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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.

interleaver

Note that time delay will occur.

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

YES!

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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.

10log10(Preceived/Pnoise)

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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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