Digital modulation schemes

[gregoost]

what are the advantages of higher level digital modulations schems (PSK, ASK, FSK) compared to the binary modulation schemes?????

 

[deepuj]

Higher modulation schemes give you more bits per symbol... and thus higher data rates!

But you need higher SNRs to reliably transfer higher modulation schemes (or BER will suffer).

 

[Lupin]

Does anyone have experience on modulation simmetry? I am performing a ToA measurement and somebody told me that the modulation simmetry plays a role in the error figure.. can anyone illuminate me?

Thanks in advance.

Lupin

 

[mpanilkumar]

Gregoost,
I think your question is about using larger number of constellation points in modulation. As mentioned by deepuj, higher order constellations require more snr for maintaining the same bit error rate as that obtained in binary signalling schemes. You could have binary signalling in PSK also, where the symbols are anti-podal ie, the 0 and 1 are 180 degrees out of phase with each other. This is Binary Phase Shift Keying. If we increase the number of constellations to 4, we get QPSK, but for the same BER, we would now require double the SNR (+3dB). Here the symbols would be 90 degrees apart.

In my experience, in very noisy channles such as HF radio, higher order constellations are difficult to demodulate. Further more, due to the poor group delay response characteristics of HF radio, adjacent symbols 'spread' and interfere with each other. In such cases, antipodal signalling gives the best results, even though the bit rate is low.

I guess it depends on the channel you intend to use the modem on. If the SNR is very good, you must go for higher order constellations, because it would improve the data transfer rate. If it is too noisy and non-linear, lower order constellations would be the only option.
Regards.

 

[permute]

what are the advantages of higher level digital modulations schems (PSK, ASK, FSK) compared to the binary modulation schemes?????

there are two problems. I suspect the main answer is that modulation moves the data up to higher frequencies. for wireless transmission, higher frequencies mean short wavelengths and small antannas. for infrared, it means less noise from ambient lighting sources. and in some cases, AC-coupling is done, which makes basic 2-level signalling difficult if long runs of 1's or 0's can be had.

beyond this, you have the issue of PAM vs QAM vs the equivalents in FSK/ASK, and PSK (QPSK/8PSK). eg, for QPSK, 2 bits are sent at a time. this is similar to 4-level signalling (PAM). similar things can be done for ASK/FSK. if the noise is low, then you can get lots of levels. QAM is a good way of spreading the different "constellation points" out.

the result of this is to give more than 1 bit/hz, and thus if you only have 1MHz of bandwidth, you can get more than 1Mbps.