UE power on (PHY) [Under UMTS]
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To understand what we talked in last two articles (Channel mapping and Insight into Uu access stratum), we need to look at overall working. Let us start with PHY procedures during UE power on.
After UE powers on, it scans the carrier frequencies for UMTS. If it is dual mode (supporting both GSM and UMTS radio interface), it may have to look for GSM carrier frequencies too. The carrier frequencies are programmed in UE or UE may use earlier camped frequencies. The initial UE process of choosing strongest cell (frequency) is known as "camping on a cell".
While scanning UMTS carrier frequencies, UE looks for known sequences transmitted over P-SCH (Primary Synchronisation CHannel). The sequences are same for all cells. P-SCH is divided by Node B in slots (15 slots in 10 ms frame as shown below). UE performs "correlations" between received signals and known P-SCH sequence. Highest correlation or strongest correlation peak is chosen. This also gives slot synchronisation. Note that the process may need to repeated for near perfect synchronisation.
Though slot is being determined by P-SCH, frame beginning is not known. S-SCH helps in frame synchronisation. S-SCH which is in-sync with P-SCH uses same slot structure, but it sends 15 codes over a 10 ms frame, each in one slot as shown in diagram above. UE now has to perform correlations between received signal and 16 possible (known) S-SCH codes. By this correlations, UE should be able to get the right sequence (i.e. Cs0, Cs1, ..., Cs14) and thereby achieving frame synchronisation. This particular sequence will belong to one of 64 possible scrambling code groups. So at the end of S-SCH procedure UE knows the scrambling code group to which cell belong.
Note that UMTS uses long scrambling codes in downlink. Though for 10 ms frame structure (with 3.84 Mcps chip rate) 218 - 1 codes are possible, 8,192 codes are used. Of 8,192 codes, 512 are taken as primary scrambling codes (and rest secondary). From 512 primary scrambling codes, 64 groups are formed with 8 codes each.
S-SCH tell UE to which scrambling code group cell below. Next step is to know the exact scrambling code from 8 possible codes. Here P-CPICH (Primary-Common PIlot CHannel) is useful. P-CPISCH transmits predefined bit sequence using known channelisation (orthogonal) code of 256 length. By doing 256 chip length correlations, UE can narrow down to correct downlink scrambling code.
So above steps have given us downlink scrambling code which makes possible for UE to look for P-CCPCH (Primary-Common Control Physical CHannel). Similar to P-CPICH, channelisation code for P-CCPCH is fixed (known). Cell specific radio information (known as system information) is broadcast over P-CCPCH. UMTS uses SFN (System Frame Number) to identify the frame. UE can now start reading this information.
Above steps are known as "cell search" procedure. If you look inside UE, cell search is RRC/PHY procedure. With this procedure, RRC/PHY finds available PLMNs and report it to layers above. Decision of choosing the PLMN is done by NAS layer (MM sublayer). MM itself may use PLMN information available in USIM (or SIM) to decide. After the decision from NAS, RRC/PHY choose the best cell (based on cell selection criteria) in PLMN selected by NAS.
UE now has to register to the network. We will look at this step in next article.
Note that even though initial cell selection is done, PHY layer has to regularly look for better cell (radio conditions change as user moves from one place to another). This procedure is known as "cell re-selection" procedure and needless to say, it is similar to cell selection procedure.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010 | § |
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