Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
Mobile Communications
Chapter 2: Wireless Transmission
“This book focuses on higher layer aspects of mobile communications,
the computer science elements rather than on the radio and transmission
aspects, the electrical engineering part.”
”This chapter introduces only those fundamental aspects of wireless
transmission which are necessary to understand the problems of
higher layers and the complexity needed to handle transmission impairments.”

Multiplexing, 2.5
Modulation, 2.6
 Spread spectrum, 2.7
 Cellular systems, 2.8

Frequencies, 2.1
 Signals, 2.2 (read yourselves)
 Antennas, 2.3 (read yourselves)
 Signal propagation, 2.4

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
2.1
Signal propagation ranges (Chapter 2.4)
Transmission range

communication possible
 low error rate
Detection range

detection of the signal
possible
 no communication
possible
sender
transmission
Interference range
distance

signal may not be
detected
 signal adds to the
background noise
detection
interference
This is an idealistic view!
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
2.2
2.1
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
Signal propagation
Propagation in free space always like light (straight line)
If such a straight line exists between sender and receiver it is called Line-of-Sight
Receiving power proportional to 1/d² (d = distance between sender and receiver)
Receiving power additionally influenced by
 fading (frequency dependent), the signal is faded due to distances
 Shadowing or blocking
 reflection at large obstacles
 refraction depending on the density of a medium
These effects are not only bad,
 scattering at small obstacles
some of them can be used!!
 diffraction at edges
shadowing
reflection
refraction
scattering
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
diffraction
MC SS02
2.3
Multiplexing (Chapter 2.5)
channels ki
k1
k2
k3
k4
k5
k6
Multiplexing in 4 dimensions
c

space (si)
 time (t)
 frequency (f)
 code (c)
t
c
t
s1
f
s2
f
c
t
Goal: multiple use of a shared medium
s3
Important: guard spaces needed!
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
f
2.4
2.2
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
Advanced Phase Shift Keying
Q
BPSK (Binary Phase Shift Keying):





bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used e.g. in satellite systems
1
10
11
00

01
A
t
11
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Q
I
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol
 symbol determines shift of sine wave
 needs less bandwidth compared to
BPSK
 more complex
I
0
MC SS02
10
01
00
2.5
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): combines amplitude and
phase modulation
 it is possible to code n bits using one symbol
 2n discrete levels, n=2 identical to QPSK
 bit error rate increases with n, but less errors compared to
comparable PSK schemes
Q
0010
0011
0001
0000
φ
a
I
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase φ,
but different amplitude a. 0000 and 1000 have
different phase, but same amplitude.
1000
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
2.6
2.3
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
Effects of spreading and interference
dP/df
dP/df
i)
user signal
broadband interference
narrowband interference
ii)
f
sender
f
dP/df
dP/df
dP/df
iii)
iv)
f
v)
f
receiver
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
f
MC SS02
2.7
Spreading and frequency selective fading
channel
quality
1
2
5
3
6
narrowband channels
4
frequency
narrow band
signal
guard space
channel
quality
1
2
spread
spectrum
2
2
2
2
spread spectrum channels
frequency
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
2.8
2.4
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
DSSS (Direct Sequence Spread Spectrum) I
XOR of the signal with pseudo-random number (chipping sequence)

many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages

reduces frequency selective
fading
 in cellular networks



base stations can use the
same frequency range
several base stations can
detect and recover the signal
soft handover
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
Disadvantages

=
resulting
signal
precise power control necessary
01101011001010
tb: bit period
tc: chip period
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
2.9
FHSS (Frequency Hopping Spread Spectrum) I
Discrete changes of carrier frequency

sequence of frequency changes determined via pseudo random number
sequence
Two versions

Fast Hopping:
several frequencies per user bit
 Slow Hopping:
several user bits per frequency
Advantages

frequency selective fading and interference limited to short period
 simple implementation
 uses only small portion of spectrum at any time
Disadvantages


not as robust as DSSS
simpler to detect
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
2.10
2.5
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 2: Wireless Transmission
End
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
2.11
2.6