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In this paper, designing an effective user relaying algorithm, in terms of relay node selection and power allocation, is discussed for amplify-and-forward wireless relay networks. The objective is to simplify the application of user relaying in practical wireless communication networks so that the system capacity can be improved with low computational complexity and system overhead. Beginning with the derivation of a tight threshold-based sufficient condition on the feasibility of a relay node, i.e., ensuring that user relaying via the node can achieve a larger channel capacity than direct transmission, a semi-distributed user relaying algorithm is proposed. In the proposed algorithm, each relay node can make decision on its feasibility individually, and the ultimate decision on the relay node selection among multiple feasible ones is made in a centralized manner. Since there is no need on exchanging channel state information among different network nodes, the proposed algorithm is simple for implementation and suitable for practical applications, which have stringent constraints on system overhead. By comparing with the centralized user relaying algorithm, which requires global channel state information of the whole network, the proposed semi-distributed algorithm can provide comparable system capacity, but has significantly reduced computational complexity.