On the Zero-Outage Secrecy-Capacity of Dependent Fading Wiretap Channels.

It is known that for a slow fading Gaussian wiretap channel without channel state information at the transmitter and with statistically independent fading channels, the outage probability of any given target secrecy rate is non-zero, in general. This implies that the so-called zero-outage secrecy capacity (ZOSC) is zero and we cannot transmit at any positive data rate reliably and confidentially. When the fading legitimate and eavesdropper channels are statistically dependent, this conclusion changes significantly. Our work shows that there exist dependency structures for which positive zero-outage secrecy rates (ZOSR) are achievable. In this paper, we are interested in the characterization of these dependency structures and we study the system parameters in terms of the number of observations at legitimate receiver and eavesdropper as well as average channel gains for which positive ZOSR are achieved. First, we consider the setting that there are two paths from the transmitter to the legitimate receiver and one path to the eavesdropper. We show that by introducing a proper dependence structure among the fading gains of the three paths, we can achieve a zero secrecy outage probability (SOP) for some positive secrecy rate. In this way, we can achieve a non-zero ZOSR. We conjecture that the proposed dependency structure achieves maximum ZOSR. To better understand the underlying dependence structure, we further consider the case where the channel gains are from finite alphabets and systematically and globally solve the ZOSC. In addition, we apply the rearrangement algorithm to solve the ZOSR for continuous channel gains. The results indicate that the legitimate link must have an advantage in terms of the number of antennas and average channel gains to obtain positive ZOSR. The results motivate further studies into the optimal dependency structures.

Stealthy Secret Key Generation.

In order to make a warden, Willie, unaware of the existence of meaningful communications, there have been different schemes proposed including covert and stealth communications. When legitimate users have no channel advantage over Willie, the legitimate users may need additional secret keys to confuse Willie, if the stealth or covert communication is still possible. However, secret key generation (SKG) may raise Willie's attention since it has a public discussion, which is observable by Willie. To prevent Willie's attention, we consider the source model for SKG under a strong secrecy constraint, which has further to fulfill a stealth constraint. Our first contribution is that, if the stochastic dependence between the observations at Alice and Bob fulfills the strict more capable criterion with respect to the stochastic dependence between the observations at Alice and Willie or between Bob and Willie, then a positive stealthy secret key rate is identical to the one without the stealth constraint. Our second contribution is that, if the random variables observed at Alice, Bob, and Willie induced by the common random source form a Markov chain, then the key capacity of the source model SKG with the strong secrecy constraint and the stealth constraint is equal to the key capacity with the strong secrecy constraint, but without the stealth constraint. For the case of fast fading models, a sufficient condition for the existence of an equivalent model, which is degraded, is provided, based on stochastic orders. Furthermore, we present an example to illustrate our results.

Comparisons of mortality and rehospitalization between hip-fractured elderly with outpatient rehabilitation and those without: A STROBE-compliant article.

Geriatric patients with hip fractures have high mortality. This study aimed to compare the mortality and rehospitalization of recipient and nonrecipient of outpatient rehabilitation in hip-fractured elderly.This retrospective cohort study used nationwide claims data in Taiwan and included 3585 senior citizen patients admitted for hip fractures between January 1, 2005, and December 31, 2012. Patients were divided into the recipient (717) and nonrecipient (2868) of outpatient rehabilitation during the first 3 months after hospital discharge. Each patient was followed up for 1 year. Mortality rates of hip-fractured elderly after discharge during the first 3-month period in different groups were analyzed with Chi-square test. Cox proportional hazards regression model was employed for both death and rehospitalization risk analyses.The mortality rate of the rehabilitation group was lower than that of the nonrehabilitation group (12.69% vs 16.70%, P < .05). A more beneficial effect was observed for patients receiving continuous rehabilitation. The rehabilitation group had a lower adjusted risk of death [hazard ratio (HR) = 0.74; 95% confidence interval (95% CI): 0.59-0.94] than that of the nonrehabilitation group. However, the rehabilitation group was at a higher risk of rehospitalization (HR = 1.37; 95% CI: 1.22-1.55).Hip-fractured elderly receiving outpatient rehabilitation have a lower risk of death but a higher risk of rehospitalization than those not receiving rehabilitation within 1 year after fracture.