1. MSIT | Master of Science in Information Technology MSIT 413: Wireless Technologies Week 3 Michael L. Honig Department of EECS Northwestern University January 2011

2. MSIT | Master of Science in Information Technology Cellular Frequency Assignments E D F C A E B G D C E A B C D F A G B A G D C B A F G CG D A D B C F E G B cell cluster (contains all channels) co-channel cells 2

3. MSIT | Master of Science in Information Technology 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 C D F A B A F G CG D A D B C F E G B cell cluster 3

4. MSIT | Master of Science in Information Technology 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 4

5. MSIT | Master of Science in Information Technology Voice Capacity Defined as –Example: SE = 0.1 Erlang/MHz/km 2  10 MHz needed to support 1 Erlang/km 2 (Note that 1 km 2 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) 5

6. MSIT | Master of Science in Information Technology 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) Train station Shopping mall 6

7. MSIT | Master of Science in Information Technology 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 assigns fixed set of channels to each cell –Drawback? Suburb (lightly loaded) Train station mall 7

8. MSIT | Master of Science in Information Technology 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 assigns fixed 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) Train station mall 8

9. MSIT | Master of Science in Information Technology Dynamic Channel Allocation 2,6 1,3 Channels already in use 5 1,3,4 new call: search for channel 9

10. MSIT | Master of Science in Information Technology 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! 2,6 1,3 Channels already in use 5 3,4 new call: search for channel 4 10

11. MSIT | Master of Science in Information Technology 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 Channels: 1611 Comparison table 11

12. MSIT | Master of Science in Information Technology Dynamic Channel Allocation: 802.11b/g Channel 6 Channel 1 Channel 11 12

13. MSIT | Master of Science in Information Technology Dynamic Channel Allocation: 802.11b/g Channel 6 Channel 1 Channel 11 Channel 6 Add new router interference 13

14. MSIT | Master of Science in Information Technology Dynamic Channel Allocation: 802.11b/g Switches to channel 1Channel 1 Channel 11 Channel 6 interference 14

15. MSIT | Master of Science in Information Technology Dynamic Channel Allocation: 802.11b/g channel 1 Switches to channel 6 Channel 11 Channel 6 interference 15

16. MSIT | Master of Science in Information Technology Dynamic Channel Allocation: 802.11b/g channel 1 channel 6 Channel 11 Switches to channel 1 interference Dynamic channel assignment becomes unstable! 16

17. MSIT | Master of Science in Information Technology Overlapping Channel Assignment channel 1 channel 6 Channel 11 Channel 3 Interference (less than before) Interference (less than before) power frequency Channels: 136 17

18. MSIT | Master of Science in Information Technology FCA vs. 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 DCAFCA 18

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

20. MSIT | Master of Science in Information Technology Why Study Radio Propagation?

21. MSIT | Master of Science in Information Technology 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 Need “small-scale” models Received Power time Deep fades may cause an outage

22. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology Propagation Mechanisms: 1. Free Space reference distance d 0 =1 distance d Reference power at reference distance d 0 Path loss exponent=2 In dB: P r = P 0 (dB) – 20 log (d) P r (dB) log (d) slope = -20 dB per decade P0P0 0 P 0 = G t G r ( /4  ) 2 antenna gains wavelength

25. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology Dipole Antenna cable from transmitter wire (radiator) 802.11 dipole antenna

27. MSIT | Master of Science in Information Technology Radiation Pattern: Dipole Antenna Dipole axis Electromagnetic wave radiates out from the dipole axis. Cross-section of doughnut pattern

28. MSIT | Master of Science in Information Technology Antenna Gain Pattern Dipole pattern (close to isotropic) 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

29. MSIT | Master of Science in Information Technology Antenna Gain Pattern Dipole pattern (vertical)90 degree sector

30. MSIT | Master of Science in Information Technology Attenuation: Wireless vs. Wired 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 Short distance  Wired has less path loss. Large distance  Wireless has less path loss. Unshielded Twisted Pair 1 GHz Radio (free space)

31. MSIT | Master of Science in Information Technology Propagation Mechanisms 2. Reflection 3. Diffraction 4. Scattering Incident E-M wave  reflected wave transmitted wave Length of boundary >> wavelength Hill Signal loss depends on geometry

32. MSIT | Master of Science in Information Technology Why Use > 500 MHz?

33. MSIT | Master of Science in Information Technology Why Use > 500 MHz? There is more spectrum available above 500 MHz. Lower frequencies require larger antennas –Antenna dimension is on the order of a wavelength = (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. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology 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…

36. MSIT | Master of Science in Information Technology

37. C Band Debate Currently service providers in the U.S. do not allow any services, applications, or handsets from unauthorized 3 rd 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. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology 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. MSIT | Master of Science in Information Technology Radio Channels TroposcatterMicrowave LOS Mobile radioIndoor radio TT

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

43. MSIT | Master of Science in Information Technology Two Signal Paths s 1 (t) s 2 (t) Received signal r(t) = s 1 (t) + s 2 (t) Suppose s 1 (t) = sin 2  f t. Then s 2 (t) = h s 1 (t -  ) = h sin 2  f (t -  ) attenuation (e.g., h could be ½) delay (e.g.,  could be 1 microsec.)

44. MSIT | Master of Science in Information Technology Sinusoid Addition (Constructive) += s 1 (t) s 2 (t) r(t) Adding two sinusoids with the same frequency gives another sinusoid with the same frequency!

45. MSIT | Master of Science in Information Technology Sinusoid Addition (Destructive) += Signal is faded. s 1 (t) s 2 (t) r(t)

46. MSIT | Master of Science in Information Technology Indoor Propagation Measurements Hypothetical large indoor environment Ceiling Normalized received power vs. distance

47. MSIT | Master of Science in Information Technology Power Attenuation In dB: P r = P 0 (dB) – 10 n log (d) reference distance d 0 =1 distance d Reference power at reference distance d 0 Path loss exponent P r (dB) log (d) slope (n=2) = -20 dB per decade P0P0 0 slope = -40 (n=4)

48. MSIT | Master of Science in Information Technology Path Loss Exponents ENVIRONMENTPATH LOSS EXPONENT, n Free space2 Urban cellular radio2.7 to 3.5 Shadowed urban cellular radio3 to 5 In building line-of-site1.6 to 1.8 Obstructed in building4 to 6 Obstructed in factories2 to 3

49. MSIT | Master of Science in Information Technology Large-Scale Path Loss (Scatter Plot) Average Received Power (dBm) Distance (meters)

50. MSIT | Master of Science in Information Technology Shadow Fading Random variations in path loss as mobile moves around buildings, trees, etc. Modeled as an additional random variable: P r = P 0 – 10 n log d + X “log-normal” random variable standard deviation   received power in dB “normal” (Gaussian) probability distribution For cellular:  is about 8 dB

51. MSIT | Master of Science in Information Technology Large-Scale Path Loss (Scatter Plot) Most points are less than  dB from the mean

52. MSIT | Master of Science in Information Technology Empirical Path Loss Models Propagation studies must take into account: –Environment (rural, suburban, urban) –Building characteristics (high-rise, houses, shopping malls) –Vegetation density –Terrain (mountainous, hilly, flat) Okumura’s model (based on measurements in and around Tokyo) –Median path loss = free-space loss + urban loss + antenna gains + corrections –Obtained from graphs –Additional corrections for street orientation, irregular terrain Numerous indoor propagation studies for 802.11

53. MSIT | Master of Science in Information Technology SINR Measurements: 1xEV-DO drive test plots

54. MSIT | Master of Science in Information Technology Link Budget How much power is required to achieve target S/I? dBs add: Target S/I (dB) + path loss (dB) + other losses (components) (dB) - antenna gains (dB) Total Power needed at transmitter (dB) Actual power depends on noise level. –Given 1 microwatt noise power, if the transmit power is 60 dB above the noise level then the transmit power is 1 Watt.

55. MSIT | Master of Science in Information Technology Example Recall that dBm measures the signal power relative to 1 mW (milliwatt) = 0.001 Watt. To convert from S Watts to dBm, use S (dBm) = 10 log (S / 0.001) Transmitted power (dBm) = -30 + 40 = 10 dBm = 10 mW What if the received signal-to-noise ratio must be 5 dB, and the noise power is -45 dBm? Transmitter Receiver wireless channel 40 dB attenuation Received power must be > -30 dBm What is the required Transmit power?

56. MSIT | Master of Science in Information Technology Urban Multipath No direct Line of Sight between mobile and base Radio wave scatters off of buildings, cars, etc. Severe multipath

57. MSIT | Master of Science in Information Technology Narrowband vs. Wideband Narrowband means that the bandwidth of the transmitted signal is small (e.g., < 100 kHz for cellular). It therefore looks “almost” like a sinusoid. –Multipath changes the amplitude and phase. Wideband means that the transmitted signal has a large bandwidth (e.g., > 1 MHz for cellular). –Multipath causes “self-interference”.

58. MSIT | Master of Science in Information Technology Narrowband Fading Received signal r(t) = h 1 s(t -  1 ) + h 2 s(t -  2 ) + h 3 s(t -  3 ) + … attenuation for path 1 (random) delay for path 1 (random) If the transmitted signal is sinusoidal (narrowband), s(t) = sin 2  f t, then the received signal is also sinusoidal, but with a different (random) amplitude and (random) phase: r(t) = A sin (2  f t +  ) A,  depend on environment, location of transmitter/receiver Transmitted s(t) Received r(t)

59. MSIT | Master of Science in Information Technology Rayleigh Fading Can show: A has a “Rayleigh” distribution  has a “uniform” distribution (all phase shifts are equally likely) Probability (A < a) = 1 – e -a 2 /P 0 where P 0 is the average received power (averaged over different locations) Ex: P 0 =1, a=1: Pr(A<1) = 1 – e -1 = 0.63 (probability that signal is faded) P 0 = 1, a=0.1: Pr(A<0.1) = 1 – e -1/100 ≈ 0.01 (prob that signal is severely faded) a Prob(A < a) 1 1-e -a 2 /P 0

60. MSIT | Master of Science in Information Technology Small-Scale Fading a = 0.1

61. MSIT | Master of Science in Information Technology Small-Scale Fading Fade rate depends on Mobile speed Speed of surrounding objects Frequency

62. MSIT | Master of Science in Information Technology Short- vs. Long-Term Fading Long-term (large-scale) fading: Distance attenuation Shadowing (blocked Line of Sight (LOS)) Variations of signal strength over distances on the order of a wavelength Time (t) Signal Strength (dB) TT Short-term fading Long-term fading

63. MSIT | Master of Science in Information Technology Combined Fading and Attenuation Time (mobile is moving away from base) Received power P r (dB) distance attenuation

64. MSIT | Master of Science in Information Technology Combined Fading and Attenuation Time (mobile is moving away from base) shadowing Received power P r (dB) distance attenuation

65. MSIT | Master of Science in Information Technology Combined Fading and Attenuation Time (mobile is moving away from base) shadowing Received power P r (dB) distance attenuation Rayleigh fading

66. MSIT | Master of Science in Information Technology Example Diagnostic Measurements: 1XEV-DO drive test measurements drive path

67. MSIT | Master of Science in Information Technology Time Variations: Doppler Shift Audio clip (train station)

68. MSIT | Master of Science in Information Technology Time Variations: Doppler Shift Propagation delay = distance d / speed of light c = vt/c Received signal r(t) = sin 2  f (t- vt/c) = sin 2  (f – fv/c) t velocity v distance d = v t transmitted signal s(t) received signal r(t) propagation delay received frequency Doppler shift f d = -fv/c delay increases

69. MSIT | Master of Science in Information Technology Doppler Shift (Ex) Mobile moving away from base  v > 0, Doppler shift < 0 Mobile moving towards base  v 0 Carrier frequency f = 900 MHz, v = 60 miles/hour = 26.82 meters/sec Mobile  Base: f d = fv/c = (900 × 10 6 ) × 26.82 / (3 × 10 8 ) ≈ 80 Hz meters/sec

70. MSIT | Master of Science in Information Technology Doppler (Frequency) Shift Frequency= 1/50 Frequency= 1/45 in phase out of phase ½ Doppler “cycle”

71. MSIT | Master of Science in Information Technology Doppler Shift (Ex) Mobile moving away from base  v > 0, Doppler shift < 0 Mobile moving towards base  v 0 Carrier frequency f = 900 MHz, v = 60 miles/hour = 26.82 meters/sec Mobile  Base: f d = fv/c = (900 × 10 6 ) × 26.82 / (3 × 10 8 ) ≈ 80 Hz Suppose the data rate is 9600 bits/sec, 80 Hz Doppler shift  phase inversion every (9600/80)/2 = 60 bits! As the data rate increases, Doppler shift becomes less significant, i.e., channel is stable over more transmitted bits. IS-136 data rate: 48.6 kbpsGSM data rate 270 kbps

72. MSIT | Master of Science in Information Technology Application of Doppler Shift: Astronomy Doppler shift determines relative velocity of distant objects (e.g., stars, galaxies…) “red shift”: object is moving away “blue shift” object is moving closer sun light spectrumspectrum of galaxy supercluster Observed “spectral lines” (radiation is emitted at discrete frequencies)

73. MSIT | Master of Science in Information Technology Application of Doppler Shift: Police Radar Doppler shift can be used to compute relative speed.

74. MSIT | Master of Science in Information Technology Scattering: Doppler Spectrum distance d = v t transmitted signal s(t) Received signal is the sum of all scattered waves Doppler shift for each path depends on angle (vf cos  c ) Typically assume that the received energy is the same from all directions (uniform scattering) received signal ?? freq. power frequency of s(t)

75. MSIT | Master of Science in Information Technology Scattering: Doppler Spectrum distance d = v t transmitted signal s(t) frequency power frequency of s(t) frequency power frequency of s(t) + Doppler shift f d Doppler shift f d 2f d Doppler Spectrum (shows relative strengths of Doppler shifts)

76. MSIT | Master of Science in Information Technology Scattering: Doppler Spectrum distance d = v t transmitted signal s(t) frequency power frequency of s(t) power frequency of s(t) + Doppler shift f d Doppler spectrum 2f d

77. MSIT | Master of Science in Information Technology Rayleigh Fading phase shift deep fade Received waveformAmplitude (dB)

78. MSIT | Master of Science in Information Technology Channel Coherence Time Coherence Time: Amplitude and phase are nearly constant. Rate of time variations depends on Doppler shift: (velocity X carrier frequency)/(speed of light) Coherence Time varies as 1/(Doppler shift).

79. MSIT | Master of Science in Information Technology Fast vs. Slow Fading Fast fading: channel changes every few symbols. Coherence time is less than roughly 100 symbols. Slow fading: Coherence time lasts more than a few 100 symbols. time received amplitude transmitted bits

80. MSIT | Master of Science in Information Technology Fade Rate (Ex) f c = 900 MHz, v = 60 miles/hour  Doppler shift ≈ 80 Hz. Coherence time is roughly 1/80, or 10 msec Data rate (voice): 10 kbps or 0.1 msec/bit  100 bits within a coherence time (fast fading) GSM data rate: 270 kbps  about 3000 bits within a coherence time (slow fading)

81. MSIT | Master of Science in Information Technology Channel Characterizations: Time vs. Frequency Frequency-domain description Time-domain description Multipath channel input s(t) is a sinusoid “narrowband” signal Amplitude attenuation, Delay (phase shift) Multipath channel input s(t) is an impulse (very short pulse) “wideband” signal (Note: an impulse has zero duration and infinite bandwidth!) time t s(t) time t r(t) multipath components

82. MSIT | Master of Science in Information Technology Pulse Width vs. Bandwidth frequency Power bandwidth = 1/T frequency Power bandwidth = 1/T time signal pulse time signal pulse T T Narrowband Wideband

83. MSIT | Master of Science in Information Technology Power-Delay Profile delay spread  Received power vs. time in response to a transmitted short pulse. For cellular systems (outdoors), the delay spread is typically a few microseconds.

84. MSIT | Master of Science in Information Technology Two-Ray Impulse Response direct path (path 1) reflection (path 2) time t s(t) time t r(t) reflection is attenuated 

85. MSIT | Master of Science in Information Technology Two-Ray Impulse Response direct path (path 1) reflection (path 2)  = [(length of path 2) – (length of path 1)]/c time t s(t) time t r(t) reflection is attenuated 

86. MSIT | Master of Science in Information Technology Urban Multipath time t s(t) r(t) time t r(t) different location for receiver Spacing and attenuation of multipath components depend on location and environment.

87. MSIT | Master of Science in Information Technology Delay Spread and Intersymbol Interference Multipath channel time t s(t) time t r(t) Multipath channel r(t) Time between pulses is >> delay spread, therefore the received pulses do not interfere. Time between pulses is < delay spread, which causes intersymbol interference. The rate at which symbols can be transmitted without intersymbol interference is 1 / delay spread. s(t) time t

88. MSIT | Master of Science in Information Technology Coherence Bandwidth frequency channel gain The channel gain is approximately constant within a coherence bandwidth B c. Frequencies f 1 and f 2 fade independently if |f 1 – f 2 | >> B c. coherence bandwidth B c f1f1 f2f2 Frequencies far outside the coherence bandwidth are affected differently by multipath. 88

89. MSIT | Master of Science in Information Technology Coherence Bandwidth and Delay Spread delay spread  frequency channel gain coherence bandwidth B c Coherence bandwidth is inversely proportional to delay spread: B c ≈ 1/  frequency channel gain coherence bandwidth B c delay spread  89

90. MSIT | Master of Science in Information Technology Narrowband Signal frequency channel gain The signal power is confined within a coherence band. Flat fading: all signal frequencies are affected the same way. coherence bandwidth B c f1f1 f2f2 Frequencies far outside the coherence bandwidth are affected differently by multipath. signal power (narrowband) 90

91. MSIT | Master of Science in Information Technology Wideband Signals frequency channel gain A wideband signal spans many coherence bands. Frequency-selective fading: different parts of the signal (in frequency) are affected differently by the channel. coherence bandwidth B c f1f1 f2f2 Frequencies far outside the coherence bandwidth are affected differently by multipath. signal power (wideband) 91

92. MSIT | Master of Science in Information Technology Frequency Diversity frequency channel gain Wideband signals exploit frequency diversity. Spreading power across many coherence bands reduces the chances of severe fading. Wideband signals are distorted by the channel fading (distortion causes intersymbol interference). coherence bandwidth B c f1f1 f2f2 Frequencies far outside the coherence bandwidth are affected differently by multipath. signal power (wideband) 92

93. MSIT | Master of Science in Information Technology Coherence Bandwidth for Cellular frequency channel gain For the cellular band, B c is around 100 to 300 kHz. How does this compare with the bandwidth of cellular systems? coherence bandwidth B c f1f1 f2f2 Frequencies far outside the coherence bandwidth are affected differently by multipath. signal power (wideband) 93

94. MSIT | Master of Science in Information Technology Fading Experienced by Wireless Systems Standard Bandwidth Fade rate AMPS 30 kHz (NB) Fast IS-136 30 kHz Fast GSM 200 kHz Slow IS-95 (CDMA) 1.25 MHz (WB) Fast 3G 1.25-5 MHz Slow to Fast (depends on rate) LTE up to 20 MHz Slow 802.11 > 20 MHz Slow Bluetooth > 5 MHz (?) Slow 94

95. MSIT | Master of Science in Information Technology Pulse Width vs. Bandwidth frequency Power bandwidth = 1/T frequency Power bandwidth = 1/T time signal pulse time signal pulse T T Narrowband Wideband

96. MSIT | Master of Science in Information Technology Bandwidth and Multipath Resolution direct path (path 1) reflection (path 2) signal pulse Narrow bandwidth  low resolution Receiver cannot distinguish the two paths. signal pulse Wide bandwidth  high resolution Receiver can clearly distinguish two paths. multipath components are resolvable  (delay spread)  T T <  T > 

97. MSIT | Master of Science in Information Technology Bandwidth and Multipath Resolution direct path (path 1) reflection (path 2) signal pulse  Wide bandwidth  high resolution Receiver can clearly distinguish two paths. multipath components are resolvable The receiver can easily distinguish the two paths provided that they are separated by much more than the pulse width . Since the signal bandwidth B ≈ 1/ , this implies B >> 1/  or B >> B c..

98. MSIT | Master of Science in Information Technology Multipath Resolution and Diversity direct path (path 1) reflection (path 2) signal pulse  Wide bandwidth  high resolution Receiver can clearly distinguish two paths. multipath components are resolvable Each path may undergo independent fading (i.e., due to Doppler). If one path is faded, the receiver may be able to detect the other path. In the frequency domain, this corresponds to independent fading in different coherence bands.

99. MSIT | Master of Science in Information Technology Fading Experienced by Wireless Systems StandardFlat/Freq.-Sel. Fast/Slow AMPS Flat Fast IS-136 Flat Fast GSM F-S Slow IS-95 (CDMA) F-S Fast 3G/4G F-S Slow to Fast (depends on rate) 802.11 F-S Slow Bluetooth F-S Slow

100. MSIT | Master of Science in Information Technology Radar Pulse Bandwidth time t r(t) s(t) delay  = 2 x distance/c reflection time t r(t) s(t) delay  Narrow bandwidth pulse High bandwidth pulse

101. MSIT | Master of Science in Information Technology Bandwidth and Resolution time t r(t) s(t) delay  = 2 x distance/c The resolution of the delay measurement is roughly the width of the pulse. Low bandwidth  wide pulse  low resolution High bandwidth  narrow pulse  high resolution If the delay measurement changes by 1 microsec, the distance error is c x 10 -6 /2 = 150 meters! reflection

102. MSIT | Master of Science in Information Technology Propagation and Handoff Received Signal Strength (RSS) from left BST from right BST unacceptable (call is dropped) time

103. MSIT | Master of Science in Information Technology Propagation and Handoff Received Signal Strength (RSS) from left BST from right BST unacceptable (call is dropped) time with handoff handoff threshold

104. MSIT | Master of Science in Information Technology Propagation and Handoff Received Signal Strength (RSS) from left BST from right BST unacceptable (call is dropped) time with handoff handoff threshold time needed for handoff RSS margin

105. MSIT | Master of Science in Information Technology Propagation and Handoff Received Signal Strength (RSS) from left BST from right BST unacceptable (call is dropped) time handoff threshold time needed for handoff RSS margin

106. MSIT | Master of Science in Information Technology Handoff Threshold Received Signal Strength (RSS) from left BST from right BST unacceptable (call is dropped) time handoff threshold time needed for handoff RSS margin Handoff threshold too high  too many handoffs (ping pong) Handoff threshold too low  dropped calls are likely Threshold should depend on slope on vehicle speed (Doppler).

107. MSIT | Master of Science in Information Technology Handoff Measurements (3G) Mobile maintains a list of neighbor cells to monitor. Mobile periodically measures signal strength from BST pilot signals. Mobile sends measurements to network to request handoff. Handoff decision is made by network. B A D C

108. MSIT | Master of Science in Information Technology Handoff Measurements (3G) Mobile maintains a list of neighbor cells to monitor. Mobile periodically measures signal strength from BST pilot signals. Mobile sends measurements to network to request handoff. Handoff decision is made by network. B A D C Pilot signals (transmitted continuously)

109. MSIT | Master of Science in Information Technology Handoff Measurements (3G) Mobile maintains a list of neighbor cells to monitor. Mobile periodically measures signal strength from BST pilot signals. Mobile sends measurements to network to request handoff. Handoff decision is made by network. B A D C active link request handoff

110. MSIT | Master of Science in Information Technology Handoff Measurements (3G) Mobile maintains a list of neighbor cells to monitor. Mobile periodically measures signal strength from BST pilot signals. Mobile sends measurements to network to request handoff. Handoff decision is made by network. B A D C link is broken network activates link

111. MSIT | Master of Science in Information Technology Handoff Decision Depends on RSS, time to execute handoff, hysteresis, and dwell (duration of RSS) –Proprietary methods –Handoff may also be initiated for balancing traffic. 1G (AMPS): Network Controlled Handoff (NCHO) –Handoff is based on measurements at BS, supervised by MSC. 2G, GPRS: Mobile Assisted Handoff (MAHO) –Handoff relies on measurements at mobile –Enables faster handoff Mobile data, WLANs (802.11): Mobile Controlled Handoff (MCHO) –Handoff controlled by mobile

112. MSIT | Master of Science in Information Technology Soft Handoff (CDMA) ”Make before break” MSC BSC MSC BSC MSC BSC Hard Handoff (TDMA) MSC BSC MSC BSC MSC BSC BEFORE DURINGAFTER

113. MSIT | Master of Science in Information Technology Types of Small-Scale Fading Small-Scale Fading Based on multipath time delay spread Flat Fading 1. BW of signal < BW of channel 2. Delay spread < Symbol period Frequency Selective Fading 1. BW of signal > BW of channel 2. Delay spread > Symbol period Small-Scale Fading Based on Doppler spread Fast Fading 1. High Doppler spread 2. Coherence time < Symbol period 3. Channel variations faster than base- band signal variations Slow Fading 1. Low Doppler spread 2. Coherence time > Symbol period 3. Channel variations slower than base- band signal variations

114. MSIT | Master of Science in Information Technology Types of Small-Scale Signal Fading as a Function of Symbol Period and Signal Bandwidth TsTs  delay spread T c (coherence time) TsTs Flat Fast Fading Flat Slow Fading Frequency-Selective Slow Fading Frequency-Selective Fast Fading Symbol Period Relative to delay spread BsBs B d = f d (Doppler shift) BsBs Flat Fast Fading Flat Slow Fading Frequency Selective Slow Fading Frequency Selective Fast Fading BcBc Symbol Period relative to coherence time. Signal BW relative to channel BW  coherence BW Signal bandwidth relative to Doppler shift