Modulation is used in communication systems such as radio, microwave, Wi-Fi and fiber.
The main objective of the modulation process is to modify the signal that can travel through the medium to carry the data to be transmitted.
The signal that is modified is called carrier
Why Modulation?
Communication systems generate a signal called a carrier. It is this signal that carries information from the sender to the receiver.
When the carrier signal is generated, it has a single form that repeats at a specific frequency.
This sign is modified to have more than one form, and each form means a specific symbol.
These symbols indicate the digital data to be sent and received.
What will the signal look like before modulation? What is meant by symbols?
The following figure shows us the shape of the signal before modification (carrier signal) and then the shape of the signal after its modification (after the modulation work).
The type of modulation used here is called ASK or Amplitude Shift Key.
This type has a very simple idea. When bit 1 is to be transmitted, the carrier signal is transmitted without any modification in the amplitude of the signal.
When bit 0 is to be transmitted, nothing is transmitted, meaning that the amplitude of the signal is zero.
So the property used in this type of modulation is the amplitude property.
So the symbols used here are:
There is signal which means bit 1
No sign which means bit 0
Why is ASK not used in high-speed communication systems?
The ASK modulation system has only two symbols, either there is a signal or there is no signal.
The first represents bit 1 and the second represents bit 0. It is therefore used in simple control applications.
If we assume that the frequency is fixed, the data rate will be very slow if we compare it to another modulation system with more symbols.
What other modulation systems work with only two codes?
Like modifying with ASK, we can use the following methods to represent 0 and 1.
FSK or Frequency Shift Key
PSK or Phase Shift Key
When using FSK, the frequency of the carrier signal is changed to represent bit 1 and the signal is left unchanged to represent bit 0.
When we use PSK, the phase angle of the carrier signal is shifted by 180 degrees to represent bit 0. The carrier signal's phase angle is also left unchanged to represent bit 1 .
Modulation using QPSK
In the case of PSK, we had only two symbols: a phase angle of 180 degrees and a sign of zero phase.
In the case of QPSK, we will have four signals, each with a different phase angle.
Each sign or symbol represents 2 bits.
What is QAM?
QAM stands for Quadrature Amplitude Modulation. QAM takes advantage of two signaling properties:
Amplitude
phase angle
So, modulation of the signal using QAM means that the signal is modulated in its amplitude & phase, thus obtaining more symbols to represent the bits.
In the case of QPSK, we were able to get four symbols that allowed us to send 2 bits through each symbol, noting that we did not change the amplitude of the signal and only changed the phase angle.
To simplify, let's take two copies of the symbols used in QPSK, the first copy will be in full amplitude, and the second copy will be in half or a quarter of the amplitude, and put them together to form another type of modulation.
This type is called 8QAM and in it we will have eight tokens. There are always two symbols that have the same angle but differ in amplitude.
In another way, we can describe 8QAM as the QPSK, but once at full amplitude and again at half or quarter amplitude.
Now we have more symbols, more bits, and a faster data transfer rate.
In the case of 8QAM each symbol will represent 3 bits.
Signal quality and QAM
Data rate increase depends on choosing a high modulation such as 256QAM or higher (this is in addition to bandwidths)
In this case the signal quality should be very high, otherwise the symbols will interfere with each other.
The higher we go to the high modulation, the symbols become very close to each other (note the figure for 256QAM), and therefore the task of the receiver will become very complicated, especially if the signal quality is not high.
So focusing on signal quality will be very important.
Many systems implement what is known as Adaptive Modulation, and here the modulation is determined based on the quality of the signal.
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