  The new 5G communications standard increases data rates and supports
low-latency communication that places constraints on the computational
complexity of channel decoders. 5G low-density parity-check (LDPC) codes have
the so-called protograph-based raptor-like (PBRL) structure which offers
inherent rate-compatibility and excellent performance. Practical LDPC decoder
implementations use message-passing decoding with finite precision, which
becomes coarse as complexity is more severely constrained. Performance degrades
as the precision becomes more coarse. Recently, the information bottleneck (IB)
method was used to design mutual-information-maximizing lookup tables that
replace conventional finite-precision node computations. Additionally, the IB
approach exchanges messages represented by integers with very small bit width.
This paper extends the IB principle to the flexible class of PBRL LDPC codes as
standardized in 5G. The extensions includes puncturing and rate-compatible IB
decoder design. As an example of the new approach, a 4-bit information
bottleneck decoder is evaluated for PBRL LDPC codes over a typical range of
rates. Bit error rate simulations show that the proposed scheme outperforms
offset min-sum decoding algorithms and operates within 0.2 dB of
double-precision sum-product belief propagation decoding.
