Construction of Near-Optimum Burst Erasure Correcting Low-Density Parity-Check Codes

In this paper, a simple, general-purpose and effective tool for the design of low-density parity-check (LDPC) codes for iterative correction of bursts of erasures is presented. The design method consists in starting from the parity-check matrix of an LDPC code and developing an optimized parity-check matrix, with the same performance on the memory-less erasure channel, and suitable also for the iterative correction of single bursts of erasures. The parity-check matrix optimization is performed by an algorithm called pivot searching and swapping (PSS) algorithm, which executes permutations of carefully chosen columns of the parity-check matrix, after a local analysis of particular variable nodes called stopping set pivots. This algorithm can be in principle applied to any LDPC code. If the input parity-check matrix is designed for achieving good performance on the memory-less erasure channel, then the code obtained after the application of the PSS algorithm provides good joint correction of independent erasures and single erasure bursts. Numerical results are provided in order to show the effectiveness of the PSS algorithm when applied to different categories of LDPC codes.Comment: 15 pages, 4 figures. IEEE Trans. on Communications, accepted (submitted in Feb. 2007

Generalized Stability Condition for Generalized and Doubly-Generalized LDPC Codes

In this paper, the stability condition for low-density parity-check (LDPC) codes on the binary erasure channel (BEC) is extended to generalized LDPC (GLDPC) codes and doublygeneralized LDPC (D-GLDPC) codes. It is proved that, in both cases, the stability condition only involves the component codes with minimum distance 2. The stability condition for GLDPC codes is always expressed as an upper bound to the decoding threshold. This is not possible for D-GLDPC codes, unless all the generalized variable nodes have minimum distance at least 3. Furthermore, a condition called derivative matching is defined in the paper. This condition is sufficient for a GLDPC or DGLDPC code to achieve the stability condition with equality. If this condition is satisfied, the threshold of D-GLDPC codes (whose generalized variable nodes have all minimum distance at least 3) and GLDPC codes can be expressed in closed form.Comment: 5 pages, 2 figures, to appear in Proc. of IEEE ISIT 200

Participatory, Visible and Sustainable. Designing a Community Website for a Minority Group

This paper tackles three aspects of community-based technological initiatives aimed to support minority groups’ public expression and communication: participation, visibility and sustainability. Participation requires\ud the active involvement of the community members in various project phases (from design to evaluation), sharing decisional power with project leaders. Visibility\ud refers to the capacity of community messages to reach a relevant audience outside the boundaries of the community itself. Sustainability indicates the capacity of a project to continue, under the control and management of the local community, beyond its “supported” lifetime. The mutual influence of these three dimensions is examined in general and also in the light of a specific case study: an initiative involving a Romani community in rural Romania, having as main outcome the development of a community website (www.romanivoices.com/podoleni)

Exploiting Pilot Mixtures in Coded Random Access

The construction of preamble sequences for channel estimation by superposition of orthogonal pilots can improve performance of massive grant-free uplink from machine-type devices. In this letter, a technique is proposed to obtain full benefit from these 'pilot mixtures' in presence of a base station with a massive number of antennas. The proposed technique consists of combining pilot mixtures with an intra-slot successive interference cancellation (SIC) algorithm, referred to as inner SIC, to increase the number of decoded messages per slot. In framed systems, the synergic effect of inner SIC and of an outer SIC algorithm across slots, typical of coded random access protocols, allows achieving a very high reliability with a low number of packet replicas per active user

Achieving sub-electron readout noise in Skipper CCDs

The readout noise for Charge-Coupled Devices (CCDs) has been the main limitation when using these detectors for measuring small amplitude signals. A readout system for a new scientific, low noise CCD is presented in this paper. The Skipper CCD architecture, and its advantages for low noise applications are discussed. A technique for obtaining sub-electron readout noise levels is presented, and its noise and signal characteristics are derived. We demonstrate a very low readout noise of $0.2e^{-}$ RMS. Also, we show the results using the detector in a low-energy X-ray detection experiment.Comment: 16 pages, 11 figure

Identification-detection group testing protocols for COVID-19 at high prevalence

Group testing allows saving chemical reagents, analysis time, and costs, by testing pools of samples instead of individual samples. We introduce a class of group testing protocols with small dilution, suited to operate even at high prevalence (5–10%), and maximizing the fraction of samples classified positive/negative within the first round of tests. Precisely, if the tested group has exactly one positive sample then the protocols identify it without further individual tests. The protocols also detect the presence of two or more positives in the group, in which case a second round could be applied to identify the positive individuals. With a prevalence of 5 % and maximum dilution 6, with 100 tests we classify 242 individuals, 92 % of them in one round and 8 % requiring a second individual test. In comparison, the Dorfman’s scheme can test 229 individuals with 100 tests, with a second round for 18.5 % of the individuals

Massive Grant-Free Access with Massive MIMO and Spatially Coupled Replicas

Massive multiple access schemes, capable of serving a large number of uncoordinated devices while fulfilling reliability and latency constraints, are proposed. The schemes belong to the class of grant-free coded random access protocols and are tailored to massive multiple input multiple output (MIMO) base station processing. High reliability is obtained owing to an intra-frame spatial coupling effect, triggered by a simple device access protocol combined with acknowledgements (ACKs) from the base station. To provide system design guidelines, analytical bounds on error floor and latency are also derived. The proposed schemes are particularly interesting to address the challenges of massive machine-type communications in the framework of next generation massive multiple access systems

An example of an infinite Steiner tree connecting an uncountable set

We construct an example of a Steiner tree with an infinite number of branching points connecting an uncountable set of points. Such a tree is proven to be the unique solution to a Steiner problem for the given set of points. As a byproduct we get the whole family of explicitly defined finite Steiner trees, which are unique connected solutions of the Steiner problem for some given finite sets of points, and with growing complexity (i.e. the number of branching points)