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The aim of this paper is to provide a framework for cross-layer management of packet-data transmissions in MIMO systems employing orthogonal space-time block coding (STBC) over Nakagami fading channels. The framework integrates physical layer adaptive modulation and link layer truncated automatic repeat request (T-ARQ), in an attempt to maximize the system's spectral efficiency under prescribed quality of service requirements such as delay and error rate constraints. The performance of the proposed scheme is assessed via three metrics for which we derive closed-form expressions. These metrics consist of the average packet error rate, the average spectral efficiency and the outage probability. We illustrate our analysis through numerical results showing how the combined cross-layer design significantly outperforms both physical layer adaptive modulation and link layer T-ARQ when solely one of these adaptive techniques is used in conjunction with STBC.