1. Mobile Computing and Wireless Networking Lec 02
03/03/2010

2. Outline Characteristic of wireless channels
Antennas and Signal Propagation
Multiplexing

3. ideal periodical digital signal
Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics
Time domain
Frequency domain 1 1 t t
ideal periodical digital signal
decomposition
Two representations:
time domain; frequency domain
Knowing one can recover the other

4. Try spectrum1.m and spectrum2.m
Examples
Try spectrum1.m and spectrum2.m

5. Recap: Modulation Objective Basic schemes
encode digital data into analog signals at the right frequency range
Basic schemes
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)

6. Modulation Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):
Frequency Shift Keying (FSK):
Phase Shift Keying (PSK): 1 t

7. Example Suppose fc = 1 GHz (fc1 = 1 GHz, fc0 = 900 MHz for FSK)
Bit rate is 1 Mbps
Encode one bit at a time
Bit seq:
Q: How does the wave look like for each scheme? 1 t t

8. Phase Shift Keying: BPSK
BPSK (Binary Phase Shift Keying):
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
Properties
robust, used e.g. in satellite systems Q I 1
Q: What is the spectrum usage of BPSK?

9. Spectral Density of BPSK
Spectral Density = bit rate width of spectrum used b
fc : freq. of carrier
Rb =Bb = 1/Tb b fc

10. Phase Shift Keying: QPSK11 10 00 01 Q I A t
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol
symbol determines shift of sine wave
often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK

11. Antennas and Signal Propagation

12. Antennas: Isotropic Radiator
Isotropic radiator: a single point
equal radiation in all directions (three dimensional)
only a theoretical reference antenna
Radiation pattern: measurement of radiation around an antenna z y z
ideal
isotropic
radiator y x x
Q: how does power level decrease as a function of d, the distance
from the transmitter to the receiver?

13. Antennas: directed and sectorized
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Antennas: directed and sectorized
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z
antenna
directed x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane) z z
antenna
sectorized x x
top view, 3 sector
top view, 6 sector
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

14. Signal propagation ranges
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Signal propagation ranges
Transmission range
communication possible
low error rate
Detection range
detection of the signal possible
no communication possible
Interference range
signal may not be detected
signal adds to the background noise
sender
transmission
distance
detection
interference
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

15. Free-Space Isotropic Signal Propagation
In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)
Suppose transmitted signal is x, received signal y = h x, where h is proportional to 1/d²
Pr: received power
Pt: transmitted power
Gr, Gt: receiver and transmitter antenna gain
 (=c/f): wave length
Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

16. Free Space Signal Propagation1 t
at distance d ?

17. Real Antennas Q: Assume frequency 1 Ghz,  = ?
Real antennas are not isotropic radiators
Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2  size of antenna proportional to wavelength for better transmission/receiving
/4
/2
Q: Assume frequency 1 Ghz,  = ?

18. Dipole: Radiation Pattern of a Dipole

19. Why Not Digital Signal (revisited)
Not good for spectrum usage/sharing
The wavelength can be extremely large to build portal devices
e.g., T = 1 us -> f=1/T = 1MHz -> wavelength = 3x108/106 = 300m

20. Signal Propagation Receiving power additionally influenced by
shadowing (e.g. through a wall or a door)
refraction depending on the density of a medium
reflection at large obstacles
scattering at small obstacles
diffraction at edges
diffraction
reflection
refraction
scattering
shadow fading

21. Signal Propagation: Scenarios
Details of signal propagation are very complicated
We want to understand the key characteristics that are important to our objective

22. i.e. reduces to ¼ of signal 10 log(1/4) = -6.02
Shadowing
Signal strength loss after passing through obstacles
Some sample numbers
i.e. reduces to ¼ of signal 10 log(1/4) = -6.02

23. Multipath
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

24. Multipath Can Reduce Signal Strength
Example: reflection from the ground: received power decreases proportional to 1/d4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground
ground
For detail, see page 9:

25. Multipath Can Spread Delay
signal at sender
LOS pulse
Time dispersion: signal is dispersed over time
multipath
pulses
signal at receiver
LOS: Line Of Sight

26. Multipath Can Cause ISI
dispersed signal can cause interference between “neighbor” symbols, Inter Symbol Interference (ISI)
Assume 300 meters delay spread, the arrival time difference is /3x108 = 1 ms
if symbol rate > 1 Ms/sec, we will have serious ISI
In practice, fractional ISI can already substantially increase loss rate
signal at sender
LOS pulse
multipath
pulses
signal at receiver
LOS: Line Of Sight

27. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Multiplexing
channels ki
Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed! k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

28. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages
no dynamic coordination necessary
works also for analog signals
Disadvantages
waste of bandwidth if the traffic is distributed unevenly
inflexible k1 k2 k3 k4 k5 k6 c f
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

29. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages
only one carrier in the medium at any time
throughput high even for many users
Disadvantages
precise synchronization necessary k1 k2 k3 k4 k5 k6 c f
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

30. Time and frequency multiplex
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Time and frequency multiplex
Combination of both methods
A channel gets a certain frequency band for a certain amount of time
Example: GSM
Advantages
better protection against tapping
protection against frequency selective interference
but: precise coordination required k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

31. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Code multiplex
Each channel has a unique code
All channels use the same spectrum at the same time
Advantages
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Disadvantages
varying user data rates
more complex signal regeneration
Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller