A two-level rate superposition coding relaying (SC-relaying) scheme is discussed, in which only the relay-to-source channel state information feedback (CSF) is considered and, at source, a long power constraint is imposed on the basic signal S 0 and the additional signal S 1 whose multiplexing gains are r 0 and r 1 , respectively. The CSF index is constructed based on the correct decoding of the signal S 0 at relay. For the SC-relaying scheme, we propose a multilevel iteration constructing rule for the CSF index. With the proposed CSF constructing rule, the allocation method of the power signal-to-noise ratio exponents is first obtained by the investigation of the optimal outage exponent of the signal S 0 . Secondly, we obtain the value range of p 11 , p 11 2 OE1; 1=r 0 , by studying the outage exponent lower bound of the signal S 0 , where p 11 denotes the minimum of the power signal-to-noise ratio exponent of the signal S . Lastly, to observe the interaction between the multiplexing gains r 0 and r 1 , we discuss the closed-form solution of the lower bound of the optimal outage exponent for the signal S 1 , through which we find that the multiplexing gain constraint is r 1 Ä 1 r 0 p 11 , p 11 1. The substantial numerical analyses show that the results in this paper are reasonable.