When the UE(mobile phone) is switched on, it doesnot have any idea regarding the Primary scrambling code that is used in the cell and it is not having any synchronisation with the network.
Now lets have a look at how the UE gets the time slot synchronisation and learns about Primary Scrambling code of the Cell.
When the UE is switched on, initially it will be trying to attain time slot synchronisation. For that it will start listening to Primary Synchronisation Channel (P-SCH). The slot timing of the cell can be obtained by receiving the primary synchronization channel (P-SCH) and detecting peaks in the output of a filter that is matched to this universal synchronization code. This P-SCH is sent during the first 256 chips of every slot. the whole slot is 2560 chips long. Thus the UE can identify when a time slot starts, i.e by decoding the first 256 chips, but still the UE cannot determine the slot number and hence it doesn’t have an idea regarding the radio frame boundary also.In UMTS there are 15 time slots in each radio frame.Each radio frame is of 10ms duration.
Thereafter the UE correlates the received signal from the secondary synchronization channel (S-SCH) with all secondary synchronization codes (SSC), and identifies the maximum correlation value. The S-SCH is also only sent during the first 256 chips of every slot. One SSC is sent in every time slot. There are 16 different SSCs, and they can form 64 unique secondary SCH sequences. One sequence consists of 15 SSCs, and these sequences are arranged in such a way that in any nonzero cyclic shift less than 15 of any of the 64 sequences is not equivalent to some other sequence. This means that once the UE has identified 15 successive SSCs, it can determine the code group used as well as the frame boundaries (i.e., frame synchronization).
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