  This paper applies error-exponent and dispersion-style analyses to derive
finite-blocklength achievability bounds for low-density parity-check (LDPC)
codes over the point-to-point channel (PPC) and multiple access channel (MAC).
The error-exponent analysis applies Gallager's error exponent to bound
achievable symmetrical and asymmetrical rates in the MAC. The dispersion-style
analysis begins with a generalization of the random coding union (RCU) bound
from random code ensembles with i.i.d. codewords to random code ensembles in
which codewords may be statistically dependent; this generalization is useful
since the codewords of random linear codes such as random LDPC codes are
dependent. Application of the RCU bound yields improved finite-blocklength
error bounds and asymptotic achievability results for i.i.d. random codes and
new finite-blocklength error bounds and achievability results for LDPC codes.
For discrete, memoryless channels, these results show that LDPC codes achieve
first- and second-order performance that is optimal for the PPC and identical
to the best-prior results for the MAC.
