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Post on 12-May-2017
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Signal Propagation
sender
Propagation: How the Signal are spreading from the receiver to sender.
Transmitted to the Receiver in the spherical shape.
• When the signal transmit from sender to receiver , the character of signal may not be changed in the wired network.
•But wired signal there is a change in character, like packet loss, external factors .etc,.
Signal Propagation Ranges
distance
sender
transmission
detection
interference
Transmission range communication possible low error rate
Detection range detection of the signal
possible no communication
possible
Interference range signal may not be
detected signal adds to the
background noise
Path Loss of Radio Signals
When Propagation in free space always like light (straight line).
The signal passes sender to receiver in a straight line is called Line-of-sight (LOS)
If there is a vacuum between sender receiver , it’s a free space loss.
Received power by the receiver is P. Distance between sender and receiver is d.
The received power by the receiver
Pr is proportional to 1/d2.
Pr α 1/d2.
The sender emits signals with certain energy.
If there is a vacuum space, the sends at the speed of light with spherical shape.
If there is no obstacle continuously grows the surface of sphere (s) and it moves to the
particular distance d. According to the equation
s=4πd2
Path Loss of Radio Signals
The additional parameters are important .
The receiver power depends on the wavelength and gain of receiver and transmitter.
The external factors are affects the signals . The path loss not occures in short distance.
The atmosphere influences in long distance travel signal.
Heavy Rain - Mobile Phone
Depending upon the frequency , it penetrate objects
A short wave length with lower frequencies
penetrate easily
A long wave length with higher frequencies can
be blocked by external factors
FUNDAMENTAL PROPAGATION BEHAVIORS
Ground Wave
• < 2 MHz . LF and MF • Low frequency follow the earth surface • Used in Submarine Communication and AM Radio
Sky Wave
• 2 to 20 MHz • Waves bounce from the ionosphere to earth. The short waves are reflected• Used in International broadcasts
SIGNAL PROPAGATION
Shadowing Making the signal weak
Reflection at large obstacles Refraction
Depending on the density of a medium Changing in Direction
Scattering at small obstacles The signal meets one point and scatter as many
Diffraction at edges
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction. i.e radio signals reaching the receiving antenna by two or more paths
The signals travelling along different path, different length and different time. This effects is delay spread
MULTIPATH PROPAGATION
A Real Example of Multiplexing
How ???
MULTIPLEXING
Transmission of multiple data communication sessions over a common wire or medium
Minimum of interference and a maximum of medium utilization.
s2
s3
s1
Space Division Multiplexing
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channels ki•The channels k1 to k3 can be mapped onto the three ‘spaces’ s1 to s3•Separate the channels and prevent the interference ranges from overlapping
k4 to k6 is allocated for some others.
Analog Telephone
Frequency Division Multiplexing
• The total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands
• Each of which is used to carry a separate signal
Frequency Division Multiplex
Separation of the whole spectrum into smaller frequency bands
Advantages: no dynamic coordination
necessary works also for analog signals
Disadvantages: waste of bandwidth
if the traffic is distributed unevenly
inflexible guard spaces
E.g FM Radio
k2 k3 k4 k5 k6k1
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TIME DIVISION MULTIPLEXING
• The time domain is divided into several recurrent time slots of fixed length• A sample byte sub-channel 1 is transmitted during time slot 1, sub- channel 2 during time slot 2, etc
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Time Division Multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages: only one carrier in the
medium at any time throughput high even
for many users
Mobile Phone for a Time
f
Time and frequency multiplex
Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM
Advantages: better protection against
tapping protection against frequency
selective interference higher data rates compared to
code multiplex but: precise coordination
required
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Code Division Multiplexing (CDM)Also known as code division multiple access (CDMA)
An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes
Used for mobile communications.
Each mobile device is assigned a unique 64-bit code (chip spreading code)
To send a binary 1, mobile device transmits the unique code
To send a binary 0, mobile device transmits the inverse of code
Code Division Multiplex
Each channel has a unique code
All channels use the same spectrum at the same time
Advantages: bandwidth efficient no coordination and synchronization
necessary good protection against interference and
tapping Disadvantages:
lower user data rates more complex signal regeneration
Implemented using spread spectrum technology
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