CS5530 Mobile/Wireless Systems Key Wireless Physical Layer Concepts - - PowerPoint PPT Presentation

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CS5530 Mobile/Wireless Systems Key Wireless Physical Layer Concepts - - PowerPoint PPT Presentation

Aug 03, 2023 533 likes •766 views

CS5530 Mobile/Wireless Systems Key Wireless Physical Layer Concepts Yanyan Zhuang Department of Computer Science http://www.cs.uccs.edu/~yzhuang UC. Colorado Springs CS5530 Ref. CN5E, NT@UW, WUSTL Outline Electromagnetic spectrum

slide-1
SLIDE 1
  • Ref. CN5E, NT@UW, WUSTL

CS5530

CS5530 Mobile/Wireless Systems

Key Wireless Physical Layer Concepts

Yanyan Zhuang

Department of Computer Science http://www.cs.uccs.edu/~yzhuang

  • UC. Colorado Springs
slide-2
SLIDE 2

Outline

  • Ref. CN5E, NT@UW, WUSTL

2 CS5530

  • Electromagnetic spectrum
  • Reflection, diffraction and scattering of signals
  • Multipath, Doppler shift
  • Digital modulation and multiplexing
  • Noise
slide-3
SLIDE 3

Electromagnetic Spectrum

  • Ref. CN5E, NT@UW, WUSTL

3 CS5530

  • Wireless communication
  • 100 kHz to 60 GHz
  • Higher frequency: only go in a straight line, can’t go far

Image: http://www2.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

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SLIDE 4

Antenna

  • Ref. CN5E, NT@UW

4 CS5530

  • Transmitter converts electrical energy to electromagnetic waves
  • Receiver converts electromagnetic waves to electrical energy
  • Same antenna used for transmission and reception
  • Signal of same frequency cause interference
  • At receiver side
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SLIDE 5

Reflection, diffraction and scattering

  • Ref. CN5E, NT@UW, WUSTL

5 CS5530

slide-6
SLIDE 6

Reflection, diffraction and scattering

  • Ref. CN5E, NT@UW, WUSTL

6 CS5530

  • Reflection
  • Surface large relative to the wavelength of signal
  • Diffraction
  • Edge of impenetrable body is large relative to the wavelength of signal
  • Scattering
  • Obstacle size in order of wavelength (lamp post)
  • LOS
  • Diffracted and scattered signals are not significant
  • Non-LOS
  • Diffraction and scattering are primary means of reception
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SLIDE 7

Multipath Propagation

  • Ref. CN5E, NT@UW, WUSTL

7 CS5530

  • Each propagation path travels from transmitting antenna to receiving antenna

while interacting with physical objects in the environment

  • Signals bounce off objects and take multiple paths

Image: http://www.wica.intec.ugent.be/ research/propagation/physical- radio-channel-models

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SLIDE 8

Doppler Shift

  • Ref. CN5E, NT@UW, WUSTL

8 CS5530

  • If transmitter or receiver is mobile, the frequency of

received signal changes

  • Moving towards each other: higher frequency
  • Moving away from each other: lower frequency
slide-9
SLIDE 9

Doppler Shift (cont.)

  • Ref. CN5E, NT@UW, WUSTL

9 CS5530

  • Frequency difference = speed (m/sec) / wavelength (m)
  • Example:
  • 2.4 GHz: wavelength = speed of light / frequency = 3*10^8/2.4*10^9

= 0.125 m

  • 120 km/h (75 m/h) = 120*10^3/3600 = 33.3 m/s
  • Frequency difference = 33.3/0.125 = 267 Hz
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SLIDE 10

Doppler Shift (cont.)

  • Ref. CN5E, NT@UW, WUSTL

10 CS5530

  • Frequency difference = speed (m/sec) / wavelength (m)
  • Example:
  • 2.4 GHz: wavelength = speed of light / frequency = 3*10^8/2.4*10^9

= 0.125 m

  • 120 km/h (75 m/h) = 120*10^3/3600 = 33.3 m/s
  • Frequency difference = 33.3/0.125 = 267 Hz
  • Why important?
  • Mobile environment: walking, driving
  • Example: WiMax is only designed for speed lower than 60 km/h

(37.5 m/h)

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SLIDE 11

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

11 CS5530

  • Digital Modulation
  • Process of converting between bits and signals that represent them
  • Regulate amplitude, phase, or frequency of a signal to convey bits

} ASK, PSK, FSK

  • Multiplexing
  • Use a single medium to carry several signals
slide-12
SLIDE 12

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

12 CS5530

  • Digital Modulation
  • ASK (Amplitude Shift Keying)

} Two different amplitudes: 0/1

  • FSK (Frequency Shift Keying)

} Two different frequencies

  • PSK (Phase Shift Keying)

} Wave is shifted 0 or 180 degrees

  • Only one of frequency / phase

can be modulated at a time: they are related

  • Amplitude and phase can be modulated in combination
slide-13
SLIDE 13

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

13 CS5530

  • Digital Modulation
  • PSK (Phase Shift Keying)

} Wave is shifted 0 or 180 degrees: BPSK } Wave is shifted 0/90/180/270 degrees: QPSK

  • QAM (Quadrature Amplitude Modulation)

} Amplitude and phase are modulated in combination

(a) QPSK. (b) 16 QAM. (c) 64 QAM.

slide-14
SLIDE 14

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

14 CS5530

  • Multiplexing
  • FDM (Frequency Division Multiplexing)

} Divides spectrum into frequency bands, with each user having

exclusive possession of some band to send their signal

slide-15
SLIDE 15

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

15 CS5530

  • Multiplexing
  • FDM (Frequency Division Multiplexing)
  • TDM (Time Division Multiplexing)

} Users take turns (round-robin), each one periodically getting entire

bandwidth for a little burst of time

slide-16
SLIDE 16

Digital Modulation and Multiplexing

  • Ref. CN5E, NT@UW, WUSTL

16 CS5530

  • Multiplexing
  • FDM (Frequency Division Multiplexing)
  • TDM (Time Division Multiplexing)
  • CDM (Code Division Multiplexing)

} A signal is spread out over a wider frequency band } More tolerant of interference } Allow multiple signals to share the same frequency band

¨ CDMA (Code Division Multiple Access)

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SLIDE 17

Frequency Hopping Spread Spectrum (FHSS)

  • Ref. CN5E, NT@UW, WUSTL

17 CS5530

  • Transmitting signals by rapidly switching among

many frequency channels

  • Using a pseudorandom sequence known to only transmitter and

receiver: training signal before transmission

  • Developed initially for military (prevent jamming and collision)
  • Spreads power over a wide spectrum (spread spectrum)

Hedy Lamarr

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SLIDE 18

Direct Sequence Spread Spectrum (DSSS)

  • Ref. CN5E, NT@UW, WUSTL

18 CS5530

  • Each bit is represented by multiple bits using a

spreading code or chipping code

  • Transmitters XOR the chipping code with data to be transmitted
  • 10-100 bit chipping code: longer chipping code, more secure

FHSS and DSSS are two methods

  • f CDMA
slide-19
SLIDE 19

Applications

  • Ref. CN5E, NT@UW, WUSTL

19 CS5530

  • FHSS in 802.11
  • A typical FHSS WLAN will subdivide the bandwidth into 79 non-overlapping

channels, each 1MHz wide

  • 802.11 standard defines 78 different hopping patterns
  • The patterns allow for 26 networks to be co-located and still operate

simultaneously

slide-20
SLIDE 20

Noise and Different Sources

  • Ref. CN5E, NT@UW, WUSTL

20 CS5530

  • Noise has 3 different sources
  • Thermal noise

} Proportional to absolute temperature } Temperature measured from absolute zero in kelvins

  • Spurious emissions

} Car ignition and electronic devices } More noise in urban areas

  • Receiver noise

} Amplifier adds noise } Noise generated before the amplifier also gets amplified

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SLIDE 21

Summary

  • Ref. CN5E, NT@UW

21 CS5530

  • Electromagnetic Spectrum
  • Reflection, diffraction and scattering of signals
  • Multipath, Doppler shift
  • Digital modulation and multiplexing
  • Noise