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

[Wunsch, Felix (CEL)]

Hi Alphonso,

I implemented a transmitter for the DRM(30) standard in GNU Radio and published it on github (https://github.com/kit-cel/gr-drm).

To (partly) answer your questions:

1) DRM only accepts specific audio sample rates, make sure to read the standard documents carefully.
2) Mixing is usually done by the USRP. You as the user only deal with the complex baseband centered around DC. 12 kHz would also be a VERY low carrier frequency, short-wave radio usually uses frequencies between 3 and 30 MHz.

--Felix

On 03.06.2015 12:57, address@hidden wrote:

hello,

i'm trying to implement an ofdm transmitter (and afterwards a receiver) for demonstration. the parameters should match the drm standard. so my first try was to implement the transmitter with the following main parameters:

 

ofdm transmitter block

fft length = 256 (--> fft length >= occupied carriers)

cyclic prefix = 64 (--> 256/4)

occupied carriers = 228 (--> (-114, -113, ..., 113, 114))

 

for this try no pilot carriers and symbols and no sync words. everything works fine but there is a problem with the spectrum of this ofdm signal. from older posts i have the following equation to set the bandwidth (bw) of my ofdm output signal:

 

bw = sampling_rate * occupied_carriers / fft_length

 

with the parameters above and the wish of a bandwidth of 10khz (to match the drm standard) i will get the following sampling rate:

 

sampling_rate = bw * fft_length / occupied_carriers = 10khz * 256 / 228 = 11.228khz

 

now i have two questions:

 

1) if there is an input source with a sampling rate higher than 11.228khz (maybe from my sound card (44.1khz) or a wav file (48 khz)) should i use a resampler block because there is a problem with nyquist? how can i handle this problem?

 

2) the same problem goes with the output. if i mix up the ofdm signal to a carrier frequency (for example 12khz) how can i handle this? there is also a problem with nyquist.

 

thanks in advance!

 

alphonso

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

Felix Wunsch, M. Sc.
Research Associate

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