In this paper, multiple-input-single-output (MISO) point-to-point communication system is considered, in which a multiple-input-multiple-output (MIMO) energy recycling (ER) transmitter is designed such that, each antenna can transmit information (multi-input) or recycle energy (multi-output) at any given point in time. The rate by such an ER-MISO communication system under an average transmission power constraint is shown to be achievable. Moreover, the optimal power allocation and the dynamic antenna selection policies that achieve the maximum communication rate are derived. The optimal dynamic antenna selection policy carefully switches the mode of the antennas between active-antennas (transmitting) and ER-antennas, where most of the harvested energy occurs from the neighboring antennas' transmissions, i.e., recycling. In addition, it is shown that, with ER, the achievable rate exceeds the capacity of the classical non-recycling counterpart. Since the complexity of the optimal dynamic antenna selection is exponential with the number of antennas, a linearithmic algorithm that has a minimal degradation in the achievable rate is proposed. Numerical results show that notable gain can be obtained by enabling ER. To address the major questions on the capability of ER and the impact of antenna coupling, hardware setup and experimental results for a four-antenna ER-transmitter are developed, based on a uniform linear array (ULA). As a result, hardware measurements indicate that the loss in the rate due to antenna coupling can be eliminated with sufficient antenna spacing and therefore, ER can achieve a significant gain.

We consider a single-cell downlink time-division duplex-based massive MIMO communication in the presence of an adversary capable of jamming and eavesdropping simultaneously. We show that the massive MIMO communication is naturally resilient to no training-phase jamming attack in which the adversary jams only the data communication and eavesdrops both the data communication and the training. Specifically, we show that the secure degrees of freedom (SDoF) attained in the presence of such an attack are identical to the maximum DoF attainable under no attack. Furthermore, we evaluate the number of base station (BS) antennas necessary in order to establish information theoretic security without even a need for Wyner encoding for a given rate of information leakage to the attacker. Next, we show that things are completely different once the adversary starts jamming the training phase. Specifically, we consider the pilot contamination attack, called training-phase jamming in which the adversary jams and eavesdrops both the training and the data communication. We show that under such an attack, the maximum achieved SDoF is identical to zero. Furthermore, the maximum achievable secure rates of users also vanish, even in the asymptotic regime in the number of the BS antennas. We finally address this attack and show that, under training-phase jamming, if the number of pilot signals is scaled in a certain way and the pilot signal assignments can be hidden from the adversary, the users achieve an SDoF identical to the maximum achievable DoF under no attack.

There has been numerous studies on the security of vehicular networks, focusing mainly at higher layers of the network stack. Vulnerabilities of vehicular communications at the physical layer have not been explored thoroughly. To that end, we study internal and external vehicular communication under a variety of attack strategies involving jamming at the physical layer. We consider a couple of attack strategies, differing in how the attacker jams the resource blocks within a targeted spectrum. In particular, we focus on flat and random jamming, which are oblivious to the existing assignments and smart jamming, which senses the assignments and allocates power in frequency accordingly. For intra- vehicular setting, we consider an attacker attached to various locations within the vehicle. We used software-defined radios in an actual vehicle to emulate potential realistic attack setups and tested the effectiveness of various attack strategies. We show that, the smart jamming attack can lead to significant degradations in communication performance, to the extent of a complete blockage even with a low jamming power, in most setups. For external communication setting, we consider a busy intersection and study uplink communication in a multicellular LTE network. Using a system-level LTE simulator we developed, we show that a flat jamming attack by a static attacker can degrade the global performance significantly, while the smart jamming attack by a stalking attacker can almost block the communication initiated from a targeted vehicle. In short, we demonstrate via real-world experiments and system-level simulations that, with an appropriate strategy, the impact of a physical layer jamming attacker can be highly detrimental to both internal and external vehicular communication, seriously threatening the security of vehicular networks.

We consider a block fading wiretap channel, where a transmitter attempts to send messages securely to a receiver in the presence of a hybrid half-duplex adversary, which arbitrarily decides to either jam or eavesdrop the transmitter-to-receiver channel. We provide bounds to the secrecy capacity for various possibilities on receiver feedback and show special cases where the bounds are tight. We show that, without any feedback from the receiver, the secrecy capacity is zero if the transmitter-to-adversary channel stochastically dominates the effective transmitter-to-receiver channel. However, the secrecy capacity is nonzero even when the receiver is allowed to feed back only one bit at the end of each block. Our novel achievable strategy improves the rates proposed in the literature for the nonhybrid adversarial model. We also analyze the effect of multiple adversaries and delay constraints on the secrecy capacity. We show that our novel time sharing approach leads to positive secrecy rates even under strict delay constraints.

We consider the wiretap channel model under the presence of a hybrid, half duplex adversary that is capable of either jamming or eavesdropping at a given time. We analyzed the achievable rates under a variety of scenarios involving different methods for obtaining transmitter CSI. Each method provides a different grade of information, not only to the transmitter on the main channel, but also to the adversary on all channels. Our analysis shows that main CSI is more valuable for the adversary than the jamming CSI in delay-limited scenarios. Similarly, in certain cases under the ergodic scenario, interestingly, no CSI may lead to higher achievable secrecy rates than with CSI.