Solution:

pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission.

Transmitting a signal at a high modulation rate through a band-limited channel can create intersymbol interference.

As the modulation rate increases, the signal's bandwidth increases. When the signal's bandwidth becomes larger than the channel bandwidth, the channel starts to introduce distortion to the signal. This distortion usually manifests itself as intersymbol interference.

The signal's spectrum is determined by the pulse-shaping filter used by the transmitter. Usually, the transmitted symbols are represented as a time sequence of Dirac delta pulses.

This theoretical signal is then filtered with the pulse shaping filter, producing the transmitted signal. The spectrum of the transmission is thus determined by the filter.

In many baseband communication systems, the pulse shaping filter is implicitly a boxcar filter. Its Fourier transform is of the form sin(x)/x and has significant signal power at frequencies higher than the symbol rate.

This is not a big problem when optical fiber or even twisted pair cable is used as the communication channel.

However, in communications, this would waste bandwidth, and only tightly specified frequency bands are used for single transmissions. In other words, the channel for the signal is band-limited.

Therefore better filters have been developed, which attempt to minimize the bandwidth needed for a certain symbol rate