LTE Application Framework: PSS signal and UE synchronization problem

[adrikayak]

NI Software

LabView Communications System Design Suite 1.1

LTE Application Framework 1.1

Windows 7 Enterprise

NI Hardware

USRP RIO 2943R

USRP RIO 2953R

Problem Description

I am currently implementing an eNB which is allocated with a PCI = 1. I am facing problems when the UE (allocated with the same PCI = 1) is synchronizing with the eNB because it can not detect the PSS.

In the eNB part, I have been investigating how the PSS is generated and transmitted. I have found that LTE DL TX PSS.gcdl is the function in charge of generating one or another PSS sequence among the 3 available, depending on the (PCI mod 3) value of the eNB.

In the UE part, I have been investigating how the PSS is received. I have found that LTE Cross Correlation.gcdl is the function in charge of detecting the PSS.

The idea that I have in my mind is that the received signal should be cross correlated with the 3 PSS possible sequence signals or at least the PSS sequence defined for the (PCI mod 3) value coming from a predefined PCI for the UE (NOTE: the PCI value is not fixed for UE in real LTE but as LTE App Framework has been developed only for PCI = 0, chances are that the received signal is only cross correlated with the PSS sequence defined for (PCI mod 3) = 0, being this a bit weird since the eNB has been given the chance to transmit the 3 PSS sequences, but this is another story).

My problem is that when I open LTE Cross Correlation.gcdl I only see that the received signal is the input of two obscure FIR filters and at the end we get the value of the cross correlation. I do not see how the PSS sequence is used here, I do not see any calling to the PSS sequences lookup table. Therefore, I do not know how to cross correlate the signal with a different PSS sequence (assuming that it is only using the PSS sequence for (PCI mod 3) = 0, which I think is the most likely case since the UE does not synchronize when the eNB transmits the PSS sequence for (PCI mod 3) = 1 or (PCI mod 3) = 2)

Could I get some technical explanation about how the UE synchronization is done by means of the PSS?

Troubleshooting

I have checked that the problem only arises when the eNB is transmitting a PSS sequence for (PCI mod 3) = 1 and (PCI mod 3) = 2, not for (PCI mod 3) = 0. This means that if PCI = {0, 3, 6, 9...} the synchronization works fine. This is where my guessing that the UE only tries to cross correlate the received signal with PSS sequence for (PCI mod 3) = 0 comes from.

Thank you in advance,

Adrián

[adrikayak · Accepted solution]

Just as I supposed, the received samples on the receiver side are only cross correlated with the PSS sequence for (PCI mod 3) = 0, that is the reason why the synchronization does not work when the eNB transmits with a (PCI mod 3) other than 0. The PSS sequence for (PCI mod 3) = 0 is integrated as a fixed time domain signal within the FIR filters though (both the one handling I and the one handling Q of the complex valued received signal). To change this and implement a different PSS sequence, one must open the Xilinx Core Generator of the FIR blocks and change the sequence.

I have got from the LabVIEW Communication support guys an updated version of the function "LTE Cross Correlation.gcdl" that implements a selective control for the (PCI mod 3) value and seems to change the PSS sequence inside the FIR blocks. I post it here should anyone encounters the same problem. Although, I have not tried it myself yet.