COMMUNICATION SYSTEMS.
A communication system conveys information from its source to a destination some
distance away. The goal of a communication system is to reproduce at the
destination an
acceptable replica of the source message. There are many kinds of information
sources,
including machines as well as people, and messages appear in various forms.
Nonetheless, we
can identify two distinct message categories: Analog and Digital.
An Analog message is a physical quantity that varies with time, usually in a smooth
and
continuous fashion e.g. the acoustic pressure produced when you speak. A Digital
message is an
ordered sequence of symbols selected from a finite set of discrete elements, e.g. a
list of hourly
temperature reading.
ELEMENTS OF A COMMUNICATION SYSTEM.
Figure 1 depicts the elements of a communication system. There are three essential
parts
of any communication system, the transmitter, transmission channel and the
receiver. Each part
plays a particular role in signal transmission as follows:
The Transmitter: it processes the input signal to produce a transmitted signal
suited to the
characteristics of the transmission channel. Signal processing for transmission
involves
modulation and may also include coding.
The Transmission channel: This is the electrical medium that bridges the distance
from source
to destination. It may be a pair of wires a coaxial cable or a radio wave or laser
beam. Every
channel introduces some amount of transmission loss of attenuation, so the signal
power, in
general progressively decreases with increasing distance.
The Receiver: it operates on the output signal from the channel in preparation for
delivery to
transducer at the destination. Receiver operations include amplification to
compensate for
transmission loss and demodulation and decoding to reverse the signal processing
performed at
the transmitter. Filtering is another important function at the receiver.
Various unwanted undesirable effects crop up in the course of signal transmission.
Attenuation: This is the reduction in signal strength. Is undesirable, since it
reduces signal
strength at the receiver.
More serious, however are distortion, interference, noise, which appear as
alterations of
the signal wave shape or spectrum. Although, such contaminations may occur at any
point, the
standard convention is to lump them entirely on the channel, treating the
transmitter and receiver
as being ideal. Figure 2a is a graph of an ideal 1101001 binary sequence as it
leaves the
transmitter. Figure 2b through 2d show the contaminating effects of distortion,
interference and
noise respectively.
Figure 2: Contamination of a signal transmitting a 1101001 sequence (a) Original
signal as it
leaves the transmitter (b) effects of distortion (c) effects of interference (d)
effects of noise.
Distortion: This is waveform (wave shape) changes caused by imperfect response of
the system
to the desire signal itself. Unlike noise and interference, distortion disappears
when the signal is
turned off. If the channel has a linear but distorting response, then distortion
maybe corrected or
at least reduced, with the help of special filters called equalizers
Interference: This is the contamination by extraneous signals from human sources –
other
transmitters, power lines, machinery, circuits etc. Interference occurs most often
in radio
systems, whose receiving antennas usually intercept several signals at the same
time.
Noise: This refers to random and unpredictable electrical signals produced by
natural processes
both internal and external to the system. Filtering reduces noise contamination,
but there
inevitably remains some amount of noise that cannot be eliminated.
FUNDAMENTAL LIMITATIONS.
An engineer faces two general kinds of constraints when designing a communication
system. Firstly, technological problems, including such diverse consideration as
hardware
availability, economic factor, governmental regulations etc. These are problems of
feasibility that
can be solved in theory, even though perfect solutions may not be practical,
secondly,
fundamental limitations, the laws of nature as they pertain to the task in
question. These
limitations ultimately dictate what can or cannot be accomplished irrespective of
the
technological problems. Two fundamental limitations of information transmission by
electrical
means are bandwidth and noise.
Bandwidth: Bandwidth can be defined in two ways. Firstly, in terms of Hertz: this
is the range
of frequencies contained in a composite signal or the range of frequencies a
channel can pass.
Secondly, in terms of bit per second (bps): this is the number of bits a link, a
channel, even a
network can transmit in one second. The concept of bandwidth applies to both
signals and
systems as a measure of speed. Every communication system has a finite bandwidth B,
that
limits the rate of signal variations. Communication under real-time conditions
requires sufficient
transmission bandwidth to accommodate the signal spectrum, otherwise, severe
distortion will
result.
Noise: it imposes a second limitation on information transmission, especially
thermal noise.
There are other types of noise, but thermal noise appears in every communication
system. We
measure noise relative to an information signal in terms of the signal-to-noise
power ration (S/N
or SNR). Thermal noise power is ordinarily quite small and S/N can be so large that
the noise
goes unnoticed. At lower values of S/N, however, noise degrades fidelity in Analog
communication and produces error in Digital communication and produced error in
Digital
communication. These problems become most severe on long distance links when the
transmission loss reduces the received signal power down close to the noise level.
Amplification
at the receiver is then to no avail, because the noise will be amplified along with
the signal.
Taking both limitations into account, Shannon (1948) stated that, the rate of
information
transmission cannot exceed the channel capacity. Therefore, channel capacity
C = Blog2(1 + S/N) = 3.32 Blog10(1 + S/N).
BENEFITS OF MODULATION
1. Modulation for efficient transmission.
2. Modulation to overcome hardware limitations.
3. Modulation to reduce noise and interference.
4. Modulation for frequency assignment.
5. Modulation for multiplexing.
