No abstract

The calculation of shrinkage-induced stresses and evaluation of cracking risk in concrete structures require information on shrinkage strain that always relates to the moisture decrease caused by drying or cement hydration. In the present paper, the development of internal relative humidity (RH) and global deformation in early-age normal and high-strength concrete were experimentally measured simultaneously by monitoring humidity and deformation sensors, respectively, starting from the time of concrete casting. The experimental results show that the development of the internal RH of concrete first follows a vapour-saturated stage (RH = 100%, stage I), followed by a gradually reducing stage (RH ≤ssthan 100%, stage II). Meanwhile, the global deformation behaves first as a plastic swelling for the initial several hours after casting and then shrinking occurs with a gradually reduced rate. The end of swelling normally corresponds to the point of concrete setting, at which point the transformation of fresh concrete from a liquid-like plastic state to a solid state is finished. Shrinkage has been measured even in the vapour-saturated stage with RH = 100%. In the humidity reduction stage, a linear relationship between free shrinkage strain and humidity reduction exists. In practice, the relationship of free shrinkage strain and local RH may be used to calculate the free shrinkage strain field and further to calculate the shrinkage-induced stresses in structures as well.

This is the first part of a two-part paper that studies the problem of jamming in a fixed-rate transmission system with fading, under the general assumption that the jammer has no knowledge about either the codebook used by the legitimate communication terminals, or the source's output. Both transmitter and jammer are subject to power constraints which can be enforced over each codeword (short-term / peak) or over all codewords (long-term / average), hence generating different scenarios. All our jamming problems are formulated as zero-sum games, having the probability of outage as pay-off function and power control functions as strategies. The paper aims at providing a comprehensive coverage of these problems, under fast and slow fading, peak and average power constraints, pure and mixed strategies, with and without channel state information (CSI) feedback. In this first part we study the fast fading scenario. We first assume full CSI to be available to all parties. For peak power constraints, a Nash equilibrium of pure strategies is found. For average power constraints, both pure and mixed strategies are investigated. With pure strategies, we derive the optimal power control functions for both intra-frame and inter-frame power allocation. Maximin and minimax solutions are found and shown to be different, which implies the non-existence of a saddle point. In addition we provide alternative perspectives in obtaining the optimal intra-frame power control functions under the long-term power constraints. With mixed strategies, the Nash equilibrium is found by solving the generalized form of an older problem dating back to Bell and Cover [1]. Finally, for comparison purposes, we derive a Nash equilibrium of the game in which no CSI is fed back from the receiver. We show that full channel state information brings only a very slight improvement in the system's performance. 5 an optimal solution has already been reached and show it has to satisfy a set of properties. We first assume these properties are not satisfied, and then show that under this assumption there is room for improvement. Thus we prove that any solution not satisfying our set of properties cannot be optimal (i.e. the properties are necessary). We pick the properties in such a manner that they are sufficient for the complete characterization of the optimal solution. That is, we make sure that the system of necessary properties has a unique solution.In the maximin case (when jammer plays first), assume that the jammer has already allocated some power J M to a given frame. Depending on the value of J M , and its own power constraints, the transmitter decides whether it wants to achieve reliable communication over that frame. If it decides to transmit, it needs to spend as little power as possible (the transmitter will be able to use the saved power for achieving reliable communication over another set of frames, and thus to decrease the probability of outage). Therefore, the transmitter's objective is to minimize the power P M spent for achieving reliable c...

In this paper we study the problem of halfduplex active eavesdropping in fast fading channels. The active eavesdropper is a more powerful adversary than the classical eavesdropper. It can choose between two functional modes: eavesdropping the transmission between the legitimate parties (Ex mode), and jamming it (Jx mode) -the active eavesdropper cannot function in full duplex mode. We consider a conservative scenario, when the active eavesdropper can choose its strategy based on the legitimate transmitter-receiver pair's strategyand thus the transmitter and legitimate receiver have to plan for the worst. We show that conventional physical-layer secrecy approaches perform poorly (if at all), and we introduce a novel encoding scheme, based on very limited and unsecured feedback -the Block-Markov Wyner (BMW) encoding scheme -which outperforms any schemes currently available.

Synchronization of relay nodes is an important and critical issue in exploiting cooperative diversity in wireless networks. In this paper, two asynchronous cooperative diversity schemes are proposed, namely, distributed delay diversity and asynchronous space-time coded cooperative diversity schemes. In terms of the overall diversity-multiplexing (DM) tradeoff function, we show that the proposed independent coding based distributed delay diversity and asynchronous space-time coded cooperative diversity schemes achieve the same performance as the synchronous space-time coded approach which requires an accurate symbol-level timing synchronization to ensure signals arriving at the destination from different relay nodes are perfectly synchronized. This demonstrates diversity order is maintained even at the presence of asynchronism between relay node. Moreover, when all relay nodes succeed in decoding the source information, the asynchronous space-time coded approach is capable of achieving better DM-tradeoff than synchronous schemes and performs equivalently to transmitting information through a parallel fading channel as far as the DM-tradeoff is concerned. Our results suggest the benefits of fully exploiting the space-time degrees of freedom in multiple antenna systems by employing asynchronous space-time codes even in a frequency flat fading channel. In addition, it is shown asynchronous spacetime coded systems are able to achieve higher mutual information than synchronous space-time coded systems for any finite signalto-noise-ratio (SNR) when properly selected baseband waveforms are employed.