Technical Issues
The technical problems that must be solved to deliver high-performance wireless systems extend acrossall levels of the system design. At the hardware level the terminal must have multiple modes of operation
to support the different applications and media. Desktop computers currently have the capability to
process voice, image, text, and video data, but breakthroughs in circuit design are required to implement
multimode operation in a small, lightweight, handheld device. Since most people don’t want to carry
around a twenty pound battery, the signal processing and communications hardware of the portable
terminal must consume very little power, which will impact higher levels of the system design. Many of
the signal processing techniques required for efficient spectral utilization and networking demand much
processing power, precluding the use of low power devices. Hardware advances for low power circuits
with high processing ability will relieve some of these limitations. However, placing the processing burden
on fixed sites with large power resources has and will continue to dominate wireless system designs. The
associated bottlenecks and single points-of-failure are clearly undesirable for the overall system. Moreover,
in some applications (e.g. sensors) network nodes will not be able to recharge their batteries. In this
case the finite battery energy must be allocated efficiently across all layers of the network protocol stack
[5]. The finite bandwidth and random variations of the communication channel will also require robust
compression schemes which degrade gracefully as the channel degrades.
The wireless communication channel is an unpredictable and difficult communications medium. First
of all, the radio spectrum is a scarce resource that must be allocated to many different applications and
systems. For this reason spectrum is controlled by regulatory bodies both regionally and globally. In 
the U.S. spectrum is allocated by the FCC, in Europe the equivalent body is the European Telecommu-
nications Standards Institute (ETSI), and globally spectrum is controlled by the International Telecom-
munications Union (ITU). A regional or global system operating in a given frequency band must obey
the restrictions for that band set forth by the corresponding regulatory body as well as any standards
adopted for that spectrum. Spectrum can also be very expensive since in most countries, including the
U.S., spectral licenses are now auctioned to the highest bidder. In the 2 GHz spectral auctions of the
early 90s, companies spent over nine billion dollars for licenses, and the recent auctions in Europe for 3G
spectrum garnered over 100 billion dollars. The spectrum obtained through these auctions must be used
extremely efficiently to get a reasonable return on its investment, and it must also be reused over and
over in the same geographical area, thus requiring cellular system designs with high capacity and good
performance. At frequencies around several Gigahertz wireless radio components with reasonable size,
power consumption, and cost are available. However, the spectrum in this frequency range is extremely
crowded. Thus, technological breakthroughs to enable higher frequency systems with the same cost and
performance would greatly reduce the spectrum shortage, although path loss at these higher frequencies
increases, thereby limiting range.
As a signal propagates through a wireless channel, it experiences random fluctuations in time if the
transmitter or receiver is moving, due to changing reflections and attenuation. Thus, the characteristics
of the channel appear to change randomly with time, which makes it difficult to design reliable systems
with guaranteed performance. Security is also more difficult to implement in wireless systems, since
the airwaves are susceptible to snooping from anyone with an RF antenna. The analog cellular systems
have no security, and you can easily listen in on conversations by scanning the analog cellular frequency
band. All digital cellular systems implement some level of encryption. However, with enough knowledge,
time and determination most of these encryption methods can be cracked and, indeed, several have been compromised. To support applications like electronic commerce and credit card transactions, the wireless
network must be secure against such listeners.
Wireless networking is also a significant challenge. The network must be able to locate a given user
wherever it is amongst millions of globally-distributed mobile terminals. It must then route a call to that
user as it moves at speeds of up to 100 mph. The finite resources of the network must be allocated in
a fair and efficient manner relative to changing user demands and locations. Moreover, there currently
exists a tremendous infrastructure of wired networks: the telephone system, the Internet, and fiber optic
cable, which should be used to connect wireless systems together into a global network. However, wireless
systems with mobile users will never be able to compete with wired systems in terms of data rate and
reliability. The design of protocols to interface between wireless and wired networks with vastly different
performance capabilities remains a challenging topic of research.
Perhaps the most significant technical challenge in wireless network design is an overhaul of the
design process itself. Wired networks are mostly designed according to the layers of the OSI model.
The most relevant layers of this model for wireless systems are the link or physical layer, which handles
bit transmissions over the communications medium, the multiple access layer, which handles shared
access to the communications medium, the network layer, which routes data across the networks, and
the application layer, which dictates the end-to-end data rates and delay constraints associated with
the application. In the OSI model each layer of the protocol stack is designed independent from the
other layers with baseline mechanisms to interface between layers. This methodology greatly simplifies
network design, although it leads to some inefficiency and performance loss due to the lack of a global
design optimization. However, the large capacity and good reliability of wired network links make it
easier to buffer high-level network protocols from the lower level protocols for link transmission and
access, and the performance loss resulting from this isolated protocol design is fairly low. However, the
situation is very different in a wireless network. Wireless links can exhibit very poor performance, and
this performance along with user connectivity and network topology changes over time. In fact, the very
notion of a wireless link is somewhat fuzzy due to the nature of radio propagation. The dynamic nature
and poor performance of the underlying wireless communication channel indicates that high-performance
wireless networks must be optimized for this channel and must adapt to its variations as well as to user
mobility. Thus, these networks will require an integrated and adaptive protocol stack across all layers
of the OSI model, from the link layer to the application layer. This cross-layer design approach draws
from many areas of expertise, including physics, communications, signal processing, network theory and
design, software design, and hardware design. Moreover, given the fundamental limitations of the wireless
channels and the explosive demand for its utilization, communication between these interdisciplinary
groups is necessary to implement systems that can achieve the wireless vision described in the previous
section.
In the next section we give an overview of the wireless systems in operation today. It will be clear
from this overview that the wireless vision remains a distant goal, with many challenges remaining before
it will be realized. Many of these challenges will be examined in detail in later chapters.
the U.S. spectrum is allocated by the FCC, in Europe the equivalent body is the European Telecommu-nications Standards Institute (ETSI), and globally spectrum is controlled by the International Telecom-
munications Union (ITU). A regional or global system operating in a given frequency band must obey
the restrictions for that band set forth by the corresponding regulatory body as well as any standards
adopted for that spectrum. Spectrum can also be very expensive since in most countries, including the
U.S., spectral licenses are now auctioned to the highest bidder. In the 2 GHz spectral auctions of the
early 90s, companies spent over nine billion dollars for licenses, and the recent auctions in Europe for 3G
spectrum garnered over 100 billion dollars. The spectrum obtained through these auctions must be used
extremely efficiently to get a reasonable return on its investment, and it must also be reused over and
over in the same geographical area, thus requiring cellular system designs with high capacity and good
performance. At frequencies around several Gigahertz wireless radio components with reasonable size,
power consumption, and cost are available. However, the spectrum in this frequency range is extremely
crowded. Thus, technological breakthroughs to enable higher frequency systems with the same cost and
performance would greatly reduce the spectrum shortage, although path loss at these higher frequencies
increases, thereby limiting range.
As a signal propagates through a wireless channel, it experiences random fluctuations in time if the
transmitter or receiver is moving, due to changing reflections and attenuation. Thus, the characteristics
of the channel appear to change randomly with time, which makes it difficult to design reliable systems
with guaranteed performance. Security is also more difficult to implement in wireless systems, since
the airwaves are susceptible to snooping from anyone with an RF antenna. The analog cellular systems
have no security, and you can easily listen in on conversations by scanning the analog cellular frequency
band. All digital cellular systems implement some level of encryption. However, with enough knowledge,
time and determination most of these encryption methods can be cracked and, indeed, several have been compromised. To support applications like electronic commerce and credit card transactions, the wireless
network must be secure against such listeners.
Wireless networking is also a significant challenge. The network must be able to locate a given user
wherever it is amongst millions of globally-distributed mobile terminals. It must then route a call to that
user as it moves at speeds of up to 100 mph. The finite resources of the network must be allocated in
a fair and efficient manner relative to changing user demands and locations. Moreover, there currently
exists a tremendous infrastructure of wired networks: the telephone system, the Internet, and fiber optic
cable, which should be used to connect wireless systems together into a global network. However, wireless
systems with mobile users will never be able to compete with wired systems in terms of data rate and
reliability. The design of protocols to interface between wireless and wired networks with vastly different
performance capabilities remains a challenging topic of research.
Perhaps the most significant technical challenge in wireless network design is an overhaul of the
design process itself. Wired networks are mostly designed according to the layers of the OSI model.
The most relevant layers of this model for wireless systems are the link or physical layer, which handles
bit transmissions over the communications medium, the multiple access layer, which handles shared
access to the communications medium, the network layer, which routes data across the networks, and
the application layer, which dictates the end-to-end data rates and delay constraints associated with
the application. In the OSI model each layer of the protocol stack is designed independent from the
other layers with baseline mechanisms to interface between layers. This methodology greatly simplifies
network design, although it leads to some inefficiency and performance loss due to the lack of a global
design optimization. However, the large capacity and good reliability of wired network links make it
easier to buffer high-level network protocols from the lower level protocols for link transmission and
access, and the performance loss resulting from this isolated protocol design is fairly low. However, the
situation is very different in a wireless network. Wireless links can exhibit very poor performance, and
this performance along with user connectivity and network topology changes over time. In fact, the very
notion of a wireless link is somewhat fuzzy due to the nature of radio propagation. The dynamic nature
and poor performance of the underlying wireless communication channel indicates that high-performance
wireless networks must be optimized for this channel and must adapt to its variations as well as to user
mobility. Thus, these networks will require an integrated and adaptive protocol stack across all layers
of the OSI model, from the link layer to the application layer. This cross-layer design approach draws
from many areas of expertise, including physics, communications, signal processing, network theory and
design, software design, and hardware design. Moreover, given the fundamental limitations of the wireless
channels and the explosive demand for its utilization, communication between these interdisciplinary
groups is necessary to implement systems that can achieve the wireless vision described in the previous
section.
In the next section we give an overview of the wireless systems in operation today. It will be clear
from this overview that the wireless vision remains a distant goal, with many challenges remaining before
it will be realized. Many of these challenges will be examined in detail in later chapters.
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