Patent application publication US2009010312, entitled METHOD OF PERFORMING CELL SEARCH IN WIRELESS COMMUNICATION SYSTEM, is a candidate for LTE DL SSs essential patent.
The cell search is the procedure by which a user equipment (UE) acquires time and frequency synchronization with a cell and detects the cell identity of the cell. The initial cell is determined according to a location of the UE at a time when the power is supplied. In general, the initial cell indicates a cell of a base station (BS) corresponding to the greatest one of signal components of all BSs, which are included in a downlink reception signal of the UE.
To facilitate the cell search, a wireless system uses a downlink channel including a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). The PSS is used to allow the UE to acquire slot synchronization. The SSS is used to allow the UE to acquire frame synchronization and a scrambling code group.
In general, cell search is classified into initial cell search, which is initially performed when a UE is powered on, and non-initial search which performs handover or neighbor cell measurement.
Wide code division multiple access (WCDMA) systems of the 3rd generation partnership project (3GPP) use a total of 512 long pseudo-noise (PN) scrambling codes in order to identify BSs. As a scrambling code of a downlink channel, each BS uses a different long PN scrambling code. When power is supplied to a UE, the UE performs downlink synchronization of a cell and acquires a long PN scrambling code identifier (ID) of the cell. To facilitate the cell search, a WCDMA system divides 512 long PN scrambling codes into 64 code groups.
In the WCDMA system, the cell search is accomplished in three steps. In the first step, a UE acquires slot synchronization by using a PSS including a primary synchronization code (PSC). A frame includes 15 slots, and each BS transmits the frame by including a PSC. Herein, the same PSC is used for the 15 slots, and all BSs use the same PSC. The UE acquires the slot synchronization by using a matched filter suitable for the PSC. In the second step, a long PN scrambling code group and frame synchronization are acquired by using the slot synchronization and also by using a SSS including a secondary synchronization code (SSC). In the third step, by using a common pilot channel code correlator on the basis of the frame synchronization and the long PN scrambling code group, the UE detects a long PN scrambling code ID corresponding to a long PN scrambling code used by the initial cell. That is, since 8 long PN scrambling codes are mapped to one long PN scrambling code group, the UE computes correlation values of all of the 8 long PN scrambling codes belonging to a code group of the UE. On the basis of the computation result, the UE detects the long PN scrambling code ID of the initial cell.
If errors occur while detecting the SSS, delay occurs when a UE performs cell search. Therefore, there is a need to improve channel detection performance in the cell search procedure.
Embodiments of the invention provide a method of performing cell search in a wireless communication system. A method is sought for improving detection performance by performing scrambling in such a manner that different scrambling codes are used for a secondary synchronization signal. A method is also sought for performing a reliable cell search by improving detection performance on the secondary synchronization signal.
The method includes acquiring an unique identity from the PSS, receiving SSS which is associated with a cell identity group, the SSS comprising two secondary synchronization codes (SSCs) and acquiring a cell identity which is defined by the unique identity within the cell identity group, wherein the two SSCs are respectively scrambled by using two different scrambling codes, the two SSCs are defined by m-sequences.
1. A method of performing cell search in a wireless communication system, the method comprising: receiving a primary synchronization signal (PSS) comprising a primary synchronization code (PSC); acquiring an unique identity from the PSS; receiving a secondary synchronization signal (SSS) which is associated with a cell identity group, the SSS comprising two secondary synchronization codes (SSCs); an acquiring a cell identity which is defined by the unique identity within the cell identity group, wherein the two SSCs are respectively scrambled by using two different scrambling codes, the two SSCs are defined by m-sequences generated by a generating polynomial x.sup.5+x.sup.2+1, and the two scrambling codes are defined by m-sequences generated by a generating polynomial x.sup.5+x.sup.3+1:
DL SSs specification in 3GPP TS36.211, V890, Section 22.214.171.124 for SSS sequence generation
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