## 3 State-of-the-Art MIMO-SDR Systems
### Mimo-Sdr Waveforms
#### 3.2.4 MIMO-OFDM Variants Enhancement Studies
The initial channel values are estimated by the pilot symbol. Accordingly, the LLR detector can localize the data position. Finally, there is the turbo equalization, where the mean and variance of the output signal are supplied to the developed Kalman channel estimation block to conduct calculations for the next iterations. Figure 12: MIMO-RDWT-OFDM transceiver. non-gray bit-mapping algorithm. Thereafter, UFMC is applied for every sub-band, with specific filtering parameters according to every user requirement. The recommended transceiver is tested under AWGN as well as Rayleigh fading environments. As described in Figure 13, the zero padding is performed to increase the length of the transmitted symbols to give them as input to the 2N-point discrete Fourier transform (DFT) block. A zero-forcing equalization was considered to correct the channel effect. In the demodulation part, LSUs threaten the HSU user signals as noise to extract the LSU original symbols. However, the HSU signals are reconstructed by demodulating and modulating the received signals to subsequently reconstruct the LSU signals, which are subtracted from the received signal to finally obtain the HSU data to be demodulated. [PERSON] and [PERSON] [101] propose a new FFT-FBMC scheme with less interference terms in comparison to the original OFDM algorithm. The FFT-FBMC deploys DFT and CP insertion blocks (with a lower CP length), which make the scheme more complex but highly compatible with MIMO signal-processing algorithms. The transmission strategy uses guard intervals as well to isolate the adjacent subcarriers. The scheme was tested with maximum likelihood detection and Alamouti space-time coding algorithms. [PERSON], [PERSON] and [PERSON] [102] have introduced a novel FBMC-OQAM scheme to minimize inter-block interference by performing a single-band (SSB) filtering on the edge subcarriers of the resource blocks. This filtering procedure helps to preserve the orthogonality between the sub-symbols within the same block and allows the transceiver to reach the complex field orthogonality at the edges without the need to insert any guard intervals. In this context, a novel resource mapping scheme was chosen, which is shown in Figure 14. This scheme allocates Sc 0 (referring to the SSB filtered edge carrier) as well to the two initial resources in an alternating way. Figure 13: MIMO NOMORA system: P/S and S/P are abbreviations for parallel to serial and serial to parallel, respectively. F1,..., FB (this refers to the filter used for sub-band B) filter parameters depend on the users’ requirements. Another enhanced FBMC based waveform, proposed by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [104], introduces a new conjugated FBMC-OQAM scheme. The authors take advantage of the fact that the pulse-shaping filter of the FBMC-OQAM systems exhibits odd symmetry, and the interferences that destruct the orthogonality (the intrinsic inter-symbol and the inter-carrier interferences) are strictly related to the neighboring symbols. Therefore, the idea was to divide the time-frequency resources into two parts, allowing the transmission of the original symbols and the corresponding conjugated version of the original ones. A unique signal-processing procedure is used to combine two conveniently separated sub-blocks. The recommended FBMC-OQAM system achieves an optimizing diversity gain. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [103] were among the multiple other authors that theoretically proved that the filter bank OFDM (FB-OFDM) was compatible with MIMO systems. The baseband diagram of the transceiver is presented in Figure 15. The main difference between this scheme and the one of the traditional OFDM technique is the filtering part. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [105] proposed to remove the CP from the OFDM in massive MIMO systems.