How is the rhyme scheme of the couplet represented?

Typically, couplets rhyme and have the same meter. They make up a unit or complete thought.

Hush, little baby, don't you cry.

Mama's gonna sing you a lullaby.

A Rhyme Scheme is usually repeated in each Stanza of a given work, so if the first Stanza is an ABAB, generally the subsequent Stanzas are also.