## 4 SNR Estimation  
This section presents the development of the preamble-based CAZAC-TD and CAZAC-FD SNR estimation algorithms, along with the SNR threshold for modulation level selection in an adaptive modulation scheme. Both algorithms are derived under the assumption of signal transmission over an Additive White Gaussian Noise (AWGN) channel. To  
Figure 1: STBC-assisted MIMO-OFDM system with adaptive modulation block diagram.  
assess their robustness, simulations are conducted considering signal transmission over an Stanford University Interim-5 (SUI-5) channel in an NLOS environment. The results show consistent performance across both channel models, suggesting that the algorithms may generalize well to more complex, real-world scenarios. However, further validation in dynamic and practical conditions is needed to fully assess their applicability.  
This work proposes employing CAZAC-TD and CAZAC-FD SNR estimators for \(2\times 2\) and \(4\times 4\) STBC-assisted MIMO-OFDM systems. Several preamble-based SNR estimators that utilized synchronization preambles for OFDM systems have been proposed in the literature to mitigate incurred throughput penalties in DA-SNR, as discussed in Section 1.  
The innovative preamble structure for timing synchronization in OFDM systems was proposed by [PERSON] et al. [32]. For the purpose of time synchronization improvement, the proposed preamble was composed of one OFDM symbol with a short preamble structure having four equal parts, each of \(N_{S}/4\) length, as depicted in Figure 2, where \(N_{S}\) is the IFFT size, which represents the total sub-carriers per OFDM symbol. \(C_{Z}\) denotes the CAZAC sequence, \(P_{N}\) is the pseudo-noise (PN) sequence, and \(W\) denotes the weighted CAZAC sequence, which is obtained by bit-wise multiplication of \(C_{Z}\) with the same length of \(P_{N}\), while \(S(W)\) represents the multiplication of the weighted CAZAC sequence, with \(W\) a scrambling function, which aims to mitigate the narrow-band interference effect. Let us denote this preamble structure as the Suparna preamble structure.  
As a further development, the preamble structure is modified as illustrated in Figure 3, where the CAZAC sequence is used because of its good autocorrelation properties [42]. Figure 4 shows the proposed preamble structure with the added cyclic prefix \(C_{P}\) of \(N_{S}/4\) length that is utilized in the proposed CAZAC-TD and CAZAC-FD SNR estimators. Using IEEE802.16d OFDM standard, the CAZAC preamble is loaded on even sub-carriers, and the odd sub-carriers are nullified. The modified preamble structure is composed of one OFDM symbol having four equal parts, each part with \(N_{S}/4=64\) bits length, where \(N_{S}=256\) bits is the length of the signal without the cyclic prefix, while the cyclic prefix \(C_{P}\) is added to mitigate inter-symbol interference and inter-carrier interference between the received OFDM symbols.  
For both the CAZAC-TD and CAZAC-FD estimators, the modified CAZAC preamble is loaded in even sub-carriers, and the odd sub-carriers are not used (nulled). The proposed CAZAC-TD and CAZAC-FD SNR estimators are compared with the Milan-FD SNR estimator, presented in [18]. The Milan-FD SNR estimator utilized the preamble structure proposed in [31], which was composed of one OFDM symbol having \(Q\) equal parts, each of \(N_{S}/Q\) length, where \(Q>2\) and \(N_{S}\) is the IFFT size. For the sake of benchmarking, the proposed SNR estimators are evaluated against the Milan-FD SNR estimator, which utilized the preamble structure in Figure 5, with \(Q=4\) and \(N_{S}=256\) bits.  
Figure 3: Proposed modified preamble structure for CAZAC-TD and CAZAC-FD SNR estimators.  
Figure 2: Suparna preamble structure proposed for time synchronization [32].