Wideband On-The-Move Satellite Communications Ground Terminal Frankie S. Sutton, Jae H. Kim, Dave F. Diener, Mike P. Pounds, and Robert E. Welling Phantom Works The Boeing Company P.O. Box 3999, MS 85-13, Seattle, WA 98124-2499 (253) 773-1502 (TEL), 773-6624 (FAX) frankie.s.sutton@boeing.com ABSTRACT Boeing has recently demonstrated a mobile satellite ground terminal that will support satellite communications on-the-move as well as on-thehalt. The terminal (known as Mobile Wideband Testbed ) is a mobile satellite communications ground terminal implemented on a High Mobility Multipurpose Wheeled Vehicle. The testbed is capable of simulating and testing a wide range of networks (e.g., ATM, IP) and protocols (e.g., TCP, mobile IP) over satellite links and wireless LANs. This paper will describe testbed implementation details, operation, and advanced features. 1. INTRODUCTION To investigate On-The-Move (OTM) as well as On-The-Halt (OTH) Satellite communications (SATCOM), Boeing has been working on the wideband mobile satellite communications ground terminal on a High Mobility Multipurpose Wheeled Vehicle (HMMWV), a.k.a., Mobile Wideband Testbed, using Boeing s Transmit/Receive (Tx/Rx) Phased-Array Antennas (PAAs). The issues under investigation related to SATCOM include the effects of disadvantaged links associated with bandwidth and latency asymmetries, and the implementation of protocols for reliable transmission control and fast link recovery from the channel blockage in a variety of military and commercial environments. Applications involve providing a full range of information services to users on-board mobile platforms on a global scale, and battlefield networks serving a combination of mobile and fixed elements in-theater. This paper describes the implementation details, operation, and advanced features of the wideband mobile HMMWV testbed. 2. WIDEBAND OTM SATCOM Wideband OTM SATCOM imposes three main requirements: 1. The ability to establish full duplex Radio Frequency (RF) communications with a satellite using frequencies in the microwave range (e.g., Ku and Ka), which in turn requires a directional antenna or agile beam antenna. 2. The ability to maintain the RF connection while the vehicle is moving by accurately pointing the antenna/antenna beam. 3. The ability to establish a data link using appropriate protocols and quickly recover that link when it is disrupted. HMMWV Mobile Platform OTM SATCOM presupposes the existence of a mobile vehicle capable of establishing linkage with a satellite and maintaining that linkage while moving. A Network Operating Center (NOC) is also required to route data to and from legacy networks to the satellite or other relay platform. Boeing has established a NOC at Tukwilla, Washington, and a ground site at Brewster, Washington. The ground site has the capability of receiving and transmitting both video and data at the same time. The establishment of linkage with a satellite and the maintenance of that linkage involves precise pointing of receive and transmit antennas (or antenna beams). Precise pointing of an antenna further requires knowledge of both the location of the vehicle and the location of the target satellite. Since the location of the vehicle changes as it moves, it is necessary to
periodically update the antenna/antenna beam pointing mechanism so that it can adjust the pointing angle. The ability of the antenna pointing mechanism to keep up with the dynamics of vehicle motion depends on (1) the rate of updates to the pointing mechanism and (2) the rate at which the mechanism can adjust the antenna pointing angles. The inputs to the pointing mechanism usually come from an Inertial Reference Unit (IRU), which has the ability to sense both linear and angular accelerations. The IRU may also receive assistance from other devices, such as the Global Positioning System (GPS) or other measurement devices, to minimize drift and improve accuracy. Communications Sequence Digital communications can be described with reference to the Open Systems Interconnection (OSI) model. The sequence of events required for the establishment of a digital satellite communications link includes: signal lockup (physical layer), RF protocol lockup (link layer), and digital protocol lockup (transport layer). Signal lockup occurs when the antenna is pointed in the correct direction. RF protocol lockup occurs when the electronic signals synchronize. Digital protocol lockup occurs when recognizable packets are being transmitted and received. If the communications link is disrupted, the communications sequence must be repeated. How rapidly this occurs depends on steering of the antenna, the RF protocol, and the digital protocol. How quickly the RF protocol is reestablished depends, in turn, on the components in the RF suite such as the modem. Directional antennas are necessary to establish the communications link and maintain it while on the move. 3. MOBILE WIDEBAND TESTBED The testbed was designed for a self-contained, mobile communications system, capable of meeting many communications requirements as configured. It was based on an open architecture consisting of Commercial-Off-The-Shelf (COTS) components for both hardware and software. It is capable of using standard communications protocols (e.g., TCP and UDP) and integrating them with legacy protocols (e.g., Link-16, Link- 11, TADIL-J, EPLRS, CEC) where necessary for satellite communications. The legacy protocols can be supported through the provision of onboard computers with appropriate interface hardware, running special gateway applications such as the Rosetta Technology from ANZUS Inc. The open architecture design facilitates rapid reconfiguration to meet special needs and integrate new technology. Physical Layout The Mobile Wideband Testbed is incorporated in a Gichner S-710 mobile shelter, mountable on a High Mobility Multipurpose Wheeled Vehicle (HMMWV) as shown in Figure 1. The shelter is completely self-contained in terms of power and air conditioning. An environmental control unit is mounted on the front of the shelter, and a 15 kw, 115 V, 60 Hz generator is mounted in a tunnel at the front of the shelter. The tunnel also incorporates the ability to connect commercial power when available. The communications equipment is mounted in shock-isolated racks installed in the S-710 shelter. Figure 1: Wideband Mobile Testbed RF Suite and Communications Equipment The RF Suite (Figure 2) incorporates agile beam PAAs that allow for the establishment of an asymmetrical Wide Area Network (WAN) using commercial Ku-band geosynchronous satellites. Other antennas, either conventional or phased array antennas, can also be mounted to establish links with other relay platforms such as unmanned aerial vehicles. The communications approach is similar to that used with Connexion by Boeing SM and is being developed in the same 2
laboratory. The receive and transmit antennas are nominally capable of 6 Mbps and 256 kbps throughput, respectively, although actual throughput depends on several factors such as weather, lookup angle, the forward error correction mechanism, and the satellite transponder in use. The IRU is provided to allow tracking of target relay platforms while the vehicle is moving. 11.7-12.2 GHz Rx PA Antenna 14.0-14.5 GHz Tx PA Antenna Server, and miscellaneous software such as Voice over IP (VoIP) and NetMeeting. The configuration is mix and match, as required. The current RF suite configuration is optimized for operation at Ku-band frequencies (11.7 to 12.2 GHz for receive, 14.0 to 14.5 GHz for transmit). However, the testbed can be easily reconfigured for additional communication systems. It is also possible to use multiple simultaneous systems to transmit and receive information in order to increase the overall bandwidth of the system. Power Rx Power Supply Inertial Ref. Unit Nav. Data Power PC Power Ku-Band Linear Polar Tx Power Converter Supply Ku-Band Low Noise Block L-Band Phased Array ler L-Band Antenna Switch Unit L-Band Router PowerAmp/ Upconverter Airborne Modem Intgr. Rx. Assy. Decoder RS449 Data Ethernet Ethernet Hub Figure 2: RF Suite RS232 RS449 Data Ethernet Ku-Band 70MHz External ATM Input ATM Link Processing Remove/insert pad cells RF Bit rate matching Suite Framing ATM Switch Byte interleaving FEC WaveLAN System 11 Mbps Server Bandwidth ws ws ws Buffer Scheduler Mail Server ws ws External IP Input Shelter LAN Hub IP Link Router Processing Framing Byte interleaving FEC Figure 3: Communications Equipment The communications equipment (Figure 3) includes servers to provide network services, an Ethernet Hub/Router, IEEE 802.11B wireless LAN access points, and mobile computers (e.g., laptop, handheld PCs) to allow for the establishment of a mobile LAN. Equipment is provided for handling either IP or ATM traffic. Other communications equipment includes workstations and flat panel displays. The operating system software is based on Microsoft NT 4.0 and Windows 2000 server (or workstation) as appropriate. Key components of this software may include: Internet Information Server (IIS), Microsoft Exchange Server, Point-to- Point Tunneling Protocol (PPTP) server, Proxy Ku-band Phased Array Antennas A Ku-band PAA system is currently installed as part of the Mobile Wideband Testbed and is being used for baseline testing. It includes a 1515-element receive antenna and a 256- element transmit antenna. The communications suite provides downlink capabilities of 6 Mbps (1 Mbps Data, 5 Mbps Streaming Video) and uplink data rates close to 256 kbps. Raw uplink data rates greater than 300 kbps have been reached using the current transmit antenna with a different spread spectrum modem. The uplink communication channel contains ½ Viterbi encoding, encapsulated within Reed-Solomon 3
forward error correction to minimize data losses due to environmental conditions such as weather condition and vehicle motion. The waveform is direct-sequence spread spectrum, spread across 25 MHz. This allows low observable transmission. 4. ADVANCED FEATURES Advanced features are also being investigated such as link asymmetry and fast link recovery protocol, high data rate antenna systems, fast recovery MODEM, and vehicle motion impact. Link Asymmetry and Fast Link Recovery Extensive studies of link asymmetry (where the reverse link is the bottleneck) show on the three immediate impacts on TCP algorithms [1-3]: a. Forward transmission burstiness due to ACK losses in the reverse link. b. Slow congestion window growth because only a fraction of ACKs generated by the receiver actually reach the sender. c. Reduced effectiveness of the fast-retransmit algorithm because it depends on the reception of at least 3 ACKs to be triggered. A potential solution with minimal modifications of the TCP stack can be to dynamically adjust the number of packets (acknowledged) per ACK (PPA). The mechanism is based on the receiver estimate of the current congestion window size of the sender (using one of several possible techniques) and adjusting the receiver s PPA based on two predefined configuration parameters; namely, a minimum number of ACKs per window and a maximum PPA. To recover a link and data quickly from signal blockages due to man-made objects, terrain, and foliage, the fast link recovery protocol is being investigated. For sufficiently small downtimes and large TCP buffer size, the TCP transmission rate may be maintained until the satellite link connection is reestablished. However, depending on the downtime for the satellite link, it will be necessary to have a transport-layer slowdown mechanism, reducing the TCP source transmission rate without running into timeouts. High Data Rate SATCOM Antenna Directional antennas come in a variety of forms, but in general they fall into two classes: mechanically steered and electronically steered (or agile beam). There are exceptions, however, which exhibit the characteristics of both classes. Such an exception is the Boeing s Mechanically Augmented Phased Array (MAPA) antenna. Electronically steered phased array antennas are the preferred solution for high-data-rate (HDR) OTM SATCOM, especially for mobile ground units. Phased arrays offer low profile, multiple simultaneous beam capability, extremely agile pointing, tracking, and little to no maintenance. The phased arrays for ground vehicles may be fielded in either a pure, planar electronic mode (Figure 4) or a hybrid, mechanically assisted mode. The main advantages of the planar implementation include very good scan/track agility over the entire field of regard, extremely low profile, and no moving parts. Boeing phased arrays offer the largest cone of any arrays built to date: up to 140 deg. about antenna normal, with near theoretical, cos θ, scan loss. Scan from vertical to below the horizon can be achieved with the MAPA antenna. Figure 4: Planar Phased Array Antenna installed on HMMWV 4
An additional benefit of the MAPA configuration is the potential for reduced antenna size due to a reduction in the required range of electronic scan. The MAPA retains (1) the electronic tracking features of the planar array, (2) the ability to track multiple satellites, and (3) the agile electronic scan and track (albeit over a restricted field of regard), but does require moving parts. The MAPA configuration also has a considerably higher profile than a planar array. Practical frequencies for OTM SATCOM applications using high-gain antennas lie between SHF (7 GHz) and EHF (44 GHz). High-gain antennas are too large to be generally feasible for OTM SATCOM at lower frequencies; frequencies above EHF are generally not useful for OTM communications, and certainly not OTM SATCOM due to very high atmospheric attenuation. The most immediate opportunity for high-datarate OTM SATCOM to small mobile platforms such as HMMWVs lies at Ku-band (10.7 to 12.2 GHz, forward; 14.0 to 14.5 GHz, reverse). A Kuband infrastructure, capable of supporting HDR communications to small mobile ground platforms and equipped with practical-size antennas, covers most of the world s landmass. Receive PAAs are in commercial production and in service on a number of private and military platforms operating around the world. Prototype transmit phased array antennas have been successfully demonstrated. Future Phased Array Antennas Ku-band transmit phased array antennas capable of providing T1 data rates are being developed at Boeing. In conjunction with a capability with multiple satellite relay systems, other antennas such as Ka-band and military X-band antennas as well as new Ku-band antennas are also under development. The phased array antennas have been demonstrated in other applications, with a proven, extensive operational track record. The development work is ongoing and these antennas may also be mounted on the Mobile Wideband Testbed for multiple satellite interfaces. Fast Recovery Modems Boeing is working with L3 Communications on the development of a spread-spectrum modem for use with the phased array system. The version of this modem on the Mobile Wideband Testbed is currently configured to support data rates of 128 kbps. The current design may be reconfigured to support higher data rates. Recovery by the receiver after drop-outs is automatic. Due to the spread spectrum coding, the modem is used in a Code Division Multiple Access (CDMA) scheme. Presently, the modems support five codes, but can be configured to support more. Testing will be necessary to determine the specific number of codes the system can support. 5. SUMMARY Wideband satellite communications OTM/OTH ground terminal (a.k.a., Mobile Wideband Testbed ) was implemented on a HMMWV platform. The testbed is capable of simulating and testing a wide range of networks and protocols over satellite links and wireless LANs. This testbed implementation details, operation, and advanced features were described. ACKNOWLEDGMENTS The authors would like to thank Geoffrey White and his Phased Array Antenna group for their support of the phased array antennas. This work was supported by Boeing Independent R&D and Capital Programs. REFERENCES [1] Nasir Ghani and Sudhir Dixit, TCP/IP Enhancements for Satellite Networks, IEEE Comm. Magazine, p.64, July 1999. [2] H. Balakrishnan, V. Padmanabhan, and R. Katz, The Effects of Asymmetry on TCP Performance, Mobile Networks & Appl., vol.4, p.219, 1999. [3] M. Albuquerque, J.H. Kim, and S. Roy, Effects of Packet Size on TCP-Reno Performance over Lossy, Congested Links, MILCOM 2001, Oct. 2001. 5