We consider signal recovery from Fourier measurements using compressed sensing (CS) with wavelets. For discrete signals with structured sparse Haar wavelet coefficients, we give the first proof of near-optimal recovery from discrete Fourier samples taken according to an appropriate variable density sampling scheme. Crucially, in taking into account such structured sparsity-known as sparsity in levels-as opposed to just sparsity, this result yields recovery guarantees that agree with the empirically observed recovery properties of CS in this setting. This result complements a recent theorem in Adcock et al. [Breaking the coherence barrier: A new theory for compressed sensing, arXiv preprint arXiv: 1302.0561, 2014.], which addressed the case of continuous time signals. Moreover, we provide a significantly shorter and more expositional argument, which clearly illustrates the key factors governing recovery in this setting: namely the division of frequency space into dyadic bands corresponding to wavelet scales, the near-block diagonality of the Fourier/wavelet cross-Gramian matrix, and the structured sparsity of wavelet coefficients.

Live migration of virtual machines is now commonplace but the issue of synchronising storage remains an obstacle to wide-area migration between datacentres. We discuss the range of possible designs for synchronising disk images and argue that we have to be adaptive at the sector level to efficiently transfer disk-state to the destination. We show, using a number of production workloads, that delaying the transmission of "hot"sectors can achieve considerable bandwidth reduction over a naive eager strategy. We introduce the Activity Based Sector Synchronisation (ABSS) algorithm and show by trace-driven simulations that ABSS is able to deliver a useful trade-off with a data transmission overhead between 0.3% and 2.4% of a gigabit link whilst incurring in the majority of cases a migration latency of less than 1.1 s.

The increasing demand for wireless access in vehicular environments (WAVE) supporting a wide range of applications such as traffic safety, surveying, infotainment etc., makes robust channel access schemes a high priority. The presence of selective fading, variable topologies, high density of nodes and feasibility issues represent important challenges in vehicular networks. We present Multi-Carrier Burst Contention, a cross-layer protocol based on a contention scheme that spans both time and frequency domains, employing short and unmodulated energy bursts and a randomized and recursive node-elimination mechanism in order to resolve collisions. It can overcome many of the vehicular environment challenges and provide desirable WAVE features such as scalability, robustness, prioritized access and others. We address physical layer related challenges, present an analytical model, hardware implementation and performance results from theoretical analysis, hardware measurements and simulations, which were run in comparison with the IEEE 802.11p. The results show high scalability and resilience to channel fading and variable topologies and a considerable performance improvement over IEEE 802.11p.

With the prevalence of wireless capability in mobile devices and the increasing number of wireless network deployments where protocols using carrier sensing (such as IEEE 802.11) are employed, it is important to understand the impact of fading during the channel sensing period and its direct effects on performance in realistic environments. In this paper we evaluate the performance of IEEE 802.11 and that of our proposed cross-layer Multi-Carrier Burst Contention (MCBC) protocol, which we have implemented in hardware, in realistic indoor fading environments with no direct line of sight. We present thorough simulation results, backed up by hardware and real-world measurements, performance issues for each protocol and describe methods for increasing resilience to fading during channel sensing in order to improve performance.

With the rapid growth of WLAN capability for mobile devices such as laptops, handhelds, mobile phones and vehicles, we will witness WLANs with very large numbers of active nodes for which very efficient medium access control techniques will be needed to cope with high loads and mobility. We propose a high performance solution based on an innovative node elimination algorithm that uses short and unmodulated bursts of energy during contention - no data is exchanged. We also present a modified OFDM PHY layer, based on IEEE 802.11a, which allows sensing and bursting on individual subcarriers. We show that the protocol maintains a very low overhead and collision probability which lead to high and virtually constant network throughput at all analyzed network loads, even beyond 500 nodes. The protocol is validated by extensive simulation, comparing it against the IEEE 802.11a and SYN-MAC protocols.