# Communication Systems/What is Modulation?

Modulation is a term that is going to be used very frequently in this
book. So much in fact, that we could almost have renamed this book
\"Principles of Modulation\", without having to delete too many
chapters. So, the logical question arises: What exactly is modulation?

## Definition

**Modulation** is a process of mixing a signal with a sinusoid to
produce a new signal. This new signal, conceivably, will have certain
benefits over an un-modulated signal. Mixing of low frequency signal
with high frequency carrier signal is called modulation.

$$f(t) = A \sin(\omega t + \phi)$$

we can see that this sinusoid has 3 parameters that can be altered, to
affect the shape of the graph. The first term, A, is called the
magnitude, or amplitude of the sinusoid. The next term, $\omega$ is
known as the frequency, and the last term, $\phi$ is known as the phase
angle. All 3 parameters can be altered to transmit data.

The sinusoidal signal that is used in the modulation is known as the
**carrier signal**, or simply \"the carrier\". The signal that is used
in modulating the carrier signal(or sinusoidal signal) is known as the
\"data signal\" or the \"message signal\". It is important to notice
that a simple sinusoidal carrier contains no information of its own.

In other words we can say that modulation is used because some data
signals are not always suitable for direct transmission, but the
modulated signal may be more suitable.

## Types of Modulation

There are 3 basic types of modulation: Amplitude modulation, Frequency
modulation, and Phase modulation.

amplitude modulation
:   a type of modulation where the amplitude of the carrier signal is
    modulated (changed) in proportion to the message signal while the
    frequency and phase are kept constant.

frequency modulation
:   a type of modulation where the frequency of the carrier signal is
    modulated (changed) in proportion to the message signal while the
    amplitude and phase are kept constant.

phase modulation
:   a type of modulation where the phase of the carrier signal is varied
    accordance to the low frequency of the message signal is known as
    phase modulation.

## Why Use Modulation?

Why use modulation at all? To answer this question, let\'s consider a
channel that essentially acts like a bandpass filter: both the lowest
frequency components and the highest frequency components are attenuated
or unusable in some way, with transmission only being practical over
some intermediate frequency range. If we can\'t send low-frequency
signals, then we need to shift our signal up the frequency ladder.
Modulation allows us to send a signal over a bandpass frequency range.
If every signal gets its own frequency range, then we can transmit
multiple signals *simultaneously* over a single channel, all using
different frequency ranges.

Another reason to modulate a signal is to allow the use of a smaller
antenna. A baseband (low frequency) signal would need a huge antenna
because in order to be efficient, the antenna needs to be about 1/10th
the length of the wavelength. Modulation shifts the baseband signal up
to a much higher frequency, which has much smaller wavelengths and
allows the use of a much smaller antenna.

## Examples

Think about your car radio. There are more than a dozen (or so) channels
on the radio at any time, each with a given frequency: 100.1 MHz,
102.5 MHz etc\... Each channel gets a certain range (usually about
0.22 MHz), and the entire bayot gets transmitted over that range.
Modulation makes it all possible, because it allows us to send voice and
music (which are essential baseband signals) over a bandpass (or
\"Broadband\") channel.

## Non-sinusoidal modulation

A sine wave at one frequency can be separated from a sine wave at
another frequency (or a cosine wave at the same frequency) because the
two signals are \"orthogonal\".

There are other sets of signals, such that every signal in the set is
orthogonal to every other signal in the set.

A simple orthogonal set is time multiplexed division (TDM) \-- only one
transmitter is active at any one time.

Other more complicated sets of orthogonal waveforms---Walsh codes and
various pseudo-noise codes such as Gold codes and maximum length
sequences---are also used in some communication systems.

The process of combining these waveforms with data signals is sometimes
called \"modulation\", because it is so very similar to the way
modulation combines sine waves with data signals.

## Further reading

-   Data Coding Theory/Spectrum
    Spreading
-   Wikipedia:Walsh code
-   Wikipedia:Gold code
-   Wikipedia:pseudonoise code
-   Wikipedia:maximum length
    sequence
