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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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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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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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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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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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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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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FIGURE 2-1 Spectral link efficiency can be
greatly increased using adaptive techniques. Theoretical
efficiencies are shown.
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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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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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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).
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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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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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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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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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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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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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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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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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.
Representative terms from entire chapter:
received signal
