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

[Anderson, Douglas J.]

Thanks Johannes,

At some point I'd like to pick up a B210 and try out the MIMO flowgraph, but 
for now I'm stuck with an N210.

So what I've got now is going straight from the LTE OFDM hier block into Decode 
PBCH hier block.

I've run into just a couple snags in the PBCH hier block.

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.

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 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.

Again, thanks for taking the time to help me understand gr-lte, it's really 
impressive.

-Doug

[1] 
https://raw.githubusercontent.com/kit-cel/gr-lte/master/docs/decode_pbch_37.grc.png
[2] 
https://github.com/kit-cel/gr-lte/blob/master/lib/qpsk_soft_demod_vcvf_impl.cc#L48
[3] 
https://github.com/kit-cel/gr-lte/blob/master/lib/pbch_descrambler_vfvf_impl.cc#L48

________________________________________
From: address@hidden address@hidden on behalf of Johannes Demel address@hidden
Sent: Thursday, July 02, 2015 1:22 AM
To: address@hidden
Subject: Re: [Discuss-gnuradio] gr-lte chanel estimation

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
>

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