Optimal power allocation in block fading Gaussian channels with causal CSI and secrecy constraints

The optimal power allocation that maximizes the secrecy capacity of block\nfading Gaussian (BF-Gaussian) networks with causal channel state information\n(CSI), M-block delay tolerance and a frame based power constraint is examined.\nIn particular, we formulate the secrecy capacity maximization as a dynamic\nprogram. We propose suitable linear approximations of the secrecy capacity\ndensity in the low SNR, the high SNR and the intermediate SNR regimes,\naccording to the overall available power budget. Our findings indicate that\nwhen the available power resources are very low (low SNR case) the optimal\nstrategy is a threshold policy. On the other hand when the available power\nbudget is infinite (high SNR case) a constant power policy maximizes the frame\nsecrecy capacity. Finally, when the power budget is finite (medium SNR case),\nan approximate tractable power allocation policy is derived.\n