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Why is bandpass modulation needed for radio transmission?

In other words, why is it necessary to carry a message waveform onto a carrier wave of much larger frequency than that of the message?
Is it possible to configure a radio with only a microphone, power amplifier and antenna?
wavelenth = light speed / frequency
Thus, for human voice (whose frequency is around 3000Hz),
wavelength = 3*10^8 / 3000Hz=100000m = 100km
The antenna size should be really large.
If carrier frequency is 3*10^9, then the wavelength is 0.1 meter which is a feasible and reasonable antenna size.
Furthermore, how you can differentiate your local voice and far away voice?

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What is CRC?

CRC: Cyclic Redundancy Check
Some redundant bits are attached to each message to detect the existence of errors at receiving side
Basic concept:
Given a k-bit block of bits (or messages), the transmitter generates an (n-k) bit sequence, known as a frame check sequence (FCS),
such that the resulting frame (consisting of n bits)  is exactly divisible by some predetermined number.
The receiver then divides the incoming frame by that number and, if there is no remainder, receiver can assume there was no error.

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What is for interleaving ?

If there are consecutive 4 bit errors  in a byte, it is difficult to reconstruct original byte even using channel coding.

Interleaving provides time diversity, which spreads signal in time domain.


Note that time delay will occur.

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Does a receiver keep sending and receiving at the same time in CDMA ?


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Why use dB in communication?

Because many signals have a very wide dynamic range, SNRs are usually expressed in terms of the logarithmic decibel scale.

In decibels, the SNR is, by definition, 10 times the logarithm of the power ratio.


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What for Eb/N0 vs. BER (Bit error probability) ? That is, how to use it?

Different communication systems will have different curve of Eb/No vs. BER
Two ways to compare– vertical line and horizontal line:
1. horizontal line:
(for a given BER) some system needs less energy, which is good.
2. vertical line:
(for the same amount of Eb/No) some system provides less BER, which is good.
Furthermore, based on the graph, transmit power can be calculated.

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