Where is modulation used?

Modulation / modulation method

A large area of ​​application in communications engineering is signal transmission. It is about how much information can be transmitted over one transmission path with as little loss as possible. When transmitting different signals on the same transmission path, signal conditioning is necessary before the signal is transmitted. For this purpose, modulation methods are used to convert information and data into electrical signals in such a way that they are suitable for transmission.

Modulation is:

  • Frequency adjustment
  • Multiple use of the transmission medium
  • Increase in immunity to interference

A modulation method describes how data must be mapped so that it can be transmitted on a cable or over the air.

How does modulation work?

From a mathematical point of view, modulation is a multiplication of the carrier and information signals. The function is carried out by a modulator, which is also referred to as a mixer. The modulator is available as a function, component or discrete circuit.
In order to combine the carrier and information signals, the signals are added in circuitry. Then this signal (linear frequency) is converted into a signal with a non-linear frequency. In the simplest case, this is done by the base-emitter path of a transistor. Since this diode path is non-linear (diode characteristic), a modulated signal is created that contains other frequencies that are not contained in the original signal.

Every electrical signal has three characteristics (signal parameters): amplitude, frequency and phase (polarity). During the modulation, one or more of these signal parameters (of the carrier signal) is changed or modulated by the information signal. The information signal, which is also referred to as the modulation signal, is impressed on the carrier signal. The signal shape changes in the process.

Simple modulation methods change the signal parameters only once per transmission step. In the case of digital information signals, in the simplest case the signal only oscillates between two states. One bit is transmitted per transmission step (symbol).
More complex modulation methods change the signal characteristics several times per step (symbol). More than one bit is transmitted per symbol. In order to increase the effectiveness and efficiency, modulation methods are combined with one another. The modulation density increases. This means that more data can be transferred per transfer step.
A typical example is pulse code modulation (PCM), in which pulse amplitude modulation (PAM) does the preparatory work. A combination of amplitude modulation (AM) and phase modulation (PM), which is referred to as quadrature amplitude modulation (QAM), is also possible.
The consequence of this is that the sensitivity of the modulated signal to interference increases as the modulation density increases. Modulation methods are subject to physical limits.

Note on AM: What is referred to here as Am is already correct. However, since only two digital values ​​are switched, it is an amplitude shift keying (ASK - Amplitude Shift Keying). The ASK is the simplest digital modulation method. With 2-ASK, the amplitude of the carrier signal has two states (0 and 1). With the 4-ASK, the amplitude of the carrier signal has four states (00, 01, 11 and 01).

Note on FM: What is referred to as FM here is already correct. However, since only two digital values, i.e. two discrete frequencies, are switched, it is actually an FSK modulation (Frequency Shift Keying).

Increase in the coding rate

In order to achieve a higher transmission rate, the data can be encoded more strongly with a higher-quality modulation method. The more valuable a modulation method, the more useful data can be transmitted per transmission step. This increases the spectral efficiency and the maximum data rate of the system.

However, a higher coding rate is at the expense of interference immunity. High coding rates only make sense if the distance between the base station and the end device is short. If the radio connection deteriorates, the coding rate has to be reduced again.

The use of a higher-quality modulation method therefore only makes sense with very good radio channels. The quality of a radio channel depends on the distance between the remote stations and the local error rate. The worse a radio channel, the higher the error rate and lower the data throughput.
In general, the quality of a radio channel can be influenced by the distance and number of antennas.

Demodulation

Since there is also a receiver (E) in addition to the transmitter (S) during signal transmission, the modulated signal must also be converted back into the original signal. This process is called demodulation. In the simplest case, the original signal can be restored with a filter. With most modulation methods, the modulation process is much more complex.

Classification of the modulation methods

There are different modulation methods. Their classification is based on the carrier signal. A basic distinction is made between time-continuous and time-discrete carrier signals.
A continuous-time carrier signal has a sinusoidal waveform. A time-discrete carrier signal has a rectangular periodic signal shape.
There is also a division into analog and digital modulation signals (information signal). Analog modulation signal means that the information signal has a typically analog sinusoidal signal form. Digital modulation signal means that the information signal has a typically digital rectangular signal shape.

Modulation method with time-continuous carrier

Modulation method with discrete-time carriers

Digital modulation methods

In analog signal transmission, the original signal tends to be drowned out in the noise and crackling. Digital modulation processes digitize the analog signal and map it as coded information in the form of "symbols". A symbol stands for a bit or a bit sequence. A digital signal transmission shows no loss of quality until the system-related signal-to-noise ratio (S / N) becomes too bad. This can be recognized by the fact that there are only interruptions and in the worst case the transmission comes to a complete standstill.

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