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2
Technology Limits, Trade-offs, and Challenges

Wireless communications networks incorporate a broad range of technologies, including electrochemical materials, electronic devices and circuits, antennas, digital signal processing algorithms, network control protocols, and cryptography. Although all of these technologies are well advanced in other applications, wireless systems introduce a set of constraints and challenges beyond those addressed in the evolution of other communications networks, such as the (wireline) public switched telephone network and the Internet. These special constraints make it exceedingly difficult to design affordable wireless systems that meet every need. The challenges can be grouped into three categories: mobility, connectivity, and energy.

Mobility is a fundamental feature of untethered communications networks. Portable, wireless communications devices significantly enhance the mobility of users, but they also pose network design difficulties. As the communications devices move, the network has to rearrange itself. To deliver information to a mobile terminal, the network has to learn the new location(s) of the terminal and change the routing of information accordingly, sometimes at very high speeds. The rerouting must be done seamlessly without any perceived interruption of service.

A wide variety of problems arise when mobile wireless communications terminals send and receive signals over the air. The signals of all the terminals are subject to mutual interference. The characteristics of the propagation medium change randomly as users move, and the mobile radio channel introduces random variation in the received signal power



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Page 56 2 Technology Limits, Trade-offs, and Challenges Wireless communications networks incorporate a broad range of technologies, including electrochemical materials, electronic devices and circuits, antennas, digital signal processing algorithms, network control protocols, and cryptography. Although all of these technologies are well advanced in other applications, wireless systems introduce a set of constraints and challenges beyond those addressed in the evolution of other communications networks, such as the (wireline) public switched telephone network and the Internet. These special constraints make it exceedingly difficult to design affordable wireless systems that meet every need. The challenges can be grouped into three categories: mobility, connectivity, and energy. Mobility is a fundamental feature of untethered communications networks. Portable, wireless communications devices significantly enhance the mobility of users, but they also pose network design difficulties. As the communications devices move, the network has to rearrange itself. To deliver information to a mobile terminal, the network has to learn the new location(s) of the terminal and change the routing of information accordingly, sometimes at very high speeds. The rerouting must be done seamlessly without any perceived interruption of service. A wide variety of problems arise when mobile wireless communications terminals send and receive signals over the air. The signals of all the terminals are subject to mutual interference. The characteristics of the propagation medium change randomly as users move, and the mobile radio channel introduces random variation in the received signal power

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Page 57 and other distortions, such as frequency shifts and the spreading of signals over time. Signals that travel over the air are also more vulnerable to jamming and interception than are those transmitted through wires or fibers. These limitations are often addressed with a combination of sophisticated signal processing techniques and antennas. However, these solutions add to the complexity of portable communications devices and increase power requirements. Wireless systems pose two types of power challenges. First, when power is radiated from an antenna, very little of it typically reaches the receiver, a phenomenon known as path loss. This problem can be partly overcome with increased transmit power, special types of antennas, and other solutions. Second, wireless terminals often carry their own power supplies in the form of batteries. Battery life is limited and is influenced by many aspects of terminal design as well as the technology of the network infrastructure. Scarce power constrains the signal processing capabilities and transmit power of the mobile terminal, motivating efforts to keep these units as simple as possible. However, a low-power design cannot accommodate the most sophisticated techniques available to cope with the vagaries of the wireless channel and support the network protocols of mobility management. In the absence of research breakthroughs that simplify these techniques, the only solution is to increase the complexity of the network, which needs to compensate for the simplicity of portable communications devices. The challenges related to mobility, connectivity, and energy have stimulated a high level of R&D activity in the telecommunications industry and academia. Still, a chasm remains between the capabilities of wired and wireless communications systems. Even as commercial wireless systems evolve, additional features will be needed to meet military requirements for untethered communications. Military applications introduce additional challenges because the systems need to be rapidly deployable on mobile platforms in any one of a diverse range of operating environments; they need to interoperate with other systems; and they need protection against enemy attempts to jam, intercept, and alter information. This chapter provides the technical basis for the analysis of military-commercial synergy in Chapter 3 by examining the challenges of mobility, connectivity, and energy and the technologies devised to overcome them. The discussion refers to the various layers of a network as defined in the Open Systems Interconnection (OSI) model (see Box 2-1). Section 2.1 is a tutorial on the wireless channel, its capacity limits, techniques for overcoming channel impairments, and the access and operational issues that arise when multiple users share the same channel. The next three sections address network, system, and hardware issues with an emphasis

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Page 58

BOX 2-1 Open Systems Interconnection Model The Open Systems Interconnection model identifies seven layers, some or all of which are implemented by virtually any network system. The physical layer includes the mechanical, electrical, and procedural interfaces to the transmission medium. The link layer converts the transmission medium into a stream that appears to be free of undetected errors. This layer includes error-correction mechanisms and the protocols used to gain access to shared channels. The network layer chooses a route from the sender to the receiver and deals with congestion and address issues. The IP protocol falls into this layer. The transport layer is responsible for the end-to-end delivery of data. The TCP protocol falls into this layer. The session layer allows multiple transport-layer connections to be managed as a single unit. The presentation layer chooses common representations (typically application dependent) for the data being carried. The applications layer deals with application-specific protocol issues. on military needs. Section 2.2 examines network design issues including architecture, resource allocation and discovery, inoperability, mobility management, and simulation and modeling tools. Section 2.3 addresses end-to-end systems design issues including application-level adaptation, quality of service, and security. Section 2.4 reviews hardware issues of particular military concern, focusing on radio components. 2.1 Communication Link Design The ideal wireless communications system would provide high data rates with high reliability and yet use minimum bandwidth and power. It would perform well in wireless propagation environments despite multiple channel impairments such as signal fading and interference. The ideal system would accommodate hardware constraints such as imperfect timing and nonlinear amplifiers. The mobile units would have low power requirements and yet still provide adequate transmit power and signal processing. In addition, despite the system complexity required to achieve this performance level, both the transmitter and receiver would be affordable. Such a system has yet to be built. In fact, many of the desired properties are mutually exclusive, meaning that trade-offs need to be made in system design. A case in point is the choice of approaches for overcoming the limitations and impairments of the wireless channel. The impairments inherent in any wireless channel include the rate at which received signal power decreases relative to transmitter-receiver distance (path loss),

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Page 59 attenuation caused by objects blocking the signal transmission (shadow fading), and rapid variations in received signal power (flat fading). The impairments determine the types of applications that can be supported in different propagation environments. Applications require different data rates and bit-error rates (BER, or the probability that a bit is received in error). For example, voice applications require data rates on the order of 8 to 32 kbps and a maximum tolerable BER of 10-3, whereas database access and remote file transfer require data rates up to 1 Mbps and a maximum tolerable BER of 10-7.1Both sets of specifications are difficult to achieve in many radio environments. In general, systems designed for the worst-case propagation conditions assume high error rates, which limit their capability to support high-speed data and video teleconferencing applications. The random nature of the radio channel makes it difficult to guarantee quality and performance for demanding applications. However, a wireless system can be designed to adapt to the varying link quality at both the link and network level, such that the system can support improved data rates and quality. Applications can also be designed to adapt to deteriorating channel conditions to minimize the degradation perceived by the user. The overall system can be optimized by making trade-offs among various performance measures such as BER, outage probability, and spectral and power efficiency. These trade-offs dictate the choice of modulation, signal processing, and antenna techniques used to mitigate channel impairments. These techniques require fairly intensive digital signal processing at the mobile unit. The extent of the computation that can be performed is limited by the power available to drive the DSP chips and the microprocessor. Thus, in addition to being power limited, the mobile unit is also complexity limited, which means that trade-offs need to be made in designing the communication link. For example, the transmit power requirements of the mobile unit can be reduced if error-correction coding is used, but then additional power is needed to drive the encoding and decoding hardware. In cellular systems it is preferable to place much of the computational burden at the base station, which has fewer power restrictions than do the mobile units. Research aimed at simplifying DSP and antenna processing techniques (Section 2.4) can also help mitigate the computational burden. The remainder of this section outlines the characteristics of the wireless channel, focusing on fading and interference problems (Section 2.1.1); key communications technologies, including modulation and coding (Sections 2.1.2 through 2.1.4); the countermeasures available to address fading and interference (Section 2.1.5); and the various ways in which users access wireless systems (Section 2.1.6).

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Page 60 2.1.1 Characteristics of the Wireless Channel The characteristics of the radio channel impose fundamental limits on the range, data rate, and quality of wireless communications. The performance limits are influenced by several factors, most significantly the propagation environment and user mobility pattern. For example, the indoor radio channels typically support higher data rates with better reliability than does the outdoor channel used by persons moving rapidly. Electromagnetic signals can be characterized by the features of the waveform: amplitude (the power, or magnitude, of the signal); phase (the timing of the peak or trough of the signal variations); and frequency (the number of repetitions of the signal per second).2The effects of the wireless channel on the received signal power are typically divided into large-scale and small-scale effects. Large-scale effects involve the variation of the mean received signal power over large distances relative to the signal wavelength, whereas small-scale effects involve the fluctuations of the received signal power over distances commensurate with the wavelength. Path loss effects are noticeable over large distances (i.e., distances on the order of 100 m or more). Signal power variations due to obstacles such as building or terrain features are observable over distances that are proportional to the length of the obstructing object. Very rapid variations result from multipath reflections, which are copies of the transmitted signal that reflect or diffract off surrounding objects before arriving by different paths at the receiver. These reflections arrive at a receiver later than the nonreflected signal path and are often shifted in phase as well. The multipath reflections either reinforce or cancel each other and the nonreflected signal path depending on the exact position of the receiver (if moving) or the transmitter (if moving). The overall effects of multipath propagation involving a moving terminal are rapid variation in the received signal power and nonuniform distortion of the frequency components of the signal. The first four subsections below discuss path loss, fading, and various sources of interference as they apply to the path between two terrestrial RF devices. The fifth subsection details the characteristics of satellite RF links. 2.1.1.1 Path Loss Path loss is equal to the received power divided by the transmitted power, and this loss is a function of the transmitter-receiver separation. For a given transmit power, a path loss model3predicts the received power level at some distance from the transmitter. The simplest model for path loss, which captures the key characteristics for most channels, is an exponential relationship: The received signal power is proportional to the transmit power and inversely proportional to the square of the transmission

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Page 61 frequency and the transmitter-receiver distance raised to the power of a ''path loss exponent."4In free space the path loss exponent is 2, whereas for typical outdoor environments it ranges from 3 to 5. In environments with dense buildings or trees, path loss exponents can exceed 8. Thus, systems designed for typical suburban or low-density urban outdoor environments require much higher transmit power to achieve the same desired performance in a dense jungle or downtown area packed with tall buildings. The BER of a wireless link is determined by the received signal power, noise introduced by the receiver hardware, interference, and channel characteristics. The noise is typically proportional to the RF bandwidth. For the exponential path loss model just described, the received signal-to-noise ratio (SNR) is the product of the transmit power and path loss, divided by the noise power. The SNR required for faithful reception depends on the communications technique used, the channel characteristics, and the required BER. Because path loss affects the received SNR, path loss imposes limits on the data rate and signal range for a given BER. In general, for a given BER, high-data-rate applications typically require more transmit power or have a smaller coverage range (sometimes both) than do low-data-rate applications. For example, given a transmit power of 1 W, a transmit frequency of 1 GHz, and an omnidirectional antenna, the transfer of data through free space (for which the path loss exponent is 2) at 1 Mbps and 10-7 BER can be accomplished between radios that are 728 m apart, whereas in a jungle (for which the path loss exponent is 10) the range can be as low as 4 m. 2.1.1.2 Shadow Fading A received signal is often blocked by hills or buildings outdoors and furniture or walls indoors. The received signal power is in fact a random variable that depends on the number and dielectric properties of the obstructing objects. Signal variation due to these obstructions is called shadow fading. Measurements have shown that the power, measured in decibels (dB), of signals subject to shadow fading exhibits a Gaussian (i.e., normal) distribution, a pattern referred to as long-normal shadowing. The random attenuation of shadow fading changes as the mobile unit moves past or around the obstructing object. Because the signal coverage is not uniform even at equal distances from the transmitter, the transmit power needs to be increased to ensure that the received-SNR requirements are met uniformly throughout the coverage region. The power increase imposes additional burdens on the transmitter battery and can cause interference for other users of the same frequency band.

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Page 62 2.1.1.3 Small-Scale (Multipath) Fading Small-scale fading is caused by interference between multiple versions of the signal that arrive at the receiver at different times. Multipath can be helpful if the signals add constructively to produce a higher power (a random event), but more often it results in harmful interference. The overall effect is a standing wave pattern of the received signal power. Harmful interference can cause the received signal power to drop by a factor of 1,000 below its average value at nulls in the standing wave pattern. Moreover, for practical speeds of wireless terminals, the changes in signal power are extremely rapid: At a frequency of 900 MHz the signal power changes every 30 centimeters, or every 23 milliseconds if the terminal is moving at 50 km per hour. In many practical environments, these changes are referred to as "Rayleigh fading" because the received signal amplitude conforms to a Rayleigh probability density function. Signal fading can be characterized by determining the delay spread of the fading relative to the signal bandwidth. The delay spread is defined as the time delay between the direct-path signal component and the component that takes the longest path from the transmitter to the receiver. Because the delay spread is a random variable, it is often characterized by its standard deviation, called the root mean square (RMS) delay spread of the channel. If the product of the RMS delay spread and the signal bandwidth is much less than 1, then the fading is called flat fading. In this case the received signal envelope has a random amplitude and phase (commonly described by a Rayleigh distribution), but there is no additional signal distortion. When the product of the RMS delay spread and signal bandwidth is greater than 1, the fading becomes frequency selective. Frequency-selective fading introduces self-interference because the delay spread is so large that multipath reflections corresponding to a given bit transmission arrive at the receiver simultaneously with subsequent data bits. This intersymbol interference (ISI) establishes an "error floor" in the received bits that cannot be reduced by an increase in signal power because doing so also increases the self-interference. Without compensation, the ISI forces a reduction in the data rate such that the product of the RMS delay spread and signal bandwidth is less than 0.1. For a 10-3 BER and a rural environment, the delay spread is approximately 25 microseconds and the corresponding maximum data rate is only 8 kbps; the data rates for lower BERs are even more limited. Some form of compensation, either signal processing or sophisticated antenna design, clearly is needed to achieve high-rate data transmission in the presence of ISI. These techniques impose additional complexity and power requirements on the receiver.

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Page 63 Movement of a receiver relative to the transmitter (or vice versa) causes the received signal to be frequency shifted relative to the transmitted signal. The frequency shift, or Doppler frequency, is proportional to the mobile velocity and the frequency of the transmitted signal. For a transmitted signal frequency of 900 MHz and a receiver or transmitter speed of 96 km per hour, the Doppler frequency is roughly 80 Hz. This Doppler shift creates an irreducible error floor for noncoherent detection techniques (which use the previous bit to obtain a phase reference for the current bit). In general the irreducible BER is not a problem when data are transmitted at high speed (faster than 1 Mbps), but it is an issue for moderate-rate (slower than 100 kbps) data applications. In general the signal changes slowly with time because of path loss, more quickly because of shadow fading, and very quickly because of multipath flat fading; all of these effects are simultaneously superimposed on the transmitted signal. As noted above, the shadow fading needs to be addressed by an increase in transmit power. The deep fades in signal power caused by flat fading also need to be counterbalanced by an increase in transmit power or some other approach (see Section 2.1.5.1). Otherwise the transmitted signal typically exhibits bursts of errors that are difficult to correct. 2.1.1.4 Interference Users of wireless communications systems can experience interference from various sources. One source is frequency reuse, a popular technique for increasing the number of users in a given region who can be supported by a particular set of frequencies. Cellular systems reuse frequencies at spatially separated locations, taking advantage of the falloff in received signal power with distance (which is indicated by the path loss model). The downside of frequency reuse is the introduction of co-channel interference (see Section 2.2.1.1), which increases the noise floor and degrades performance. Other sources of interference include adjacent channels and narrow bands of problem frequencies. Adjacent-channel interference can be mitigated by the introduction of guard channels between users, although this technique consumes bandwidth. Narrowband interference can be removed by notch filters or spread-spectrum techniques. Notch filters are simple devices that block the band of frequencies containing the interference; these devices are effective only if the specific frequencies of concern are known. Spread-spectrum techniques (see Section 2.1.5.2), which spread a signal across a larger band of frequencies than is required for normal transmission, can reduce the effect of interference and hostile jamming signals.

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Page 64 2.1.1.5 Satellite Channels Satellite channels (the link between a receiver or transmitter on Earth and an orbiting receiver or transmitter) have inherent advantages over terrestrial radio channels. Multipath fading is rare because a signal propagating skyward does not experience much reflection from surrounding objects (except in downtown areas with densely packed buildings). Moreover, most satellite systems operate in the gigahertz frequency range, allowing for large-bandwidth communication links that support very high bit rates. The primary limitation of satellite channels is very high path loss, which generally follows the formula described earlier in this chapter. For satellites the path loss exponent is 2. Because satellites operate at high frequencies and the path distance is long (500 to 2,000 km for a LEO satellite), much higher transmit power is needed than is the case for terrestrial systems operating at the same data rate. Satellite signals are also subject to attenuation by Earth's atmosphere. The effects are especially adverse at frequencies above 10 GHz, where oxygen and water vapor, rain, clouds, fog, and scintillation cause random variations in signal amplitude, phase, polarization, and angle of arrival (similar to the adverse effects of multipath fading in terrestrial propagation). Satellite systems compensate somewhat for the large path loss and adverse atmospheric effects by using high-gain directional antennas to boost the received power. 2.1.2 Capacity Limits of Wireless Channels The pioneering work of Claude Shannon determined the total capacity limits for simple wired and wireless channel models: These limits established an upper bound on the maximum spectral link efficiency, measured as the data rate per unit of bandwidth as a function of the received SNR. For a channel without fading, ISI, or Doppler shift, this maximum bandwidth efficiency was identified by Shannon to be the logarithm of the term [SNR + 1] (Shannon, 1949). Determining the capacity limits of wireless channels with all the impairments outlined in the previous section is quite challenging. A relatively simple lower bound for a channel capacity that varies over time is the Shannon capacity under the worst-case propagation conditions. This is often a good bound to apply in practice because many communication links are designed to have acceptable performance even under the worst conditions. However, this design wastes resources because typical operating conditions are generally much better than the worst-case scenario. For channels that exhibit shadow fading or multipath fading, the channel

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Page 65 capacity under worst-case fading conditions is close to zero. The capacity of these fading channels increases greatly when the data rate, power, and transmission are adapted using sophisticated modulation techniques, which are discussed in the next section. As measured by spectral link efficiency, these adaptive techniques in both Rayleigh fading and lognormal-shadowed channels can support much higher data rates than are typical in today's wireless systems. For example, typical digital voice systems deliver 8 kbps in a 30-kHz channel, which corresponds to a spectral link efficiency of 8/30, far less than 1. If this channel experiences Rayleigh fading, then an SNR of approximately 30 dB is required. At this SNR, a spectral link efficiency of approximately 8 can be achieved in Rayleigh fading by using adaptive techniques—a 30-fold improvement over the typical voice system of today (see Figure 2-1).5 2.1.3 Modulation Modulation is the process of encoding information into the amplitude, phase, and/or frequency of a transmitted signal (Ziemer and Tranter, 1995). This encoding process affects the bandwidth of the transmitted signal and its robustness under impaired channel conditions. In the case of bandwidth-limited channels, digital modulation techniques encode several bits into one symbol. The rate of symbol transmission determines the bandwidth of the transmitted signal: the larger the number of bits encoded per symbol, the more efficient the use of bandwidth but the greater the power requirement for a given BER in the presence of noise. Modulation techniques fall into two categories: linear and nonlinear. In general, linear modulation techniques use less bandwidth than do nonlinear techniques. However, linear modulation techniques also tend to produce large fluctuations in signal amplitude. This is a disadvantage when using nonlinear amplifiers such as class C amplifiers (the least expensive, most readily available, and most power-efficient amplifiers), because they distort linear modulation signals. Thus, the bandwidth efficiency of linear modulation is generally obtained at the expense of the additional power needed for very linear amplifiers (and reduced battery life). 2.1.4 Channel Coding and Link-Layer Retransmission Channel coding improves performance by adding redundant bits in the transmitted bit stream that are used by the receiver to correct errors introduced by the channel, thus reducing the average BER. This approach enables a reduction in the transmit power required to achieve a target BER. Conventional forward-error-correction (FEC) codes, which reduce

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Page 66 image FIGURE 2-1 Spectral link efficiency can be greatly increased using adaptive techniques. Theoretical efficiencies are shown.

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Page 97 TABLE 2-2 Use of Digital Components in Commercial Communications Products Product RFa Amplifier Mixer Filter Demodulator Decoder Car radio with equalizer Analog Analog Analog Analog Analog DirecTV receiver Analog Analog Analog Digital Digital Dual-mode cell phone Analog Analog Digital Digital Digital PC telephone modem Analog Digital Digital Digital Digital FDDIb modem Digital n/a n/a Digital Digital a Radio frequency. b Fiber-distributed data interface. the future as new spectrum is allocated at higher frequencies (above 2 GHz) and multiband radios become available. For the time being, the DOD may need to fund its own R&D in this area. 2.4.2 Other Radio Components The evolution of digital technology is transforming radios. Other than antennas, all the components of the radio system—RF amplifier, mixer, filter, demodulator, and decoder—are amenable to either analog or digital implementation. Many commercial radios and other communications products already use programmable digital modules (see Table 2-2). There are many advantages to replacing analog hardware with programmable digital technology, although trade-offs are involved. As noted above, digital technology offers inherent security advantages. Another benefit is time to market: As with PCs, product development time can be reduced because changes and improvements can be implemented through software. Digital technologies also make it easier to achieve temperature stability and reliability and to manufacture, support, and test equipment. Digital radios can be designed for performance peaks, whereas analog radios de-optimize performance because the filters are detuned to make the system easier to manufacture and tolerant of component variability. Finally, digital components can reduce costs by providing increased functionality per unit and reducing the need for multiple types of radios. The design of wideband (i.e., multiband) digital radios has been enabled by rapid advances in microelectronics, including DSPs, A/D converters, ASICs, and field-programmable gate arrays (FPGAs). In new radio architectures, referred to variously as software-defined radio, programmable radio, or simply software radio, analog functions such as tuning,

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Page 98 filtering, demodulating, and decoding are replaced with software directing the digital equivalents. The mixers and filters can process multiple modulations spanning multiple bandwidths; the demodulation and decoding processes are programmed; and modulation and coding are usually performed using DSP chips. The wideband A/D conversion of the software radio enables the implementation in handsets of direct frequency conversion (i.e., eliminating the typical intermediate steps between the baseband and transmit frequencies, thereby reducing noise and the need for filtering). This design is not yet appropriate for commercial systems, because it is not feasible at this point to service a large number of subscribers using such receivers simultaneously. In the handset, the RF amplifier is required to obtain a reasonable noise figure and input intercept. The anti-alias filter selects which of the multiple subbands to digitize. The wideband (multiband) digitizer converts all RF signals into a digital representation. The processor uses software to implement all legacy and future radio systems. The processor is capable of implementing multiple simultaneous radios, much like a PC can run multiple applications simultaneously. The use of digital radio hardware still presents challenges and requires trade-offs that may not be readily apparent. A digitally implemented radio needs to be at least as good as the analog radio it replaces in terms of QoS parameters such as reception sensitivity or power. This challenge is being met: Coding and decoding improvements, driven by DSP advances, are making digital radio systems not only equal to analog systems but also better. However, four limiting technologies need to be developed further if wideband (i.e., multiband) software radios are to become a practical reality: advanced A/D converters, DSP chips, filters, and RF amplifier components. 2.4.2.1 Analog-to-Digital Converters The key enabling component, and the most complex and misunderstood element of wideband software radios, is the A/D converter. Most A/D converters are characterized by maximum clock rate and number of output bits, digital metrics similar to those used to characterize microprocessors or memory devices. However, because signal quality is key, A/D converters are better characterized using analog characteristics such as SNR, signal range free from spurious noise, and usable bandwidth (known as Nyquist bandwidth). The use of these metrics helps ensure that the critical A/D transition can be accomplished with minimum degradation in signal quality. An A/D converter can be implemented in many different architectures; the three significant modern architectures are known as flash (or

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Page 99 parallel), subranging, and sigma-delta. The choice involves trade-offs between the accuracy and conversion rates. The flash A/D uses the most hardware and power but also operates at the fastest sampling rates and produces the largest usable Nyquist bandwidth. These converters are generally inadequate because of a lack of dynamic range. Subranging A/D converters are slower but offer both the bandwidth and dynamic range needed for software radios. Sigma-delta A/D converters generally have very low sample rates and are appropriate for narrowband applications. It is not clear at this stage which technology can be improved most readily to provide the requisite low-power converters with high dynamic range that process bits as rapidly as possible. A demonstration of ultrafast A/D converters was planned as part of the DOD-funded Millennium program. The commercial sector is also performing R&D in this area and is likely to produce advances that would be appropriate for military applications. 2.4.2.2 Digital Signal Processors For the past few decades semiconductor manufacturing has followed Moore's Law, which predicts that the number of devices on an IC will double every 18 months. This trend is directly related to the steady reductions in the minimum feature size, or linewidth, that can be manufactured in large volumes. The Semiconductor Industry Association road map calls for the production of devices with more than a billion transistors by the year 2010. Such densities could enable entire systems to be built on a few or even a single chip. Indeed, software radio architectures may be implemented increasingly in small numbers of ICs (see Figure 2-4). image FIGURE 2-4 Future wideband (i.e., multiband) software radios may implement some functions, such as analog-to-digital conversion and signal processing, in single integrated circuits.

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Page 100 The several general-purpose, programmable DSPs now on the market are primarily a customized segment of the IC industry. Although DSP speed is improving every year, single-chip performance is still very limited for software radio applications. High speed can be achieved with large arrays of DSPs but the size, weight, power, and cost of this design are not attractive for small or handheld radio applications. Because filters are critical to the performance of a software radio, gains could be achieved through the use of integrated FIR-filtering ICs. These devices, developed very recently, could perform the processing function at a small fraction of the complexity and cost of a programmable DSP.
24Regardless, the rapid commercial advances in signal processing technology are likely to produce chips that meet military needs. 2.4.2.3 Filters Filters influence not only a radio's signal-processing speed but also its sensitivity, dynamic range, and capability to avert co-site interference. Their importance is reflected in their physical presence: Filters constitute 25 percent of the volume of a typical software radio, in part because several different filters are needed (i.e., for receive preselectors, amplifier output, local oscillators, and mixers). Improvements in frequency-tuning range and selectivity as well as miniaturization would be helpful, especially for application in handheld devices. The commercial sector continues to rely on older technology (e.g., mechanical filters are used in cellular telephone systems) whereas the military has unique needs to reduce co-site interference, both within software radios and across multisystem platforms, and cover wide frequency ranges. Existing radios that span wide frequency ranges require combined filters made of new materials that have remarkably flexible and adaptive electrical properties, far beyond older static inductors and capacitors. The new materials and modern filter fabrication techniques will lead to new and smaller implementations of wideband filtering based on the fundamentals of transmission-line techniques. Thus, filters may merit a significant military R&D investment. 2.4.2.4 Radio Frequency Amplifiers The commercial sector is designing ultralinear amplifiers that will process many signals from multiple transmitters and add them coherently to achieve good fidelity. These designs will improve power efficiency and consume less space than traditional amplifiers. However, the commercial sector is unlikely to produce multiband amplifiers, which will be very expensive, anytime soon. Alternative materials might offer

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Page 101 advantages in the design of future military systems and could be a topic for DOD-funded research. The most important recent development in RF technology is the reemergence of semiconductors (i.e., silicon) as an alternative to the semi-insulator materials (e.g., gallium arsenide) traditionally used for RF device manufacturing. Semiconductors offer two advantages. First, they form a natural ground plane on an IC such that microwave devices can be fabricated much closer together, resulting in smaller chips and enabling the design of circuits that cost less and support higher frequencies and performance than do conventional circuits. Second, semiconductors cost less than semi-insulators because they are produced in higher volumes (they are also used in the most advanced CMOS microprocessors and random-access memory [RAM] chips). As a consequence, silicon is now being used in moderate-performance RF front ends for cellular and personal communication systems. Further, a new technology involving the implantation of germanium atoms in silicon to create heterojunction bipolar transistors promises an extremely low cost, silicon-based approach for RF (30 MHz to 2 GHz) and microwave (2 GHz to 40 GHz and above) analog front ends and power amplifiers. 2.4.3 Portable Terminal Design The small size and portability of wireless communicators provide obvious benefits for users but also introduce challenges for system designers because they limit display, processing, power, and storage capabilities. The following subsections review the limitations and the new technologies designed to overcome them. The commercial sector is making rapid advances in all these areas that the DOD can exploit to good advantage. 2.4.3.1 Displays, User Interfaces, and Input Devices Small, highly portable devices contain relatively low quality displays. There are three reasons for this. First, portable devices have limited physical space and power available for the display. Second, display pixels cannot be smaller than the resolving limit of the human eye, meaning that the number of pixels in a given display (i.e., the resolution) is limited. In addition, bright colors can be produced only if there is sufficient power for backlights and display elements; otherwise the display is dim and monochrome. For these reasons the user interface of a portable device needs to be designed for monochrome presentations in a very small screen area—a significant impediment to the display of video or high-quality

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Page 102 images. Nevertheless, significant market forces are fueling a trend toward ubiquitous information displays, and commercial displays offering high resolution, full color, and reduced power requirements are likely to be developed. Because portable devices lack the space for standard keyboards, icon-based interfaces and pen-based input have been considered as alternatives. In some devices the keyboard is replaced with a small number of function-specific buttons. These devices still support functionally specific virtual keyboards, which are displayed on a touch screen and can be operated by applying pressure to the keys with a stylus. The pen-based devices either support handwriting recognition or simply capture pen strokes ("digital ink"). Ideally, mobile communication devices will be able to send images from remote sites. This capability will be enabled by charge-coupled devices (CCDs)25and CMOS camera chips. These chips are already used in commercial camcorders and have become inexpensive and widespread as a result. Highly integrated cameras have been declining in price, and such a camera is integrated into at least one state-of-the-art Japanese PDA. 2.4.3.2 Processors The successful development of low-power devices with long battery life has placed limits on the raw performance of embedded processors because processing speed and clock cycle directly influence power consumption. New metrics are therefore required to measure the performance of processors for portable applications: millions of instructions per second (MIPS) per watt, a measure of the impact on battery life and heat dissipation in highly integrated systems; MIPS per square millimeter, a measure of the silicon manufacturing costs of the processor; and bytes per task, a measure of the amount of memory that devices need to incorporate to perform signal-processing functions. Because consumers are demanding highly integrated yet portable computing devices, the commercial sector is performing R&D with the aim of increasing processor capabilities while also reducing power requirements. 2.4.3.3 Batteries The commercial sector has made tremendous strides in battery technology in recent years because it plays a role in many technologies, ranging from surgical implants to electric cars. Nickel cadmium (NiCd) batteries are the most widely used rechargeable batteries, found in many consumer electronic devices. Most laptop computers now use nickel metal hydride batteries, which have slightly better energy storage per weight

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Page 103 and substantially improved energy storage per volume. Lithium ion (LiIon) batteries are used in some new portable products, such as small cellular telephones. The energy-storage capacity of LiIon batteries is more than twice that of NiCd technology by both weight and volume. Lithium polymer (LiPoly) batteries are about 10 percent more efficient than are LiIon batteries. Both LiIon and LiPoly batteries use solid electrolytes, making it possible to form the battery into arbitrary shapes, a significant improvement over other battery technologies. 2.4.3.4 Storage The disk-drive capacity of information processing devices continues to increase while physical size shrinks, but the 2.5-inch disk widely used in notebook and laptop computers is still too large for handheld devices. In PDAs the disk is replaced by RAM in the form of battery-backed-up static RAM and flash RAM on personal computer multiple component interface access (PCMCIA, or just PC) cards, which can cost 30 times more than disk storage per megabyte. Commercial R&D in this area is producing steady, impressive advances that are likely to meet military needs. 2.5 Summary The design of wireless communications systems presents countless challenges. Some solutions are available and many more are on the horizon. Although the review presented in this chapter is general in nature, consideration of this information in the context of DOD's needs suggests a number of areas deserving careful attention in the design of future military systems. Specifically, network architecture is a fundamental issue that defines all other aspects of the system design. The basic choice is between a peer-to-peer and base-station-oriented design, but there are also other questions related to how infrastructure elements are connected and the nature of communications with other networks. The commercial and defense sectors have differed in their choice of network architectures in the past and continue to have some different needs and concerns. The selection of an optimal military network design could be assisted by simulation and modeling. However, current tools are inadequate to the task of modeling an untethered communications system that uses wideband signals and advanced components such as software radios. The DOD also has unique needs for interoperability and security of communications systems, although commercial concerns about system integrity and service availability are growing. The evolution of software radios will enable interoperability among advanced and legacy systems,

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Page 104 but this technology presents co-site interference problems that will require new solutions. Similarly, the available AJ and LPD/I technologies will need to be complemented with security advances that accommodate global, heterogeneous communications systems and multiple security levels. The emergence of wideband, programmable radios for military applications will also depend on advances in hardware components such as antennas, which need to be designed for mobile units, and filters, which need to be miniaturized and designed for wideband applications. These issues are examined further in Chapter 3, which explores the opportunities for synergy between the commercial and military sectors in the development of advanced wireless communications systems. Notes 1. These error rates are associated with the link layer of the OSI model and are commonly accepted as tolerable for these applications. At higher levels of the OSI model the use of error-correction protocols can improve the rates. 2. These definitions apply only to unmodulated waveforms. Modulation changes the phase and frequency with time (see Section 2.1.3) such that the definitions are no longer accurate. 3. There are numerous path loss models that conform to a variety of propagation mechanisms, including free space, reflection, diffraction, scattering, or some combination of these (Rappaport, 1996). 4. The relationship is L = Pr/Pt = K/f2dn, where Pr is received power, Pt is transmitter power, f is the center frequency of the transmitted signal, and K is a constant that depends on the average path loss at a reference distance d0 from the transmitter (d0 is the far field of the antenna, typically 1 m for indoor environments and 0.1–1 km for outdoor environments). The exponent n is the path loss exponent. 5. This analysis is based on the assumption that the channel is changing slowly enough to allow for adaptation, and that the channel fading can be estimated accurately at the receiver and this information fed back to the transmitter with minimal delay. 6. A RAKE receiver produces a coherent sum of individual multipath components of the received signal. The components can be weighted based on their signal strength to maximize the SNR of the RAKE output. The sum provides an estimate of the transmit signal. A RAKE receiver is essentially another form of diversity because the spreading code induces a time diversity on the transmitted signal such that independent multipath components can be resolved. 7. If multiple systems share the same bandwidth without any channel access coordination and are not interoperable, then some technique is still needed to enable efficient operations. Etiquette rules permit incompatible systems to coexist when using the same bandwidth (whereas interoperability requires standardization—agreement on all waveforms and protocols before systems are built and deployed). The Wireless Information Network Forum, an industry group, has

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Page 105 defined etiquette rules for the unlicensed personal communications bands and has taken the same basic approach for the 60-GHz spectrum allocation (Steer, 1994). The key elements of etiquette rules are (1) listen before transmitting to ensure that the transmitter is the only user of the spectrum, thereby minimizing the possibility of interfering with other spectrum users; (2) limit transmission time to allow others to use the spectrum in a fair manner; and (3) limit transmitter power so as not to interfere with users in a nearby spectrum. 8. For many networks, including voice-oriented cellular networks, the number of transmitters active at any one time is much smaller than the total number of possible transmitters that need be recognized by the hub station. The unpredictable and dynamic nature of the set of active transmitters clearly precludes the fixed assignment of separate channels to each transmitter. 9. Simultaneous detection of multiple users is not currently possible because of the increased complexity required in the receiver. Multiuser detection schemes also require low BERs because bits that are incorrectly detected are subtracted from the signals of other users, possibly causing those signals to be decoded in error as well. 10. These analyses are based on simplifying assumptions about the hardware and communications environment; many of these assumptions would break down in a real operating environment. Moreover, it is not known which technique has a higher spectral efficiency in flat or frequency-selective fading, particularly when countermeasures to fading are used. 11. Techniques are available to avert the delay. For example, a certain number of packet slots can be allocated for unreserved transmissions using a contention scheme. The successful sending of a packet in this slot is taken as a request for a reserved slot (or two) in the next round-trip. As long as the slots are used this reservation continues to be available, and as long as there is capacity the reservation is allowed to grow. But the reservation is abandoned as soon as the sender does not use the slot. This approach involves no delay (except for contention failures on the first packet), poses contention issues only for the first packet in a burst, and matches the natural behavior of the TCP slow-start phase (which is described later in this chapter). However, for applications that alternate a short message in each direction (e.g., transaction processing) the procedure still produces latency equal to one round-trip for each message, and, assuming fixed-length slots and a perfect fit between the data to be transmitted and a slot, has a fundamental throughput limit of 33 percent. If the transmission is smaller than a slot, then the throughput will be even lower (and lower still in many realistic applications with short transaction times). If the information to be transmitted is less than or even comparable to the amount of information required to set up the DAMA resources, then efficiency will be compromised. 12. Assuming that a collision results in the loss of two packets, the maximum throughput in an ALOHA channel is about 18 percent of the peak data rate if the probability of a collision is to be reduced to a level acceptable to the user. Various modifications of ALOHA channels, such as slotted ALOHA or CSMA/CD, can increase efficiency, but they also impose restrictions on data transmission. 13. The latency of Mobile IP is typically much less than a second—the time it

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Page 106 takes for one round-trip between the foreign agent and home agent, or perhaps two round-trips counting the time for message receipt verification. 14. Route optimization is an enhancement to the base specification for Mobile IP and has not to date reached an equivalent level of standardization within the IETF. Mobile IP is a proposed standard (RFC 2002-2006), whereas route optimization has yet to be standardized or shown to be interoperable in multiple implementations. 15. A typical simulator accepts as input a description of network topology, protocols, workload, and control parameters. The output includes a variety of statistics, such as the number of packets sent by each source of data, the queuing delay at each queuing point, and the number of dropped and retransmitted packets. Visualization packages have been developed to allow the simulator's dynamic execution history to be made visible to the network designer. The simulators are designed in such a way that they can modified easily by users. 16. Recent research has investigated the adaptation of compression algorithms to a channel of varying quality (i.e., a channel with fading or varying noise or interference levels). Such adaptation can reduce distortion significantly. This design is based on the idea that, because the transmission rate is constant, this rate needs to be divided between the compression algorithm and the channel code. The optimal way to divide the transmission rate and minimize distortion is the following: On a channel with high SNR, no channel coding is needed, and all the rate is allocated to the compression scheme; as the channel quality degrades, more of the rate is allocated to the channel coding to remove most of the effects of channel errors. However, joint compression and channel coding creates some problems. First, this approach requires that the compression algorithms, which typically sit at the application layer, have access to information about the link layer, which means that the layer separation of the open-systems interconnection model breaks down. Second, the design can become very complicated. It is often easier to design the compression algorithms and the channel coding independently and then ''glue" them together (the compression and coding communities prefer this approach because they have developed separate languages and perspectives, which make it difficult for them to work together). Some future cellular systems will implement a crude form of this joint design using "vocoders" (compression schemes for voice) that operate at multiple rates. If the channel has a high SNR, then the higher-rate vocoder (which performs poorly at low SNRs) is used, and the vocoder rate is decreased as the channel quality decreases. 17. The Internet community is carrying forward two proposals for real-time service: Guaranteed service provides per-flow hard guarantees (i.e., no statistical aggregation or probabilistic bounds), whereas controlled-load service provides a probabilistic bound based on aggregation of a number of real-time flows into one scheduling class. Although guaranteed service provides a delay bound that is computed in advance, controlled load provides a bound that is stable but not explicitly computed. The application must adapt to the service it receives. Both are set up using RSVP. The soft and hard states differ in terms of what happens when a route fails. In ATM the connection is cleared and no traffic is delivered until a new connection is established. In Internet/RSVP the packets start flowing once the routing tables have found a new route, but only with default QoS until

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Page 107 RSVP reestablishes the state. In both cases the new request may fail if there is not enough capacity after the failure. 18. The Wireless ATM Working Group of the ATM Forum (an industry group) is addressing the problems of end user mobility. This effort may be the only avenue for extending ATM to the end user. 19. Latencies in the wireless channel are not only high but also variable over time because of fluctuations in retransmission. Forward error correction can mitigate this problem somewhat but imposes a penalty even when the channel quality is good. 20. Because of the error characteristics of wireless links, some of the QoS issues need to be addressed locally at the link layer rather than from an end-to-end perspective. The DARPA PRNet had a strategy of accomplishing enough at the link level that TCP could handle the remaining reliability issues. However, this approach requires interaction between the link layer and higher layers (e.g., if the link layer needs to implement a stronger channel code, then its transmission rate may be reduced or its delay increased). In addition, the wireless channel may be so degraded that little can be done at the link level to improve matters. There needs to be a way to cope with this situation through higher-layer protocols. 21. Software security is another category but it is not unique to wireless communications and therefore is not addressed here. 22. Some security concerns are being alleviated in the transition from analog to digital systems, which offer an inherent advantage because the meaning of a pattern of 1s and 0s cannot be casually discerned. 23. For example, systems based on the GSM standard keep the key in a separate smart card, not in the telephone. 24. For example, most contemporary software radios use commercial filters by Graychip, Inc., or Harris Corp. for highly programable channel access to FDMA, TDMA, and CDMA systems with the low size, weight, and power of ASICs. 25. A CCD detector turns light into an electric charge, which is then transformed into the binary code recognized by computers. Some commercial cameras use this technology, but they remain expensive.