  We analyze the achievable rate of the superposition of block Markov encoding
(decode-and-forward) and side information encoding (compress-and-forward) for
the three-node Gaussian relay channel. It is generally believed that the
superposition can out perform decode-and-forward or compress-and-forward due to
its generality. We prove that within the class of Gaussian distributions, this
is not the case: the superposition scheme only achieves a rate that is equal to
the maximum of the rates achieved by decode-and-forward or compress-and-forward
individually. We also present a superposition scheme that combines broadcast
with decode-and-forward, which even though does not achieve a higher rate than
decode-and-forward, provides us the insight to the main result mentioned above.
