129 research outputs found

    Second-order parameter estimation

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    This work provides a general framework for the design of second-order blind estimators without adopting any approximation about the observation statistics or the a priori distribution of the parameters. The proposed solution is obtained minimizing the estimator variance subject to some constraints on the estimator bias. The resulting optimal estimator is found to depend on the observation fourth-order moments that can be calculated analytically from the known signal model. Unfortunately, in most cases, the performance of this estimator is severely limited by the residual bias inherent to nonlinear estimation problems. To overcome this limitation, the second-order minimum variance unbiased estimator is deduced from the general solution by assuming accurate prior information on the vector of parameters. This small-error approximation is adopted to design iterative estimators or trackers. It is shown that the associated variance constitutes the lower bound for the variance of any unbiased estimator based on the sample covariance matrix. The paper formulation is then applied to track the angle-of-arrival (AoA) of multiple digitally-modulated sources by means of a uniform linear array. The optimal second-order tracker is compared with the classical maximum likelihood (ML) blind methods that are shown to be quadratic in the observed data as well. Simulations have confirmed that the discrete nature of the transmitted symbols can be exploited to improve considerably the discrimination of near sources in medium-to-high SNR scenarios.Peer Reviewe

    The Gaussian assumption in second-order estimation problems in digital communications

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    This paper deals with the goodness of the Gaussian assumption when designing second-order blind estimation methods in the context of digital communications. The low- and high-signal-to-noise ratio (SNR) asymptotic performance of the maximum likelihood estimator - derived assuming Gaussian transmitted symbols - is compared with the performance of the optimal second-order estimator, which exploits the actual distribution of the discrete constellation. The asymptotic study concludes that the Gaussian assumption leads to the optimal second-order solution if the SNR is very low or if the symbols belong to a multilevel constellation such as quadrature-amplitude modulation (QAM) or amplitude-phase-shift keying (APSK). On the other hand, the Gaussian assumption can yield important losses at high SNR if the transmitted symbols are drawn from a constant modulus constellation such as phase-shift keying (PSK) or continuous-phase modulations (CPM). These conclusions are illustrated for the problem of direction-of-arrival (DOA) estimation of multiple digitally-modulated signals.Peer ReviewedPostprint (published version

    SINR profile for spectral efficiency optimization of SIC receivers in the many-user regime

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    © 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In dense wireless scenarios, and particularly under high traffic loads, the design of efficient random access protocols is necessary. Some candidate solutions are based on Direct- Sequence Spread Spectrum (DS-SS) combined with a Successive Interference Cancellation (SIC) demodulator, but the perfor- mance of these techniques is highly related to the distribution of the users received power. In that context, this paper presents a theoretical analysis to calculate the optimum user SINR profile at the decoder maximizing the spectral efficiency in bps/Hz for a specific modulation and practical Forward Error Correction (FEC) code. This solution is achieved by means of Variational Calculus operating in the asymptotic large-user case. Although a constant SINR function has been typically assumed in the literature (the one maximizing capacity), the theoretical results evidence that the optimum SINR profile must be an increasing function of the users received power. Its performance is compared with that of the uniform profile for two representative scenarios with different channel codes in a slightly overloaded system. The numerical results show that the optimum solution regulates the network load preventing the aggregate throughput from collapsing when the system is overloaded. In scenarios with a large number of transmitters, this optimum solution can be implemented in an uncoordinated manner with the knowledge of a few public system parameters.Peer ReviewedPostprint (published version

    Optimal power control law for equal-rate DS-CDMA networks governed by a successive soft interference cancellation scheme

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    ©2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper studies the throughput maximization of a dense multiple access network of low-rate subscribers that share the same practical Forward Error Correction (FEC) code and modulation scheme, and transmit to a central node that implements a Successive Soft Interference Cancellation (soft SIC) strategy in order to mitigate Multiple Access Interference (MAI). In the user-asymptotic case, we make use of Variational Calculus (VC) tools to derive, in terms of the Packet Error Rate (PER) of the shared encoder and the Residual Energy (RE) from imperfect cancellation, the optimum energy profile that maximizes the network spectral efficiency, when a sum power constraint at the SIC input is enforced. Comparative performance analyses using a representative encoder are carried out. Simulation results show the benefit of the adopted soft SIC scheme in front of other SIC strategies, obtaining relevant throughput gains under high traffic loads.Peer ReviewedPostprint (author's final draft

    Joint energy and rate allocation for successive interference cancellation in the finite blocklength regime

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    © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This work addresses the optimization of the network spectral efficiency (SE) under successive interference cancellation (SIC) at a given blocklength n. We adopt a proof-of-concept satellite scenario where network users can vary their transmission power and select their transmission rate from a set of encoders, for which decoding is characterized by a known packet error rate (PER) function. In the large-system limit, we apply variational calculus (VC) to obtain the user-energy distribution, the assigned per-user rate and the SIC decoding order maximizing the network SE under a sum-power constraint at the SIC input. We analyze two encoder sets: (i) an infinite set of encoders achieving information-theoretic finite blocklength PER results over a continuum of code rates, where the large-n second order expansion of the maximal channel coding rate is used; (ii) a feasible finite set of encoders. Simulations quantify the performance gap between the two schemes.Peer ReviewedPostprint (author's final draft

    Multipolling and OFDMA Reservation Protocol for IEEE 802.11 Networks

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    This paper presents an efficient multipolling scheme that improves the IEEE 802.11 MAC protocol efficiency and does not suffer from the hidden terminal problem or inefficient channel usage problems. To design an efficient polling scheme, the access point (AP) needs to obtain information about the current transmission status and channel state for each station. In this new scheme, we propose the use of multipolling frames in the downlink and Orthogonal Frequency Division Multiple Access (OFDMA) for the uplink reservation frames in order to reduce signaling overhead.Postprint (published version

    Decentralized random energy allocation for massive non-orthogonal code-division multiple access

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    © 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This work studies the spectral efficiency achievable when a very large number of terminals are connected simultaneously to a central node (uplink) through independent and identically-distributed flat-fading channels. Assuming that terminals only have statistical channel state information (CSI), the optimum random transmitted-energy allocation is formulated considering a non-orthogonal direct-sequence code-division multiple access (DS-CDMA) where all users transmit using the same modulation and error correcting code and the receiver implements successive interference cancellation (SIC). Focusing on low-power terminals, optimization is carried out by imposing constraints on both the average and peak peruser transmitted energy. Simulations have revealed that a limited number of random energy levels, whose number is determined by the channel power gain variance, is sufficient to achieve approximately the maximum spectral efficiency that would be obtained under direct optimization of the received energy profile.Peer ReviewedPostprint (author's final draft

    Channel-aware energy allocation for throughput maximization in massive low-rate multiple access

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    © 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.A multiple access (MA) optimization technique for massive low-rate direct-sequence spread spectrum communications is analyzed in this work. A dense network of users transmitting at the same rate to a common central node under channelaware energy allocation is evaluated. At reception, successive interference cancellation (SIC) aided by channel decoding is adopted. Our contribution focuses on wireless scenarios involving a vast number of users for which the provided user-asymptotic model holds. Variational calculus (VC) is employed to derive the energy allocation function that, via user-power imbalance, maximizes the network spectral efficiency (SE) when perfect channel state information at transmission (CSIT) is available and both average and maximum per-user energy constraints are set. Monte Carlo simulations at chip-level of a SIC receiver using a real decoder assess the proposed optimization method.Peer ReviewedPostprint (published version

    Energy and rate allocation for massive multiple access with interference cancelation

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    This article addresses the problem of energy and code allocation to many users accessing, under spreading-based nonorthogonal multiple access, a wireless node set up with a successive interference cancellation architecture aided by redundancy-check error control. As an application, we consider the asynchronous access of a delay-tolerant satellite system, where users employ finite-length channel codes and are subject to a known power unbalance induced by the known distribution of the channel’s attenuation. The article develops, as a mathematically tractable approximation to massively populated systems, a unified framework to compute the best energy and code allocation rules that maximize the spectral efficiency of a network that handles asymptotically many users. Concretely, the presented approach circumvents the exponential complexity in the number of users when modeling the propagation of packet decoding failures through the receiver’s decoding scheme. It also enables a deterministic analysis of the more complex features affecting the receiver, making the related performance optimization problem amenable to systematic tools from differential and variational calculus. The derived expressions evidence the most favorable three-way unbalance between energy, rate, and reliability for receiver performance. Low-level system simulations are carried out for validation.This work was supported in part by the Spanish Ministry of Science and Innovation through project RODIN (PID2019-105717RB-C22/AEI/10.13039/501100011033) and in part by Grant 2017 SGR 578.Peer ReviewedPostprint (published version
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