## 5 Results and Discussion
### Adaptive Modulation Scheme Performance  
This section presents two sets of performance results to evaluate the proposed adaptive scheme. The first set compares the BER and channel capacity performance of the adaptive AM-CAZAC-TD-MIMO system against a non-adaptive MIMO-OFDM scheme, highlighting the advantages of the adaptive approach. The second set examines the BER and throughput performance of the adaptive MIMO-OFDM scheme, comparing it with the adaptive SISO-OFDM scheme. Figures 17 and 18 show the performance of adaptive modulation in a \(2\times 2\) STBC-assisted MIMO-OFDM system using the CAZAC-TD SNR estimator in terms of BER and channel capacity. Let us refer to this adaptive scheme as the AM-CAZAC-TD-MIMO system. The AM-CAZAC-TD-MIMO system uses \(M\)-PSK, with a switching SNR threshold, as shown in Table 2. More specifically, the adaptive MIMO-OFDM system adjusts its modulation level  
Figure 16: The BER performance of the non-adaptive STBC-MIMO-OFDM system for the SUI-5 channel. based on the estimated channel SNR, as outlined in Table 2. Its performance is compared to that of the non-adaptive \(2\times 2\) STBC-assisted MIMO-OFDM system. Figure 17 shows that, at BER = \(10^{-4}\), the AM-CAZAC-TD-MIMO system with \(M\)-PSK outperformed the non-adaptive STBC-assisted MIMO-OFDM system using 8-PSK by about 2 dB. This improvement is clearly reflected in the performance curves, where the adaptive AM-CAZAC-TD-MIMO system requires a lower SNR to achieve the same target BER. Moreover, when targeting a specific BER, the adaptive MIMO-OFDM system shows greater SNR reductions compared to the non-adaptive MIMO-OFDM system with a fixed modulation level, as the modulation order \(M\) increases. This suggests that the adaptive MIMO-OFDM scheme requires less SNR to maintain reliable performance across various modulation schemes. Figure 17: The proposed AM-CAZAC-TD-MIMO system’s BER performance for the SUI-5 channel. Figure 18: The proposed AM-CAZAC-TD-MIMO system’s channel capacity performance for the SUI-5 channel. The channel capacity is quantified using the following equation [51]:  
\[C_{stlv-MIMO}=R\cdot\log_{2}\left(1+SNR_{MIMO-Est}\right) \tag{24}\]  
where \(SNR_{MIMO-Est}\) for the CAZAC-TD SNR estimator can be calculated using Equations (14) and (21) and \(R\) is the STBC code rate with \(R=1\) for Alamouti's code of a \(2\times 2\) MIMO system [4; 5]. Figure 18 shows that the AM-CAZAC-TD-MIMO system performed well in terms of channel capacity, reflecting variations in the level of modulation of \(M\)-PSK based on the SNR threshold in Table 2. For channel SNRs below 9 dB, the AM-CAZAC-TD-MIMO system performed similarly to the non-adaptive STBC-assisted MIMO-OFDM system using QPSK modulation in terms of channel capacity. The channel capacity performance improved significantly for channel SNRs ranging from 9 dB to 15 dB. Eventually, it achieved channel performance comparable to the non-adaptive STBC-assisted MIMO-OFDM system using 64-PSK modulation for channel SNRs greater than 15 dB. Thus, the enhanced robustness, coupled with the increased spectral efficiency, showcases the system's ability to deliver consistent and reliable performance, even under varying channel conditions. The second set of results focuses on the adaptive MIMO-OFDM system's BER and channel capacity performance, comparing it against an adaptive SISO-OFDM scheme to demonstrate the benefits of employing multiple antennas. The adaptive SISO-OFDM system, which invokes the CAZAC-TD SNR estimator, is denoted as the AM-CAZAC-TD-SISO system. Both of these adaptive modulation systems use the \(M\)-PSK modulation technique. In contrast to the AM-CAZAC-TD-SISO system, the AM-CAZAC-TD-MIMO system demonstrated an SNR gain of about 4 dB, as shown in Figure 19.  
Furthermore, Figure 20 illustrates that the AM-CAZAC-TD-MIMO system and the AM-CAZAC-TD-SISO system exhibit a notable improvement in channel capacity. Therefore, it was shown that the spatial diversity of the MIMO-OFDM system is key for the AM-CAZAC-TD-MIMO system's improved performance.