###### Abstract
The two most common DT strategies with respect to the selection of DT are the adaptive/dynamic detection threshold (DDT) and the nonadaptive/fixed detection threshold (FDT) selection. The FDT is determined using a fixed noise level above the noise floor and it remains unchanged during the SS process while the DDT is dynamically determined based on the instantaneous level of NU in the environment. The SU can better deal with NU when using DDT adjustments as part of the detection of the PU signals. This makes the ED process more accurate. Although in communications systems, Orthogonal Frequency Division Multiplexing (OFDM) technology improves the spectral efficiency and robustness against inter-symbol interference (ISI) and frequency selective fading, while Multiple-Input Multiple-Output (MIMO) transmission technology improves the system capacity and reliability [7], the influence of NU challenges the accurate estimation of the channel occupancy even when DDT is employed in the MIMO-OFDM communication system [8; 9; 10]. To improve the accuracy of ED, various diversity techniques can be used to exploit multiple receive (Rx) antennas on the SU side of the MIMO communication system. Among the proposed diversity techniques, of particular interest is the square-law combining (SLC) diversity approach. The SLC is a straightforward diversity technique that exploits merging the energy of the received signals from every Rx antenna of the SU. It represents one of the diversity approaches with the lowest computing complexity [11]. It is expected that exploiting the diversity of the SLC technique during the ED process can improve the signal detection of PU in MIMO-OFDM systems affected by the NU. The main aim of this article was therefore to analyse the SS accuracy of the ED method employing the SLC approach when the ED process is affected by NU variations and performed using DDT adaptations in MIMO-OFDM communication systems. For pursuing the ED performance analyses, the receiver operating characteristic (ROC) curves indicating an interdependence between the probability of detection and false alarm probability are used as a common approach to analyse the detection efficiency of the SS method. The following is a summary of this paper's main contributions:  
* A mathematical modelling of the parameters for the assessment of the effectiveness of the ED process employing the SLC technique in MIMO-OFDM transmission systems affected by NU changes and DDT adaptation. * The verifying of an algorithm that enables the simulation of the ED process assessment in SISO- and MIMO-OFDM transmission systems. * An elaboration on how the different levels of DDT adjustment and NU variations affect the ROC curves when the ED process is performed with a different number of transmitting and receiving antennas, as well as varied sensing sample totals, PU transmitting powers, modulation techniques, and SNR values. The rest of the paper contains the following sections. Section 2 provides a literature review of the ED in MIMO-OFDM transmission systems. Section 3 describes the mathematical formulation of the parameters used for the assessment of the ED efficiency affected by a distinct range of NU and DDT levels. Section 4 describes the algorithm used for evaluating the ED performance in MIMO-OFDM transmission systems. The extensive discussion on the simulation results presented using ROC curves is shown in Section 5. Finally, the conclusion is given in Section 6 of the paper.