Designing energy-efficient beamformers is a major challenge to the deployment of battery-powered devices transmitting short data packets at millimeter-wave frequencies. In general, directly maximizing energy efficiency will not meet the stringent reliability requirements of certain Internet-of-Things applications. We focus on designing analog beamformers in a device-to-device setting under short packets, a finite encoder set, and imperfect channel knowledge. To guarantee the required error decoding performance, a two-step procedure is proposed such that beamformers are designed to minimize the power consumption subject to reliability constraints under worst-case channel estimation errors, and then the channel encoder maximizing the energy efficiency is selected. The proposed design enjoys a moderate loss under practical low-resolution phase shifters, and exhibits robustness to inaccurate channel estimators, which is crucial to the tradeoff between energy efficiency and error decoding performance.

This paper deals with scenario-aware, uncoordinated, and distributed signaling techniques in the context of feedforward opportunistic communications, that is, when the opportunistic transmitting node does not cooperate with any other node in a heterogeneous communication context. In this signaling technique, each network node individually follows a transmission strategy based on the locally sensed occupied and unused physical-layer network resources to minimize the induced interference onto other coexisting networks, taking into account the impact of the sensing errors and the locality of the sensing information. The paper identifies and characterizes critical invariance properties of the transmitted pulse shaping waveforms that guarantee the detectability of the feedforward transmitted signal by the uncoordinated receiving nodes, irrespective of the sensing signal space basis. The paper also shows that, under mild operating conditions, the proposed transmission scheme asymptotically defines efficient alternatives in the frequency domain, such as the circulant-shaping TDMA (CS-TDMA) modulation, and all of them admit a direct adaptation to frequency-selective channels. Numerical evaluation of the proposed schemes validates the provided theoretical models.

Inter-system interference may limit the performance of co-existing systems in dense heterogeneous wireless networks. Context-aware waveform design can profitably overcome this limitation. However, the latter substantially depends on degrees-of-freedom (DoF) sensing mechanisms. In this work, we show that total least-squares (TLS)-based waveform design is robust against sensing uncertainties. Given the equivalence of minimum norm and TLS, the latter exhibits the good properties of linear predictors, which are of paramount importance to guarantee minimum inter-system interference and detectability by neighboring nodes. Additionally, since derived solution relies on orthogonal projector onto the so- called noise subspace, we can efficiently and iteratively construct an orthogonal waveform-book enabling the presented transmission scheme in multi-carrier scenarios. Simulation results are presented to support our theoretical contributions, and to highlight any potential advantage of proposed solution in crowded heterogenous wireless networks.

The uncoordinated design of pulse shaping filters for opportunistic communications is addressed. We show that under degrees-of-freedom sensing uncertainties the waveform design problem can be cast as a minimum-norm optimization, admitting hence a closed-form expression. Because designed waveforms are adapted to scenario working conditions, proposed design scheme may be considered in pilot reference signals design to achieve orthogonality, regardless the traditionally considered pilot symbols orthogonality. Hence, the effect of interferences such as pilot contamination is diminished. However, a crucial aspect relies on their detectability. Since each node uses only local observations from the wireless network, the sensed degrees-of-freedom may slightly differ from one node to others. In this paper we prove that, thanks to the existence of some invariances, designed waveforms can be detected by neighboring nodes. Even though degrees-of-freedom sensing uncertainties may incur in a performance loss, we propose a least-squares constrained basis pursuit algorithm to reduce the effect of uncertainties by considering only the degrees-of-freedom subspace intersection.