Designer’s Guide Community

steven Community Member Offline Posts: 59	IQ mismatch calibration methods Feb 28^th, 2006, 5:59pm Hi, I want to know some calibration methods on IQ mismatches. References are welcome. It seems that there are no too much literatures on this topic. Thanks.
Back to top	IP Logged

Jess Chen

Community Fellow

Offline

Posts: 380
California Bay Area

Re: IQ mismatch calibration methods
Reply #1 - Mar 1^st, 2006, 11:15am

I believe there are quite a few papers out there on IQ mismatch calibration methods. I am fairly certain you can find some in the IEEE Journal on Solid State Circuits. You could also try the patent applications. I know of at least three. You may not want to infringe on someone's patent but it would at least get you up to speed, if you can wade through the legaleeze. You may also find other references cited in some of the applications. You could also try the internet. I just did a quick search on the internet and found a few papers.

-Jess

Back to top

IP Logged

steven

Community Member

Offline

Posts: 59

Re: IQ mismatch calibration methods
Reply #2 - Mar 2^nd, 2006, 11:07am

Jess Chen wrote on Mar 1^st, 2006, 11:15am:

I believe there are quite a few papers out there on IQ mismatch calibration methods. I am fairly certain you can find some in the IEEE Journal on Solid State Circuits. You could also try the patent applications. I know of at least three. You may not want to infringe on someone's patent but it would at least get you up to speed, if you can wade through the legaleeze. You may also find other references cited in some of the applications. You could also try the internet. I just did a quick search on the internet and found a few papers.

-Jess

Hi Jess,

You are right, I grapped some papers on this topic online later on. It is a little bit not clear that superheterodyne structure doesn't have I/Q mismatch problem. On the other hand, direct conversion architecture suffers this a lot. I think both of them have to do down-conversion to baseband, they would have the similar problem, right?

Thanks,
Steven

Back to top

IP Logged

Jess Chen Community Fellow Offline Posts: 380 California Bay Area	Re: IQ mismatch calibration methods Reply #3 - Mar 2^nd, 2006, 11:16am Right. It's only the final conversion to baseband that introduces the IQ mismatch problem. -Jess
Back to top	IP Logged

steven

Community Member

Offline

Posts: 59

Re: IQ mismatch calibration methods
Reply #4 - Mar 2^nd, 2006, 12:08pm

Then it would be the problem of severe magnitude of IQ mismatch in the two methods. Comparing techniques using test sequences and without test sequences, e.g., LMS method, what constraints would be in applying them? What I think is that since there must be some lag in calibrating the mismatch, the test sequence may be well budgeted during the calibration process while the LMS method is more robust. Is there any popular choice in the current technology?

Thanks.

Back to top

IP Logged

Jess Chen

Community Fellow

Offline

Posts: 380
California Bay Area

Re: IQ mismatch calibration methods
Reply #5 - Mar 2^nd, 2006, 3:11pm

Steven,

You need to back up a bit for me. What two methods are you talking about? And are you working with OFDM preambles?

As for choice in algorithms, my personal belief is that the choice of method is primarily driven by:

1. when in the design flow the problem is identified and given attention and
2. whether the baseband and RF teams work at the same company.

For example, if the problem is identified late in the design and the RF and baseband chips are being designed together, the solutions tend to involve the micro processor and some complex algorithms. However, if the problem is recognized early, and/or the product is just an RF chip, the solutions tend to be more circuit oriented and the computational algorithm tends to be much simpler. My experience is with the former. Our orignal thought was to calibrate in the factory. After we saw how expensive that was, we enlisted help from the microprocessor to implement an automatic calibration/compensation scheme. Another driving force is schedule. We had access to some potentially more robust but also more complex algorithms but we lacked the time to pursue them.

-Jess

Back to top

IP Logged

steven

Community Member

Offline

Posts: 59

Re: IQ mismatch calibration methods
Reply #6 - Mar 2^nd, 2006, 6:21pm

Jess Chen wrote on Mar 2^nd, 2006, 3:11pm:

Steven,

You need to back up a bit for me. What two methods are you talking about? And are you working with OFDM preambles?

As for choice in algorithms, my personal belief is that the choice of method is primarily driven by:

1. when in the design flow the problem is identified and given attention and
2. whether the baseband and RF teams work at the same company.

For example, if the problem is identified late in the design and the RF and baseband chips are being designed together, the solutions tend to involve the micro processor and some complex algorithms. However, if the problem is recognized early, and/or the product is just an RF chip, the solutions tend to be more circuit oriented and the computational algorithm tends to be much simpler. My experience is with the former. Our orignal thought was to calibrate in the factory. After we saw how expensive that was, we enlisted help from the microprocessor to implement an automatic calibration/compensation scheme. Another driving force is schedule. We had access to some potentially more robust but also more complex algorithms but we lacked the time to pursue them.

-Jess

Hi Jess,

I am sorry I didn't express clear enough. The two methods I referred to are:

1. Using particular calibrating stage to perform IQ correction. This method may not be OFDM. One reference is

"Circuits and algorithms for wireless communications", Messerschmitt et al. During idle time, an image tone generator produces calibration.

2. For OFDM, the one I am looking at is
"Compensation schemes and performance analysis of IQ imbalance in OFDM receiver", Tarighat et al, IEEE trans on signal processing Aug. 2005. In which there is no particular calibration stage, I am not sure it only works on preamble though.

Maybe what I said should be categorized in the former case you pointed out: automatic calibration with the help of microprocessor. One thing I don't quite understand, when the problem is recognized early, the solution would be simple. Is simple in a sense that the calibration can be embedded into the chip? It seems some popular algorithms would be used in this case, right?

What I think is for wireless channel, the calibration is still running 'cause the mismatches are random but (asumed)slow varying. So the calibration is either on microprocessor side or the chip side. Am I right?

Thanks,
Steven

Back to top

IP Logged

Jess Chen

Community Fellow

Offline

Posts: 380
California Bay Area

Re: IQ mismatch calibration methods
Reply #7 - Mar 3^rd, 2006, 12:34pm

Hi Steven,

Yes, by "simple" algorithm I meant something embedded in the RF chip. I've seen some papers describing such methods. But I've also heard of such methods being passed up because of area/power costs. In those cases, calibration relies much more on the microprocessor. Also, as you mentioned, if the concern is over slow drifts in IQ mismatch, a microprocessor approach may be able to track those out. I've seen it both ways. I think sometimes the decision is driven by history, sometimes by foresight, and sometimes by cost/schedule/area. I personally don't think there's a clear cut optimal way to do IQ compensation that applies to all situations.

-Jess

Back to top

IP Logged

steven

Community Member

Offline

Posts: 59

Re: IQ mismatch calibration methods
Reply #8 - Mar 3^rd, 2006, 2:29pm

Hi Jess,

Ok, your answer cleared something up. By the way, what are the "simple" and "complex" method you referred to in your previous post? Since you also mentioned it bogged down as a circuit issue, what kind of circuit design issues have to be addressed before the design actually starts? What is usual ways to isolate the IQ mismatch and circuit component mismatch when coming on to circuit design. I am sure you have experience on this.

Thanks.

Back to top

IP Logged

Jess Chen

Community Fellow

Offline

Posts: 380
California Bay Area

Re: IQ mismatch calibration methods
Reply #9 - Mar 4^th, 2006, 5:36pm

Hi Steven,

Quote:

what are the "simple" and "complex" method you referred to

I was referring to the computational aspects of the compensation. For example, if for a receiver, one includes circuitry to generate an offset RF tone at the mixer inputs, the math to extract the sideband is much simpler than if you try to extract the compensation matrix from the preamble and/or pilots.

Quote:

what kind of circuit design issues have to be addressed before the design actually starts

To continue with the example above, you would have to make sure you have power and area available for the offset tone generator. �If your transmitter and receiver are on the same chip and you try to use a tx-rx leakage to inject a tone for calibration, you would have to make sure the desired leakage path dominates. These are just the issues I know. I'm sure there are more.

Quote:

What is usual ways to isolate the IQ mismatch and circuit component mismatch when coming on to circuit design.

I have not kept up on the papers in this area but I believe they are all distinguished by the stimulus used to measure the mismatch. I have heard of some using the offset input RF tone. There's a patent application for one using noise. The algorithms that work with the received signal usually involve the microprocessor. I hope this helps.

-Jess

Back to top

IP Logged