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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