An Orthogonal Frequency Division Multiple Access (OFDMA) downlink system with secure transmissions and delay constraints is investigated, in which a base station (BS) transmits both open data with delay limitations and private data with secrecy demands to each user. The time-varying channel is modeled as slow-fading and all channel state information (CSI) is assumed to be known to BS. An online cross-layer scheduling algorithm composed of flow control and resource allocation is proposed in this paper to maximize the downlink throughput with delay and power constraint. Furthermore, the algorithm makes decisions on current CSI instead of channel statistics. In addition, virtual queues of delay and power are constructed to track them so that those time-average constraints are fulfilled. In the end, it is proven by means of Lyapunov optimization technique that our algorithms will obtain a performance which can be extremely close to optimality and reduce operation complexity significantly.
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