Why can waves with a very low frequency follow the earth's surface? Why are they not used for data transmission in computer networks?
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Radio waves which are below 2 MHz follow the ground. Following are the reason why its happened,
These can be some reason that why Low frequency are not used for data transmission in computer networks.
Low frequencies - à lower data rates ( Nyquist/Shannon)
As more power is needed to transmit and receive low frequencies, for this large antenna are needed.
Penetration of Lower frequencies to material is high.
Why does the ITU-R only regulate 'lower' frequencies(up to some hundred GHz) and not higher frequencies (in the THz range)?
Answers:
THz range frequencies, such as infrared, visible light, don't interfere with other transmission because ther are are blocked easily when obstacles are presents.
So only the standard safety regulations applied to the systems.
Its not easy to generate the higer freuencies when the radio systems stay beyond the 100 GHz.
What are the different approaches in reulation regarding mobile phone systems in Europe and the US? What are the consequences?
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European approach
American Approach
Affects of different Approaches
Why is the international availability of the same ISM bands important?
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International availability of same ISM band is important because of
What are the main problems of signal propagation? Why do radio waves not always follow a straight line? Why is reflection both useful and harmful?
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Following are the reason which cause the signal propagation,
In the presence of large buildings, without multipath propagation or reflection of signal we can't receive radio signal. Inter symbol interference (ISI) is due to multipath propagation.
Why, typically, is digital modulation not enough for radio transmission? What are general goals for digital modulation? What are typical schemes?
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FDM is used worldwide for separating different systems. So all the radio system must use analogue modulation for modulating the digital signal onto a carrier frequency. Most of the radio stations want to transmit at the same time, because of this reason all the signal should be modulation onto different carrier frequencies.
Other reason behind digital modulation,
Characteristics for digital modulation are
Typical schemes are
What are the main benefits of a spread spectrum system? How can spreading be achieved? How can DSSS systems benefit from multi-path propagation?
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Benefits of a Spread Spectrum System:
By Chipping sequence or frequency hopping (XORing) a bit, we can achieve Spreading.
Guard spaces are now the orthogonality of the chipping sequences or hopping patterns.
Higher Orthogonality-à Lower the correlation of spread signals, or lower the collision probability of frequency hopping systems.
To recombining signals travelling along different paths, DSSS system typically use rake receivers.
What limits the number of simultaneous users in a TDM/FDM system compared to a CDM system? What happens to the transmission quality of connections if the load gets higher in a cell, i.e., how does an additional user influence the other users in the cell?
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