The figure depicts the Shannon’s model of communication.
Shannon recognized that interference (noise) plays significant role in communication system.
The analysis of channel capacity is incomplete without including noise in the calculation.
dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in [...]
It’s a jargon engineers or physicists are using.
It’s a promise.
It’s about describing a ratio between two quantities.
How big is the one compared to the other?
For example, Chan’s height is 40 inch and Han’s height is 80 inch.
Han is two times higher than Chan.
Using dB, we say
10 log (p2/p1) = 10 log 2 = 3 dB [...]
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 = [...]
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 [...]
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.
YES!
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)
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 [...]
Q: What is Eb/N0?
Eb/No is S/N (Signal to Noise ratio) divided by spectral efficiency.
“SNR per bit”
Q: Why is Eb/N0 important in digital communication?
Motivation:
- analog waveform is a power signal (infinite energy, finite average power)
- digital waveform is an energy signal (zero average power, non-zero finite energy) since a waveform is transmitted only during each symbol [...]
