This paper introduces hierarchical overlapped tiling, a transformation that applies loop tiling and fusion to conventional loops. Overlapped tiling is a useful transformation to reduce communication overhead, but it may also generate a significant amount of redundant computation. Hierarchical overlapped tiling performs overlapped tiling hierarchically to balance communication overhead and redundant computation, and thus has the potential to provide better performance.In this paper, we describe the hierarchical overlapped tiling optimization and its implementation in an OpenCL compiler. We also evaluate the effectiveness of this optimization using 8 programs that implement different forms of stencil computation. Our results show that hierarchical overlapped tiling achieves an average 37% speedup over traditional tiling on a 32-core workstation.
<p>Jamming attacks significantly degrade the performance of wireless communication systems and can lead to significant overhead in terms of re-transmissions and increased power consumption. Although different jamming techniques are discussed in the literature, numerous open-source implementations have used expensive equipment in the range of thousands of dollars with the exception of a few. These implementations have also tended to be partial band, and do not cover the whole available bandwidth of the system under attack. In this work, we demonstrate that flexible, reliable, and low priced software-defined radio (SDR) jamming is feasible by designing and implementing different types of jammers against IEEE 802.11n networks. First, to demonstrate the optimal jamming waveform, we present an analytical bit error rate expression of the system under attack by employing two common jamming waveforms: Gaussian noise and digitally modulated. Then, we validate this analysis through simulations using the MATLAB WLAN toolbox. Afterwards, we implement JamRF, a toolkit that employs a low-cost SDR to implement numerous types of jammers to validate the analysis. Obtained results showed that, to jam the whole 2.4GHz spectrum, a stateful-reactive jammer employing random channel hopping jamming strategy, achieves a packet loss ratio above 90%. </p>
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