Thursday, February 16, 2012

Initial UE Radio Access

Cell search is a procedure for synchronizing time and frequency to a base station sector. Additionally, cell search and synchronization include deriving basic information of the target cell.
LTE defines a hierarchical cell search as it is deployed with WCDMA UMTS. PSS and SSS provide radio frame and slot synchronization, as well as information like the duplex mode TDD or FDD and the physical layer group and c-ID.
UEs synchronizing to a new LTE cell start searching for a PSS which is a Zadoff–Chu sequence. Three sequences are defined indicating the physical cell group ID. Three physical layer c-ID groups with 168 physical layer c-IDs each are defined. After successfully detecting the PSS with its physical layer c-ID group and slot timing, the SSS is decoded which is broadcasted one OFDM symbol prior to the PSS. Now the UE retrieved DL slot, radio frame timing and frequency synchronization. With decoding successfully PSS and SSS, it obtained as well the complete 9-bit physical layer c-ID together with the radio frame type (either type 1 for FDD or type 2 for TDD) and the CP length.
Figure 1 illustrates the initial steps of cell synchronization and access. After synchronization, the UE is ready to detect and decode the PBCH in order to derive the system bandwidth, PHICH configuration, and the current System Frame Number (SFN). Other common system information now needs to be retrieved from the DL-SCH. SIBs are scheduled on regular shared channel resources by using a special C-RNTI = 0xFFFF. SIBs provide general system configuration information like UL configuration and random access configuration.

Figure 1: Initial cell access with level of retrieved information

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