Hello, In LTE specification, OFDM is implemented using FFT of sizes 128/256/512/1024/1536/2048. These correspond to Downlink bandwidth of 1.4/3/5/10/15/20 MHz. I don't know how these N point FFT are selected for OFDM implementation. Let us take 1.4 MHz and 128 point FFT for example. 128 point FFT has N value of (2^N = 128) of 7. With a sub-carrier spacing of 15 KHz, this covers = 7*15 KHz = 105 KHz. So, somehow, I am not able to see the co-relation between Downlink Bandwidth and N points selected for the FFT/iFFT. Thanks a lot ... --------------------------------------- Posted through http://www.DSPRelated.com
LTE, FFT, OFDM
Started by ●November 15, 2015
Reply by ●November 15, 20152015-11-15
>Hello, > >In LTE specification, OFDM is implemented using FFT of sizes >128/256/512/1024/1536/2048. These correspond to Downlink bandwidth of >1.4/3/5/10/15/20 MHz. > >I don't know how these N point FFT are selected for OFDM implementation. >Let us take 1.4 MHz and 128 point FFT for example. >128 point FFT has N value of (2^N = 128) of 7. >With a sub-carrier spacing of 15 KHz, this covers = 7*15 KHz = 105 KHz. > >So, somehow, I am not able to see the co-relation between Downlink >Bandwidth and N points selected for the FFT/iFFT. > >Thanks a lot ... > >--------------------------------------- >Posted through http://www.DSPRelated.comsampling rate of 1.4 is 1.92Msps. 1.92 * 75/128 = 1.125MHz (nominal 1.4) Kaz --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 15, 20152015-11-15
On Sun, 15 Nov 2015 06:50:02 -0600, "Sharan123" <99077@DSPRelated> wrote:>Hello, > >In LTE specification, OFDM is implemented using FFT of sizes >128/256/512/1024/1536/2048. These correspond to Downlink bandwidth of >1.4/3/5/10/15/20 MHz. > >I don't know how these N point FFT are selected for OFDM implementation. >Let us take 1.4 MHz and 128 point FFT for example. >128 point FFT has N value of (2^N = 128) of 7. >With a sub-carrier spacing of 15 KHz, this covers = 7*15 KHz = 105 KHz.Not sure what N=7 has to do with it. Not all subcarriers are populated. Some are left for guard band. On this site: http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm under "bandwidths" the number of populated subcarriers for each bandwidth is shown. Taking 15kHz times the number of populated subcarriers pluts about 10% for guard band gives the indicated channel bandwidths.>So, somehow, I am not able to see the co-relation between Downlink >Bandwidth and N points selected for the FFT/iFFT. > >Thanks a lot ... > >--------------------------------------- >Posted through http://www.DSPRelated.comEric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●November 16, 20152015-11-16
>Not sure what N=7 has to do with it. > >Not all subcarriers are populated. Some are left for guard band. > >On this site: > >http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm > >under "bandwidths" the number of populated subcarriers for each >bandwidth is shown. Taking 15kHz times the number of populated >subcarriers pluts about 10% for guard band gives the indicated channel >bandwidths.Dear Eric, N point in FFT was just my assumption. But I would like to know how channel bandwidth is co-related with the FFT size since it is the FFT stage that does actual sub-carrier modulation --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 16, 20152015-11-16
>sampling rate of 1.4 is 1.92Msps. > >1.92 * 75/128 = 1.125MHz (nominal 1.4) > >KazDear Kaz, Can you elaborate what is 75 in the equation above? --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 16, 20152015-11-16
>>sampling rate of 1.4 is 1.92Msps. >> >>1.92 * 75/128 = 1.125MHz (nominal 1.4) >> >>Kaz > >Dear Kaz, > >Can you elaborate what is 75 in the equation above? > >--------------------------------------- >Posted through http://www.DSPRelated.comFFT points (bins) represents all digital domain (-Fs/2 ~ Fs/2) I assumed only 75 carriers are occupied ffor lte (it could be 72 or so). The ration of normalised bandwidth is 75/128 This translates in real world according to applied Fs i.e. 1.92Msps in this case of lte 1.4 Kaz --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 16, 20152015-11-16
On Mon, 16 Nov 2015 02:21:08 -0600, "Sharan123" <99077@DSPRelated> wrote:>>Not sure what N=7 has to do with it. >> >>Not all subcarriers are populated. Some are left for guard band. >> >>On this site: >> >>http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm >> >>under "bandwidths" the number of populated subcarriers for each >>bandwidth is shown. Taking 15kHz times the number of populated >>subcarriers pluts about 10% for guard band gives the indicated channel >>bandwidths. > >Dear Eric, > >N point in FFT was just my assumption. But I would like to know how >channel bandwidth is co-related with the FFT size since it is the FFT >stage that does actual sub-carrier modulationUse the number of populated subcarriers from the link above, and take the next largest power-of-two N for the FFT size. e.g., for the 5MHz BW case there are 301 subcarriers. Use N=512, populate the 301 subcarriers around DC, and leave the remaining 211 subcarriers as zeros for guard band. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●November 16, 20152015-11-16
On Monday, November 16, 2015 at 11:06:29 AM UTC-5, Eric Jacobsen wrote:> On Mon, 16 Nov 2015 02:21:08 -0600, "Sharan123" <99077@DSPRelated> > wrote: > > >>Not sure what N=7 has to do with it. > >> > >>Not all subcarriers are populated. Some are left for guard band. > >> > >>On this site: > >> > >>http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm > >> > >>under "bandwidths" the number of populated subcarriers for each > >>bandwidth is shown. Taking 15kHz times the number of populated > >>subcarriers pluts about 10% for guard band gives the indicated channel > >>bandwidths. > > > >Dear Eric, > > > >N point in FFT was just my assumption. But I would like to know how > >channel bandwidth is co-related with the FFT size since it is the FFT > >stage that does actual sub-carrier modulation > > Use the number of populated subcarriers from the link above, and take > the next largest power-of-two N for the FFT size. e.g., for the 5MHz > BW case there are 301 subcarriers. Use N=512, populate the 301 > subcarriers around DC, and leave the remaining 211 subcarriers as > zeros for guard band. > > > Eric Jacobsen > Anchor Hill Communications > http://www.anchorhill.comHere's another link that may (or may not) help (alternately, Google for "LTE in a Nutshell" + "Telesystem Innovations" to get the link below): http://www.tsiwireless.com/docs/whitepapers/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf Table 1 on p. 5 of the above shows bandwidth, FFT size, sub-carriers, etc. Kevin McGee
Reply by ●November 17, 20152015-11-17
>On Monday, November 16, 2015 at 11:06:29 AM UTC-5, Eric Jacobsen wrote: >> On Mon, 16 Nov 2015 02:21:08 -0600, "Sharan123" <99077@DSPRelated> >> wrote: >> >> >>Not sure what N=7 has to do with it. >> >> >> >>Not all subcarriers are populated. Some are left for guard band. >> >> >> >>On this site: >> >> >> >>>http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm >> >> >> >>under "bandwidths" the number of populated subcarriers for each >> >>bandwidth is shown. Taking 15kHz times the number of populated >> >>subcarriers pluts about 10% for guard band gives the indicatedchannel>> >>bandwidths. >> > >> >Dear Eric, >> > >> >N point in FFT was just my assumption. But I would like to know how >> >channel bandwidth is co-related with the FFT size since it is the FFT >> >stage that does actual sub-carrier modulation >> >> Use the number of populated subcarriers from the link above, and take >> the next largest power-of-two N for the FFT size. e.g., for the 5MHz >> BW case there are 301 subcarriers. Use NQ2, populate the 301 >> subcarriers around DC, and leave the remaining 211 subcarriers as >> zeros for guard band. >> >> >> Eric Jacobsen >> Anchor Hill Communications >> http://www.anchorhill.com > >Here's another link that may (or may not) help (alternately, Google for"LTE>in a Nutshell" + "Telesystem Innovations" to get the link below): > >http://www.tsiwireless.com/docs/whitepapers/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf > >Table 1 on p. 5 of the above shows bandwidth, FFT size, sub-carriers,etc.> >Kevin McGeeI am lost now about carrier numbers for downlink lte 1.4MHz first link says 73 carriers. second link says 76? which one is correct. Kaz --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 18, 20152015-11-18
On Tuesday, November 17, 2015 at 5:44:42 AM UTC-5, kaz wrote:> >On Monday, November 16, 2015 at 11:06:29 AM UTC-5, Eric Jacobsen wrote: > >> On Mon, 16 Nov 2015 02:21:08 -0600, "Sharan123" <99077@DSPRelated> > >> wrote: > >> > >> >>Not sure what N=7 has to do with it. > >> >> > >> >>Not all subcarriers are populated. Some are left for guard band. > >> >> > >> >>On this site: > >> >> > >> > >>>http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte/content/lte_overview.htm > >> >> > >> >>under "bandwidths" the number of populated subcarriers for each > >> >>bandwidth is shown. Taking 15kHz times the number of populated > >> >>subcarriers pluts about 10% for guard band gives the indicated > channel > >> >>bandwidths. > >> > > >> >Dear Eric, > >> > > >> >N point in FFT was just my assumption. But I would like to know how > >> >channel bandwidth is co-related with the FFT size since it is the FFT > >> >stage that does actual sub-carrier modulation > >> > >> Use the number of populated subcarriers from the link above, and take > >> the next largest power-of-two N for the FFT size. e.g., for the 5MHz > >> BW case there are 301 subcarriers. Use NQ2, populate the 301 > >> subcarriers around DC, and leave the remaining 211 subcarriers as > >> zeros for guard band. > >> > >> > >> Eric Jacobsen > >> Anchor Hill Communications > >> http://www.anchorhill.com > > > >Here's another link that may (or may not) help (alternately, Google for > "LTE > >in a Nutshell" + "Telesystem Innovations" to get the link below): > > > >http://www.tsiwireless.com/docs/whitepapers/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf > > > >Table 1 on p. 5 of the above shows bandwidth, FFT size, sub-carriers, > etc. > > > >Kevin McGee > > I am lost now about carrier numbers for downlink lte 1.4MHz > first link says 73 carriers. second link says 76? which one is correct. > > Kaz > --------------------------------------- > Posted through http://www.DSPRelated.comI've seen the same author(Rayal) use the 76 number in another publication, but others use the 73 number (or 72 if not including the DC sub-carrier), so I think that 73 is the correct one: http://www.informit.com/articles/article.aspx?p=1627062&seqNum=3 And I've always seen others use the number 15 instead of 12 as the number of resource blocks for the 2.5 MHz Channel. I don't know if he was using an earlier version of the spec, or some kind of custom implementation. Some errors/typos are easier to spot than others (eg: #sub-carriers + #guard sub-carriers = FFT size). Kevin McGee
