N210 + XCVR2450 Lost Constellation Aspects in Communication between 2 USRPs ( using GNU Radio )

[Hompeaz]

Hi All,

I am trying to build a communication system using 16QAM modular using 2 USRPs ( both N210 + XCVR2450 ), but the constellation aspects somehow lost ( shown in the figs below )

Transmitter :

Received :

The flowgraph in gnu radio are listed below : 

Transmitter : 

Receiver :

Center Freq is set to 5.5e9 & Bandwidth is set to 36,000,000. I tried to change the frequency or the bandwidth but there was no help.

Do you have any idea what might be the cause?

Thank you so mush.

[ni_hj]

Hi Hompeaz,

Sample rate can not be less than bandwidth. Sample rate must be at least two times higher than bandwidth, "Nyquist rate".

for N210:

• 50 MHz of RF bandwidth with 8 bit samples
• 25 MHz of RF bandwidth with 16 bit samples

check what is size of your samples, and then follow it. Although in practice you need to set less than it.

So, increase sample rate and decrease bandwidth accordingly.

[Hompeaz]

Thank you for the reply. I tried several bandwidth and sample frequency but all the figures will stuck if the sample rate sets too high. ( The waterfall sink stop flowing and every other figures no longer changes over time. ) 

While the sample rate is less than 10,000,000 Hz it works fine. However, in either case, the constellation aspects stays the same. 

[ni_hj]

Hi,

you can not use sample rate as 50,000,000 Msps, decrease it to 10,000,000 Msps.
it seems your flowgraphs are not correct, please double check. check online for sample code for 16QAM encoder/decode examples.

also increase sample per symbol from 2 to 4 or 8.
in your receiver scale the IQ plot, receive signal will be weaker than transmit signal.

check your antenna port for TX and RX, and in your code set correctly.

it seems your ratio for bandwidth, sample rate, and others is not correct, try to solve the warning you have in the left down corner!

[Hompeaz]

Thank you so much for the suggestions. My advisor asked me to do this. I don't even know much about either communications or USRPs. I'll try my best to solve this problem following your suggestions. 

[Hompeaz · Accepted solution]

Sorry for asking questions again. I really appreciate your patient.

The USRP I used might have a bandwidth limit :

So that sample rate can not reach 2 * bandwidth ( since I have to decrease the sample rate to 10,000,000 Msps )

I've searching for gnuradio 16QAM examples these days but there's no luck. I tried with some examples but they are either have bugs or outdated.

e.g. This example looks okay : https://wiki.gnuradio.org/index.php/Constellation_Object

but there may be something wrong with the "root_raised_cosine"