Modulation is a signal-processing operation that is basic
to the transmission of an information-bearing signal over a communication
channel, whether in the context of digital or analog communications. This
operation is accomplished by changing some parameter of a carrier wave in
accordance with the information-bearing (message) signal. The carrier wave may
take one of two basic forms, depending on the application of interest:
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Sinusoidal carrier wave, whose amplitude, phases, or
frequency is the parameter chosen for modification by the information-bearing
signal.
Periodic sequence of pulses, whose amplitude, width, or
position is the parameter chosen for modification by the information-bearing signal.
The IEEE defines modulation as
“a process whereby certain characteristics of a wave,
often called the carrier, are varied or selected in accordance with a
modulating function.”
The modulating function is the information baseband.
Reasons why modulation is necessary in communications
system:-
We can see that baseband signals are in compatible for
direct transmission over the medium so; we have to use modulation To convey
(baseband) signals from one place to another.
Allows
frequency translation
(I.e. translate the signal from
one region in the frequency domain to another) The benefits
of frequency translation are:
Frequency Multiplexing
Reduce the height of antenna
Avoids mixing of signals
Narrow banding (convert wideband signal into narrowband
signals)
Common processing
We can
make efficient transmission
Quite a few wireless channels have their own appropriate pass
bands. For efficient transmission, it would be necessary to shift the message spectrum
into the pass band of the channel intended. Ground wave propagation (from the
lower atmosphere) is possible only up to about 2 MHz Long distance ionosphere
propagation is possible for frequencies in the range 2 to 30 MHz beyond
30 MHz, the propagation is line of sight. Preferred frequencies for
satellite communication are around 3 to 6 GHz. By choosing an appropriate
carrier frequency and modulation technique, it is possible for us to translate
the baseband message spectrum into a suitable slot in the pass band of the
channel intended. That is, modulation results in frequency translation.
Uses for ease of radiation
Consider again transmission of good quality audio.
Assume we choose the carrier frequency to be 1MHz. The linear modulation
schemes that would be discussed shortly give rise to a maximum frequency
spread (of the modulated signal) of 40 kHz, the spectrum of the modulated
signal extending from(1000 - 20) = 980 kHz to (1000 + 20) = 1020 kHz. If the
antenna is designed for 1000 kHz, it can easily take care of the entire range
of frequencies involved because modulation process has rendered the signal
into a NBBP signal.
Allows
frequency assignment
You can tune radio or TV stations by using filtering
because each station has owned assigned carrier frequency
Allows
multiplexing of signals
Multiplexing Is a method of sharing a bandwidth with
other independent data Channels. So, we can combine several signals for simultaneous
transmission on one channel .Ex. FDM uses CW modulation TDM
uses pulse modulation CDM, WDM and so on.
Allows multiple access of signals
Multiple accesses is a variation
of multiplexing which involves a fixed assignment of the common communications
resource at local level, it also involves the remote sharing of the resource. Ex.
FDMATDMACDMA and so on.
Allows
adjustments in the bandwidth
You can make a smaller or larger bandwidth than the
original signal to control the SNR in the receiver
Reduced
noise and interference
The signals communication using modulation techniques
reduce the effect of noise and interference to great extent so the quality
of reception improve
Overcomes
hardware limitation
Communication system design may by constrained by the cost and availability of hardware
For example hardware that depend on frequency you can put
the signals in some frequency that avoids hardware limitation
Increases
the range of communication
At low frequencies radiation is poor and signal gets
highly attenuated. Baseband signals have significant spectral content
around DC. Some of the baseband signals that are of interest to us are: a)
Speech b) music and c) video (TV signals).Approximate spectral widths of
these signals are: Speech: 5 kHz, Audio: 20 kHz, Video: 5 MHz Therefore
baseband signals cannot be transmitted directly over long distance.
Modulation effectively increases the frequency of the signal to
be radiated and so increase the distance of communication
Increases
the speed of communication
You can speed the communication by using aproper method of modulation and multiplexing.
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CONTACT US
Dingshengwei Electronics Co., Ltd
Company Address: Building A, the first industry park of
Guanlong, Xili Town, Nanshan, Shenzhen, Guangdong, China
Tel: +86 0755 26909863
Fax: +86 0755 26984949
Phone: +86 13410066011
Email:ken@soukacatv.com
Skype: soukaken
Source: zh.scribd
