The literal meaning of PCFICH is the Physical Channel that carries CFI (Control Format Indicator). As you see in Downlink Frame Structure page, it is described as follows :

• It carries the number of symbols that can be used for control channels (PDCCH and PHICH).
• Mapped to the first OFDM symbol in each of the downlink sub-frameThis contains the information on number of OFDM symbols for PDCCH and PHICH symbol duration received from the PBCHUE decode this channel to figure out how many OFDM symbols are assigned for PDCCH
• It is 16 data subcarriers of the first OFDM symbol of the subframe.
• PCFICH data is carried by 4 REGs and these four REGs are evenly distributed across the whole band regardless of the bandwidth.
• The exact position of PCFICH is determined by cell ID and bandwidth.

The PCFICH is the first channel a UE decodes in every downlink subframe. Until it knows the CFI, it cannot tell where the control region ends, so it cannot find its PDCCH or the start of its PDSCH. A wrong CFI therefore loses the whole subframe for that UE, and this is why so few bits are protected so heavily.

Followings are the topics to be covered in this page.

• Overall Channel Processing
• Information Carried by PCFICH
• Location of PCFICH
• Reference

Overall Channel Processing

The steps to process PCFICH (Converting CFI into physical symbols) is illustrated as follows. If you are interested seriously in details of physical layer channel processing, this can be one of the good place to start. It is much simpler than PDSCH or PBCH processing, but still has all the essense of channel processing.

If you want to have some hands-on this process and visualize the final result of this process, refer to Matlab : Toolbox : PCFICH.

The diagram below shows the five steps from the CFI value to the four symbol quadruplets that go on the air. Each box names the clause of 36.211 or 36.212 that defines the step, and the labels between the boxes show how many values each step produces.

PCFICH processing chain. 2 bits of CFI become 32 coded bits, 16 QPSK symbols and 4 quadruplets z(0) to z(3).

Channel coding in 36.212 v19.3.0 clause 5.3.4 turns the CFI into a fixed 32-bit code word. The next section shows the code words. Scrambling in 36.211 v19.3.0 clause 6.7.1 uses a cell-specific sequence. It is initialised at the start of every subframe with c_init = (floor(n_s/2) + 1)(2N_ID^cell + 1)2⁹ + N_ID^cell. The PCFICH of a neighbour cell therefore looks like noise to the UE, even when both cells send the same CFI.

Modulation is always QPSK (Table 6.7.2-1), so the 32 scrambled bits give 16 symbols. Layer mapping and precoding follow clause 6.7.3. With one antenna port the symbols pass through unchanged, and with two or four ports they use transmit diversity. The PCFICH is sent on the same set of antenna ports as the PBCH. The 16 symbols then form four quadruplets, and each quadruplet fills one REG in the first OFDM symbol.

• CFI to 32 bits : a fixed code word from 36.212 Table 5.3.4-1.
• Cell-specific scrambling : re-initialised in every subframe from the PCI and the slot number.
• QPSK only : 16 symbols in 4 quadruplets.
• Same antenna ports as the PBCH : transmit diversity with 2 or 4 ports.

Information Carried by PCFICH

With high level view, the information carried by PCFICH is simple. It is CFI number which can be 1 or 2 or 3. (For the detailed meaning and implication of CFI, refer to CFI page)

The table below is 36.212 Table 5.3.4-1, which gives one 32-bit code word for each CFI value. The arrows show the CFI entering channel coding and the code word leaving it as b₀ to b₃₁.

36.212 Table 5.3.4-1. Each code word is a 3-bit pattern repeated: 011 for CFI 1, 101 for CFI 2 and 110 for CFI 3. CFI 4 is reserved.

The code is very strong for its size. Any two of the three valid code words differ in 21 or 22 of the 32 bits. The UE can therefore still pick the right CFI with up to 10 of the 32 bits wrong. The table in the current release is the same as in Release 8.

The CFI value does not always equal the number of control symbols. 36.211 v19.3.0 Table 6.7-1 allows 1, 2 or 3 PDCCH symbols when the carrier has more than 10 RB, and 2, 3 or 4 symbols when it has 10 RB or fewer. So at 1.4 MHz, CFI 1 means 2 symbols. An extended PHICH duration also sets a lower limit on the control region, as 36.213 v19.4.0 clause 9.1.3 states.

In some cases the UE takes the CFI from higher layers instead of the PCFICH. 36.213 clause 9.1.3 lists the RRC parameters cfi-SubframeNonMBSFN, cfi-SubframeMBSFN and their slot and subslot versions for short TTI. With cross-carrier scheduling, the start symbol of the PDSCH on the scheduled cell comes from pdsch-Start in RRC, not from that cell's PCFICH.

• CFI 1, 2 or 3 : CFI 4 is reserved.
• 21 or 22 bits of distance : between any two of the 32-bit code words.
• CFI + 1 symbols at 10 RB or fewer : 36.211 Table 6.7-1.
• CFI from RRC in some cases : short TTI and cross-carrier scheduling.

Location of PCFICH

Regardless of System Bandwidth, PCFICH is always carried by 4 REGs (16 REs) at the first symbol of each subframe. The exact location of these four REGs for PCFICH is determined by Physical Cell ID and System BW according to formula as shown below.

The picture below shows the formula of 36.211 clause 6.7.4 on the left and a subframe grid on the right. Quadruplet z(0) goes to the base position k-bar, and z(1) to z(3) go roughly one quarter, one half and three quarters of the band further up.

36.211 clause 6.7.4. k-bar = (N_sc^RB/2) x (N_ID^cell mod 2N_RB^DL), and the other three REGs are offset by multiples of floor(N_RB^DL/2) x N_sc^RB/2.

Note : Click here to download the Excel Spreadsheet for this calculation (The subrame tructure shown above was created by LTE Resource Grid)

Since N_sc^RB/2 = 6, k-bar is always a multiple of 6 subcarriers. That is the width of one REG in the first OFDM symbol, where each REG has 6 subcarriers with 2 of them taken by the CRS. The additions are modulo N_RB^DL x N_sc^RB, so a large k-bar wraps around to the bottom of the band. The table below works the formula out for a few cells.

Each value in the table is the first subcarrier of a REG. PCI 0 and PCI 1 place their PCFICH one REG apart, so neighbour cells with adjacent PCIs do not collide. The shift repeats every 2N_RB^DL PCIs, however. At 1.4 MHz this is only 12 PCIs, so PCI 1 and PCI 13 use the same REGs. In the PCI 99 and PCI 301 rows, the positions after the modulo wrap restart near subcarrier 0.

• 4 REGs spread over the band : about one quarter of the band apart.
• k-bar set by the PCI : PCI mod 2N_RB^DL, in steps of 6 subcarriers.
• Positions repeat every 2N_RB^DL PCIs : every 12 PCIs at 1.4 MHz.

Reference

[1] 3GPP TS 36.211 v19.3.0 - clause 6.7, Physical control format indicator channel, and Table 6.7-1

[2] 3GPP TS 36.212 v19.3.0 - clause 5.3.4, Control format indicator

[3] 3GPP TS 36.213 v19.4.0 - clause 9.1.3, Control Format Indicator assignment procedure