Cell synchronization is the very first step when UE wants to camp on any cell. From this, UE acquires physical cell id (PCI), time slot and frame synchronization, which will enable UE to read system information blocks from a particular network.
UE will tune it radio turn by turning to different frequency channels depending upon which bands it is
supporting. Assuming that it is currently tuned to a specific band / channel, UE first finds the primary
synchronization signal (PSS) which is located in the last OFDM symbol of first time slot of the first subframe (subframe 0) of radio frame as shown in figure (green squares). This enables UE to be synchronized on subframe level. The PSS is repeated in subframe 5 which means UE is synchronized on 5ms basis since each subframe is 1ms. From PSS, UE is also able to obtain physical layer identity (0 to 2).
In the next step UE finds the secondary synchronization signal (SSS). SSS symbols are also located in the same subframe of PSS but in the symbol before PSS as shown in the figure(yellow squares). From SSS, UE is able to obtain physical layer cell identity group number (0 to 167).
Using physical layer identity and cell identity group number, UE knows the PCI for this cell now. In LTE 504 physical layer cell identities (PCI) are allowed and are divided into unique 168 cell layer identity groups where each group consist of three physical layer identity. As mentioned earlier, UE detects physical layer identity from PSS and physical layer cell identity group from SSS. Assuming physical layer identity = 1 and cell identity group=2 then the PCI for given cell is
PCI = 3*(Physical layer cell identity group)+ physical layer identity = 3*2+1 = 7
Once UE knows the PCI for a given cell, it also knows the location of cell Reference signals as shown in figure (red and black squares). Reference signals are used in channel estimation, cell selection / reselection and handover procedures.
|LTE Resource Grid (Source: http://paul.wad.homepage.dk/LTE/lte_resource_grid.html)|