Hi Bastian,
Yes thats correct. I believe this problem will be there if I do soft bit
MRC i.e. independent decoding of the two branches. Just combining the
long sync is not the solution :(
On an other note I have following question :
Inside the general_work in sync_long.cc, *ninput* is computed as follows.
* int ninput = std::min(std::min(ninput_items[0], ninput_items[1]),
8192);*
This is the case when there are two inputs to sync_long i.e. *in* and
*in_delayed*.
How shall I compute *ninput* when I have another branch coming say *in1*
and *in1_delayed*.
In sync_short I faced the same issue when I input another branch. There
I modified as follows :
*int ninput = std::min(std::min(ninput_items[2]
,std::min(ninput_items[0], ninput_items[1])), std::min(ninput_items[4],
ninput_items[3]));*
where previously it was
*int ninput = std::min(std::min(ninput_items[0], ninput_items[1]),
ninput_items[2]);*
Regards
Sumit
**
On 02/05/2018 08:26, Bastian Bloessl wrote:
The problem with this configuration is that the "Soft frame equalizer"
blocks are not synced. It's the same problem as with the Sync Short
block, just at a later stage. Consider what happens, if one branch
manages to decode the signal field and one doesn't. Or one thinks it's
a 100 Byte BPSK 1/2 and the other thinks it's a 200 Byte 64-QAM 3/4
Best,
Bastian
On 04/29/2018 03:30 PM, Sumit Kumar wrote:
"In essence, you have to make sure that all branches start the
synchronization process if one branch detects a frame."
I am doing only slightly different from above. Frame detection is
happening with the combined value of correlation. And once frame is
detected, both of the branches start the synchronization process.
I have created a dual channel short_sync which uses
*(a[n1]+a[n2])/(p[n1]+p[n2])* to compute *in_cor.
*
If this in_cor > d_threshold, i declare that WiFi has started on both
the branches by calling a modified *insert_tag *function as follows:*
*
void insert_tag(uint64_t item, double freq_offset, *double
freq_offset_1*, uint64_t input_item) {
mylog(boost::format("frame start at in: %2% out: %1%") % item %
input_item);
*
* const pmt::pmt_t key = pmt::string_to_symbol("wifi_start");*
* const pmt::pmt_t value = pmt::from_double(freq_offset);*
const pmt::pmt_t value_1 = pmt::from_double(freq_offset_1);
* const pmt::pmt_t srcid = pmt::string_to_symbol(name());*
* add_item_tag(0, item, key, value, srcid); // tag branch -1 *
add_item_tag(1, item, key, value_1, srcid); ***//** tag branch -2**
**
*}*
*freq_offset* for both the branches are computed independently too. *
*
The dual channel short sync block outputs two streams which are fed
to the usual WiFi Long Sync-> FFT -> Equalizer-> etc etc steps.
With this approach simulator works flawlessly now. Also with USRP it
works good. But after some time, say 2-3 minutes or so, USRP stops
receiving signal. There is no warning or error or overruns etc making
it difficult for me to debug further.
Regards
Sumit
On 29/04/2018 14:49, Bastian Bloessl wrote:
Hi,
On 28. Apr 2018, at 17:02, Sumit Kumar<sumit.ku...@eurecom.fr> wrote:
So basically I will be combining the correlation values from all
antennas to find the start of WiFi frame. With this approach, I
believe, there wont be any need of synchronization between
branches. Let me know your view on this.
That depends at what stage you want to combine the signal. I’d do it
at a later stage, i.e., do frame detection and synchronization in
each branch independently and combine the subcarriers after
equalization. With that approach, you wouldn’t combine correlations
values. In essence, you have to make sure that all branches start
the synchronization process if one branch detects a frame.
Best,
Bastian
Regards
Sumit
On 27/04/2018 11:25, Bastian Bloessl wrote:
I don't know about such an implementation. IIRC, in the paper, we
recorded the IQ samples and processed the data offline.
If you are interested in the code you could write the first
author, but since it was not real-time and for a single-carrier
scheme, it might not be too helpful for your project.
If you come up with a solution, let me know.
Best,
Bastian
On 04/27/2018 11:15 AM, Sumit Kumar wrote:
Ok I understand now. Could you point me how to approach for such
synchronization between the two branches. Or if there are any
existing open source example if you know.
For this implementation, I was following one of your recently
co-authored paper "Low-Complexity Soft-Bit Diversity Combining
for Ultra-Low Power Wildlife Monitoring". However it seems that
the source code is not open yet.
Sumit
On 27/04/2018 11:00, Sumit Kumar wrote:
Yes indeed, this could also happen! I note this in my to-do list.
But as of now there are no warnings of overruns etc. I recorded
it. What is making USRP to stop receiving.
https://www.youtube.com/watch?v=SPXLJ3iEWg8&feature=youtu.be
Sumit
On 27/04/2018 10:41, Bastian Bloessl wrote:
Hi,
I'm not sure if I get it, but don't you need some
synchronization logic between the branches. Consider what
happens if one branch receives frames while the other one
doesn't, then data queues up in the add block, which will
sooner or later lead to overruns, independent from the buffer
size.
Best,
Bastian
On 04/27/2018 09:54 AM, Sumit Kumar wrote:
Hi,
I am working on soft bit maximal ratio combiner (SBMRC). It's
basically a MRC but instead of combining the actual signals
according to their SNR, we combine the LLRs from separate
branches and send them to Soft Decision Viterbi Decoder
(SDVD). For this, I have modified gr-ieee 80211 which
generates soft bits and integrated a SDVD with it. It works
good when I use either single branch or both branch separately.
In order to implement SBMRC, for every OFDM symbol decoded, a
vector of LLR (size 48 X 1) is generated from both the
receiver branches. These two vectors of LLR are further added
and sent to SDVD. I configured the ADD block to take 48 floats
as input.
First I made a simulator for SBMRC, but even after increasing
the output buffer size to 500000, warnings of buffer over flow
kept coming.
Then I used USRP, it simply refuses to work. I am using NI
2901 Tx/Rx A and Tx/Rx B ports as my receive branches. The
LEDs go green for a second then stop. No error no warning.
Looks like the *ADD *block is the cause. I have never seen
this, so I am clueless where to debug. Am I missing something
fundamental here ?
The attached picture *usrp_sbmrc* says details of my schematic
and the results when I use USRP.
The attached picture *sbmrc_sim* says details of my schematic
and the results when I use simulations.
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Dipl.-Inform. Bastian Bloessl
CONNECT Center
Trinity College Dublin
GitHub/Twitter: @bastibl
https://www.bastibl.net/
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