On Mon, 23 Jun 2014 21:08:55 -0700 (PDT), Rakesh Joshi <rakesh@saankhyalabs.com> wrote:>On Tuesday, June 24, 2014 8:49:05 AM UTC+5:30, Eric Jacobsen wrote: >> On Mon, 23 Jun 2014 11:59:16 -0700 (PDT), Rakesh Joshi >> wrote: >>=20 >>=20 >>=20 >> >Thanks Eric. Does having a dc carrier also affect the dagc response if i= >ts employed?=20 >>=20 >>=20 >>=20 >> Typically DC offset and AGC are detected and adjusted separately from >>=20 >> each other. They can be made independent regardless of whether the >>=20 >> DC subcarrier is populated or not.=20 >>=20 >>=20 >>=20 >>=20 >>=20 >> Eric Jacobsen >>=20 >> Anchor Hill Communications >>=20 >> http://www.anchorhill.com > >As per my understanding, DC offset that is corrected is due to the ADC comp= >onents. A long window averaging is taken over the signal, and the estimated= > DC offset is compensated. But, if there is a dc as part of the ofdm symbol= >, its effect would last for an entire symbol duration. This gets seen by th= >e AGC. If the AGC is quick to react to this dc, the AGC gain can falsely va= >ry. Hence for the AGC to lock to the proper gain values, one may have to de= >sign the AGC to react slowly. Does this increase the AGC settling time? If = >so, by knocking off the DC carrier component in the ofdm symbol, does it he= >lp reduce the AGC settling time? I hope Im clear. Please correct me if you = >find me wrong.The DC offset due to the ADC(s) is not the same as the DC-bleed-through from the mixer into the center carrier. The carrier bleed-through is DC relative to the other subcarriers, so even after the overall DC offset at the ADCs is removed, there can still be a residual offset contaminating the center subcarrier. This is why the center subcarrier cannot be used reliably with direct conversion (until somebody comes up with a downconverter with zero mixer bleed-through). So ADC DC offset and AGC in a direct-conversion OFDM demodulator generally don't need to be any different than for any other system. There are always tradeoffs and issues around developing both DC-offset removal and AGC loop bandwidths, and they are often specific to the system or even the implementation. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
why center tone is null in input of IFFT in OFDM systems
Started by ●June 16, 2014
Reply by ●June 24, 20142014-06-24
Reply by ●June 27, 20142014-06-27
On Tuesday, June 17, 2014 1:29:16 AM UTC+5:30, harishm wrote:> why center tone is null in input of IFFT in OFDM systems ? > > > > is it for reducing PAPR ...? > > > > or > > > > to reduce complexity in OFDM receiver ? > > > > _____________________________ > > Posted through www.DSPRelated.comHello Eric, Im unable to catch words like 'mixer bleed through', although I feel like I understand something vaguely. Can you point me to some literature that briefly explains this situation so that its more clear to me. Thanks for your valuable guidance!
Reply by ●June 28, 20142014-06-28
On Fri, 27 Jun 2014 07:28:08 -0700 (PDT), Rakesh Joshi <rakesh@saankhyalabs.com> wrote:>On Tuesday, June 17, 2014 1:29:16 AM UTC+5:30, harishm wrote: >> why center tone is null in input of IFFT in OFDM systems ? >> >> >> >> is it for reducing PAPR ...? >> >> >> >> or >> >> >> >> to reduce complexity in OFDM receiver ? >> >> >> >> _____________________________ >> >> Posted through www.DSPRelated.com > >Hello Eric, >Im unable to catch words like 'mixer bleed through', although I feel like I understand something vaguely. Can you point me to some literature that briefly explains this situation so that its more clear to me. >Thanks for your valuable guidance!A better search term would be "direct conversion carrier leakage". In a heterodyne system the local oscillator mixing products other than the desired image wind up out-of band. Since the local oscillator frequency in a direct conversion system equals the rf carrier frequency, any leakage of the local oscillator into the signal at rf winds up at DC, and exactly at DC if the receiver phase-locks to the carrier. So it's really just a technology problem in that we don't yet have low-leakage mixer implementations that are cheap enough to use in consumer products like WiFi, etc. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
