  This paper elaborates on the achievable schemes of interference alignment in
constant channels via finite amplify-and-forward (AF) relays. Consider $K$
sources communicating with $K$ destinations without direct links besides the
relay connections. The total number of relays is finite. The objective is to
achieve interference alignment for all user pairs to obtain half of their
interference-free degrees of freedom. In general, two strategies are employed:
coding at the edge and coding in the middle, in which relays show different
roles. The contributions are that two fundamental and critical elements are
captured to enable interference alignment in this network: channel randomness
or relativity; subspace dimension suppression.
