Synchronisation Channel (SCH)
The Synchronisation Channel (SCH) is a downlink signal used for cell search. The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. Figure illustrates the structure of the SCH radio frame
Figure: Structure of Synchronisation Channel (SCH)The Primary SCH consists of a modulated code of length 256 chips, the Primary Synchronisation Code (PSC) denoted cp in figure, transmitted once every slot. The PSC is the same for every cell in the system.The Secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted csi,k in figure 18, where i = 0, 1, …, 63 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell’s downlink scrambling code belongs to. The primary and secondary synchronization codes are modulated by the symbol a shown in figure 18, which indicates the presence/ absence of STTD encoding on the P-CCPCH and is given by the following table:
| P-CCPCH STTD encoded | a = +1 |
| P-CCPCH not STTD encoded | a = -1 |
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