Communication Systems/What is Modulation?
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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 "Principals 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 of an un-modulated signal, especially during transmission. If we look at a general function for a sinusoid:
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, is known as the frequency, and the last term, 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 being modulated is known as the "data signal". It is important to notice that a simple sinusoidal carrier contains no information of it's own.
Types of Modulation
There are 3 different types of modulation: Amplitude modulation, Frequency modulation, and Phase modulation. We will talk about the differences and uses of all three in a later chapter.
Why Use Modulation?
Clearly the concept of modulation can be a little tricky, especially for the people who don't like trigonometry. Why then do we bother to use modulation at all? To answer this question, let's consider a channel that essentially acts like a bandpass filter: The lowest frequency components and the highest frequency components are attenuated or unusable, in some way. 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 it's own frequency range, then we can transmit multiple signals simultaneously over a single channel, all using different frequency ranges. Another use of modulation is that it allows the efficient use of antenna, a baseband signal to be transmitted will need an antenaa of huge length, whereas once modulated the antenna length will be reduced dramatically (antenna length almost equal 1/10 of the signal wavelength
Examples
Think about your car radio. There are more then a dozen (or so) channels on the radio at any time, each with a given frequency: 100.1MHz, 102.5MHz etc... Each channel gets a certain range (usually about 0.2MHz), and the entire station gets transmitted over that range. Modulation makes it all possible, because it allows us to send voice and music (which are essentiall baseband signals) over a bandpass (or "Broadband") channel.
non-sinusoidal modulation
A sine wave at one frequency can 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 pseudonoise 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 are with data signals.