This document provides an overview of key concepts in mobile computing. It discusses:
1) Modes of evaluation including problem-based learning, continuous assessments, and term-end exams.
2) Types of mobility including user mobility (e.g. Vodafone services) and device mobility (e.g. mobile phones).
3) Characteristics of communication devices ranging from fixed/wired to mobile/wireless and applications of mobile technologies in various domains like vehicles, emergencies, and replacing wired networks.
5. Characteristic of communication devices
• Fixed & wired eg:Desktop
• Mobile & wireless eg:Laptop
• Fixed & wireless eg:wireless on historical
buildings
• Mobile and wireless eg:GSM
12. Signals
• Signals are function of time & location
• Signal Parameters represent the data values.
• Carrier signals
• Periodic signals(sine waves)
• A-amplitude
• F-frequency
• ϕ-phase swift
15. Signal reconstruction by Fourier
• C-determine direct current component of
signal
• an & bn amplitudes of nth sine & cosine
functions
16. Frequency domain &Phase domain
M - Amplitude Of Signal
I – In-Phase(phase 0)
Q - Quadrature(90’ Phase Shift)
Representation of Signals
17. Antennas
• It couples electromagnetic energy to and from
space to and from a wire or a coaxial cable
• Theoretical reference : it is isotropic radiator
• i.e – a point in a space radiating equal power
in all directions
• Radiation pattern is symmetric in all directions
18. • Isotropic antennas does not exist in reality
• Real antennas exhibit directive effect(intensity
is not same in all directions)
• Types of antennas
– Dipole
– Omni directional
– Directional
– Sectorized
– Diversity
19. • Dipole antennas
– Consist of two collinear conductors of equal
length , separated by small gap
20. • Omni – directional
– Radiation pattern in one plane
– This is used to overcome environmental
challenges by boosting power level of signal
– Challenges could be mountains, valley, building etc
23. • Diversity combining
– Constitutes a power of all signals to produce gain
– Phase is corrected to avoid cancellation
24. Signal propagation
• One can determine the behavior of a signal
travelling along wire, e.g., received power
depending on the length of wire
• For wireless ,the above prediction is valid only
in vacuum
25. • Transmission Range: with this radius of the sender
transmission is possible
• Detection range: within a second radius , detection
of transmission is possible.
i.e transmitted power is large enough to differ from
background noise
• Interference range:background noise is added with
the transmission.
26. Path Loss of radio signals
• Line-of-sight
• Path loss in vacuum
– Received energy pr =1/d2
– d is distance between sender & receiver
• Sender in a space radiate energy in spherical
shape,surface area is s=4πd2 increasing results
loss
• Received power depends on the wavelength,
gain of the receiver & transmitter antennas
27. • Penetration of signal on object depends on
the frequency
• Lower frequency better penetration
3 fundamental behavior of radio waves
• Ground waves
• Sky waves
• Line of sight
28. Ground waves(<2MHz)
– It follows earths surface and propagate long
distance
Sky waves (2-30 MHz)-(short waves)
– Reflected by ionosphere
Line of sight (>30MHz)
– Follows straight line of sight
– No reflection in ionosphere
– Cable of bending due to refraction
– Mobile phone system, satellite system, cordless
phone
29. Additional Signal propagation effects
• Blocking or shadowing of radio signals due to
large obstacles
• Reflection
• Refraction-bending effect of signal
30. • Scattering :size of obstacle is in order of the
wavelength or less,radio waves get scattered
• Diffraction: Deflection at the edge of object
and propagate in different direction
32. • Along with direct transmission from a sender
to a receiver the propagation effects like
scattering, reflection, difraction leads to one
of the severe effect called Multi-path
propagation
33. • signals travelling along different paths with
different lengths arrive at the receiver at
different times
• This effect is called delay spread
• Typical values for delay spread are
approximately 3 μs in cities, up to 12 μs
• GSM, for example, can tolerate up to 16 μs of
delay spread, i.e., almost a 5 km path
difference
34. Effects of spread delay
• A short impulse will be smeared out into a
broader impulse, or rather into several weaker
impulses.
• Inter-symbol-Interference(ISI) - At the
receiver, both impulses interfere, i.e., they
overlap in time
35. Avoiding ISI
• Knowing the channel characteristic
• If sender knows the delay of different path ,it
can compensate the distortion caused by the
channel
• How?
– sender may first transmit a training sequence
known by the receiver. The receiver then
compares the received signal to the original
training sequence and programs an equalizer that
compensates for the distortion
36. Fading
• The power of the received signal changes
considerably over time
• Short time fading-quick changes in the
received power are called short-term fading
– signals may have a different phase due to different
paths and cancel each other
37. • Long term fading:
– The average power over time, is caused by, for
example, varying distance to the sender
– senders can compensate for long-term fading by
increasing/decreasing sending power
38. MULTIPLEXING
• Multiplexing describes how several users can
share a medium with minimum or no
interference
• For wireless communication, multiplexing can
be carried out in four dimensions:
– space, time, frequency, and code