Mobile Communications

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Mobile Communications

• Wireless Physical Layer

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Learning outcomes

• You have learnt this if you can
• Compare wireless and fixed networks
• Draw and label a sine wave
• Sketch the Electromagnetic spectrum
• Explain why a transmitted signal is distorted
• Sketch 3 forms of modulation

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Mobile computing wireless?

• Access to services anytime anyplace
• Examples
• Travelling information
• Disaster relief support
• Games
• Wire-free house
• Push information (e.g. share price changes)

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Mobile Technical Issues

• Energy consumption
• Radio has a high energy consumption
• Limited computing power
• Lower quality displays
• Small/no disks
• Small size
• Portable
• Harder to use
• Little space for an antenna

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Wireless vs. Fixed Networks

• Higher error rates from interference
• emissions from engines, lightning
• Government regulation of frequencies
• Lower transmission rates
• local 50Mbit/s, regional 50kbit/s with GSM
• Set up delays connection set-up time
• GSM 20 second, Bluetooth 2-10s
• Lower security, simpler active attacking
• E.g. simulate base station

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A Layered Approach
Regulating amount of data sent
e.g. noise, speed
yellow pages
Switching between transmitters
Identifying the sender receiver
Deciding who uses a channel
Sharing a channel
Combining a signal with a carrier moving the
signal to a different frequency band
Noise unwanted signals
Decreasing amplitude
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Physical Layer

• Electromagnetic waves
• Interacting electric and magnetic fields
• The magnitude of the field varies with time
• The simplest wave is a sine wave

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Sine waves

• signal(t) A sin(2p f t p) At a location
• signal(x) A sin(2p x / ? p) At an instant
• amplitude A
• frequency f 1 Hz 1 cycle per second
• period T 1/f
• wavelength ? with ?f c (3108 m/s)
• phase p

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Sine waves at a location

• signal(t) A sin(2p f t p)
• Imagine you are standing still in the sea
• The sea rises and falls about you
• The time between two high levels is the period
• Your friend stands slightly away from you
• The sea rises and falls, but slightly out of step
• If he moves further away,
• the water is in step
• The distance is the wavelength

Frequency The number of times per second that
you get water up your nose. The longer the
period, the shorter the frequency
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Sine Waves at an instant

• signal(x) A sin(2p x / ? p)
• Imagine the sea has instantly frozen
• As you move away, the height changes
• Wavelength
• The distance between two peaks

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Sine Wave
? - wavelength
A - amplitude
P phase difference
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Frequency

• Frequency number of waves per second
• Measured in Hertz
• If 1 cycle is 0.5 seconds f1/0.5 2Hz
• If 1 cycle is 0.125 seconds, f ?
• If frequency 3400Hz,
• how many waves in one second?
• f ? c speed of light 3x108 ms-1

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Electromagnetic Spectrum
Gamma rays
Visible light
Microwaves
Television
Ultraviolet
FM Radio
Infrared
X-Rays
Radar
Radio
106
1012
108
1018
1010
1014
1016
Low frequency Long wavelength
High frequency Short wavelength
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Fourier Theory

• Any signal can be created by adding infinitely
  many sine waves of different frequencies and
  appropriate phases.

Different sine waves will stay as sine waves but
the amplitude and phase will be changed
differently during transmission. This will alter
the shape of the signal - distortion
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Generating a Triangular Wave
Triangular wave from Constant 4 sine waves
3.5
Result
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2.5
2
If a triangular wave can be made up from a
combination of sine waves, its believable that
any waveform can be made from sines
1.5
Fundamental
1
Multiples of original frequency
0.5
0
0
100
200
300
400
500
600
700
800
-0.5
-1
Harmonics
-1.5
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Distortion

• Channels dont have infinite bandwidth
• Cant reproduce full signal
• High frequency sine waves are lost
• Different frequencies - different losses
• Received signal isnt just lower amplitude
  version of transmitted
• Reflection, scattering, refraction, diffraction,
  attenuation
• Different paths may add echo

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Distortion
Refraction
Attenuation
Echo
Diffraction
Scattering
Reflection
Shadowing
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Modulation

• Fit signal into different frequency band
• Share frequencies bands are regulated
• Different transmission features
• Long wave bends around the earth
• Short wave straight line only
• Modulation Techniques
• Amplitude Modulation
• Frequency Modulation
• Phase Modulation

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Modulation shifts frequencies
High frequency carrier
Amplitude Modulation
Low frequency signal
Fourier Analysis
Frequencies
Real signals have many frequencies
Frequency Shift
Carrier
Original signal
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Modulation Demodulation
analog baseband signal
digital data
digital modulation
analog modulation
radio transmitter
101101001
radio carrier
analog baseband signal
digital data
digital demodulation
analog demodulation
radio receiver
101101001
radio carrier
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Modulation of digital data
1
0
1

• Amplitude Shift Keying (ASK)
• very simple
• low bandwidth requirements
• susceptible to interference
• Frequency Shift Keying (FSK)
• needs larger bandwidth
• Phase Shift Keying (PSK)
• more complex
• robust against interference

t
1
0
1
t
1
0
1
t
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Summary

• Wireless increases mobility
• Wireless protocols use a layered approach
• Data link media access error checking
• Physical use electromagnetic waves (radio)
• Distortion
• Attenuation, reflection, refraction, echo
• Modulation
• Combine signal with carrier
• For digital data ASK, FSK, PSK