Page 776 - 5G Basics - Core Network Aspects

2 Transport aspects

label corresponds to the uppermost branch for each state. The trellis diagram may be used when decoding
the trellis code by the Viterbi algorithm.

10.2.1.4 Constellation mapper
The constellation mapper maps a set of bits to a constellation point. For each symbol, L' bits shall be
extracted from the encoded data buffer (see Figure 10-5, reference point B) as defined in clause 10.2.1.4.1.
The extracted bits shall be mapped to constellation points as defined in clause 10.2.1.4.2.
10.2.1.4.1 Bit extraction

The bit extraction mechanism described in this clause is applicable only for RMC symbols and data symbols
(during showtime); for other types of symbols, see clause 10.2.2. Groups of bits shall be extracted from the
incoming data frames or from a pseudo random binary sequence (PRBS) generator for mapping to
individual subcarriers, based on the subcarrier order defined by the re-ordered tone table t' (see
clause 10.2.1.2).

For each subcarrier i of the MEDLEY set with bi > 0, the mapper shall extract b = bi bits from the data frame.
The number of bits extracted for each subcarrier is determined by the original bit allocation table b. The set
of b extracted bits shall be represented as a binary word (vb−1 vb−2 ... v1 v0), where the first bit extracted shall
be v0, the LSB. The encoder shall select a point (X, Y) from the constellation based on the b-bit word
(vb−1 vb−2 ... v1 v0) as defined in clause 10.2.1.4.2.

For each subcarrier of the MEDLEY set with bi = 0 (monitored tones, pilot tones, and sub-carrier with gi=0,
see Table 10-5), no bits shall be extracted from the data frame. Instead, for each of those subcarriers, the
encoder shall extract b = 2 bits (v1 v0) from the PRBS generator, where the first bit extracted (LSB) shall be
v0. For the pilot tone subcarrier(s), the bits extracted from the PRBS generator shall be overwritten by bits
00 (i.e., the two bits from the PRBS generator are effectively ignored).
The output bits dn of the PRBS generator shall be defined by:
d n 1  for n 1  to n 23 and

d n  d  n 18 d 3 2  n for n 23.

The PRBS generator shall be restarted at the symbol with index 0 of the first logical frame of each
superframe. The index n is incremented after each bit extraction from the PRBS generator. Upon the restart
of the PRBS, d1 shall be the first bit extracted, followed by d2, d3, etc. For each symbol position of the NOI
and DOI interval except the positions in the TA interval, the sync symbol position, and symbol positions
outside of TBUDGET, the number of bits extracted from the PRBS generator shall be twice the number of
subcarriers in the MEDLEY set with bi=0 that would be needed if the symbol position contains a data symbol
or an RMC symbol with the respective bit loading table. No bits shall be extracted from the PRBS generator
during sync symbols (see clause 10.2.2.1).

For subcarriers that are not in the MEDLEY set (bi = 0 by definition), no bits shall be extracted from the
encoded data buffer and no bits shall be extracted from the PRBS generator. Instead, the constellation
mapper may select a vendor-discretionary (X, Y) point (which may change from symbol to symbol and
which does not necessarily coincide with any of the constellation points defined in this Recommendation).
10.2.1.4.2 Constellations

The defined algorithmic constellation mapper shall be used to construct subcarrier quadrature amplitude
modulation (QAM) constellations with a minimum number of bits equal to one and a maximum number of
bits equal to 12, 13 or 14 (based on the "FTU-O maximum bit loading" and "FTU-R maximum bit loading"
capability indicated during initialization in the O-MSG 1 and R-MSG 2 messages for the FTU-O and FTU-R,
respectively.
NOTE – Supporting a maximum bit loading of 13 or 14 may lead to increased power consumption at the transmitter or
the receiver or both relative to supporting a maximum bit loading of 12.

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