The ETSI S-band Mobile Interactive Multimedia standard [1] defines a hybrid satellite/terrestrial mobile system, bundling up a range of protocols aimed at providing interactive broadcast, multicast, data acquisition and two-way real-time services to subscribers. The non real-time interactivity is guaranteed by an asynchronous messaging protocol particularly designed for the satellite return link, either for GEO or nonGEO (only if specific frequency pre-compensation systems are deployed to countermand the Doppler effects). The EUTELSA T lOA S-band payload perfectly fits the requirements of the standard and it has been used to test the effectiveness of the S-MIM messaging protocol. The random access radio interface is best suited for the low bit rate and low duty cycle applications in the so-called intelligent devices, whose forthcoming exponential growth in the automotive and domotics worlds will require a low-cost high efficient system for non-real-time communications. Telemetry, environment and traffic monitoring, emergency alerts, fleet management, highway tolling, forecast predictions are just few examples among the possible applications. In particular, the Intelligent Transportation Systems (ITS) and the Machine-to-Machine (M2M) worlds will be the drawing powers in the very near future [2] [10].
The good spectral efficiency of the modified W-CDMA Random Access scheme at the basis of the S-MIM messaging protocol has been widely proved in recent works [3] [4]. The aim of this paper is to evaluate the system performances in a real satellite environment.
In Section II a system overview is reported, focusing on the description of the ground hub and on the system parameters.
In Section III the reference link budget is presented, while in Section IV prominent relevance is given to the performance analysis over satellite with respect to theoretical and simulated tests results, considering different SNR values and two distinct channelization modes.
II. SYSTEM OVERVIEW
The S-MIM messaging protocol is based on the Enhanced Spread Spectrum Aloha defined in [4]. It is able to reach good performances in terms of spectral efficiency, up to now inconceivable for a system exploiting a totally random and unsynchronized access scheme. The performance assessment is expressed throughout the study in terms of Packet Loss Ratio (PLR) versus the MAC-Load (A.), as it is one of the most relevant parameters.
). = (N *8) / B
Where N is the number of packets sent per second, S the total packet length (preamble + useful bits + FEC bits) and B the total bandwidth including the roll-off.
A. Ground Hub
The ground platform under analysis, hosted at Eutelsat premises in the Rambouillet teleport, constitutes a crucial point for the validation of the system performances. In fact, in order to effectively test the capabilities of the demodulator and validate the protocol behaviour, the platform shall operate in the vicinity of the link layer saturation threshold, at very high MAC Load. Thousands of messages shall be simultaneously received, each one having its own EIRP set by the user. This
translates into the need of having a coherent traffic emulator, as close to the reality as possible.
Figure I. S-MIM compliant platform at Eutelsat premises. The three sections represent the three configurations under analysis: the laboratory RF setup,
including the insertion of an A WGN channel, the laboratory RF setup with a satellite channel simulator and the satellite configuration.
The traffic generator serves this purpose at a great extent. It is a sort of master terminal, able to emulate thousands of simultaneously emitting terminals, each one with its own SNR value. The implementation difficulty, with respect to a commonly employed channel emulator, resides in the fact that terminal parameters shall reflect distinct geographical location, corresponding to different G/T and different power/frequency/phase parameters. With the aim of matching real conditions, the traffic is generated according to a Lognormal power distribution, whose standard deviation can be changed at will, and it follows a Poisson arrival rate, whose average value Ie represents the MAC-Load in packets per second.
Both the traffic generator and the demodulator have been implemented using a general purpose Software Defined Radio interface, able to work in a wide frequency range, from 50 MHz to 2200 MHz.
The signal is generated at an intermediate frequency (70 MHz) and then up converted to reach the S-band frequencies assigned to Solaris Mobile to provide Mobile Satellite Services over Europe, a 15 MHz slot from 1995 to 2010 MHz. A high power amplifier is inserted before the parabolic antenna in order to emulate an aggregated traffic up to 7000 transmitting terminals per second, each one with a fixed EIRP equivalent to an average uplink SNR. In addition to the traffic emulator, a pilot generator transmits a reference packet per second over the uplink chain. Connected to a very accurate clock, the reference packet is needed at the hub receiving side to compensate the frequency errors introduced by the whole chain, including the satellite Doppler, and to centre the demodulator frequency with an error less than 1 KHz, the upper bound of the tolerated frequency error, beyond whom performance degradation appears, as shown in [5].
While the uplink is operated in S-band, the corresponding feeder link is received by the hub in Ku-band. This asymmetry is peculiar to EUTELSA T lOA.
The downlink RF chain includes an antenna of 3.7 m of diameter, a down-converter that sorts out an L-band frequency signal and the core of the ground hub, i.e. the demodulator.
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The demodulator used for our experiments has been developed by MBI s.r.l. under Eutelsat specifications.
Although the demodulator is able to operate in real-time, the results reported here have been obtained in a non-real-time mode. The packets have been generated, the aggregated traffic samples transmitted through EUTELSA Tl OA and then stored on a local disk at the hub side to be demodulated. Moreover, the number of successive interference cancellation and the computational resources are quite large, consequently allowing testing the benchmarks of the protocol under analysis, in total disregard of the limits of the chosen implementation.
B. System Parameters
In Table I, the most relevant system parameters are reported. Only two modes defined in the S-MIM standard have been implemented. The first one, the Mode 1, is the reference mode that employs the 5 MHz channelization, while the second one, the Mode 7, has been tested in order to understand the effectiveness with respect to the previous mode. In some cases, due to the scarcity of the spectrum resources in S-band, it could be useful to operate over 2.5 MHz.
TABLE!. SYSTEM PARAMETERS
Device System Parameters
Name Model Mode 7
Bandwidth 5 MHz 2.5 MHz
Roll-off 0.22
Chip-rate 3.84 Mcps 1.92 Mcps
Spreading Factor 256 128 Mod
Number of spreading sequences I
Information bits per packet 1200 bits
Coded bits per packet 3600 bits
Preamble length 96 bits
Demod Turbo code iterations 6
III. LINK BUDGET
Here after is reported the link budget of the platform hosted at Eutelsat premises. Both Mode 1 and Mode 7 have been considered. In order to work at a reference value of SNR = -16 dB, which has been taken as reference threshold in [6], the corresponding average EIRP value for a terminal located in Rambouillet is approximately 3 dBW. A probe, inserted at the output of the traffic generator and remotely controlled, allows to measure the real-time power at the input of the antenna and, consequently, to change at will the aggregated EIRP emitted towards EUTELSAT lOA. Even if the MAC-Load reported in Table II is the same in both cases (1000 packets/s), it is important to underline that due to the different occupied bandwidth, the resulting CIN and Et/Nt are different and, when comparing Mode 1 and Mode 7 performances throughout this paper, the same EblNthas been used.
TABLE II. LINK BUDGET
Parameters Name Model Mode 7
Uplink frequency 20Hz
Occupied Bandwidth 4.68 MHz 2.34 MHz
Chip-rate 3.84 Mcps 1.92 Mcps
Packet duration 240 ms
Code-rate 113
Despreading gain 24.08 dB 2 1.07 dB
Single terminal EIRpa 3 dBW
Number of emulated 1000 pis
terminals
Aggregated EIRP 27.8 dBW
Free Space Loss 190.25 dB
IPDF - 159.8 dBW/m2
Satellite O/T at terminal 9 dB/K
location
Received CIT - 153.55 dB/K
Boltzmann constant -228.6 dBW/HzlK
Single terminal uplink CIN" - 16 dB - 13 dB
Aggregated uplink CIN 8.80 dB 1 1.80 dB
Aggregated downlink CIN 17.90 dB 20.90 dB
Total CIN 8.30 dB 1l.30 dB
Single terminal CIN - 16 dB - 13 dB
EJN, 4.44 dB 1.73 dB
a Theoretical average values, not measured'
IV. PERFORMANCE ANALYSIS
A. Theoretical Vs. Experimental RF Results
In order to start the performance analysis, it is important to assess the S-MIM prototype performances with respect to the analytical results given in [6]. The MAC-Load was normalized to the chip-rate (3.84 Mcps), consequently in this first comparison the same approximation was done to rigorously compare both results. It is important to stress that the results in the following section are normalized to the effectively used bandwidth, taking into account the roll-off (4.68 MHz and 2.34 MHz). The simulation conditions are the same as in [6]. The generated traffic follows a Poisson distribution in time, while the packet power follows a log-normal distribution, with a fixed standard deviation of 3 dB, the optimal value highlighted in [5]. The only differences reside in some of the implementation choices, e.g. an increased packet size of 1200 information bits and the use of a hierarchical preamble, and the introduction of frequency variations between the packets in order to better match a real environment. For this test, an adhoc A WGN is added to the aggregated traffic before the digital-to-analogue conversion phase. The resulting signal is then sent at 70 MHz to the receiver, which is connected via a cable in a local loop.
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The comparison between the theoretical protocol performances and the simulated results is presented in Fig.2.
The resulting performances are not so different. The maximum reachable MAC-Load is about 2 bitslslHz, proving that the high theoretical throughput of Enhanced Spread Spectrum Aloha (E-SSA), at the basis of the S-MIM messaging protocol, is confirmed, even through a real RF transmission over cable. However, PLR degradation at increasing MACLoad can be noticed with the current implementation, These performance losses may be explained by the performance of the Successive Interference Cancellation (SIC). In a real RF environment, the residual power increases at each packet cancellation and the SNR is consequently lower at each demodulation cycle. Thus the SNR degradation affects the PLR proportionally to the MAC-Load.
B. Mode 1 Vs. Mode 7
This section has the purpose of comparing the system performances between two different S-MIM modes, Mode 1 and Mode 7 [1], respectively at 5 and 2.5 MHz. The tests reported in this section have been performed in experimental RF mode via cable,
The EUTELSA T lOA satellite embeds S-band transponders of 5 MHz bandwidth each and Solaris Mobile Ltd. was granted with 15 (5 x 3) MHz over Europe, It was therefore taken as an initial assumption that the reference occupation bandwidth for the Mobile Satellite Services in S-band shall also be 5 MHz.
As defined in the S-MIM standard, it may occur that the messaging protocol has to share the available bandwidth with other services (e.g, real-time emergency applications exploiting QS-CMDA waveforms). Due to that, certain system flexibility is required, The Mode 7 was defined to allow a smart reuse of the available frequencies, thanks to narrower channelizations that permit to accommodate other services.
In this section, two sets of tests have been performed in both modes, at varying MAC-Load,
As the bandwidth employed by the 2.5 MHz mode is only half of the bandwidth employed by the 5 MHz mode, the amount of introduced noise will be also halved, but on the other hand the resulting de spreading gain is divided by a factor 2. Nevertheless, the transmitted power remains the same; therefore we have to compare simulation at a different SNR, with a step of 3 dB. We will compare the results obtained at SNR = -16 dB in Mode 1 to the ones at SNR = -13 dB in Mode 7.
Fig.3 shows that in both cases the behaviour of the system remains very similar. This proves that the S-MIM messaging system has a great potential in terms of flexibility.
0,00001 -'-------------------------------1 MAC Load [h1.fHlI
Figure 3. Packet Loss Ratio (Mode I SNR= - 16 dB, Mode 7 SNR= - 13 dB)
C. Satellite Channel Vs. Emulated Channel
This section deals with the results through a real satellite channel. In order to precisely characterize the behaviour of the S-MIM messaging system, two types of tests have been executed. On the one hand, the propagation channel is simulated thanks to the VEGA RF channel simulator placed between the traffic emulator and the receiver [8]. This equipment is able to simulate several physical phenomena introduced by a satellite link:
• Satellite delay variations
• Satellite frequency variations
• Satellite phase noise
• Satellite A WGN
The delay and frequency variations have been chosen in order to perfectly match the measured values on EUTELSA T lOA and the satellite simulator emulates a typical phase noise, as described in the DVB-SH guidelines [9]. On the other hand, a real satellite link is used thanks to the platform described in Section II [3].
From that point, three transmission modes are possible: "A WGN" mode (used in the previous parts), "simulated satellite channel" mode and "real satellite channel" mode.
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Fig. 4 and Fig. 5 summarize the experimental results respectively for the Mode 1 and Mode 7 at the same EbINb whereas Fig. 6 shows results obtained in Mode 1 at lower EblNt. In all cases, the best performance can be observed for the in-lab tests, obtained with the introduction of an emulated A WGN channel. In this case, the signal does not suffer from channel variations. Frequency, phase, power and chip-time are constant over the whole E-SSA packet. It is the best case for the system in terms of demodulation and interference cancellation, but unfortunately it does not match the real environment.
The introduction of an emulated satellite channel implies a decrease of performance in terms of maximum throughput. This is due to the fact that in real conditions the channel estimation is more difficult. Consequently, at each SIC, the residual power of cancelled packets is higher. At the end, the transmitted packets with the lowest SNR are affected from the residual cancellation noise that translates in an increased interference and the PLR increases. It is interesting to notice that the difference between in-lab results and channel emulator results is higher in Mode 7 than in Mode 1.
The behaviour observed when performing the test over EUTELSA T lOA is closer to the simulated channel, except for higher MAC-Loads in Mode 1. This is caused by non-linear effects introduced by the High Power Amplifier in the uplink chain. Using a reduced power (average SNR= -19 dB), allows to mitigate this effect, as can be seen in Fig. 6.
In Mode 7, results obtained with EUTELSA T lOA closely match results obtained with emulated channel, as can be observed in Fig. 5
Figure 6. Packet Loss Ratio (Mode 1, SNR= - 19 dB, comparison between local mode with A WGN, local mode with Satellite Channel Emulator and
satellite mode over EUTELSAT lOA)
D. Satellite Results for different SNR
The last step of the performance analysis is the comparison of the results with different average SNR.
Indeed, the aim of these tests was to assess the performances for different classes of terminals and to define appropriate requirements in terms of needed output power at the terminal and consistent antenna gain, Throughout this paragraph three reference EIRP were considered: -3, 0 and 3 dBW, which translate into SNR average values of -22, -19 and -16 dB at the employed terminal location (the traffic emulator within the hub).
The first results are given for an SNR= -22 dB, corresponding to an average terminal EIRP of -3 dBW emitting from Rambouillet (taking into account the corresponding G/T value of 9 dB/K), At this EIRP, the performance of the protocol over a large population of distributed terminals is limited, and the PLR is slightly higher than 10-2. However, results become interesting for terminals transmitting at 0 dBW (SNR= -19 dBW), A PLR approximately around 10-2 (the
1 1
reference value to meet the minimum QoS target at application level) becomes sustainable until 1.6 bits/s/Hz. Terminals with EIRP equal to 3 dBW allow reaching a very low PLR at moderate MAC-Loads. Unfortunately, the PLR at high MACLoad is increased due to the effect previously described (nonlinearities in the high power amplifier),
Fig. 7 well synthetises the PLR performances of the three chosen classes of terminals, when transmitting over the satellite, while in Fig. 8 the maximum reachable throughput is plotted at different MAC-Loads. It is interesting to notice that the curves for SNR= -16 dB and SNR= -19 dB are almost superposed.
In order to foresee real performances with average EIRP at 3 dBW, it is possible to repeat the test in Mode 7 at the same EllNt. which we have seen to be equivalent in terms of maximum allowed throughput. The results reported in Fig. 5 show that the obtained PLR is lower than 10-2 up to 1.6 bits/s/Hz.
Figure 8. Throughput Vs. MAC-Load (Mode I, satellite mode over EUTELSA T lOA)
ACKNOWLEDGMENT
The authors would like to acknowledge Solaris Mobile Ltd. for allowing the opportunity to use EUTELSA T 10A payload for the purpose of this study. A special thank to MBI s.r.l, for the collaboration in the execution of the trials and in the resolution of problems that arose.
CONCLUSIONS
The detailed performance analysis of the tests done on the first S-MIM compliant platform over a real S-band satellite (EUTELSAT lOA) has been shown in this paper. The results of the satellite tests match the performance assessment obtained with the laboratory platform. Moreover, the results confirm the effectiveness of the return link asynchronous access protocol for messaging applications and open the way for an optimization phase of the available prototype. In fact, quite high throughputs (up to 1.6 b/s/Hz) can be achieved even at relatively low SNR, allowing the use of low-cost emitting terminals embedding off-the-shelf power amplifiers, typically employed in the 30 devices. Furthermore, the huge advancements in the field of the antennas, both on-board and on the terminal side, could reduce in the very next future the need in terms of power amplifier.
It is important to underline that some deployment constraints in the ground hub, such as non-linearities introduced by the high power amplifier in the uplink chain, mitigate global results obtained in these experiments, while they will not affect the demodulator performances in the commercial scenario with real emitting terminals.
Future works will deal with the introduction of mobile conditions at the channel emulator side and the implementation
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of the power control algorithm, as defined in [1], in order to complete the performance assessment in mobility.
REFERENCES
[I] ETSI TS 102 72 1, "Satellite Earth Stations and Systems (SES); Airlnterface for S-band Mobile Interactive Multimedia (S-MIM)", v 1. 1.I, (201 1- 12).
[2] G. Fremont, S. Grazzini, A Sasse, A Beeharee, "The SafeTRIP project: improving road safety for passenger vehicles using 2-way satellite communications, ITS World Congress, Busan, 2010.
[3] Demonstrator Emergency aNd Interactive S-band sErvices (DENISE), Phase I, Artes 3-4 Projects, Internal Reports, http://telecom.esa.int (2009-20 II).
[4] O. Del Rio Herrero, R. De Gaudenzi, "Methods, Apparatuses and System for Asynchronous Spread-Spectrum Communication", European Patent EP2 159926 (AI).
[5] F.Collard, A Recchia, N. Antip, A Arcidiacono, D. Finocchiaro, O. Pulvirenti, "Performance analysis of an Enhanced Spread Spectrum Aloha system", 4ti' International ICST Conference on Personal Satellite Services, Bradford, 2012, in press.
[6] O. Del Rio Herrero, R. De Gaudenzi, "A High Efficiency Multiple Access Scheme for Machine-to-Machine Communications", IEEE Transactions on Aerospace and Electronic Systems, 2012.
[7] F. Perez-Fontan, M.A. Vazquez-Castro; Cabado, C.E.; Garcia, J.P.; Kubista, E., "Statistical Modelling of the LMS Channel", IEEE Transactions on Vehicular Technology, Vo1.50, No 6, Nov 2001, pp. 1549-1567.
[8] VEGA channel emulator, Teamcast, http://www . ascendant.com . tw/download/other/vega e. pdf
[9] ETSI TS 102 584, "Digital Video Broadcasting (DVB) DVB-SH ImplementationGuidelines", v. 1.2. I, (20 I I - I).
[10] AArcidiacono, D. Finocchiaro, A Geurtz, O. Pulvirenti, G. Schluter, "S-band: A new age in Mobile Satellite Services", IEEE Conference
Publications, 41h Advanced Satellite Mobile Systems, 200S, pp. I 2S- I 33.
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