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Design >> RF Design >> WiMax DC offset problem
https://designers-guide.org/forum/YaBB.pl?num=1219389737

Message started by aaron_do on Aug 22^nd, 2008, 12:22am

Title: WiMax DC offset problem
Post by aaron_do on Aug 22^nd, 2008, 12:22am

Hi all,

I read that WiMax has a DC offset issue since the time given to switch from TX mode to RX mode is relatively short. This makes it difficult for the DC offset correction loop to settle in time. However, i noticed at least two vendors are using zero-IF RX for WiMax. Just wondering how they got around the problem. I suppose it is possible to simply turn on the RX in advance to allow the DC offset correction loop to at least partially settle...

Any thoughts?

thanks,
Aaron

Title: Re: WiMax DC offset problem
Post by loose-electron on Aug 28^th, 2008, 8:32am

Aaron:

I haven't done an 802.16 system, but there are lots of way of dealing with this - including:

Keeping the RX path up and running all the time, but squelch the input to it in TX mode

Fast attack control of offset - you change the loop BW from high (aquire offset) to low (maintain offset)

Control memory - the offset loop maintains its prior value (DAC setting) when restarting in order to have shorter acquisition times.

those are the ones that come quickly to mind.
Jerry

Title: Re: WiMax DC offset problem
Post by rf-design on Aug 28^th, 2008, 2:11pm

As similar to 802.11ag all DC-receivers use a coordinated offset rejection in the radio and the BB.

The question is what to reject. If the RX path from the switch is still disconnected it could be a different offset than if the switch is on the air. If the rejection operate on noise it could sample the noise instance.

BB have further rejection build in. The cleverness is that the BB offset rejection operate in the presence of frequiency shifts.

That is where the companies differentiate.

The technical best system is a LowIF-RX and ZeroIF-TX. But in development there are other flavors making this decision.