# Emerging Topics in Joint Radio-Based Positioning, Sensing, and Communications
## 3 Emerging Topics in JPSAC Context
### ELAA, XLMIMO, mMIMO, umMIMO, and LIS  
ELAA, Extremely Large-scale Multiple Inputs-Multiple Outputs (XLMIMO), mMIMO, umMIMO, and Large Intelligent Surfaces (LIS) are different antenna categories that fall under at least one of these conditions: the antenna dimension is much larger than the wavelength (e.g., ELAA and LIS) or the number of antenna elements is massive (i.e., in the order of hundreds) or ultra-massive (thousands). The definitions of these terms are sometimes a bit ambiguous in the current literature with respect to their differences and similarities, though these terms are not exactly synonymous with each other. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Year** & **Brief Description** & **Methods** & **Wireless Technologies** & **Metrics** \\ \hline \multirow{10}{*}{[29]} & \multirow{5}{*}{2024} & \multirow{5}{*}{2024} & Detailed mathematical models; 3 GPP standardization overview; a wide overview of methodologies for waveform and codebook design and channel estimation in ISAC; ML-based solutions for radio SLAM; definitions of snapshot, filtering, and smoothing approaches, etc. & Accuracy measured in terms of positioning errors, target detection probabilities, SINR, positioning and sensing ranges, sum rate, SPEB,... \\ \cline{1-1}  & & & Three addressed cases: sensing-centric, communication-centric, and joint communication and sensing via a sum-weighted approach; the integration of ISAC with NOMA; the exploitation of spatial resolution and advanced beamforming of near-field & SNR, beam-pattern gain, sum rate \\ \cline{1-1}  & & & Challenges in THz bands, such as beam-split or beam-quint effects, near-field effects, distance-dependent bandwidths, and the broadening of the absorption lines; umMIMO and RIS are proposed as solutions to counter the effects of the huge path losses & \\ \hline \multirow{10}{*}{[45]} & \multirow{5}{*}{2024} & \multirow{5}{*}{2024} & Both single-waveform and dual-waveform (DFRC/(RCC) designs are outlined by defining various optimization problems in terms of the objective function and constraints) & \multirow{5}{*}{5G} & RMSE, detection probability, SINR, radar estimation rate and communication rate,... \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._The ELAA concept [47; 48] refers to the deployment of a very large number of antenna elements over a large physical area. This can be achieved through distributed antenna systems or large-scale antenna arrays. ELAA can provide high spatial resolution and improved SNR by leveraging the large aperture, and this is particularly useful for applications requiring precise beamforming and high-resolution sensing. A holographic MIMO antenna [29; 49] uses a nearly continuous surface of reconfigurable and sub-wavelength-spaced antenna elements to create a holographic image of the electromagnetic field, allowing for the precise control of the wave fronts. XLMIMO [50] refers to both systems with a massive number of antennas, often in the hundreds or thousands, thus covering the mMIMO and umMIMO categories, as well as to antennas with very large dimensions compared to the wavelength, thus covering the ELAA category. The mMIMO and umMIMO [51] categories refer to antennas with a large and very large number of elements, though there is no unanimous agreement in the literature with respect to what represents a 'large' or a'very large' number. A coarse rule of thumb is that antennas with few dozen to a few hundred elements fall into the mMIMO category, while antennas with more than a few hundred antenna elements fall into the umMIMO category. The massive diversity and refined spatial resolution achieved by the ELAA, XLIMIMO, mMIMO, umMIMO, and LIS antennas indicate a yet-to-be-harnessed potential for JPSAC.