Pulse shaping

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The pulse shaping ( English pulse shaping is) a method in communications technology in which the impulse response of the symbols used for transmission on the channel is adjusted.

The pulse shaping is realized with pulse shaping filters. This can be, for example, analog filter circuits or digital filters with which almost any impulse responses can be formed. When selecting the filter for a given channel, one typically tries to find a compromise between the transmission rate and the bandwidth required for it. It is therefore a practical matter to find a filter (i.e. an impulse response) which can achieve the highest transmission rate for a given channel. This is achieved by the filter converting the discrete-value symbol sequence of the transmitter into a signal with the lowest possible symbol crosstalk. In addition to the bandwidth, other restrictions of the channel can be exhausted with suitable filters, such as echoes in radio channels.

In signal processing , the pulse shaping is usually carried out after the line coding and before the modulation .

example

Square wave signal in red and smooth edges behind a low pass in black

The maximum transfer rate for a discrete value and time (i.e. a digital ) signal is achieved without any signal shaping. This means that a sequence of square-wave pulses, such as a square-wave signal, is transmitted on the channel . However, there are then infinitely steep transitions between the discrete levels of the signal , since one level changes to the next level without delay. However, these infinitely steep transitions also mean that the signal occupies an infinite amount of bandwidth . The less bandwidth of the channel is available, the more slurred transitions - the slope of the signal decreases and the intersymbol interference increases.

Many electronic channels have low-pass characteristics and are therefore bandwidth-limited. A square-wave signal of (relatively) low frequency can easily pass through such a channel, while the signal approaches a direct voltage with increasing frequency. The symbol crosstalk is then at a maximum because the symbols can no longer be distinguished.

Examples of pulse shaping filters are the raised cosine filter , root raised cosine filter and the Gaussian filter

See also

• Impulse technique
• Pulse (electrical engineering)
• Optimal filter

literature

• Erwin Hölzler, Herbert Holzwarth: Applications and systems (= pulse technology. Volume 2). Springer, Berlin / Heidelberg 1976, ISBN 978-3-642-96294-3 (Chapter 2, pulse shaping and generation ).