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Re: [Discuss-gnuradio] problem with ofdm bandwidth and sampling rate


From: BHN66
Subject: Re: [Discuss-gnuradio] problem with ofdm bandwidth and sampling rate
Date: Fri, 5 Jun 2015 10:15:19 +0200

hi felix,
i have tried it with the resampler block but it didn't work well.
 
on the tx side i have used it to resample from 11khz, which i need for my 10khz bandwidth, to 48khz. on the rx side i can see the spectrum with 48khz and the bandwidth of my signal still remains at 10khz (as requested). the problem is that i can't detect any signal at rx side. when i adjust both parts (tx and rx) with a sampling rate at 11khz it works.
 
for better understanding i have attached my flowgraphs and sink outputs.
 
alphonso
Gesendet: Donnerstag, 04. Juni 2015 um 23:40 Uhr
Von: "Wunsch, Felix (CEL)" <address@hidden>
An: "address@hidden" <address@hidden>, "address@hidden" <address@hidden>
Betreff: Re: [Discuss-gnuradio] problem with ofdm bandwidth and sampling rate
 
On 03.06.2015 17:29, address@hidden wrote:
hello felix,
>>please remember to always reply to the list.
i'm sorry, i'm new at this. i have checked my configuration and i hope it works the next time.
 
to mix up the baseband signal to 12khz or higher to short wave (3-30mhz) should be the same procedure. i have tested it by using a multiply block and a signal source block as the carrier frequency.
 
but there is an other question: if i configured my system as described, can i be sure that the bandwidth of each sub carrier has the bandwidth and symbol duration as shown by the equations?
 
used_bw = occupied_carriers * df
df = used_bw / occupied_carriers = 10khz / 228
df = 43.86hz   -->   tsym = 22.8ms
 
it's very confusing why i have to adjust my used bw by changing the sampling rate. when i follow the equations and the standard i choose one mode (RM A, SO 3). that means i have to use 228 carriers
and have a symbol duration with 24ms which leads to 41.66hz carrier spacing. so the used bw is 228 * 41.66hz = 9.5khz (approx. 10khz). this bw should be fix and should not change by the sampling rate.
 
sorry for the maybe silly questions but i want to keep on that stuff.
Well, the measurable on-air bandwidth is always defined by the speed/rate at which your soundcard processes your digital samples. If you remember that B ~ 1/T and that the rate of your soundcard controls T it becomes obvious that changing the sample rate changes the bandwidth. If you don't want that, you could use a resampler that adapts its resampling ratio to the sampling rate of the soundcard.

Felix
 
@Marcus
you mean if there were pilots for a channel estimation in the transmitted signal?
 
alphonso
 
 
-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

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