Technical challenges of wireless communications

Transcription

Chapter 2
Technical challenges of wireless
communications
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
29
The major challenges
• Multipath propagation
• Spectrum limitations
• Limited energy
• User mobility
30
Multipath propagation
31
Small-scale fading
Signal on reflected
path arrives later
Signal on direct
path arrives first
TX
Constructive (self-)interference
+
=
RX
Destructive (self-)interference
+
=
32
Large-scale fading
Received power at distance d [log scale]
D
C
d
Position
A
B
C
B
A
C
33
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
34
Intersymbol interference (1)
• Channel impulse response is delay-dispersive
35
Intersymbol interference (2)
36
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
37
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
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
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)
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)
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)
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
14
FDMA Channel Allocation
User 1
User 2
…
User n
Mobile Stations
Frequency 1
Frequency 2
…
Frequency n
Base Station
15
Frequency Hopping
Frequency
Frame
one technique used in spread spectrum
Slot
f1
f2
f3
f4
f5
Time
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)
43
TDMA
(Time Division Multiple Access)
…
User n
User 2
User 1
Frequency
Time
16
TDMA Frame Structure
…
1
2
3
4
n
Time
Frame
17
TDMA Frame Illustration
for Multiple Users
Time 1
…
User 2
User n
Time 2
…
User 1
…
Time n
Mobile Stations
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
44
CDMA
(Code Division Multiple Access)
.
User 1
..
User 2
User n
Frequency
Time
Code
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
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)
45
OFDM
(Orthogonal Frequency Division Multiplexing)
Frequency
Conventional multicarrier modulation used in FDMA
Frequency
Orthogonal multicarrier modulation used in OFDM
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
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
12
Cellular System Infrastructure
Service area (Zone)
BS
Early wireless system: Large zone
1st Generation of Cell System which was an analog system
23
Cellular System: Small Zone
BS
BS
BS
BS
BS
Service area
BS
BS
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
BS MS
BS MS
25
Control and Traffic Channels
Mobile Station
(MS)
Base Station (BS)
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
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
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
29

Technical challenges of wireless communications

Transcription

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