Re: [Discuss-gnuradio] gr-lte chanel estimation

[Johannes Demel]

Hey Doug,

> First one should be pretty easy for you: a couple of the blocks ask
> for input "rxant", and the pre-decoder asks for both rxant and
> N_ant. I scanned the source and looks like "1" will be an okay
> value for both, but just out of curiosity, what is actual different
> between rxant and N_ant? I'm assuming one is the total number of
> antennas being used, and one is the actual antenna being fed to
> that block, but it's a little unclear by the name of the variables
> which is which.
Name choice was probably suboptimal here. 'rxant' represents the
number of actually used antennas. It was added after multi antenna
support was added. 'N_ant' represents the system configuration. Say,
the system is supposed to work with 2x2 MIMO, then N_ant=2.


> Second issue is this: in your example flowgraph [1], you show QPSK
> soft demod taking a vector of 240 complex, and that block produces
> an output vector 2x the input vector length, so 480 floats [2].
The blocks are already configured correctly. PBCH descrambler is a
hier block. First thing it does is repeat every vector of size 480
four times and then process a vector of size 1920.
It is very specific to the LTE PBCH. PBCH is spread over 4 frames but
should be decodable with just one frame. In order to do correct
descrambling, this repetition and descrambling scheme must be applied.
Afterwards you can detect the 2 LSB of a frame number by the position
the correctly decoded PBCH frame had.

Hope that helps

Cheers
Johannes

> The next block that you show in the flowgraph is the PBCH
> descambler, which takes 1920 floats, so it's expecting 4 rather
> than 2 vectors from QPSK soft demod.
> 
> I just wanted to double check with you that changing the vlen*2 to
> vlen*4 throughout qpsk_soft_demod will be the appropriate fix for
> this, or if there's some subtlety I'm missing.
> 
> 
> Hey Doug,
> 
> This particular part of the flowgraph deals with reception of a 2 
> antenna transmitter. That implies that an Alamouti code is applied 
> before transmission. Thus, for correct reception, it is necessary
> to perform the inverse Alamouti code operation. This is generally
> called Precoding. For data channels, some other schemes are used as
> well. They all require to do an inverse operation. So, the
> flowgraph does channel estimation for all antennas. Also, RS
> symbols are transmitted differently for each antenna layer. Without
> a second receiver antenna, this whole channel estimation, precoding
> thing does not result in better reception. But you could use a B210
> or a similar device which offers to RX channels in order to exploit
> this extra diversity. Hope that sheds some light on why and how
> things are done. Also let me know about specifics which are unclear
> in those blocks. So I'll add some doc.
> 
> Cheers Johannes
> 
> On 01.07.2015 23:16, Anderson, Douglas J. wrote:
>> Hi all,
>> 
>> I'm working at putting together a flowgraph based on gr-lte.
>> 
>> For the channel estimator hier block [1], it looks like there
>> are two RS map generators, one for each of 2 "antenna ports".
>> 
>> I only have one antenna connected to my USRP N210. Is this a 
>> problem? Do I still use 2 RS map generators with antenna port 0
>> and 1?
>> 
>> -Doug
>> 
>> [1] 
>> https://raw.githubusercontent.com/kit-cel/gr-lte/master/docs/lte_estimator_hier.grc.png
>>
>
>> 
> _______________________________________________ Discuss-gnuradio
> mailing list address@hidden 
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>