4G ue doesn't sync well

Even with perfect I/Q, the UE doesn't always sync:
sudo ./lte-softmodem -O enb.band40.tm1.50PRB.FairScheduler.usrpb210.conf --rfsim --rfsimulator.serveraddr server
sudo ./lte-uesoftmodem -C 2350000000 -r 50 --rfsim -T -d --ue-scan-carrier

--ue-scan-carrier seems to improve the success rate because the ue change frequency (void function with rfsim), so it makes several synchro trials