Achieving consensus on common global parameters through totally decentralized algorithms is a topic that has attracted considerable attention in the last few years, in view of its potential application in sensor networks. Several algorithms, along with their convergence properties, have been studied in the literature, among which the most popular are the (weighted) average consensus based schemes. One of the most critical aspects of these algorithms is that they suffer from catastrophic noise propagation. We show that the noise affecting the system state variables has a variance that grows linearly with the time index. In addition, we prove that encoding the information on the first forward difference of the state variables rather than on the state itself improves noise resilience, since it guarantees that the asymptotic value of the consensus is affected by noise with bounded variance. The results of our in-depth analysis of the effect of additive noise on consensus algorithms are valid regardless of the noise statistics and for arbitrary network topologies, i.e., arbitrary Laplacian matrices, and contain as special cases previously known results.

In this paper we consider multiple access systems where users and access points are equipped with multiple antennas. In order to exploit some of the MIMO potentials we have to resort to space-time coding. Unfortunately, such a processing may induce severe loss in terms of achievable information rates. In this paper we prove the necessary and sufficient condition ensuring that the space-time coding is information lossless, in the sense that it does not induce any modification in certain regions of achievable rates. In particular information lossless property is guaranteed if each user makes use of a trace-orthogonal design (TOD), that is a linear space-time code whose encoding matrices are orthogonal with respect to the trace inner product. Noteworthy, users can also use the same set of encoding matrices.

Trace-orthogonality is an important property of linear space-time encoders that has emerged relatively recently. In this work we carry out the theoretical performance analysis of a low complexity decoder for trace-orthogonal space-time codes based on the linear MMSE estimator. We derive the diversity order of such a decoder in the case of MIMO systems affected by uncorrelated flat Rayleigh fading, when information symbols are carved from a QPSK constellation, and are encoded using what we term a unitary trace-orthogonal design. Then we propose an iterative decoder that dramatically improves the performance over the linear MMSE decoder. Simulation results give evidence of the effectiveness of the proposed scheme

In this paper we prove that trace-orthogonal space-time coding provides a necessary and sufficient condition for information lossless coding in a multiple access system, where each user encodes its own symbols independently of the other users. Then, we show that the sub-optimal MMSE decoder can be implemented very simply as a set of scalar decoders and we prove that, under the assumption of using the (sub-optimal) MMSE decoder, the trace-orthogonal design with scaled unitary coding matrices yields minimum BER for each user.

We present a novel iterative (turbo) receiver with tunable complexity for reliable detection of (uncoded) payload data transmitted over long intersymbol interference (ISI) channels affected by crosstalk, as those typically encountered in emerging HDSL2 services standardized by ANSI T1.418 recommendation. The proposed receiver combines in an original way "minimum mean square error (MMSE) estimation principle," "turbo-processing principle" and "crosstalk-prediction principle" for achieving both suboptimal maximum a posteriori probability channel equalization and reliable soft-mitigation of ISI tail plus crosstalk. More in detail, according to the turbo-processing principle, at each iteration suitable extrinsic information is extracted from the equalizing and interference-canceling modules and used as "a priori information" for the next iteration. Several simulation results on typical HDSL-like test-loops confirm the superiority of the proposed turbo-detector (TD) over current solutions based on conventional MMSE decision-feedback equalizers and precoders (such as the Tomlinson-Harashima precoder). The numerical tests also point out that performance of the presented TD on typical high bit-rate digital subscriber lines (HDSL) is not limited by error-floor phenomena, even at error-probabilities below 10/sup -7/.

We develop the solution for optimal loading of digital subscriber loop (DSL) multicarrier (MC) systems that present constraints both on overall available energy and maximum energy per carrier. In the emerging ADSL3 systems planned for high-throughput multimedia applications the constraint on the peak-energy arises from spectral compatibility issues. However, up till now none of the published loading algorithms apparently account for peak-energy limitations neither optimal solutions for loading peak-energy constrained MC systems seem available. Therefore, starting from some results about the maximization of /spl cap/-convex functions which generalize the usual Kuhn-Tucker conditions, we present the analytical relationships characterizing the optimal peak-energy limited Loading. Thus, a low-complexity iterative algorithm implementing the optimal solution is also developed and its performance is numerically tested on an ANSI-standard ADSL3-type DSL impaired by crosstalk. Furthermore, a suitably modified version of the presented loading algorithm which guarantees integer-bit rates is presented.

In this article, quickly computable upper and lower bounds are presented on the symmetric capacity of flat-faded Rice and Nakagami channels with side information (SI) for data-transmissions via finite-size quadrature amplitude modulation (QAM) constellations. The proposed bounds exhibit the appealing feature to be tight and asymptotically exact both for high and low signal-to-noise ratios (SNRs). Furthermore, exponentially tight Chernoff-like formulas are also presented for an analytical evaluation of the resulting system outage probabilities when interleaved packet transmissions are carried out.

In this contribution, a novel version of the reduced-state maximum a posteriori probability/decision-feedback (MAP/DF) equalizer for intersymbol interference channels with long impulse responses is presented. The main feature of the proposed equalizer is that the soft statistics generated by the MAP receiver are employed to recursively compute a suitable index of the actual reliability of the (soft) decisions feeding the feedback filter. Therefore, in the presented equalizer, the usual (over-optimistic) assumption of error-free decisions at the input of the feedback filter is relaxed and improved performances are experienced, especially on channels with long impulse responses.

In this contribution, novel symbol-by-symbol (SbS)-type maximum a posteriori probability (MAP) receivers are presented for non spread-spectrum (SS) multiple-access (MAC) narrowband systems impaired by ISI and co-channel interference (CCI). The proposed decentralized type receivers minimize the error probability in the detection of the desired user message on a per-symbol basis. A practical application environment for the presented receivers can be constituted by Ethernet-type local area networks (LANs) where central offices are linked to subscribers via multitapped buses.

The symmetric cut-off rate is derived for quadrature-amplitude-modulated (QAM) block-coded data-transmissions over noisy links impaired by time-correlated multipath-phenomena which give rise to Rayleigh-distributed randomly time-variant intersymbol Interference (ISI) effects. The presented formulas for the cut-off rate are new; they are derived from a random coding bound developed "ad hoc" for bounding the ensemble-averaged block-error probability of a maximum likelihood (ML) decoder supported by perfect channel-state-information (CSI). The assumption of time-correlated ISI phenomena introduces memory in the link and makes the underlying ISI-channel a continuous-state channel with memory.