandwidth.
Furthermore, the optical signals in the co-propagating channels
have different initial waveform distortions and undergo different
additional distortions during propagation along the fiber. As a result, the
signals have different power levels, OSNRs, and degrees of distortion
when they arrive at the respective receivers, if they had equal power levels
at the corresponding transmitters.
WDM networks, and particularly SONET/SDH WDM networks,
are widely spread and the custom demand for these networks is growing
fast. They provide faster bit rates, and are more flexible in terms of the
bandwidth per channel and complexity than the previous single-channel
systems. Network providers are looking for features such as user-friendly
installation, operation and maintenance, and thus, an equalization
procedure that is simple and reliable will greatly simplify the set-up and
hence reduce the maintenance costs of the communication system.
It has been shown that the OSNRs at the output of an amplified
WDM system can be equalized by adjusting the input optical power for all
channels. For example, United States Patent No. 5,225,922 (Chraplyvy et
al.), issued on July 6, 1993 to AT&T Bell Laboratories, provides for
measuring the output OSNRs directly and then iteratively adjusting the
input powers to achieve equal OSNRs.
Figure 1 shows a block diagram of a four-channel unidirectional
wavelength division multiplexed (WDM) transmission link deployed
between terminals 11 and 17, using OSNR equalization according to the
above identified patent.
There are four unidirectional channels ~,(1)-~,(4) illustrated on
Figure 1, carrying traffic in the direction West -to-East. A short discussion
of this method follows for a better understanding of the present
invention. Terminal 11 comprises transmitters Tl to T4 and terminal 17
comprises receivers Rl to R4, connected over optical amplifiers 10, 20, 30,
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40 and 50 and fiber spans 10', 20', 30' and 40'. The optical amplifiers are
arranged at a suitable distance from each other, typically 100 km, to
compensate for the attenuation of the signal with the distance. An optical
amplifier amplifies all four signals, as it is well known.
The lasers of the transmitters Tl to T4 are modulated with signals
D1 to D4, respectively, to produce optical signals S1 to S4. A multiplexer 13
at the site of terminal 11, combines optical signals Sl- S4 into a multi-
channel signal S, which is amplified in post-amplifier 10 before being
launched over the transmission link. At reception, the mufti-channel
signal is amplified by pre-amplifier 50 and separated thereafter into signals
S'1- S'4 with demultiplexer 15. Each receiver at terminal 17 converts the
respective optical signal into an output electrical signal D'1 - D'4,
corresponding to input signals Dl to D4.
The US Patent No. 5,225,922 teaches establishing a telemetry link
between two terminals 11 and 17 of a transmission network, for providing
the measurements obtained at one terminal to the other. The patent
indicates that the telemetry link may be provided with a control unit 5 (a
microprocessor) that receives the measured input powers of signals S1 to
S4 and the total output power or OSNR of mufti-channel signal S, and
adjusts the input power accordingly. This method also takes into account
the known relative values of the gain for each channel. However, the
method disclosed in the above patent has three disad~-antages: (1) it
equalizes OSNR, which is only one parameter of several that affect the
performance of an optical transmission system, (2) measuring the OSNR
requires additional equipment, such as an optical spectrum analyzer,
outside of the SONET/SDH standards, and (3) it cannot be used to equalize
systems where channels with different wavelengths carry traffic with
different bit rates, since in such cases each channel has different OSNR
requirements.
Figure 1B shows the optical spectrum of a 4-channel WDM system,
showing how the power of the channels varies with the wavelength.
As indicated above, in a typical WDM system the co-propagating
channels do not have the same performance in terms of bit error rate
(BER), because of different component losses, different transmitter and
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receiver characteristics, different path distortions, and also because the
gain and noise of optical amplifiers in the system are channel-dependent.
The BER is the ratio between the number of erroneous bits counted at a
site of interest over the total number of bits received.
There is a need for providing a method for equalizing WDM
systems that is more accurate and easier to implement than the current
methods.
SUMMARY OF THE INVENTION
It is a primary object of the invention to provide a method for
equalizing the BER performance of a WDM system, that alleviates totally
or in part the drawbacks of the prior art methods.
It is another object of this invention to provide a method for
equalizing a multi-channel communication system based on monitoring
the BER values for all co-propagating channels.
It is still another object of the invention to pro~~ide a method for
equalizing a WDM system that corrects the performance differences of the
network elements without special instrumentation nor physical access to
remote terminal sites.
Still another object of the invention is to determine the margins to
the failure point of all channels, regardless of their bit rates, ~~hich is an
important parameter for the customer when deploying the network.
Accordingly, there is provided a method for equalizing the
performance of (J) transmission channels of a WDM link connecting a first
terminal and a second terminal, comprising the steps of, (a) identifying an
error threshold level E(j)pail of an error count indicator E(j) for a signal
S(j), the E(j) characterizing the distortion of the signal S(j) between the
first
and the second terminal, (b) determining an attenuation A(j) of a
parameter of interest P(j) of the signal S(j) between the first and the second
terminal, (c) repeating steps (a) to (b) for all the J channels of the WDM
system, (d) at the first terminal, adjusting the parameter P(j) according to
all the attenuations A(j), and (e) repeating step (d) for all the signals S(j)
for
obtaining substantially equal values of the parameter for all the J signals at
the second terminal.
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Further, there is provided a method for equalizing a plurality of (J)
signals S(j), travelling on a WDM link between a first terminal and a
second terminal comprising the steps of, for each channel ~,(j) of the WDM
link, measuring a distance to failure A(j) for a parameter P(j) of a signal
5 S(j) travelling on the channel ~,(j), and adjusting the parameter P(j) at
the
first terminal for obtaining equal distances to failure for all J channels.
Advantageously, equalizing the BER value for all channels is
preferable to equalization of any other parameter such as OSNR, in that
the BER value accounts for all factors that affect the signal in both its
electrical and optical states. The BER is the ratio between the number of
erroneous bits counted at a site of interest over the total number of bits
received, giving a measure of all errors introduced into a signal along an
entire transmitter-receiver link.
Furthermore, some systems (for example SONET/SDH) are
specified in terms of the BER and therefore the BER value is available at
reception.
The method according to the invention performs field equalization
to optimize a system in the field, and therefore a higher system margin is
used than for equalization based on the average system parameters.
In addition, the method of the present invention does not require
necessarily simultaneous access to both the transmitter and receiver ends,
but requires physical access to both, one or none of the terminal sites. The
method could be automated by a software interfacing between the
terminals. Requiring simultaneous access to both terminal sites is
disadvantageous, because it requires at least two persons communicating
over long distances.
As indicated above, OSNR alone does not accurately characterize
the system performance. The degradation of a signal is, on the other hand,
fully expressed by the BER (bit error rate), which by definition accounts for
all above listed signal degradation factors.
Furthermore, SONET/SDH systems are typically guaranteed in
terms of a minimum BER requirement at the system end of life (EOL) and
as such the BER measurement is generally available at any receiver site.
To guarantee the EOL performance, there must be enough margin built in
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the system at the start of life (SOL), since many of the system parameters
change in time or with environmental changes.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular description
of the preferred embodiments, as illustrated in the appended drawings,
where:
Figure 1A is a block diagram showing a four-channel amplified
WDM system;
Figure 1B shows an example of the optical spectrum of the WDM
system;
Figure 2A is a block diagram of an optical receiver provided with
performance monitoring;
Figure 2B is a block diagram of an optical transmitter;
Figure 3 is a block diagram of an amplified two-channel system for
equalization of the received power according to the invention;
Figure 4 is a flow-chart showing the method of the invention for
equalizing the system of Figure 3;
Figure 5 is a flow-chart showing another method for equalizing a
mufti-channel WDM system;
Figure 6 is a block diagram of an amplified two-channel system for
determining the various margins according to the invention;
Figure 7 shows a WDM system with multiple terminal sites, where
equalization of the channels is performed automatically;
Figures 8A, 8B and 8C are flow-charts showing methods for
equalizing a mufti-channel WDM system with multiple terminal sites.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a method for equalization of a
transmission link in terms of BER, which is a more accurate and easier
solution than what prior art provides. Also, this invention provides for
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measurement of an optimum margin to failure for all the channels at the
system SOL.
The invention is applicable to networks equipped with performance
monitoring capabilities. More precisely, the method of the invention is
applicable to modern WDM transmission systems, which are in general
provided with means for measuring the system BER at various network
elements of interest.
The invention is described next using examples of SONET/SDH
WDM transmission systems, but it is to be understood that it may also be
applied to other technologies. The BER of a SONET/SDH system can be
measured using the standard feature of this systems called the
performance monitor (PM). In the following, a block diagram of a typical
receiver and a typical transmitter for the transmission systems to which
this invention pertains are provided for a better understanding of the
invention.
A block diagram of a typical receiver is shown in Figure 2A. An
optical receiver generally comprises an optical-to-electrical converter 2,
that could be an avalanche photodiode (APD), or a high performance PIN
photodiode. A data regenerator and clock recovery block 3 extracts the
information from the converted signal, based on a threshold level VTh.
The threshold is selected such as to provide the best error rate for a
predetermined signal power level. For example, the levels over VTh are
interpreted by the receiver as logic "1"s, while those under, as logic "0"s.
The errors in regenerated data Dl', namely the BER value, are counted
using an error detector 4.
It is known to generate a control code at the transmission site which
is then transmitted with the information along the communication link.
Error detection is based in general on comparison between the transmitted
and the received control code.
For example, the error detection in SONET/SDH determines the
BER of the respective signal based on the information in the B1 and B2
fields of the transport overhead of the SONET/SDH frame, as well as field
B3 of the path overhead for the respective section, line and path.
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The threshold level VTh applied to data regenerator 3 may be
adjusted with a controller 6, so as to obtain BER values under a
provisioned value of the respective system. The error count and control
data are input to a performance monitor 7, connected to some or all
remote elements of the network over a bus 60.
As the requirement for essentially error free operation for fiber
systems became more stringent, sophisticated performance monitors are
provided at the receiver site, which perform optimization routines for
lowering the BER of the recovered signal.
Current transmitters are also able to monitor performance, as
shown in the block diagram of Figure 2B. A transmitter, e.g. Tl comprises
a CW (continuous wave) laser 12, which is fed into the external
modulator 8 and is modulated with e.g. a signal Dl, to provide the optical
signal S1. Before transmission, this signal is amplified in an EDFA
(erbium doped fiber amplifier) 32, which is provided with a pump laser 42
of a controlled power and wavelength. The input, output and backplane
po~~er of laser 42 are measured by diverting a fraction (approximately 3%)
of the optical signal with taps 31, 33, and 34, respectively, and converting
the respective fraction into a corresponding electrical signal with O/E
converters 35, 36 and 37. The measurements are processed by a
transmission controller 9, which in turn adjusts the pump laser 42 and the
external modulator 8, and communicates with the performance monitor 7
and other remote elements of the network over bus 60.
In this way, the SONET/SDH performance monitor allows the user
to remotely measure the channel performance anywhere in the network.
The equalization of the channels according to the invention is based
on equalizing the BER values measured in a point of interest of the
transmission link, so that it accounts for all factors affecting the system
performance. Since bit errors occur at random times, a minimum number
of errors, e.g. 10, is required to estimate the BER for a respective
transmission channel within an acceptable confidence level. For example,
if the testing interval is chosen to be 1 minute and the bit rate is 2.5 Gb/s,
then 15 errors/ minute corresponds to a BER of 10n~. In general, the
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network owner prepares BER-Power curves for all channels at installation,
so that these curves are also available for equalization purposes.
To accelerate the procedure, equalization is preferably done at a high
BER. Since it is difficult to adjust the system parameters to obtain an exact
value for the BER, the failure rate is defined within an order of magnitude
for the BER. For example, the point of failure for a channel is defined
herein when between 2 and 150 errors are counted in a minute, which
corresponds to a BER range from 1.33 x 10n1 to 10-9 for a 2.5Gb/s system. It
is to be noted that the definition of failure point for the equalization
procedure is different from the guaranteed minimum BER requirement of
the system in operation.
The method according to the invention is next described for the
two-channel system of Figure 3 with regard to the flowchart of Figure 4.
Figure 3 illustrates a two-channel unidirectional amplified
transmission link connecting transmitters Tl and T2 at terminal 11, with
receivers R1 and R2 at terminal 17; however, the invention equally applies
to bi-directional systems with more than two channels.
The link includes fiber spans 10', 20', 30' and 40' connecting optical
amplifiers 10, 20, 30, 40, and 50 for amplifying channels ~,(1) and ~,(2). We
note in the following the optical power output by transmitter Tl on
channel ~,(1) with P(1)T, the loss introduced by span 20' to signal S1
travelling on channel ~,(1) with L(1)S, the power of signal Sl at the output
of amplifier 20 with P(1)A, and the power at the input of receiver Rl with
P(1)R. BERG) is the bit error rate measured after detection of the signal at
the output of receiver Rl, while BER(2) is the bit error rate measured after
detection of the signal at the output of receiver R2. Individual notations
are used for BERG) and BER(2) for distinguishing the channels in the
process of describing the equalization method of the invention.
Same definitions apply to elements and parameters for channel
~,(2), or to any other channels that a transmission system under
consideration may have.
In this specification, the received power margin of a channel is the
amount that the received power can be reduced until the channel fails.
The transmitter power margin of a channel is the amount that the
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transmitter power of a channel can be reduced until the channel fails.
Similarly, the amplifier power margin of a channel is the amount that the
output power P(j)p of one or more of the amplifiers can be reduced until
the channel fails, and the span margin of a channel is the amount of loss
5 L(j)S that can be added to one or more of the fiber spans until the channel
fails, where (j) is the range of the channel between 1 and J, the total
number of channels of the WDM system.
In the embodiment of Figure 3, the equalization is performed
manually at the transmitter site 11, based on a the BER curves prepared at
10 installation, that predict how BER varies with the power input to the link.
Alternatively, the actual readings of the BERG) and BER(2) may be used, if
the network is provided with a communication channel between the
transmitter site 11 and the receiver site 17.
For example, the communication between the sites may take place
along a bidirectional service channel illustrated in Figure 3 by dotted line
22. A network monitoring unit may be connected over this channel for
processing the information received from terminals 11 and 17, or for any
other network elements of interest in the respective link.
Figure 4 shows a flow-chart of the method of the invention for
equalizing the system of Figure 3. First, the link is installed/
commissioned to work in the current operating point, without
equalization, as shown in step 100. We note the range of a channel with
(j) and the total number of channels with (J). In the example of Figure 3
J=2 and (j) takes the values 1 and 2.
In step 105, a BER failing point is defined for all channels,
corresponding to a BER that is denoted with BERFaiI~ As indicated above,
for example, the failing point of a channel is defined by a range of BER
values or a fixed value of at about BERgail = 10-9. The chosen failure point
depends on the bit rate and system requirements.
The transmitter-receiver pairs are identified, from the transmitter
site, namely it is identified which transmitter and receiver communicate
along a channel; Tl and Rl operate~on the frequency of channel ~,(1), and
T2 and R2 operate on the frequency of channel ~,(2). This is illustrated in
step 110.
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Then, the output power of both transmitters is set at maximum,
PMax% which is the maximum power of the weakest channel. The
maximum value PMax or PMax is measured and recorded, as shown in step
115.
With one channel at the maximum power, e.g. with transmitter T2
transmitting at PMax, the power of the first transmitter Tl is lowered until
the first channel fails, as shown in step 125 for j=1. This measurement
takes place under operating conditions of the link, and as such it accounts
for the rate of transmission and other parameters of the respective
channel. The value of the power for which the channel failed is denoted
with P(1)Fail~
An added attenuation A(1) is determined in terms of the power
level in step 130. A(1) is calculated as:
A(1)= PMax- P(1)Fail (1)
Then, in step 135, the power of Tl is increased back to PMax
This measurement is repeated for T2 in steps 125 - 145. Again, P(T2)
is reduced until BER(2)Fail is obtained at the receiver R2. An added
attenuation A(2) is determined from the measured P(2)Fail and PMax
according to the relation:
A(2) = PMax - P(2)Fail (2)
and the power of T2 is increased back to PMax in step 135. If the
system has more than two channels, i.e. j>2, steps 125 - 145 are repeated for
each channel (j) and the respective attenuation is determined and
recorded.
Attenuations A(1) and A(2) are used to determine the optimum
transmitter bias level of all channels, i.e. to select the operation point of
the transmitter, shown in steps 150 to 170 for the example of Figure 3.
A(1), in dB, is the added attenuation required to fail channel ~,(1), and
A(2),
in dB, is the added attenuation to fail channel ~,(2). The stronger channel
can now be identified, as shown in steps 150 and 160 to determine the
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required bias for the transmitters. The power of the transmitter of the
weak channel is then set to the maximum, and the power in the stronger
channel is adjusted so that the difference between the output powers of
the two transmitters equals the required power difference.
More precisely, the system should be biased as follows:
If A(1) > A(2), then the channel ~,(1) should be attenuated by:
(A(1)-A(2))/2 (in dB) (3)
and the channel ~,(2) set at full power, as shown in steps 150 and 155.
If A(1) < A(2), then the channel ~,(1) should be set at full power and
the channel ~,(2) should be attenuated by:
(A(2)-A(1))/2 (in dB), (4)
as shown at 160 and 165. For example, if channel ~,(1) requires 10
dB attenuation to fail, and channel ~,(2) requires 6 dB attenuation to fail,
then 2 dB attenuation on channel ~,(1) should be used during operation.
Step 170 accounts for the case when the two attenuations are equal.
Figure 5 shows an alternative equalization method based on BER
for a WDM system with J (J>2) optical channels propagating in the same
direction, called herein the extrapolation method, as shown for example
in Figure 7 for four channels. The required output power for a channel (j),
where j E [1, J] could be determined using the relation:
P(1)T = PMax- rl (A(j) -AMin) (5)
where the distance to failure A(j) of channel (j) is calculated as in
Eq(1) and Eq(2), namely A(j) = PMax - P(1)Fail~ where PMax is the maximum
output power of transmitter Tj, P(j)pail is the transmit output power of
channel (j) at which the BER reaches the predefined BERFaiI value. AMin
in Eq. (5) is the minimum of A(j), and r~ is a coefficient depending on the
number of channels. For example, the optimal value for this coefficient,
for a 3 or 4 ~, system is r~=0.8.
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To implement this extrapolation method, the following steps are
performed, as illustrated in Figure 5.
As in the case shown in Figure 4, the transmission link is
installed/commissioned in step 100, T~ - R~ pairs are identified in step 105.
and the BER for each channel in the failure point is identified, which
depends on the receiver. For example, different values are provided for
the receiver of a OC-192 channel than for a receiver for a OC-48 channel.
This is shown in step 110.
Thereafter, all transmitters are set to a maximum output power in
step 115.
Next, the distance to failure in terms of attenuation for all (J)
channels is determined in steps 180 - 205. When the system is provided
with an external monitoring network, such as is service channel 22 and
monitoring unit 24, the power monitors screens of receivers R~ may be
brought-up at terminal 11 site for reading the power values for all
channels. The output power is reduced for the first transmitter until this
channel fails, i.e. a BERFaiI of 10-8 - 10-9 is reached at the output of Rl.
It is
to be noted that the value of BERpail has been selected in the above range
as an example only, other targets may be used in a similar way. This is
illustrated by step 185 for channel ~,(j). After the P(j)Fail is recorded, the
output power of T~ is reset to PMax, as shown in step 190.
Monitoring unit 24 determines the distance to failure A(j) for
channel (1) according to Eq(1) in step 195. Steps 185 to 195 are repeated for
all (J) channels, as shown by 200 and 205.
When all distances to failure are available, the minimum A(j) is
determined in step 210 and denoted with AMin. Equalization of channels
is next performed by adjusting the output power of all transmitters
according to Eq. (5), as shown by step 220. Step 220 is repeated for all J
channels, as shown at 225 and 230. In steps 240 decision is made if the
channels should be further equalized (fine tuning) or not. Thus, for fine
tuning, steps 185 to 240 are repeated with the powers found in step 220, as
indicated in box 235.
Figure 6 illustrates an example of a network provided with an
external monitoring network. In the example of Figure 6, the external
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monitoring network comprises a service channel 22 and a monitoring
unit 24, but the external monitoring may be effected in any other way. It is
also to be understood that service channel 22 can be carried through the
same fiber with the user traffic, or may be a separate communication
network.
The equalization for the link between transmitter terminal 11 and
receiver terminal 17 is performed automatically, in that the BER measured
in the failing point of the channels and the powers of the transmitters are
processed by monitoring unit 24 which calculates the attenuations A(1) or
A(2) and adjusts the operating point of transmitters T1 and T2 accordingly.
The service channel 22 conveys the measured BERs to unit 24 processor
that calculates the power margins and performs the required adjustments
of the power for each transmitter.
In an optically amplified system as shown in Figure 6, the power
margin in a point of interest may be measured using attenuators. As well,
attenuators may be used for the case when the WDM system is equipped
with fixed power transmitters.
For measuring the received power margin of the entire
transmission link between terminals 11 and 17, a variable-optical-
attenuator (VOA) 21 is connected just before demultiplexer 15. The
attenuation of the VOA 21 for channel ~,(1) is increased until the BERG)
reaches the failure condition denoted with BERFaii~ The transmitter
powers are adjusted until all channels fail at the same VOA attenuation,
in which case all channels will have the same power margin.
Similarly, amplifier power margin can be measured by monitoring
the BER at the output of a channel as the power of one or more amplifiers
is reduced. The span margin can be measured by inserting a VOA into one
or more spans.
The transmitter power margin can be measured by setting all
transmit output powers at maximum, then reducing the powers, one
channel at a time until that channel fails. The measured distance to
failure, in terms of attenuation or power level, is then used to select the
biasing levels for the transmitters.
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The various margins are very useful in that they give the network
owner a good measure of how much more equipment/fiber can be added
to the link, or how the link may be reconfigured or upgraded. The
presence of the external monitoring network simplifies the adjustment, in
5 that all power values measured for the system and the corresponding
BERs may be collected in the point of interest, and also, the adjustment of
any network element may be effected remotely by the monitoring unit 24
based on the data collected from the elements of the link.
The transmitter power adjustment may be done manually or
10 automatically, using a model that predicts how the BER varies with the
input power, while incorporating other relevant system constraints. With
a control feedback loop through monitoring unit 24, the transmitter power
margins of the co-propagating channels can be equalized automatically, as
the measurements can be effected by the power monitor.
15 In case of more channels, the BER of all the channels can be
equalized by adjusting the transmitted powers of the various channels
taking into account the transmitter dynamic ranges, the system input
power dynamic range, and the measured margins to failure. The margins
for the new values of input powers can be re-measured and the input
powers can be iteratively adjusted to obtain a more accurate bias point. For
most cases, adequate equalization can be obtained by going through only
one or two iterations.
Figure 7 depicts a schematic diagram of a typical WDM transmission
link with multiple terminals. An Add/Drop multiplexer (ADM) 65 drops
signal Sl' travelling on channel ~,(1) from the mufti-channel signal S.
Receiver R1 is located at the site of ADM 65 in this example. A fifth signal,
S5, which has the same band as signal S1, is added on channel ~,1 to multi-
channel signal S, and transmitted from ADM 65 to terminal 17 where it is
detected by a receiver R5.
Equalization of a network as shown in Figure 7 or a network with
any number of channels to be dropped/added at the ADM site may be
performed using the principle illustrated in Figure 5 and disclosed in the
accompanying text.
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A first way of equalizing performance of this network is shown in
Figure 8A. After the network is installed/commissioned, as shown in step
100, all (K) channels to be dropped at the ADM 65 site, here designated
with (k), are connected through the ADM coupler so as to by-pass the
ADM. More precisely, the drop port and the add port of the coupler are
connected using a short patch-cord for each of the channels to be dropped,
so that the channels travel all the way from terminal 11 to terminal 17. In
the example of Figure 7, channel S1 dropped at ADM 65 is rather
connected to by-pass the ADM to arrive at terminal 17. This is shown in
step 300.
Steps 302 and 305 indicate that the procedure marked in Figure 5
with A, performed for equalizing the performance of J end-to-end
channels (four in this example) are now carried for signals S1-S4
transmitted by terminal 11.
Next, ADM 65 is reconnected so that the K drop channels end at that
site, as shown in step 310. In the example of Figure 7, channel S1 is
reconnected to the drop port of ADM 65.
The power of each drop channels (k) is measured in step 320, and
the channel is added back at the ADM site, with the transmit power equal
to the received power, step 325. Steps 320 and 325 are repeated for all
dropped channels, as shown at 315-335. In the present case, the power of
added signal S5 transmitted over channel ~,1 is adjusted to be equal to the
power of signal S1' measured at the ADM site.
Figure SB shows another variant of how the system of Figure 7 can
be equalized. In this variant, optical paths (a), (b) and (c) shown on this
figure are considered namely: path (a), includes M channels directly
connecting terminal 11 with terminal 17, passing through ADM 65; path
(b) includes K channels connecting terminal 11 with ADM 65, and dropped
at the ADM site; and path (c) includes K channels added at ADM 65,
bet~Teen ADM site and terminal 17. In this example, the number of
channels dropped and added as equal.
This method employs again the procedure in Figure 5 for optical
path (a) and (b) with the channels dropped/added at the ADM site. Figure
8B shows step 100, whereby the link between terminals 11 and 17 is
CA 02261509 1999-02-11
17
installed/commissioned, followed by equalization of all M direct (end-to-
end) channels on link (a), as shown in steps 400 and 405. In steps 410 and
415 channels K dropped at ADM 65 are equalized using the same
procedure as in Figure 5. Then, the power of each of the K received
(dropped) channels is measured at the ADM site, as shown in step 425.
Finally, at the ADM site, the transmit power of each added channel
P(k)a is set equal to the power of the corresponding dropped channel P(k)d,
shown in step 425.
Another method of equalization of performance for the link of
Figure 7 is disclosed next in connection with Figure 8C. The procedure in
Figure 5 is applied to optical paths (a), for all M direct channels between
terminals 11 and 17, in steps 500 - 505, then to path (b) including all K
dropped channels, in steps 510-515 and then to path (c) including all K
added channels, in steps 520-525, by considering the channels added back to
the link through the ADM coupler as extra channels when using Eq(5).
To equalize a WDM network with multiple transmitter and
receiver terminals, external monitoring network iterates the steps above
until the powers of all channels converge to within predetermined
guidelines. Since one or more channels may be operating error-free before
equalization, the performance of these channels is degraded during
equalization to measure the received power margin, transmitted power
margin, amplifier power margin, or span margin.
The method may be used for network with channels of different bit
rates. Monitoring unit 24 equalizes the BER at the receivers by adjusting
the transmitter powers, either by reducing the power of the strongest
transmitter, or by increasing the power of the weaker transmitters.
It is recommendable to carry out the equalization procedure when a
ne~~ mufti-channel system is initially set up or upgraded to more
channels. The method ensures that channels within each transmission
band are matched in terms of BER performance. This procedure is
particularly suited for transmitters having adjustable output power, but
can be also used for fixed power transmitters, by using VOAs to adjust the
transmitter powers.
CA 02261509 1999-02-11
18
For a multi-channel system, it is possible that one or more channels
operate before equalization at a BER higher than the specified BERpail~
This happens because the ripple and gain tilt introduced by optical
amplifiers in the path of the optical signal vary with the wavelength.
There are two ways to approach this problem. A first solution is to
redefine the failure point and continue with the method described above.
As indicated above, failure point is a user pre-defined point, so that a
higher BER value may be selected for defining the failing point, to operate
all channels above it. Another solution is to attenuate the transmitter
output power of all other channels, which are initially running at a BER
lower than the failure point. In this way, the performance of the failed
channels) is brought back in the vicinity of the failure point. The output
power of these transmitters is then used as the initial setting.
Another way to define failure of a channel is to use a loss-of signal
(LOS) or signal-degradation (SD) alarms at the receiver. This alternative
method has the advantage that the LOS or SD alarm are raised
automatically and immediately, so it gives a faster measure of the channel
failure point. On the other hand, this second alternative has some
drawbacks, such as: (1) the equalization would be performed with the
channels functioning away from the operating point so that the results are
less accurate, (2) it requires a larger transmitter power dynamic range to
reach the failure point, and (3) the LOS or SD alarm are not as accurate a
measure of system performance as is the BER.
While the invention has been described with reference to particular
example embodiments, further modifications and improvements which
will occur to those skilled in the art, may be made within the purview of
the appended claims, without departing from the scope of the invention
in its broader aspect.
