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

[Wunsch, Felix (CEL)]

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

 

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Karlsruhe Institute of Technology (KIT)
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