[SOLVED] Q and I part in QAM modulation

[ashugtiwari]

I have one confusion in QAM about in-phase I and quadrature Q part, i.e. do they transmit identical data or not? For e.g. if i have 11001001 as a byte to be transferred, so will 4-QAM separate it like?

I 01 00

Q 10 11

or both I and Q are similar at any instant of time?

Thanks for answers

 

[blade88]

Hi,

I had the same questions (and more) a while ago, about IQ.
I'll give it a try and any correction is highly welcomed!

No, they don't have to transmit identical data.

In QAM modulation, the IQ signals modulate the amplitude and phase of a carrier (RF signal).

I and Q are two different signals, orthogonal to each other, so there is no interference between them.
Suppose it is I(t)=a(t)*cos(θ) and Q(t)=a(t)*sin(θ).
Cos and sin are orthogonal, since they have pi/2 phase shift with each other.

The final QAM signal is just the sum of the I/Q signals:
x_qam = I(t)*cos(w_c t)-Q(t)*sin(w_c t),

where w_c is the carrier frequency.

This x_qam signal is called a "symbol".

In 4-QAM, there are 4 possible symbols (waveforms), each "carrying" two bits.

The bits 0,1 are just two voltage levels, that show the amplitude of each I/Q signal.

Changing the amplitudes of IQ, changes the amplitude and phase of the final RF signal,
resulting in a different symbol.

In higher orders of modulation, 16/64 QAM, each symbol "carries" 4/6 bits.
In these cases, you just have more voltage levels (I/Q amplitudes), 16 in the first case, 64 in the second.

Now, regarding how the bits are separated in IQ, I think this has to do with the mapping you use.

As an example from real life, PSK31 is a digital communications scheme,
that uses the I signal for the information data and the Q signal for the error correction coding data:

https://www.arrl.org/psk31-has-rtty-s-replacement-arrived

This is mentioned in the section "From BPSK to QPSK".

 

[777arc]

In modulation schemes that have an I and Q part, they pretty much never transmit identical data. If they did then the constellation would look like a y=x line and you would not be getting the most out of your transmit power.

Also 4-QAM is usually referred to as QPSK.

 

[ashugtiwari]

Thanks blade88 and 777arc. So, I and Q are actually two different bits but not same bit used twice. That cleared the problem. And Thanks blade88 for very good and detailed explanation, lot of other questions too got answered in your post.