Re: [Discuss-gnuradio] IEEE 802.11 a/g/p Transceiver module - Making changes in WiFi PHY Hier Block

[Bastian Bloessl]

Hi,

On 03/24/2017 08:44 PM, Qurat-Ul-Ann Akbar wrote:
> How is a particular polarity picked for the pilot tones in each symbol
> with 52 subcarriers in GNU Radio? I can see the list in the OFDM Carrier
> Allocator block but where is this exactly happening in the code?

the pilots are copied here:

https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/ofdm_carrier_allocator_cvc_impl.cc#L185-L189

The vector "d_pilot_carriers" is created from the list of pilots that
you mentioned (i.e. the argument of the carrier allocator block).
This list contains pilots according to the polarity pattern given in the
standard.

Best,
Bastian

> 
> 
> 
> On Mon, Mar 20, 2017 at 3:37 PM, Bastian Bloessl <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     Hi,
> 
>     On 03/20/2017 09:17 PM, Qurat-Ul-Ann Akbar wrote:
>     > Hi,
>     >
>     > Thank you. I got it! I have another question. In the OFDM Carrier
>     > Allocator block, there are 127 values for pilot symbols each of them
>     > either (1,1,1,-1) or (-1,-1,-1,1). The pilot frequencies are
>     > (-27,-7,7,21) according to IEEE 802.11 standard. But I do not understand
>     > why are there 127 symbol values. Can you kindly explain that ?
> 
>     The four values indicate the four pilot symbols (on subcarriers -21, -7,
>     7, 21). With 802.11, the pilots change polarity based on a predefined
>     sequence (of length 127). It is in this exact format also in the
>     standard (Section 18.3.5.10 of the 2012 version).
> 
>     Best,
>     Bastian
> 
> 
> 
>     >
>     > On Mon, Mar 20, 2017 at 12:09 PM, Bastian Bloessl <address@hidden 
> <mailto:address@hidden>
>     > <mailto:address@hidden <mailto:address@hidden>>> wrote:
>     >
>     >     Hi,
>     >
>     >
>     >     > On 20 Mar 2017, at 17:50, Qurat-Ul-Ann Akbar <address@hidden
>     <mailto:address@hidden>
>     >     <mailto:address@hidden
>     <mailto:address@hidden>>> wrote:
>     >     >
>     >     > If I want to make changes in the PHY hierarchical block, can
>     I just make changes in the grc file available in the examples folder
>     and then recompile ? Because I cant file a .cc file in the lib
>     folder in IEEE_802.11 module and the source code for most blocks is
>     in there.
>     >     >
>     >
>     >     I’m afraid I don’t get the question, but if you don’t see any .cc
>     >     files in the lib folder, you might want to clone the repository
>     >     again, because actually there are some
>     >
>     >     https://github.com/bastibl/gr-ieee802-11/tree/next/lib
>     <https://github.com/bastibl/gr-ieee802-11/tree/next/lib>
>     >     <https://github.com/bastibl/gr-ieee802-11/tree/next/lib
>     <https://github.com/bastibl/gr-ieee802-11/tree/next/lib>>
>     >
>     >     If you only change the flow graph in GRC, you don’t have to
>     >     recompile the module (i.e., recompile c++ code). Just click on the
>     >     “generate” button (or press F5) this will rebuild the PHY.
>     Then the
>     >     other flow graphs will automatically use the new version of
>     the PHY.
>     >
>     >     Best,
>     >     Bastian
>     >
>     >
> 
>