1. نیمسال اوّل 92-91 افشین همّت یار دانشکده مهندسی کامپیوتر مخابرات سیّار (626-40) معرفی

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, 2 nd Edition, Prentice Hall.  J. Fayyaz, Radio Design of Cellular Networks, Naghoos Press, 2011. 2

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

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

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

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 6

7. Propagation Principle electric field magnetic field propagation direction 7

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

9. Propagation in the “Real World” a wave can be absorbed reflect penetrate bend 9

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

11. Electromagnetic Spectrum Propagation characteristics are different in each frequency band. UV 1 MHz 1 kHz 1 GHz 1 THz 1 PHz 1 EHz infrared visible X rays Gamma rays LFHF VHFUHFSHFEHF MF AM radio S/W radio FM radio TV cellular 902 – 928 Mhz 2.4 – 2.4835 Ghz 5.725 – 5.785 Ghz ISM band 30kHz300kHz 3MHz 30MHz 300MHz 30GHz300GHz 10km 1km 100m 10m 1m 10cm 1cm 100mm 3GHz 11

12. Evaluating Frequencies  50 MHz-250 MHz Good for range outdoors (antenna size, bending and penetrating), no foliage problems. “Sees” metallic building structures, doesn’t pass through windows or down corridors, needs large antenna (2 meter). TV.  450 MHz to 2 GHz - Good compromise for cellular- type systems. Antenna small, but big enough for outdoor range. Minor foliage effects. OK for windows walls and corridors. (450 might be best, but...) (Range issue for 2 GHz systems- more bases)  5-20 GHz- Antenna too small for outdoor range. Foliage and rain effects. Indoor microcells, Point-to- point, and Satellites to ground stations. 12

13. Unlicensed Radio Spectrum (ISM: Industrial, Science, Medicine) 902 Mhz 928 Mhz 26 Mhz 83.5 Mhz 125 Mhz 2.4 Ghz 2.4835 Ghz 5.725 Ghz 5.850 Ghz cordless phones baby monitors WaveLan 802.11b Bluetooth Microwave oven 802.11a 33cm 12cm 5cm 13

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 lossLong wave length (low frequency) has less loss Short wave length (high frequency) has more lossShort wave length (high frequency) has more loss 14

15. Other Path-loss Exponents  Path-Loss Exponent Depends on environment: Free spacen = 2 Urban area cellularn = 2.7 to 3.5 Shadowed urban celln = 3 to 5 In building LOSn = 1.6 to 1.8 Obstructed in buildingn = 4 to 6 Obstructed in factoriesn = 2 to 3 15

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

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

18. Modulation  Modulation allows the wave to carry information by adjusting its properties in a time varying way AmplitudeAmplitude FrequencyFrequency PhasePhase  Digital modulation using discrete “steps” so that information can be recovered despite noise/interference 8VSB - US HDTV8VSB - US HDTV BFSK - Mote Sensor NetworksBFSK - Mote Sensor Networks QPSK - 2 Mbps 802.11 & CMDA(IS-95)QPSK - 2 Mbps 802.11 & CMDA(IS-95) 18

19. 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

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 Throughput (bits/sec)Throughput (bits/sec) Spectrum usage (Hz)Spectrum usage (Hz)  Inter-symbol interference (ISI) occurs unless delay spread << symbol time 20

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 noise floor: -174 dBm/Hz transmitting signal strength falls below noise floor: -174 dBm/Hz 21

22. Thermal Noise Calculation  Depends on channel bandwidth = -174dBm/Hz + 10log(bandwidth in Hz) = -174dBm/Hz + 10log(bandwidth in Hz)  So for 802.11 About 25 MHz for 802.11b or 802.11a channelAbout 25 MHz for 802.11b or 802.11a channel Noise Floor is about -100 dBmNoise Floor is about -100 dBm -100 dBm = 10log(0.0000000000001 Watts )-100 dBm = 10log(0.0000000000001 Watts ) 22

23. Physical Channel Properties Review  Wireless signal strength Transmit powerTransmit power Loss over distance (falls off by d 2 )Loss over distance (falls off by d 2 ) Shadowing (e.g. absorption by walls)Shadowing (e.g. absorption by walls) Multi-path (e.g. bouncing off of metal objects)Multi-path (e.g. bouncing off of metal objects)  Noise Thermal noise floorThermal noise floor Environmental noise (e.g. microwave ovens)Environmental noise (e.g. microwave ovens)  Channel Quality Related to Signal to Noise RatioRelated to Signal to Noise Ratio 23

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

25. Cellular Telephony (1) 25

26. Cellular Telephony (2) 26

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

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

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) 29

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

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)  802.11 now is emerging as competitor.  Bluetooth has larger delay and shorter range. 31

32. Satellite Communications  Cover very large areas.  Different orbit heights o GEOs (36000 Km) ---- LEOs (2000 Km)  Optimized for one-way transmission o Radio (DAB) and TV (DVB-S) broadcast  Two-way systems? o Expensive alternative to terrestrial systems o A few ambitious systems on the horizon 32

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) 33

34. Moving toward Real Multimedia VoiceDataVideo Delay< 100 mSec- Packet loss< 1%0 BER10 -3 10 -6 Data Rate8-32Kbps1-100 Mbps1-20 Mbps TrafficContinuousBurstyContinuous 34

35. Design Challenges  Wireless channels are capacity-limited broadcast communication medium. o 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. 35

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

37. Ad-hoc Wireless Networks (1) 37

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

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. 39

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 100000 nodes.  Data highly correlated in transmission.  Nodes can cooperate in transmission, reception, compression, and signal processing. 41

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

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. 43

44.  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? Can we make distributed control robust to the network? Yes, by a radical redesign of the controller and the network. Distributed Control (2) 44

45. Cooperative Networks (1) 45

46.  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 Cooperative Networks (2) 46

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

48. Cognitive Radio (2) 48

49. Cognitive Radio (3)  Sense, learn, and exploit the environment  One “simple” option: o Use “un-used” spectrum o Agile Radios o 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: o Game Theory o Near-Noise Level Signal detection o BW transactions o Trust theories (how to identify users with “bad” intensions?) 49

50. Summary  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. 50