several new technologies. Massive MIMO [1], is one innovative technology that consists of several antennas which enhance the spectral efficiency. Moreover, Orthogonal Frequency Division Multiplexing (OFDM) combined with MIMO has given an interesting performance. MIMO-OFDM systems have drawn the attention of researchers over the last decade. Several MIMO-OFDM review papers and surveys, shown in Table 1, have been published to give a comprehensive analysis of the different challenging issues for the scientific community. There were some papers proposing a simple overview of some important research axes and pointing out the recent technology evolutions [2] concerning MIMO systems, namely MIMO 5G candidate waveforms [3,4], channel models [5], and coding schemes [6] relative to MIMO technology. Other papers, which propose a deeper review of MIMO systems in general, are classified in Table 1 as well. \begin{table}
\begin{tabular}{c c c c} \hline
**Year** & \begin{tabular}{c} **Details Paper** \\ **Reference** \\ \end{tabular} & \
\begin{tabular}{c} **Special Focus/General Vision of** \\ **the Paper** \\ \end{tabular} &
\begin{tabular}{c} **MIMO-SDR System** \\ **Research Axes** \\ \end{tabular} \\ \hline
2019 & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} 5G standards, specifications, and \\ massive MIMO testbed, including \\ transceiver design models using \\ QAM modulation scheme \\ \end{tabular} & - &
\begin{tabular}{c} Implemented massive \\ MIMO testbeds \\ 5G specifications \\ \end{tabular} \\ \hline
2017 & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} 5G standards, trials, challenges, \\ deployment, and practice \\ \end{tabular} & - &
\begin{tabular}{c} Channel characteristics \\ and access \\ Waveforms for the 5G \\ \end{tabular} \\ \hline
**Simple overview** & 2014 & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} Modulation formats and waveforms \\ for 5G networks \\ \end{tabular} & - &
\begin{tabular}{c} Waveforms for 5G \\ networks including \\ MIMO systems \\ MIMO channel effect \\ \end{tabular} \\ \hline
2014 & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} Key technologies for 5G wireless \\ communications \\ \end{tabular} & - &
\begin{tabular}{c} 5G MIMO technologies \\ General view of MIMO \\ channel models \\ \end{tabular} \\ \hline
2012 & \
\begin{tabular}{c} [PERSON] and \\ [PERSON] [6] \\ \end{tabular} &
\begin{tabular}{c} Space–time coding scheme for \\ MIMO system \\ \end{tabular} & - & Time Coding Scheme \\ \hline
2022 & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} \\ \end{tabular} & - &
\begin{tabular}{c} New MIMO trends \\ (hardware and \\ architecture innovations) \\ \end{tabular} \\ \hline
2022 & Our paper & MIMO-SDR OFDM systems & - & MIMO waveforms \\ \end{tabular} & - &
\begin{tabular}{c} MIMO-OFDM systems \\ advancements: channel \\ estimation and \\ equalization. \\ \end{tabular} \\ \hline
2021 & \begin{tabular}{c} \\ \end{tabular} & \begin{tabular}{c} \\ \end{tabular} & \
\begin{tabular}{c} - \\ Massive MIMO systems \\ \end{tabular} & - &
\begin{tabular}{c} Algorithms for resource \\ allocation \\ Channel estimation \\ - \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: Review paper concerning MIMO-SDR systems. Only a few papers [8] discuss the waveforms tested using MIMO systems. Nonetheless, several authors [9; 10; 11; 12; 13; 14] have analyzed the MIMO channel estimation and modeling problem. However, to the best of our knowledge, there has been until now no review paper that specifically discusses the recent works concerning channel estimation and the equalization process for MIMO-OFDM systems. The exception is that there are some review papers [16] on the MIMO-OFDM systems used especially for the underwater acoustic communication concept, where the transmission conditions are completely different to those of over-air transmission. In addition, there are some surveys of channel estimation and signal processing over massive MIMO systems [7].