## 3 State-of-the-Art MIMO-SDR Systems
### Mimo-Sdr Waveforms
#### 3.2.4 MIMO-OFDM Variants Enhancement Studies  
In addition to the existing OFDM waveform variants described before, several authors have suggested additional enhancements to those structures. In this paragraph, we are going to give a brief insight into the most significant additions to the literature. Table 5 groups those studies, with reference to the base waveform scheme. [PERSON] and [PERSON] [98] recommend a GFDM receiver scheme based on index modulation (IM) [107], considering a fading channel variable in time. The designed architecture (please refer to Figure 11) overcomes the defects introduced by shaping filters used with GFDM, namely self-created inter-carrier interference (ICI) and ISI as well. It deploys two-phase rotations to work for multipath transmission in fast-fading environments. The optimal phase rotations are obtained by maximizing the carrier-to-interference ratio (CIR). In fact, the authors suggested a new algorithm that uses the adaptive normalized block least-mean-squared (NBLMS) method [108], which allows the avoidance of the feedback of the carrier frequency offset. The Kalman algorithm was chosen to model the time variation of the channel using the pilot-symbol-aided estimation technique [109], combined with the turbo minimum mean square error (MMSE) equalizer [110]. In fact, the turbo equalizer deploys an iterative scheme to update the Kalman channel estimator, equalizer, and decoder blocks, given values until the convergence is achieved. The log-likelihood ratio (LLR) is used to determine the current active carrier. It was proven that the current receiver scheme could reduce the decoding process for the current active subcarrier. [PERSON] and [PERSON] [99] have suggested a new redundant discrete wavelet-based MIMO-OFDM (MIMO-RDWT-OFDM) framework, depicted in Figure 12. The utilization of the new introduced redundant discrete wavelet transform, which does not have the down-sampling step in the decomposition tree, widely improves the spectral efficiency of the transceiver, and it could be deployed in Additive White Gaussian Noise (AWGN) channels such as the Rayleigh fading model as it is more robust for noise [99]. Figure 10: FBMC-QAM transceiver. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Enhancement of** & **OFDM** & **CP-OFDM** & **UFMC** & **GFDM** & **F-OFDM** & **FBMC** \\ \hline [PERSON] and [PERSON] [98] & & & & X & & \\ [PERSON] and [PERSON] [99] & X & & & & & \\ [PERSON], et al. [100] & & & X & & & \\ [PERSON] and [PERSON] [101] & & & & & & X \\ [PERSON], et al. [102] & & & & & & X\({}^{1}\) \\ [PERSON], et al. [103] & & & & & & & X \\ [PERSON], et al. [104] & & & & & & \\ [PERSON], et al. [105] & X & & & & & \\ [PERSON], et al. [106] & X & & & & & \\ \hline \hline \end{tabular} \({}^{\dagger}\) It is theoretically proven that it is compatible with MIMO systems, but not tested experimentally.  
\end{table} 
Table 5: MIMO-OFDM variants enhanced schemes. In addition, the MIMO-RDWT-OFDM model deploys an optimal genetic algorithm (OGA) to obtain optimized weights for the ICI cancellation scheme. [PERSON], [PERSON] and [PERSON] [100] have recommended a NOMA-UFMC-based Radio Access (NOMURA) downlink scheme (please refer to Figure 13) to enhance the throughput and system capacity. The base station groups the end users, following their Signal to Interference Noise Ratio (SINR), into two groups: Lower SINR Users (LSUs) and Higher SINR Users (HSUs). The LSU and HSU information is concatenated using superposition coding (SC) with a power ratio value of \(\gamma\) and \(1-\gamma\), respectively. SC is deployed with a Multiuser Superposition Transmission (MUST) Category 1 technique previously deployed in the 3 GPP specifications [111], where HSU symbols are superimposed over LSUs with a  
Figure 11: MIMO turbo GFDM-IM receiver: first, two-phase rotations are computed on initial carrier frequency offset values. Afterwards, the phase value for ICI interference correction is computed by an adaptive recommended NBLMS algorithm.