Frequency Selectiveness in Data Communication Principles
Explore the concept of frequency selectiveness in data communication principles, where mediums act as filters with selective responses based on frequency ranges. Learn about coherence bandwidth, flat fading, and frequency selective fading, and the use of equalizers to optimize signal transmission.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
You are allowed to download the files provided on this website for personal or commercial use, subject to the condition that they are used lawfully. All files are the property of their respective owners.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author.
E N D
Presentation Transcript
DATA COMMUNICATION Principles : Frequency Selectiveness
Frequency Selectiveness Under the performance models for PHY, it has been said earlier in this chapter that the medium acts like a filter. A filter is a device that has selective response. A frequency filter is a device whose response is different for different frequency ranges. Usually cables have a smooth response over a certain range of frequencies and gradually attenuating response outside this band. This makes it easy to define a bandwidth for a cable. The wireless channel, however, has highly frequency selective response. Figure 9-13 shows an imaginary cable and wireless band. Due to the frequency selective behavior of a wireless channel, signal distortion is more prominent for some frequency components than the others. This makes it highly difficult to use a repeater or amplifier to boost the signal power. The repeaters or amplifiers boost a certain range of band equally, thus pronouncing those parts of signals that have been much less attenuated. Instead of amplification, equalization is the usual recourse to deal with frequency selective channel behavior. An equalizer understands the channel behavior and then mimics its inverse so that only those parts of the signal spectrum get boosted that are attenuated more.
In order to avoid the use of equalizer, we must know the range of frequencies over which the channel response remains fairly smooth. Such a bandwidth is called the coherence bandwidth of the channel. If B is the coherence bandwidth of a channel, it can allow digital signal consisting of pulses approximately 1/ B seconds in width. Thus, the coherence bandwidth upper limits the data rate without using special circuits to equalize the frequency selective response. A signal that is within the coherence bandwidth will be attenuated in such a way that all of its frequency components will be attenuated equally. Such a fading of signal is called flat fading and it can be cured with the help of repeaters or amplifiers. Alternatively, if a signal bandwidth is larger than the coherence bandwidth, it may fade only in selective part of the signal spectrum. Such a fading is called frequency selective fading. As said earlier, the frequency selective fading is harder to cure and equalization is used for this purpose to invert the channel effect.
REFERENCES Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_..-Academic Press
