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Wireless Communication Fundamentals

Wireless Communication Fundamentals. Wired Vs. Wireless Communication. Each cable is a different channel. One media (cable) shared by all. Signal attenuation is low. High signal attenuation. No interference. High interference

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Wireless Communication Fundamentals

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  1. Wireless Communication Fundamentals

  2. Wired Vs. Wireless Communication Each cable is a different channel One media (cable) shared by all Signal attenuation is low Highsignal attenuation No interference High interference noise; co-channel interference; adjacent channel interference

  3. Why go wireless ? Advantages • Sometimes it is impractical to lay cables • User mobility • Cost Limitations • Bandwidth • Fidelity • Power • Security

  4. 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

  5. Wave Propagation Example electric field propagation direction magnetic field

  6. Radio Propagation Three basic propagation mechanisms Scattering λ >> D Diffraction λ  D Reflection λ << D • Propagation effects depend on not only on the specific portion of spectrum used for transmission, but also on the bandwidth (or spectral occupancy) of the signal being transmitted

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

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

  9. ISM band 902 – 928 Mhz 2.4 – 2.4835 Ghz FM radio S/W radio TV TV AM radio cellular 5.725 – 5.785 Ghz VHF UHF SHF EHF LF MF HF  300MHz 30MHz 30GHz 300GHz 3GHz 3MHz 30kHz 300kHz  100mm 10cm 10m 1cm 1m 100m 10km 1km EM Spectrum X rays Gamma rays visible UV infrared  1 MHz 1 GHz 1 kHz 1 THz 1 EHz 1 PHz Propagation characteristics are different in each frequency band

  10. 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 range. Foliage and rain effects. Indoor microcells, Point-to-point, and Satellites to ground stations.

  11. Unlicensed Radio Spectrum  12cm 5cm 33cm 26 Mhz 83.5 Mhz 125 Mhz 902 Mhz 2.4 Ghz 5.725 Ghz 2.4835 Ghz 5.850 Ghz 928 Mhz 802.11a 802.11b Bluetooth Microwave oven cordless phones baby monitors WaveLan

  12. Propagating sinusoidal wave with some frequency/wavelength C (speed of light) = 3x108 m/s Frequency & Public Use Bands

  13. 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 loss • Short wave length (high frequency) has more loss

  14. Other Path-loss Exponents • Path-Loss Exponent Depends on environment: • Free space 2 • Urban area cellular 2.7 to 3.5 • Shadowed urban cell 3 to 5 • In building LOS 1.6 to 1.8 • Obstructed in building 4 to 6 • Obstructed in factories 2 to 3

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

  16. Multi-Path Effect • Multi-path components are delayed depending on path length (delay spread) • Phase shift causes frequency dependent constructive / destructive interference Amplitude Amplitude Time Frequency

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

  18. 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. 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) • Spectrum usage (Hz) 􀂇 Inter-symbol interference (ISI) occurs unless delay spread << symbol time

  20. 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

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

  22. Physical Channel Properties Review 􀂇 Wireless signal strength • Transmit power • Loss over distance (falls off by d2) • Shadowing (e.g. absorption by walls) • Multi-path (e.g. bouncing off of metal objects) 􀂇 Noise • Thermal noise floor • Environmental noise (e.g. microwave ovens) 􀂇 Channel quality • Related to signal to noise ratio

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