Wireless Communications:

1
Wireless Communications Trends and Challenges
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OUTLINE
Introduction Radio Environment
Physical Layer Issues Channel Access
Issues Network Issues Standards and
Future Systems Summary
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WIRELESS DATA VISION
Seamless Multimedia Networks with Mobility and
Freedom from Tethers
R. Katz, "Does Wireless Data Have a Future?",
Plenary Talk, INFOCOM '96
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VOICE VERSUS DATA VERSUS VIDEO
Wired Networks Trying to Integrate
(ATM, SONET, Multimedia Services)
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WHAT IS THE FUTURE OF WIRELESS DATA?
100
USA market
90
80
70
cellular PCS subs
Internet users
60
millions
50
paging subs
40
30
dedicated wireless
laptop users
data subs
20
10
annual laptop sales
0
1995
2000
Estimates as of 1996
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THE ISSUE IS PERFORMANCE
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GAP BETWEEN WIRED AND WIRELESS NETWORK
CAPABILITIES
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TECHNICAL CHALLENGES
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RADIO ENVIRONMENT
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PATH LOSS MODEL
Different, often complicated, models are
used for different environments.
A simple model for path loss, L, is
Pr 1 Pt f2da
K
L

where Pr is the local mean received signal
power, Pt is the transmitted power, d is the
transmitter-receiver distance, f is
frequency, and K is a transmission constant.
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SHADOW FADING
Pr (dB) Pr (dB) Gs
where Gs N(0, ss ), 4 ss 10 dB.
2
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MULTIPATH
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DELAY SPREAD TIME DOMAIN INTERPRETATION
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PHYSICAL LAYER ISSUES
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LINK PERFORMANCE MEASURES PROBABILITY OF BIT ERROR
The probability of bit error, Pb, in a radio
environment is a random variable.
Typically only one of these measures is
useful, depending on the Doppler frequency
and the bit rate.
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LINK PERFORMANCE MEASURES EFFICIENCY
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GOALS OF MODULATION TECHNIQUES
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DIGITAL MODULATION
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LINEAR MODULATION TECHNIQUES
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SIGNAL CONSTELLATIONS
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PULSE SHAPING
Rectangular pulses are spectrally inefficient

pulse shaping
intersymbol interference (ISI)
non-constant envelope
Nyquist pulses

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RAISED COSINE PULSE SHAPING
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DEMODULATION
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FREQUENCY SHIFT KEYING
Continuous Phase FSK (CPFSK)
digital data encoded in the frequency
shift
typically implemented with frequency
modulator to maintain continuous phase
nonlinear modulation but
constant-envelope
Minimum Shift Keying (MSK)
minimum bandwidth, sidelobes large
can be implemented using I-Q receiver
Gaussian Minimum Shift Keying (GMSK)
reduces sidelobes of MSK using a
premodulation filter
used by RAM Mobile Data, CDPD,
and HIPERLAN
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SPECTRAL CHARACTERISTICS
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BIT ERROR PROBABILITY AWGN CHANNEL
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BIT ERROR PROBABILITY
FADING CHANNEL
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BIT ERROR PROBABILITY EFFECTS OF DOPPLER SPREAD
Doppler causes an irreducible error floor when
differential
detection is used Þ decorrelation of
reference signal.
The irreducible Pb depends on the data rate and
the Doppler.
For fD 80 Hz,
data rate T Pbfloor

10 kbps 10-4s 3x10-4
100 kbps 10-5s 3x10-6
1 Mbps 10-6s 3x10-8
The implication is that Doppler is not an issue
for high-speed
wireless data.
M. D. Yacoub, Foundations of Mobile Radio
Engineering , CRC Press, 1993
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BIT ERROR PROBABILITY EFFECTS OF DELAY SPREAD
ISI causes an irreducible error floor.
The rms delay spread imposes a limit on the
maximum bit rate
in a multipath environment. For
example, for QPSK,
t Maximum Bit Rate
Mobile (rural) 25 msec 8 kbps
Mobile (city) 2.5 msec 80 kbps
Microcells 500 nsec 400 kbps
Large Building 100 nsec 2 Mbps
J. C.-I. Chuang, "The Effects of Time Delay
Spread on Portable Radio
Communications Channels with Digital Modulation,"
IEEE JSAC, June 1987
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SUMMARY OF MODULATION ISSUES
Tradeoffs linear versus nonlinear
modulation constant envelope versus
non-constant envelope coherent versus
differential detection power efficiency versus
spectral efficiency Limitations flat
fading doppler
delay spread
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HOW DO WE OVERCOME THE LIMITATIONS IMPOSED BY
THE RADIO CHANNEL?
Flat Fading Countermeasures
Fade Margin Diversity Coding and
Interleaving Adaptive Techniques
Delay Spread Countermeasures
Equalization Multicarrier Spread Spectrum
Antenna Solutions
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DIVERSITY

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DIVERSITY COMBINING TECHNIQUES
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DIVERSITY PERFORMANCE
There is dramatic improvement even with
two-branch
selection combining.
The output SNR with Maximal-Ratio Combining
improves
linearly with the number of diversity
branches, M Þ the
complexity becomes prohibitive.
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CHANNEL CODING
-2
10
5
BPSK
2
-3
10
5
For Pb 10-6
2
Uncoded 10.5 dB
-4
Hamming 10.0 dB
10
BCH 6.5 dB
5
Uncoded
Pb
Conv. 5.0 dB
2
Hamming
(7,4,1)
-5
10
BCH
5
(127,64,10)
Conv.
2
1/2 rate
-6
(k7)
10
5
2
-7
10
0
2
4
6
8
10
12
14
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CODING PERFORMANCE FADING CHANNEL
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CODING PERFORMANCE FADING CHANNEL
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CODING AND INTERLEAVING
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ADVANCED CODING TECHNIQUES
Trellis Codes reduce Pb without
bandwidth expansion through joint design
of the channel code and signal
constellation can be designed with
built-in time diversity
Turbo Codes
exhibit enormous coding gains
interleaving inherent to code design very
complex with large delays not
well-understood for fading channels
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CODING PERFORMANCE TCM
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ADAPTIVE TECHNIQUES
Adaptive Modulation Automatic Repeat
Request
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ADAPTIVE MODULATION
Potential for large increase in spectral
efficiency
Can be combined with adaptive compression
increases transmitter and receiver complexity
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AUTOMATIC REPEAT REQUEST (ARQ)
Method of "self-adapting" the data rate to
the channel conditions
Used in combination with error-detecting code
Variations of ARQ used in Mobitex and CDPD
Types Stop-and-Wait, Go-Back-N, Selective-
Repeat
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DELAY SPREAD COUNTERMEASURES

Signal Processing
at the receiver, to alleviate the
problems
caused by delay spread
(equalization)
at the transmitter, to make the signal
less
sensitive to delay spread
(multicarrier,
spread spectrum)

Antenna Solutions
change the environment to reduce, or
eliminate, the delay spread
(distributed
antenna system, small cells, directive
antennas)
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EQUALIZER TYPES AND STRUCTURES
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LINEAR EQUALIZER
A linear equalizer effectively inverts the
channel.
The linear equalizer is usually implemented as
a
tapped delay line.
On a channel with deep spectral nulls, this
equalizer
enhances the noise.
poor performance on frequency-selective
fading channels
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DECISION FEEDBACK EQUALIZER
DFE
n(t)

x(t)
x(t)

Forward
Hc(f)
Filter
-
Feedback
Filter
The DFE determines the ISI from the previously
detected
symbols and subtracts it from the incoming
symbols.
This equalizer does not suffer from noise
enhancement
because it estimates the channel rather than
inverting it.
Þ The DFE has better performance than the linear
equalizer in a frequency-selective fading
channel.
The DFE is subject to error propagation if
decisions are
made incorrectly.
Þ no coding gain
Decisions are made on coded symbols.

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MAXIMUM LIKELIHOOD SEQUENCE ESTIMATION
MLSE has theoretically optimum performance.
It requires knowledge of the channel parameters
and
the noise distribution.

The implementation complexity grows
exponentially
with the length of the channel impulse
response Þ
not practical for high bit rates.
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EQUALIZER ISSUES FOR HIGH-SPEED WIRELESS DATA
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EQUALIZER PERFORMANCE
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MULTICARRIER MODULATION
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OFDM RECEIVER STRUCTURE
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WHAT TO DO WITH BAD SUBCHANNELS?
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MULTICARRIER MODULATION ISSUES FOR
HIGH-SPEED WIRELESS DATA
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CURRENT AND PROPOSED APPLICATIONS OF OFDM
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SPREAD SPECTRUM
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DIRECT SEQUENCE SPREAD SPECTRUM
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RAKE RECEIVER
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PERFORMANCE OF RAKE RECEIVER FADING CHANNEL
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SPREAD SPECTRUM ISSUES FOR HIGH-SPEED WIRELESS
DATA
Hardware Complexity
synchronization
high processing speeds for high
bit rates
RAKE receiver
High Required Bandwidth to Accommodate
Spreading
Spread spectrum is difficult at
high bit rates and not really needed.
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ANTENNA SOLUTIONS
Goal Reduce (or eliminate) delay spread
Distributed Antenna System
Very Small Cells Þ antenna in every room
Sectorization
Directive Antennas/Beam Steering
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DISTRIBUTED ANTENNA SYSTEM
A. A. M. Saleh, A. J. Rustako, Jr., and R. S.
Roman, "Distributed Antennas
for Indoor Radio Communications," IEEE Trans. on
Commun., December 1987
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EXAMPLES OF PERFORMANCE IMPROVEMENTS
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SUMMARY OF COUNTERMEASURES


These techniques can be combined.
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COMBINED EQUALIZATION AND SECTORED ANTENNAS
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CHANNEL ACCESS ISSUES
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MULTIPLE ACCESS TECHNIQUES
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FDMA
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TDMA
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CDMA
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IMPLICATIONS FOR HIGH-SPEED WIRELESS DATA
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RANDOM ACCESS TECHNIQUES
ALOHA Carrier-Sense Techniques
Reservation Protocols Implication for
High-Speed Wireless Data
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ALOHA
Data is packetized.
Retransmission is required when packets
collide.
Pure ALOHA
send packet whenever data is available
a collision occurs for any partial overlap of
packets
Slotted ALOHA
send packets during predefined timeslots
avoids partial overlap of packets
Slotted Aloha
Pure Aloha
Comments
inefficient for heavily loaded systems
capture effect improves efficiency
combining SS with ALOHA reduces collisions
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CARRIER-SENSE TECHNIQUES
Channel is sensed before transmission to
determine
if it is occupied.
More efficient than ALOHA Þ fewer
retransmissions
Carrier sensing is often combined with
collision
detection in wired networks (e.g., Ethernet).
Þ not possible in a radio environment

Wired Network

Wireless Network
Collision avoidance is used in current
wireless LANs.
(WaveLAN, IEEE802.11, Spectral Etiquette)
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RESERVATION PROTOCOLS
DemandBased Assignment a common
reservation channel is used to assign
bandwidth on demand reservation channel
requires extra bandwidth very efficient if
overhead traffic is a small percentage of the
message traffic Packet Reservation Multiple
Access (PRMA) similar to reservation ALOHA
uses a slotted channel structure all
unreserved slots are open for contention a
successful transmission in an unreserved slot
effectively reserves that slot for future
transmissions
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EXAMPLES
ARDIS slotted CSMA RAM Mobile Data
slotted CSMA CDPD DSMA/CD - Digital Sense
Multiple Access collisions detected at
receiver and transmitted back WaveLAN
CSMA/CA
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IMPLICATIONS FOR HIGH SPEED WIRELESS DATA
Retransmissions are power and spectrally
inefficient. ALOHA cannot satisfy
high-speed data throughput requirements.
Reservation protocols are also ineffective for
short messaging. Delay constraints impose
throughput limitations.
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FREQUENCY REUSE
Frequencies (or time slots or codes) are
reused at spatially-separated locations.
Introduces interference Þ system capacity is
interference-limited. Mainly designed for
circuit-switched communications Base stations
perform centralized control functions. (call
setup, handoff, routing, etc.)
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DESIGN CONSIDERATIONS
Reuse Distance (D)
distance between cells using the same
frequency, time slot, or code
smaller reuse distance packs more users
into a given area, but also increases their
co-channel interference
Cell Radius
decreasing the cell size increases system
capacity, but complicates the network
functions of handoff and routing
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CHANNEL ASSIGNMENT
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METHODS TO IMPROVE SPECTRUM UTILIZATION
Interference Averaging (CDMA)
Interference Reduction (power adaption,
sectorization) Interference
Cancellation (smart antennas, multiuser
detection) Interference Avoidance (dynamic
resource allocation)
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Ad-Hoc Networks

• Each node generates independent data.
• Source-destination pairs are chosen at random.
• Routing can be multihop.
• Topology is dynamic
• Generally a fully connected network with
  different link SNRs
• Can allocate resources dynamically (rate, power,
  BW, routes,)

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NETWORK ISSUES
Network Architectures Mobility
Management Network Reliability
Internetworking Security
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NETWORK ARCHITECTURES
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NETWORK CONTROL
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MOBILITY MANAGEMENT
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NETWORK RELIABILITY
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APPROACHES TO NETWORK RELIABILITY
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QUALITY OF SERVICE (QoS)
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INTERNETWORKING
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STANDARDS AND FUTURE SYSTEMS
Bluetooth Wireless LANs High-Speed
Digital Cellular (3G) 4G Cellular Wireless
"Cable" Multichannel Multipoint
Distribution Service (2.2 GHz) Local
Multipoint Distribution Service (28 GHz)
Satellite Networks - Iridium, Globalstar,
Others HomeRF
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BLUETOOTH
Cable replacement RF technology Short range
(10 meters) 2.4 GHz band 1 Data (700 Kbps)
and 3 Voice channels Supported by over 200
telecommunications and computer companies
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802.11 Wireless LANs
802.11b standard for 2.4 GHz ISM band
Frequency hopped spread spectrum 1.6 Mbps data
rates, 500 foot range Star or peer-to-peer
architecture 802.11a extends rates to 10-70
Mbps Extensions trying to add QoS
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HIPERLAN
Types 1-4 for different user types - Frequency
bands 5.15-5.3 GHz, 17.1- 17.3 GHz Type
1 - 5.15-5.3 GHz band - 23 Mbps, 20 MHz
Channels - 150 foot range (local access only) -
Protocol support similar to 802.11 - Peer to
peer architecture - ALOHA channel access
Types 2-3 - Wireless ATM - Local access and
wide area services - Standard under
development - Two components access and
mobility support
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HIGH-SPEED DIGITAL CELLULAR
North American Digital Cellular CDMA (IS-95)
enhancements TDMA (IS-136) enhancements
IS-136 Þ 32-64 kbps IS-136HS Þ 384
kbps GSM General Packet Radio System
(GPRS) Enhanced Data Rates for GSM Evolution
(EDGE)
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EDGE

• Evolution of GSM / GPRS
• ETSI standardization as GSM evolution chosen
  for data services for IS136HS
• Higher-level modulation (adaptive)
• 200 kHz carrier spacing
• Up to 384 kbps in 200 kHz

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WIDEBAND CDMA (3G)
The W-CDMA concept 4.096 Mcps Direct
Sequence CDMA Variable spreading and multicode
operation Coherent in both up-and downlink
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W-CDMAKEY TECHNICAL FEATURES
High bit-rate services require
wideband Flexibility for different
services Optimized for packet data
transfer Capacity and coverage gain from
frequency diversity Built in support for
adaptive antenna arrays multi-user
detection hierarchical cell structures
transmitter diversity Low infrastructure cost
(many users/ transceiver) BS synchronization
not required
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SUMMARY
The desire for mobility coupled with the
demand for Internet and multimedia services
indicate a bright future for wireless
data. Current products and services have
unsatisfactory performance for high-speed
wireless data applications. The inherent
limitations of the radio channel can be
significantly reduced using signal processing
and architectural techniques, at the expense
of cost and complexity. The network-level
design must take into account the physical
layer limitations of the wireless channel, as
well as the impact of user mobility.
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