Embed Size (px)
DESCRIPTION
good for 4G
Citation preview
29Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Chapter 2
Technical challenges of wireless
communications
30Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
The major challenges
• Multipath propagation
• Spectrum limitations
• Limited energy
• User mobility
31Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Multipath propagation
32Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Small-scale fading
Signal on directpath arrives first
Signal on reflectedpath arrives later
Constructive (self-)interference Destructive (self-)interference
+ = + =
RXTX
33Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Large-scale fading
d
Received power at distance d [log scale]
Position
A B C C
A
B
C
D
34Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Consequences of fading
• Error probability is dominated by probability of being in a
fading dip
• Error probability decreases only linearly with increasing
SNR
• Fighting the effects of fading becomes essential for wireless
transceiver design
• Deterministic modeling of channel at each point very
difficult
• Statistical modeling of propagation and system behavior
35Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Intersymbol interference (1)
• Channel impulse response is delay-dispersive
36Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Intersymbol interference (2)
37Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Spectrum assignment
• <100 MHz: CB radio, pagers, and analogue cordless phones.
• 100-800 MHz: broadcast (radio and TV)
• 400-500 MHz: cellular and trunking radio systems
• 800-1000 MHz: cellular systems (analogue and second-generation digital); emergency communications
• 1.8-2.0 GHz: main frequency band for cellular and cordless
• 2.4-2.5 GHz: cordless phones, wireless LANs and wireless PANs (personal area networks); other devices, e.g., microwave ovens.
• 3.3-3.8 GHz: fixed wireless access systems
• 4.8-5.8 GHz: wireless LANs
• 11-15 GHz: satellite TV
38Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Frequency reuse
• Available spectrum is limited
• -> the same frequency (range) has to be used at many
different locations
• Regulated spectrum:
– a single operator owns the spectrum and can determine where to
put TXs
– cell planning so that interference adheres to certain limits
• Unregulated spectrum:
– Often only one type of service allowed,
– Nobody can control location of interferers
– Power of interferers is limited by regulations
39Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Duplexing and multiple access
• Within each frequency band, multiple users need to
communicate with one BS (multiple access)
• Cellphones have to be able to transmit and receive voice
communications (duplexing)
Mobile telephony, wireless LAN, ...
40Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
DUPLEX
Frequency-division Duplex (FDD)
DuplexfilterU
p lin
k
Do
wn
lin
k
Receiver
Transmitter
FDD gives a more complexsolution (the duplex filter).
Can be used for continuoustransmission.
Frequency
Examples: Nodic Mobile Telephony (NMT), Global System for Mobile communications (GSM),
Wideband CDMA (WCDMA)
41Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
DUPLEX
Time-division duplex (TDD)
Up
lin
k
Do
wn
lin
k
Up
lin
k
Do
wn
lin
k
Up
lin
k
Do
wn
lin
kTransmitter
Receiver
Duplex switch
TDD gives a low complexitysolution (the duplex switch).
Cannot be used for continuoustransmission.
Time
Examples: Global System for Mobile communications (GSM),
Wideband CDMA (WCDMA)
42Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
MULTIPLE ACCESS
Frequency-division multiple access (FDMA)C
ode
Users are separatedin frequency bands.
Examples: Nordic Mobile Telephony (NMT), Advanced Mobile Phone System (AMPS)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 13
FDMA (Frequency Division Multiple Access)
User 1 User 2
User n
…
Time
Frequency
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 14
FDMA Bandwidth Structure
1 2 3 … n Frequency
Total bandwidth
4
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 15
FDMA Channel Allocation
Frequency 1 User 1
Frequency 2 User 2
Base Station
Frequency n User n
… …
Mobile Stations
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 22
Frequency Hopping Frequency
f5
f4
f3
f2
f1
Frame Slot
Time
43Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
MULTIPLE ACCESS
Time-division multiple access (TDMA)C
ode US
ER
1
US
ER
2
US
ER
3
US
ER
1
US
ER
2
Users are separatedin time slots.
Example: Global System for Mobile communications (GSM)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 16
TDMA (Time Division Multiple Access)
Use
r 1
Use
r 2
Use
r n
…
Time
Frequency
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 17
TDMA Frame Structure
…
Time
Frame
1 2 3 n 4
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 18
TDMA Frame Illustration for Multiple Users
Time 1
Time 2
Time n
… …
Base Station
User 1
User 2
User n
…
Mobile Stations
44Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
MULTIPLE ACCESS
Code-division multiple access (CDMA)C
ode
Users are separatedby spreading codes.
Examples: CdmaOne, Wideband CDMA (WCDMA), Cdma2000
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 19
CDMA (Code Division Multiple Access)
Time
Frequency
Use
r n
Code U
ser
1 U
ser
2
. . .
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 20
Transmitted and Received Signals in a CDMA System
Information bits
Code at transmitting end
Transmitted signal
Received signal
Code at receiving end
Decoded signal at the receiver
45Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
MULTIPLE ACCESS
Carrier-sense multiple access (CSMA)C
ode
US
ER
1
US
ER
3
Users are separatedin time but not in
an organized way.The terminal listens to
the channel, andtransmits a
packet if it’s free.U
SE
R 2
US
ER
2Collissions
can
occur and
data is lost.
Example: IEEE 802.11 (WLAN)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 21
OFDM (Orthogonal Frequency Division Multiplexing)
Conventional multicarrier modulation used in FDMA
Orthogonal multicarrier modulation used in OFDM Frequency
Frequency
46Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
User mobility
• User can change position
• Mobility within one cell (i.e., maintaining a link to a certain
BS): biggest impact on channel propagation - fading
• Mobility from cell to cell:
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 12
Fundamentals of Cellular Systems
Illustration of a cell with a mobile station and a base station
BS
MS
Cell MS
Alternative shape of a cell
Ideal cell area (2-10 km radius)
Hexagonal cell area used in most models
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 23
Cellular System Infrastructure
BS
Service area (Zone)
Early wireless system: Large zone
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 24
Cellular System: Small Zone
BS BS
BS BS BS
BS BS
Service area
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 25
PSTN
Home phone
BSC
…
… … BSC
…
BSC
…
BSC
…
BS MS BS MS BS MS BS MS BS MS BS MS BS MS BS MS
MSC … MSC
MS, BS, BSC, MSC, and PSTN
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 26
Control and Traffic Channels
Base Station (BS) Mobile Station (MS)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 27
Call Setup from MS (Cell Phone) to BS?
BS MS
1. Need to establish path
2. Frequency/time slot/code assigned (FDMA/TDMA/CDMA)
3. Control Information Acknowledgement 4. Start communication
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 28
Steps for A Call Setup from BS to MS
BS MS
2. Ready to establish a path
3. Use frequency/time slot/code (FDMA/TDMA/CDMA) 4. Ready for communication
5. Start communication
1. Call for MS # pending
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved 29
A Simplified Wireless Communications System Representation
Information received
(Voice/Data)
Information to be
transmitted (Voice/Data)
Coding Modulator Transmitter
Decoding Demodulator Receiver
Antenna
Antenna Carrier
Carrier
