Descrambler

The descrambling is performed at the receiver to undo the effect of scrambling. THe descrambling is carried out in two different ways

  1. When the scrambling is carried out between the channel coder and symbol mapper, and soft information is generated from the symbol mapper for channel decoding

  2. When either the scrambling is carried out before the channel coder or the hard information is conveyed by the symbol mapper to channel coder.

This module is used for first case only. As scrambling is an inverse process, hence for the second case the descrambling can be carried out simply by passing the information through a scrambler itself.

Important

  • Scrambling and descrambling are inverse process of itself.

Tip

  • For bit information, the descrambling can be carried out using Scrambler itself.

  • For soft information (log likelihood ratios), the DeScrambler class is used.

Tip

  • For executing the following code, the output of a scrambler must be bi-polarized before passing it to the object of a DeScrambler class.

Descrambling for PBCH:

## Bits to be scrambled
cellID  = 471 # Physical cell ID
Lmax    = 8   # Maximum number of candidate SS/PBCH blocks in a half frame

# Decrambler Object for PBCH
pbchdeScr = DeScrambler("PBCH", cellID, Lmax)
# Descrambling the input Bits using the object
descrBits = pbchdeScr(scrBits)

Descrambling for PDCCH:

rnti    = 1051  # RNTI value
nID     = 18548 # Scrambling ID
# Decrambler Object for PBCH
pbchdeScr = DeScrambler("PDCCH", rnti, nID)
# Descrambling the input Bits using the object
descrBits = pbchdeScr(scrBits)

Descrambling for PDSCH:

rnti    = 1151  # RNTI value
cbIndex = 0     # Codeblock Index
nID     = 39742 # Scrambling ID
# Decrambler Object for PBCH
pbchScr = DeScrambler("PDSCH", rnti, cbIndex, nID)
# Descrambling the input Bits using the object
descrBits = pbchdeScr(scrBits)

The details about the input-output interface of the Descrambler is provided below.

class toolkit5G.Scrambler.DeScrambler(purpose, *args)[source]

Descrambles the input soft values passed to the call method. The soft inputs are descrambled using values randomly generated using a seed. This seed is calculated using the parameters passed as input the constructor. The first variable purpose defines the physical channel for whom scrambling is performed and *args accepts variable inputs with different understanding based on the purpose.

Important

  • The descrambler is used to undo the effect of scrambling when soft decoding is used at the receiver. The input and output of the descrabler are soft.

  • If the information to be descrambled is hard then scrambler itself can be used.

Parameters:

  • purpose (str) – Defines the physical channel for which the scrambling is being performed. It can take 6 values currently {“PBCH-1”, “PDCCH”, “PDSCH”, “PBCH-2”, “PSBCH”, “PSCCH”}.

  • *args – Its a Variable length argument list which accepts values based on the parameter purpose.

    If purpose == “PBCH-1”:

    • arg[0] is N_cell_ID (int) - physical cell ID \(N_{ID}^{cell} \in \{0,1,...,1007\}\)

    • arg[1] is Lmax (int) - maximum number of candidate SS/PBCH blocks in a half frame (\(L_{max}\)), \(\in \{4, 8, 64\}\).

    If purpose == “PBCH-2”:

    • arg[0] is N_cell_ID (int) - physical cell ID \(N_{ID}^{cell} \in \{0,1,...,1007\}\)

    • arg[1] is Lmax (int) - maximum number of candidate SS/PBCH blocks in a half frame (\(L_{max}\)), :math:`in {4, 8, 64}.

    • arg[2] is ssbIndex (int or Numpy array, np.int) - It is the index pointing towards the SS/PBCH Block and must be lesser than (\(L_{max}\))

    If purpose == “PDCCH”:

    • arg[0] is rnti (int or NumPy array, np.int) - It defines the radio network temporary identifier (RNTI) \(\in \{1,2,..,65519\}\).

    • arg[1] is id (int) - It is defined by pdcch-DMRS-ScramblingID \(\in \{0,1,2,..,65535\}\).

    If purpose == “PSCCH”:

    • No arguments required

    If purpose == “PDSCH”:

    • arg[0] is rnti (int or NumPy array, np.int) - It defines the radio network temporary identifier (RNTI) \(\in \{1,2,..,65519\}\).

    • arg[1] is q (int or NumPy array, np.int) - It defines codeword index \(\in \{0,1\}\).

    • arg[2] is nID (int or NumPy array, np.int) - It defines dataScramblingIdentityPDSCH(2), \(\text{n}_{\text{ID}} \in \{0,1,2,..,65535\}\).

    If purpose == “PSBCH”:

    -arg[0] is N_cell_ID (int) - Physical-layer sidelink synchronization identity. \(N_{ID}^{cell} \in \{0,1,...,671\}\).

Important

The array parameters passed for descrambling should have equal lengths.

Input:

[…, length], np.float – Input (scrambled) sequence (llrs)

Output:

[…, length], np.float – descrambled sequence (llrs).

Note

For PBCH, the length must be 32.

Raises:

  • ValueError – [Error-DeScrambler]: Invalid choice for purpose!n Valid inputs are “”PBCH-1””, “”PDCCH””, “”PBCH-2””, “”PSBCH””, “”PSCCH”” and, “”PDSCH””

  • ValueError – [Error-DeScrambler]: input must be an NumPy array of numbers!

  • ValueError – [Error-DeScrambler]: Number of MIB bits must be 32 only!

  • ValueError – [Error-DeScrambler]: rnti in PD(S|C)CHdescrambler must be an integer/array!

  • ValueError – [Error-DeScrambler]: rnti in PD(S|C)CHdescrambler must be +ve integer/array!

  • ValueError – [Error-DeScrambler]: q ‘(codeword-Index)’ in PD(S|C)CHdescrambler must be an integer/array!

  • ValueError – [Error-DeScrambler]: q ‘(codeword-Index)’ in PD(S|C)CHdescrambler must be +ve integer/array!

  • ValueError – [Error-DeScrambler]: q ‘(codeword-Index)’ in PD(S|C)CHdescrambler must be have size same as ‘rnti’!

  • ValueError – [Error-DeScrambler]: nID in PD(S|C)CHdescrambler must be an integer/array!

  • ValueError – [Error-DeScrambler]: nID in PD(S|C)CHdescrambler must be +ve integer/array!

  • ValueError – [Error-DeScrambler]: nID in PD(S|C)CHdescrambler must be have size same as ‘rnti’!

  • ValueError – [Error-DeScrambler]: rnti in pdcchdescrambler must be +ve integer!

  • ValueError – [Error-DeScrambler]: ‘PDCCH’:id in pdcchdescrambler must be +ve integer!

  • ValueError – [Error-DeScrambler]: Lmax in pdcchdescrambler must be +ve integer!

  • ValueError – [Error-Scrambler]: ssbIndex in scrambler must be lesser than (\(L_{max}\))!

  • ValueError – [Error-DeScrambler]: A: ‘size of payload’ in pdcchdescrambler must be +ve integer!

property Lmax

Defines the maximum number of candidate SS/PBCH blocks in a half frame (\(L_{max}\)), \(\in \{4, 8, 64\}\).

property c_init

Defines the seed used for generating the random variables for scrambling.

property id

Defines the pdcch-DMRS-ScramblingID \(\in \{0,1,2,..,65535\}\).

property mu

\({\nu}\) is internally calculated for scrambling PBCH-2.

property nID

Defines dataScramblingIdentityPDSCH(2), \(\text{n}_{\text{ID}} \in \{0,1,2,..,65535\}\).

property purpose

Defines the physical channel for which the scrambling is being performed. It can take 3 values currently {“PBCH-1”, “PBCH-2”, “PDCCH”,”PSCCH”, “PDSCH”}.

property q

Defines codeword index \(\in \{0,1\}\).

property rnti

Defines the radio network temporary identifier (RNTI) \(\in \{1,2,..,65519\}\).

property ssbIndex

Defines the maximum number of candidate SS/PBCH blocks in a half frame (\(L_{max}\)), \(\in \{4, 8, 64\}\).

The examples of Descrambling and scrambling can be found in Tutorials [?].