This document provides an overview of principles of communications, including: - The basic components and models of communication systems, including the purpose of transferring information between locations. - A timeline of major developments in electronic communications from the telegraph to satellites. - Descriptions of simplex, half-duplex, and full-duplex transmission modes. - Explanations of decibel, dBm, and bel units for measuring signal power levels and gains/losses. - An overview of the electromagnetic spectrum and the different frequency bands used for communication. - Definitions of bandwidth, information capacity, and Shannon's limit relating capacity to bandwidth and signal-to-noise ratio.

1. 1
PRINCIPLES OF
COMMUNICATIONS

2. Electronic Communications System
 Main purpose of an electronic communications system is to transfer
information from one place to another.
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3. 3
DATE EVENT
1844 Morse patents the telegraph.
1876 Bell invents and patents the telephone.
1887 Hertz discovers radio waves.
1901 Marconi makes first transatlantic radio
transmission.
1923 Television is invented.
1962 First communications satellite

4. 1.1 Basic Communication Model
 Basic communication models shows the communication flows between 2
points.
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5. 1.2 Communication / Transmission Mode
 Communication system can be designed for transmitting information in
one or both direction. Generally, the mode of communication can be
divided into 3 types :
 Simplex System : the system capable of sending information in one
direction only where only the sender can send the information and only the
recipient can receive the information.
 Half-duplex System : the system capable to carry information in both
direction, but only one direction is allowed at a time. The sender transmits to
the intended receiver, and then reverse their roles.
 Full-duplex System : Information can be carried in both direction at the
same time. The 2 directions of information travel are independent of each other.
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6. 1.2 Communication Transmission/Mode
 Half-duplex System vs Full-duplex System
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7. 1.3 Power Measurement (dB, dBm & Bel)
 Magnitudes of communication signals span a very wide range causing a
drawbacks as follow :
 Extremely large scale (graph/drawing)
 Hard calculation (too big vs too small numbers)
 Prone to errors (e.g. 0.0001 vs 0.00001)
 Hard to compare the signals
 As a solution, logarithmic scale is used !
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8. 1.3.1 Decibel (dB)
 Used to measure the ratio between 2 values – value to be measured
relative to a reference value
 In the electronic communication field, decibel is normally used to define
the power ratios between 2 signals
 In the common usage, it also used to express the ratios of voltage and
current
 If 2 powers are expressed in the same units (e.g. watt, miliwatt), their
ratio is a dimensionless quantity that can be expressed in decibel form
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9. 1.3.1 Decibel (dB)
 Positive (++) dB value indicates the output power is greater than the input
power, which indicates power gain or amplification
 Negative (--) dB value indicates the output power is less than the input power
which indicates power lossloss or attenuation
 If Pout == Pin, the absolute power gain is 1, which means dB power gain is 0
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10. 1.3.1 dB
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11. 1.3.1 dB
 Ex : Convert the absolute power ratio of 200 to a power gain in dB.
 Ex : Convert a power gain Ap = 30 dB to an absolute power ratio.
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12. 1.3.2 dBm
 A dBm is a unit of measurement used to indicate the ratio of power level
with respect to a fixed reference level. With dBm, the reference level is 1
mW (miliwatt).
 Ex : Convert a power level of 200 mW to dBm
 Ex : Convert a power level of 30 dBm to an absolute power
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13. 1.3.2 dBm
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14. 1.3.3 Bel
 A Bel is one-tenth of a decibel
(5)
 The Decibel unit was originated from the Bel unit, in honor of Alexander
Graham Bell.
 Bel unit compressed absolute ratios of 0.00000001 to 100000000 to a
ridiculously low range of only 16 Bel (-8 Bel to + 8 Bel).
 Difficult to relate Bel unit to true magnitudes of large ratios and
impossible to express small differences with any accuracy.
 To overcome this, Bel was simply multiplied by 10, creating a decibel.






=
in
out
P
P
Bel 10log
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15. 1.3.4 Power levels, Gains and Losses
 When power levels are given in watts and power gains are given as
absolute values, the output power is determined by multiplying the input
power with the power gains.
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16. 1.3.4 Power levels, Gains and Losses
 Exercise::
 For a 3-stages system with an input power Pin = -20 dBm and the power
gains/loss of the 3-stages as AP1 = 13 dB, AP2 = 16 dB and AP3 = -6 dB,
determine the output power (Pout) in dBm and watts.
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17. Channel Noise and Noise Weighting
Noise signals with equal interfering effects are assigned equal
magnitudes. To accomplish this effect, the AT & T developed a weighting
network called C-message weighting.
dBrnc is similar to dBrn except dBrnc is the dB value of noise with respect to
reference noise using C-message weighting.
dBrn is the dB level of noise with respect to reference noise (-90 dBm).
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18. Transmission Level Point
is defined as the optimum level of a test tone on a channel at
some point in a communications system.
Data Level Point
is a parameter equivalent to TLP except TLP is used for voice
circuits, whereas DLP is used as a reference for data transmission. The DLP is
always 13 dB below the voice level for the same point.
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19. 1.4 Electromagnetic Frequency Spectrum
 The electromagnetic spectrum is the range of all possible frequencies of
electromagnetic radiation. It is a continuum of all electromagnetic waves
arranged according to frequency and wavelength.
 Communicating the information between two or more location is done by
converting the original information into electromagnetic energy and then
transmitting it to the receiver where it is converted back to its original
form
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20. 20
The Electromagnetic Spectrum

21. 1.4 Electromagnetic Frequency Spectrum
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22. 1.4 Electromagnetic Frequency Spectrum
 The spectrum is divided into bands, with each band having a different
name and boundary.
 The radio frequency band (30Hz ~300GHz) is divided into narrower band
as follow.
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23. 1.4 Electromagnetic Frequency Spectrum
 WavelengthWavelength : is the length that one cycle of electromagnetic wave
occupies in space. It is inversely proportional to the frequency of the
wave and directly proportional to the velocity of propagation.
 Total electromagnetic wavelength spectrum is shown below.
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24. 1.4 Electromagnetic Frequency Spectrum
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25. 1.4 Electromagnetic Frequency Spectrum
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26. 1.5 Bandwidth
 Bandwidth of an information signal is the difference between the highest
and the lowest frequency contained in that signal.
 Bandwidth of a communication channel is a difference between the
highest and the lowest frequency that the channel will allow to pass
through it.
 Bandwidth of a communication channel must be equal or greater than the
bandwidth of the information.
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27. 1.6 Information Capacity
 Information capacity is a measure of how much information can be
propagated through a communication system.
 It can be expressed in the function of bandwidth and transmission time.
 It represents the number of independent symbols that can be carried
through a system in a given unit of time.
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28. 1.6 Information Capacity
 In 1948, Claude E. Shannon published what is called as Shannon limit for
information capacity
 Based on this law, the information capacity of any communication channel
is related to its bandwidth and the signal-to-noise ratio.
 The higher the signal-to-noise ratio, the better the performance and the
higher the information capacity is.
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29. 1.6 Information Capacity
 Ex: For a standard telephone circuit with a signal-to-noise ratio of 30 dB
and a bandwidth of 2.7 kHz, determine the Shannon limit for information
capacity.
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