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MSIT 413: Wireless Technologies Week 4

MSIT 413: Wireless Technologies Week 4. Michael L. Honig Department of EECS Northwestern University. January 2009. Outline. Brief review of radio propagation Applications: location tracking, handoffs Digital modulation. Radio Channels. T. T. Troposcatter. Microwave LOS. Mobile radio.

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MSIT 413: Wireless Technologies Week 4

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  1. MSIT 413: Wireless TechnologiesWeek 4 Michael L. Honig Department of EECS Northwestern University January 2009

  2. Outline • Brief review of radio propagation • Applications: location tracking, handoffs • Digital modulation

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

  4. 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

  5. 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

  6. Large-Scale Path Loss (Scatter Plot) Most points are less than dB from the mean

  7. Urban Multipath • No direct Line of Sight between mobile and base • Radio wave scatters off of buildings, cars, etc. • Severe multipath

  8. Narrowband Fading Received signal r(t) = h1 s(t - 1 ) + h2 s(t - 2) + h3 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 + ) Received r(t) Transmitted s(t) A,  depend on environment, location of transmitter/receiver

  9. Time Variations: Doppler Shift Audio clip (train station)

  10. Scattering: Doppler Spectrum distance d = v t transmitted signal s(t) Doppler Spectrum (shows relative strengths of Doppler shifts) Doppler shift fd power power 2fd frequency frequency of s(t) frequency frequency of s(t) + Doppler shift fd

  11. Rayleigh Fading deep fade phase shift Received waveform Amplitude (dB)

  12. Fast vs. Slow Fading time received amplitude transmitted bits time 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.

  13. Channel Characterizations:Narrowband vs. Wideband Narrowband signal (sinusoid) Wideband signal (impulse) Multipath channel Amplitude attenuation, Delay (phase shift) infinite duration, zero bandwidth delay spread r(t) s(t) Multipath channel time t time t multipath components zero duration, infinite bandwidth

  14. Delay Spread and Intersymbol Interference s(t) r(t) Multipath channel time t time t Time between pulses is >> delay spread, therefore the received pulses do not interfere. r(t) s(t) Multipath channel time t 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.

  15. Coherence Bandwidth coherence bandwidth Bc channel gain Frequencies far outside the coherence bandwidth are affected differently by multipath. frequency f1 f2 The channel gain is approximately constant within a coherence bandwidth Bc. Frequencies f1 and f2 fade independently if |f1 – f2 | >> Bc. If the signal bandwidth < coherence bandwidth Bc, then the channel is called flat fading, and the transmitted signal is regarded as narrowband. If the signal bandwidth > Bc, then the channel is called frequency-selective and the signal is regarded as wideband.

  16. channel gain coherence bandwidth Bc frequency Coherence Bandwidth and Delay Spread delay spread  channel gain delay spread  coherence bandwidth Bc frequency Coherence bandwidth is inversely proportional to delay spread: Bc≈ 1/.

  17. Coherence Bandwidth and Diversity signal power (wideband) coherence bandwidth Bc channel gain Frequencies far outside the coherence bandwidth are affected differently by multipath. frequency f1 f2 Frequency-selective fading: different parts of the signal (in frequency) are affected differently by fading. 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).

  18. Narrowband Signal signal power (narrowband) coherence bandwidth Bc channel gain Frequencies far outside the coherence bandwidth are affected differently by multipath. frequency f1 f2 Flat fading: the narrowband signal fades uniformly, hence does not benefit from frequency diversity. For the cellular band, Bc is around 100 to 300 kHz. How does this compare with the bandwidth of cellular systems?

  19. 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

  20. Bandwidth and Radar reflection delay  = 2 x distance/c delay  s(t) s(t) r(t) r(t) time t Narrow bandwidth pulse time t High bandwidth pulse

  21. Bandwidth and Radar reflection delay  = 2 x distance/c s(t) 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 r(t) time t Ex: If the delay measurement changes by 1 microsec, the distance error Is c x 10-6 = 300 meters!

  22. Propagation and Handoff Received Signal Strength (RSS) from right BST from left BST unacceptable (call is dropped) time

  23. Propagation and Handoff Received Signal Strength (RSS) from right BST with handoff handoff threshold from left BST unacceptable (call is dropped) time

  24. Propagation and Handoff Received Signal Strength (RSS) from right BST with handoff handoff threshold from left BST RSS margin unacceptable (call is dropped) time time needed for handoff

  25. Propagation and Handoff Received Signal Strength (RSS) from right BST handoff threshold from left BST RSS margin unacceptable (call is dropped) time time needed for handoff

  26. Handoff Threshold • 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). Received Signal Strength (RSS) from right BST handoff threshold from left BST RSS margin unacceptable (call is dropped) time time needed for handoff

  27. 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. • Depends on available resources (e.g., channels/time slots/codes). Handoffs take priority over new requests (why?). • Hysteresis needed to avoid handoffs due to rapid variations in signal strength.

  28. 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, 3G: 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

  29. Soft Handoff (CDMA) ”Make before break” DURING AFTER BEFORE MSC MSC MSC BSC BSC BSC BSC BSC BSC Hard Handoff (TDMA) MSC MSC MSC BSC BSC BSC BSC BSC BSC

  30. SINR Measurements: 1xEV-DO drive test plots

  31. Why Digital Communications? 1G (analog)  2G (digital)  3G (digital) Digitized voice requires about 64 kbps, therefore the required bandwidth is >> the bandwidth of the voice signal (3—4 kHz)!

  32. Why Digital Communications? • Can combine with sophisticated signal processing (voice compression) and error protection. • Greater immunity to noise/channel impairments. • Can multiplex different traffic (voice, data, video). • Security through digital encryption. • Flexible design possible (software radio). 1G (analog)  2G (digital)  3G (digital) Digitized voice requires about 64 kbps, therefore the required bandwidth is >> the bandwidth of the voice signal (3—4 kHz)! VLSI + special purpose digital signal processing  digital is more cost-effective than analog!

  33. Binary Frequency-Shift Keying (FSK) Bits: 10110

  34. Quadrature Phase Shift Keying (QPSK) Bits: 0001 10 11

  35. Binary Phase Shift Keying (BPSK) Bits: 101 10 Baseband signal

  36. Amplitude Shift Keying (4-Level ASK) Bits: 0001 10 11 Baseband signal symbol duration

  37. Baseband  RF Conversion Passband (RF) signal Baseband signal sin 2fct time X T Modulate to the carrier frequency fc Power Power signal bandwidth is roughly 1/T frequency frequency fc 0  0

  38. Why Modulate?

  39. Why Modulate? • The baseband spectrum is centered around f=0. Without modulation all signals would occupy low frequencies and interfere with each other. • It is difficult to build effective antennas at low frequencies since the dimension should be on the order of a wavelength. • Low frequencies propagate further, causing more interference.

  40. Selection Criteria How do we decide on which modulation technique to use?

  41. Selection Criteria How do we decide on which modulation technique to use? • Performance: probability of error Pe. • Probability that a 0 (1) is transmitted and the receiver decodes as a 1 (0). • Complexity: how difficult is it for the receiver to recover the bits (demodulate)? • FSK was used in early voiceband modems because it is simple to implement. • Bandwidth or spectral efficiency: bandwidth (B) needed to accommodate data rate R bps, i.e., R/B measured in bits per second per Hz. • Power efficiency: energy needed per bit to achieve a satisfactory Pe. • Performance in the presence of fading, multipath, and interference.

  42. Example: Binary vs. 4-Level ASK 3A A A -A -A -3A Rate = 1/T symbols/sec Bandwidth is roughly 1/T Hz Bandwidth efficiency = 1 bps/Hz Rate = 2/T symbols/sec Bandwidth is roughly 1/T Hz Bandwidth efficiency = 2 bps/Hz What about power efficiency?

  43. Noisy Baseband Signals 3A A A -A -A -3A Rate = 1/T symbols/sec Bandwidth is roughly 1/T Hz Bandwidth efficiency = 1 bps/Hz Power =A2 (amplitude squared). Rate = 2/T symbols/sec Bandwidth is roughly 1/T Bandwidth efficiency = 2 bps/Hz Power = (A2 + 9A2)/2 = 5A2 What about probability of error vs transmitted power?

  44. Probability of Error 4-ASK BPSK 7 dB (factor of 5) Signal-to-Noise Ratio (dB)

  45. How to Increase Bandwidth Efficiency?

  46. How to Increase Bandwidth Efficiency? • Increase number of signal levels. • Use more bandwidth efficient modulation scheme (e.g., PSK). • Apply coding techniques: protect against errors by adding redundant bits. • Note that reducing T increases the symbol rate, but also increases the signal bandwidth. There is a fundamental tradeoff between power efficiency and bandwidth efficiency.

  47. The Fundamental Question Given: B Hz of bandwidth S Watts of transmitted signal power N Watts per Hz of background noise (or interference) power What is the maximum achievable data rate? (Note: depends on Pe.)

  48. Claude Shannon (1916-2001) Father of “Information Theory” Shannon’s 1948 paper “A Mathematical Theory of Communications” laid the foundations for modern communications and networking: “The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point… transmitter receiver

  49. Claude Shannon (1916-2001) Father of “Information Theory” Shannon’s 1948 paper “A Mathematical Theory of Communications” laid the foundations for modern communications and networking: “The significant aspect is that the actual message is one selected from a set of possible messages. The system must be designed to operate for each possible selection, not just the one which will actually be chosen since this is unknown at the time of design.” transmitter receiver

  50. Claude Shannon (1916-2001) Father of “Information Theory” Shannon’s 1948 paper “A Mathematical Theory of Communications” laid the foundations for modern communications and networking: “The choice of a logarithm base corresponds to the choice of a unit for measuring information. If the base 2 is used the resulting units may be called binary digits, or more briefly bits, a word suggested by J. W. Tukey. log2M bits Transmitter (M possible messages) receiver

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