  In this paper, we consider the asymptotic and finite-length performance of
block Markov superposition transmission~(BMST) of short codes, which can be
viewed as a new class of spatially coupled~(SC) codes with the generator
matrices of short codes~(referred to as {\em basic codes}) coupled. A modified
extrinsic information transfer~(EXIT) chart analysis that takes into account
the relation between mutual information~(MI) and bit-error-rate~(BER) is
presented to study the convergence behavior of BMST codes. Using the modified
EXIT chart analysis, we investigate the impact of various parameters on BMST
code performance, thereby providing theoretical guidance for designing and
implementing practical BMST codes suitable for sliding window decoding. Then,
we present a performance comparison of BMST codes and SC low-density
parity-check (SC-LDPC) codes on the basis of equal decoding latency. Also
presented is a comparison of computational complexity. Simulation results show
that, under the equal decoding latency constraint, BMST codes using the
repetition code as the basic code can outperform $(3,6)$-regular SC-LDPC codes
in the waterfall region but have a higher computational complexity.
