Which one is better QAM or QPSK?

[senaydud]

Hello,

Which one do you prefer and Why? A QPSK symbol constellation with the following set;
symbols = sqrt(2)*{-1, +1, -j, +j}
Or a QAM with the following;
symbols = { (-1-j), (+1-j), (-1+j), (+1+j) }

 

[flatulent]

qam vs qpsk

These will be identical performance at identical power levels. Your main problem will be in reducing the "implementation loss" of the real decoding hardware.

 

[senaydud]

qpsk vs qam

Thanks for the reply, Could you please compare the two schemes with respect to "Implemetation loss in decoding hardware"? I was wondering that the case where you use QAM might be more convenient because you both make use of In-phase and Q-phase components of your transmitted signal.

 

[m_pourfathi]

difference between qam and qpsk

That is correct. QPSK is a harder to implement due to the 45 degrees phase difference between quadrature components. it is preferred to use a IQ modem So in conclusion, they are both the same theoretically but the implementation is different. I'd prefer QAM as I said. you do not have to implement the 45 degree phase shift in both transmitter and receiver which might degrade performance.

 

[senaydud]

what are the differences between qpsk and 4-qam

What a minute guys, there is something that I misunderstood. Look at my constellations again, I think they are both QPSK, but with different constellation points. Sorry for misleading you..

 

[m_pourfathi]

differences between qpsk and qam

it is! but they way you look at it is important. it can also be interpreted as some sort of qam or maybe psk. constellation points are extremely important. they might look the same theoretically, but when implementing the IQ modulator and demodulator would work differently, as I said you have add a phase shifter to your modem. which increase probable error. it depends how you look at it.

 

[senaydud]

what is the difference between qpsk and qam

Thanks m_pourfathi, if you dont mind I would like to understand what you mean. You say there are 45 degrees phase difference between components of QPSK. However QPSK has 4 constellations (+1, -1, j and -j) each of which has 90 degrees difference with neighbouring constellation points. How come that 45 degrees introduce to our system of QPSK? May be a little Maths would work.

 

[m_pourfathi]

difference qam and qpsk

look. the quadrature components can be described as follow:

S(t) = I(t)*cos(Wc*t) - Q(t)*sin(Wc*t)

in which Wc is the carrier or IF frequency of the system. this is the output of a quadrature modulator. Imagine the 2 dimensional signal space of which the bases are cos(Wc*t) and -sin(Wc*t). Thus, Q(t) defines the imaginary part of the constellation point while I(t) describes the real part of the constellation point.
when you're using a QPSK modualtor of which the constellation points are {-1,+1,+j,-j} each symbol uses only cos(Wc*t) or sin(Wc*t). you just need an oscillator and a 90 degrees phase shifter to introduce the quadrature component.
when you are using the constellation points 1/sqrt(2) * {+1+j, -1-j, +1-j, -1+j} each symbol has both real and imaginary parts. you have shift it like 45 degrees compared to the latter case, to achieve this. or you may also be able to use the second case. you have to be careful when demodulation because each symbol has a 45 degrees phase shift compared to the quadrature components. that's the 45 degrees I'm talking about not the constelltion points compared to each other. while demodulation, you have keep this 45 degrees very accurate not to confront error. I hope I have made my point.

 

[senaydud]

qpsk constellation

Yep, I got your point. Thanks for your valuable time m_pourfathi..

 

[m_pourfathi]

difference between qam and psk

Don't mention it! Here's all about extending our knowledge. Even explaining a thing helps understand the subject more!