1. CS395T: Wireless Networking
Lili Qiu
UT Austin
Aug. 30, 2006

2. Course Information Instructor: Lili Qiu, lili@cs.utexas.edu
Office: ACES 6.242
Lecture: Mon. & Wed. 3 – 4:30 pm
Office hour: Mon. & Wed. 4:30 – 5:30 pm or by appt.
TA’s office hour: Tue. 1-2pm & Thur. 2-3pm, ESB 229, Desk 1
Course homepage:
Mailing list:
Subscribe mailing list: subscribe cs395t-wireless2006 YourFirstName YourLastName

3. Class Goals Learn wireless networking fundamentals
Discuss challenges and opportunities in wireless networking research
Obtain hands-on wireless research experience

4. Course Material Suggested references
Mobile Communications by Jochen Schiller
Wireless Networks: The Definitive Guide by Matthew S. Gast
Wireless Communications Principles and Practice by Ted Rappaport
Selected conference and journal papers
Other resources
MOBICOM, SIGCOMM, INFOCOM proceedings

5. Course Workload Grading Classroom participation: Homework Mid-term
Course project: 40%
Classroom participation:
Actively participate in class discussions
Make insightful comments and/or initiate interesting discussions
Homework
Assignment
Paper review: submit a review for one paper of your choice at the beginning of each class (1-2 pages)
Review form
Mid-term
Oct. 16 in class, open books/open notes
Does Anyone have class after 4:30pm?

6. Course Workload (Cont.)
Course project
Goal: obtain hands-on experience in wireless networking research
Work in a group of 2-3
I’ll hand out a list of project topics next week
You may also choose your own topic approved by me
Initial report + Mid-point report + final report + presentation
Vote for best project

7. Course Overview Part I: Introduction to wireless networks
Physical layer
MAC
Introduction to MAC and IEEE
Channel assignment and channel hopping
Power control
Rate control
Multi-radio
Routing
Mobile IP
DSR and AODV
TCP over wireless
Problems with TCP over wireless
Other proposals

8. Course Overview (Cont.)
Part II: Different types of wireless networks
Wireless mesh networks
Sensor networks
Cellular networks
Delay tolerant networks
RFID
WiMax

9. Course Overview (Cont.)
Part III: Wireless network management and security
Localization
Wireless network usage studies
Wireless network diagnosis
Wireless network security

10. Class Time
Due to conference travel, I have to reschedule classes on Sept. 13, 25, 27
Will Sept. 8, 22, 29 (Friday) work for you?
What time?

11. Introduction to Wireless Networks

12. Mobile and Wireless Services – Always Best Connected
UMTS, GSM
115 kbit/s
LAN
100 Mbit/s,
WLAN
54 Mbit/s
GSM 53 kbit/s
Bluetooth 500 kbit/s
LAN, WLAN
780 kbit/s
UMTS,
DECT
2 Mbit/s
GSM/EDGE 384 kbit/s,
WLAN 780 kbit/s
UMTS, GSM
384 kbit/s
GSM 115 kbit/s,
WLAN 11 Mbit/s

13. On the Road UMTS, WLAN, ad hoc DAB, GSM, cdma2000, TETRA, ...
Vehicles
transmission of news, road condition, weather, music via DAB
personal communication using GSM
position via GPS
local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy
vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance
Emergencies
early transmission of patient data to the hospital, current status, first diagnosis
replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.
crisis, war, ...
GSM, WLAN, satellite, Vehicle networks, emergency networks,
Personal Travel Assistant,
DAB, PDA, laptop,
GSM, UMTS, WLAN,
Bluetooth, ...

14. Home Networking iPod Game WiFi WiFi Surveillance UWB WiFi HDTV
Camcorder
High-quality
speaker
WiFi
WiFi
Surveillance
Game
Surveillance

15. Last-Mile Many users still don’t have broadband
End of 2002
Worldwide: 46 million broadband subscribers
US: 17% household have broadband
Reasons: out of service area; some consider expensive
Broadband speed is still limited
DSL: 1-3 Mbps download, and Kbps upload
Cable modem: depends on your neighbors
Insufficient for several applications (e.g., high-quality video streaming)

16. Disaster Recovery Network
9/11, Tsunami, Hurricane Katrina, South Asian earthquake …
Wireless communication capability can make a difference between life and death!
How to enable efficient, flexible, and resilient communication?
Rapid deployment
Efficient resource and energy usage
Flexible: unicast, broadcast, multicast, anycast
Resilient: survive in unfavorable and untrusted environment

17. Environmental Monitoring
Micro-sensors, on-board processing, wireless interfaces feasible at very small scale--can monitor phenomena “up close”
Enables spatially and temporally dense environmental monitoring
Embedded Networked Sensing will reveal previously unobservable phenomena
Ecosystems, Biocomplexity
Contaminant Transport
Marine Microorganisms
Seismic Structure Response

18. Challenges in Wireless Networking Research

19. Challenge 1: Unreliable and Unpredictable Wireless Links
Asymmetry vs. Power
Reception v. Distance
Standard Deviation v. Reception rate
*Cerpa, Busek et. al
What Robert Poor (Ember)
calls “The good, the bad and the ugly”
Wireless links are less reliable
They may vary over time and space

20. Challenge 2: Open Wireless Medium
Wireless interference S1 R1 S2 R1

21. Challenge 2: Open Wireless Medium
Wireless interference
Hidden terminals S1 R1 S2 R1 S1 R1 R2

22. Challenge 2: Open Wireless Medium
Wireless interference
Hidden terminals
Exposed terminal S1 R1 S2 R1 S1 R1 R2 R1 S1 S2 R2

23. Challenge 2: Open Wireless Medium
Wireless interference
Hidden terminals and
Exposed terminal
Wireless security
Eavesdropping, Denial of service, … R1 S1 S2 R1 S1 R1 R2 R1 S1 S2 R2

24. Challenge 3: Intermittent Connectivity
Reasons for intermittent connectivity:
Mobility
Environmental changes
Existing networking protocols assume always-on networks
Under intermittent connected networks
Routing, TCP, and applications all break
Need in-network storage to support communication under such environments

25. Challenge 4: Limited Resources
Limited battery power
Limited bandwidth
Limited processing and storage power
PDA
data
simpler graphical displays
802.11
Mobile phones
voice, data
simple graphical displays
GSM
Laptop
fully functional
standard applications
battery;
Sensors,
embedded
controllers

26. Introduction to Wireless Networking

27. Internet Protocol Stack
Application: supporting network applications
FTP, SMTP, HTTP
Transport: host-host data transfer
TCP, UDP
Network: routing of datagrams from source to destination
IP, routing protocols
Link: data transfer between neighboring network elements
WiFi, Ethernet
Physical: bits “on the wire”
Radios, coaxial cable, optical fibers
application
transport
network
link
physical

28. Physical Layer

29. Outline Signal Frequency allocation Signal propagation Multiplexing
Modulation
Spread Spectrum

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

31. Signals Physical representation of data Function of time and location
Classification
continuous time/discrete time
continuous values/discrete values
analog signal = continuous time and continuous values
digital signal = discrete time and discrete values

32. Signals (Cont.) Signal parameters of periodic signals:
period T, frequency f=1/T
amplitude A
phase shift 
sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t) 1 t

33. ideal periodical digital signal
Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics 1 1 t t
ideal periodical digital signal
decomposition
The more harmonics used, the smaller the approximation error.

35. Why Not Send Digital Signal in Wireless Communications?
Digital signals need
infinite frequencies for perfect transmission
however, we have limited frequencies in wireless communications

36. Frequencies for Communication
twisted pair
coax cable
optical transmission
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
VLF LF MF HF
VHF
UHF
SHF
EHF
infrared
visible light UV
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequency and wave length:  = c/f , wave length , speed of light c  3x108m/s, frequency f
LF: submarine
MF & HF: radio (AM & FM)
VHF & UHF: TV
UHF mobile phoes, cordless phones, 3G
SHF: microwave, wifi

37. Frequencies and Regulations
ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

38. Why Need A Wide Spectrum: Shannon Channel Capacity
The maximum number of bits that can be transmitted per second by a physical channel is:
where W is the frequency range that the media allows to pass through, S/N is the signal noise ratio

39. Signal, Noise, and Interference
Signal (S)
Noise (N)
Includes thermal noise and background radiation
Often modeled as additive white Gaussian noise
Interference (I)
Signals from other transmitting sources
SINR = S/(N+I) (sometimes also denoted as SNR)

40. dB and Power conversion
Denote the difference between two power levels
(P2/P1)[dB] = 10 * log10 (P2/P1)
P2/P1 = 10^(A/10)
Example: P2 = 100 P1
dBm and dBW
Denote the power level relative to 1 mW or 1 W
P[dBm] = 10*log10(P/1mW)
P[dB] = 10*log10(P/1W)
Example: P = mW, P = 100 W

41. Outline Signal Frequency allocation Signal propagation Multiplexing
Modulation
Spread Spectrum

42. 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
These ranges are not circular in the real world.
detection
interference

43. Signal Propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d² (d = distance between sender and receiver)
Receiving power additionally influenced by
shadowing
reflection at large obstacles
refraction depending on the density of a medium
scattering at small obstacles
diffraction at edges
fading (frequency dependent)
shadowing
reflection
refraction
scattering
diffraction

44. Path Loss Free space model Two-ray ground reflection model
Log-normal shadowing
Indoor model
P = 1 mW at d0=1m, what’s Pr at d=2m?

45. Multipath Propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
 interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
 distorted signal based on the phases of different parts
LOS pulses
multipath
pulses
LOS: Line Of Sight
signal at sender
signal at receiver

46. Fading Channel characteristics change over time and location
e.g., movement of sender, receiver and/or scatters
 quick changes in the power received (short term/fast fading)
Additional changes in
distance to sender
obstacles further away
 slow changes in the average power received (long term/slow fading)
long term
fading
power t
short term fading

47. Typical Picture Received Signal Power (dB) path loss shadow fading
Rayleigh fading
path loss
log (distance)
Received
Signal
Power
(dB)

48. Real world example

49. Outline Signal Frequency allocation Signal propagation Multiplexing
Modulation
Spread Spectrum

50. Multiplexing Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!

51. Space Multiplexing Assign each region a channel Pros Cons
channels ki
Assign each region a channel
Pros
no dynamic coordination necessary
works also for analog signals
Cons
Inefficient resource utilization k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f

52. Frequency Multiplexing
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Pros:
no dynamic coordination necessary
works also for analog signals
Cons:
waste of bandwidth if the traffic is distributed unevenly
Inflexible
guard spaces k1 k2 k3 k4 k5 k6 c f t

53. Time Multiplex
A channel gets the whole spectrum for a certain amount of time
Pros:
only one carrier in the medium at any time
throughput high even for many users
Cons:
precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t

54. Time and Frequency Multiplexing
Combination of both methods
A channel gets a certain frequency band for a certain amount of time (e.g., GSM)
Pros:
better protection against tapping
protection against frequency selective interference
higher data rates compared to code multiplex
Cons:
precise coordination required k1 k2 k3 k4 k5 k6 c f t

55. Code Multiplexing Each channel has a unique code
All channels use the same spectrum simultaneously
Pros:
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Cons:
lower user data rates
more complex signal regeneration
Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t

56. Outline Signal Frequency allocation Signal propagation Multiplexing
Modulation
Spread Spectrum

57. Modulation I Digital modulation Analog modulation
digital data is translated into an analog signal (baseband)
differences in spectral efficiency, power efficiency, robustness
Analog modulation
shifts center frequency of baseband signal up to the radio carrier
Reasons
Antenna size is on the order of signal’s wavelength
More bandwidth available at higher carrier frequency
Medium characteristics: path loss, shadowing, reflection, scattering, diffraction depend on the signal’s wavelength

58. Modulation and Demodulation
digital
modulation
data
analog
radio
carrier
baseband
signal
radio transmitter
synchronization
decision
digital
data
analog
demodulation
radio
carrier
baseband
signal
radio receiver

59. Modulation Schemes Amplitude Modulation (AM) Frequency Modulation (FM)
Phase Modulation (PM)

60. Digital Modulation Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):
Pros: simple
Cons: susceptible to noise
Example: optical system, IFR 1 1 t

61. Digital Modulation II Frequency Shift Keying (FSK):
Pros: less susceptible to noise
Cons: requires larger bandwidth 1 1 t 1 1

62. Digital Modulation III
Phase Shift Keying (PSK):
Pros:
Less susceptible to noise
Bandwidth efficient
Cons:
Require synchronization in frequency and phase  complicates receivers and transmitter t

63. Phase Shift Keying BPSK (Binary Phase Shift Keying):
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used in satellite systems Q I 1 11 10 00 01 Q I A t
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol
needs less bandwidth compared to BPSK
symbol determines shift of sine wave
Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK

64. 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
bit error rate increases with n
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
Used in Modem

65. Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
Side effects:
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence, Frequency Hopping
signal
power
interference
spread signal
power
spread
interference
detection at
receiver f f

66. DSSS (Direct Sequence Spread Spectrum)
XOR of the signal with pseudo-random number (chipping sequence)
generate a signal with a wider range of frequency: spread spectrum
user data
chipping
sequence
resulting
signal 1
XOR = tb tc
tb: bit period
tc: chip period

67. FHSS (Frequency Hopping Spread Spectrum)
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

68. FHSS: Example tb: bit period td: dwell time user data slow hopping
(3 bits/hop)
fast
(3 hops/bit) 1 tb t f f1 f2 f3 td
tb: bit period td: dwell time

69. Comparison between Slow Hopping and Fast Hopping
Pros: cheaper
Cons: less immune to narrowband interference
Fast hopping
Pros: more immune to narrowband interference
Cons: tight synchronization  increased complexity