>Multiple-level quadrature modulation,
>"constellation modulation",
>is most common for packing
>lots of bits per Hz of bandwidth.
>The more you pack,
>the better the s/n ratio has to be.
>
>http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
>
>http://en.wikipedia.org/wiki/Constellation_diagram
Does this mean that Quadrature Modulation and Constellation Modulation can be used -- at least in theory -- to give dial-up modem connections around the same fast speeds provided by Broadband cable modems?
Thanks,
Green
Answer Hi,
Green
As I have said earlier there are 3 basic type of modulation amplitude, phase, and frequency.
QAM is one kind of the amplitude modulation, constellation is not a modulation but rather the phases that a particular modulation is going trough, a QAM has the same constellation as a QPSK, for the simple reason that they have both 4 legs of a constellation if you have carefully read, the materials that you provided regarding modulation and Shannon’s law clearly states that there is a limitation in capacity and transmission rate versus noise.
Designers would always have to make a decision regarding what type of network the signal would have to go trough as well as noise and how much data redundancy and FEC is needed in order to compensate for burst error, better defined as BER. Keep in mind though that the more data or users there are in a network, the higher the noise will be.
As it is defined that the better the signal quality the higher data rate can be transmitted simply because there are lesser data redundancy for burst error recovery and more information bits, and vice versa.
In simple analogy, if you can hear the speakers words clearly, then there is no need for him to repeat what he said, then he can say more words for a given period of time, and when the environment is noisy then he may have to repeat his words in order for you to understand what he said, this is also true when the listener is a distance away, he cannot also hear the speakers words, then he may have to ask him to repeat the same words, then there is less information conveyed for the given period of time.
