Random Access in Massive MIMO by Exploiting Timing Offsets and Excess Antennas

Massive MIMO systems, where base stations are equipped with hundreds of antennas, are an attractive way to handle the rapid growth of data traffic. As the number of user equipments (UEs) increases, the initial access and handover in contemporary networks will be flooded by user collisions. In this paper, a random access protocol is proposed that resolves collisions and performs timing estimation by simply utilizing the large number of antennas envisioned in Massive MIMO networks. UEs entering the network perform spreading in both time and frequency domains, and their timing offsets are estimated at the base station in closed-form using a subspace decomposition approach. This information is used to compute channel estimates that are subsequently employed by the base station to communicate with the detected UEs. The favorable propagation conditions of Massive MIMO suppress interference among UEs whereas the inherent timing misalignments improve the detection capabilities of the protocol. Numerical results are used to validate the performance of the proposed procedure in cellular networks under uncorrelated and correlated fading channels. With $2.5\times10^3$ UEs that may simultaneously become active with probability 1\% and a total of $16$ frequency-time codes (in a given random access block), it turns out that, with $100$ antennas, the proposed procedure successfully detects a given UE with probability 75\% while providing reliable timing estimates.Comment: 30 pages, 6 figures, 1 table, submitted to Transactions on Communication

Random Access in Uplink Massive MIMO Systems: How to exploit asynchronicity and excess antennas

Massive MIMO systems, where the base stations are equipped with hundreds of antennas, are an attractive way to handle the rapid growth of data traffic. As the number of users increases, the initial access and handover in contemporary networks will be flooded by user collisions. In this work, we propose a random access procedure that resolves collisions and also performs timing, channel, and power estimation by simply utilizing the large number of antennas envisioned in massive MIMO systems and the inherent timing misalignments of uplink signals during network access and handover. Numerical results are used to validate the performance of the proposed solution under different settings. It turns out that the proposed solution can detect all collisions with a probability higher than 90%, at the same time providing reliable timing and channel estimates. Moreover, numerical results demonstrate that it is robust to overloaded situations.Comment: submitted to IEEE Globecom 2016, Washington, DC US

Kleptoplast photoacclimation state modulates the photobehaviour of the solar-powered sea slug Elysia viridis.

Some sacoglossan sea slugs incorporate intracellular functional algal chloroplasts (kleptoplasty) for periods ranging from a few days to several months. Whether this association modulates the photobehaviour of solar-powered sea slugs is unknown. In this study, the long-term retention species Elysia viridis showed avoidance of dark independently of light acclimation state. On the contrary, Placida dendritica, which shows non-functional retention of kleptoplasts, showed no preference over dark, low or high light. High light acclimated (HLac) E. viridis showed a higher preference for high light than low light acclimated (LLac) conspecifics. The position of the lateral folds (parapodia) was modulated by irradiance, with increasing light levels leading to a closure of parapodia and protection of kleptoplasts from high light exposure. Furthermore, closure of parapodia occurred at higher irradiances in HLac E. viridis. Our results strongly indicate that kleptoplasts photoacclimation state modulates the photobehaviour of the solar-powered sea slug E. viridis

Aspetti fisici e matematici della teoria della relatività ristretta

Questa tesi contiene un’esposizione della teoria della relatività ristretta. Partendo dalla fisica di fine ottocento si è descritto lo sviluppo della teoria a partire dal lavoro di Einstein del 1905. In seguito si sono considerate le principali caratteristiche possedute dalla formulazione di una meccanica coerente con la teoria della relatività. Per concludere si è data un’esposizione relativistica dell’elettromagnetismo terminando con una breve introduzione alla teoria dei campi. Si è prestata particolare attenzione ad alcuni aspetti matematici della teoria prevalentemente riconducibili ai concetti di invarianza e simmetria

DOA Estimation in the Uplink of Multicarrier CDMA Systems

We consider the uplink of a multicarrier code-division multiple-access (MC-CDMA) network and assume that the base station is endowed with a uniform linear array. Transmission takes place over a multipath channel and the goal is the estimation of the directions of arrival (DOAs) of the signal from the active users. In a multiuser scenario, difficulties are primarily due to the large number of parameters involved in the estimation of the DOAs which makes this problem much more challenging than in single-user transmissions. The solution we propose allows estimating the DOAs of different users independently, thereby leading to a significant reduction in the system complexity. In the presence of multipath propagation, however, estimating the DOAs of a given user through maximum-likelihood methods remains a formidable task since it involves a search over a multidimensional domain. Therefore, we look for simpler solutions and discuss two alternative schemes based on the SAGE and ESPRIT algorithms

An Improved Scheme for Initial Ranging in OFDMA-based Networks

An efficient scheme for initial ranging has recently been proposed by X. Fu et al. in the context of orthogonal frequency-division multiple-access (OFDMA) networks based on the IEEE 802.16e-2005 standard. The proposed solution aims at estimating the power levels and timing offsets of the ranging subscriber stations (RSSs) without taking into account the effect of possible carrier frequency offsets (CFOs) between the received signals and the base station local reference. Motivated by the above problem, in the present work we design a novel ranging scheme for OFDMA in which the ranging signals are assumed to be misaligned both in time and frequency. Our goal is to estimate the timing errors and CFOs of each active RSS. Specifically, CFO estimation is accomplished by resorting to subspacebased methods while a least-squares approach is employed for timing recovery. Computer simulations are used to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives.Comment: 6 pages, 3 figures, To appear in the Proceedings of the 2008 IEEE International Conference on Communications, Beijing, May 19 - 23, 200

A system of innovation to activate practices on open data:The Open4Citizens project

The increasing production of data is encouraging government institutions to consider the potential of open data as a public resource and to publish a large number of public datasets. This is configuring a new scenario in which open data are likely to play an important role for democracy and transparen-cy and for new innovation possibilities, in relation to the creation of a new generation of public services based on open data.In this context, though, it is possible to observe an asymmetry between the supply side of open data and the demand side. While more and more insti-tutions are producing and publishing data, there is no public awareness of the way in which such data can be used, nor is there a diffuse practice to work with those data. The definition of a practice for a large use of data is the aim of the Open4Citizens project, which promoted initiatives at different levels: at the level of immediate interaction between citizens, experts and open data, at the level of the creation of an ecosystem to work with data and at a level that could support the institutionalisation and consolidation of the new practice.<br/

A costly commitment: populism, economic performance, and the quality of bureaucracy

We study the consequences of populism for economic performance and the quality of bureaucracy. When voters lose trust in representative democracy, populists strategically supply unconditional policy commitments that are easier to monitor for voters. When in power, populists try to implement their policy commitments regardless of financial constraints and expert assessment of the feasibility of their policies, worsening government economic performance and dismantling resistance from expert bureaucrats. With novel data on more than 8,000 Italian municipalities covering more than 20 years, we estimate the effect of electing a populist mayor with a close-election regression discontinuity design. We find that the election of a populist mayor leads to smaller repayments of debts, a larger share of procurement contracts with cost overruns, higher turnover among top bureaucrats – driven by forced rather than voluntary departures – and a sharp decrease in the percentage of graduate bureaucrats