Contents I. Introduction
The use of reconfigurable intelligent surfaces (RISs) in wireless networks is a solution with significant potentiality in terms of performance improvement as these surfaces are able to act on the propagation environment through controllable signal reflection, enhancing the signal reception at desired locations [1], [2]. The spatial reconfiguration and control of the signal quality makes RISs a desirable addition to be exploited also in localization schemes. In [3] the basic characteristics of a localization scheme exploiting a passive RIS, i.e., the RIS elements can configure only the phase of the reflected signal, are described and analyzed. References [4], [5] explore adaptive localization in a millimeter-wave (mmWave) RIS-aided multiple input multiple output (MIMO) system using a feedback communication link between the mobile station (MS) and the RIS and hierarchical codebooks for the active beamforming at the base station (BS) and at the MS. The use of multiple controlled transmissions in presence of RISs is able not only to efficiently assist the localization procedure, but also leads to the accuracy improvement for estimation of clock bias [6], [7] and MS’s orientation [8], a crucial parameter that needs to be estimated especially in MIMO settings. The Cramér-Rao lower bounds (CRLBs) of the position and orientation estimation errors are efficient tools for the analysis of localization schemes and they are derived for the passive RIS and compared with the performance of actual estimators in references [6], [8], [9]. To the best of the authors’ knowledge prior research in recent literature focusing on the use of RISs for localization does not consider a possible amplification of the reflected signal. The use of active RIS, i.e., the RIS elements can control both amplitude and phase of the reflected signals, was investigated in literature for communications and channel estimation, some examples are references [10]–[12]. However, the potentialities of active RISs were not yet analyzed in literature from the localization perspective. Indeed, the multiplicative fading phenomenon often makes the introduction of a passive RIS irrelevant as the line of sight (LoS) path is overwhelmingly more powerful. This unbalanced signal power between the direct and reflected paths creates the need for an RIS composed of active elements, where the total available power is split between the BS and the RIS.
