Wireless communication overview of basic concepts
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Wireless Communication: Overview of basic concepts. Narayan Mandayam. Wired Vs. Wireless Communication. Each cable is a different channel. One media (cable) shared by all. High signal attenuation. Signal attenuation is low. High interference

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Wireless Communication: Overview of basic concepts

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Wireless communication overview of basic concepts

Wireless Communication: Overview of basic concepts

Narayan Mandayam


Wired vs wireless communication

Wired Vs. Wireless Communication

Each cable is a different channel

One media (cable) shared by all

Highsignal attenuation

Signal attenuation is low

High interference

noise; co-channel interference; adjacent channel interference

No interference


Why go wireless

Why go wireless ?

Advantages

  • Sometimes it is impractical to lay cables

  • User mobility

  • Cost

Limitations

  • Bandwidth

  • Fidelity

  • Power

  • (In)security


Em spectrum

EM Spectrum

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

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


Unlicensed radio spectrum

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


Understanding wireless communication

Understanding wireless communication

Tx

Rx

  • How does signal propagate ?

  • How much attenuation take place ?

  • How does signal look like at the receiver ?


Radio propagation

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

  • Spatial separation of Tx-Rx


Propagation in the real world

Propagation in the “Real World”

EE

a wave

can

be absorbed

penetrate

reflect

bend


Propagation

Propagation

EE

And, the higher frequencies will

usually encounter more “loss”

in “real world” situations

(again, smaller cells?;

more base stations?)


The cluttered world of radio waves

The Cluttered World of Radio Waves

hills

girders

rain

hallways

windows

vehicles

trees

walls


Evaluating frequencies

Evaluating Frequencies

  • 50 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? Satellites to ground stations?


Path loss in db

Path loss in dB

10 W

P1

dB = 10 log (----)

P2

101

Power

1 mW

10-3

10,000 times

1,000 times

1 W

40 dB

30 dB

10-6

d1

source

d2

Path loss from source to d2 = 70dB


Dbm absolute measure of power

dBm ( absolute measure of power)

10 W

P1

dBm = 10 log (-------)

1mW

101

Power

1 mW

10-3

1 W

10-6

d1

source

d2

= 40 dBm

+ 10,000 times

= 0 dBm

- 1,000 times

= -30 dBm


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