The difference between 2 QAM constellation for channel ?

[m_nasiriyan]

QAM constellation

Hi.
Please consider two constellations of for example 16-QAM :an uniform
and a nonuniform (inwhich all of distances aren't equal) . As the
output we allocate 00, 01, 10 and 11 to the I axe respectively from
the left and the same to the Q axe from the down and we send 4
bits.This allocation is the same for both systems. So what is the
difference between two constellation for channel?

 

[zorro]

Re: QAM constellation

Hi m_nasiriyan,

The probability to (erronously) detect a certain symbol when an adjacent symbol is transmitted depends mainly on the distance between them (more distance = less error probability). For that, the error probability of the system is dominated by the minimum distance between symbols.
For a given average symbol energy, and in the case of a 16-QAM like constellation, the minimum distance is maximized when the distances between adjacent symbols are uniform. In this way, the performance of the system (bit error probability vs. Eb/No) is maximized with the uniform constellation, giving a performance that is 4 dB worse than BPSK.

Regards

Z