In this article, we present a main memory system for improving the lifetime and security of phase-change main memories. Storing encrypted data increases the bit-flip rates in memory cells, which adversely affects the lifetime of the phase-change memory cells. Thus, to improve the lifetime and security, the proposed system reduces the bit-flip rates by introducing two techniques. The first technique is a memory-side encryption which provides security against DIMM stealing attacks. To prevent unauthorized accesses, in this technique, the encrypted data are not saved in the main memory. As the second technique, we suggest an adaptive partial encryption approach, which makes use of behavior tracking of the application in the CPU side to minimize the latency overhead of the first technique. Additionally, it prevents the loss of data against application-based attacks. This technique uses a recurrent neural network (RNN) to do sequence classification and detect malicious applications. In addition, an auxiliary method, called periodic encryption (PE), which overcomes the security loss in some applications induced by the low accuracy of the employed neural network, is presented. The efficacy of the proposed method is evaluated using gem5 simulator and some benchmarks. Compared to DEUCE and Crypto-Comp methods, the results for the lifetime evaluation show an average bit-flip rate reduction of 11%. In addition, the security improvements against the DIMM stealing and application-based attacks are about 100% and 92.5%, respectively.

This paper addresses the degraded discrete-time Poisson wiretap channel (DT-PWC) in an optical wireless communication system based on intensity modulation and direct detection (IM-DD). Subject to nonnegativity, average-intensity, and bandwidth constraints, we find that the secrecy capacity and the entire boundary of the rate-equivocation region are attained by discrete distributions with a countably infinite number of mass points, but with finitely many mass points in any bounded interval. Additionally, we shed light on the asymptotic behavior of the secrecy capacity in the regimes where the average intensity constraint either tends to zero (low-intensity) or tends to infinity (high-intensity). In the low-intensity regime, we observe that: when the channel gains of the legitimate receiver and the eavesdropper are identical, the secrecy capacity scales linearly in the average-intensity ε; whereas when the channel gains are different, the secrecy capacity scales, to within a constant, like$(\alpha_{B}-\alpha_{E})\mathcal{E}\log\log\frac{1}{\mathcal{E}}$, where αBand αEare the legitimate receiver's and the eavesdropper's channel gains, respectively. In the high-intensity regime, we establish that the secrecy capacity does not scale with the average intensity constraint.

In this article, we present a simple, yet energy- and area-efficient method for tolerating the stuck-at faults caused by an endurance issue in secure-resistive main memories. In the proposed method, by employing the random characteristics of the encrypted data encoded by the Advanced Encryption Standard (AES) as well as a rotational shift operation, a large number of memory locations with stuck-at faults could be employed for correctly storing the data. Due to the simple hardware implementation of the proposed method, its energy consumption is considerably smaller than that of other recently proposed methods. The technique may be employed along with other error correction methods, including the error correction code (ECC) and the error correction pointer (ECP). To assess the efficacy of the proposed method, it is implemented in a phase-change memory (PCM)based main memory system and compared with three error tolerating methods. The results reveal that for a stuck-at fault occurrence rate of 10-2and with the uncorrected bit error rate of 2 × 10-3, the proposed method achieves 82% energy reduction compared to the state-of-the-art method. More generally, using a simulation analysis technique, we show that the fault coverage of the proposed method is similar to that of the state-of-the-art method.

In optical wireless communication (OWC) systems, channel estimation and detection of the transmitted symbols have been conventionally performed using analytical methods assuming that the optical channel follows a certain model, e.g., free-space model, input-dependent noise model, or Poisson model. In practical OWC systems, channels do not necessarily follow a specific model. Hence, it is difficult, if not impossible, to derive analytical models that provide optimal performance in realistic optical channels. Motivated by the success of neural networks in estimation and classification in various fields, we propose a neural network-based methodology for detection and estimation for OWC that does not rely on a channel model. Simulation results show that the proposed learning-based estimation and detection schemes achieve the optimal performance of the maximum likelihood detector under different channel state information assumptions.

This paper addresses the discrete-time Poisson wiretap channel (DT-PWC) in an optical wireless communications system based on intensity modulation and direct detection. Subject to nonnegativity and peak intensity as well as bandwidth constraints imposed on the channel input, we study the secrecy-capacity-achieving input distribution of this wiretap channel and prove it to be unique and discrete with a finite number of mass points. Furthermore, we establish that every point on the boundary of the rate-equivocation region of this wiretap channel is also obtained by a unique and discrete input distribution with a finite support. In general, the number of mass point of the optimal distributions are greater than two. This is in contrast with the continuous-time PWC where the secrecy capacity and the entire boundary of the rate-equivocation region are achieved by binary distributions when the signaling bandwidth is not restricted. Additionally, we shed light on the asymptotic behavior of the secrecy capacity in the low intensity regime and observe that the secrecy capacity scales quadratically with the peak intensity constraint. Finally, Our numerical results indicate that there is a tradeoff between the secrecy capacity and the capacity in the sense that both may not be achieved simultaneously.

This paper addresses a two-user intensity-modulation direct-detection discrete memoryless free space optical broadcast channel (DM-FSOBC) with nonnegativity, peak and average intensity constraints at the transmitter. It is shown that superposition coding along with generating the codebooks of both users according to discrete distributions with a finite number of mass points, achieve the capacity region of the DM-FSOBC. Although the capacity-achieving distributions are not necessarily unique, it is established that only discrete distributions with a finite number of mass points can achieve the points on the boundary of the capacity region.

This paper studies the optical wiretap channel with input-dependent Gaussian noise, in which the main distortion is caused by an additive Gaussian noise whose variance depends on the current signal strength. Subject to nonnegativity and peak-intensity constraints on the channel input, we first present a practical optical wireless communication scenario for which the considered wiretap channel is stochastically degraded. We then study the secrecy-capacity-achieving input distribution of this wiretap channel and prove it to be discrete with a finite number of mass points, one of them located at the origin. Moreover, we show that the entire rate-equivocation region of this wiretap channel is also obtained by discrete input distributions with a finite support. Similar to the case of the Gaussian wiretap channel under a peak-power constraint, here too, we observe that under nonnegativity and peak-intensity constraints, there is a tradeoff between the secrecy capacity and the capacity in the sense that both may not be achieved simultaneously. Furthermore, we prove the optimality of discrete input distributions in the presence of an additional average intensity constraint. Finally, we shed light on the asymptotic behavior of the secrecy capacity in the low- and high-intensity regimes. In the low-intensity regime, the secrecy capacity scales quadratically with the peak-intensity constraint. On the other hand, in the high-intensity regime, the secrecy capacity does not scale with the constraint.

Recently, work has been done on using nonvolatile cells, such as Spin Transfer Torque RAM (STT-RAM) or Magnetic RAM (M-RAM), to construct last level caches (LLC). These structures mitigate the leakage power and density problem found in traditional SRAM cells. However, the low endurance of nonvolatile caches decreases the lifetime of the LLC. Therefore, an effective wear-leveling technique is required to tackle this issue. In this paper, we propose the inter-set algorithm that distributes the write traffic to all portions of the cache. Our method is based on cluster mapping that dynamically replaces two clusters during the operation of system. Since the inter-set algorithm is based on data movement, a large amount of data must transfer in each replacement. For an efficient data movement with a minimum effect on performance, we develop the novel scheduling technique that utilizes the idle time of the LLC in the computation phase of the processors. Our approach effectively improves the lifetime of LLC with negligible performance and area overhead. Using these methods in a quad core system with 2MB LLC, we can improve the lifetime of non-volatile LLC by 30% on average.

In this work, the total spectrum sensing duration required for cognitive radios in multiband environments is studied to minimize the reactive handoff latency. Two spectrum sensing strategies, namely window-based and sample-based sensing, are evaluated to estimate channel workload and idle time probability. Channel workload is the percentage of time the band is used by other wireless networks. Idle time probability is defined as the probability of usable durations for the cognitive radio communications. The mean square error performance of the estimations is provided for both sensing strategies in the case of energy detection based sensing and realistic interarrival time distribution of packets. It is shown that, sample-based strategy requires half of the total multiband spectrum sensing duration compared to its window-based counterpart, if the hardware switching delay is under a specific threshold.

Raising concerns about the security of wireless communication led researchers develop new concepts to keep information secret from eavesdroppers. Among them, physical layer security relies on the features of the wireless channels. Most of the work in physical layer security does focus on data transmission. On the other hand the focus of this work is to use manipulation of pilot tones to enhance communication security and to reduce eavesdroppers' ability to estimate the wireless channel. Particularly, we are introducing two novel algorithms, which manipulate pilot tones according to legitimate channels' phase and amplitude characteristics. Both algorithms decrease the channel estimation quality of the eavesdropper considerably, while the amplitude based algorithm provides high quality reception at the legitimate receiver. We provide resulting pilot error rates due to proposed algorithms. In addition, we show the effect of threshold selection to channel estimation quality both at the legitimate receiver and eavesdropper.