OPTIMIZED LOADING OF New NETWORKS ON WGS
Chris McLain
Lino Gonzalez
LinQuest Corporation
Christopher Swenarton
PM WIN-T
Fort Monmouth, NJ
and
ABSTRACT
LINK OPTIMIZATION
The Wideband Global Satellite (WGS) system represents
an order of magnitude increase in capacity over the existing Defense Support Communications Satellite (DSCS)
system. In addition to adding capability in the X-band and
aKa-band, WGS adds a unique digital channelizer. The
digital channelizer greatly increases the flexibility of the
WGS payload over previous satellite systems. It allows an
individual 125 MHz transponder to be decomposed into as
many as 48 sub-channels, each of which can be given its
own transfer gain, be cross-banded between X-band and
Ka-band, be combined with other sub-channels (fannedin), or be duplicated in multiple downlinks (fanned-out).
However, with increased flexibility comes the following
planning challenge. Given all of the possible options, how
does an operator configure the WGS satellite to most efficiently serve a given network?
The principle difference between WGS and a conventional
transponded satellite is its digital channelizer. The digital
channelizer allows a 125 MHz transponder to be decomposed into 48 sub-channels of 2.6 MHz each. Each subchannel can be assigned its own gain state so that, in principle, individual links can be optimized. This provides a
great deal of flexibility over a conventional transponded
satellite, which usually only have one, rarely adjusted gain
setting per transponder which must be shared by all of the
carriers in the transponder. With more flexibility comes
more difficulty in optimizing the link setup. The objective
of this section is to derive an algorithm for selecting optimum gain states for a specific link.
This planning challenge becomes particularly acute for the
Network Centric Waveform (NCW). NCW is an advance
mesh waveform used by programs such as WIN-T and
Prophet. The NCW can support networks of heterogeneous terminals ranging from small mobile terminals to fixed
regional hub nodes, which span multiple beams, different
bands, and many discontinuous sub-channels. NCW can
take full advantage of the flexibility of WGS but doing so
presents the operator with a dizzying number of options
when setting up a network.
This paper describes an algorithm for optimally loading
NCW links and networks on WGS.
INTRODUCTION
The purpose of this paper is to derive a method for optimized the loading ofNCW networks on WGS. This analysis has been divided into two parts: I) the optimization of
an individual link, and II) the optimization of loading a
collection of links that constitute a network.
This analysis will be restricted to the case of a network
that resides in a single transponder with a co-coverage area
uplink and downlink. Networks with multiple coverage
areas, cross-banding, and fanned-in or fanned-out bandwidth segments will be considered in a future analysis.
Before proposing a specific optimization method, we will
first examine the effect of changing the gain state on an
example link: a SNE 1 terminal communicating with another SNE. For this analysis, the transmitter is operated at
its maximum ERIP 2 , 42 dBW, and the receiver has a G/T
of 11.9 dB/K. The terminals are located in a WGS NCA
beam where the G/T is 7.7 dB/K and the saturated EIRP is
60.7 dBW. The uplink center frequency is 30.5 GHz and
the downlink center frequency is 20.5 GHz. The transfer
gain of the satellite is varied between 177.8 dB and 227.8
dB, reflecting a 50 dB range in gain state in the digital
channelizer.
C/No Ratios
The effect of changing the transfer gain on the C/N 0 ratios
of an individual link is shown in Figure 1 for the example
link. The uplink C/No is shown as a horizontal, solid blue
line, which is not affected by transfer gain. The downlink
C/No is a sloped, dashed blue line. As transfer gain increases the downlink C/No also increases. The end-to-end
C/No is shown as a solid red curve. The behavior of the
end-to-end C/No suggest three regions: 1) the downlink
limited region (less than about 197 dB) where the end-toend C/No asymptotically approaches the downlink C/No,
2) the uplink limited region (greater than about 217 dB)
where the end-to-end C/No asymptotically approaches the
1
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978-1-4244-5239-2/09/$26.00 ©2009 IEEE
10f7
PM WIN-T Soldier Network Extension (SNE) SOTM product
NeW always attempts maximum supportable burst-rate
uplink C/No, and 3) a transition between them (between
about 197 dB and 217 dB). These regions will figure
prominently in our later discussions.
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The downlink power required to achieve the data rates
above is shown in Figure 3 for the example link. The solid
blue line shows the signal EIRP, which is linear with transfer gain . There is no transition in behavior between regions , so the signal EIRP keeps increasing linearly in the
uplink limited region even though the end-to-end C/No is
no longer increasing and, more importantly, the data rate is
no longer increasing. This is an undesirable condition
since downlink EIRP is a limited resource.
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supported data rate, 32 kbps, requires a transfer gain of at
least 193 dB. Data rate increases as a step function with
transfer gain until it reaches a maximum of 768 kbps,
which is still well below the capability of the modem. After this point, no increase in transfer gain will result in an
increase in data rate.
225
55
Transfer Gain (dB)
Figure 1. SNE to SNE link C/No versus transfer gain.
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gain for the example link. The solid blue line shows the
data rate for a 4.6 dB Eb/No without quantization. The
data rate increases linearly with the transfer gain in the
downlink limited region . The linear relationship breaks
down in the transition region where the data rate increases
at a lower rate than the transfer gain, as would be expected
from the end-to-end C/No curve in Figure 1. By the onset
of the uplink limited region, there is essentially no increase
in data rate with increasing transfer gain; the data rate asymptotically approaches a maximum.
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Figure 3. SNE to SNE downlink EIRP versus transfer gain.
The situation is even worse when the total satellite down
link EIRP is considered, which is shown in Figure 3 as a
red dashed line. Total EIRP is the signal EIRP plus the
reradiated uplink noise. The difference between the signal
EIRP and total EIRP is negligible so very little noise is
being reradiated in the downlink limited region . This
changes in the transition region where the total EIRP begins increasing faster than the signal EIRP. The difference
approaches a value of lO*loglO(l /(Eb/No) + 1) in dB in
the uplink limited region. For an Eb/No of 4.6 dB the difference in Total EIRP and signal EIRP is 1.3 dB. This
means that not only is the total EIRP increasing in the uplink limited region without any increased in data rate, but
the reradiated noise increases the total EIRP by 35% above
the signal EIRP.
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Figure 2. SNE to SNE data rate versus transfer gain.
Downlink Power Efficiency
The solid red line shows the quantized data rate based on
the thresholds in MIL-STD-188-EEE. Note that the lowest
The disconnect between increasing downlink EIRP and
fixed data rate in the uplink limited region leads naturally
20f7
to the concept of downlink power efficiency. For this
analysis the power efficiency will be calculated as:
eq 1
Powerliff = SignalDataRate/l o Signa/OwpIIHackO.ffIlO
Based on this definition, it is possible to calculate the fraction of the transponder total power consumed by a signal
as its data rate divided by its power efficiency.
Figure 4 shows the downlink power efficiency of the example link as a function of transfer gain. The power efficiency is greatest at minimum transfer gain and remains
essentially constant across the downlink limited region .
The power required in this region only varies in proportion
to the data rate. The power efficiency begins to drop off in
the transition region . Data rates continue to increase in
this region but the power required increases faster and at
an accelerating rate. Power efficiency continues to fall in
the uplink limited region as downlink EIRP increases but
data rate does not. Obviously, this situation and the uplink
limited region should be avoided when optimizing a link.
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Given these goals, the optimum will likely fall in the transition region . The maximum data rate possible will occur
at transfer gains towards the uplink limited edge of the
transition region. The maximum power efficient data rate
will occur at transfer gains around the downlink limited
edge of the transition region. On occasion, the most favorable data rate may occur in the downlink limited region,
but only because NCW does not support a higher data rate.
Certainly the optimum points are not likely to ever occur
in the uplink limited region because the data rate is no
longer increasing in this region and the power efficiency is
dropping rapidly.
The difference between the downlink and the end-to-end
carrier to noise-density ratio, delta-ClNo, can be used as a
proxy for trade between power efficiency and data rate .
We can define the lower edge of the transition region as
occurring at a delta-ClNo of 0.5 dB and the upper end at a
delta-ClNo of 10 dB. The delta-ClNo corresponds directly
to the loss in power efficiency so a 0.5 dB delta-ClNo is a
~ 10% loss in power efficiency relative to the maximum
and a 10 dB delta-ClNo is a ~90% loss in power efficiency. This makes delta-ClNo a good ' knob' for setting
the optim ization goal. If the user would like the best data
rate without losing power efficiency then the permissible
delta-ClNo would be set to 0.5 dB. Alternatively, if the
user would like the best possible data rate then the permissible delta-ClNo would be set to 10 dB. A balance between the goals of power efficiency and maximum data
rate might be a delta-ClNo of 3 dB.
An example case where the delta-ClNo is equal to 3 dB is
shown in Figure 5. Note that the delta-ClNo of 3 dB,
which is marked with a red dot, occurs where the uplink
and downlink ClNo 's are equal. This corresponds to a
transfer gain of207.28 dB.
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power. The optimum gain state will be a balance between
maximum data rate and maximum power efficiency.
225
Figure 4. SNE to SNE downlink power efficiency
Selecting an Optimum Transfer Gain
After looking at the effect of changing the gain state for an
example case, we will now return to the question of how to
choose an optimum gain state and apply the insights we
have gained .
Of the possible goals for optimizing a link, two stand out:
maximum data rate, or maximum power efficiency. Of
these, maximum data rate is the most obvious. Maximizing data rate maximizes the possible throughput for a given terminal. Still, there may be cases where the operating
with the maximum data rate is undesirable. This can be
seen as the data rate approaches the uplink limited region
and the power efficiency plummets. If the transponder
loading is power constrained, then operating highest possible data rate will consume a disproportionate amount of
The corresponding data rate is 384 kbps as shown on Figure 6. An inset in Figure 6 shows the data rate versus
transfer gain in more detail around the 207.28 dB point.
There are several points of interest to note within the inset
in addition to the 3 dB delta-ClNo. First, the 3 dB deltaClNo point, marked with a red dot, is not the lowest transfer gain that could support 384 kbps. The transfer gain
could be lowered to 206 dB, which is marked with a green
dot, and still maintain the same data rate. This point
would actually be more power efficient than using the
207.28 dB and would be a better choice for transfer gain.
The only downside of this point is that the link is right on
the edge of dropping to a lower data rate. Any small negative perturbation in the link could cause it to fall back in
data rate. A compromise to avoid this would be to add a
small amount of margin to minimum transfer gain to im-
3 of?
prove the stability of the data-rate in the link. The minimum transfer gain plus a half dB of margin, 206.5 dB, is
shown as a blue dot in the inset. This is represents the best
choice for an optimum transfer gain when the permissible
delta-ClNo is 3 dB.
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Add the transfer gain margin to the minimum transfer
gain to yield the optimum transfer gain.
Step 2 could speed up by using a binary search through the
range of transfer gains to find the maximum permissible
delta-ClNo.
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1. Compute the uplink, downlink, end-to-end ClNo, and
supportable data rate at the minimum transfer gain.
3. Calculate the minimum transfer gain that will support
the optimum link data rate.
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The steps in the algorithm are:
2. Increase the transfer gain incrementally and repeat
Step 1 until the difference between the downlink
ClNo and the end-to-end ClNo equals the maximum
permissible delta-ClNo. The data rate at this point
will be the optimum link data rate.
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to the minimum transfer gain for a given data rate.
Optimum Transfer Gains fOr Various Antenna Pairs
225
Transfer Gain (dB)
Using an implementation of the proposed algorithm, we
can now compute for various antenna types.
The link
budget assumptions are the same as was described for the
example case. The EIRP and the G/T of the antenna types
analyzed are shown in Table 13 •
Figure 5. SNE to SNE link ClNo versus of transfer gain
with the 3 dB delta-ClNo point marked.
. tics
Tabl e 1 A ntenna T vpes an dCh arac t efts
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Table 2. Optimum transfer gain for various antenna types
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EIRP (dBW)
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Table 2 gives the optimum transfer gain for a delta-ClNo
of 10 dB (maximum data rate). The selected data rates for
the optimum transfer gains are shown in Table 3 and the
corresponding power efficiencies are shown in Table 4.
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Figure 6. The 3 dB delta-ClNo point and possible optimum
transfer gains
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Table 3. Data rate at the optimum transfer gain
Top Level Algorithm fOr Optimum Transfer Gain
Receiver
From the discussion in the last section we can derive an
algorithm for selecting an optimum transfer gain.
5TT+
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768 .0
3072 .0
PoP!TCN
LRH
3072.0
(kbps)
This algorithm has two inputs : 1) the maximum permissible delta-ClNo, and 2) the transfer gain margin. The maximum permissible delta-ClNo has a range of 0.5 dB to 10
dB and sets the preference between power efficiency and
maximum data rate. The transfer gain margin has a range
of 0 to 1.5 dB (approximately the spacing between the data
rate increments) this sets how close the system will operate
3
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5NE
3072.0
768.0
3072 .0
3072.0
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768 .0
3072 .0
3072 .0
LRH
3072 .0
768 .0
3072 .0
3072 .0
STT+, SNE, poP/TeN and LRH are all PM WIN-T satellite terminal s
the number of bandwidth segments also minimizes the setup and management complexity for the network.
Table 4. Power efficiency at the optimum transfer gain
Receiver
(kbps/Xpdr) STI+
STI+
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PoP/TCN
LRH
SNE
7,393,766
1,280,446
6,418,318
7,393,766
286,036
42,400
212,530
284,722
LRH
PoP/TCN
532,245
23,238,684
88,585
4,439,778
444,039
22,254,673
594,869
23,238,684
Optimization Constraints and Inputs
NETWORK OPTIMIZATION
NCW networks on WGS will consist of multiple links. If
the terminals in a network are of different types or are located at different satellite antenna gain contours, then the
links between them will have a different optimum gain
states. NCW can accommodate up to 63 bandwidth segments in a network with each bandwidth segment having a
different gain state. Networks with more than 63 unique
terminal type / satellite gain combinations will require
multiple links to share a gain state, which means that some
links will have to be operated off of their optimum gain
state. Even with networks smaller than this there will frequently be reasons to consolidate multiple links with different optimum gain states into a single bandwidth segment as a result of finite size of WGS sub-channels. WGS
sub-channels are limited to increments of 2.6 MHz. If a
particular link has a small or negligible data rate then allocating a 2.6 MHz sub-channel to that link is very bandwidth inefficient. This section will deal with the optimization of a whole network in terms of how many bandwidth
segments to create and how to optimize their gain states.
Optimization Objectives
The three most common optimization objectives are:
A key constraint when optimizing the network bandwidth
gain states is that no link be assigned to a bandwidth segment with a transfer gain lower than its optimum transfer
gain. This assumes that the link optimum transfer gains
have been chosen to meet certain link objectives (e.g. maximum data rate) and providing a lower transfer gain could
preclude those objectives from being achieved. The implication of this is that at least one bandwidth segment must
have a transfer gain equal to the highest transfer gain of all
of the links in the network. This is also the case if there is
only one bandwidth segment in the network.
The optimization is also constrained by the maximum
power and bandwidth available. These will generally be
set by the maximum transponder power and bandwidth. If
the optimization objective is the minimum bandwidth, it
must be the minimum bandwidth that can be supported
within the maximum power available.
The optimization is also constrained by the number of
bandwidth segments available. While the modem can
support up to 63 bandwidth segments, doing so for a single
network would generally be undesirable. Based on experience, most single transponder, co-coverage networks can
be optimized with one to five bandwidth segments. As a
result we include a user settable maximum number of
bandwidth segments. The default value is five segments.
Finally, we have observed that as the optimization progresses, the network power will asymptotically approach
the ideal minimum power for the network. Rather than
continue the optimization past the point of diminishing
returns, we include a user settable convergence threshold
for terminating the optimization within a certain percent of
the ideal power. The default convergence threshold is
10%.
1. Minimum Network Bandwidth - If excess satellite
power is available or the amount of bandwidth available is very limited, then the objective of the loading
may be to occupy as little satellite bandwidth as possible.
2. Minimum Network Power - Conversely, if the satellite is relatively power constrained, then the objective
of loading the network may be to minimize the power
required by the network.
Calculation ofNetwork Power
Calculating the network power and bandwidth is essential
to optimizing the network. Network power is the sum of
the bandwidth segment powers. The segment power is the
sum of the throughputs of each link assigned to the segment divided by the power efficiency of the link at the at
the bandwidth segment transfer gain:
3. Minimum Total Resources (Bandwidth and Power) More often, the objective will to minimize the limiting resource - either power or bandwidth - so that the
network requires the least overall resources. Since
power and bandwidth are tradable to some degree,
this will frequently lead to a 'balanced' loading where
the percentage of transponder power and bandwidth
occupied are similar.
n_segments
NetPower ==
LSegPower(i)
eq2
i=1
.
A major secondary goal is to mmmuze the number of
bandwidth segments; fewer and larger bandwidth segments
will cope with the variability in traffic on a given segment
better than many small bandwidth segments. Minimizing
m_links
SegPower(z)= L
)=1
LinklrataThroughputcjs
------~­
LinkPowerEff(SegGain(j))
The link power efficiency as a function of transfer gain is
obtained during the link optimization process as shown in
50f7
Figure 4.
Calculation ofNetwork Bandwidth
It can be seen from Figure 4 that the power efficiency of a
link decreases if the link is assigned to a higher transfer
gain than its optimum. It follows from this that the ideal
minimum power for a network is achieved when there is
one bandwidth segment for every unique optimum link
transfer gain in the network and every link is assigned to
its optimum transfer gain. The ability to compute the ideal
minimum power is very useful when determining the quality of the optimization and when to terminate the process.
As with network power, network bandwidth is also calculated as the sum of the bandwidth segment bandwidths.
The bandwidth segment bandwidth is also calculated as the
sum of the link throughputs divided by an efficiency term,
in this case the spectral efficiency of the link. The only
additional step is that the segment bandwidths are quantized into 2.6 MHz sub-channels:
n
eq3
NetBW = ISegBW(i)
An example calculation of the ideal minimum network
power can be done for a delta-CzNo of 10 dB using the
example power efficiency values in Table 4 and a set of
example throughputs in Table 5. The optimum link transfer gains from Table 2 are plotted against the cumulative
network power as a percent of the transponder power in
Figure 7. The total for ideal minimum power amounts to
3.9% of the transponder power in this example.
SegB wei) = 2.6MHz * ceil(m
Table 5. Example scenario data throughputs
Receiv er
STI+
(kbps)
SNE
STI+
SNE
PoP/TCN
LRH
PoP/TCN
929
730
977
579
86
262
1,602
29
LRH
237
459
1,926
547
S21
232
978
624
215
210
_ 205
ell
.~
195
f-
190
~
185
180
fh
} =1
LinkDataThroughput(j)
LinkPowerFJJ(SegGairi.,j)) * 2.6MHz
J
The quantization of sub-channels becomes important when
the number of bandwidth segments becomes large and
their individual throughput is small. To achieve the ideal
minimum network power of3.9% of the transponder power from the example, it was necessary to have all twelve
unique transfer gains in the network their own bandwidth
segment. Some of these bandwidth segments carried very
little throughput. As a result, the required bandwidth for
this case would be 25.0% of the total transponder bandwidth .
These cases demonstrate that minimum power requires the
maximum bandwidth, and the minimum bandwidth requires the maximum power.
200
CJ
-*
i=l
Minimizing the quantization penalty requires minimizing
the number of bandwidth segments. If all of the links in
the example network were lumped into a single bandwidth
segment, then they would require 8.3% of the bandwidth
of WGS transponder. On the other hand, as was pointed
out before, this case would require 28.2% of the transponder power.
220 ,-----.-------.---~-~-~--,____-_,_____-___,
E
~segmenls
1
Optimization Starting Point
I
r-----r
175 '------
o
-L-Q5
--'---1
---'---1.5
----'-2
----" -
L-3
Cumulative Percentage of Transponder Power (%)
~5
-
:-'--~5
---'
4
Figure 7. Minimum cumulative power versus transfer gain
This example gives every link its optimum transfer gain.
Consider what would happen if all of the links in Figure 7
were placed in a single bandwidth segment with a transfer
gain equal to the maximum gain link in the network,
215.4 dB. A substantial fraction of the cumulative power
occurs in links with optimum transfer gains between 179
dB and 205 dB. Operating these links at transfer gains that
are 10 dB to 25 dB than their optimum transfer gains will
substantially reduce their power efficiencies. As a result,
the cumulative network power would increase to 28.2% of
the transponder power - a substantial penalty. This motivates the use of more than one bandwidth segment.
The network optimization is an iterative, numerical process. Choosing the correct starting point for the optimization will have a substantial effect on how long it takes to
converge. The two obvious alternatives are to start at the
minimum power point (maximum number of segments) or
the minimum bandwidth point (minimum number of segments). Given our secondary goal of minimizing the number of total bandwidth segments, starting with the minimum bandwidth point makes the most sense.
Top Level Algorithm for Network Optimization
The basic algorithm starts with a single bandwidth segment with the transfer gain of the highest optimum link
transfer gain. The power and bandwidth are then computed for this case.
If the optimization objective is minimum bandwidth and
the power constraints for the network are already met, then
the optimization stops immediately. Adding segments will
60f7
only increase the occupied bandwidth. Similarly, if the
objective is minimum total resources and the percentage of
transponder power occupied by the network is already
smaller than the percentage of transponder bandwidth, the
optimization stops immediately.
However, if the objective is to minimize power, or to minimize bandwidth with a constrained power, or to minimize total resources when the power required exceeds the
bandwidth required for one bandwidth segment, then it is
necessary to add bandwidth segments.
comes within the convergence threshold of the ideal network power.
Example Optimization
Figure 8 shows the results of an example optimization.
The power threshold has been set to zero and the maximum number of segments has been set artificially high to
show what happens with the optimization is allowed to run
to the minimum power point.
- - Required Power
~
Bandwidth segments are added by splitting an existing
segment according to the following steps :
I.
g
i
Re-compute the network bandwidth and power based
on the new segment transfer gains .
Total Resource
,
,
"
MinimumTotal
\ ,l Resourc/es
15
____
.gj
8. 10
~ 5~ .
~
~
-,
~ over M inimum
Minimum
Bandwidth
oL....::...=.:..:.:::..:..:..:..:::..::..:.:.......L_
1
Minimum
Power
Power < 10%
c
"'
3. Compute the delta power for each bandwidth segment
between the actual power required and the ideal power required. This is a measure of how much power
could be recovered from the bandwidth segment by
splitting it into smaller segments and assigning them
transfer gains that are closer to the optimum link
transfer gains .
5.
,
~
2. Compute the ideal power for each bandwidth segment
by computing the data throughput for each link in the
segment and the power efficiency for each link at its
optimum link transfer gain.
Split the segment with the highest delta power into
two segments with roughly equal ideal powers. The
upper half of the old segment will have the transfer
gain of the old segment. The lower half will have a
transfer gain between that of the old segment and the
segment with the next lowest transfer gain.
- - Required Bandwidth
,
a:
Compute the actual power for each bandwidth segment based on its assigned transfer gain using the data throughput for each link in the segment and the
power efficiency for each link at that transfer gain .
4.
- - - - Minimum Power
"5 25 ,
2
3
4
-'-----l' - -- ' -_
5
6
7
-'-----'-_
-'----'-_
8
10
9
11
-'
12
Number of Bandwidth Sea men ts
Figure 8. Example network optimization
Several points on Figure 8 are notable. First, as expected
the minimum bandwidth occurs with only one segment.
The required power falls off steeply as the first few additional are added. The minimum total resources occur
when there are only two segments. While the required
power and minimum power lines appear on top of each
other from five segments and higher, the two do not actually converge until twelve segments. For practical purposes however, there is no improvement in the network
power after four or five segments. The bandwidth however still climbs as segments are added .
This example emphasizes the point that only a few bandwidth segments per network are normally required to minimize the network power. Seeking to further optimize the
power only results in wasted bandwidth. This validates
constraining the maximum number of segments in the optimization and including a convergence threshold.
6. Iteratively adjust the segment gains upwards or
downwards and repeat step 5 until the network power
is minimized for the number of segments or an iteration limit is reached.
Steps I through 6 are repeated until the optimization objectives are met or the maximum segment constraint is
reached. If the objective is minimum bandwidth within a
constrained power, then the process continues until the
power required is reduced to within the constraint. If the
objective is a minimum total resource, then the process
continues until the percentage of transponder power required falls below the percentage of transponder bandwidth required. Finally , if the objective is a minimum
power, then the process continues until the network power
CONCLUSION
was has the potential to provide an enormous amount of
potential capacity to support NCW networks. However,
these resources can only be used efficiently with optimized
gain states . This paper has proposed a method for optimizing individual link transfer gains for NCW links and the
bandwidth segment transfer gains for NCW networks.
70f 7