Delivering broadband internet access for high speed trains passengers using the new WiFi. standard n for train-to-ground communications

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1 Delivering broadband internet access for high speed trains passengers using the new WiFi standard n for train-to-ground communications Hassan GHANNOUM 1, David SANZ 1, Bernadette VILLEFORCEIX 2, Henri PHILIPPE 1, Pascal MERCIER 1 1 SNCF, Paris, France; 2 Orange Labs, Issy Les Moulineaux, France Summary This paper presents an experimentation carried out by SNCF and Orange Labs which aimed at evaluating the performance of new generation WiFi IEEE802.11n technology for the implementation of broadband train-to-ground communications solutions. The major results of the project are presented, showing n potentialities for the foreseen services. This experimentation was part of the TGV Communicant programme, launched by SNCF in 2003 in order to find technical solutions for the delivery of broadband Internet access services for passengers onboard its TGV high speed trains, and which was crucial in the validation of technical solutions having lead to the first commercial Internet service for passengers onboard SNCF s TGV (Box TGV), which has been opened in Q onboard the trains of the TGV East European Service. Index Terms: Train to ground communications, WiFi n, broadband Internet access service. 1. INTRODUCTION During the last few years, many railway operators have been experimenting, testing, developing, and some of them even launching commercial services based on train-to-ground broadband connectivity solutions aiming at providing onboard internet services to train passengers, high speed trains included. Through these projects, various types of communication solutions have been investigated, including cellular solutions, WLAN-based solutions, hybrid terrestrial/satellite solutions and two-way satellite solutions as well. The solution that seems to be the most suitable in France at the moment, combines bidirectional satellite communications with terrestrial solutions for blind spot areas (stations, tunnels, ) in order to provide continuous internet connectivity. Based on these technical choices, the OCEA consortium has implemented the end to end connectivity solution that has been deployed in the first SNCF commercial service delivering Internet services to passengers onboard TGV high speed trains. This commercial service has been launched on the 1 st of December The 52 trainsets of the TGV East European Service have been equipped with hybrid terrestrial/satellite solution (two-way satellite as primary communication and WiFi and 3G for blind spot areas). Existing solutions are able to deliver connectivity speeds of a few Mbps to a single train. These throughputs are comfortable for the delivery of Internet access to some tens of passengers, but needed data-rates are going to grow very fast along with user s demand, who are more and more used to the large data-rates provided by DSL at home, HSPA on mobile phones, WiFi in hotspots in coffees, hotels, airports, train stations The number of passengers willing to connect will grow very rapidly in the next years, in particular due to the very large success of smartphones. To meet this future growing demand for this kind of services onboard trains, mainly in terms of bandwidth, future communication systems will certainly need to rely more and more on terrestrial technologies, at least in dense traffic areas, to provide the required quality of service. Such a solution could also enable a reduction in operational costs related to satellite bandwidth.

2 The objective of this collaborative project between SNCF and Orange was to study the potentialities of new generation WiFi IEEE802.11n technology for the implementation of broadband train-to-ground communications solutions. Following chapters of the paper will introduce the objectives of the project, the experimentation campaigns carried out and the obtained results. 2. PROJECT GOALS As previously introduced, meeting future growing demand in terms of throughput will certainly need to rely more and more on terrestrial technologies. However, terrestrial train to ground communication solutions rely on long and complex deployments of infrastructures along the track which are in general very expensive. In order to reduce deployment costs, it is essential to try to minimize the number of sites required to ensure radio coverage of the high speed railway network. The radio coverage is thus one of the main features to be taken into account when choosing a terrestrial communication technology. Another key feature to be considered is data rate offered by that technology, since the objective is to provide the highest possible throughputs for high speed train passengers. These two features, radio coverage and data rate, are closely related. Therefore, the more suitable technologies will be the ones offering the best trade off between range and throughput. Obviously, other features have to be studied as well, including reliability, security and need for a licensed radio spectrum. Among the various terrestrial technologies, 3G and WiMAX may be considered but the data rate offered by these technologies decreases considerably as speed increases. Besides, these technologies are subject to licensing. Proprietary technologies like RedLine Communications, Telefunken Racoms, Flarion or Reicom may be considered as well. These technologies offer good performance and in some cases, have been specifically designed for railway applications. However, the major drawbacks of these technologies are their dependence on a single manufacturer and their deployment costs which are considerably higher than those in case of WiFi for example. Among the various terrestrial technologies, WiFi IEEE is a very interesting candidate. Indeed, it s an unlicensed, well known and inexpensive technology delivering very good performance even at high mobile speeds. Previous WiFi-a/b/g standards were studied few years ago by SNCF and Orange Labs and the tests carried out in early 2005 showed that WiFi is one of the more suitable terrestrial technologies that can be used for train-ground high data rate communications. The most recent generation of WiFi standards is WiFi n, which offers a better performance trade-off in terms of coverage and throughputs thanks to the use of state of the art techniques on antennas and signal processing, at the physical and MAC layers. Compared to previous IEEE standards a/b/g, n major enhancements are: - Channel bonding allowing channel bandwidth of 40 MHz instead of only 20 MHz channel (increase of capacity). - Multiple Input Multiple Output (MIMO) scheme allowing multiple streams and capacity increase. - Best robustness due to FEC in the PHY layer and new Modulation and Coding - Greater number of OFDM sub carriers - ACM allowing better link budget and best robustness to fading - Two guard interval sizes - Frame aggregation at MAC layer to limit overhead and enhance efficiency - Power save mode to avoid contention in the channel The experimentation campaign was carried out by SNCF and Orange Labs between Q and Q1 2010, using some of the first n products in the market (respecting the pre-standard draft 2 implementation of the technology) in order to implement broadband train-to-ground communications. The main objective of our project was to evaluate the performance in terms of range and throughputs that could be achieved between a TGV high speed train and a WiFi n trackside radio infrastructure along a French High Speed Line.

3 Next chapter introduces the experimented architecture. 3. EXPERIMENTATION 3.1 Tests location The measurements have been performed along the French Atlantic High Speed Line. Two WiFi access points were installed along that line and a Test&Measure TGV high speed train was equipped with the mobile terminals. The measurements were done with trains running at speeds of up to 300 km/h using MIMO configurations for train-to-ground communications. The following figure shows the location of the experimental network deployed for the field tests. The 2 access points pilot network achieved with WiFi n technology were installed at 6,3 km from each other in locations of Auneau and Santeuil. Figure 1: Field tests location 3.2 Radio equipments The pilot network consists of a terrestrial network composed of 2 WiFi n (pre-standard, draft 2) access points operating in the 2.4 and 5 GHz bands and an antenna network deployed in each site as depicted in figure 2. The backbone link between the two sites was achieved with Motorola s Canopy technology, a point to point proprietary radio technology operating at 5 GHz frequency band and allowing the implementation of a 90 Mbps capacity link over this 6.3 km distance configuration.

4 Figure 2: Pilot network The composition of the pilot network is summarized in the following list: 1 ground network, 2 sites : Auneau (at the top of a 25m meter high mast) and Santeuil (at the top of a 35m meter high cereal silo) 2 Motorola CANOPY equipments (one per site): Point to point backbone at 5 GHz frequency band 2 AP Cisco 1250 (one per site): 2 radios 2.4 and 5 GHz per site 3 directional 2.4 GHz antennas and 3 directional 5 GHz antennas, for each side of the railway line (in both sites) 3 omni-directional 2.4 GHz antennas and 3 omni-directional 5 GHz antennas, for each side of the railway line (in Santeuil site) 1 on-board network on SNCF s IRIS320. IRIS320 is a Test&Measure train used for infrastructure maintenance of High Speed Lines in France: 3 bidirectional antennas 5 GHz 2 bidirectional antennas 2.4 GHz 3 omni-directional antennas in 2.4/5 GHz 1 WiFi n client at 2.4 and 5 GHz: Intel card 4965AGN and Ubiquity card SR71C The ground network antenna system for the 2 sites of Auneau and Santeuil is shown in figures 3 and 4 respectively.

5 Figure 3: Auneau site antenna system Figure 4: Second site (Santeuil) antenna system The on-board antenna system, designed by Orange Labs, was set up on an experimental SNCF TGV IRIS 320 as depicted in figure 5. Figure 5: On-board antenna system installed on the roof of SNCF s IRIS 320 Test&Measure train

6 3.3 Network architecture The network architecture deployed in the pilot network is composed of the following elements: LAN: WLAN network based on the two n access points linked by the radio-backbone implemented with the two Canopy equipments. The LAN at Auneau site had also a wired component, in which a switch, an application server and the WAN administration link were installed. WAN: secured 2 Mbps link relying the LAN installed on technical premises at Auneau to SNCF supervision centre Application and traffic generator servers In the next section, the different configurations that have been tested and the achieved performance are presented. 4. RESULTS The IRIS320 Test&Measure train runs across the entire French high speed lines network twice every two weeks. Hence, the opportunities of experimentation for train to ground communications in the chosen geographic area were of only 2 roundtrips (2 passages in each direction of the track) every two weeks. Preliminary tests were carried out in order to choose the more appropriate antennas, MIMO configuration, and the other n parameters (band, bandwidth, guard interval ) both onboard and at the ground. These tests confirmed the poor propagation performance of the 5 GHz band, in which the implementation of the train-to-ground link did not allow to achieve a large range of coverage, even with high directivity antennas at the ground. This band was hence abandoned to focus on the 2.45 GHz frequency band. Due to configuration capabilities of the available n clients onboard (only 2x MIMO configurations were possible, only 20 MHz channels were possible in 2.45 GHz band, spatial multiplexing was not supported) and to the LOS configuration of the test, multi stream capabilities at access point level were not activated. The following results are given for communications in the 2.45 GHz frequency band, using directional antennas both at the ground and onboard, and using 20 MHz bandwidth channels: Figure 6: UDP instantaneous throughput (3 bidirectional antennae at the ground, 2 directional antennae onboard)

7 Figure 7: UDP instantaneous throughput (3 bidirectional antennae at the ground, 2 bidirectional antennae onboard) Figure 8: UDP instantaneous throughput (3 bidirectional antennae at the ground, 2 bidirectional antennae onboard) Access points logs has recorded association times of up to 3 minutes and 30 seconds, which represents a coverage distance of about 17 km at train speed of 300 km/h. Data transfers shown in previous figures show however that effective throughputs were available during around 120 seconds (except in the case of the pink curve, where data transfers were recorded for over 180 seconds). The curves show also: The dependence between train distance to access point and throughput (large data-rates are obtained when train is close to the access point). A hole in the observed data-rates area approximately in the middle of the covered area between the two sites. This hole is due to a very weak signal to noise ratio, which causes a loss of efficiency in the communications but the access points stayed associated even in this area. An important fading in the data rates when the train passes very close to the access point. This fading is caused by a loss of signal due to the use of directional and non-optimally oriented omnidirectional antennas at the ground.

8 Large variability / low stability of the achieved data-rates. Orange Labs and SNCF estimate however that stable throughputs of between 10 and 20 Mbps could be achieved if the adequate radio engineering optimization measures were put in place. In Figure 9, a zone of the WiFi n radio coverage is shown on a Google maps view, in which the reception levels (RSSI: Received Signal Strength Indicator) are represented with the following colour code: Green: RSSI > -80 dbm Yellow: -85 dbm < RSSI < -80 dbm Red: RSSI < -85 dbm The sensitivity of the WiFi clients used in the experimentation was typically of about -95 dbm. Figure 9: WiFi n coverage in the central segment of the experimentation zone In Figure 10, a 3D representation shows the WiFi n radio coverage for the entire experimentation area: yellow bars represent coverage delivered by Auneau access point, and blue bars represent coverage delivered by Santeuil access point.

9 Figure 10: 3D view of the WiFi n coverage in the entire experimentation zone 5. CONCLUSIONS AND PERSPECTIVES This project has allowed Orange Labs and SNCF to carry out field experimentations with WiFi n technology for the implementation of broadband train-to-ground communications. From a technical point of view, the lessons learnt were the following: When the experimentation started in 2009, there was a poor availability of WiFi cards equipped with MIMO functionalities and options, compared to the standard capability. Actually, the available cards were the first ones (compatible with the pre-standard draft2 of IEEE n), with poor implementation of the standard, specifically, in MIMO transmission modes and poor configuration capabilities. This explains that all expected performances were not achieved. In particular, the cards only supported 20 MHz channel bandwidth in 2.4 GHz frequency band, and MIMO spatial multiplexing was not implemented in the chipsets. At access point side, the tests were performed with Cisco 1250, also with less capability than the complete standard. Best propagation at 2.4 GHz: the use of this frequency band for the train-to-ground is advised. Tests were achieved in MIMO configurations but in LOS conditions. MIMO is more useful in NLOS scenarios, where spatial diversity gains can be expected due to multi path propagation. Performance in terms of coverage was very good, as expected, a single access point being able to cover between 5 and 9 km of railway line. The perfect overlapping of the two sites is shown in figure 9.

10 Interesting performance in terms of throughput: increase of throughput foreseen with client card capable of managing spatial multiplexing and with a more optimized radio engineering of each site. The lack of throughput performance compared to the expected one was essentially due to two essential reasons: - LOS propagation conditions, no multi-path, only one stream - MIMO, no spatial multiplexing These two parameters are correlated, and well combined at both transmitter and receiver sides, can contribute to the generation of multi streams with MIMO technology, thus allowing an increase of the throughput. In conclusion, further tests should be necessary with more recent equipments in order to complete a good knowledge of this technology in the railway domain. From a service point of view, in the perspective of implementing broadband connectivity services for passengers onboard high speed trains, the experimentations carried out has shown n potentialities for this purpose. Internet service for passengers like the one deployed by SNCF on its East European service will need to evolve in the coming years to meet the foreseen evolution in passengers demand. Cheap and performing terrestrial technologies like IEEE n could be a good candidate since the trade-off between coverage and throughput delivered by this new version of the WiFi standard is very good: some tens of Mbps could be reached over 5 km areas. References [1] D. Sanz, P. Pasquet, P. Mercier, B. Villeforceix, D. Duchange. TGV Communicant Research Program: from Research to industrialization of onboard broadband Internet services for high-speed trains, In 8 th World Congress on Railway Research WCRR 2008, Seoul-Korea, May [2] [3] [4] [5] [6] [7] [8] [9] [10]