1. Wireless Channels
Y. Richard Yang
01/12/2011

2. Outline
Recap
Characteristic of wireless channels

3. Recap: Wireless and Mobile Computing
Driven by technology and infrastructure
wireless communication technology
global infrastructure
device miniaturization and capabilities
software development platforms
Challenges:
wireless channel: unreliable, open access
mobility
portability
changing environment
heterogeneity

4. Recap: Overview of Wireless Transmissions
source coding
bit
stream
channel coding
analog
signal
sender
modulation
receiver
bit
stream
source decoding
channel decoding
demodulation

5. 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

6. Try spectrum1.m and spectrum2.m
Examples
Try spectrum1.m and spectrum2.m

7. 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)

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

9. Example Suppose fc = 1 GHz (fc1 = 1 GHz, fc0 = 900 GHz 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

10. 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?

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

12. 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

13. Phase Shift Keying: Comparison
fc: carrier freq.
Rb: freq. of data
10dB = 10; 20dB =100
BPSK A
QPSK t 11 10 00 01

14. 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
0000
0001
0011
1000 Q I
0010 φ a
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have same amplitude but different phase
Q: why would any one use BPSK, but the highest QAM?

15. Antennas and Signal Propagation

16. 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?

17. 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)

18. Free Space Signal Propagation1 t
at distance d ?

19. 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,  = ?

20. Dipole: Radiation Pattern of a Dipole

21. 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

22. Figure for Thought: Real Measurements

23. 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

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

25. 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

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

27. 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:

28. Multipath Fading
Due to constructive and destructive interference of multiple transmitted waves, signal strength may vary widely as a function of receiver position

29. Multipath Fading: A Simple Two-path Example
receiver
- Wavelength is about 0.3 m for 1 GHz cellular

30. More detail see page 16 Eqn. (2.13):
Multipath Fading with Mobility: A Simple Two-path Example
r(t) = r0 + v t, assume transmitter sends out signal cos(2 fc t) r0
More detail see page 16 Eqn. (2.13):

31. Received Waveform v = 65 miles/h, fc = 1 GHz:
10 ms
deep fade
v = 65 miles/h, fc = 1 GHz:
fc v/c = 109 * 30 / 3x108 = 100 Hz
Why is fast multipath fading bad?

32. Small-Scale Fading

33. 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

34. RMS: root-mean-square
Delay Spread
RMS: root-mean-square

35. 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

36. Summary: Wireless Channels
Channel characteristics change over location, time, and frequency
Received
Signal
Large-scale
fading
Power
power
(dB)
path loss
log (distance)
time
small-scale fading
frequency