This paper studies the information-theoretic secrecy rates of wireless two-way relay systems where two users wish to exchange information through a single relay with an eavesdropper observing all communications. We formulate and compare the achievable secrecy rates of the system that employs one of the three common relay protocols: conventional decode-and-forward (DF), DF with network coding (NC), and compute-and-forward (CF) based on physical-layer network coding (PNC). We show that CF based on PNC achieves the highest secrecy rate at high signal-to-noise ratio (SNR), while, interestingly, the other two protocols have mixed performance depending on the power allocation scheme and the network topology. Our study offers insights into designing wireless two-way relay protocols from a secrecy perspective.
Understanding the implications in smartphone usage and the power breakdown among hardware components has led to various energy-efficient designs for mobile systems. While energy consumption has been extensively explored, one critical dimension is often overlooked - unperceived activities that could steal a significant amount of energy behind users' back potentially. In this paper, we conduct the first exploration of unperceived activities in mobile systems. Specifically, we design a series of experiments to reveal, characterize, and analyze unperceived activities invoked by popular resident applications when an Android smartphone is left unused. We draw possible solutions inspired by the exploration and demonstrate that even an immediate remedy can mitigate energy dissipation to some extent.
Mobile systems will increasingly feature emerging OLED displays, whose power consumption is highly dependent on the image content. Existing OLED power-saving techniques change users' visual experience or degrade images' visual quality in exchange for power reduction, or seek a chance to also enhance image quality by employing a compound objective function. This paper presents a win-win scheme that always enhances image quality and reduces power consumption simultaneously. We define metrics to assess the profit and the cost for potential image enhancement and power reduction. Then, we propose algorithms that ensure the transformation of images into their quality-enhanced power-saving versions. Finally, the proposed scheme is realized as a practical camera application on mobile devices. The results of experiments conducted on a commercial tablet with a popular image database are very encouraging and provide valuable insights for future research and practices.
Reducing the communication energy is essential to facilitate the growth of emerging mobile applications. In this paper, we introduce signal strength into location-based applications to reduce the energy consumption of mobile devices for data reception. First, we model the problem of data fetch scheduling, with the objective of minimizing the energy required to fetch location-based information without impacting the application's semantics adversely. To solve the fundamental problem, we propose a dynamic-programming algorithm and prove its optimality in terms of energy savings. Then, we perform postoptimal analysis to explore the tolerance of the algorithm to signal strength fluctuations. Finally, based on the algorithm, we consider implementation issues. We have also developed a virtual tour system integrated with existing Web applications to validate the practicability of the proposed concept. The results of experiments conducted based on real-world case studies are very encouraging and demonstrate the applicability of the proposed algorithm toward signal strength fluctuations.
Improving the performance of storage systems without losing the reliability and sanity/integrity of file systems is a major issue in storage system designs. In contrast to existing storage architectures, we consider a PCM-based storage architecture to enhance the reliability of storage systems. In PCM-based storage systems, the major challenge falls on how to prevent the frequently updated (meta)data from wearing out their residing PCM cells without excessively searching and moving metadata around the PCM space and without extensively updating the index structures of file systems. In this work, we propose an adaptive wear-leveling mechanism to prevent any PCM cell from being worn out prematurely by selecting appropriate data for swapping with constant search/sort cost. Meanwhile, the concept of indirect pointers is designed in the proposed mechanism to swap data without any modification to the file system's indexes. Experiments were conducted based on well-known benchmarks and realistic workloads to evaluate the effectiveness of the proposed design, for which the results are encouraging.
This paper considers a noncoherent distributed space-frequency coded (SFC) wireless relay system with multiple relays. Each relay adopts a censoring scheme to determine whether the relay will decode and forward the source's information towards the destination. We analytically obtain the achievable diversity for both cases of perfect and imperfect relay censoring. With perfect censoring, we show that the same diversity of a conventional noncoherent SFC MIMO-OFDM system is achievable in the considered noncoherent distributed SFC system with maximum likelihood (ML) decoding, regardless of whether partial information of channel statistics and relay decoding status is available at the destination. With imperfect censoring, we analytically investigate how censoring errors affect the achievability of the system's diversity. We show that the two types of censoring errors, which correspond to useless and harmful relays, respectively, can decrease the achievable diversity significantly. Our analytical insights and numerical simulations demonstrate that the noncoherent distributed system can offer a comparable diversity as the conventional MIMO-OFDM system if relay censoring is carefully implemented.
In recent years, advances in virtualization technology have enabled multiple virtual machines to run on a physical machine, such that each virtual machine can perform independently with its own operating system. The IT industry has adopted virtualization technology because of its ability to improve hardware resource utilization, achieve low-power consumption, support concurrent applications, simplify device management, and reduce maintenance costs. However, because of the hardware limitation of storage devices, the I/O capacity could cause performance bottlenecks. To address the problem, we propose a hybrid storage access framework that exploits solid-state drives (SSDs) to improve the I/O performance in a virtualization environment.
In this paper, we study the sum degrees of freedom (DoF) of an uplink two-cell multiuser MIMO interference network with asymmetric number of users in the cells. The achievable DoF is devised based on a two-dimensional space-time spreading code framework with linear precoding/decoding design and finite channel extension. The derivation of the achievable DoF is shown related to a rank minimization problem, which corresponds to the minimization of the dimension of the interference subspace. The problem is solved by the proposed grouping algorithm (GA) based on aligning interfering signals into a low-dimensional subspace as a group and attaining the minimum number of groups. The achievable sum DoF derived based on the proposed GA is shown to be greater than prior arts and achieves the theoretic upper bound in several cases. We also give a closed-form expression of the maximum achievable sum DoF when there is the maximum number of admissible users in the considered finite diversity environment.
Native Mandarin normal-hearing (NH) listeners can easily perceive lexical tones even under conditions of great voice pitch variations across speakers by using the pitch contrast between context and target stimuli. It is however unclear whether cochlear implant (CI) users with limited access to pitch cues can make similar use of context pitch cues for tone normalization. In this study, native Mandarin NH listeners and pre-lingually deafened unilaterally implanted CI users were asked to recognize a series of Mandarin tones varying from Tone 1 (high-flat) to Tone 2 (mid-rising) with or without a preceding sentence context. Most of the CI subjects used a hearing aid (HA) in the non-implanted ear (i.e., bimodal users) and were tested both with CI alone and CI + HA. In the test without context, typical S-shaped tone recognition functions were observed for most CI subjects and the function slopes and perceptual boundaries were similar with either CI alone or CI + HA. Compared to NH subjects, CI subjects were less sensitive to the pitch changes in target tones. In the test with context, NH subjects had more (resp. fewer) Tone-2 responses in a context with high (resp. low) fundamental frequencies, known as the contrastive context effect. For CI subjects, a similar contrastive context effect was found statistically significant for tone recognition with CI + HA but not with CI alone. The results suggest that the pitch cues from CIs may not be sufficient to consistently support the pitch contrast processing for tone normalization. The additional pitch cues from aided residual acoustic hearing can however provide CI users with a similar tone normalization capability as NH listeners.
Organic light-emitting diode (OLED) technology is considered as a promising alternative to mobile displays. This paper ex- plores how to reduce the OLED power consumption by exploiting visual attention. First, we model the problem of OLED im- age scaling optimization, with the objective of minimizing the power required to display an image without adversely impacting the user’s visual experience. Then, we propose an algorithm to solve the fundamental problem, and prove its optimality even without the accurate power model. Finally, based on the algorithm, we consider implementation issues and realize two application scenarios on a commercial OLED mobile tablet. The results of experiments conducted on the tablet with real images demonstrate that the proposed methodology can achieve significant power savings while retaining the visual quality.