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PDSCH modulationV15.8.0

Toolbox: PDSCH signal generator
Input bits (0,1,...)
scrambling seq (0,1,...)
scrambled bits (0,1,...)
Modulation scheme
Modulated signal
Layer mapped x[0]
Layer mapped x[1]


Up to two codewords q ∈ {0,1} can be transmitted. In case of single-codeword transmission, q = 0.

For each codeword q, the UE shall assume the block of bits , where is the number of bits in codeword q transmitted on the physical channel, are scrambled prior to modulation, resulting in a block of scrambled bits according to

where the scrambling sequence is given by clause 5.2.1 Pseudo-random sequence generation. The scrambling sequence generator shall be initialized with


  • equals the higher-layer parameter dataScramblingIdentityPDSCH if configured and the RNTI equals the C-RNTI, MCS-C-RNTI, or CS-RNTI, and the transmission is not scheduled using DCI format 1_0 in a common search space,
  • otherwise

and where nRNTI corresponds to the RNTI associated with the PDSCH transmission as described in clause 5.1 of TS 38.214. RNTI occupies 16 bits, ranges from 1 to 65535.

What it means: scrambling sequence has length same as the input bits. For each input bit, scrambling means to logically XOR it with the corresponding scrambling bit. On the receiver side, the XOR operation is performed again with the same scrambling sequence to recover the original bits. The scrambling scequence is only known by the transmitter and the intended receiver, thus offering protection agains information leakage.


For each codeword q, the UE shall assume the block of scrambled bits are modulated as described in using one of the modulation schemes in Table, resulting in a block of complex-valued modulation symbols .

Table Supported modulation schemes.
Modulation schemeModulation order Qm

Layer mapping

The UE shall assume that complex-valued modulation symbols for each of the codewords to be transmitted are mapped onto one or several layers according to Table Complex-valued modulation symbols for codeword q shall be mapped onto the layers where 𝜐 is the number of layers and is the number of modulation symbols per layer.

What it means:

  • PDSCH with ≤ 4 layers uses only one codeword. Each layer has 1/v the number of total modulation symbols.
  • PDSCH with > 4 layers uses two codewords. Codeword 0 maps onto the first layers, with size of ; Codeword 1 maps onto the last layers, with size of .
Table Codeword-to-layer mapping for spatial multiplexing.
Number of layersNumber of codewordscodeword-to-layer mapping