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+---- Thread: future work of analog and digital hybrid modulation (/showthread.php?tid=65118)
future work of analog and digital hybrid modulation - mahendra - 10-06-2017
let us consider the transmission of an analog signal over
a band-limited channel. This could be possible by two
conventional techniques: analog transmission, and digital
transmission, of which the latter uses sampling and
quantization principles. Analog Modulation techniques
such as Frequency and Phase Modulations provide
significant noise immunity as known and provide SNR
improvement proportional to the square root of modulation
index, and are thus able to trade off bandwidth for SNR.
However, the SNR improvement provided by these
techniques is much lower than the ideal performance as
shown by the Shannon s capacity theorem.
On the other hand, Digital techniques of transmission
can utilize error-correction codes that provide performance
close to theoretical prediction. However, the major
disadvantage of digital transmission techniques is the
inherent quantization error introduced, which is imminent
all the while the signal is relayed. This error causes
distortion in the original signal being relayed and cannot
be later recovered by any means possible.
If we quantized the sampled signal using QAM or any
other method, using a fixed number of bits, a fixed digital
distortion is introduced in the developmental stage itself.
This distortion is present regardless of the transmission
quality of the channel being used. Thus the original signal
can be considered to be permanently impaired.
Communications systems are normally constructed for
SNR much higher than the minimum that is required, so as
to leave a margin for fading and other effects, which might
occasionally reduce the SNR .
So, it is essential to design a communications system
where the output SNR increases as the channel SNR
increases. While, as already stated, this technique is not
feasible through digital modulation, it is an inherent
property in analog modulation.