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This paper proposes adaptive discrete modulation (ADM) for opportunistic amplify-and-forward relaying networks, designed to improve system spectral efficiency while still ensuring quality of service (QoS) constraints, i.e., the target bit error rate. Based on the limited feedback signal sent by the destination, the source adjusts the transmitted modulation constellation size according to the total instantaneous received signal-to-noise ratio (SNR) at the destination. To qualify the proposed system performance, we develop performance upper bounds for the outage probability, bit error rate and achievable spectral efficiency. From numerical results, it is confirmed that the analytic results agree closely with the empirical ones and the proposed scheme with optimal switching provides a better spectral efficiency as compared to that with fixed switching at medium SNR regime.