不晓得你有没底下这篇文献,
内容可能有叙述estimate procedure,
应该没那么容易限制系数就可估计出来
Have the constraints on PPP relaxed over time? Some evidence from Japan.pdf
A standard model with transportation costs implies that real exchange rates
should follow a band threshold model where the process is a random walk within
the bands and mean-reverting outside them. Because of technological improvements,
these bands should narrow over time. The evidence indicates that such a feature
is not supported by the data, casting doubts on the relevance of such models
to describe the behavior of exchange rates.