Secure communication in K-user multi-antenna broadcast channel with state feedback

Abstract : In this paper, we consider the secure communication in a K-user multi-antenna broadcast channel (BC) with state feedback. We characterize the optimal secure degrees of freedom (SDoF) region of multiple-input single-output (MISO) channel. The SDoF region is achievable by a secret key based linear scheme, which generates analog secret keys by sending artificial noise and then performs space-time alignment scheme secured by these secret keys. The optimality is proved by deriving a new outer bound on the capacity region in a systematic way. Interestingly, the proposed outer bounding technique also applies to the erasure broadcast channel and provides a simpler proof as compared to the existing one. Finally, an explicit connection between the multi-antenna BC and the erasure BC is revealed. We show that, with a large number of users, secrecy cannot be guaranteed in the erasure BC, while it comes almost " for free " in the multi-antenna BC when the number of transmit antennas grows accordingly. I. INTRODUCTION In wireless communication systems, channel state information at transmitter is usually obtained via feedback from receivers. Hence, it may not be accurate due to the time-varying nature of the underlying channel as well as limited resource for the channel estimation/feedback. In fast fading scenarios, the state information may be even outdated when it becomes available at the transmitter. Recent works have shown the usefulness of such feedback, even if it is completely outdated. In [4], [5], the authors have studied the state feedback in the context of the K-user broadcast erasure channel and characterized the capacity region under some symmetry conditions. In [1], Maddah-Ali and Tse have considered the K-user multi-input single-output (MISO) broadcast channel (BC) with K antennas and characterized its degrees of freedom (DoF) region, i.e., the prelog factor of capacity at high signal-to-noise ratio (SNR), for most cases of interest. The main finding of these works is that by exploiting the multicasting opportunity created by the overheard signals (side information) at receivers, an unbounded capacity gain can be achieved. Such idea of the opportunistic multicasting has been widely applied in various multiuser networks (see e.g. [2], [6], [8] and references therein). In this work, we consider the state feedback in the K-user secured MISO broadcast channel so that the transmitter must convey each message to its intended receiver reliably while keeping it secret to all other receivers. As for the non-secured communication setups, a number of recent contributions have
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Sheng Yang, Mari Kobayashi. Secure communication in K-user multi-antenna broadcast channel with state feedback. IEEE International Symposium on Information Theory - (ISIT 2015), Jun 2015, Hong Kong, China. pp.1976-1980, ⟨10.1109/ISIT.2015.7282801⟩. ⟨hal-01261220⟩



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