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Abstract-In orthogonal frequency division multiplexing (OFDM) systems over fast-varying fading channels, channel estimation and tracking is generally carried out by transmitting known pilot symbols in given positions of the frequency-time grid. The traditional approach consists of two steps. First, the least-squares (LS) estimate is obtained over the pilot subcarriers. Then, this preliminary estimate is interpolated/smoothed over the entire frequency-time grid. In this paper, we propose to add an intermediate step, whose purpose is to increase the accuracy of the estimate over the pilot subcarriers. The presented techniques are based on the observation that the wireless radio channel can be parametrized as a combination of paths, each characterized by a delay and a complex amplitude. The amplitudes show fast temporal variations due to the mobility of terminals while the delays (and their associated delay-subspace) are almost constant over a large number of OFDM symbols. We propose to track the delay-subspace by a subspace tracking algorithm and the amplitudes by the least mean square algorithm (or modifications of the latter). The approach can be extended to multiple input multiple output OFDM or multicarrier code-division multiple-access systems. Analytical results and simulations prove the relevant benefits of the novel structure.Index Terms-Channel estimation, fading radio channel, multiple input multiple output (MIMO), orthogonal frequency-division multiplexing (OFDM), subspace tracking.

The design of Medium Access Control (MAC) protocols for wireless sensor networks (WSNs) has been conventionally tackled by assuming battery-powered devices and by adopting the network lifetime as the main performance criterion. While WSNs operated by energy-harvesting (EH) devices are not limited by network lifetime, they pose new design challenges due to the uncertain amount of harvestable energy. Novel design criteria are thus required to capture the trade-offs between the potentially infinite network lifetime and the uncertain energy availability. This paper addresses the analysis and design of WSNs with EH devices by focusing on conventional MAC protocols, namely TDMA, Framed-ALOHA (FA) and Dynamic-FA (DFA), and by accounting for the performance trade-offs and design issues arising due to EH. A novel metric, referred to as delivery probability, is introduced to measure the capability of a MAC protocol to deliver the measure of any sensor in the network to the intended destination (or fusion center, FC). The interplay between delivery efficiency and time efficiency (i.e., the data collection rate at the FC), is investigated analytically using Markov models. Numerical results validate the analysis and emphasize the critical importance of accounting for both delivery probability and time efficiency in the design of EH-WSNs. Index TermsWireless sensor networks, multiaccess communication, energy harvesting, dynamic framed ALOHA. ). U. Spagnolini is with Politecnico di Milano. O. Simeone is with the CWCSPR, NJIT. Recent advances in low-power electronics and energy-harvesting (EH) technologies enable the design of self-sustained devices that collect part, or all, of the needed energy from the surrounding environment. Several systems can take advantage of EH technologies, ranging from portable devices to wireless sensor networks (WSNs) [1]. However, EH devices open new design issues that are different from conventional battery-powered (BP) systems [2], where the main concern is the network lifetime [3]. In fact, EH potentially allows for perpetual operation of the network, but it might not guarantee short-term activities due to temporary energy shortages [2].This calls for the development of energy management techniques tailored to the EH dynamics. While such techniques have been mostly studied at a single-device level [4], in wireless scenarioswhere multiple EH devices interact with each other, the design of EH-aware solutions needs to account for a system-level approach [5] [6]. This is the motivation of this work.In this paper, we focus on system-level design considerations for WSNs operated by EHcapable devices. In particular, we address the analysis and design of medium access control (MAC) protocols for single-hop WSNs (see Fig. 1) where a fusion center (FC) collects data from sensors in its surrounding. Specifically, we investigate how performance and design of MAC protocols routinely used in WSNs, such as TDMA [7], Framed-ALOHA (FA) and Dynamic-FA (DFA) [8], are influenced by the discontinuous energy availabi...