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Underwater wireless sensor networks are gaining popularity since supporting a broad range of applications, both military and civilian. Wireless acoustics is the most widespread technology adopted in underwater networks, the realization of which must face several challenges induced by channel propagation like signal attenuation, multipath and latency. In order to address such issues, the attention of researchers has recently focused on the concept of cooperative communication and networking, borrowed from terrestrial systems and to be conveniently recast in the underwater scenario. In this paper, we present a comprehensive literature review about cooperative underwater wireless sensor networks, investigating how nodes cooperation can be exploited at the different levels of the network protocol stack. Specifically, we review the diversity techniques employable at the physical layer, error and medium access control link layer protocols, and routing strategies defined at the network layer. We also provide numerical results and performance comparisons among the most widespread approaches. Finally, we present the current and future trends in cooperative underwater networks, considering the use of machine learning algorithms to efficiently manage the different aspects of nodes cooperation. [ABSTRACT FROM AUTHOR]

This paper deals with the maximum-likelihood (ML) noncoherent data-aided (e.g., no blind) synchronization of multiple-antenna ultrawideband impulse-radio (UWB-IR) terminals that operate over broadband channels and are affected by multipath fading with a priori unknown number of paths and path-gain statistics. The synchronizer that we developed achieves the ML data-aided joint estimate of the number of paths and their arrival times (e.g., time delays), without requiring any a priori knowledge and/or a posteriori estimate of the amplitude (e.g., module and sign) of the channel gains. The ultimate performance of the proposed synchronizer is evaluated (in closed form) by developing the corresponding Cramer-Rao bound (CRB), and the analytical conditions for achieving this bound are provided. The performance gain for the synchronization accuracy of multipath-affected UWB-IR signals arising from the exploitation of the multiple-antenna paradigm is (analytically) evaluated. Furthermore, a low-cost sequential implementation of the proposed synchronizer is detailed. It requires an all-analog front-end circuitry composed of a bank of sliding-window correlators, whose number is fully independent from the number of paths comprising the underlying multiple-antenna channel. Finally, the actual performance of the proposed synchronizer is numerically tested under both the signal acquisition and tracking operating conditions. Index Terms--Cramer-Rao Bound (CRB), multipath fading, multiple antennas, noncoherent maximum-likelihood (ML) synchronization, optimized signaling strategy, ultrawideband impulse radio (UWB-IR).

In this contribution, we consider emerging wireless content delivery networks (CDNs), where multiple (possibly nomadic) clients download large-size files from battery-powered proxy servers via faded links that are composed of multiple slotted orthogonal bearers (e.g., logical subchannels). Since the considered transmit proxy servers are battery-powered mobile routers, a still open basic question deals with searching for optimal energy-allocation (e.g., energy scheduling) policies that efficiently split the available energy over the (faded) bearers. The target is to minimize the resulting (average) download time when constraints on the average available energy per information unit (IU), peak-energy per slot, and minimum energy per bearer (e.g., rate-induced constraints) are simultaneously active. The performance and the robustness of the resulting optimal energy scheduler are tested on the last hop of Rayleigh-faded mesh networks that adopt the so-called 'dirty paper strategy' for broadcasting multiple traffic flows that are generated by proxy servers equipped with multiple antennas. Index Terms--Broadcast (BC) network, content delivery network (CDN), convex calculus of variations, download time, energy management, multiantenna link, multichannel downlink, quality-of-service (QoS) requirements.

This paper presents an optimized spatial signal shaping for multiple-input--multiple-output (MIMO) 'ad hoc'-like networks. It is adopted for maximizing the information throughput of pilot-based multiantenna systems that are affected by spatially colored multiple access interference (MAI) and channel estimation errors. After deriving the architecture of the minimum mean square error (MMSE) MIMO channel estimator, closed-form expressions for the maximum information throughput that is sustained by the MAI-affected MIMO links are provided. Then, we present a novel power allocation algorithm for achieving the resulting link capacity. Several numerical results are provided to compare the performance achieved by the proposed power-allocation algorithm with that of the corresponding MIMO system that works in MAI-free environments and is equipped with error free (e.g., perfect) channel estimates. By so doing, we are able to give an insight on the ultimate performance loss that is induced in MIMO systems by spatially colored MAI and imperfect channel estimates. Finally, we point out some implications about space-division multiple access strategies arising from the proposed power allocation algorithm. Index Terms--Imperfect channel estimation, multiantenna, multiple access interference (MAI), signal shaping, space-division multiple access.

This paper focuses on the competitively optimal power control and signal shaping for 'ad hoc' networks composed by multiple-antenna noncooperative transmit/receive terminals affected by spatially colored multiple-access interference (MAI). The target is the competitive maximization of the information throughput sustained by each link that is active over the network. For this purpose, the MAI-impaired network is modeled as a noncooperative strategic game, and sufficient conditions for the existence and uniqueness of the Nash equilibrium (NE) are provided. Furthermore, iterative power-control and signal-shaping algorithms are presented to efficiently achieve the NE under both best-effort and 'contracted QoS' policies. The presented algorithms also account for the effect of (possibly) imperfect channel estimates available at the transmit/receive units active over the network, they are fully scalable, and they may be implemented in a fully distributed and asynchronous way. The presented numerical results support the conclusion that the proposed distributed algorithms may be able to outperform the conventional centralized orthogonal MAC strategies (as time division multiple access, frequency division multiple access, and code division multiple access) in terms of a sustained network throughput, especially in operating scenarios affected by a strong MAI. Index Terms--Competitive optimality, game theory, multiple-access interference (MAI), multiple antennas, power allocation, spatial signal shaping.

A study on the throughput analysis, outage evaluation and optimized power allocation for Multiple-Input Multiple-Output (MIMO) pilot-based wireless systems subject to short-term constraints on the radiated power is presented. The results show that the power allocation algorithm gives rise to noticeable improvements in the conveyed ergodic throughput when the estimates available at the receiver are reliable.

Some novel contributions about architecture, optimal design and performance of multiple-antenna systems with space-time block codes (STBCs) and partially coherent decoding are presented. New performance bounds and optimized design criteria together with a new family of robust space-time unitary block codes that 'self-match' to channel-estimation errors is presented after considering emerging fourth-generation wireless local area networks (4GWLANs) as a possible application scenario.

In the last few years, ultra-wideband (UWB) systems became an appealing technology for wireless communication applications. Unfortunately, when the transmission channel is affected by intersymbol interference (ISI), system performance of UWB systems equipped with receivers based on conventional matched filters presents error-floor phenomena. Aimed by these considerations, in this letter, we present a novel transmit-receive scheme allowing blind channel estimation and minimum mean-square error linear channel equalization. Essentially, the proposed scheme exploits a very short duration of the UWB pulse for achieving reliable blind deconvolution of the received signal. A nice feature of the resulting system is that blind deconvolution of the received signal is achieved without power and throughput losses. Simulation results support the effectiveness of the proposed scheme, and show that it is able to gain about 8 dB over current UWB receivers based on matched filtering on several test channels impaired by ISI. Index Terms--Blind deconvolution, detection, equalization, intersymbol interference (ISI), system performance, ultra-wideband (UWB).

We here focus on the optimized design and performance of Rayleigh-faded multiple-antenna multiple-input multiple-output systems when both transmitterand receiver share imperfect (i.e., error impaired) channel estimates computed via training sequences. In particular, according to the emerging principle of the so-called synchronized detection, for this operating scenario we propose a simple water-filling (WF)-like strategy for allocating power over transmit antennas and test its actual performance. Afterward, we introduce and evaluate some related figures of merit summarizing the overall system's performance and use these last for noting effective system's design guidelines. particular, some optimized solutions for power- versus-bandwidth efficiency tradeoff are presented and their validity limits are debated. All the developed results explicitly take into account the actual reliability of the achievable channel estimates. Index Terms--Bandwidth-versus-power tradeoff, channel estimation, multiple antenna, optimized system design, power-allocation policy, synchronized detection, water filling.

Solutions are found for loading digital subscriber loop (DSL) multicarrier (MC) systems with constraints on overall available energy and maximum energy per carrier. An analytical characterization of the solution for optimal loading that is valid for the general case of concave rate-functions is described, and it is applied to the case where the 'gap analysis' holds.