  Kudekar et al. proved an interesting result in low-density parity-check
(LDPC) convolutional codes: The belief-propagation (BP) threshold is boosted to
the maximum-a-posteriori (MAP) threshold by spatial coupling. Furthermore, the
authors showed that the BP threshold for code-division multiple-access (CDMA)
systems is improved up to the optimal one via spatial coupling. In this letter,
a phenomenological model for elucidating the essence of these phenomenon,
called threshold improvement, is proposed. The main result implies that
threshold improvement occurs for spatially-coupled general graphical models.
