Demapper

Demapper class is used for demodulating the estimated “qam”, “bpsk” or “\(\frac{\pi}{2}\)-bpsk” symbols. The demapper supports two implementations

  1. Max-log-MAP: Sub-optimal (3dB performance loss for AWGN channel) but has an order lower complexity than Log-MAP. It is configured using demapping_method = “maxlog”

  2. Log-MAP: Optimal performance for AWGN channels. It is configured using demapping_method = “app”

The decoder can be configured to return soft or hard output by using hard_out flag. The demapper for “qam” methods is imported from Sionna-demapper.

Important

  • When the output of demapper is passed to a channel coder or descrambler, the channel coder should be configured to either accept the hard inputs or the hard inputs must be converted to bipolar \(\{-1, 1\}\) format.

The demapping is performed as follows:

Log-MAP bpsk Demapper generating soft output:

demapMethod = "app" # Demapping method
constType = "bpsk"  # Symbol mapping type
modOrder  = 1       # Mordulation order or number of bits per symbol
hard_out  = False   # Whether to return hard output or soft output

# Symbol demapping object
demapper  = Demapper(demapMethod, constType, modOrder, hard_out = hard_out)
llr       = demapper([symbols, 1/snr]) # LLR Computation

Max-Log-MAP QAM Demapping generating hard output:

demapMethod = "maxlog" # Demapping method
constType = "qam"  # Symbol mapping type
modOrder  = 4       # Mordulation order or number of bits per symbol
hard_out  = True   # Whether to return hard output or soft output

# Symbol demapping object
demapper  = Demapper(demapMethod, constType, modOrder, hard_out = hard_out)
bitEst    = demapper([symbols, 1/snr]) # LLR Computation

The details about the input-output interface of the Symbol demapper detailed below.

class toolkit5G.SymbolMapping.Demapper(*args, **kwargs)[source]

Generates the channel state information reference sequence (CSI-RS) for 5G system. The constructor expects scramblingID, slot index and, symbol index. The call method expect the length to be generated for the PRS and returns 1 or multiple CSI-RS based on the size of slot index and symbol index passed as input. The details about the implementation of the sequence are provided in section 7.4.1.7 of [3GPPTS38211_CSIRS].

Parameters:

  • demapping_method (str) – Defines the demapping method \(\in \{\)“app”, “maxlog”}.

  • constellation_type (str) – Defines symbol modulation/mapping type \(\in \{\)“qam”,”bpsk”,”pi/2-bpsk”\(\}\).

  • num_bits_per_symbol (int) – Defines the modulation order (number of bits per symbol).

  • constellation (sionna constellation class) – Defines the sionna constellation class object. It is used for defining custom symbol mapping.

  • hard_out (bool) – Defines the type of output demapper will return.

  • dtype (NumPy data type) – Defines the data type for the output. The type conversion is supported as per NumPy type conversion.

  • **kwargs (keyValue) – keyvalue pairs for Sionna Demapper.

Input:

inputs ([…, numSymbols], np.complex64/np.complex128) – Define input symbols to be mapped to either log likelihood ratios or bits.

Output:

[…, numSymbols* num_bits_per_symbol], np.float32 or np.int8 – Bit estimate (when hard_out is False) or log-likelihood (when hard_out is True)

Raises:

Exception – [Error-Demapper]: ‘inputs’ must be a list of length 2