-
1.3 Technical Issues
Many
technical challenges must be addressed to enable
the wireless applications of the future.
These challenges extend across all
aspects of the system design. As wireless
terminals add more
features, these
small devices must incorporate multiple modes of
operation to support the different
applications and media. Computers
process voice, image, text, and video data, but
breakthroughs
in circuit design are
required to implement the same multimode operation
in a cheap, lightweight,
handhe
ld
device.
Since
consumers
don’t
want
large
batteries
that
frequently
need
recharging,
transmission
and
signal
processing
in
the
portable
terminal
must
consume
minimal
power.
The
signal
processing
required
to
support
multimedia
applications
and
networking
functions
can
be
power-intensive. Thus, wireless
infrastructure-based networks, such as wireless
LANs and cellular
systems,
place
as
much
of
the
processing
burden
as
possible
on
?xed
sites
with
large
power
resources. The
associated bottlenecks and single points-of-
failure are clearly undesirable for the
overall
system.
Ad
hoc
wireless
networks
without
infrastructure
are
highly
appealing
for
many
ap
plications due to their
?exibility and robustness. For these networks all
processing
and control
must
be
performed
by
the
network
nodes
in
a
distributed
fashion,
making
energy-
ef?ciency
challenging to achieve. Energy is a
particularly critical resource in networks where
nodes cannot
recharge
their
batteries,
for
example
in
sensing
applications.
Network
design
to
meet
the
application requirements under such
hard energy constraints remains a big
technological hurdle.
The ?nite
bandwidth and random variations
of
wireless channels also requires robust
applications
that degrade gracefully as
network performance degrades.
Design
of
wireless
networks
differs
fundamentally
from
wired
network
design
due
to
the
nature
of
the
wireless
channel.
This
channel
is
an
unpred
ictable
and
dif?cult
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.
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. Spectrum
can
also
be
very
expensive
since
in
many
countries
spectral
licenses
are
often
auctioned
to
the
highest
bidder. In the U.S. companies spent over nine
billion dollars for second generation cellular
licenses, and the auctions in Europe
for third generation cellular spectrum garnered
around 100
billion dollars. The
spectrum obtained through these
aucti
ons must be used extremely
ef?ciently 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. However, path loss at
these
higher frequencies is larger, thereby limiting
range, unless directional antennas are used.
As a signal propagates through a
wireless channel, it experiences
random?uctuations in time
if
the
transmitter,
receiver,
or
surrounding
objects
are
moving,
due
to
changing
re?ections
and
attenuation. Thus, the characteristics
of the channel appear to change randomly with
time, which
makes it dif?cult to design
reliable
systems with guaranteed
performance. Security is also more
dif?cult
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
one
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.
W
ireless
networking
is
also
a
signi?cant
challenge.
The
network
must
be
able
to
locate
a
given
user
wherever
it
is
among
billions
of
globally-distributed
mobile
terminals.
It
must
then
route
a
call
to
that
user
as
it
moves
at
speeds
of
up
to
100
Km/hr.
The
?
nite
resources
of
the
network must be allocated in a fair and
ef?cient manner
relative to changing
user demands and
locations.
Moreover,
there
currently
exists
a
tremendous
infrastructure
of
wired
networks:
the
telephone
system,
the
Internet,
and
?ber
opt
ic
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 rates
and reliability. Interfacing between
wireless and wired networks with
vastly different performance
capabilities is a dif?cult problem.
Perhaps the most signi?cant technical
challenge in wireless network design is an
overhaul of
the
design
process
itself.
Wired
networks
are
mostly
designed
according
to
a
layered
approach,
whereby
protocols
associated
with
different
layers
of
the
system
operation
are
designed
in
isolation, with baseline mechanisms to
interface between layers. The layers in a wireless
systems
include
the
link
or
physical
layer,
which
handles
bit
transmissions
over
the
communications
medium,
the
access
layer,
which
handles
shared
access
to
the
communications
medium,the
network
and
transport
layers,
which
routes
data
across
the
network
and
insure
end-to-end
connectivity and data delivery, and the
application layer, which dictates the end-to-end
data rates
and
delay
constraints
associated
with
the
application.
While
a
layering
methodology
reduces
complexity
and
facilitates
modularity
and
standardization,
it
also
leads
to
inef?ciency
and
performance
loss
due
to
the
lack
of
a
global
design
optimization.
The
large
capacity
and
good
reliability of wired networks make
these inef?ciencies relatively benign for many
wired network
applications,
although it does preclude good performance of
delay-constrained applications such
as
voice and video. 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
and
broadcasting.
The
dynamic
nature
and
poor
performance
of
the
underlying
wireless
communication
channel
indicates
that
high-performance
networks
must
be
optimized for this channel and must be
robust and adaptive to its variations, as well as
to network
dynamics. Thus, these
networks require integrated and adaptive protocols
at all layers, from the
link
layer
to
the
application
layer.
This
cross-layer
protocol
design
requires
interdiciplinary
expertise in communications, signal
processing, and network theory and design.
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
technical
challenges to
overcome. These challenges will be examined in
detail throughout the book.
1.4
CurrentWireless Systems
This
section
provides
a
brief
overview
of
current
wireless
systems
in
operation
today.
The
design
details of these system are constantly evolving,
with new systems emerging and old ones
going by the wayside. Thus, we will
focus mainly on the high-level design aspects of
the most
common systems. More details
on wireless system standards can be found in [1,
2, 3] A summary
of the main wireless
system standards is given in Appendix D.
1.4.1 Cellular Telephone Systems
Cellular
telephone
systems
are
extremely
popular
and
lucrative
worldwide:
these
are
the
systems
that
ignited
the
wireless
revolution.
Cellular
systems
provide
two-way
voice
and
data
communication with regional, national,
or international coverage. Cellular systems were
initially
designed for mobile terminals
inside vehicles with antennas mounted on the
vehicle roof. Today
these systems have
evolved to support lightweight handheld mobile
terminals operating inside and
outside
buildings at both pedestrian and vehicle speeds.
The basic premise behind cellular
system design is frequency reuse, which exploits
the fact
that
signal
power
falls
off
with
distance
to
reuse
the
same
frequency
spectrum
at
spatially-
separated
locations.
Speci?cally,
the
coverage
area
of
a
cellular
system
is
divided
into
nonoverlapping cells where some set of
channels is assigned to each cell. This same
channel set is
used in another cell
some distance away, as shown in Figure 1.1, where
Ci denotes the channel set
used
in
a
particular
cell.
Operation
within
a
cell
is
controlled
by
a
centralized
base
station,
as
described in more detail below. The
interference caused by users in different cells
operating on the
same channel set is
called intercell interference. The spatial
separation of cells that reuse the same
channel set, the reuse distance, should
be as small as possible so that frequencies are
reused as
often
as
possible,
thereby
maximizing
spectral
ef?ciency.
However,
as
the
reuse
distance
decreases,
intercell
interference
increases,
due
to
the
smaller
propagation
distance
between
interfering cells.
Since intercell interference must remain below a
given threshold for acceptable
system
performance, reuse distance cannot be reduced
below some minimum value. In practice it
is
quite
dif?cult
to
determine
this
minimum
value
since
both
the
transmitting
and
interfering
signals
experience
random
power
variations
due
to
the
characteristics
of
wireless
signal
propagation. In order to determine the
best reuse distance and base station placement, an
accurate
characterization of signal
propagation within the cells is needed.
Initial
cellular
system
designs
were
mainly
driven
by
the
high
cost
of
base
stations,
approximately one million dollars
apiece. For this reason early cellular systems
used a relatively
small number of cells
to cover an entire city or region. The cell base
stations were placed on tall
buildings
or
mountains
and
transmitted
at
very
high
power
with
cell
coverage
areas
of
several
square miles. These
large cells are called macrocells. Signal power
was radiated uniformly in all
directions,
so
a
mobile
moving
in
a
circle
around
the
base
station
would
have
approximately
constant
received
power
if
the
signal
was
not
blocked
by
an
attenuating
object.
This
circular
contour
of constant
power yields
a
hexagonal cell
shape
for
the
system,
since
a
hexagon
is
the
closest shape to a circle that can
cover a given area with multiple nonoverlapping
cells.
Cellular systems in urban areas
now mostly use smaller cells with base stations
close to street
level
transmitting
at
much
lower
power.
These
smaller
cells
are
called
microcells
or
picocells,
depending
on
their
size.
This
evolution
to
smaller
cells
occured
for
two
reasons:
the
need
for
higher
capacity
in
areas
with
high
user
density
and
the
reduced
size
and
cost
of
base
station
electronics. A cell
of any size can support roughly the same number of
users if the system is scaled
accordingly. Thus, for a given coverage
area a system with many microcells has a higher
number
of users per unit area than a
system with just a few macrocells. In addition,
less power is required
at the mobile
terminals in microcellular systems, since the
terminals are closer to the base stations.
However, the evolution to smaller cells
has complicated network design. Mobiles traverse a
small
cell
more
quickly
than
a
large
cell,
and
therefore
handoffs
must
be
processed
more
quickly.
In
addition,
location
management
becomes
more
complicated,
since
there
are
more
cells
within
a