Demapper

The constellation demapper takes packets of received constellation points as an input, and outputs the corresponding soft decision bitstream.

This module implements the de-mapping of constellation points (I/Q symbols) into bits or log-likelihood ratios (LLR). DVB-S2X specifies additional modulation schemes, including BPSK and 64APSK to 256APSK. For DVB-S2, some receivers, especially used for DTH reception, are limited to QPSK and 8-PSK modulations; this allows a rather simple (e.g. table based) hard-decision de-mapping, without resulting in major implementation loss. However, support for the higher order modulations in DVB-S2 and DVB-S2X requires more complex algorithms, producing »soft« LLR information for use in the subsequent LDPC decoder. Depending on the target technology, the de-mapper may limit the MODEM throughput, leading to pipelined or multi-instance implementation for carrier sizes significantly above 100 MSps.