Cell Search Procedure in LTE

Cell Search – will search for PSS Normally a UE would find multiple cells in this process
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In LTE, there are two downlink synchronization signals which are used by the UE to obtain the cell identity and frame timing.

• Primary synchronization signal (#PSS)
• Secondary synchronization signal (#SSS)

The division into two signals is aimed to reduce the complexity of the cell search process

Primary and Secondary Secondary Sync: 

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. First, the UE has to synchronize with the network at the frame and slot level.

i) Detect PSS (Primary Synchronization Signal) for symbol timing

• #The Primary Synchronization Signal is responsible for slot synchronization.
• #The Synchronization Signal are sent only on time slots (1st and 11th) of each frame.
• ##PSS is send at the last OFDM symbol of the 1st and 11th slot of each frame.

ii)  Identification of cell ID (0,1,2) :  From #PSS, UE is also able to obtain physical layer identity (0 to 2).

Query_1: How does UE know to look for the #PSS synchronization signal?

Well, UE doesn’t need to worry much for this. As, the synchronization signal are always sent only on the center 62 sub carriers irrespective of the  channel bandwidth (1.25,3,5,10,20). Therefore, UE will look for the central sub carriers, i.e at the last OFDM symbol of the 1st time slot and again at the last OFDM symbol of the 11th slot. With this UE synchronizes at the slot level.

SSS (Secondary Synchronization Signal : 

Detect SSS (Secondary Synchronization Signal) for radio frame timing :

• The Secondary Synchronization Signal is responsible for frame synchronization.
• Using both these signals, the UE derives the Physical Cell ID of the corresponding cell.

Question_2 : #How does UE know to look for the #SSS synchronization signal?

Once, when the #PSS is identified, #SSS is always send at the slot before the #PSS is present. In other words, #SSS immediately precedes the #PSS.

Question_3: Let’s see how the UE derives the Cell ID using these two signals:

From PSS: PHYSICAL LAYER CELL IDENTITY is derived. It carries the value of 0, 1 and 2.
From SSS: PHYSICAL LAYER CELL IDENTITY GROUP is derived. It can take the value to 0 to 167.

i) Detection of cell ID group (0, ….,167): From #SSS, UE receive 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.

ii) physical layer cell identities (PCI) : 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 .

Reference signals are used in channel estimation, cell selection / reselection and handover procedures.                                                              

So total number of ID become 168 x 3 = 504

Once the UE synchronizes with the network, it proceeds to read MIB and then requires the following info for camping on a cell:

• #PLMN id
• #Tracking Area Id
• #Cell Id
• #Radio and Core n/w capabilities (for it’s n/w selection)

The above information is being broadcast by the network in order to help the UE for it’s selection process.

This broadcast is known as SYSTEM INFORMATION which is MIB and SIB’s.

The call flow for the UE to read the System information is mentioned below: