Cell id or also known as Physical Cell Id (PCI) is an unique identity for a cell. LTE has 504 cell ids where 5G has 1008 cell ids.
In this post we will find out the answers to our following questions:
- How these cell ids are assigned to cells?
- Can we assign any random cell id to a cell? If not then what are the rules?
Cell Id is a combination of 2 different Ids: PSS Id and SSS Id.
Cell Id = PSS + 3 * SSS
Where PSS (Primary Synchronization Signal) takes values 0, 1 & 2 and SSS (Secondary Synchronization Signal) takes values 0 to 335. Therefore after every 3 Cell Ids PSS will repeat since PSS takes only 3 values.
What happens if we assign same cell Id to neighboring cells?
Cell Id is not just an identity of the cell, it is used in almost all the physical layer channels and signals to generate uncorrelated sequence to cancel out interference. Cell id use in scrambling
Even if the neighboring cells are at different frequency bands UEs which are at the overlapping zones of both the cells may find it difficult to synchronize in DL. When a UE moves from one cell to another cell handover will fail if both of them have same cell ID.
What happens if we assign same cell Id to multiple neighbors of a cells?
Here cell 02 has 2 neighbors cell 01 and cell 03 both with cell ID 10. Though both the cells 01 & 03 are not neighbors but they will create confusion for cell 02 while doing handover. Whenever a cell handover a UE to another cell, both the cells exchange lots of information.
What happens if we assign different cell Ids having same PSS Ids to the neighboring cells?
Lets consider 3 different cells 01, 02 & 03 with cell Ids 10, 13 & 16. All 3 cells have the same PSS Id and different SSS Id.
Cell 01: PSS = 1, SSS = 3
Cell 02: PSS = 1, SSS = 4
Cell 03: PSS = 1, SSS = 5
If all three cells are operating at different frequency bands then using the same PSS id won’t create any collision or interference but if all three cells are operating at the same band then the UE which is in the overlapping zone will get interference while doing the DL synchronization.
PSS is used for time and frequency synchronization in downlink and it is the first signal UE look for. Two cells with same cell id but with different PSS id will transmit exactly same PSS signal. Given that the multi path channel between both the cells and UE, distance of the cells from the UE and transmit time of PSS might differ for both of the cells, PSS transmitted from one cell may generate significant interference for the PSS transmitted from the other cell.
PBCH-DMRS collision because of the cell id
Location of the DMRS used for PBCH depends only on physical cell id. There are 4 different DMRS location inside the PBCH allocation. So if (cell id) mode 4 is same for the neighboring cells then PBCH-DMRS of both the cells might collide which may result in failure of PBCH decoding.
Failure of Uplink signals/channels decoding because of the cell id
In 5G DMRS signals used for channel estimation of PUSCH and PUCCH channels uses ZC sequence. SRS signals also generated using the ZC sequence. This ZC sequence has 30 groups of roots. These roots are associated with the cell id. So if (cell id) mode 30 is same for the neighboring cells then it might cause interference between UL signals which may cause failure of decoding at gNB.
