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
why center tone is null in input of IFFT in OFDM systems
Started by ●June 16, 2014
Reply by ●June 16, 20142014-06-16
On 2014-06-16 12:59, harishm wrote:> why center tone is null in input of IFFT in OFDM systems ? > > is it for reducing PAPR ...?Not sure what you deem "center tone" but if it's frequency 0 (mirrored around y-axis), it is to remove DC components. Peter
Reply by ●June 23, 20142014-06-23
On Tuesday, June 17, 2014 2:05:25 AM UTC+5:30, Peter Mairhofer wrote:> On 2014-06-16 12:59, harishm wrote: > > > why center tone is null in input of IFFT in OFDM systems ? > > > > > > is it for reducing PAPR ...? > > > > Not sure what you deem "center tone" but if it's frequency 0 (mirrored > > around y-axis), it is to remove DC components. > > > > PeterI dont see such a measure taken in DVB-T or ISDB-T DTV stds. Is it there as part of WiMax/WiFi stds? -Rakesh
Reply by ●June 23, 20142014-06-23
In downlink LTE centre subcarrier is not transmitted due to higher interference possibilty. In uplink LTE it does not exist as whole spectrum is shifted by half subcarrier spacing and becomes symmetrical around dc. _____________________________ Posted through www.DSPRelated.com
Reply by ●June 23, 20142014-06-23
On Mon, 23 Jun 2014 06:10:51 -0700 (PDT), rakesh@saankhyalabs.com wrote:>On Tuesday, June 17, 2014 2:05:25 AM UTC+5:30, Peter Mairhofer wrote: >> On 2014-06-16 12:59, harishm wrote: >> >> > why center tone is null in input of IFFT in OFDM systems ? >> >> > >> >> > is it for reducing PAPR ...? >> >> >> >> Not sure what you deem "center tone" but if it's frequency 0 (mirrored >> >> around y-axis), it is to remove DC components. >> >> >> >> Peter > >I dont see such a measure taken in DVB-T or ISDB-T DTV stds. Is it there as part of WiMax/WiFi stds? > >-RakeshSystems that are expected to use a direct-conversion rf architecture in order to reduce cost have to leave the center carrier unpopulated since it will be subject to a DC offset due to bleed-through of the mixing oscillator. WiFi falls into that category, as do many other cost-sensitive consumer products. Since televisions have historically used super-heterodyne architectures, for those cases the center carrier can be used for information. So DVB-T and other standards that are not expected to use direct-conversion architectures will populate the center subcarrier. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 23, 20142014-06-23
Reply by ●June 23, 20142014-06-23
Reply by ●June 23, 20142014-06-23
Eric Jacobsen <eric.jacobsen@ieee.org> wrote: (snip)> Systems that are expected to use a direct-conversion rf architecture > in order to reduce cost have to leave the center carrier unpopulated > since it will be subject to a DC offset due to bleed-through of the > mixing oscillator.(snip)> Since televisions have historically used super-heterodyne > architectures, for those cases the center carrier can be used for > information.Though for vestigial side-band, the carrier isn't in the center. -- glen
Reply by ●June 24, 20142014-06-24
On Mon, 23 Jun 2014 11:59:16 -0700 (PDT), Rakesh Joshi <rakesh@saankhyalabs.com> wrote:>Thanks Eric. Does having a dc carrier also affect the dagc response if its employed?Typically DC offset and AGC are detected and adjusted separately from each other. They can be made independent regardless of whether the DC subcarrier is populated or not. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 24, 20142014-06-24
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: > > > > >Thanks Eric. Does having a dc carrier also affect the dagc response if its employed? > > > > Typically DC offset and AGC are detected and adjusted separately from > > each other. They can be made independent regardless of whether the > > DC subcarrier is populated or not. > > > > > > Eric Jacobsen > > Anchor Hill Communications > > http://www.anchorhill.comAs per my understanding, DC offset that is corrected is due to the ADC components. 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 the AGC. If the AGC is quick to react to this dc, the AGC gain can falsely vary. Hence for the AGC to lock to the proper gain values, one may have to design 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 help reduce the AGC settling time? I hope Im clear. Please correct me if you find me wrong.
