A MODULAR SIMULATION TOOL OF INTERFERENCE AND FADE MITIGATION

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					COST 280                                       -1-                                            PM3-007
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L. Castanet - J. Lemorton - M. Bousquet      July 2002



                                    COST Action 280
           “Propagation Impairment Mitigation for Millimetre Wave Radio Systems”


                A MODULAR SIMULATION TOOL
      OF INTERFERENCE AND FADE MITIGATION TECHNIQUES
        APPLIED TO MILLIMETER-WAVE SATCOM SYSTEMS


                                           ABSTRACT
       A simulation tool has been developed by ONERA to simulate the behaviour of the air
interface of satellite communication systems operating at ku-band and above and more
particularly to design and test Fade Mitigation Techniques (FMT) in order to improve link
performances and Quality of Service (QoS) for the end-user. The objective of this contribution
is to give a description of the functionality of this simulation tool and to illustrate it with some
examples.



1    INTRODUCTION
A simulation tool has been developed by ONERA in the framework of : the European action
COST 255 “Radiowave propagation modelling for new SatCom services at Ku-band and
above”, the RNRT project SAGAM of the French Ministry of Industry, and the European IST
project GEOCAST. This tool enables the behaviour of the air interface of satellite
communication systems operating at ku-band and above to be simulated and more particularly
Fade Mitigation Techniques (FMT) to be designed and tested in order to improve link
performances and Quality of Service (QoS) for the end-user.
The objective of this contribution is to give a description of the functionality of this
simulation tool and to illustrate it with some examples. In a first part a general description of
the configurations implemented in to the simulator such as system architectures, FMT and
interference issues. In a second part, the internal functions of the FMT control loop are
described : firstly the detection schemes that can be simulated, secondly the short-time
prediction of the propagation channel and thirdly a description of the decision process in
terms of FMT activation thresholds and time delay issues. In a third part, typical statistical
simulation results are presented, including example of control loop internal parameters
optimisations and instantaneous performances and statistical results.



    L. Castanet, J. Lemorton                                     Prof M. Bousquet
    ONERA                                                        SUPAERO
    DEMR / APR                                                   AECP
    2 av. E. Belin - BP 4025                                     10 av. E. Belin - BP 4032
    F-31055 Toulouse cedex 4                                     F-31055 Toulouse cedex
    Tel : +33.5.6225.2729                                        Tel : +33.5. 6217.8086
    Fax : +33.5.6225.2577                                        Fax : +33.5. 6217.8345
    E-m : castanet@onera.fr                                      E-m : bousquet@supaero.fr,
           lemorton@onera.fr
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2     SATCOM SYSTEM AIR INTERFACE SIMULATOR
The simulator developed at ONERA [Castanet, December 2001] allows to simulate the
performances of the air interface of SatCom systems operating in the 10 GHz – 50 GHz
frequency range. In the current version of this simulator, the upper layers are considered only
in terms of the constraints they impose on the FMT system configuration such as the detection
scheme or the decision process. It means that the objective is not to simulate network
performances or quality of service but more pragmatically quality of link.
The simulation principle relies on a time-driven schedule, i.e. using a constant sampling
period. The simulation is carried out with a sampling rate compatible with the variation of the
propagation channel, that is at least several samples per second. This sampling rate allows to
take into account the reaction time necessary for the system to adapt its configuration
(transmitted power evolution, coding or modulation switches, signalling exchanges between
components of the system, …) to the propagation channel fluctuations and not to evaluate fine
performance like packet loss.

2.1   SatCom system architectures and performance assessment

System architectures :
Different system architectures can be taken into account through the implementation of their
link budget. In particular, the modularity of the simulator allows regenerative and transparent
configurations to be considered in the simulation. Up to now the performances of three
systems have been assessed with the ONERA FMT simulator in different frameworks :
   • A Ka-band videoconferencing VSAT system characterised by a regenerative payload
      and a mesh VSAT network, studied in the framework of the European action COST 255
      “Radiowave propagation modelling for new SatCom services at Ku-band and above
      [COST 255 Final Report, Chapter 6.2] [Mertens & Castanet, 2000],
   • A Ka-band ATM switch system aiming at providing multimedia application for the
      mass market using a regenerative payload and a meshed VSAT network, studied in the
      framework of the SAGAM project funded by the French Ministry of Industry [Castanet
      et al., October 2001],
   • A Ka-band packet switch system aiming at demonstrating the feasibility of multicast per
      satellite, using a regenerative payload and a star network, currently studied in the
      framework of the IST project GEOCAST [Castanet et al., September 2002].
As far as regenerative payloads are concerned, the implementation of the link budget into the
simulator is relatively simple : it is performed only through the introduction of the link
margins and of the Earth station receiver main characteristics for the downlink (in order to
calculate the degradation of the figure-of merit of the Earth station in presence of clouds. If
transparent configuration are of interest, the whole link budget has to be implemented in order
to model the characteristics of the on-board TWTA.
Current activity on the simulator aims first of all at introducing the whole link budget into the
simulation in order to be able to take into account for interference perturbations, and at
modelling the performances of a Ka-band bent-pipe system with a star network configuration.

Performance and capacity criteria :
The end-to-end performance of the link is evaluated from the introduction of propagation time
series in the link budget. When the instantaneous BER of the received signal is higher than the
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minimum required objective (or when the instantaneous Eb/N0 is lower than the required one),
it is considered that the link does not perform satisfactorily and consequently that an outage
occurs. Figure 1 hereafter gives an example of the performance of the air interface of the
GEOCAST system (right) [Castanet et al., September 2002] submitted to a strong propagation
event (left) measured during the Olympus campaign by University Catholic of Louvain-la-
Neuve (Belgium) [Vanhoenacker et al., 1990].




                  Figure 1 : Example of air interface performance without FMT

On the basis of this error-rate criterion, a number of parameters are calculated to measure the
efficiency of FMTs in the simulation. The main parameter categories are as follows :
      (i)   Link Outage. The following parameters are computed :
            • number of outage higher than 10 s, lower than 10 s and total number of outage
            • duration of outage higher than 10 s, lower than 10 s and total duration of
               outage
            • fraction of time with outage higher than 10 s, lower than 10 s and total fraction
               of time with outage detected.
      (ii) Availability, that is the percentage of time for which the link is available. Based
            on Recommendation ITU-T G.826, the link is considered available if it operates
            properly (BER < 1.4 10-9) for more than ten consecutive seconds. The parameters
            considered in this study include : total number of availability period, total duration
            of availability period and ITU-T availability.
      (iii) Unavailability, that is the percentage of time for which the link is unavailable. In
            the same sense as availability parameter, the link is considered unavailable if an
            outage occurs (BER > 1.4 10-9 ) for more than ten consecutive seconds. The
            parameters under this category are : total number of unavailability period, total
            duration of unavailability period and ITU-T unavailability
      (iv) Duration of Return periods, that are defined as the time intervals between two
            unavailability periods. It differs from the availability period by the fact that it
            includes very short time intervals lasting for less than 10 seconds as well as very
            long time intervals comprising more one or more outages lasting for less than 10
            seconds. The parameters are : total number of return period, total duration of
            return period and total fraction of time of return period
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      (v)  Mean duration, which includes the following parameters : mean duration of
           outage period, mean duration of availability period, mean duration of
           unavailability period and mean duration of return period
Another set of parameters has been identified as relevant to characterise the behaviour of the
system and to evaluate the efficiency of FMTs :
      (i)   The percentage of time the system operates in each of the various level of
            power transmitted in case of ULPC
      (ii) The relative throughput denoted in percentage of the time, that is the ratio of the
            total number of bits effectively transmitted when the link is available to the
            maximum number of bits that would have been transmitted if the system operated
            in nominal conditions (without outage). This parameter allows an insight into the
            actual system capacity to be obtained.
      (iii) The percentage of time the system operates in each coding rate when adaptive
            coding is considered. As for the relative throughput, this parameter allows the
            actual system capacity to be estimated.
      (iv) The average switching rate, expressed in number per hour, defined as the ratio
            of the number of switches from one mode (SSPA output power, information data
            rate or coding rate) to another during the test sequence.


2.2   Fade Mitigation Techniques
Among all FMT identified up to now [Willis and Evans, 1988] [Tartara, 1989] [Allnutt and
Rogers, 1993] [Gallois, 1993] [Acosta, 1997] [Castanet et al., 1998], etc., it has been chosen
to implement the most promising one according to the studied systems (see § 2.1). These
FMT rely on the principles of Up-Link Power Control, Data Rate Reduction and Adaptive
Coding.

Up-Link Power Control (ULPC) :
With ULPC, the output power of a transmitting Earth station is matched to up-link or down-
link (in case of non-regenerative repeater) impairments. In the case of regenerative repeaters,
up and down links budgets are independent, so ULPC acts only on the up-link budget. ULPC
is used to keep a constant level of all the carriers at the input of the repeater, while
maintaining the uplink budget close to target. Transmitter power is increased to counteract
fade or decreased when more favourable propagation conditions are recovered so as to
optimise satellite capacity.
This FMT is simple to implement since it requires only the introduction of the minimum and
the maximum power of the Earth station power amplifier, as well as the power increment. At
this level it is important to recall that for this technique it is possible to play on the granularity
(power increment) which is not possible so easily with other FMT.

Data Rate Reduction (DRR) :
Another technique implemented in the simulator consists in decreasing the information data
rate at constant BER. The technique is called Data Rate Reduction. Here, user data rates
should be matched to propagation conditions : nominal data rates are used under clear sky
conditions (no degradation of the service quality with respect to the system margin), whereas
reductions of data rates are introduced according to fade levels.
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As for ULPC, this technique is relatively easy to implement : it requires also to introduce the
nominal data rate, the maximum acceptable degradation in terms of quality of service and the
data rate variation step.


Adaptive Coding (AC) :
The introduction of redundant bits to the information bits when a link is experiencing fading,
allows detection and correction of errors (FEC, …) caused by propagation impairments and
leads to a reduction of the required energy per information bit. Adaptive coding consists in
implementing a variable coding rate matched to impairments originating from propagation
conditions.
AC coding can be implemented in two ways : on the one hand alone which involves to be able
to have the bandwidth vary accordingly to the extra coding and on the other hand in
combination with DRR in order to work at constant bandwidth. Both solutions have been
implemented in the ONERA FMT simulator.

Joint FMT :
Joint FMT [Castanet, December 2001] is a very promising solution to improve the
performance of a SatCom system. The possibility to combine FMT has been introduced in the
ONERA FMT simulator. Figure 2 (left) shows an example of FMT activation for the event
given in § 2.1 : with ULPC, a combination of AC and DRR and a combination of the three
techniques. The interest of such kind of combination clearly appears on these graphs.




         Figure 2 : examples of FMT activation, ULPC (left), ULPC+AC+DRR (right)

2.3   Interference issues
With multibeam satellite communication systems, the level of interference impacts strongly of
the performance of the physical layer [Sleight et al., 2001]. If interference is taken into
account, it has a significant impact on the behaviour of the system submitted to strong
propagation fading. Indeed, Figure 3 hereafter shows that the behaviour of the Eb/N0+I0 with
and without interference differs especially for the area around the required Eb/N0. Of course,
the link availability is lower with interference than without interference. In addition, the slope
of the curves are also different, indicating a non-linear effect of the variation of the Eb/N0+I0
with respect to the time percentage and therefore with respect to the fading level (linear
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variation without interference for the uplink or without interference and without figure-of-
merit degradation for the downlink).
In the ONERA FMT simulator, three kinds of interference are being implemented : multi-
beam interference and single-beam interference that are internal interference (that is due to
components of the considered system), and adjacent system interference that are external
interference (that is due to components belonging to another system).




                         Figure 3 : example of up and down link budget
                  from propagation loss prediction and interference calculation


3     INTERNAL FUNCTIONS OF THE FMT CONTROL LOOP
The aim of a FMT control loop is to track the variations of the propagation channel in real
time and to compensate propagation impairments either to increase its availability or to
improve its instantaneous performance. For this purpose, it is first necessary to detect when a
fade is occurring in order to assess if the quality of link is going to be degraded or if an outage
is going to occur. Secondly, whenever an event supposed to lead to an outage is detected, it is
necessary to check if the terminal is authorised to set up the mitigation, and upon reception of
the clearance, to trigger the mitigation process. Another step can consist in performing a real
time prediction of the propagation channel in order to compensate the reaction time of the
system to obtain a better control loop behaviour. The following three functions are therefore
implemented in the ONERA FMT simulator : the detection function, the prediction function
and the decision function.

3.1   Detection schemes
Two kinds of detection schemes are modelled in the simulator : on the one hand an open-loop
ground detection scheme and on the other hand an hybrid-loop on-board detection scheme.
The open-loop ground detection scheme relies on the estimation by the Earth station of the
uplink (or downlink) impairment from a measurement of the propagation conditions. In the
simulator, the open-loop ground detection scheme is based on the measurement of a downlink
beacon (in general the satellite TT&R beacon) that could be in the downlink frequency band
or in another band (for instance Ku-band). Once the downlink attenuated signal has been
measured, the use of instantaneous frequency scaling algorithms between uplink and
downlink frequencies allows the uplink fade to be estimated in real-time, for the downlink,
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the impairment is directly measured by the Earth station (description of this process in the
next section).
The interest of the hybrid-loop on-board detection scheme is on the one hand to avoid to
implement a beacon receiver into the Earth station (especially for consumer or SOHO
terminals for which the cost is a major constraint), and on the other hand to avoid to use
frequency scaling algorithms (as in open-loop or in closed-loop detection schemes) thanks to
a direct estimate of the uplink impairment. Here, the uplink impairment is estimated by the
payload from a measurement of the power level of each individual carrier. Therefore, this
detection scheme is implemented for regenerative repeaters only, for which it is possible to
estimate the carrier power level directly on board. Indeed, as the signal is demodulated on-
board the satellite, the Eb/N0+I0 ratio can be estimated in base-band and the transmitting Earth
station can be identified from the signalling headers examination.

3.2     Short-time prediction of the propagation channel
The detection concept defined previously can be considered is a "a posteriori" technique,
since the system is able to react (through an appropriate FMT) only after detection of an
event. Therefore, extra time delay is introduced when the detection of an event and the
estimation of the necessary mitigation are carried out, in addition to the delay necessary for
the activation of the FMT. These extra time delays lead to error contributions, that could be
reduced through the implementation of short-term predictions into the FMT control loop. The
architecture of the propagation channel short-time predictor is given at Figure 4 hereafter.




                     Figure 4 : Architecture of the FMT control loop predictor

3.2.1    Attenuation prediction
The attenuation prediction follows two or three consecutive steps : first of all a filtering of the
fast fluctuating component of the monitored signal, afterwards a frequency scaling of the
monitored signal (in the open loop scheme only) and finally a real time prediction.
The filtering is performed with different types of filters : moving average window, rectangular
filter, exponential filter, raised cosine filter, butterworth… The input parameters are the
number of coefficients of the filter as well as the cut-off frequency.
As far as frequency scaling is concerned, several methods have been implemented in the
simulator such as : exponential, ITU-R, COST 205 (fixed and variable), Hodge, Laster-
Stutzman, Rücker, or Gremont EFSR and IFSR methods (see the COST 255 final report).
Regarding the short-term prediction method, only a rough technique relying on the estimation
of the instantaneous fade slope has been implemented up to now. However, the modular
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architecture of the simulator allows more sophisticated prediction techniques to be
implemented if necessary.

3.2.2    Scintillation variable margin
As scintillation is unpredictable in real time, a predictor of the envelope of the scintillation
component relying on a variable detection margin has been proposed by UCL [Mertens &
Castanet, 2000] and implmented in the simulator (see Figure 4). The objective of this
prediction method is to add to the predicted attenuation a variable margin, which corresponds
to the slow varying envelope of the scintillation (due to temperature, humidity and cloud
integrated liquid water content).
After having filtered out the fast varying component of the monitored signal, its log-amplitude
is loaded into a FIFO shift register containing the last 21 samples upon which standard
deviation is returned. To estimate the severity of scintillation log-amplitudes at the
transmission link frequency, the expected joint stochastic process of clear-air amplitude
scintillation along with rapid fluctuations of rain attenuation, supposed to be approximately
gaussian with zero-mean and with a RMS intensity, is estimated by moving standard
deviation of past fluctuation amplitudes of the monitored signal. If needed (open-loop
scheme) a frequency scaling law can be applied on the estimated scintillation standard
deviation (see next paragraph). Scintillation fades at the higher frequency are therefore
statistically compensated by biasing the rain fade predictor with the scintillation amplitude
level not exceeded for some prescribed percentage of time (scintillation signal envelope
definition).

3.3     Decision process
When a fade is occurring, a primary concern is to initiate in due time the appropriate
compensation. The control algorithm implemented in this simulator makes use of a detection
margin. The aim of such a detection margin in a FMT control loop allows to prevent the
system from : firstly prediction errors arising in the detection / prediction process, secondly
errors due to the frequency scaling method (open-loop scheme) and thirdly disturbances due
to the fast fluctuating component of the received signal.




        Figure 5 : example of optimisation of a FMT control loop from [Castanet, 2001]
When more favourable propagation conditions are recovered, FMT are gradually disabled and
the nominal operating mode is restored. An additional hysteresis margin HM is employed to
prevent repeated mode switching if the predicted signal fluctuates around the fade detection
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threshold. Another way to take into account the hysteresis effect would consist in introducing
a hysteresis delay, which is not yet implemented in the simulator. Figure 5 presents an
example of sensitivity analysis carried out with respect to detection and hysteresis margins.
Time delay issues is also an important problem to deal with. Different time delays are taken
into account in the simulator : the first contribution to the delay budget is due to the detection
phase (single or double hop depending on the detection scheme), the second one is the delay
corresponding to the sampling rate of the measurement, the third one is the filtering delay
(that depends on the number of coefficients). As far as FMTs are concerned, depending if the
authorisation of the Network Control Centre has to be requested, a new delay is introduced as
well as for the reaction time of the DAMA, which has to wait for the upgrade of its
connection control matrix.


4    TYPICAL SIMULATION RESULTS
Examples of simulation results obtained with this simulator have been presented in previous
sections (see Figures 1 to 5) to perform FMT control loop optimisation and event-based
analyses. Figure 6 hereafter shows an example of statistical analysis performed from the
introduction of several months of propagation data.

                           ITU-T availability                                                   System Throughput
    (%)                                                                     (%)

           100                                                                  100

           99.5                                                                   99
      A                                                                     u
      v                                                                     t     98
            99
      a                                                                     i
                                                                        N         97
      i    98.5                                                             l
                                                                        o
      l                                                                     i     96
      a      98                                                             z
                                                     No FMT             F         95
      b                                                                     a                                               ULPC
                                                     ULPC               M
      i    97.5                                                             t     94                                        UL DRR
                                                     UL DRR             T
      l                                                                     i
             97                                                                   93                                        both DRR
      t                                              both DRR               o
      y     96.5                                                            n     92

             96                                                                    91
                                                                                   90
            95.5
                   1                                                                    1
                       3                                                                    3
                              5                                                                    5
                                    7                                                                       7
                                           10                                                                   10
                           Month                11                                               Month               11


          Figure 6 : example of long-term analysis performed in the framework of COST 255


5    CONCLUSION AND FURTHER WORK
A simulation tool has been developed by ONERA in the framework of the projects :
COST 255, SAGAM and GEOCAST. This tool enable the behaviour of the air interface of
satellite communication systems operating at ku-band and above to be simulated and more
particularly Fade Mitigation Techniques (FMT) to be designed and tested in order to improve
link performances and Quality of Service (QoS) for the end-user.
This simulation tool allows different link budgets to be taken into account and both statistical
and dynamics analyses to be performed in order to design FMT and optimise system
performances. Statistical analysis enables the interest to implement FMT to be assessed
through estimates of propagation impairments that the system will have to face within the
coverage area of interest or system availability with respect to different FMT schemes.
Dynamics analysis enables the performances of a SatCom system to be studied through the
introduction of propagation time series. These performances are estimated in terms of link
availability (outage, ITU-T availability, ITU-T unavailability, …) and in terms of parameters
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having a strong impact on the system behaviour (for instance number of switches between
states) or on the system capacity (throughput as information data rate supplied to the end-
user).
This simulation tool has been developed on a modular way, it is therefore possible to upgrade
it with new models. At the moment, different configurations can be simulated such as : Ku,
Ka or EHF systems, transparent or regenerative payloads, single or multispot beams. Different
FMT can be simulated such as Up-Link Power Control, Data Rate Reduction, Adaptive
Coding or Adaptive Modulation. For a given FMT, different control loop configuration can be
taken into account such as open-loop, closed-loop or hybrid-loop with ground or on-board
detection.
The key-actions for the on-going activity on this simulator in the framework of COST 280
will be to address interference and signalling issues. Once these aspects will have been dealt
with, other FMT such as Site Diversity, Frequency Diversity or Time Diversity could be
implemented into the simulator. In addition, new short-term prediction models are due to be
developed in order to improve the accuracy of the control loop.


VI      REFERENCES
Acosta R.J. : "Rain fade compensation alternatives for Ka-band communication satellites", 3rd Ka-band
Utilization Conference, Sorrento, Italy, 15-18 Sept. 1997.
Allnutt J.E. - Rogers D.V. : "Recent developments in propagation counter-measures for VSAT services",
ICAP’93, Heriot-Watt University, UK, 30 March - 2 April 1993.
Castanet L., Lemorton J., Bousquet M. : "Fade Mitigation techniques for New SatCom services at Ku-band and
above : a Review", Fourth Ka-band Utilization Conference, Venice, 2-4 November 1998.
Castanet L., Lemorton J., Bousquet M., Garnier B. : "Illustration of the interest to use propagation information to
design Fade Mitigation Techniques : application to the SAGAM system", COST 280 2nd Management Commitee
Meeting, doc. PM2-011, Toulouse, France, 29-30 October 2001.
Castanet L. : "Fade Mitigation Techniques for new SatCom systems operating at Ka and V bands", Ph’D of
SUPAERO, Toulouse, France, 18 Decembre 2001.
Castanet L., Lemorton J., Bousquet M., Claverotte L. : "A joint Fade Mitigation Technique applied to the
regenerative packet switch payload of the GEOCAST system", 8th Ka-band Utilization Conference, Baveno,
Italy, 25-27 September 2002.
COST 255 : "Ka-band videoconference VSAT system", COST 255 Final Report, Chapter 6.2, to be published.
Gallois A.P. : "Fade countermeasure techniques for satellite communication links", Int. Symp. on Comms Theory
and Applications, July 1993.
Mertens D., Castanet L. : "Performance simulation of an adaptive data rate scheme for rain fade compensation in
a Ka-band VSAT videoconferencing system", HF (Belgian Journal of Electronics & Communications), nº1,
2000.
Sleight S., Toth F., Thorburn M., Fashano M. : "Parametric report of interference in Ka-band multiple-beam
broadband satellite payload architectures", 7th Ka-band Utilisation Conference, Portofino, Italy, 26-28 September
2001.
Tartara G. : "Fade countermeasures in millimetre-wave satellite communications : a survey of methods and
problems", Proc. Olympus Util. Conference, Vienna, Austria, April 1989.
Vanhoenacker D., Matagne J., Vyncke C., Vander Vorst A. : "Preliminary Results of the Belgian Olympus
Experiment," presented at the 13th Meeting of Olympus propagation Experimenters, ESTEC, Noordwijk, March
1990, pp. 105-122.
Willis M.J. - Evans B.G. : "Fade countermeasures at Ka-band for OLYMPUS", Int. Jour. of Sat. Com., Vol. 6,
June 88, pp. 301-311.