With regard to GPS signals, what is the significance of I-parts and Q-parts of Complex signal?

Question

What is the significance of I-part & Q-part magnitudes and imbalances in the screenshot? They are listed next to the red Complex signal diagram.

Imbalance of I-part: 4.7%

Imbalance of Q-part: 25.8%

Magnitude of I-part: 56.5%

Magnitude of Q-part: 55.7%

I am running uBlox uCenter and looking at the Extended Hardware Status message.

Is there an ideal percentage of I and Q parts?

Answer 1

For direct conversion, you demodulate down to zero and use two ADCs, which means the system will have separate DC offsets and gain factors for the I and Q channels. For demodulation, these need to be compensated for (in a superheterodyne system, this problem doesn't exist, because there is only one ADC, and the DC offset of the system is outside the analyzed signal).

The DC offset can be determined by a low-pass-filter -- simple averaging over some time is usually enough, and works for everything that isn't a CW signal near the LO frequency, just subtract it, and the I/Q constellation is properly centered. That is usually a fairly large error, so the compensation is required in order to get a usable baseband signal.

There are other imperfections in signal reconstruction as well, the most obvious ones in the constellation diagram are

• gain difference between I and Q channels (stretching in I or Q direction)
• phase offset between I and Q channels not exactly 90 degrees (shearing)

The phase offset seems to be ignored in that application, because it's usually small and doesn't really matter for GPS reception.

Answer 2

Not much to go on but here goes: -

I suspect the imbalances relate to the accumulated data amplitude (over a limited period of time) not averaging to zero. Data should average to zero to make it easier for costas loops (or whatever demodulation method employed) to align the I and Q local oscillator(s) precisely.

Without correct alignment, demodulation produces significant errors. If the average value of I and Q data streams are both zero you can attempt to demodulate and use an integrator to force the the phase-locked-loop and VCO in the right direction to obtain near-perfect alignment and therefore near-perfect demodulation (ignoring noise of course).

The magnitudes are just precisely what they say they are - the RMS magnitudes of I and Q respectively.

Hope this helps.