Train-to-Ground Wireless Broadband Communication for … · Train-to-Ground Wireless Broadband...

Train-to-Ground Wireless

Broadband Communication for High-Speed Railways

Fluidity™ Technology

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Summary 1. Introduction 1 2. Physical Layer 2 3. MAC Layer 3 4. Network Layer 4 5. Conclusion 7 References 9

Appendix A 1

Ver. 1 Fluidity Technology

1 RISERVATO – NON DISTRIBUIRE © 2013 Fluidmesh Networks, LLC

1. Introduction Train-to-ground wireless communication in high-speed vehicular mobility scenarios poses complex technical challenges across several layers of the ISO-OSI stack [1], from the physical layer up to the network layer. The physical layer specifies the actual modulation / demodulation technique used to transmit packets over the air. Distortion effects like the Doppler effect [2] may negatively impact the performance of the physical layer. Modern modulation techniques like OFDM (Orthogonal Frequency Division Multiplexing) and diversity transmission techniques like MIMO (Multiple Input, Multiple Output) [4,5] can dramatically reduce the impact of distortive phenomena on the channel and, thus, allow obtaining good transmission performance even in high-speed vehicular communication scenarios. Fluidmesh employs a MIMO-OFDM physical layer operating at 5 GHz that provides good throughput performance at several miles distance using fairly directive antennas on the trackside. In Europe, a significant portion of the 5 GHz spectrum falls within the ISM band (ITU-R sections 5.138, 5.150 and 5.280) as prescribed by the European Commission based on the CEPT recommendations and the ETSI standards. The European standard EN301893 version 1.7.1 regulates the use of the 5.15-5.75 GHz band. Channel bandwidths supported are 20 and 40 MHz and the maximum E.I.R.P allowed is 1W (30 dBm). Based on the signal-to-noise ratio, several modulations and coding schemes (MCSs) can be used ranging from 6 Mbps to 300 Mbps raw throughput. Right above the physical layer, Medium Access Control (MAC) layer is in charge of coordinating the access to the channel by defining a suite of protocols and algorithms that are in common among the wireless devices. Moreover, the MAC layer selects the MCSs to be used by the physical layer according to rate selection algorithms [10]. MCS selection depends on several parameters including the signal level and its time variability. It is thus of paramount importance designing rate adaptation algorithms specifically tailored to the application scenario [6]. Fluidmesh has developed a proprietary rate adaptation algorithm based on prediction techniques specifically designed for high-speed vehicular mobility scenarios that guarantees high throughput in presence of high channel variability. The proposed solution is described in Section 0. The MAC layer also contains all the necessary procedures to handle the handoff, the mechanism used by the mobile units to roam from one infrastructure AP to next one. The handoff procedure is critical to provide a fluid connection and a continuous service across the whole train path. State-of-the-art technologies allows handoff times in the order of a few hundreds milliseconds [11] and they are usually unable to maintain a persistent connection during the process, like in the case of the IEEE 802.11 (commonly known as Wi-Fi) [11]. This level of service is insufficient to provide an acceptable performance in high-speed mobility scenarios. For instance, a train traveling at 300 Km/h covers roughly 80 meters in 1 second. Therefore, a handoff time of 1 second, as for the IEEE 802.11, it is insufficient in order to provide a good level of service. The train will be too far away from the location where the handoff has been initiated, finding completely different channel conditions that might even prevent the handoff from successfully complete. Using Fluidity™, the proprietary Fluidmesh mobility protocol, the handoff is performed within 3 ms, thus, the whole roaming procedure is concluded within a few tens of centimeters at 300 Km/h. It is worth noting that the “overlapping zone” of two subsequent APs’ signals covers a few tens of meters. Therefore, Fluidity small handoff time is more than enough to virtually guarantee the correct network operation with much higher speeds than 300 Km/h. The Network layer contains all the procedures to route the packets across the network. High-speed mobility scenarios exhibit extreme challenges for standard network layer protocols. IP address reconfiguration mechanisms like Mobile IP [12] are not fast enough to react to network changes. Fluidity, based on derivation of MPLS [13], provides a micro-mobility solution [14] that allows terminating the network layer handoff without the need of reconfiguring the IP address or the IP packet routes, thus, providing a seamless service without disruption. In the following sections the Fluidmesh solution is described tackling the specific challenges of each layer according to the ISO-OSI stack model.


2 CONFIDENTIAL – DO NOT DISTRIBUTE © 2013 Fluidmesh Networks, LLC

2. Physical Layer Modulation and demodulation of the data transmitted over the air occur in the physical layer. Modern modulation techniques are usually based on OFDM (Orthogonal Frequency Division Multiplexing) including those adopted by the standards IEEE 802.11 (WiFi) and LTE. OFDM is very robust in adverse channel conditions with high interference levels [15]. Additionally, diversity transmission techniques, such as MIMO (Multiple Input Multiple Output) [16], are increasingly adopted. The main advantages of MIMO are the increased spectral efficiency and the improved robustness of the link thanks to space and time diversity. At the physical layer, Doppler effect is considered one of the main challenges [2]. For example, in the IEEE 802.11 standard, trailing symbols are used to recover the Channel State Information (CSI) for channel estimation. CSI is supposed to be invariant during the whole packet reception because it is used to keep track of the signal phase. This assumption is usually true in low-speed vehicular mobility scenarios in which the channel coherence time can be a few ms with a negligible variation during the packet transmission. In high-speed mobility contexts, the Doppler effect can be seen as a rapid channel variation that can break the channel coherence in the time domain. The literature on the impact of mobility and Doppler effect on IEEE 802.11-based systems is rather limited [3, 4, 5]. All the works are mainly focused on the performance evaluation of WiFi systems. In [3], the authors show that WiFi performance are good even when the speed is fairly high but they also indicate that the analysis becomes complex due to the lack of a fast enough mechanism to perform the handoff among APs in standard WiFi systems. The handoff is indeed a cornerstone in the Fluidity architecture as described in sections 3 e Errore. Il segnalibro non è definito.. The literature [4, 5] describes some mechanisms to improve the channel estimation in presence of rapid channel variations to increase the performance. In general, Doppler effect has an indirect effect on performance because it tends to reduce the coherence time of the channel more rapidly. However, if the signal level is sufficiently high, this impact is rather limited. Fluidmesh employs the MIMO-OFDM technology with larger OFDM subcarrier guard intervals to better handle Doppler effect in high-speed mobility scenarios. Train-to-ground communication in tunnels MIMO technology exploits signal multi-path to maximize performance, i.e. the multiple paths a signal can take to reach the receiver due to reflections. Therefore, the higher the number of uncorrelated reflections occurring on obstacles, the better is the capacity of MIMO to equalize and recover the received signals. This feature is particularly useful in tunnels. It is possible to exploit constructively the large number of reflections occurring by simply using panel or omnidirectional antennas inside the tunnel. The authors in [17] prove, through an extensive experimental campaign, that WiFi communication in tunnels offer very good performance thanks to the coverage offered by the high number of signal reflections.



3. MAC Layer

The MAC layer defines all the protocols and procedures to control the access to the medium including the handoff and rate selection mechanisms.

Fluidmesh proprietary “layer 2 handoff” is completely distributed. The mobile unit monitors the wireless channel by means of probes sent by the infrastructural units at any point in time. Specifically, the mobile units record the received signals from the static units and autonomously decide if, when and which static AP to handoff to.

Rate selection is also critical to obtain good performance. Several algorithms and solutions have been proposed in the literature. In a mobile scenario, the rate selection cannot be performed a posteriori, i.e. reacting to events already occurred such as packet retransmissions, because these approaches will not be fast enough to reach to the rapid channel variation. Other metrics like the signal level or, better, the RSSI (Received Signal Strength Indication) should be used instead [6, 7, 8, 9]. Specifically, the authors of [9], describe an approach based on RSSI prediction using a initial training phase. Fluidmesh adopts a predictive approach without neither the need of an initial training phase nor the use of GPS localization of mobile or static units. The algorithm details are confidential and under review by the European Patent Office (EPO). The predicted vs. the real RSSI is reported in Fig. 2 for a real-life test with prediction set at 100 ms. The RSSI prediction is fairly accurate and the system also filters out all the instantaneus RSSI variation that can have a detrimental impact on the choice of the transmission rate. The transmission rate is then selected according to the prediction of the estimated RSSI.

Figura 1. Handoff occurred in a real-life scenario



4. Network Layer The network layer is maybe the most challenging layer in high-speed mobility scenarios because the standard protocols are usually too slow and cannot react fast enough to the topology changes that occur to the network within a few tens of seconds. This clearly limits the applicability of standard solutions to high-speed mobile environments. Fluidity eliminates protocol latencies due to network topology changes thanks it proprietary system, derived from MPLS [13], that hides network disruptive events, like the handoff, to the attached network devices giving them no perception that the network is actually mobile. The Fluidity allows completing the handoff procedure within a few milliseconds, thus guaranteeing a fluid service across the whole path. The network architecture is based on Prodigy 2.0, MPLS-based technology, which is used to deliver IP-encapsulated data. MPLS provides an end-to-end packet delivery service operating between levels 2 and 3 of the ISO-OSI network stack. It relies on label identifiers, rather than the network destination address as in traditional IP routing, to determine the sequence of nodes to be traversed to reach the end of the path. An MPLS-enabled device is also called a Label Switched Router (LSR). A sequence of LSR nodes configured to deliver packets from the ingress to the egress using label switching is denoted as a Label Switched Path (LSP), or “tunnel”. LSRs situated on the border of an MPLS-enabled network and / or other traditional IP-based devices are also called a Label Edge Router (LER). The ingress node of the path classifies incoming packet according to a set of Forwarding Equivalence Classes (FEC); when a packet matches a class, it is marked with the label associated with the particular class and then forwarded to the next node of the sequence, according to the information configured into the Forwarding Information Table (FIB) of the node. Subsequently, each intermediate node manipulates the MPLS label(s) stored into the packed and then it forwards the data to the next node. The egress node finally removes the label and handles the packet using normal IP routing functions. The FIBs on the different nodes of the network are managed by means of a Label Distribution Protocol (LDP) that is the primary component of the so-called network control plane. Fluidity relies on a custom label distribution protocol that provides automated installation of LSPs among the different nodes of the network; this ensures that each node can be reached from any other node.

R2A0BK (FLUIDRC)

Figura 2. RSSI prediction set at 100ms.



In traditional MPLS networks, whenever the network topology changes for any reason, the FIBs of the nodes involved must be reconfigured to adapt to the new paths. This is usually performed using the standard label distribution protocol signaling available. In a mobility network scenario, the handoff process can be assimilated to a network topology change, where a link is broken and a new one is created. The standard mechanisms to detect the change and reconfigure the nodes are, however, too slow and data-intensive to provide adequate performance in a real-time constrained scenario such as high-speed mobility. In particular, the whole reconfiguration latency and the number of messages exchanged should be minimized to reduce the chances that some data packets are lost in the process. In order to mitigate the aforementioned issues, Fluidity implements a fast handoff solution that is able to provide very fast path reconfiguration with latency in the order of one millisecond. This mechanism is an extension to the existing control plane of the network and it is based on a specific manipulation technique concerning the MPLS FIB tables of the nodes. This scheme allows mobile nodes, and client devices attached to them, to maintain their IP address throughout the mobility process. Besides, all nodes are part of a single layer-2 mesh network. This kind of management is often referred to as “micro-mobility” in literature. The layer-3 handoff process is seamless in the sense that, thanks to a make-before-break strategy, the availability of at least one valid LSP is ensured during the handoff transitory as the network is reconfigured. Therefore, Fluidity guarantees that the handoff (and the network reconfiguration) is performed with latencies smaller than 3 ms. For the rest of this white paper we will make reference to Fig. 3. All routers belonging to the same solid system are said to form a “mobility domain”. The backbone and each individual vehicle are examples of distinct mobility domains, which are represented in the picture as boxes Backbone Architecture The LSR nodes of the backbone install automatic LSPs to ensure full connectivity across the network. On-board Network Architecture The vehicle can be equipped with multiple mobile nodes for improving network redundancy and resiliency. The routers and the devices are connected together using a layer 2 connection, e.g. a network switch. The

S1 S2 S3

M2M1

SB

MB

Vehicle V1

Static backbone

MM M2M1

MB’

Vehicle V1

MM

SB’

Figura 3. Example network architecture.



nodes inside each vehicle install automatic LSPs among them. One router MB is designed as the vehicle’s border router and elected as master among the mobile nodes on the same vehicle. MB processes all traffic flowing between the onboard devices and the backbone hosts, manages the wireless link currently used by the vehicle to connect to the backbone taking the appropriate decisions with respect to which wireless node is selected for the communication to and from the backbone among those available onboard. LSPs connecting to the static backbone are installed and updated whenever the vehicle performs the handoff procedure using the dedicated signaling described below.

Layer-3 hand-off management

This section describes the allocation of MPLS label switched paths to provide seamless and efficient hand-off at the network layer. From the network point of view, the handoff procedure is performed using a combination of MPLS label stacking and label stitching techniques.

Nodes grant full connectivity to all devices by establishing a full mesh of LSP among them. For each LSP installed, the label distribution agent ensures that appropriate entries are present inside the nodes’ tables so that traffic arriving from an upstream LSP is properly forwarded into the considered tunnel.

The handoff command triggers the execution of the layer-3 handoff protocol, which involves the transmission of some few data messages among the various nodes of the network. The messages carry label mapping information and other status data which allow the involved nodes to configure their MPLS forwarding tables so that bi-directional LSP between static and mobile nodes can be installed without requiring a full LSP setup process using the standard LDP procedures, which would take an excessive amount of time and would also require too much data to be exchanged in the network, increasing the chance of incurring into transmission errors.

The layer-3 handoff protocol consists of three phases: 1) the handoff request, issued from the mobile border router MB to the static border router SB; 2) the handoff response, sent by SB to MB; 3) the handoff notification, performed independently and simultaneously both by MB and SB. This message exchange sequence is local to SB and MP nodes and does not involve any network reconfiguration because the LSPs to carry the traffic coming from the mobile nodes are already in place before the handoff process occurred. The goal of the message exchange is therefore to “stitch” together the backbone and the mobile-to-backbone LSP segments. From a technical standpoint, this is obtained by using the well-know standard MPLS

S1 S2 S3

M2 M3M1

SB

MB

SB’

MB’MM

L2M1

L1S2

L2M1

L3S2

LM2M1

LM1

LSP_S1S2 LSP_S2M2 LSP_S3S2

LSP_S2M2

Figura 4. LSPs configuration after the handoff between MB and SB.



technique, i.e. “label stacking”. Once the LSP are stitched together, traffic can be transmitted across the newly created LSP. Fig. 4 reports the configuration of the LSPs after the handoff procedure occurred on between MB and SB. The LSPs are indicated using different colors and names. As can be seen, the blue LSP in “stacked” into the green and red ones thanks to the LSP stitching occurred during the handoff procedure. Packets transmitted by the mobile nodes will flow across the backbone using a double MPLS encapsulation provided by label stacking mechanism. Please note that for the three destinations M1, M2 and M3 the outer label L2M1 is always the same and only the inner label varies.

This handoff technique represents a quantum leap in terms of performance compared to any standard procedure like the one used by the IEEE 802.11 standard. Fig. 5 reports the handoff latency comparison between IEEE 802.11 and Fluidity. Results are obtained using the same test environment.

Notably, the described technique provides the following additional major advantages that preserve networking operations and prevents any service disruption:

• No additional MPLS table entries are required on the other nodes along the path. • All devices, including label-switching routers and client devices, maintain the same IP address

during mobility, preserving data connections intact.

5. Conclusion Broadband high-speed train-to-ground communication is extremely challenging and requires a cross-layer approach that addresses all the issues related to the each layer of the ISO-OSI stack. WiFi or LTE have not been designed to tackle all those issues. Fluidity has been specifically designed to address the high-speed train-to-ground communication challenges and it’s the results of almost 3 years of research in the field of wireless communications. Fluidity brings optimizations and solutions to each layer of the ISO-OSI stack from the physical layer to network layer. Fluidmesh has built a strong intellectual property portfolio around

Fig 5. Average handoff time comparison between Fluidity™ and IEEE 802.11



Fluidity by filing several patents both in Europe and in the US.



References [1] ISO 7498-2, “Information processing systems -- Open Systems Interconnection -- Basic Reference Model”, 1989 [2] J. G. Proakis, “Digital communications fourth edition,” 2001. [3] Janis Jansons, Arturs Barancevs , “Using wireless networking for vehicular environment: IEEE 802.11a standard performance”, 2012. [4] Iulia Ivan, Philippe Besnier, Matthieu Crussière, M’hamed Drissi, Lois Le Danvic, Mickael Huard, and Eric Lardjane, “Physical Layer Performance Analysis of V2V Communications in High Velocity Context”, 2009. [5] Zhipeng, ZHAO, “Wi-Fi in High-Speed Transport Communications”, 2009. [6] Joseph Camp and Edward Knightly, “Modulation Rate Adaptation in Urban and Vehicular Environments: Cross-layer Implementation and Experimental Evaluation”, 2008. [7] Pralhad Deshpande, Samir Das, “BRAVE: Bit-Rate Adaptation in Vehicular Environments”, 2010. [8] Xi Chen, Prateek Gangwal, Daji Qiao,“RAM: Rate Adaptation in Mobile Environments”, 2012. [9] Pravin Shankar, Tamer Nadeem, Justinian Rosca, Liviu Iftode, “CARS: Context-Aware Rate Selection for Vehicular Networks”, 2008 [10] D. A. Bell, “Information Theory; and its Engineering Applications (3rd ed.)”, 1962. [11] Yanfeng Zhang, Yongqiang Liu,Yong Xia, Quan Huang ,“LeapFrog: Fast, Timely WiFi Handoff”, 2007. [12] IETF RFC 5944, “IP Mobility Support for IPv4, Revised”, 2010. [13] IETF draft “Multiprotocol Label Switching”, 2013. [14] Junn-Yen Hu, Chen-Fu Chou, Min-Shi Sha, Ing-Chau Chang, Chung-Yi Lai, “On the Design of Micro-mobility for Mobile Network”, 2007. [15] webe.org - 2GHz BAS Relocation Tech-Fair, COFDM Technology Basics. 2007-03-02. [16] Thomas Kailath, Arogyaswami J. Paulraj, “Increasing capacity in wireless broadcast systems using distributed transmission/directional reception (DTDR)”, 1993. [17] Mohamed Kassab, Martine Wahl, Mauricio Casanova, Marion Berbineau, Marina Aguado, “IEEE 802.11a performance for infrastructure-to- train communications in an underground tunnel”, 2009.

Appendix A Contacts

Worldwide Headquarters

Fluidmesh Networks, LLC 1327 Barclay Boulevard Buffalo Grove, IL 60089

U.S.A.

Tel. +1 (617) 209 -6080 Fax. +1 (866) 458-1522

[email protected]

www.fluidmesh.com

EMEA Headquarters (Italy) Tel. +39 02 0061 6189

UK Branch

Tel. +44 2078 553 132

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