- LTE Synchronization Signals (SCH)

Used for Initial System Acquisition
• Time/frequency synchronization
– Carrier frequency determination
– OFDM symbol/subframe/frame timing
• Cell ID determination (504 Possible cell IDs)
– Cell-IDs are grouped into 168 unique physical layer cell-identity groups, each group
contains 3 unique IDs (168*3 = 504) Transmitted in
• Time: subframe 0 and 5 in every radio-frame
• Frequency: middle 1.08MHz (6 RBs)
Primary synchronization signal (TX on P-SCH)
• Provides the cell ID within the cell ID group (1 of 3)
• Transmitted in last OFDM symbol of subframes 0 and 5 (provides sub-frame timing)
Secondary synchronization signal (TX on S-SCH)
• Provides the 168 cell ID group information
• M-sequences with scrambling (different “generation” method for SF0 and SF5, provides frame timing)

Cell ID Determination using P-SCH and S-SCH Signals
There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cellidentity
groups, each group containing 3 unique identities (168*3 = 504) . The grouping is such that each physical-layer cell identity is part
of one and only one physical-layer cell-identity group. A physical-layer cell identity is thus uniquely defined by a number in the range of 0
to 167, representing the physical-layer cell-identity group, and a number in the range of 0 to 2, representing the physical-layer identity
within the physical-layer cell-identity group.
Acquisition Procedure
􀁺 The initial cell search begins after the UE is switched on. At this time the UE does not know anything about the
cells around it and it begins to look for strong cells in the DL band.
􀁺 Once the UE has found a good candidate with strong 72 subcarriers (6x12) that might carry the synchronization
sequences and the BCH, the UE performs a rough frequency synchronization.
􀁺 The UE looks for the Primary Synchronization Channel (P-SCH). Once it has found it, it knows the exact carrier
frequency and the timing of the slot 0 or 10. By trial and error, the UE also knows the CP configuration.
􀁺 During the previous process, the UE also performed the first step to find the Physical Layer cell ID. Each of the 3
P-SCH sequences is linked to one group member of the 168 different cell ID groups.
􀁺 The next step is to detect the S-SCH. The SCH is transmitted 1 OFDM symbol before the
P-SCH. Once S-SCH is detected, the radio frame and the Physical Layer cell ID are perfectly known.
􀁺 The UE is ready to read BCH at this time and can also read the PLMN ID from the system information on the
BCCH.
􀁺 UE registers in the cell.
Additional information about Synchronization Sequences
Primary Synchronization Signal uses:
- Frequency domain Zadoff-Chu sequence of length 62
Secondary Synchronization Signal uses:
- Interleaved concatenation of two length-31 binary sequences. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal.
- The combination of the two length-31 sequences defining the secondary synchronization signal differs between subframe 0 and
subframe 5. according tothe method of interleaving (This difference helps the UE to identify the start of a new frame)