![]() The SIB-1 tells about the other SIBs (SI periodicity and SI Window length), including SIB-2 which tells about the RACH information required for uplink synchronization. The copies of SIB-1 after 20 ms are different redundacny versions of the same SIB-1. The SIB-1 is sent after every 20 ms but the TTI is 80 ms (like PBCH – comes every 10 ms but the TTI is 40 ms). The UE looks for the DCI for SIB-1 by decoding the DCI masked with CRC of SI-RNTI. Now that the UE knows PHICH, PCFICH and RS location, all the other REs belong to PDCCH. PBCH tells about the system BW, System frame number, PHICH config and number of Tx. Then it goes for PBCH which is after the PSS of the first subframe. This lets the UE get the RSRP and verify that the cell is above the cell selection threshold. Once, the UE has decoded both the PSS and SSS, it can derive the PCI which tells the location of the RS and the PCFICH. The PCI (Physical Cell Identity) is made up of a combination of PSS & SSS with the following equation So, this ensures that the UE is synchronized at both frame and symbol level. That is why the SSS is different in both subframes and therefore, after decoding the SSS, the UE can understand which is the 1st subframe. ![]() As SSS & PSS exist in both Subframe 1 and 6, so the UE needs to know which of the frame is the 1st subframe. ![]() Next thing to find out is the start of the frame.Since the location of SSS and PSS differs in TDD and FDD so the UE can also find out the frame type.UE gets to know the CP length as it has the duration of two consecutive symbols (SSS and PSS) so it can derive whether the eNB is using normal CP or extended CP.After decoding SSS, the UE gets the following information. ![]() The PSS is on the 7th symbol of the 1st slot of 1st & 6th subframe while SSS lies on the 6th symbol. It provides the center frequency as it is around the DC carrier.Īfter this, the UE starts looking for the Secondary Synchronization Signal (SSS) which is just one symbol before the PSS.PSS gives the slot boundary timing independant of the CP length, so syncs at slot level.So, once the UE has decoded the PSS, it gets the following information. The location of PSS is in the 1st and 6th subframe of LTE and within the subframe it exists on the last symbol of the slot. The first thing UE needs to do is to search for Primary Synchronization Signal (PSS). ![]()
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