This document summarizes key concepts from a lecture on communication basics, including: - Signals can be represented in the time domain (amplitude over time) or frequency domain (amplitude by frequency). Periodic signals repeat over a set time period while aperiodic signals do not. - Distortion sources that can impact signals include attenuation reducing strength, bandwidth limitations filtering frequencies, delay of some frequencies, and noise from random perturbations. - Other impairments include atmospheric absorption, shadowing from obstacles blocking signals, and fading from multiple reflected signal versions adding or canceling out.

1. Lecture 1
Communication basics.
(2 hours)
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2. Contents
Further readings: Behrouz, Chapters 3, 4, 5, 7 (section 7.1), 10
• Signals
– Amplitude
– Frequency
– Phase
• Distortion sources
– Attenuation
– Bandwidth limitation
– Delay
– Noise
– Fading
– Shadowing
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3. Signals - Definition
• Signals - Electromagnetic Waves that carry
information.
• Information is encoded by the shape of the
signals.
• Analog signals vary smoothly over time.
• Discreet signals maintain constant level for some
time then change to another constant level.
Digital signals have only 2 levels.
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4. Time
Amplitude
Time
Amplitude
Analog signal
Discreet signal
Amplitude
Time
Digital signal
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5. Signals, Representation (1)
• Time domain: Amplitude (A) (height of the signal,
representing the power) against time.
– Periodic Signals: Repeated pattern over a time
distance called the period T. The rate of the
repetition is called the frequency (f = 1/T). The
distance between the two points of repetition is
called the wavelength ( = c/f) (c is the speed of the
signal). The phase () is the shift of the signal in time
from the origin.
– Periodic signals can be expressed as series of sin and
cos, S(t) = A sin (2ft + ) + …
– Aperiodic signals: do not repeat.
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6. Periodic signal
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7. Concept of phase shifts relative to time zero
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8. Signals, Representation (2)
• Frequency Domain: Amplitude against the
frequencies. The frequency is the number of
changes that the signal make every second. The
signal is made up of many frequencies, which are
multiple of the Fundamental frequency.
• Spectrum of a signal: The range of all frequencies
that the signal contains. The bandwidth
represents the range of the frequencies where
most the signal strength is located.
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9. Frequency domain
Amplitude
Fundamental
Bandwidth
Spectrum
Example: S(t) = 15sin(10t) + 4 sin(20 t) + 2 sin (30t) + 0.5sin(40t)
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10. Distortion Sources (1)
• Attenuation: the strength of the signal
decreases as it propagates throughout a
transmission medium.
– Repeater and Equalizers are normally used.
– Loss (dB) = 10 log10 (Pout/Pin) (guided media)
• Limited Bandwidth: the bandwidth of the
medium filters some of the frequency
components of the signal.
– Nyquist theorem: To recover the signal the rate of
transmission C should be twice the bandwidth W.
• C = 2 Wlog2M
• M is the levels of the signal (binary M =2)
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11. Distortion Sources (2)
• Delay distortion: Some frequency components
of a signal arrive at different time, so the signal is
made up of various delayed frequencies.
• Noise: in the absence of transmission there are
random perturbations on the line.
– Shannon theorem: in a noisy channel the maximum
data rate C = W log2(1+S/N) bps, with the signal to
noise ratio = 10log10 (S/N)dB.
– Sources of Noise: Cross talk, Impulse noise, inter-modulation,
thermal (white) noise (N=KTW thermal
noise in watts present in a channel of bandwidth W at
temperature T kevin , K=1,3808x10-23J/K)
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12. Other Impairments
• Atmospheric absorption – water vapor and oxygen
contribute to attenuation
• Shadowing – Obstacles on the way of the transmission
absorb the strength from the signal.
• Fading – Reception of multiple version of the same signal.
– Reflected signals may arrive relatively simultaneously ( local
scattering around the receiver.), they could create combined or
destructive effects, deep fading (explain…)
– Reflected signals may arrive relatively with long intervals, dues
to multiple paths.
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13. Shadowing
(1)
(2)
• Buildings between the transmitter and receiver
may cause temporary degradation to the signal
strength. (1)
• Temporary line-of sight transmission path
would results in abnormally high received
power.(2)
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14. Multipath Propagation
– Reflection - occurs when signal encounters a surface
that is large relative to the wavelength of the signal
– Diffraction - occurs at the edge of an impenetrable
body that is large compared to wavelength of radio
wave
– Scattering – occurs when incoming signal hits an
object whose size is in the order of the wavelength of
the signal or less
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15. Quiz
• What is the difference between frequency
domain and time domain representations?
• Sketch a periodic and aperiodic signals.
• What happens in the attenuation?
• What does -2dB mean? Give an example.
• What is the difference between shadowing and
fading?
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