Autonomous Tactical Communications

Transcription

1 Autonomous Tactical Communications Possibilities and Problems Lars Ahlin Jens Zander Div. of Communication Systems, Radio Communication Systems Department of Command and Dept. of Signals, Sensors and Systems Control Warfare Technology KTH FOA Summary On the battlefield of the future, more and more information will be available for making decisions on a tactical level, provided that this information can be dispersed rapidly and accurately. Sophisticated electronic equipment for communication, information processing and for collection of sensor data are becoming light-weight, small and inexpensive. As a consequence, advanced tactical decision support that now is limited to advanced platforms (e.g. combat aircrafts) will become available at a much lower level, maybe down to the individual soldier. In such a scenario, the number of communicating entities is one or several orders of magnitude larger than in todays tactical systems. Establishing reliable wireless communications in such a large group constitutes a considerable engineering challenge. In this paper we investigate the specific engineering challenges and the fundamental limitations of such low level, autonomous communication systems. Our conclusions are that mainly distributed computing complexity, device power consumption and available bandwidth constitute the fundamental problems. 1. Introduction The aim of the paper is to illustrate the requirements that the future battlefield will set on the communications networks. Based on the concept the digital battlefield, we will present some ideas concerning a future typical battle scenario and draw some conclusions based on possible techniques and corresponding requirements of a communication system. The warfare of tomorrow will surely be quite different compared to the the battle of today. Military encounters will be fragmentary and mobile and the speed of the battle will increase. New type of sensors will be used, together with advanced decision support facilities. The number of weapon platforms will be smaller but more qualified. There will be an integration between different type of forces and strong demands of cooperation between army, navy and airborne forces. It will be very important for the forces to have the same understandings of the actual situation. The forces will need faster and correct information to fulfill the mission. Information will flow between different kinds of platforms which will put new demands on the communication systems. The requirement of capacity will raise to a very high level, which, together with the need of jamming protection and stealth properties of the radio signals, will give rise to extreme requirement of available bandwidth.

2 2.The digital battlefield The future battle is often illustrated by looking at the digital battlefield. But, what is the digital battlefield? In USA, the development of a so called digital brigade is going on, with the main purpose to give all units a common view (hopefully correct) of the actual battlefield situation [1]. With the help of, basically, commercial available hard- and software, a number of data bases will be tied together and connected to different units through digital communications. What we refer to when discussing the digital battlefield is rather a future scenario, where data from different advanced sensors are gathered and processed and compiled for target identifying and classification by using multi sensor data analysis and data fusion. A lot of new sensor equipment will be used, such as laser radar, low frequency radar, IR, etc. There will surely be a need of distributed data bases. Methods for target handover will be very important for combined arms task forces. A co-ordination of reconnaissance and weapon activities will give rise to a lot of information transmissions over the battlefield within an electronic warfare environment. To meet the operational requirements, the need of robust and reliable communications is very essential. A main question is; what requirements will the changes of the battlefield give rise to regarding the flow of information over the battlefield? Will it be possible to solve the problems or is communication the major bottleneck in the concept? It is obvious that we will see a trend towards emphasis on information instead of weapon. We are now talking about information warfare, command and control warfare and electronic warfare. The part that controls the electromagnetic spectra will have a great advantage in the battles of the future. 3. Problems and possibilities Tactical communication systems of today are geared to rather wide area coverage and moderate bandwidth demands providing services like voice communications and low data rate applications. As discussed above, the tactical telecommunication services required in the battlefield of tomorrow might be of a completely different breed offering advanced multimedia applications provided by means of low-power, personal, wearable equipment [2]. These services require (at least instantaneously) high bandwidths. A wireless tactical system should be transparent to the user and thus highly integrated with the fixed networks in the battlefield. Clearly such systems are not found in the marketplace today. Several problems of varying degree of difficulty have to be solved. Some of these problems are of technological nature, where we by extrapolation of the current development in technology can deduct that only given time these problems may be solved. An example of this is device technology, which is expected to make even more progress in the coming years, why size reduction and complex functionality of personal devices is expected not to constitute a problem in the near future. Other technological problems include display technology. There are, however, also seemingly more fundamental problems limiting the design of high capacity tactical wireless systems, which are the concern of this section. The first three fundamental problems are associated with the wireless communications aspect of an autonomous tactical system: 1) Spectrum shortage 2) Device power consumption 3) Range and coverage These three items are closely connected. We will demonstrate that we may overcome any two of these limitations but not all three together. High bandwidth requirements and large user populations will clearly demand large portions of the frequency spectrum. This natural resource is finite but, fortunately, it can be reused as many times as we like, provided the

3 geographical distance between wireless terminals using the same portion of the spectrum is large enough. In an autonomous network with a large number of users, data messages are usually transmitted only over short distances, allowing a frequent reuse of the spectrum. Long distance transmissions may still be feasible using multihop store-and-forward techniques [3,4]. Device power consumption is another problem since supply technology is not expected to make substantial progress (i.e. more than 1-2 orders of magnitude) in the next decade, why power consumption has to be limited. As is illustrated by figure 1 and table 1, we see that the power consumption is proportional to the required data rates and grows faster than the square of the range. However, in a high user density battlefield scenario, typical transmission distances can be assumed to be rather small (< m) making substantially higher data rates (compared to todays system) feasible. Short distances also allow for a short reuse distance allowing for many terminals to share the spectrum. As this examples show, we can handle power consumption and spectrum shortage, but at the expense of low range. Long range and bandwidth efficient signalling requires a high transmitter power. These and further statements demonstrate the interrelations between the limitations 1) - 3). Obviously, there will be situations when inter-terminal distances are large, the spectrum is locally overloaded or the transmissions are subject to hostile jamming. In these situations, high data rates cannot be sustained in all links. Future systems should however be capable of handling these situations by adapting and lowering the user data rates by introducing more error protection and spectrally spreading the signals. Interfaces and applications should be aware of this and should adapt in turn in order to maintain the functionality of the communication applications, thus providing seamless services to the user. Examples of such applications are hiearchical transmission schemes such as quickly providing pictures at a lower resolution and gradually adding details. Datarate Range 30 m 100 m 1 km 20 kbit/s < 1mW < 1mW 0.3 W 1 Mbit/s < 1mW 3 mw 30 W 100 Mbit/s 3 mw 0.3 W 3000 W Table 1: Example of required transmitter power as function of distance and data rate for a typical personal communication scenario. Another critical problem may be denoted by 4) Distributed networks and mobility management In large scale autonomous networks, the sheer numbers of mobile devices, will require efficient and reliable distributed network functions. In current telecom networks, centralized decision making is used in order to assign spectrum and to keep track of all mobiles. Even though the number of terminals may be large, the decision making process is comparatively straigth-forward and thus inherently reliable. Physically however, this approach results in vulnerable networks since without the central node, the network ceases to function. Another drawback is the large volumes of control traffic that has to flow back and forth from the terminals to the central controller, and in the wireless tactical environment, exposing the terminals and consuming valuable spectrum resources. A distributed, autonomous approach would avoid these problems and create a physically very reliable infrastructure, tolerant both to physical damage and hostile jamming [5]. The drawback is the large complexity of the resulting autonomous systems with potential stabiltity problems. Research results in this area

4 are promising but regarding to stability issues, still a lot of research remains to be done in order to fully utilize the potential of the distributed networks. 4. Conclusions The battlefield of tomorrow is characterized by a high degree of mobility. In addition, tactical decision at all levels, including very low levels (i.e. the individual soldier) will be supported by large amounts of information. We believe that wireless communication systems are capable of providing these service also in hostile jamming environments. The key problem areas where to focus future research were identified. Low power equipment technology and lower power system design is one area, distributed systems design is another. 5. References [1] Pettersson G, The Digital Battlefield, proposals for further research activities, FOA report FOA-R SE, Feb 1995 (in Swedish). [2] Smailagic, A., Sieworek, D., Modalities of Interaction with CMU Wearable Computers, IEEE Personal Communications, vol 3, no 1, Feb [3] Johnson, D.B, Maltz, D.A., Protocols for Adaptive Wireless Mobile Networking, IEEE Personal Communications, vol 3, no 1, Feb [4] Kahn et al., Advances in Packet Radio Technology, Proc IEEE, vol 66, Nov [5] Zander, J., "Jamming in Slotted ALOHA Multihop Packet Radio Networks", IEEE Trans. Comm, vol 39, no 10, October Rel Terminal power consumption BW=4 BW=2 BW= ,5 1 1,5 2 Range Fig 1. Required terminal (transmitter) power as function of transmission range for different data rates/bandwidths (BW)

5 Jens Zander received the M.S degree in Electrical Engineering and the Ph.D. Degree from Linköping University, Sweden, in 1979 and 1985 respectively. From 1985 to 1989 he was a partner of SECTRA, an R&D consultant firm. His work here was mainly concerned with Aerospace and Defence applications. In 1989 he was appointed professor and head of the Radio Communication Systems Laboratory at the Royal Institute of Technology, Stockholm, Sweden. Since 1992 he also serves as Senior Scientific Advisor (associate research manager) to the National Defence Research Establishment (FOA) and on the board of TERACOM, the Swedish National broadcasting operator. Dr Zander has published numerous papers in the field of Radio Communication, in particular on resource management aspects of Personal Communication Systems. Dr Zander was the recipient of the IEEE VTS "Jack Neubauer Award" for best systems paper in He has co-authored two textbooks in Digital Radio Communication Systems, which are now the standard textbook in this subject in Swedish Universities. He is an adjoint member of the Swedish URSI committee, sections C and G/H. Lars Ahlin received the M.S. degree in Electrical Engineering from the Royal Institute of Technology, Stockholm in He worked with digital mobile radio communication at Linköping University. In 1986 he joined the National Defence Research Establishment (FOA). He has since then worked with spread-spectrum communications, error correcting codes and LPI-systems. Since 1994, he is the Head of the Division of Communication Systems. He has co-authored two textbooks in Digital Radio Communication Systems, which are now the standard textbook in this subject in Swedish Universities. He is an adjoint member of the Swedish URSI committee, sections C.