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Energy-Efficient Technologies for the Dismounted Soldier 6 Networks, Protocols, and Operations Network architecture, communications and computing protocols, and operational doctrine will set major constraints on the power requirements of future soldier systems and the digitized battlefield. This chapter addresses those constraints and considers technological approaches for reducing power requirements for future battlefield networks, protocols, and operations. Extending wireless communications to individual soldiers will increase the soldier's effectiveness dramatically. Wireless communications account for a major share of the energy used in the Land Warrior system and are likely to dominate power requirements in the future (see Chapter 5). Fundamental limitations, such as the minimum energy needed to transmit a bit of information over a required distance, will place difficult constraints on reducing power for wireless communications. Soldier communications will include access to databases, transfer of images and, very likely, video displays. Some of the technology areas under investigation for the future battlefield include personal communications systems (PCS), satellite-based communications systems, and direct broadcast systems. The Army intends to make maximum use of COTS (commercial off-the-shelf) technology to capitalize on the large market and evolutionary trends in the commercial sector (Leiner et al., 1996). Table 6-1 lists types of wireless communications and required transmission rates (Rapeli, 1995). Clearly, video displays and image-intensive access to databases will place the greatest demands on transmission rates and power. Commercial technology often advances more rapidly than equipment designed for military use partly because the huge volumes of commercial markets can defray R&D costs (Sass and Gorr, 1995). Commercial wireless communications and personal computers are cases in point. Military adaptations of commercial cellular and PCS technologies could reduce power requirements for the dismounted soldier and enable the Army to keep pace with commercial technology more economically. Several large scale commercial satellite-based PCS networks will be available for consideration as military assets in the next few years that could support widespread operations by dismounted soldiers with limited power sources. Direct broadcast satellite (DBS) systems should use protocols consistent with low power soldier system operation. The Army could leverage emerging
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Energy-Efficient Technologies for the Dismounted Soldier TABLE 6-1 Required Transmission Rates Traffic Required Throughput (kbps) Speech 8–32 Short messages 1.2–9.6 Electronic mail 1.2–64 Remote control 1.2–9.6 Video 64–384 Database access 2.4–768 commercial capabilities, such as Direct-PC, a commercial satellite system that provides unidirectional data communications at rates of several hundred kilobits/second to users with small (about 2 foot diameter) antenna terminals. The Army is also considering ''pseudo-satellite" repeater concepts using unmanned aerial vehicles (UAVs). But their economical introduction will depend on synergy with developing applications for military reconnaissance. WIRELESS TRANSMISSION TECHNIQUES AND LIMITATIONS The wireless channel that provides communications among soldiers on the battlefield will nearly always be operating in a hostile environment. Communication must be established in the presence of hills, buildings, and foliage, and interference from both hostile and friendly sources will add to the difficulty. Many techniques can be used to make the best of this environment and maximize the ability to communicate reliably. These techniques apply both to the design of individual radio links and to the design of network architectures and methods of sharing the same frequency spectrum (Pottie, 1995). The wireless channel is subject to wide variations in quality as users move (even a few inches) and as other users communicate over the shared spectrum. Mitigation techniques involve either averaging over these variations or attempting to avoid them. Error correction coding, time interleaving, and direct-sequence code division multiple access (CDMA) are examples of averaging over interference and channel variations. Dynamic channel allocation and dynamic power allocation are examples of techniques for avoiding interference. All of these approaches could be used for wireless networks on the battlefield. Most techniques that increase the capacity of the wireless channel do so at the expense of computational or hardware complexity. Incorporation of these techniques will significantly increase the overall capacity of the network (that is, the overall amount of information that can be transmitted among many potentially
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Energy-Efficient Technologies for the Dismounted Soldier interfering users). However, there are fundamental limits on the minimum amount of energy required to send a given amount of information between two points. The remainder of this section discusses these limits. The energy requirements of the RF (radio frequency) portion of a radio communications system will not follow the rapid downward trend that characterizes the energy requirements of the silicon devices and architectures discussed in Chapter 4. Bounds on DC-to-RF conversion efficiency and information channel capacity are discussed by Forney et al., (1984). The overall DC-to-RF conversion efficiency of commercial cellular and PCS handsets is between 10 and 20 percent. Improvements in RF transistors may raise this to 50 percent in five to ten years, but the fundamental limit of 100 percent conversion efficiency is immutable. Thus, a realistic upper bound on the achievable transmitter efficiency is at most a factor of four over current technology (perhaps 60 percent overall). The required signal-to-noise ratio (SNR) for simple modulation/demodulation techniques, such as quadrature phase-shift keying (QPSK) with coherent demodulation (i.e., tracking the received signal phase, the most robust demodulation method) is around 10 dB, for an error rate of about 10-2. With advanced error correction techniques, such as trellis coding, turbo codes, or low rate convolutional coding, a similar error rate for an SNR of around 6 dB can be achieved. These SNR values are realistic minima for acceptable performance because the error rate increases very rapidly as SNR decreases. The Shannon bound on Eb/No (the ratio of energy per information bit to noise spectral density, which is very close in magnitude to SNR) is -1.6 dB; that is, a zero error rate is impossible for SNR values below around -1.6 dB. The difference between the simple technology of today and the almost infinitely complex technology is then only around 10 dB. Practically speaking, even the extremely complex signal processing that will be available in 2015 will probably not permit SNRs below 1 or 2 dB for useful error rates; thus, energy requirements will be reduced, at most, by a factor of five. Together with the previous factor of four increase in transmitter efficiency, the achievable energy efficiency of state of the art wireless information transmission will increase, at most, by a factor of 20 in the future. Nevertheless, highly advanced digital signal processing techniques will significantly improve the performance of wireless communications systems. Many of today's systems (cellular and PCS systems, for instance) are severely limited by interference. In other words, the noise level at the receiver rises above the inherent thermal noise floor because of interference from other cochannel users. Advanced signal processing techniques, such as multi-user detection and interference cancellation, may significantly reduce this kind of interference. They cannot reduce the underlying thermal noise floor, however, they can ameliorate the increasing levels of self-interference as the existing spectrum is used more intensively. These techniques will also be effective in battlefield applications where intentional jamming can contribute to the overall interference level.
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Energy-Efficient Technologies for the Dismounted Soldier Adaptive arrays are also useful for minimizing the effect of cochannel interference or intentional jamming and enhancing collection of the arriving signal. Adaptive arrays are useful only at higher frequencies (above 1 GHz), where electromagnetic wavelengths are short enough to permit several antenna elements to fit within the size constraints of soldier-carried equipment. Advances in low-noise receiver front-end amplifiers can make modest improvements in the overall receiver noise floor, but manmade noise in the high frequency (HF, 3 to 30 MHz) and very high frequency (VHF, 30 to 300 MHz) bands, the frequencies used for military communications, and thermal noise from the warm earth will limit these improvements to only a few dB. Losses in the radio propagation environment will impose fundamental limits on the energy needed to communicate a message. The necessarily low and small antennas available to the dismounted soldier will contribute to these losses. Small antennas are more inefficient at lower frequencies (below around 100 MHz). Shadowing by buildings, trees, and terrain at high frequencies and cancellation from ground reflections at lower frequencies will also create significant problems. At higher frequencies (above 1 GHz), adaptive arrays promise to maximize the amount of signal collected at the soldier receiver. The characteristics of the propagation environment will limit improvements and determine the amount of signal processing needed to attain these improvements. At the present time, the interaction between antenna and medium characteristics is not well understood, and research in this area may lead to significant gains in overall energy efficiency. The power to maintain a 16-kilobit-per-second (kbs) link can be computed as a function of frequency using available propagation models (Tables 6-2 and 6-3). Table 6-2 shows results for a frequency of 75 MHz (Federal Communications Commission, 1964). Table 6-3 shows similar results for a frequency of 1.5 GHz (Devasirvatham et al., 1993). The tables assume that there is no jamming signal interference. TABLE 6-2 Transmitter Power Needed to Maintain 16-Kilobit-Per-Second Link at 75 MHz Communication Distance (km) Antenna Height (m) Required Transmitter Power (W) Required Energy (Wh/Mb) 1.5 1 0.6 0.010 1.5 5 0.025 0.00043 5 1 60 1.04 5 5 2.5 0.04 15 1 6000 104 15 5 250 4.34 Source: Federal Communications Commission, 1964.
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Energy-Efficient Technologies for the Dismounted Soldier TABLE 6-3 Transmitter Power Needed to Maintain 16-Kilobit-Per-Second Link at 1.5 GHz Communication Distance (km) Antenna Height (m) Suburban Residential Environment (inside/outside) Moderately Treed Environment Transmitter Power (W) Energy (Wh/Mb) Transmitter Power (W) Energy (Wh/Mb) 0.5 1 2 0.035 0.0001 0.0000017 0.5 5 0.08 0.0014 0.0000025 0.000000043 1.0 1 50 0.87 0.2 0.0035 1.0 5 2 0.03 0.008 0.00014 5 1 100,000 1700 > 100,000 >1700 5 5 4000 69 > 100,000 > 1700 Source: Devasirvatham et al., 1993. In both tables a "soldier" antenna height of 1 meter and a required SNR of 10 dB are assumed. At 75 MHz, a noise floor of 10 dB above the room temperature thermal level accounts for manmade noise; an antenna gain of -3 dB (with respect to an isotropic antenna) is assumed. At 1.5 GHz, a noise floor of 3 dB above thermal level is assumed. These tables suggest that higher frequencies may be more appropriate for short distance intrasquad communications, but that the VHF band now used may still be appropriate for intersquad communications. At 1.5 GHz, the significant limitations that manmade obstructions impose on wireless communications are apparent. The figures in Table 6-3 also suggest the potential benefits in terms of energy consumption of multihop system architectures, which are described later in this chapter. LAND WARRIOR SYSTEM The Land Warrior system includes radios for communications, a GPS (global positioning system) terminal with navigation capabilities, a computer for processing, a helmet-mounted display, and imaging sensors, such as a thermal weapon sight and a video camera. These subsystems can be broken down into three categories, radios, computing, and imaging. The subsystems are interconnected, with the computer serving as the system core. Soldier systems for the Army After Next (beyond the year 2025) are likely to require access to satellite-based systems. For example, one-way DBS systems providing links to the battlefield at tens of millions of bits per second (Mbps) could be combined with two-way satellite communications systems to provide tens of kbps for asymmetrical database access. UAV-based technology could also be used if development and deployment of the technology has been completed.
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Energy-Efficient Technologies for the Dismounted Soldier The radio communications network for Land Warrior includes a soldier radio operating in a peer-to-peer network configuration for each soldier. Each squad leader also has a SINCGARS-compatible (single channel ground airborne system) (frequency, waveform, and protocol) radio for communications with other squads and with higher command. The operating power requirement of the soldier radio, while transmitting, is estimated to be 6 W, and the requirement for the squad radio is estimated to be 12 W. Radio power requirements are dominated by two components. In standby (receive) mode, when the receiver is monitoring for incoming traffic, the power requirements can be as low as 1 W for each radio. The expectation is that these radios will usually be in standby mode, so that standby mode contributes about half of the power requirements during a 12-hour mission. The power requirement increases to 6 W or more during transmission, but transmission is expected to have only about a 20-percent duty cycle, so it also contributes about half of the total requirements. In the near term, voice transmissions will continue to be the predominant form of soldier communications, but data transmissions will become increasingly important as the battlefield is digitized. Networks, protocols, and operational procedures evolve much more slowly than component technology. For example, protocols for SINCGARS radios (required for the squad radio) originated in the 1960s, and they are expected to be maintained until well beyond 2000. In the meantime, rapid advances in technology have made smaller personal units that readily support faster and more efficient data communication protocols feasible. The computer includes interfaces to the various subsystems and may be enhanced through software upgrades. Power requirements for the computer are estimated to average as high as 15 W. Voice recognition and control, image processing, and communications are functions of the computer. The imaging subsystems include an integrated helmet display, a video camera, and a thermal weapons sight. The various subsystems can be placed in standby modes either automatically or manually, so the duty cycle of each subsystem is critical to determining battery life and overall power requirements. The duty cycles are largely determined by network architectures, communications and computing protocols, and operating doctrine. Power requirements for the soldier system may be dominated by power for the listening modes of various subsystems. Generally, the most direct way to make use of a capability is to operate a subsystem in "hot standby" mode, continuously monitoring for incoming stimuli. This mode supports rapid response and less complex protocols, but it does not support energy conservation. Protocols that activate downstream subsystems when an incoming stimulus is detected can minimize power requirements. Digital cellular and PCS technologies that are emerging in the commercial market support standby times that range from several days to several weeks, using batteries that store on the order of one to several watt hours (Wh) of energy. Hand-held PCs are becoming available that can operate for about 24 hours and
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Energy-Efficient Technologies for the Dismounted Soldier require only 1 or 2 Wh of energy. Rapid advances in digital technology account for most of these large gains in energy efficiency, but advances in protocols also account for significant improvements in energy conservation. NETWORKS AND PROTOCOLS Cellular and PCS radio networks are organized in a hierarchical structure, in which terminals communicate with base stations through switching centers but not directly with other terminals (Figure 6-1). These networks are based on call setup procedures and protocols, and the concept of a communications session is fundamental to their design. A base station typically includes more processors, RF circuitry, antennas, and consequent energy demand than the terminals, so the hierarchical structure works to reduce the power requirements of the terminal equipment. Figure 6-2 shows a peer-to-peer network, which is representative of the radio communications networks for Land Warrior. Terminals communicate directly with other terminals, and no base stations or fixed infrastructure are required. This arrangement is not vulnerable to the loss of supporting infrastructure, but it tends to place larger power burdens on individual terminals than a hierarchical network. The alerting protocol used in first-generation analog advanced mobile phone system (AMPS) cellular radio systems (for which there are about 40 million terminals in the United States) is a simple procedure. A terminal is placed FIGURE 6-1 Hierarchical wireless system architecture used by commercial PCSs and cellular systems.
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Energy-Efficient Technologies for the Dismounted Soldier FIGURE 6-2 Peer-to-peer (nonhierarchical) wireless system architecture representative of Land Warrior. in a continuous receive mode on an alerting channel, where it continuously monitors an alerting stream for its identification number. Cellular radio was originally conceived mainly as a service for vehicles, with terminals with access to large batteries and alternators, so that requiring a receiver to be continuously active was considered acceptable. With the development of hand-held cellular terminals that weigh only a few ounces, however, this protocol design became a major limitation. Hand-held terminals typically provide 8 to 24 hours of standby time, but many users want standby times of days or even weeks. The protocol used by early paging systems provided for longer battery life than analog cellular systems. Paging systems were designed from the beginning to support small, pocketable terminals with days or weeks of battery life based on a protocol that did not require the receiver to monitor the channel continuously to extend battery life. Instead, pagers were designed to wake up about every 10 seconds to check for the presence of an RF carrier transmitted from the base station. If a carrier was found, the pager would stay in a listening mode until the carrier was released. Base stations would collect pages, and when enough of them had accumulated (about every minute), the base station would activate its carrier.
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Energy-Efficient Technologies for the Dismounted Soldier No traffic was sent for about 10 seconds until all the pagers woke up, and then the pages were sent. This simple protocol dramatically extended battery life and made early paging systems feasible. One undesirable property, however, was that it introduced substantial latency, or delay. A more advanced alerting network is shown in Figure 6-3. Variations on this scheme are used by digital cellular and PCS systems as well as modern paging systems. An alerting stream from a base station is organized into a periodic frame structure with a large number of time slots. The system protocol distributes its alerting stream across the time slots, so that an individual terminal need only listen to a single time slot in an entire frame to receive alerting messages intended for it. This protocol can result in dramatic reductions in power consumption with only a modest increase in latency. A major requirement of this approach is that a terminal keep accurate time while powering the receiver down and then waking it back up in time to monitor the next time slot. Given the timing stabilities of even inexpensive quartz oscillators, receiver power requirements were reduced by a factor of 100 or more. Figure 6-4 illustrates, in simplified form, the protocol used by the SINCGARS radios that will be required for the squad radio and is contemplated for soldier radios. This type of peer-to peer network does not involve call setup protocols, which can take several seconds before a conversation can begin. The FIGURE 6-3 Time-slotted alerting scheme used by commercial cellular systems, PCSs, and paging systems.
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Energy-Efficient Technologies for the Dismounted Soldier FIGURE 6-4 Simplified push-to talk access protocol used by SINCGARS and other military wireless systems. protocol more closely resembles a push-to-talk protocol. Since a SINCGARS receiver can be reached by a large number of transmitters and more than one base station alerting channel, and because low latency is desirable, the SINCGARS protocol requires that a radio keep its receiver awake continuously or nearly continuously to monitor for incoming transmissions. This protocol is a major contributor to the power requirements of Land Warrior radios. The unique needs of battlefield communications—low latency, security, and the ability to continue operating if one or more nodes are destroyed—led to a protocol design based on peer-to-peer carrier-sense networks. However, the protocols for SINCGARS were developed before the widespread use of small personal communicators on the battlefield was feasible. Power requirements of the soldier and squad radios for Land Warrior could be significantly reduced with protocols and network architectures that permit receivers to sleep a large percentage of the time, but the simultaneous requirement for low latency presents a serious problem. Protocols that place larger complexity and power requirements on the transmitter while substantially reducing the complexity and power requirements on the receiver may be possible. Adaptations of commercial digital cellular technologies may also be an attractive alternative. A project called Personal Communications System for the Soldier proposes to adapt commercial PCS technologies for insertion into the Land Warrior system (Staba, 1996). This program was briefed to members of this
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Energy-Efficient Technologies for the Dismounted Soldier committee in December 1996. To date, this project has focused on the insertion of nearly complete technologies and architectures into Land Warrior. As explained earlier, PCS technologies are primarily based on hierarchical networks with fixed base stations and mobile terminals, while the Land Warrior system is planned as primarily a peer-to-peer network. A hybrid network, in which a hierarchical architecture provides "virtual" peer-to-peer capabilities, could reduce the power requirements for wireless communications for future soldier systems and meet the same functional requirements as a true peer-to-peer communications network. Such a network is described in the next section. Hybrid "Virtual" Peer-to-Peer Network Architecture Suppose one soldier radio in a squad acted as a mini-base station, repeater, or master radio for a hierarchical network. This arrangement could have several important advantages in terms of power reduction. First, the master radio transmitter could synchronize all other soldier radios, as base stations do in commercial cellular systems, and thereby provide for efficient sleep modes, which would dramatically reduce the power requirement for receivers. This system could offer quick access, equivalent to push-to-talk operation, by waking soldier radios up for only about 1 millisecond every 20 to 100 milliseconds to listen for the beginning of a transmission. The effective duty cycle of the soldier radio receiver could then be as low as 1 percent, if the time to "wake" the radio were less than the 1 millisecond "on" time. Second, the transmit power could be concentrated at the master radio. A peer-to-peer network in which every station must hear every transmission does not readily support efficient control of transmit power because each signal is transmitted to a number of terminals. With a master radio, each terminal transmits only to the master radio, so power control and reductions in requirements for transmit power can be readily achieved by feedback information from the master radio. The master radio would have to retransmit information to a number of terminals, but its power requirements would be less than for terminals in a peer-to-peer network because it could be located away from the extremes of geographical coverage. Choosing the proper master radio within a peer-to-peer group would minimize that radio's total transmitter energy and the total energy used by the other transmitters in the group. The failure of the master radio and the motion of radios within the group could be accommodated by permitting any soldier radio to serve as the master radio based on an adaptive protocol. If a reliable choice of a master radio within the group was not possible, the network could fall back to operating as a peer-to-peer network. Protocols for configuring and dynamically reconfiguring hybrid networks are not well defined yet and are unique to military communications environments. Research may significantly increase network-wide energy efficiency.
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Energy-Efficient Technologies for the Dismounted Soldier Multihop Network Architectures In a dense grid of wireless terminals, information that flows between two widely separated terminals may not be needed by terminals lying between the two. Communication may thus be established either directly between the two widely separated terminals, or the intervening terminals may be used as repeaters of the information bound for the distant terminal. Repeaters can reduce the overall energy needed to communicate. To illustrate this, consider the following example. Adding a repeater at the midpoint of a path doubles the number of radio elements involved, but the length of each path is halved. In many scenarios, this reduces the energy needed to communicate a given message by a factor of eight. Because this architecture spreads the energy needed to communicate a message over several radios, energy resources are shared more uniformly by all of the network elements. The algorithms needed to manage the connectivity in this kind of a multihop network are complex, but they have been the subject of much recent study, especially in the military research community (Leiner et al., 1996). The energy needed to perform the computations must be weighed against the benefits of the multihop architecture. Fortunately, the computational algorithms are subject to efficiency improvements, and the power requirements of the computer platforms will decrease as time goes on. Multihop architectures also offer the benefits of redundant communication paths and the possibility of reconfiguring to replace missing nodes. As long as the underlying radio transmission technologies can still maintain the economic benefits of using COTS technology, a unique military multihop architecture can still be cost effective and energy-efficient. SELECTING A SUITABLE COMMERCIAL TECHNOLOGY The next question is how commercial PCS technologies can be adapted for a hybrid network (to provide virtual peer-to-peer capabilities) because all commercial technologies use fixed hierarchical architectures. Table 6-4 lists the seven PCS technologies that have been standardized for use in the United States (Cook, 1994). Global system for mobile (GSM) communications, a digital wireless technology developed in Europe, is the most advanced PCS technology in terms of commercial development, international acceptance, and subscriber penetration. In fact, GSM is expected to reach a subscriber penetration level of about 200 million in the year 2000. GSM is deployed throughout Europe and most of Asia, including India and China, and it will be widely deployed in the United States. In addition to its frequency hopping capability, GSM (and all other digital PCS technologies) incorporate commercial bit-level encryption technology. This system should permit easy substitution of the more powerful encryption technology required for battlefield applications. Plans are already under way for
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Energy-Efficient Technologies for the Dismounted Soldier TABLE 6-4 PCS Technologies Used in the United States Standard Tiera Access Method Number of Users Worldwide (1996) Omnipoint low TDMA/CDMA/TDD none IS-95 high CDMA/FDD 1 million PACS low TDMA/FDD none (5 million PHS) IS-54 high TDMA/FDD 5 million DCS or GSM high TDMA/FDD 25 million DCT or DECT low TDMA/TDD 5 million Wide-CDMA high CDMA/FDD none a ''Low tier" refers to low power or digital cordless telephone technologies with ranges generally limited to 1,000 to 2,000 feet. "High tier" refers to conventional macrocellular technologies. continued evolution of the technology (IEEE, 1995). Thus GSM should be considered a strong candidate for adaptation to soldier systems. IS-95 code division multiple access (CDMA) technology is also expected to be widely deployed in the United States. IS-95 uses direct-sequence spread spectrum and should provide low probability of intercept (LPI), low probability of detection (LPD), and anti-jamming capabilities. However, it is highly dependent upon a centralized hierarchical structure and cannot be readily adapted to a hybrid network or to a peer-to-peer network. For example, the forward (base-to-mobile) links and reverse (mobile-to-base) links are very different from each other and use different coding and multiple-access protocols, so terminal-to-terminal communications would be very difficult to arrange. IS-95 is intended to operate in a frequency division duplex (FDD) mode with continuous activity on both the forward and reverse links, which operate on a pair of frequencies. It is not possible to modify IS-95 for single frequency operation using time division duplexing (TDD) without making extensive changes. The difficulty in obtaining frequencies for use in military operations would interfere with the simultaneous access to paired frequency bands necessary for FDD operation. Finally, IS-95 will not readily support uncoordinated access of many different channels in a common area. Because all CDMA users interfere with one another, a user near a given receiver would cause unacceptable interference to transmissions from a user farther away (the "near-far" problem). The near-far CDMA problem with IS-95 is managed by centralized feedback power control of the mobile transmitters and by soft handoffs between base stations, which must be executed when terminals cross boundaries between the coverage areas of adjacent base stations to prevent loss of access. Soft handoffs allow terminals to communicate simultaneously with more than one base station when the
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Energy-Efficient Technologies for the Dismounted Soldier transmission conditions to the base stations are comparable (e.g., near the boundaries of coverage areas). "Low-tier" technologies are intended for short-range communications and will not meet the range requirements of soldier radios that must provide communication over distances up to 1.5 km from low "soldier-mounted" antennas. IS-54 TDMA (time division multiple access) technology has a great deal in common with analog AMPS and, therefore, has advantages for commercial deployment in the United States. However, it requires operation with a continuous downlink, which would be a problem in peer-to-peer operation and in single-frequency TDD operation (use of a common frequency for both transmission directions). The personal handy-phone system (PHS), which is substantially like personal access communications systems (PACS), is now widely deployed in Japan. PHS is a low-tier technology, and thus it is not appropriate as it stands for a soldier radio system. Existing equipment supports operating in both hierarchical and peer-to-peer modes, however, so it is instructive for soldier radio systems. PHS is based on a TDMA approach with eight time-slots in each 5 millisecond frame. Because PHS also uses TDD, protocols could be implemented that support both peer-to-peer and hierarchical access. A variant of PACS technology, the PACS unlicensed B version (PACS-UB), has been optimized for TDD operation in the unlicensed PCS spectrum. Although PACS-UB does not meet the range requirement of the soldier system, elements of this technology are also instructive. GSM is also based on TDMA (with eight time slots every 4.615 msec) and uses FDD operation. Because the forward link and reverse link for GSM are the same at the physical layer (identical radio channel bit rates, modulation and coding methods, information frame structure, etc.), and because GSM is based on TDMA, it should be possible to adapt it to TDD operation, which is more attractive than FDD for peer-to-peer or hybrid operation. The GSM standard supports frequency hopping, so it should be possible to include frequency hopping in any adaptation of GSM for soldier systems to improve LPI and LPD performance. Significant reductions in power for soldier systems are possible by designing protocols to support a hybrid network based on an adaptation of commercial technology. The availability of highly optimized components for wireless terminals from the commercial sector, which will accelerate in coming years, should help to reduce power requirements. Hybrid networks should make sleep modes more efficient, thereby reducing receiver power. They should also reduce transmitter power requirements in many cases. NETWORK ARCHITECTURES ABOVE THE SOLDIER LEVEL The radio communication requirements above the soldier level (the master radio in the architecture described in the previous section) could become even more energy-hungry. The master, or squad, radio is the gateway to the higher
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Energy-Efficient Technologies for the Dismounted Soldier levels of the communications network. Because it collects information from multiple soldier radios, its total information flow may be higher than the soldier radio. Furthermore, the longer and more hostile propagation path to the next level in the network requires that even more energy be expended to send a given message. Because of this, a significant amount of information filtering should be done at the squad leader level. This process may require significant amounts of computational power, but the additional power would probably be more than offset by the reduction in the amount of data to be transmitted. Because the power requirements for computing are expected to drop more rapidly than for wireless communications, this trade-off will become more attractive as time goes by and as the volume of necessary information increases. The SINCGARS radio protocols will clearly be incapable of meeting future communications needs above the squad level; their capacities are already stressed by existing needs for voice communication. However, the SINCGARS technology will be widely deployed for at least 10 years. During that time, the power requirements for software-defined multimode radios (such as Speak Easy) are expected to fall far below their present 1-kW level. This suggests that software-defined radios at the squad level (or, more realistically, initially at higher levels in the network hierarchy) could provide backward compatibility for gracefully phasing out the SINCGARS technology. The broad range of communication requirements may be seen to form loose clusters (see Table 6-1). One higher-rate cluster encompasses transferring images, video streams, and large databases. At the other extreme is the lower-rate transfer of short messages, voice messages, and requests for access to databases. This division suggests that separate wireless technologies could be used to meet these two needs. It also offers the possibility of a "fail-soft" mode and more redundancy, in which "basic" communications are carried out on a lower bandwidth technology and high-rate needs are met through another technology. "Slant-path" communications to satellites or UAVs appears to be one way of keeping the power requirements of the high-rate technology within reason. These systems and architectures are discussed in the next section. NONTERRESTRIAL SYSTEMS AND ARCHITECTURES Mobile Satellite Systems Several commercial satellite systems are being planned and deployed to serve mobile voice and low-rate data users in areas of the world where there is no terrestrial infrastructure. The satellite orbits used by these systems range from low earth orbit (LEO) to geosynchronous orbit (GSO). One system architecture, Iridium, uses 66 low-orbit satellites with 2 GHz links to mobile users, 20 GHz links from the satellites to terrestrial gateways, and 60 GHz links between satellites. The satellites form a mesh capable of routing information from one mobile user to any other mobile user or to a terrestrial gateway. Other GSO
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Energy-Efficient Technologies for the Dismounted Soldier system architectures cover large areas without intersatellite links. Hand-held subscriber radios typically use transmitter powers of several watts. Coverage is available outdoors in areas with moderate foliage and, often, in the top floor of most wooden buildings. Several of these GSO systems are scheduled for deployment before the year 2000. Limits in satellite energy generation (solar cells) and the amount of available radio spectrum greatly limit the traffic-handling capacity of GSO systems. Therefore, "dual-mode" (dual air-interface) mobile subscriber terminals are being planned to provide both satellite and terrestrial cellular or PCS interfaces. Whenever possible, a connection would be established through the local terrestrial cellular or PCS network. Some satellite-air interfaces (e.g., Iridium) are designed with this commonality in mind. This commonality could be exploited to create COTS-based soldier or squad-level radios with both a GSM-based terrestrial mode, as described above, and a satellite-based mode for immediate low-rate access to a global network infrastructure. Direct Broadcast Satellite Systems and Architectures Direct Broadcast Satellite (DBS) systems based on the low cost commercial home video systems with 18-inch antennas have the potential to distribute huge amounts of data throughout a battlefield with bit rates of more than 20 Mbps. For future soldier systems, this technology would have to be adapted in several ways for low power operation. The requirement for 18-inch dishes is obviously a physical problem at the individual soldier level. Options for using much smaller receiver antennas should be examined. These options could include reducing individual RF channel data rates (to around 2 Mbps) or using spot-beam satellite antennas. DBS technology for high-speed broadcasting could also be combined with two-way mobile satellite systems like Iridium to provide an asymmetrical high-speed database capability. A key element missing from current DBS architectures is wireless upstream transport capability. DBS is just that—a broadcast technology. Other proposed fixed satellite technologies, however, such as Teledesic and SpaceWay, will provide bidirectional connectivity at rates up to 2 Mbps to antennas with apertures in the 12-inch range. Recognizing the need for even smaller antenna sizes, commercial technologies like these could also form the basis of a bidirectional satellite-based wideband network providing connectivity to the squad level. Unmanned Aerial Vehicle Systems and Architectures A potential architecture that would capture the benefits of both terrestrial and satellite-based networks would place "base stations" on high-flying unmanned aerial vehicles, which would fly over the area in which wide-area
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Energy-Efficient Technologies for the Dismounted Soldier communications are desired. This kind of network might provide even lower power wideband communication than satellite-based architectures because the distances from the terrestrial terminals to the airborne base station would be much smaller. However, this architecture appears to have only military applications. There is not likely to be a commercial technology base to drive down the costs of the UAV and its repeater payload. In addition, unless the airborne repeaters are deployed fairly densely (e.g., on a 10-mile grid) the low elevation angle from the terrestrial terminals at the edges of the coverage limits would lead to high path losses approaching those for terrestrial architectures, thus negating many of the advantages of the UAV architecture. UAV and aerostat technologies are being proposed for military reconnaissance. It might be fruitful to investigate these programs further to determine if their platform technologies are applicable to military communication needs and if the combined volume of the two applications could produce the needed economies of scale. OPERATIONAL CONSIDERATIONS One of the most important steps the Army can take to reduce energy use by the dismounted soldier is to develop protocols and software capabilities that address power discipline in an operational context. Notwithstanding advances in micro-electronics and in optimizing energy supply and demand, the problem of power discipline, which the Army must address from the level of the soldier and the organization in which he functions, remains. The committee was unable to find information about any educational or training programs in power conservation. Software algorithms that allow communications to wake up via a call or some type of initiator are advances that would improve the equipment a soldier carries. The equipment can be designed to manage the allocation of power so as to minimize, in absolute terms, the amount of energy expended. However, the Army must address power discipline from a standpoint of soldier training to make the soldier aware that energy is a limited resource. The Land Warrior system fits within a larger operational concept involving other command, control, communications, computers, and intelligence (C4I) systems, all working together to facilitate the dynamic transmission of battlefield information. The soldier will need a filtering capability to pull down particular pieces of information so as not to become a repository for unnecessary data. The Army is implementing a "tactical internet" to provide near-real-time access to information leaders, down to the squad leader. Possibly the easiest way to save energy is through system partitioning to move power-hungry operations and databases away from soldier-portable devices. A simple innovation, such as using a UAV as a base station, can produce major savings but would probably also require changes in traditional operational procedures and protocols.
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Energy-Efficient Technologies for the Dismounted Soldier In light of the equivalence of data and energy (explained in Chapter 5), the Army must assess its needs for battlefield data to reach a balance between energy sufficiency and combat effectiveness. This aspect of power discipline, based on the principle of energy conservation, can provide a basis for real-time control of energy use in the field as part of dismounted soldier operations. FINDINGS Networks, protocols, and operational concepts tend to have long lifetimes so they will have a long-term impact on power requirements. Much of the power requirement for Land Warrior is for subsystems listening for relatively infrequent stimuli. Protocols and networks that support powering up downstream subsystems only when they are needed can minimize power requirements. This concept of "sleep mode" is applicable to both computing systems and wireless systems. Significant gains over present computers and radios are readily attainable. Peer-to-peer architecture for radio networks and protocols at the soldier level can satisfy the Army's need for low-latency connectivity, and hierarchical architectures can satisfy the Army's power concerns and have the advantages of using COTS technology. A GSM-based hybrid architecture may offer low cost, low energy consumption, and low latency and provide opportunities for covertness through frequency hopping and encryption. The harsh radio propagation environment and the wide range of communications dictate that energy-efficient wireless networks be optimized closely to specific environments, which means a wide range of radio interfaces will be necessary. Both terrestrial and satellite-based networks can be used for communications to and from dismounted soldiers. Additional relevant findings include the following: Hybrid wireless network architectures in which one radio can serve as a repeater for several others and provide "virtual" peer-to-peer communication with a hierarchical architecture are attractive in terms of energy efficiency. GSM wireless technology and wireless access protocols can form a basis for a "virtual" peer-to-peer intrasquad network and can capture most of the economic benefits of COTS technology. Better understanding of the interaction between the harsh radio propagation environment and antenna and signal processing characteristics could lead to significant gains in energy efficiency for wireless soldier communications systems. Multihop wireless networks based on COTS wireless technology offer the benefits of energy efficiency, reliability, and reconfigurability. Satellite technologies (such as DBS, Iridium, Teledesic, SpaceWay) can play a role in supporting battlefield communications to and from the squad level. The Iridium PCS system, for example, is designed to work with GSM-based terrestrial networks. All of these systems offer potentially strong COTS product bases. The adaptation of wideband commercial satellite technologies should take into account the military need for even smaller and more mobile antennas.
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Energy-Efficient Technologies for the Dismounted Soldier Airborne platforms offer a way to reduce energy consumption by extending radio transmit ranges. UAV repeater technology has no obvious commercial market and may not be cost-effective unless military reconnaissance requirements for UAVs increase. Soldiers trained to conserve energy can increase combat effectiveness by practicing power discipline.
Representative terms from entire chapter: