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

[BHN66]

hi felix,

thanks, you are right! now it works!

 

i think i have understand the concept now: the sampling rate is valid global for the hole graph. if there is a previous/following block or source with a different rate, i have to resample it with such a resampler block. i am right?

 

but there are still two questions left concerning the sampling rate:

 

1) as written in previous posts i have to adjust my sampling rate to 11.228khz to get a bandwidth of 10khz. but with the resampler block i can only set the interpolation/decimation factor as an integer value. now i have set the sampling rate to 11khz to get afterwards a rate of 48khz by resampling (dec=11, int=48) but this will affect my bandwidth again. is there no way to adjust the exact bandwidth?

 

2) 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

 

alphonso

Gesendet: Freitag, 05. Juni 2015 um 01:28 Uhr
Von: "Wunsch, Felix (CEL)" <address@hidden>
An: "address@hidden" <address@hidden>, "address@hidden" <address@hidden>
Betreff: Re: Aw: Re: [Discuss-gnuradio] problem with ofdm bandwidth and sampling rate

Hi Alphonso,

keep in mind that if you don't resample on RX side your signal processing has to deal with the oversampled signal. If you use the same chain to demodulate your signal I can see why it only works when you resample on RX side.

-- Felix
 

On 05.06.2015 10:15, address@hidden wrote:

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)

Felix Wunsch, M. Sc.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-46276
Fax: +49 721 608-46071
E-Mail: address@hidden

www.cel.kit.edu

KIT -- University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

 

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Felix Wunsch, M. Sc.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-46276
Fax: +49 721 608-46071
E-Mail: address@hidden

www.cel.kit.edu

KIT -- University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association