1. Wireless Communication Introduction
Sharif University of Technology
Wireless Communication Introduction
Fall 2015
Afshin Hemmatyar

2. References
A. Goldsmith, Wireless Communications, Cambridge University Press, 2005.
D. Tse and D. Vaswanth, Fundamentals of Wireless Communications, Cambridge University Press, 2005.
T. Rappaport, Wireless Communications, Principles and Practice, 2nd Edition, Prentice Hall.
J. Fayyaz, Radio Design of Cellular Networks, Naghoos Press, 2011.

3. Contents Background and Preview Wireless Propagation Channel
Multiple Access Methods
Cellular Systems
Diversity Issues
Information Transmission Capacity
Multiple Antenna Technologies
Cooperative Communications

4. Wired Vs. Wireless Communications
Each cable is a different channel
One media (cable) shared by all
Signal attenuation is low
High signal attenuation
No interference
High interference
noise; co-channel interference; adjacent channel interference

5. Why Wireless? Advantages Limitations
Sometimes it is impractical to lay cables
User mobility
Cost
Limitations
Bandwidth
Fidelity
Power
Security

6. Wireless ≈ Waves Electromagnetic radiation
Emitted by sinusoidal current running through a wire (transmitting antenna)
Creates propagating sinusoidal magnetic and electric fields according to Maxwell’s equations:
Fields induce current in receiving antenna.

7. Propagation Principle
electric
field
propagation direction
magnetic
field

8. Propagation Mechanisms
Line-of-Sight S D
Non Line-of-Sight
Reflection
λ << D
Diffraction
λ  D
Scattering
λ >> D

9. Propagation in the “Real World”
a wave
can
be absorbed
penetrate
LIGHT ANALOGIES (BENT STICK, LIGHT IN CANYON, BLACK PAINT, WINDOW)
reflect
bend

10. The Cluttered World of Radio Waves
hills
girders
rain
hallways
windows
vehicles
trees
walls

11. Electromagnetic SpectrumLF HF
VHF
UHF
SHF
EHF MF
AM radio
S/W radio
FM radio TV
cellular 
902 – 928 Mhz
2.4 – Ghz
5.725 – Ghz
ISM band 
30kHz
300kHz
3MHz
30MHz
300MHz
30GHz
300GHz
10km
1km
100m
10m 1m
10cm
1cm
1mm
3GHz
X rays
Gamma rays 
infrared
visible UV
1 kHz
1 MHz
1 GHz
1 THz
1 PHz
1 EHz
Propagation characteristics are different in each frequency band.

12. Evaluating Frequencies
50 MHz-250 MHz: Good for outdoor range, large antenna size, bending and penetrating. No foliage problems. “Sees” metallic building structures, doesn’t pass through windows or down corridors.
450 MHz to 2 GHz: - Good compromise for cellular-type systems. Small antenna, but big enough for outdoor range. Minor foliage effects. OK for windows walls and corridors.
5-20 GHz: Antenna too small for outdoor range. Foliage and rain effects. Indoor microcells, Point-to-point, and Satellites to ground stations.
hallways, leaves, walls, rain, absorption, diffraction (black),
50mhz good for range (bending, penetrating) but not for windows, hallways, and long antenna (2 meter)
450 would be good for range, windows, corridors, walls, and antenna (6 inches)
GHz still OK, range diminishing but short antenna good for portables (but 900 better)
5+ GHz not good for range, penetration, leaves and even rain we don’t really need the antenna to be smaller (in buildings?)
must consider cost (technology, volume) and availability of spectrum

13. Unlicensed Radio Spectrum (ISM: Industrial, Science, Medicine)
33cm
12cm
5cm
26 Mhz
83.5 Mhz
125 Mhz
902 Mhz
2.4 Ghz
5.725 Ghz
928 Mhz
Ghz
5.850 Ghz
cordless phones
baby monitors
WaveLan
802.11b
Bluetooth
Microwave oven
802.11a

14. Free-space Path-loss
Power of wireless transmission reduces with square of distance (due to surface area increase of sphere).
Reduction also depends on wavelength:
Long wave length (low frequency) has less loss
Short wave length (high frequency) has more loss

15. Path-loss Models L(d) = L(d0)(d/d0)n
Path-Loss Exponent Depends on environment:
L(d) = L(d0)(d/d0)n
Free space n = 2
Urban area cellular n = 2.7 to 3.5
Shadowed urban cell n = 3 to 5
In building LOS n = 1.6 to 1.8
Obstructed in building n = 4 to 6
Obstructed in factories n = 2 to 3

16. Multi-path Propagation
Electromagnetic waves bounce off of conductive (metal) objects.
Reflected waves received along with direct wave.

17. Multi-Path Effect
Multi-path components are delayed depending on path length, causes delay spread.
Phase shift causes frequency dependent constructive / destructive interference.

18. Multi-transmitter Interference
Similar to multi-path
Two transmitting stations will constructively/destructively interfere with each other at the receiver.
Receiver will “hear” the sum of the two signals, which usually means garbage.

19. Modulation
Modulation allows the wave to carry information by adjusting its properties (Amplitude, Frequency, and Phase) in a time varying way.
Digital modulation using discrete “steps” so that information can be recovered well despite noise/interference.
8VSB - US HDTV
BFSK - Mote Sensor Networks
QPSK - 2 Mbps & CMDA(IS-95)

20. Symbol Rate & Bandwidth
Modulation allows transmission of one of several possible symbols (two or more).
Data stream is encoded by transmitting several symbols in succession.
Symbol rate ≈ Bandwidth
Symbol Rate or Baud Rate (symbols/sec)
Bit Rate or Throughput (bits/sec)
Spectrum Usage or Bandwidth (Hz)
Inter-symbol interference (ISI) occurs unless delay spread << symbol time.

21. Thermal Noise Ever-present thermal noise in wireless medium.
Sums with any wireless transmission.
Potentially causes errors in reception (digital) or degradation of quality (analog).
Effectively limits transmission range when
transmitting signal strength falls below a threshold.

22. Thermal Noise Calculation
Noise power depends on channel bandwidth:
Noise Power = -174dBm/Hz + 10log(BW in Hz)
So for
BW = 25 MHz for b or a channel
Thus noise power is about -100 dBm
-100 dBm = mW

23. Physical Channel Properties Review
Received signal power depends on:
Transmit power
Loss over distance (falls off by dn)
Shadowing (e.g. absorption by walls)
Multi-path (e.g. bouncing off of metal objects)
Noise power depends on:
Thermal noise
Environmental noise (e.g. microwave ovens)
Channel Quality
Related to Signal to Noise Ratio

24. Current Wireless Technologies
Cellular Telephony (GSM, CDMA2000)
Fixed Wireless Access (WiMax)
Wireless Local Area Networks
Local Networks (Bluetooth, UWB)
Satellite Communications

25. Cellular Telephony (1)

26. Cellular Telephony (2)

27. Cellular Telephony (3) Data is bursty, whereas voice is continuous.
3G widens the data pipe
384Kbps to few Mbps
Standard based of WCDMA
Packet-based switching for both voice and data
New generations
HSDPA, HSPA+, LTE
WiMAx added to 3G
4G systems start to come up
Mostly based on OFDM

28. Fixed Wireless Networks (WiMax)
Provide broadband wireless access to
homes/offices in a few Km range.
A potential replacement of ADSL
Provide high speed Internet and VOD

29. Wireless Local Area Networks (1)
WLANs connect local computers (100m to a few Km range)
Breaks data into packets
Channel is shared (random access)
Backbone internet provides best-effort service
Poor performance in some applications (e.g. video)

30. Wireless Local Area Networks (2)
802.11b (oldest)
Standard for 2.4GHz ISM band (80MHz BW)
Frequency hopped spread spectrum
1.6-10Mbps, few hundred meter range
802.11a (old)
Standard for 5.7GHz NII band (300MHz BW)
OFDM with time division
20-70Mbps, variable range
802.11g (newer)
Standard for 2.4GHz and 5.7GHz bands
OFDM
Up to 54Mbps, few hundred meter range
802.11n (current)
Up to 140Mbps
Uses smart antenna technology

31. Ultra Wide-Band Also known as “Impulse Radio”.
Use very high bandwidth to decrease power level.
Hard synchronization
No license problem, but seems to interfere with GPS
Now mainly targeted to small distance applications.
(home networks to replace Bluetooth)
Smaller delay and longer range than Bluetooth
Emerging as competitor for

32. Satellite Communications
Cover very large areas.
Different orbit heights
GEO (36000 Km), MEO and LEO (<2000 Km)
Optimized for one-way transmission
Radio (DAB) and TV (DVB-S) broadcast
Two-way systems
Expensive alternative to terrestrial systems

33. Evolution of Current Technologies
Link:
Modulation, Coding, Adaptivity, Smart Antennas
Network:
Dynamic resource allocation, Mobility support
Hardware:
Better batteries, Circuits/Processors
Applications:
Soft and adaptive QoS (main issue in real-time
multi-media services)

34. Moving toward Real Multimedia
Voice
Data
Video
Delay
< 100 mSec -
Packet loss
< 1%
<1%
BER
10-3
10-6
Data Rate
8-32Kbps
1-100 Mbps
1-20 Mbps
Traffic
Continuous
Bursty

35. Design Challenges Wireless channels are capacity-limited
broadcast communication medium.
Two main problems in wireless media:
Fading
Interference
Traffic patterns, user locations, and network
conditions are constantly changing.
Energy and delay constraints change design
principles across all layers of the protocol
stack.

36. Emerging Systems Ad-hoc Wireless Networks Wireless Sensor Networks
Distributed Control Networks
Cooperative Networks
Cognitive Radio Networks

37. Ad-hoc Wireless Networks (1)

38. Ad-hoc Wireless Networks (2)
Peer-to-peer communications
No backbone infrastructure
Multi-hop routing
Dynamic topology
Fully connected with different link SNRs

39. Ad-hoc Wireless Networks (3)
Ad-hoc Wireless Networks provide a flexible
network infrastructure for any emerging
application.
The capacity of such a network is generally
unknown.
Transmission, access, and routing strategies are
generally ad-hoc
Energy constraints impose interesting design
tradeoffs for communication and networking.

40. Wireless Sensor Networks (1)40

41. Wireless Sensor Networks (2)
Energy is the driving constraint.
Nodes powered by non-rechargeable batteries.
Data flows to central location.
Low per-node rates but up to nodes.
Data highly correlated in transmission.
Nodes can cooperate in transmission, reception,
compression, and signal processing.

42. Wireless Sensor Networks (3)
(Energy-constrained nodes)
Short-range networks must consider
transmit, circuit, and processing energy.
Sleep modes save energy but complicate
networking.
Changes everything about the network
design:
Optimization of bit allocation across all protocols
Delay vs. throughput vs. node/network lifetime
tradeoffs
Optimization of nodes cooperation
Efficient MAC layer communication and Scheduling

43. Distributed Control (1)
Packet loss and/or delay impacts controller
performance.
Controller design should be robust to network faults.
Joint application and communication network delay.
Interesting ideas in packet-based communication
and file transfer.

44. Distributed Control (2)
There is no methodology to incorporate random
delays or packet losses into control system design.
The best rate/delay tradeoff for a communication
system in distributed control can not be determined.
Current autonomous vehicle platoon controllers are
not string stable with any communication delay.
What are the best routing technologies?

45. Cooperative Networks (1)

46. Cooperative Networks (2)
Increase coverage area
Reduce number of “blind spots”
Reduce transmit power per node
Increase in transceiver complexity
More complex synchronization problems
More interference to be handled properly
Higher end-to-end delays
Additional delay to be handled in real-time applications

47. Cognitive Radio (1) Available spectrum looks scarce.
Measurements show the allocated spectrum is vastly
underutilized.

48. Cognitive Radio (2)

49. Cognitive Radio (3) Sense, learn, and exploit the environment
One “simple” option:
Use “un-used” spectrum
Agile Radios
Give priority to ”primary” users
But not receiving a signal in wireless environment
does not mean that no signal is actually transmitted
at that frequency!
Even in simplest form is very challenging.
Many ideas such as:
Game Theory
Near-Noise Level Signal detection
BW transactions
Trust theories (how to identify users with “bad” intensions?)

50. Summary exciting systems and applications.
The wireless vision encompasses many
exciting systems and applications.
Technical challenges go across all layers
of the system design.
Wireless systems today still have
limited performance and interoperability.