Wireless Communication Fundamentals Wired Vs Wireless Communication Wired

Wireless Communication Fundamentals

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

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

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

Wave Propagation Example electric field propagation direction magnetic field

Radio Propagation Three basic propagation mechanisms Reflection Diffraction λ << D λ D Scattering 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

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

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

EM Spectrum ISM band 30 k. Hz 10 km 1 km di ar VHF HF 3 MHz 30 MHz 100 m 10 m 2. 4 – 2. 4835 Ghz TV ce llu l TV FM ra ra W S/ MF 300 k. Hz o o di ra AM LF 902 – 928 Mhz 5. 725 – 5. 785 Ghz UHF 300 MHz 1 m SHF 3 GHz EHF 30 GHz 300 GHz 1 cm 100 mm 10 cm X rays infrared visible UV 1 k. Hz 1 MHz 1 GHz 1 THz 1 PHz Gamma rays 1 EHz Propagation characteristics are different in each frequency band

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 cellulartype 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.

Unlicensed Radio Spectrum 33 cm 26 Mhz 902 Mhz 12 cm 83. 5 Mhz 2. 4 Ghz 928 Mhz cordless phones baby monitors Wave. Lan 2. 4835 Ghz 802. 11 b Bluetooth Microwave oven 5 cm 125 Mhz 5. 725 Ghz 5. 850 Ghz 802. 11 a

Frequency & Public Use Bands • Propagating sinusoidal wave with some frequency/wavelength • C (speed of light) = 3 x 108 m/s Band Range Bandwidth 900 MHz 902 - 928 26 MHz 2. 4 GHz 5 GHz 2. 4 - 2. 4835 5. 15 - 5. 35 83. 5 MHz 200 MHz Wavelength. 33 m / 13. 1”. 125 m / 4. 9”. 06 m / 2. 4”

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

Other Path-loss Exponents • Path-Loss Exponent Depends on environment: Free space Urban area cellular Shadowed urban cell In building LOS Obstructed in building Obstructed in factories 2 2. 7 to 3. 5 3 to 5 1. 6 to 1. 8 4 to 6 2 to 3

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

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

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 – 8 VSB - US HDTV – BFSK - Mote Sensor Networks – QPSK - 2 Mbps 802. 11 & CMDA(IS-95)

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

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

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 d. Bm/Hz

Thermal Noise Calculation �� Depends on channel bandwidth �� = -174 d. Bm/Hz + 10 log(bandwidth in Hz) �� So for 802. 11 • About 25 MHz for 802. 11 b or 802. 11 a channel • Noise Floor is about -100 d. Bm • -100 d. Bm = 10 log(0. 0000001 Watts )

Physical Channel Properties Review �� Wireless signal strength • • Transmit power Loss over distance (falls off by d 2) 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