Utilizing MT BER

[Jdjames]

I have set up a QAM generation and acquistion system and I am trying to utilize Labview's BER tool to measure the BER.  I have my threshold set to 0.1 and in ideal situations (not adding any noise to the system) I am getting a 0.5 BER.  I double checked to make sure that the PN numbers match.  This seems extremely high and I don't know what I am doing wrong.  I am using the RFSG & RFSA QAM example programs to generate and demodulate the signals. 

Any ideas?

Thanks!

Jenn

[MicrowaveDave]

Hey,

First of all, lets see if we can't figure out it the problem is due to physical noise...or an issue of recovering the bits correctly.  Thus, I have a couple of questions.  First, is the physical channel a couple of antennas or a direct connection between the RFSG and RFSA.  Also, have you tried making EVM measurements?  If the EVM is very low, that would suggest that there is not a problem with the physical system.  Any more information about the physical channel would be helpful.

Second, I am curious about two factors relating to the bits being modulated/demodulated.  It would be helpful to know the M-ary of the QAM scheme you are using.  Also, it would be helpful to know other information such as the length of the bitstream and the samples per symbol.  Finally, it would be good to know the type of bit coding you are using as well.

At this point, it is not clear to me what exactly is going on with the system.  A little more information might be helpful.

Regards,

David Hall | Applications Engineering | National Instruments

[Jdjames]

I have a direct connection from the RFSG to the RFSA.  I am using the example programs provided with the Modulation Toolkit.  I just want to make sure the setup is working before I add in the physical channel. 

At this point I am using the defaults on all of the settings.  I believe this is 16-ary QAM, with no coding involved yet.  16 samples per symbol and I beleive the default is 1000 symbols acquired.  I apologize, I am coming up with all this by memory since I am not at my lab right now.

I will have to provide additional information on the EVM and so on tomorrow.

Thanks for your help!

Jenn

[Jdjames]

Following up:

The EVM measurements are very low, < 0.50 for the RMS, Peak of 1.28.  All the other measurements look great, the signal constellation and eye diagrams look great as well.

[MicrowaveDave]

Jen,

Thats very good information.  EVM values around 0.5 are pretty good, which explains why the constellation and eye diagrams are clean.  It seems to me this might be a software issue then.  I would try a couple of different things.  First, try generating more symbols at a lower M-ary.  In general, a certain degree of randomness is required in order to demodulate the signal correctly.  Just to make sure, you should be using a pseudo random bistream to make sure that each symbol is generated ideally more than once.  This is even more critical at higher M-aries such as 64 QAM. 

Another thing you can try is to append a defined number of random bits onto the end of your actual bistream.  For example, if you have 1000 bits of useful data, try appending 1000 random bits onto the end....and check to see if the first 1000 came through correctly.  In one of the simulation examples that I wrote recently, I noticed an issue similar to what you're experiencing.  In my example, I was generating the several symbols over and over again without hitting all of the symbols.  By adding some random bits to the end of the bistream, I was able to ensure that all symbols were generated and recover all bits correctly.

Anyway, this is just a thought.  Ideally, it seems like you should be getting a BER of 0 with EVM measurements like you mention.  It sounds like the constellation plot is locked to the appropriate phase, which is good.  At this point, I would experiment with your bistream.  Do you get the same results at all M-aries?  Or does the BER increase as the M-ary increases?  I hope this helps and let me know if you have any more questions.

Regards,

David Hall | Applications Engineering | National Instruments

[Jdjames]

David,

I appear to be getting the same results for all M-levels.

I tried changing the PN sequence number and that had no effect.  I made sure the PN sequence number matched on both VI's.  I will try and add some to the bit stream as you suggest.

Thanks for all your help thus far!

Jenn

[SteCla]

I have the exact same issue. Was this solved? How?