Chapter 2
Technical challenges of wireless
communications
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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The major challenges
• Multipath propagation
• Spectrum limitations
• Limited energy
• User mobility
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Multipath propagation
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Small-scale fading
Signal on reflected
path arrives later
Signal on direct
path arrives first
TX
Constructive (self-)interference
+
=
RX
Destructive (self-)interference
+
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
=
32
Large-scale fading
Received power at distance d [log scale]
D
C
d
Position
A
B
C
B
A
C
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Consequences of fading
• Error probability is dominated by probability of being in a
fading dip
• Error probability decreases only linearly with increasing
SNR interference limited e.g., a probalistic environment as compared to a noise limited environment
• 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
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
34
Intersymbol interference (1)
• Channel impulse response is delay-dispersive
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Intersymbol interference (2)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Spectrum assignment
(VHF/UHF/Microwave)
• <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 secondgeneration 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
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
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Frequency reuse
• Available spectrum is limited
• -> the same frequency (range) has to be used at many
different locations
• Regulated spectrum: Licensed 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:
Unlicensed spectrum
– Often only one type of service allowed,
– Nobody can control location of interferers
– Power of interferers is limited by regulations
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
38
Duplexing and multiple access
• Within each frequency band, multiple users need to
communicate with one BS (multiple access)
Mobile telephony, wireless LAN, ...
• Cellphones have to be able to transmit and receive voice
communications (duplexing) talk and listen at the same time
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
39
Down link
Up link
DUPLEX
Frequency-division Duplex (FDD)
Duplex
filter
Transmitter
Receiver
Frequency
FDD gives a more complex
solution (the duplex filter).
Can be used for continuous
transmission.
Examples: Nodic Mobile Telephony (NMT), Global System for Mobile communications (GSM),
Wideband CDMA (WCDMA)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
40
Down link
Up link
Down link
Up link
Down link
Up link
DUPLEX
Time-division duplex (TDD)
Transmitter
Receiver
Time
TDD gives a low complexity
solution (the duplex switch).
Cannot be used for continuous
transmission.
Duplex
switch
Examples: Global System for Mobile communications (GSM),
Wideband CDMA (WCDMA)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
41
MULTIPLE ACCESS
Frequency-division multiple access (FDMA)
Code
Users are separated
in frequency bands.
Examples: Nordic Mobile Telephony (NMT), Advanced Mobile Phone System (AMPS)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
42
FDMA
(Frequency Division Multiple Access)
Frequency
User n
…
User 2
User 1
Time
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
13
FDMA Bandwidth Structure
1
2
3
4
…
n
Frequency
Total bandwidth
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
14
FDMA Channel Allocation
User 1
User 2
…
User n
Mobile Stations
Frequency 1
Frequency 2
…
Frequency n
Base Station
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
15
Frequency Hopping
Frequency
Frame
one technique used in spread spectrum
Slot
f1
f2
f3
f4
f5
Time
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
22
MULTIPLE ACCESS
Time-division multiple access (TDMA)
USER 2
USER 1
USER 3
USER 2
USER 1
Code
Users are separated
in time slots.
Example: Global System for Mobile communications (GSM)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
43
TDMA
(Time Division Multiple Access)
…
User n
User 2
User 1
Frequency
Time
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
16
TDMA Frame Structure
…
1
2
3
4
n
Time
Frame
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
17
TDMA Frame Illustration
for Multiple Users
Time 1
…
User 2
User n
Time 2
…
User 1
…
Time n
Mobile Stations
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
Base Station
18
MULTIPLE ACCESS
Code-division multiple access (CDMA)
Users are separated
by spreading codes.
Code
* codes of a very special
mathematical nature - orthogonal
Examples: CdmaOne, Wideband CDMA (WCDMA), Cdma2000
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
44
CDMA
(Code Division Multiple Access)
.
User 1
..
User 2
User n
Frequency
Time
Code
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
19
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
20
MULTIPLE ACCESS
Carrier-sense multiple access (CSMA)
USER 2
USER 2
Users are separated
in time but not in
an organized way.
The terminal listens to
the channel, and
transmits a
packet if it’s free.
USER 3
Code
USER 1
Fundamental property of Ethernet
Collissions
can
occur and
data is lost.
Example: IEEE 802.11 (WLAN)
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
45
OFDM
(Orthogonal Frequency Division Multiplexing)
Frequency
Conventional multicarrier modulation used in FDMA
Frequency
Orthogonal multicarrier modulation used in OFDM
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
21
OFDM
Analog
View
Digital
Implementation
MIMO
(3 techniques)
beamforming (gain)
diversity (Tx & Rx)
spatial multiplexing*
*Each Rx antenna
will see all Tx signals
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: various techniques used to manage moving a cell
customer from one cell to another adjacent cell while
maintaining the connection
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
46
Fundamentals of Cellular Systems
Ideal cell area
(2-10 km radius)
Cell
Alternative
shape of a cell
BS
MS
MS
Hexagonal cell area
used in most models
Illustration of a cell with a mobile station and a base station
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
12
Cellular System Infrastructure
Service area (Zone)
BS
Early wireless system: Large zone
1st Generation of Cell System which was an analog system
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
23
Cellular System: Small Zone
BS
BS
BS
BS
BS
Service area
BS
BS
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
24
MS, BS, BSC, MSC, and PSTN
Majority of cell system traffic is
on a wired (digital) medium, only
base station to mobile station (user)
is actually wireless
Home phone
PSTN
Public
Switched Telephone
Network
MSC
BSC
…
Mobile
Switching
Center
BSC
…
…
BS MS
BS MS
BS MS
…
Base
Station
Controller
BS MS
MSC
…
BSC
BSC
…
BS MS
…
BS MS
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
BS MS
BS MS
25
Control and Traffic Channels
Mobile Station
(MS)
Base Station (BS)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
26
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
27
Steps for A Call Setup
from BS to MS
BS
MS
1. Call for MS # pending
2. Ready to establish a path
3. Use frequency/time slot/code
(FDMA/TDMA/CDMA)
4. Ready for communication
5. Start communication
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
28
A Simplified Wireless Communications
System Representation
Antenna
Information
to be
transmitted
(Voice/Data)
Information
received
(Voice/Data)
Coding
Modulator
Transmitter
Carrier
Decoding
Demodulator
Antenna
Receiver
Carrier
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved
29