digital communications
DESCRIPTION
Digital CommunicationsTRANSCRIPT
MSc Digital Communications
University of Baghdad-2011
Lecture -1
Introduction to Communications System Parameters
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Basic Digital Communications System
Transmitter Channel Receiver
______________________________________________________________________________________________
Information Source Source encoder Channel encoder Digital Modulation
________________________________________________________________
Transmitter
Channel
_____________________________________________________________________
Destination Source decoder Channel decoder Digital Demod
_____________________________________________________________________
Receiver
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Definitions
• Information: Binary (0, 1)
• Source encoder: remove any redundancy in
binary data (data compression)
• Channel encoder: add redundancy so it can be
used to detect / correct errors
• Digital modulator: map digital data to
analogue signal sent to destination by wire or wireless.
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Communication Channel
We can model the linear Gaussian channel in the form:
Input Output
+
Random Variables of Gaussian distribution
Equivalent Gaussian Communication Channel
Output = Input + Noise
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Digital transmission
Noise corrupt data:
_________ _________ • _______________ t _______________ t
• Digital demodulator: convert received analogue signal into digits
• Channel decoder: use added redundancy to estimate original data with possibly some errors
• Source decoder: Decode the compression at transmitter.
Communication systems always have certain, fast processors, filters, data storage.
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Performance of digital
communication system
System performance is dependable on the following system parameters:
Speed of information transmission given by Shannon channel capacity which is limited by the available spectrum (bits/sec/Hz)
Available transmission power which is expressed by energy / bit ( ) dB
Data transmission codes used
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bE
Shannon channel capacity
Shannon theorem specifies the limit on therate of error free transmission for a power-limited, band-limited channel in additivewhite Gaussian noise.
channel capacity expressed in terms ofthree key parameters: channel bandwidthB, average received power S and noisepower spectral density at the channeloutput.
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0N
Shannon channel capacity
Shannon limit for channel capacity C is
C = B ( 1 + ) bits / sec
C = 3.32 B ( 1 + ) bits / sec
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2log
10log
BN
S
0
BN
S
0
Shannon channel capacity
Show that
Where are energy/bit, data rate, and channel capacity.
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B
RN
E
b
BbR
b 2
0
CRE bb ,,
Electric field
Space that surrounds an electric charge.
An electric field or E-field exerts a force on
charged objects.
The concept of electric field was introduced
by Michael Faraday
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Electric field forces
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Magnetic field
Magnetic fields or H-field exists when there
is a changing electric field or an electric
current or permanent magnet.
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Magnetic field forces
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Radio waves
• Radio is the wireless transmission of
signals by electromagnetic (EM) waves
with frequencies below those of light.
• An EM has H & E field orthogonal to each
• A wave is a disturbance that propagates
through space and transfer energy from
one point to another.
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EM waves
Electromagnetic waves travel in space with speed of light. It has electric and magnetic components and can travel in vacuum (without media).
The electric and magnetic components of EM wave oscillate at right angle to each other and to the direction of energy propagation and are in phase with each other.
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EM Radiation
EM radiation occurs in radio waves,
microwaves, infrared radiation, visible
light, ultraviolet radiation, X-rays, and
gamma rays.
• Wave length ( ) is the distance between
successive crests (or troughs) of a wave
pattern.
•
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.f
lightofspeedmx sec/103 8
Data speed
Data rate is the speed in bits per second (bits/sec) at which information is being communicated.
• Bandwidth
• Spectrum occupied by the transmission in Hz. Bandwidth and data rate are related by system efficiency
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durationpulseR
1ateData
HzbitsB
Rb sec//
Noise and errors
• Noise is the unwanted signal accompanies the useful signal. Noise is mainly due to thermal noise but it may include interference from other transmitters.
• Error rate is the rate at which transmission error occur. When an error occurs binary ‘1’ is received as ‘0’ and binary ‘0’ is received as binary ‘1’.
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Nyquist Bandwidth
Nyquist bandwidth is the minimum
bandwidth at which data can be
transmitted and recovered correctly. In
ideal case (no noise or interference) it is
equal
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DECIBEL: A tenth of a Bel
The Bel is mostly used in acoustics invented by Bell Telephone Lab (founder Alexander Graham Bell) in 1924 to quantify the reduction in audio level over a 1 mile (1.6 km) length of standard telephone cable.
• The Bel was too large for everyday use, so the decibel (dB), equal to 0.1 Bel (B), became more commonly used.
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Power in dB
• A decibel is a relation between two values
of POWER levels.
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Definitions of dBs
• dBm power A in dB when power of
reference transmitter B = 1 mW.
• Example 1 watt in dB = 30 dBm
• dBi is gain of antenna when its radiation is
equal in all directions (omni directional
antenna). Omni directional antenna = 10
dBi when it has gain of 10 dB in all
directions
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Signal to noise ratio (SNR)
• Signal to noise ratio is the power ratio
between the information signal power and
the background noise power where signal
and noise are measured within the same
bandwidth.
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SNR in dB
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Multimedia
• Multimedia is media that uses multiple forms
of information content and processing (e.g.
text, audio, graphics, animation, video and
interactive applications such as gaming) to
inform or entertain the user or audience.
• Multimedia Systems then support the
interactive use of text, audio, still images,
video, and graphics.
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Thermal noise floor
Thermal noise floor at room temperature T is
kTB
k=Boltzmann’s Constant= J/K
T=Temperature = 300 K
kT= =-174 dBm
kTB= -174 + dB
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231038.1 x
wattsx 211014.4
)(log10 10 HzinB
Radio Link Design
Worked example• A radio system operating at centre frequency 850 MHz is detected by a
sensitive receiver located at 550 metres away and experiencing clear line of sight. You may assume the following system parameters:
• Transmitter power 15 dBm
• Transmitter antenna gain 2 dBi
• Receiver antenna gain 3.2 dBi
• Receiver filter noise bandwidth (B) 120 Hz
• Thermal noise floor -174+10 dB
• Receiver noise figure 3 dB
• Receiver threshold 5dB
• Calculate the signal margin in dB. You may assume the free line of sight loss in dB is 20 .
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