In this work we address the distance dependence of reconfigurable intelligent surfaces (RIS). As differentiating factor to other works in the literature, we focus on the array near-field, what allows us to comprehend and expose the promising potential of RIS. The latter mostly implies an interplay between the physical size of the RIS and the size of the Fresnel zones at the RIS location, highlighting the major role of the phase. To be specific, the point-like (or zero-dimensional) conventional scattering characterization results in the well-known dependence with the fourth power of the distance. On the contrary, the characterization of its near-field region exposes a reflective behavior following a dependence with the second and third power of distance, respectively, for a two-dimensional (planar) and one-dimensional (linear) RIS. Furthermore, a smart RIS implementing an optimized phase control can result in a power exponent of four that, paradoxically, outperforms free-space propagation when operated in its near-field vicinity. All these features have a major impact on the practical applicability of the RIS concept. As one contribution of this work, the article concludes by presenting a complete signal characterization for a wireless link in the presence of RIS on all such regions of operation.

Accessing the spatial domain of wireless environments through antenna arrays has become key to improve the use of energy and, in light of this, there is an increasing need for low cost multi-antenna architectures. As one alternative, the Electronically Steerable Parasitic Array Radiator (ESPAR) was proposed as a coupling-based inexpensive option; allowing to significantly reduce the amount of required radio frequency (RF) front-ends. As a caveat, due to its inherent non-linear behavior, the required computational complexity can be prohibitive. Additionally, the unavailability of precise mutual coupling and channel state information (CSI) becomes a further issue difficult to avoid in practical setups. In this regard, the current work has two main contributions: a) it deals with the computational complexity by proposing the linearization of ESPAR's system model through the truncated Taylor expansion of the admittance matrix; facilitating the spatial processing algorithm of interest. Also, the authors propose b) to face the mutual coupling and CSI unavailability issue via the joint estimation of the channel-ESPAR parameters as directly observed through the single RF front-end. Relying on the optimization of the linearized system model, the pilot-based algorithm to be introduced allows ESPAR to track the configuration that synthesizes coherent combination at a significantly low complexity. Particularly, as shown through Monte-Carlo simulation, the latter allows to obtain 4 dB of array gain with a single RF front-end via a 5-element ESPAR.

The electronically steerable parasitic array radiator (ESPAR) has been acknowledged as an inexpensive multiple antenna architecture. Nonetheless, due to the non-linear behavior on the loads used in its control, its related computational complexity can be prohibitive for practical implementations. The current paper deals with such an issue by proposing an approximation of its model via a truncated polynomial expansion of the inverse impedance matrix. The proposed approximation allows to reframe the problem of reactance optimization for beamforming, referred to as analog maximum ratio combining (A-MRC) in previous work. The method proposed in this paper provides a means to obtain a computationally efficient solution to the problem of reactance optimization and it avoids the need of an iterative-based approach. Therefore, resulting in a major computational complexity reduction with respect to related work in the literature.

The use of decode-and-forward (DAF) and amplify-and-forward (AAF) relays with the purpose of receiving and re-transmitting information has long been proposed. Nonetheless, DAF relays increase latency and AAF relays suffer from inherent stability issues due to potential echoes on the relayed signal. Alternatively, a passive relaying array (PRA) is proposed as a means of coping with both of the so-mentioned drawbacks. The role of PRA is that of capturing and scattering energy in a desired direction. The current work proposes a closed-form model for a dipole-based PRA with an electronically tunable response. The validation of the proposed model and the link budget for communication through the relay are also shown. Furthermore, given the non-linear relationship of PRA on the electronically tunable loads, the simultaneous perturbation stochastic approximation (SPSA) method is used as a means of finding the set of loads that maximizes energy transfer in a desired direction.

The electronically steerable parasitic array radiator (ESPAR) has been proposed as an inexpensive device for spatial multiplexing in transmission via the so-called beamspace modulation. On the other hand, spatial multiplexing (SM) is well known to allow a linear increase of capacity in MIMO systems with the number of antennas under rich scattering, unlike its corresponding logarithmic increase via signal to noise ratio (SNR). When it comes to ESPAR in reception for SM, nonetheless, the issue at hand is the so-called SNR degradation due to oversampling. In particular, oversampling is required given that, although all parasitic elements can be simultaneously controlled, up to a single simultaneous sample can be conveyed in reception. The counter intuitive role of the oversampling rate when seen as the number of spatial samples and its negative impact on SNR is studied in the present paper. It is shown that, from a capacity perspective, the ideal number of spatial samples depends on both the SNR and channel conditions. Furthermore, an algorithm for capacity maximization over the set of reactive loads relying on the simultaneous perturbation stochastic approximation method (SPSA) is shown. Particularly, as an extension of previous work, such an algorithm converges to analog maximum ratio combining (A-MRC) when the oversampling rate is equal to one.

The reactively loaded parasitic array radiator (RL-ESPAR) receiver has been acknowledged due to its compactness, fabrication cost and reconfigurability; showing significant beam-forming trade-off capabilities compared to conventional multi-element receiver ends. When used on reception, one relevant problem is to find the value of the reactance loadings to obtain the best signal to noise ratio performance. In the current paper, we compare the performance of digital and analog coherent combining over RL-ESPAR. Particularly, digital combining is realized by virtually rotating such a receiver and then applying the known maximum ratio combining (MRC) technique. On the other hand, analog combining is realized by synthesizing MRC on the reactance loadings via a technique that relies on the simultaneous perturbation stochastic approximation method (SPSA) method (widely applied on ESPAR in the literature). We show that analog combining exceeds digital combining and single dipole reception by around 3 dBs and 4 dBs via the proposed technique, respectively.