Re: [Discuss-gnuradio] ofdm parameters

[Marcus Müller]

Hi Sara,
On 23.06.2014 18:31, Sara Chérif wrote:
> I plot the signal before the tx and after the rx on same graph to compare
> between them while increasing noise to know how much noise can affect it
> for different types of channel & frequency offset . But when I set noise to
> zero  , I find that the 2 signals are not the same ! why ?! They may be the
> same but they are not synchronized . How to solve this problem ?
> (Attached the loopback.grc & the graph)
If one is delayed longer than the other, use the delay block to
compensate for this.

> Second , There is a variable called sync word , I know that it is for
> synchronization using schmidl and cox algorithm  & I know that is takes
> fft length & occupied carriers & pilot carriers
> as it is written in "ofdm_rxrx.py
> "if sync_word1 is None:
>             self.sync_word1 = _make_sync_word1(fft_len, occupied_carriers,
> pilot_carriers)"
>
> But why I get an error syaying that " sync words must be fft length" when I
> write in sync word 1 & sync word 2 fields in the tx & rx block : (128,
> (range(-61,-42)+range(-41, -14) + range(-13, -7) + range(-6, 0) + range(1,
> 7) + range(7, 14) + range(15, 42)+range(43,62),) ,  ((-42, -14, 14, 42,),)
> ) ?
Ok, this is a little much python parsing to do it in my head. Have you
verified that len(<your range concatenation>) is actually correct? For
my python interpreter, it's 117, not 128, just like the error said.
> Third , I tried my parameters in loopback example & I got no error , but I
> want to try tx_ofdm.grc with my parameters & put file sink after each block
> to understand , but I got an error in tx_ofdm.grc . Why I got an error
> while the same parameters don't give an error in loopback ?!!
> Only I changed these parameters to be mine in tx & rx :
> fft len=128 , CP =fft_len/8=16 , packet length=87
>  occupied carriers = (range(-61,-42)+range(-41, -14) + range(-13, -7) +
> range(-6, 0) + range(1, 7) + range(7, 14) + range(15, 42)+range(43,62),)
> pilot carriers :  ((-42, -14, 14, 42,),)
> pilot symbols : (((-1+1j),(1-1j),(-1+1j),(-1-1j),),)
> sync_word1=None , sync_word2=None
>
> Here is the full output when executing :
> Using Volk machine: sse4_2_64_orc
> Traceback (most recent call last):
>   File "/home/sara/trials/tx_ofdm.py", line 241, in <module>
>     tb = tx_ofdm()
>   File "/home/sara/trials/tx_ofdm.py", line 100, in __init__
>     self.digital_ofdm_carrier_allocator_cvc_0 =
> digital.ofdm_carrier_allocator_cvc(fft_len, occupied_carriers,
> pilot_carriers, pilot_symbols, (sync_word1, sync_word2), length_tag_key)
>   File
> "/usr/local/lib/python2.7/dist-packages/gnuradio/digital/digital_swig.py",
> line 4312, in make
>     return _digital_swig.ofdm_carrier_allocator_cvc_make(*args, **kwargs)
> TypeError: in method 'ofdm_carrier_allocator_cvc_make', argument 5 of type
> 'std::vector< std::vector< gr_complex,std::allocator< gr_complex >
>> ,std::allocator< std::vector< gr_complex,std::allocator< gr_complex > > >
>> const &'
Again, this is a type error which has to do with the content and/or
shape of your parameters. You'll have to figure this one out yourself.

> Thanks is advance
No problem! However, I feel that you tend to ask questions that, with a
little more investigative vigor, you would have answered yourself, so
maybe it would pay to work a little more thoroughly before mailing the
list. That being said, whenever you're stuck, feel absolutely free and
encouraged to ask!

Greetings,
Marcus