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 Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio