PDSCH - Physical Downlink Shared Channel

Almost every bit the eNB sends to a UE travels on the PDSCH, from user data to system information and paging. The PDCCH only tells the UE where to look and how to decode. This page follows the PDSCH from a transport block to the resource elements of one subframe.

PDSCH is a physical channel that carries user data.

Carries user specific data (DL Payload).
Carries Random Access Response Message, in which case the PDSCH is scheduled by a PDCCH with RA-RNTI.
It is using AMC with QPSK, 16 QAM and 64 QAM. The modulation scheme for each PDSCH for each user is determined by MCS value in DCI.

Two transport channels map onto the PDSCH: the DL-SCH, which carries user data, SIBs and the Random Access Response, and the PCH, which carries paging. The RNTI of the scheduling PDCCH tells them apart: C-RNTI for user data, SI-RNTI for system information, P-RNTI for paging and RA-RNTI for the Random Access Response. The list of modulations above is the Release 8 set. 36.211 v19.3.0 Table 6.3.2-1 now allows QPSK, 16QAM, 64QAM, 256QAM and 1024QAM for the PDSCH. The MCS table in use decides which ones a DCI can select (see MCS vs Modulation Order).

Followings are the topics to be covered in this page.

PDSCH Processing Chain

In terms of channel processing, PDSCH is going through the most complicated process. If you understand full details of PDSCH channel processing, you can say you have understood almost everything of LTE Physical Layer, implying that it would be almost impossible to describe everything in single page without boring you out.

Refer to Channel Coding Processing for DL SCH/PCH/MCH for step (1)~(5). Refer to Precoding page for step (8),(9).

If you are also interested more in Step (8),(9) in terms of Antenna Configuration, Refer to PHY Processing page.

If you want to have some more intuitive understanding and try follow these steps on your own, Refer to Matlab : Toolbox : PDSCH.

The diagram below shows the whole chain in ten numbered steps. Steps 1 to 5 on the left are transport channel processing in 36.212, and steps 6 to 10 on the right are physical channel processing in 36.211. The labels between the boxes name the bit or symbol sequence that each step hands to the next.

PDSCH processing chain from transport block CRC attachment to resource element mapping

PDSCH processing in ten steps. The box of step 8 reads Layer Mapping/Precoding, but its clause 6.3.3 covers layer mapping only; precoding is step 9, clause 6.3.4.

The table below lists each step with its clause and what it does. The clause numbers in the diagram still match 36.212 v19.3.0 and 36.211 v19.3.0.

Step	Process	Clause	What it does
1	Transport block CRC attachment	36.212 5.3.2.1	Adds a 24-bit CRC to the transport block
2	Code block segmentation and code block CRC	36.212 5.3.2.2	Splits blocks longer than 6144 bits and, when there is more than one code block, adds a 24-bit CRC to each
3	Channel coding	36.212 5.3.2.3	Turbo code with rate 1/3: a systematic stream and two parity streams
4	Rate matching	36.212 5.3.2.4	Reads E bits per code block from a circular buffer, starting at the redundancy version
5	Code block concatenation	36.212 5.3.2.5	Joins the code blocks into one codeword of G bits
6	Scrambling	36.211 6.3.1	XORs the codeword with a Gold sequence that depends on RNTI, codeword, slot and cell
7	Modulation	36.211 6.3.2	Maps bits to QPSK, 16QAM, 64QAM, 256QAM or 1024QAM symbols
8	Layer mapping	36.211 6.3.3	Spreads one or two codewords over one to eight layers
9	Precoding	36.211 6.3.4	Maps the layers onto the antenna ports
10	Resource element mapping	36.211 6.3.5 and 6.4	Places the symbols on the REs of the allocated resource blocks

Scrambling is what makes a PDSCH belong to one UE. 36.211 clause 6.3.1 initializes the scrambling sequence at the start of each subframe with c_init = n_RNTI x 2¹⁴ + q x 2¹³ + floor(n_s/2) x 2⁹ + N_ID^cell. Here q is the codeword index and n_s the slot number. A UE that descrambles with another RNTI gets noise, and the CRC of step 1 then fails. The sequence itself is the length-31 Gold sequence of clause 7.2 (see Pseudo Random Sequence).

Steps 1 to 7 run once per codeword, and a PDSCH carries at most two codewords in one subframe. Layer mapping is where the two paths meet. With CRS-based transmission the PDSCH uses up to 4 layers, and with UE-specific reference signals on ports 7 to 14 it uses up to 8. The number of layers never exceeds the number of antenna ports used.

Ten steps : five in 36.212 for the transport channel and five in 36.211 for the physical channel.
One or two codewords : each with its own CRC, coding, rate matching, scrambling and modulation.
Scrambling by RNTI : only the addressed UE can descramble the PDSCH.
Up to 8 layers : 4 with CRS-based precoding, 8 with ports 7 to 14.

Resource Element Mapping

Which REs of the allocated resource blocks does the PDSCH actually use? Step 10 cannot fill every RE, because other signals already occupy some of them. 36.211 clause 6.4 lists what the PDSCH has to avoid, and that list decides how many coded bits G fit into the subframe.

In time, the PDSCH starts at the OFDM symbol l_DataStart of 36.213 clause 7.1.6.4. Normally that is the first symbol after the control region that the PCFICH announces, and for cross-carrier scheduling it comes from pdsch-Start in RRC. In frequency, the PDSCH takes the resource blocks that the resource allocation field of the DCI selects.

Inside those resource blocks, the PDSCH skips the REs of the cell-specific reference signals and of its own UE-specific reference signals. When the DCI uses the C-RNTI, it also skips the zero-power and non-zero-power CSI-RS configured for the UE. The PDSCH also avoids PRB pairs carrying an EPDCCH for the same UE. With ports 5 and 7 to 14 it avoids the PRB pairs of the PBCH and the synchronization signals as well. With CRS-based ports, the PDSCH is placed around the PBCH and synchronization signals RE by RE instead (see Reference Signal - Downlink).

The count matters because rate matching must produce exactly G bits. For example, one PRB pair with 2 control symbols and 2 CRS ports has 168 - 24 - 12 = 132 REs left for the PDSCH. With 16QAM that is 528 coded bits per PRB pair. The transport block size is chosen separately from the MCS, so the code rate varies with the number of REs that are free.

Start symbol : l_DataStart, set by the CFI or by pdsch-Start.
REs skipped : CRS, own DMRS, CSI-RS and ZP CSI-RS, EPDCCH PRB pairs.
PBCH and sync : whole PRB pairs skipped for ports 5 and 7 to 14.
G bits : the number of free REs times the modulation order.

Transmission Modes and Antenna Ports

How the layers of step 8 reach the antennas depends on the transmission mode, which RRC configures per UE. The mode fixes the DCI format the UE monitors in its UE-specific search space and the transmission scheme of the PDSCH. The table below follows 36.213 v19.4.0 Table 7.1-5.

Transmission mode	DCI format	PDSCH transmission scheme
1	1	Single antenna port, port 0
2	1	Transmit diversity
3	2A	Large delay CDD or transmit diversity
4	2	Closed-loop spatial multiplexing or transmit diversity
5	1D	Multi-user MIMO
6	1B	Closed-loop spatial multiplexing with a single layer
7	1	Single antenna port, port 5
8	2B	Dual layer on ports 7 and 8, or a single port 7 or 8
9	2C	Up to 8 layers on ports 7 to 14
10	2D	Up to 8 layers on ports 7 to 14, with quasi co-location signalling

Every mode also accepts DCI format 1A, in the common and the UE-specific search space. Format 1A falls back to port 0 or transmit diversity, depending on the number of PBCH antenna ports. In MBSFN subframes, modes 9 and 10 fall back to port 7 instead. The fallback lets the eNB keep scheduling a UE while an RRC reconfiguration of the mode is under way.

Modes 1 to 6 demodulate with the CRS and use the codebooks of the Precoding page. Modes 7 to 10 demodulate with UE-specific reference signals that are precoded together with the data. The UE therefore does not need to know the precoding matrix in those modes. The power of the PDSCH relative to the CRS is set separately, by P_A and P_B (see Downlink Power Allocation).

Transmission mode : set by RRC, fixes the DCI format and the PDSCH scheme.
DCI format 1A : fallback in every mode.
Modes 1 to 6 : CRS-based, with codebook precoding.
Modes 7 to 10 : UE-specific reference signals on ports 5 or 7 to 14.

Reference

[1] 3GPP TS 36.211 v19.3.0 - clauses 6.3 and 6.4, Physical downlink shared channel

[2] 3GPP TS 36.212 v19.3.0 - clause 5.3.2, Downlink shared channel, paging channel and multicast channel

[3] 3GPP TS 36.213 v19.4.0 - clause 7.1, UE procedure for receiving the physical downlink shared channel