MSIT 413: Wireless Technologies Week 3

1. MSIT 413: Wireless TechnologiesWeek 3 Michael L. Honig Department of EECS Northwestern University January 2011

2. Cellular Frequency Assignments B A C E D G A C B F D G A C B E F D G C B E F D G A C B E D G A C B F G A D cell cluster (contains all channels) co-channel cells

3. B A C D G A F D B E F C B G A C D G F cell cluster Cellular Terminology • Cell cluster: group of N neighboring cells which use the complete set of available frequencies. • Cell cluster size: N • Frequency reuse factor: 1/N • Uplink or reverse link: Mobiles  Base station • Downlink or forward link: Base station  mobiles • Co-channel cells: cells which are assigned the same frequencies

4. Performance Measure:Signal-to-Interference-Plus-Noise Ratio (SINR) • Expressed in dB (10 log (SINR)) • Typically, the interference power dominates (ignore noise) • SINR  Signal-to-Interference Ratio (SIR or S/I) • Total interference power is the sum over all interferers: • More co-channel users  more interference

5. Voice Capacity • Defined as • Example: SE = 0.1 Erlang/MHz/km2 10 MHz needed to support 1 Erlang/km2(Note that 1 km2 may correspond to a cell.) • To convert Erlangs to users, must estimate traffic per user, e.g., if on average, each user is active 1/10 of the time, then 1 Erlang corresponds to 10 users. • Traffic per cell depends on: • Number of channels • Grade of Service (e.g., typically 2%) • S/I requirement (determined by cluster size N)

6. Channel Allocation • Consider GSM: 25 MHz (simplex), 200 kHz channels 125 channels (1 is used for control signaling) • How to allocate channels to cells? Suburb (lightly loaded) Shopping mall Train station

7. Channel Allocation • Consider GSM: 25 MHz (simplex), 200 kHz channels 125 channels (1 is used for control signaling) • How to allocate channels to cells? • Objective: equalize blocking probability(target is around 2%) • Fixed channel allocation assignsfixed set of channels to each cell • Drawback? Suburb (lightly loaded) mall Train station

8. Channel Allocation • Consider GSM: 25 MHz (simplex), 200 kHz channels 125 channels (1 is used for control signaling) • How to allocate channels to cells? • Objective: equalize blocking probability(target is around 2%) • Fixed channel allocation assignsfixed set of channels to each cell. • Drawback: traffic is time-varying • Dynamic channel allocation varies the number of channels per cell, depending on the load. Suburb (lightly loaded) mall Train station

9. Dynamic Channel Allocation Channels already in use 2,6 1,3,4 new call: search for channel 1,3 5

10. Dynamic Channel Allocation • Channels assigned from complete set of available channels • Each user searches for a channel with high SINR (low interference) • No frequency planning! Channels already in use 2,6 3,4 new call: search for channel 1,3 4 5

11. 802.11b/g Channels • 14 overlapping (staggered) channels (11 in the U.S.) • Center frequencies are separated by 5 MHz • Bandwidth/interference controlled by “spectral mask” • 30 dB attenuation 11 MHz from center frequency • 50 dB attenuation 22 MHz from center frequency Comparison table Channels: 1 6 11

12. Dynamic Channel Allocation: 802.11b/g Channel 11 Channel 1 Channel 6

13. Dynamic Channel Allocation: 802.11b/g Channel 11 Channel 1 Channel 6 interference Add new router Channel 6

14. Dynamic Channel Allocation: 802.11b/g Channel 11 Switches to channel 1 Channel 1 interference Channel 6

15. Dynamic Channel Allocation: 802.11b/g Channel 11 Switches to channel 6 channel 1 interference Channel 6

16. Dynamic Channel Allocation: 802.11b/g Channel 11 channel 6 channel 1 interference Switches to channel 1 Dynamic channel assignment becomes unstable!

17. Overlapping Channel Assignment Channel 11 channel 6 channel 1 Interference (less than before) Interference (less than before) power Channel 3 Channels: 1 3 6 frequency

18. FCA vs. DCA FCA DCA • Moderate/High complexity • Must monitor channel occupancy,traffic distribution, S/I (centralized) • Better under light/moderate traffic • Insensitive to changes in traffic • Stable grade of service • Low probability of outage (call termination) • Suitable for micro-cellular systems(e.g., cordless) • Moderate/high call setup delay • No frequency planning • Assignment can be centralized or distributed • Low complexity • Better under heavy traffic • Sensitive to changes in traffic • Variable grade of service • Higher probability of outage • Suitable for macro-cellular systems(e.g., cellular) • Low call setup delay • Requires careful frequency planning • Centralized assignment

19. Why Study Radio Propagation? • To determine coverage • Must determine path loss • Function of • Frequency • Distance • Terrain (office building, urban, hilly, rural, etc.) Can we use the same channels? Need “large-scale” models

20. Why Study Radio Propagation?

21. Why Study Radio Propagation? • To enable robust communications (MODEM design) • How can we guarantee reliable communications? • What data rate can we provide? • Must determine signal statistics: • Probability of outage • Duration of outage Received Power Deep fades may cause an outage time Need “small-scale” models

22. Will provide answers to… • What are the major causes of attenuation and fading? • Why does the achievable data rate decrease with mobility? • Why are wireless systems evolving to wider bandwidths (spread spectrum and OFDM)? • Why does the accuracy of location tracking methods increase with wider bandwidths?

23. Propagation Key Words • Large-scale effects • Path-loss exponent • Shadow fading • Small-scale effects • Rayleigh fading • Doppler shift and Doppler spectrum • Coherence time / fast vs slow fading • Narrowband vs wideband signals • Multipath delay spread and coherence bandwidth • Frequency-selective fading and frequency diversity

24. Propagation Mechanisms:1. Free Space distance d reference distance d0=1 Reference power at reference distance d0 Path loss exponent=2 P0 In dB: Pr = P0 (dB) – 20 log (d) slope = -20 dB per decade Pr (dB) P0 = Gt Gr (/4)2 log (d) 0 wavelength antenna gains

25. Wavelength • Wavelength >> size of object  signal penetrates object. • Wavelength << size of object  signal is absorbed and/or reflected by object. • Large-scale effects refers to propagation over distances of many wavelengths.Small-scale effects refers to propagation over a distances of a fraction of a wavelength.  (meters) = c (speed of light) / frequency

26. Dipole Antenna 802.11 dipole antenna cable from transmitter wire (radiator)

27. Radiation Pattern: Dipole Antenna Dipole axis Dipole axis Electromagnetic wave radiates outfrom the dipole axis. Cross-section of doughnut pattern

28. Antenna Gain Pattern Red curve shows the antenna gain versus angle relative to an isotropic pattern (perfect circle) in dB. Often referred to as dBi, dB “isotropic”. -5 dB (factor of about 1/3) relative to isotropic pattern Dipole pattern (close to isotropic)

29. Antenna Gain Pattern Dipole pattern (vertical) 90 degree sector

30. Path loss ~ 13 dB / 100 meters or 130 dB / 1 km Increases linearly with distance Requires repeaters for long distances Path loss ~ 30 dB for the first meter + 20 dB / decade 70 dB / 100 meters (2 decades) 90 dB / 1 km(3 decades) 130 dB / 100 km! Increases as log (distance) Repeaters are infeasible for satellites Attenuation: Wireless vs. Wired 1 GHz Radio (free space) Unshielded Twisted Pair Short distance  Wired has less path loss. Large distance  Wireless has less path loss.

31. Incident E-M wave reflected wave q q Length of boundary >> wavelength l transmitted wave Propagation Mechanisms 2. Reflection 3. Diffraction 4. Scattering Signal loss depends on geometry Hill

32. Why Use > 500 MHz?

33. Why Use > 500 MHz? • There is more spectrum available above 500 MHz. • Lower frequencies require larger antennas • Antenna dimension is on the order of awavelength = (speed of light/frequency) = 0.6 M @ 500 MHz • Path loss increases with frequency for the first meter • 10’s of GHz: signals are confined locally • More than 60 GHz: attenuation is too large(oxygen absorbs signal)

34. 700 MHz Auction • Broadcast TV channels 52-69 relocated in Feb. 2009. • 6 MHz channels occupying 698 – 806 MHz • Different bands were auctioned separately: • “A” and “B” bands: for exclusive use (like cellular bands) • “C” band (11 MHz): must support open handsets, software apps • “D” band (5 MHz): shared with public safety (has priority) • Commenced January 24, 2008, ended in March

35. Why all the Hubbub? • This band has excellent propagation characteristics for cellular types of services (“beach-front property”). • Carriers must decide on technologies: 3G, LTE, WiMax,… • Rules for spectrum sharing can be redefined…

37. C Band Debate • Currently service providers in the U.S. do not allow any services, applications, or handsets from unauthorized 3rd party vendors. • Google asked the FCC to stipulate that whoever wins the spectrum must support open applications, open devices, open services, open networks (net neutrality for wireless). • Verizon wants to maintain “walled-garden”. • FCC stipulated open applications and devices, but not open services and networks: spectrum owner must allow devices or applications to connect to the network as long as they do not cause harm to the network • Aggressive build-out requirements: • Significant coverage requirement in four years, which continues to grow throughout the 10-year term of the license.

38. Sold to… • Verizon • Other winners: AT&T (B block), Qualcomm (B, E blocks) • Total revenue: $19.6 B • $9.6 B from Verizon, $6.6 B from AT&T • Implications for open access, competition?

39. D Band Rules • Winner gets to use both D band and adjacent public service band (additional 12 MHz!), but service can be preempted by public safety in emergencies. • Winner must build out public safety network:must provide service to 75% of the population in 4 years, 95% in 7 years, 99.3% in 10 years • Minimum bid: $1.3 B; estimated cost to deploy network: $10-12 B • Any takers? …

40. D Band Rules • Winner gets to use both D band and adjacent public service band (additional 12 MHz!), but service can be preempted by public safety in emergencies. • Winner must build out public safety network:must provide service to 75% of the population in 4 years, 95% in 7 years, 99.3% in 10 years • Minimum bid: $1.3 B; estimated cost to deploy network: $10-12 B • Any takers? … Nope! Highest bid was well below reserve…

41. T T Radio Channels Troposcatter Microwave LOS Mobile radio Indoor radio

42. Sinusoidal Signal Electromagnetic wave s(t) = A sin (2  f t + ) Time delay = 12, Phase shift  = 12/50 cycle = 86.4 degrees Amplitude A=1 s(t) Period= 50 sec, frequency f = 1/50 cycle/sec Time t (seconds)

43. Two Signal Paths s1(t) s2(t) Received signal r(t) = s1(t) + s2(t) Suppose s1(t) = sin 2f t. Then s2(t) = h s1(t - ) = h sin 2f (t - ) attenuation (e.g., h could be ½) delay (e.g.,  could be 1 microsec.)

44. Sinusoid Addition (Constructive) s1(t) r(t) + = s2(t) Adding two sinusoids with the same frequency gives another sinusoid with the same frequency!

45. Sinusoid Addition (Destructive) s1(t) r(t) + = s2(t) Signal is faded.

46. Ceiling Hypothetical large indoor environment Indoor Propagation Measurements Normalized received power vs. distance

47. Power Attenuation distance d reference distance d0=1 Path loss exponent Reference power at reference distance d0 P0 slope (n=2) = -20 dB per decade In dB: Pr = P0 (dB) – 10 n log (d) Pr (dB) slope = -40(n=4) log (d) 0

48. Path Loss Exponents

49. Large-Scale Path Loss (Scatter Plot) Average Received Power (dBm) Distance (meters)

50. Shadow Fading • Random variations in path loss as mobile moves around buildings, trees, etc. • Modeled as an additional random variable: Pr = P0 – 10 n log d + X “normal” (Gaussian) probability distribution “log-normal” random variable standard deviation -  received power in dB For cellular:  is about 8 dB