LinkWay S2 Latest Advance in Mesh Satellite Networking

The 2010 Military Communications Conference - Unclassified Program - Systems Perspectives Track LinkWay S2 Latest Advance in Mesh Satellite Networking Lawrence E. Kingsley, Ph.D. Don Wilcoxson, Philip Chacon, Michael Geist ViaSat, Inc. 6155 El Camino Real Carlsbad, CA 92009-1699 Lawrence.kingsley@viasat.com Abstract LinkWay S2, the mesh IP satellite communication modem system for the U.S. Department of Defense, has proven itself over many years of fielded service to be the most flexible and capable mesh multi-frequency time-division multiple access (MF-TDMA) modem available for net-centric warfare. LinkWay S2 is currently used in the Army s Warfighter Information Network-Tactical (WIN-T) program, the Marine s Support Wide Area Network (SWAN) program and a number of other government programs. The latest development in the LinkWay S2 system take the IP network platform to a new level of performance, with improvements in efficiency, data handling capacity, and security, bringing even more capability to LinkWay S2 -based SATCOM terminals. This paper discusses the history of LinkWay and how development has led to the latest improvements to the LinkWay S2 system and their positive implications for the warfighter. These improvements include, for example, a new turbo coded TDMA waveform, doubling the TDMA maximum symbol rate to 10Msps; 8PSK modulation for even higher speed TDMA; pseudo-spread BPSK waveform for mobile and/or disadvantaged terminals, configurable modulation and coding on a per-link basis; embedded TRANSEC; integrated DVB-S2 receiver; increased IP packet processing capability; and enhanced IP QoS. In addition, the overall system architecture has been improved with a new network control server supporting automatic geographic redundancy. Also discussed are recent communications on-the-move (OTM) demonstrations and future enhancements including a further-developed communications on-the-move (OTM) capability through the implementation of a spread spectrum waveform, and even higher data rate handling with improved hardware. Keywords-LinkWay S2, MF-TDMA, net-centric, WIN-T, SWAN, TRANSEC, communications on-the-move, SATCOM, mesh I. BACKGROUND The LinkWay system was originally developed by COMSAT Laboratories in the 1990 s to provide enterprise/infrastructure extension satellite communications for corporations and institutions. LinkWay is a direct lineal descendent of the largescale TDMA system developed by COMSAT Labs for the INTELSAT world-wide satellite communications network in the 1980 s. The INTELSAT TDMA system was designed to provide satellite infrastructure communications between very large major satellite earth stations (for example, interconnection between national telecommunications carriers) and therefore was not a viable solution for individual corporations, banks, organizations, etc. LinkWay was envisioned as a scaled-down version of the INTELSAT TDMA system suitable for commercial applications. Originally intended and designed for the commercial communications market, LinkWay s MF-TDMA satellite access scheme, automatic data routing, and dynamic bandwidth-on-demand provided organizations with a much more cost-effective and flexible satellite data network than using traditional fixed-speed serial data modems (e.g. singlechannel-per-carrier, SCPC). However, even early on it was recognized that LinkWay had the potential for significant application to military communications. [1] The first LinkWay modem system was the LINKWAY 2000 TM, which was first demonstrated in 1996. The LINKWAY 2000 modem had a 70MHz IF interface and a MF-TDMA frequency-hopping bandwidth of 36MHz. This allowed burstto-burst frequency hopping across an entire 36MHz transponder, a notable technical achievement at that time. The LINKWAY 2000 initially operated at only one carrier symbol rate, 2.5Msps, although additional symbol rates decreasing in octaves, 1.25Msps and 625ksps, were soon added. The initial LINKWAY 2000 supported ATM DS3/E3 and ISDN PRI as networking interfaces, with serial frame relay and IP added shortly after initial release. A LINKWAY 2000 system provided true network-centric connectivity, integrating 978-1-4244-8179-8/10/$26.00 2010 IEEE 2341

seamlessly with networking applications using IP, frame relay, ATM, and ISDN, automatically routing network data via satellite. All four protocols were supported natively on the same platform in the same network simultaneously, maximizing networking options. LINKWAY 2000 networks were purchased and installed by several commercial satellite carriers, such as INTELSAT and EUTELSAT, and by large corporations, such as Telespazio in Italy and Rosneft Oil in Russia. A small number of units were sold to U.S. Government agencies, such as NASA and Joint Battle Command, for evaluation. Despite having advanced MF-TDMA technology and offering multiple networking options, the LINKWAY 2000 had limitations that prevented its widespread use. The biggest limitation was the 70 MHz IF interface, which limited the LINKWAY 2000 to operation on a single transponder, and required use of relatively expensive 70MHz up converters, down converters and power amplifiers. (However, it should be noted that in that timeframe all SCPC modems had similar limitations in their IF interfaces.) In 1999, the LINKWAY 2000 was succeeded by the LinkWay2100, which addressed some of these limitations. The LinkWay2100 provided an even more scaled-down TDMA modem designed specifically for the commercial VSAT (Very Small Aperture Terminal) market emerging at that time. The LinkWay2100 had a wideband L-band satellite interface which could utilize inexpensive amplifying block upconverters (BUCs) and lownoise amplifier/block downconverters (LNBs) while providing burst-to-burst MF-TDMA hopping across a 500 MHz IF band from 950-1450 MHz. Additional TDMA carrier symbol rates were added -- 5Msps at the high end and 312.5Ksps at the low end. The addition of the 5Msps carrier was in response to LINKWAY 2000 users need for higher data rates and modem throughput. (This trend continues today, with LinkWay users demanding ever increasing data rates.) The LinkWay2100 was a commercial success, with networks deployed worldwide. For example, one of the largest LinkWay2100 networks was operated by GCI, a telecommunications provider in Alaska. That network had more than 250 LinkWay2100 terminals using more than 40 TDMA carriers spanning 4 transponders. Small numbers of LinkWay2100 units were also purchased by U.S. Government agencies for evaluation as a mesh network solution for tactical military units to provide better bandwidth efficiency and more flexible connectivity as compared to legacy Frequency Division Multiple Access (FDMA, aka SCPC) technologies. This evaluation of the LinkWay architecture and the LinkWay2100 modems eventually led to widespread use of LinkWay2100 network architecture in several large U.S. Army exercises where it excelled on both a cost and performance basis. With the U.S Army s increasing reliance on costly commercial satellite bandwidth services during the conflicts in Iraq and Afghanistan, the U.S. Army sought to employ this new technology, in order to meet communications needs that would not have been possible with a without a bandwidth-on-demand (BoD) technology such as provided by LinkWay. The fielded LinkWay2100 with MF- TDMA technology was determined to be a highly reliable, cost-effective, readily available commercial-off-the-shelf (COTS) network solution that would provide the needed connectivity and quell the insatiable demand for satellite bandwidth resources. The U.S. Army adopted the LinkWay2100 as its current-force mesh IP modem, and since 2003 has deployed a large number of LinkWay2100 modems in the Joint Network Node (JNN) and Warfighter Information Network, Tactical (WIN-T) satellite terminals. The LinkWay2100 has proved to be an effective solution for the US Army s connectivity requirements in theater and its bandwidth-on-demand; net-centric waveform has resulted in significant satellite bandwidth savings for the US Army. For the same reasons, the LinkWay2100 has also been widely deployed by the US Marine Corp in their Support Wide Area Network (SWAN) satellite terminals. II. THE LINKWAY S2 - FIRST RELEASE A. Initial Fielding Although ViaSat has delivered thousands of LinkWay2100 modems to the warfighter since 2003, the LinkWay2100 was designed in the 1990 s, and began showing the signs of age in the late 2000 s. In response, ViaSat introduced the LinkWay S2, a successor to the LinkWay2100. Released in 2007, the LinkWay S2 immediately began replacing the legacy LinkWay2100 in the WIN-T Increment 1 architecture. The initial software/firmware release for the updated LinkWay S2 modem hardware used the same proven concatenated Convolutional/Reed-Solomon coded waveform as the LinkWay2100. While an older waveform design, its use allowed the LinkWay S2 to be fully backward compatible with the LinkWay2100 over-the-air. This greatly facilitated fielding of the LinkWay S2, allowing LinkWay S2 units to be one-for-one replacements for already fielded LinkWay2100 units, or to be installed in new satellite terminals which could be inserted into the existing network. While the LinkWay S2 did not immediately offer any advances in coding and modulation using the legacy LinkWay2100 waveform, it did provide an immediate improvement in IP packet processing power, a faster Ethernet interface, extended L-band interface (950-2050 MHz), more rugged packaging, and a DVB-S2 Integrated Receiver Decoder (IRD). B. The S2 in LinkWay S2 One new feature available with the upgrade to LinkWay S2 is the integrated DVB-S2 receiver/decoder which can provide broadband IP data from any DVB-S2 hub. Thus, this provides the capability for efficient broadband star connectivity to a central data source while simultaneously providing the same full-mesh capability already inherent in LinkWay. Operators may use LinkWay S2 with any standard EN 302 307 DVB modulator and IP encapsulator, i.e., no expensive proprietary hub equipment is required. Furthermore, this reception of data from a DVB-S2 hub is independent from, and in addition to (simultaneously), the mesh MF-TDMA capability, and in fact the DVB-S2 signal can come from a completely different 2342

network, that is, the DVB-S2 source does not need to be tied in any way to the network control center (NCC) managing the mesh MF-TDMA network. The DVB-S2 receiver in each LinkWay S2 operates from 1 to 30 Msps, is DVB-S2 compatible, and is independently tunable. The DVB carrier is not required or used for timing or synchronization. Therefore, each LinkWay S2 can receive any available standards-based DVB-S2 outbound on any frequency on the same satellite used for the TDMA mesh carriers. As an example application for the integrated DVB-S2 receiver, LinkWay S2 could support bandwidth-intensive, multidestination IP applications such as streaming video or transfers of large common-content files over the DVB-S2 link while supporting intrinsically mesh applications such as VoIP or VTC over the mesh TDMA carriers. Also, LinkWay S2 is now interoperable with ViaSat s LinkStar S2 system and also with the Joint IP Modem (JIPM) outbound (in TRANSEC-bypass mode) - providing a universal, flexible networking solution. Future enhancements for LinkWay S2 will include the ability to receive the JIPM DVB-S2 outbound with TRANSEC enabled. The US Army Battle Labs have extensively tested the embedded DVB-S2 receiver of the LinkWay S2 modem within the WIN-T Increment I architecture, evaluating simultaneous DVB-S2 and TDMA operation using the current networking/encryption schemes, and have validated its potential for delivery of high-speed communications to the warfighter. C. Extended L-band Range LinkWay S2 extends the 500 MHz L-band range of the LinkWay2100, supporting Wideband Global Satellite (WGS) IF ranges of 950 2050 MHz with 20 db MF-TDMA receive power burst-to-burst dynamic range. Unique in the industry, LinkWay S2 is a mesh MF-TDMA modem with completely independent fast-hopping transmit and receive paths. The transmit modulator and receive demodulator can each tune on a burst-to-burst basis, independently and automatically, to any of 256 carriers across an 1100 MHz frequency range spanning multiple transponders, multiple carrier rates, multiple carrier coding rates, and multiple carrier modulations. This allows the most efficient allocation of bandwidth on the network, on any available timeslot on any available carrier frequency, for the most flexible and frequency-agile system available. D. Other Initial Enhancements In addition to the extend L-band IF interface and the DVB-S2 demodulator/decoder, the initial LinkWay S2 enhancements also included a much improved packet processing capability, more rugged packaging and a faster Ethernet interface. All of these items came as a result of ViaSat responding to our customer base, i.e., the WIN-T and SWAN operational networks. The Ethernet interface in LinkWay2100 was only 10BaseT which by today s standards was unacceptable (LinkWay S2 now supports 10/100BaseT). The more rugged packaging also came about from analyzing equipment deployed in the field and analyzing government requirements for the MD-1366 Enhanced Bandwidth Efficient Modem (EBEM) and the MD- 1377 Joint IP Modem. As a result modems in the field have extremely low repair rates, despite the COTS nature of the product. Lastly, the improved packet processing capability allows the modem to fully transmit and receive all data capable across the physical air interface. That means the only limitation to MF-TDMA performance is configuration of modulation and coding from a link budget standpoint, and not the processing power of the modem s internal processor. III. THE LINKWAY S2 - ADVANCED FEATURES The initial release of the LinkWay S2 used the legacy Convolutional/Reed-Solomon coded waveform of the LinkWay2100 specifically to facilitate the rollover of WIN-T networks from the LinkWay2100 hardware to the LinkWay S2 hardware. Any real advance in MF-TDMA performance had to wait until all terminals in a network were equipped with the newer LinkWay S2 modem. Today, in 2010, we have reached the tipping point, and LinkWay S2 network operators can now make use of all the features possible with this new hardware platform, including advanced turbo coding, 8PSK and BPSK modulation, extended symbol rate (i.e., 10 Msps), embedded government-specified TRANSEC (FIPS 140-2, Level 2, AES 256 included), improved networking features, and upcoming Communications-On-The-Move (COTM) capability [3]. A. New Waveform for MF-TDMA carriers In LinkWay S2 -pure networks (no LinkWay2100 units), LinkWay S2 can now use the new waveform. While the LinkWay2100 supported only QPSK modulation, the LinkWay S2 has added both 8PSK and BPSK modulation. Convolutional/Reed-Solomon coding is replaced by advanced standards-based turbo coding (DVB-RCS codec, i.e. same as the Joint IP Modem). Turbo coding provides quasi-error-free connections with minimal carrier power requirements. The addition of 8PSK modulation provides dramatically improved spectral efficiency. Combined with an improved, shorter TDMA preamble, LinkWay S2 is up to 40% more efficient than convolutional-encoded, Reed-Solomon systems, increasing throughput, reducing station size, and reducing satellite bandwidth requirements. The BPSK, rate 1/3 waveform allows operation of the LinkWay S2 from ultra-small (sub-meter) antennas with FCC/ITU off axis interference compliance. The maximum MF-TDMA carrier rate has been doubled, from 5 Msps to 10 Msps, allowing very high-speed mesh connections between LinkWay S2 equipped terminals. For example, using 10 Msps mesh MF-TDMA carriers, LinkWay S2 equipped terminals can stream high-definition video directly to each other at high speed (up to 15 Mbps) in real time in a single satellite hop no double-hop transfer through a hub is required, using precious satellite bandwidth twice and adding delay. With the new 10 Msps carrier rate, LinkWay S2 is the highest-speed MF-TDMA mesh modem available. The modulation and coding (modcode) for each direction of link between LinkWay S2 terminals can now be specified by the network operator. LinkWay S2 modems can now switch modcode on a burst-by-burst basis based on destination. This greatly enhances the operator s ability to tailor the modcode to suit the capabilities of the various types of terminals deployed in the network. For example, smaller disadvantaged terminals could use, BPSK, rate 1/3 for mesh communications amongst 2343

themselves or with fixed medium-sized stations, but switch to QPSK, rate 1/2 when they reach back to larger earth stations. This ability is key to including ultra-small terminals or mobile terminals in a LinkWay S2 network. The new waveform is summarized in Table 1. Table 1: LinkWay S2 Waveform Summary (SW Ver 9.1) Modulation Code Rate 8PSK Turbo 6/7 8PSK Turbo 4/5 8PSK Turbo 3/4 QPSK Turbo 6/7 QPSK Turbo 3/4 QPSK Turbo 2/3 QPSK Turbo 1/2 BPSK Turbo 1/3 MF-TDMA Carrier Rates (ksps) 312.5; 625; 1250; 2500; 5000; 10000 B. Embedded TRANSEC LinkWay S2 can now supports ViaSat s V3K-100 embedable TRANSEC module which is also used in the Joint IP Modem (JIPM) and is also the key technology in the TRANSEC module for Blue Force Tracking-2 (BFT-2) system [2]. This module provides an embedded FIPS 140-2, Level 2, AES-256 based TRANSEC capability, system ensuring secure connectivity and protecting sensitive communications. The LinkWay S2 TRANSEC module bulk encrypts both unicast and multicast MF-TDMA traffic bursts and all sytem control and signalling MF-TDMA bursts. The Transmission Encryption Key (TEK) is generated fom a Transmission Security Key (TSK) and front-panel-entered passphrase. The TSK is loaded through a front-panel connector using an Simple Key Loader (SKL). LinkWay S2 TRANSEC supports over-the-air re-keying, zeroizing, and authentication messages. Dummy MF-TDMA bursts are generated to frustrate traffic analysis. Using transportable and mobile SATCOM platforms equipped with LinkWay S2 modems, deployed Warfighters can communicate securely directly with other LinkWay S2 equipped sites. Based on the TRANSEC specification for the MD-1377 Joint IP Modem, LinkWay is the first widely deployed broadband MF- TDMA modem with government specified TRANSEC capability. C. Networking and Control Features The latest LinkWay S2 software version adds several enhancements to the networking and control features of the LinkWay system, including improvements in IP Quality-of- Service (QoS) and control server redundancy. 1) IP Quality of Service The LinkWay2100 and first release of LinkWay S2 supported a Differential Services (DiffServ) compatible IP Quality of Service (QoS) scheme with 6 transmit queues. In the older scheme, IP packets were prioritized for transmission on MF- TDMA bursts based on their DiffServ codepoint, either pre-set by a terrestrial-side router or set by LinkWay itself based on an operator-defined flow profile. IP packets would be prioritized and placed in one of six transmission queues, corresponding to the 6 basic DiffServ classes, descending from Expedite Forward to Best Effort in priority. Segments would be removed from the highest priority non-empty queue first. LinkWay S2 network operators reported that the 6-queue IP QoS scheme was insufficient for the all the traffic types that require classification and prioritization. Beginning in software release 8.3, LinkWay S2 added a 16-queue IP QoS scheme using Class- Based Weighted Fair Queuing (CBWFQ). With CBWFQ, segments are removed from queues based on an operator-set relative weight. This prevents queues from being starved and makes more efficient use of available bandwidth. 2) Network Control Redundancy LinkWay S2 terminals are controlled by a full-featured Network Control Center (NCC) workstation that manages TDMA network timing, synchronization, terminal acquisition, network configuration, and bandwidth management. The NCC also acts as the Network Management System (NMS) server. The NMS is a client-server system with an easy-to-use, secure web-based graphical interface. With this approach, a PC-based remote NMS client can securely access the NCC server from anywhere in the world (subject to security restrictions). Local and manual geographic redundancy was provided for the NCC to ensure reliable network operation. Automatic NCC server switch-over and automatic network recovery was available with local NCC server redundancy, but geographic NCC redundancy required operator intervention. LinkWay S2 now supports automatic geographic NCC server redundancy for enhanced network reliability. LinkWay S2 NCC servers can be placed at geographically diverse satellite terminals and will automatically detect failure of the distant NCC and automatically assume control of the network. This feature is particularly valuable for LinkWay S2 networks where the NCC servers are operated from forward-based in-theater satellite terminals, such as those LinkWay S2 networks operated by the US Marine Corps. IV. LINKWAY S2 COMMUNICATIONS ON THE MOVE (COTM) As mentioned previously, LinkWay S2 now supports a BPSK, rate 1/3 waveform which allows communications with submeter satellite terminals while maintaining regulatory compliance. This waveform (along with extensions to add direct sequence spreading) opens up the possibility of a true COTM capability for LinkWay S2. Even without all specific modifications for COTM, initial tests and demonstrations using this BPSK waveform have been impressive. The low Eb/No BPSK, rate 1/3 waveform has already been operated with vehicle-mounted RaySat SR2000 and TracStar 0.4m effective 2344

aperture antennas. Streaming full-duplex IP video at 512 kbps has been demonstrated between a fixed 2.4m station and a vehicle equipped with a mobile antenna using 2.5Msps carriers at BPSK rate 1/3 (video transmitted from vehicle). Communications were maintained during high-speed vehicle motion. Notable was the demonstration of mesh communications while on-the-move, in particular that the OTM terminal communicated with two separate 2.4m terminals simultaneously in the same network. Using the capability that LinkWay has to multicast with a single transmission to multiple users, it can be seen the value that this might have for military operations in dissemination of time critical information from OTM platforms. Note that a 10 Msps carrier could provide close to 3 Mbps useable IP throughput, for truly highspeed COTM. An upcoming LinkWay S2 release (Fall 2010) will provide a complete COTM ability using the BPSK, rate 1/3 pseudo-spread waveform, with enhancements for communication between the mobile LinkWay S2 modem and the mobile antenna controller unit (ACU) and including a specific mobile profile for the LinkWay S2 modem that provides blockage mitigation and transmit mute for exclusion zones. The next LinkWay S2 release will provide a true spread spectrum waveform (early 2011). An initial 2X or 4X spread BPSK, rate 1/3, waveform is possible on the current LinkWay S2 hardware platform and is planned for this release. This will lower the power spectral density (PSD) requirements and allow use of LinkWay S2 COTM on a greater range of satellites, including disadvantaged satellites. With 4x spreading, the BPSK r=1/3 waveform and the low required Eb/No, the vast majority of satellites will sport COTM terminals in a regulatory compliant manner. In addition, two more low Eb/No waveforms will introduced, BPSK, rate 1/2, and BPSK, rate 2/3 to allow for higher throughput when the satellites and network configuration will allow it. LinkWay S2 network operators will have the flexibility to choose either a spread waveform or low Eb/No waveform for their OTM terminals on a link-by-link basis. VI. REFERENCES [1] Pontano, Benjamin, LinkWay TM for 21 st Century Military Communications, Proc. IEEE MILCOM, 2000, October 2000. [2] BFT-2 Award Announcement Press Release, http://www.viasat.com/news/viasat-selected-us-armysupply-next-generation-blue-force-tracking, 21 Jul 2010. [3] LinkWay S2 product description, http://www.viasat.com/governmentcommunications/high-speed-modems. V. SUMMARY In this paper, the evolution of the LinkWay family of MF- TDMA mesh modems has been presented. From the LINKWAY 2000, to the LinkWay2100, to the current LinkWay S2, the modem has become smaller, lighter, faster and more capable. From an initial design that was restricted to a 36MHz frequency-hopping range and operated only on large earth stations, we arrive at today s LinkWay S2 that offers a 1.1GHz frequency-hopping range and can even operate with a sub-meter COTM terminal. The next generation LinkWay, being planned now, will use a new hardware platform that further extends LinkWay S2 s capabilities, with MF-TDMA symbols rates perhaps up to 20 Msps and a smaller form factor for easer terminal integration. Furthermore, this next generation hardware will also support waveforms for access to Ka-band high capacity satellites (HCS), such as ViaSat-1, Eutelsat s Ka-SAT, and YahSat s satellites. 2345