constellation diagram

Hi all can someone pls explain constellation in a very simple basic way.
1)What is it?
2)When does it occur?
3)why is it used?
4)What do we understand from it?
Is it used for particular process like modulation or for any other purpose or in general for all communication systems???

I read once to design a good system we need to have the constellation diagram well so we can see how effiecient the communication will be.




What are we supposed to understand from all these points. and is there a particular way to arrange these points in the constellation.. So i can see from fig 3 that as we keep increasing from 4 to 8 to 16 qam the bit values for each points increase as in 4 qam has 2 bit values... So its in someway a representation of all values but why a circle or sometimes square??? Pls have been trying to understand this and getting little confused... Pls help... Thank you!!!

Look up trellis coding and eye patterns and Shannon's Law for SNR vs Baud per Hz compression ratios group delay distortion can skeq the constellation. Circle coordinates can give more equidistance phase margin.

Many modern communication systems use modulation types that combine amplitude and phase manipulations. For a better understanding the pattern diagrams are used. If you draw dividing lines between the spots representing "perfect" aplitude and phase positions for 0 and 1 levels, you can see how any uncertainty in amplitude and phase due to noise and jitter can degrade the modulation/demodulation structure.
Depending on position (spot) numbers we have QAM, PSK, QPSK and e.g. 256 PSK modulation systems. More spots offer a higher vulnerability by noise and interference. In the complete system all this affects the Bit Error Rate. Additional error-correction coding offers an improvement in BER for a cost of a lower transmission capacity (more bits must be transmitted for corrections than for data transmission itself).

- - - Updated - - -

To add: if the spots are arranged on a circle it means a constant signal amplitude, so the modulation is a PSK type. Squares and similar structures indicate that also amplitude is manipulated, so QAM.

Please go and read some basics like books by Feher.

https://www.google.ca/search?safe=a...5j1.6.0....0...1.1.64.img..5.1.69.9kpTEEEKtpM

more pictures instead of 1000 books.