2x2 MIMO on USRP x310 issues

[mrouili]

Hello srs community,

I am able to run 5G gNB configuration at 70Mhz TDD (n78) , 92.16 srate, using a USRP x310 without issues on SISO. I get around 200mbps on DL and about 20 on UL. However, when I try MIMO 2x2, the DL and UL rates are extremely low 0-3mbps. I tried different bandwith configurations (20Mhz,40Mhz,50Mhz) and sample rates but the problem persists. I am using the USRP's internal clock and not a GPSDO.

Any ideas on what could be causing this issue ? am I missing something to enable MIMO correctly ?

[ismagom]

Hi @mrouili, can you share the console trace output? It prints useful metrics like ri and cqi.

Also, when doing MIMO make sure the 2 TX antennas are orthogonal to each other. The TX and RX can be parallel since duplexing is TDD.

[herlesupreeth]

Hey @mrouili For the GPSDO, just make sure you are connecting the correct output (output 1/output 2) to your USRP REF IN

For the GPSDO I would suggest the following

1. Make sure the GPSDO's antenna is near a window where you can get a GPS signal
2. Then, press Find button (just ensure the output 1/ output 2 you are using has 10Mhz before pressing Find)
3. Dont worry about the GSP reference Hz, N31 and other parameters as it will be automatically populated once the GPS signal gets locked
4. Once you see PLL locked (in green) at the bottom then press Update

Then you can move away the GPSDO antenna from the window and use it for quite sometime.

[herlesupreeth]

And @mrouili you may want to remove/ comment out the following parameters in your gNB config if you are using external clock reference (GPSDO)

time_alignment_calibration: -200

[mrouili]

hi @herlesupreeth, thank you for your reply. I will make sure to report back here as soon as I am able to test this.

[pgawlowicz]

Hi @mrouili, any feedback on this issue?

[mrouili]

Hi @pgawlowicz, due to some logistical delays, not yet, unfortunately. My testbed is deployed in a lab where I can't get a strong GPS signal which requires some logistical planning to solve. I am still planning to test this so I will eventually report back when done.