3G/UMTS

 

 

 

Cell Search Procedure

 

Cell Search procecess in WCDMA would be described as follows (For the detailed understanding, I would recommend you to study each of the physical channels involved in the following description).

Before a UE can camp on a WCDMA cell or measure it, it has to find the timing and the identity of that cell. The downlink gives it no cell ID field to read. Instead, the UE must find the primary scrambling code, which is 1 of 512 codes. This page lists the steps, then shows how the SCH and the CPICH cut that search down to a few simple correlations.

What are the steps of the WCDMA cell search ?

The five items below name the channels that take part in the cell search and what the UE gets from each one. Read them as a list of tasks. The order in which a UE actually works through them comes right after the list.

i) Every cell is tranmitting its scrambling code(Primary Scrambling Code) via CPICH.

ii) UE detect the cell power, primary scrambling code and some addition info for compensating demodulation process

iii) UE detect P-SCH (Primary Synchronization Code) and figure out slot boundary (start and end of each slot)

iv) UE detect S-SCH (Secondary Synchronization Code) and figure out frame bounday (start and end of each frame)

v) UE detect P-CCPCH and decode MIB. Through the SYSTEM INFORMATION message carrying this MIB, UE can figure out SFN.

TS 25.214 Annex C describes the typical cell search in three steps, and the order differs from the numbering above. The UE uses the P-SCH first, because it cannot use any other channel before it knows the slot timing. It uses the S-SCH next, and the CPICH only after that. So items i) and ii) happen in the third step. The P-CCPCH comes last.

Figure 1 shows the three steps, and what the UE knows after each one.

slot timing unknown cell identified Step 1 - P-SCH Matched filter on the PSC, the same 256 chips in every cell and slot Result : slot timing Step 2 - S-SCH Correlate with the 64 15-slot SSC sequences and all cyclic shifts Result : frame timing and code group 0..63 Step 3 - CPICH Test the 8 primary scrambling codes of that code group Result : primary scrambling code 0..511 Then - P-CCPCH Decode the BCH on Cch,256,1, 20 ms TTI Result : SFN from SFNprime, then MIB

Figure 1. The three cell search steps of TS 25.214 Annex C. Each step narrows the search, so the UE never has to test all 512 primary scrambling codes one by one.

  • Step 1 uses the P-SCH : the PSC is the same in every cell, so one matched filter finds the slot timing of all nearby cells.
  • Step 2 uses the S-SCH : the SSC sequence gives the frame timing and 1 of 64 scrambling code groups.
  • Step 3 uses the CPICH : the UE tests only the 8 primary scrambling codes of that group.
  • A neighbour list shortens the search : if the UE already knows which scrambling codes to search for, steps 2 and 3 can be simplified.

How does the SCH give the slot timing, the frame timing and the code group ?

The first two steps need no knowledge of the cell at all. That is possible because the SCH is not scrambled with the cell's scrambling code. Let's see how its two parts divide the work.

The P-SCH and the S-SCH are sent together in the first 256 chips of every slot. A slot has 2560 chips, and a 10 ms radio frame has 15 slots. The Primary CCPCH is not transmitted during those first 256 chips, so the SCH has that part of the slot to itself.

The P-SCH carries the primary synchronisation code, PSC, which is the same for every cell in the system. So the UE correlates the received signal with one known 256 chip code. A peak in the output marks the start of a slot. Because every cell sends the same PSC, one matched filter finds the slot timing of every cell in range, and the strongest peak usually belongs to the best cell.

The S-SCH sends one of 16 secondary synchronisation codes, SSC, in each slot. Over the 15 slots of a frame, these SSCs form a sequence, and TS 25.213 Table 4 defines 64 such sequences, one per scrambling code group. The sequences are built so that their cyclic shifts are unique. So when the UE finds the sequence and its shift, it learns two things at once: the code group, and which slot is slot 0. Slot 0 marks the frame boundary.

The SCH also carries one extra bit of information. Both codes are modulated by a symbol that tells whether the P-CCPCH uses STTD, as TS 25.211 clause 5.3.3.5 defines. So the UE knows how to decode the P-CCPCH before it reaches the last step.

  • The SCH is in the first 256 chips of each slot : the P-CCPCH leaves that part free.
  • The PSC is common to all cells : it gives slot timing but no identity.
  • The SSC sequence carries the identity : 64 sequences for 64 code groups, and the cyclic shift gives the frame start.
  • The SCH also signals P-CCPCH STTD : through the modulation of the PSC and the SSC.

How does the UE find the scrambling code and the SFN ?

After step 2, the UE knows the frame timing and the code group, but not yet the exact code. Step 3 and the BCH decoding close that gap. They also give the UE the one timing value the SCH cannot give, which is the SFN.

TS 25.213 divides the 512 primary scrambling codes into 64 scrambling code groups of 8 codes each. So after step 2, only 8 candidates are left. The UE correlates the CPICH symbol by symbol with each of the 8 candidate codes. The CPICH carries a pre-defined bit sequence at a fixed rate of 30 kbps with SF 256, so the UE knows exactly what it should receive. The candidate with the highest correlation is the primary scrambling code of the cell.

With the scrambling code known, the UE can read the P-CCPCH. Its channelisation code is fixed to Cch,256,1, so there is nothing more to search. The P-CCPCH carries the BCH, and the BCH carries SYSTEM INFORMATION messages on the BCCH. These messages contain the MIB and the SIBs.

The SFN comes from the same messages. The SCH repeats every 10 ms, so it cannot tell one frame from the next. Instead, each SYSTEM INFORMATION message starts with the SFNprime IE. The BCH uses a 20 ms TTI, which covers two frames, so the value is always even. The first frame of the TTI has SFN = SFNprime, and the second frame has SFN = SFNprime + 1.

  • 8 codes per group : 64 groups x 8 codes = 512 primary scrambling codes, so step 3 has only 8 candidates.
  • The CPICH is a known sequence : the UE identifies the code by correlation, not by decoding.
  • The P-CCPCH needs no code search : it always uses Cch,256,1.
  • SFN comes from SFNprime : a field of the SYSTEM INFORMATION message on the BCH, not from the SCH.

Reference

[1] 3GPP TS 25.214 v19.0.0 - Physical layer procedures, FDD - clause 4.1 and Annex C, Cell search procedure

[2] 3GPP TS 25.213 v19.0.0 - Spreading and modulation, FDD - clause 5.2.2 scrambling code groups, clause 5.2.3 synchronisation codes and Table 4

[3] 3GPP TS 25.211 v19.0.0 - Physical channels, FDD - clause 5.3.3.5 SCH, CPICH and Primary CCPCH

[4] 3GPP TS 25.331 v19.0.1 - RRC - clause 10.2.48, SYSTEM INFORMATION, SFNprime