Introduction
Information and messages
Telecommunication is the transmission of information from one place to another in
electromagnetic form.
In the following we discuss about the message that carries the information. It can be a
waveform, a bit sequence, or other physical manifestation of information. The goal of a
communication system is to reproduce at the destination an acceptable replica of the source
message.
The original message can be analog (e.g. speech) or digital (e.g. text). Independent of the
source, the message can be sent in analog or digital form.
In analog transmission there are certain quality criteria (Signal-to-Noise Ratio SNR,
distortion, etc) which should be taken into account when designing the system.
In digital transmission, usually, the goal is to minimize the probability of error.
Elements of Communication System
Transmitted
Signa l
Source
Transmitter
Received
Signal
Channel
Receiver
Destination
N oise, I nterference
Distortion
The transmitter processes the input signal and produces a signal suited to the characteristics
of the transmission channel. This involves almost always
• Modulation
• Coding
The channel (i.e., the electrical medium that bridges the distance from source to destination)
may cause some undesired changes to the message signal such as:
• Attenuation,
• Phase and frequency distortions
• Noise (random signal)
• Interference (e.g., other channels, other users in CDMA)
Elements of Communication System (cont.)
The receiver restores the message signal to its form (via amplification, decoding,
demodulation, filtering) and minimizes the effect of non-idealities of the channel.
Transmission can be one-way or two-way:
• Simplex: one-way
• Full-duplex: simultaneous transmission in both directions
• Half-duplex: transmission in either directions but not at the same time
Transmission Channels - Examples
1. Cables
• wire pairs (ordinary telephone line)
• coaxial cable
2. Radio Transmission
• broadcasting
• microwave links
• satellite transmission
• cell networks
3. Optical Fibers
4. Magnetic Tapes/CD.
Physical Limitations
The fundamental limitations when designing a communication system are the noise and the
bandwidth.
There is always thermal noise (due to the random motion of charged particles at temperatures
above absolute zero), which is the main problem when the transmission distance increases.
Every communication system has a finite bandwidth. The bandwidth is the main problem
when the transmission speed is increased because they are directly proportional to each other.
Channel capacity is
C = B log2 (1 + S / N )
Where B is the bandwidth and S / N is the Signal-to-Noise Ratio (SNR) of the channel. This
relationship is also known as Hartley-Shannon law.
Modulation
The modulation involves two waveforms:
A modulating signal that that represents the
message, and a carrier wave that suits the particular
application.
Here are some examples of amplitude modulation
using sinusoidal and pulse train as a carrier. The
message signal can be seen in the envelope of the
modulated signal. In the receiver, the message can be
retrieved using demodulation.
a) Modulating signal; b) Sinusoidal carrier with amplitude
modulation; c) Pulse-train carrier with amplitude
modulation.
In general, the carrier frequency is much higher than
the highest frequency component of the modulating
signal. In this case, the spectrum of the modulated
signal consists of a band of frequency components
clustered around the carrier frequency. Therefore,
modulation produces frequency translation.
Modulation Benefits
Modulation for efficient transmissions
The efficiency of any transmission method depends on the frequency of the signal being transmitted.
For example, efficient line-of-sight radio propagation requires antennas whose physical dimensions are
at least 1/10 of the signal wavelength. E.g., unmodulated transmission of an audio signal at 100Hz
would require 300 km long antenna, while modulated transmission at 100MHz allows a practical
antenna size of about 1 m.
Modulation for frequency assignment
For example, since each radio/TV station has a different assigned carrier frequency, the desired signal
can be separated from the others by filtering. Radio frequencies are allocated by international
agreements.
Multiplexing
Several signals can be combined for simultaneous transmission on one channel if, e.g. the carrier
frequencies are different (frequency division multiplexing).
Modulation to overcome hardware limitations
The design of a communication system may be constrained by the cost and availability of hardware,
hardware whose performance often depends on the frequencies involved. Modulation permits the
designer to place a signal in some frequency range that avoids hardware limitations.
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The Electromagnetic
Spectrum
C
Coding
Coding is a processing of message signal for improving digital communication. Decoding is the inverse
operation.
• Channel coding (a technique used to introduce controlled redundancy to improve the performance
reliability in a noisy channel).
• Source coding (a technique that reduces the redundancy in the signal to achieve more efficiency).
Examples:
1. ASCII-code: coding of the alphanumerical characters to binary data.
2. Transmission capacity can be improved by sending 2
code words of length M (source coding).
M
level symbols that represent binary
3. By appending extra check digits to each binary code word we can detect or correct most of
the errors in the receiver (channel coding).