Re: [Discuss-gnuradio] About DPSK mod and demod

[Henry Jin]

Thanks for your effort in trying to help, Michael. I will continue to study and if I managed to get it working, I will keep you updated.

Henry

On Thu, Dec 5, 2013 at 2:23 PM, Michael Berman <address@hidden> wrote:

Henry,

I looked at this and some of the underlying code, and tried to run your example with some modifications, but all to no avail as to what you are attempting to do.  Last time I did anything like this I ended up using the mod_pkts code (found in gr-digital/python/digital/pkt.py).  Maybe there is some insight in there that may shed light on what you are trying to do.  Sorry I couldn't be of more help.

Michael

On Wed, Dec 4, 2013 at 1:31 PM, Henry Jin <address@hidden> wrote:

On Wed, Dec 4, 2013 at 12:13 PM, Michael Berman <address@hidden> wrote:

Looking at your flowchart in the original post, you have an Unpacked to Packed block after the demodulator, with bit's per symbol of 1.  This doesn't seem right to me.  I have never tried this with a random source like you have it setup, however there should be an Unpacked to Packed block prior to the modulator and a Packed to Unpacked block after the demodulator.  These should also have bits per chunck values that correspond with the bits per symbol of the modulator and demodulator.  You need to feed in the data in a chunk with the correct amount of bits that correspond to the bits per symbol of the modulation scheme being used.  In the example, it looks like you are using QPSK, and therefore the bits per chunk should be 2 (which is log2(number of constellation points)).  The modulator and demodulator work with chunks of data where each chuck corresponds to a symbol.

If not using the random source, what other sources do you think it's better? I know GLFSR source also perfectly fits into this scenario. You also mentioned I have to attach a Unpacked to Packed block prior to the modulator. But since in my flow, I already set the random source with maximum being 256. That means it's already outputting packed bytes. Thus, IMO, Unpacked to Packed block is not needed based on my settings. The reason I set the Bits per Chunk value as 1 in the Unpacked to Packed block after demodulation is that I notice in the source code, it says the output of the demodulation block is unpacked byte with only one LSB being valid. So In my understanding, it is independent of what modulations (BPSK, QPSK, etc) I'm using. 

 

For the Samples per Symbol, if I were transmitting over the air, I would raise this value to a little bit more than 2, just to ensure the receiver can lock onto the changes with given noise before the symbol changes again.  In this case of looping the modulator strait into the demodulator, this should work fine.

Thanks for the advice, I will take it. 

 

One more thing I would look at would be the Error Rate block source.  I have never used this block, but in my thinking about it in this flowchart, I would source it from the throttle instead of the random source.  This may help with keeping the data a little more somewhat aligned.

Yes, this could make things clearer. But maybe it makes no difference. I remember in one of Tom's tutorial, he said as long as there is one throttle in the flow, then all the units are throttled.

Michael

On Wed, Dec 4, 2013 at 9:52 AM, Henry Jin <address@hidden> wrote:

I tried again replacing the DPSK module with PSK module. Still cannot get the correct data. The parameter for Error Rate "Bits per Symbol" is changed to 8 since every bit carries information in my flow. The sizes of the files from the two file sinks are exactly the same, except with different data.

On Tue, Dec 3, 2013 at 10:11 PM, Nick Foster <address@hidden> wrote:

Can you try using the PSK mod/demod blocks with differential=Yes instead of the DPSK mod/demod blocks? I found an issue today with the DPSK mod/demod blocks which results in them not actually using differential encoding.

--n

On Tue, Dec 3, 2013 at 9:00 PM, Henry Jin <address@hidden> wrote:

Hi, 

I tried to build a simple flow graph of DPSK modulation and demodulation. The result is verified using the Error Rate module. The link shows the flow I'm using.

https://www.dropbox.com/s/jwmmttyi4es4alf/Screenshot%20from%202013-12-03%2021%3A50%3A58.png

I know that the output of demod module is unpacked bytes, so an "unpacked to packed" module is attached after demodulation. However, the BER is close to 50%, which surely indicates something wrong. I further analyzed the two inputs of the Error Rate module by writing info to the file sink. It clearly shows the discrepancy. So I just wonder what is wrong with this flow graph. Could someone please help me?

Also, as I noticed, in the output file generated after "unpacked to packed" module, there are many consecutive 0s at the start of the file. Does that indicate something?

Suggestions are greatly appreciated!

Henry

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