Re: [Discuss-gnuradio] Update 2: BPSK problem: intermittent inter-symbol interference

So, I've been discussing this with my colleague, and we agree that we don't really know why we have the burst shaper. That's a really honest question: Why do you shape your bursts? And: why the arbitrary resampler doing your 2 samples-per-symbol interpolation? I must admit I'm no expert in ultra-short burst communication, so I've never considered burst shapers. What's their purpose? Is it clever to have it *before* resampling?

Best regards,
Marcus

On 09/01/2017 01:37 PM, Marcus Müller wrote:

Wow! Now, that is a flowgraph that's really pretty! Thanks! Will have a look at it!

On 08/31/2017 10:58 PM, Edwin Li wrote:

Hi Marcus,

I'm sorry. That was very inconsiderate of me. I made a new grc file, merging the tx and rx into one. You can find it in the attachment. 

I want to make sure the system works with an ideal channel. But I still have the intermittent ISI problem. 

                                                Figure 1 Down sampled signal

As you can see in Figure 1. There are 5 spots with severe ISI, while others are good. 

                                               Figure 2  Packet with severe ISI

Figure 2 shows the zoom in of Figure 1. How can one packet gets so much ISI while others are good? If my RRC filters have any error, all of the packets should suffer the same problem, right?

Or could it be that the parameters still need some tweaking? I'm guessing, in an ideal channel(in simulation), I can use an decimating FIR filter to replace the polyphase clock sync, while in experiment, I must use the polyphase clock sync as well as some kind of equalizer and tweak the parameters until I lower the ISI below an acceptable level. 

Regards,

Edwin

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