LTE Synchronization Channels
LTE, like UMTS , follows a hierarchical approach to cell
search. This makes the process easier for the UE.
There are two Synchronisation channels defined for cell
search procedure. The two channels carry the information about cell ID between
them. After the UE has obtained the information from these two synchronization channels,
it has the Physical Cell ID of the cell and is Radio Frame, Subframe and Slot
aligned with the cell.
Primary
Synchronisation channel (P-SCH) carries the primary code with value 0-2. This
indicates the cell ID within a cell ID group. The sequence to be transmitted on the primary
synchronization signal corresponding to one of the cell-Ids is generated from a
frequency-domain Zadoff-Chu sequence, which results in 3 root indexes, one each
for the 3 cell-Ids.
Secondary
Synchronisation channel (S-SCH) carries the secondary code with value 0-167.
This indicates the cell ID group, one from 168 possible groups.
The above arrangement allows for 168x3 = 504 unique Physical cell IDs. The UE computes the Physical
Cell ID as follows:
Physical Cell ID = 3 x (cell-ID group)
+ cell-ID
Mapping of P-SCH
and S-SCH onto the OFDM Resource Grid
The Synchronisation channels are restricted to the 62 sub
carriers about the DC in the resource grid. This is done to allow the UE to do
cell search with FFT size of 64, which is much easier for the UE. The position of the P-SCH and S-SCH on the OFDMA grid is shown in the figure below. This figure is for LTE FDD (Frame Structure 1 with 7 OFDMA symbols/slot).
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| LTE Synchromisation Channels in the OFDMA Resource grid |
The following points are shown in the figure above:
- The P-SCH is transmitted in the last OFDMA symbol of the 0th and 11th slot i.e. in the 1st and 5th sub frame.
- The S-SCH is transmitted in the second last OFDMA symbol of the 1st and 11th slot i.e. in the 1st and 5th sub frame.
- In other words, P-SCH occupies the last symbol of the slot=0 and slot=11, and S-SCH immediatel precedes it on the second last symbol of the slot=0 and slot=11.
- Both P-SCH and S-SCH are transmitted on 62 subcarriers centered about the DC. This is irrespective of the DL bandwidth of the LTE system. The DL bandwidth in use is communicated on the Broadcast Channel(PBCH) inside the MIB.
- The periodicity of P-SCH and S-SCH is 5ms.
- Since the position of the P-SCH and S-SCH is fixed in the sub frame and slot structure of the LTE Radio Frame, the UE gets aligned to the slot level after reading the P-SCH and S-SCH
The cell search procedure tells the UE the Physical cel ID
of the cell. The next step is reading the master Information Block (MIB) on the
Physical Broadcast Channel (PBCH).