Filtering

In a digital communication system, digital information can be sent on a carrier through changes in its fundamental characteristics such as phase, frequency, and amplitude. In a physical channel, these transitions can be smoothed, depending on the filters implemented during transmission. In fact, filters play an important part in a communications channel because they can eliminate spectral leakage, reduce channel width, and eliminate adjacent symbol interference known as inter-symbol interference (ISI).

The matched filter is as important as the pulse-shaping filter. Though the pulse-shaping filter generates signals such that each symbol period does not overlap, the matched filter is important because it filters out the signal reflections that occur in the transmission process. Because a direct-path signal arrives at the receiver before a reflected signal does, it is possible for the reflected signal to overlap with a subsequent symbol period. The matched filter reduces this affect by attenuating the beginning and ending of each symbol period. Thus, it can reduce ISI.

The Modulation Toolkit provides the following types of filters:

• Raised cosine
• Root-raised cosine
• Gaussian pulse-shaping and matched filters