This paper investigates the network lifetime optimization issue for two-hop cooperative Orthogonal Frequency Division Multiplexing (OFDM) systems recruiting multiple relays.In order to maximize the network lifetime

we propose an exhaustive algorithm which takes energy pricing concept into account.Specifically

all possible combinatorial strategies of subcarrier pairing and relay selection are firstly formulated and by applying standard Lagrange technique

the optimal power assignment which minimizes the total energy consumption subject to network throughput constraints

can be theoretically derived.Accordingly

the candidate with smaller energy sacrifice is designated as the optimum solution.Since the real world applications of the proposed approach are restricted by prohibitively high computational complexity

two low complexity alternatives are consequently designed based on the price of unit Signal-to-Noise Ratio (SNR).The essence of these methods is that relay selection and subcarrier pairing are optimized in two steps.Simulation results indicate that

the network lifetime performance of the proposed algorithms significantly outperforms the existing schemes.