## 5 Simulation Results  
This section provides extensive computer simulations carried out jointly using STK and MATLAB to evaluate the performance of SU spectral efficiency for the proposed spectrum sensing scheme. For analysis, we have considered the number of cooperating SUs is \(K=4\) unless otherwise stated. The prior probabilities of presence and absence of PU are set to be \(p(\mathcal{H}_{0})=0.8\) and \(p(\mathcal{H}_{1})=1-p(\mathcal{H}_{0})=0.2\). We suppose that the number of samples collected by each SU is \(N=1000\), and compressed domain sampling frequency is \(f_{s}=10\) KHz. The PU's power is \(\sigma_{s}^{2}=10\) dBmW, the noise power \(\sigma_{n}^{2}=-10\) dBmW the channel gain between PU and \(i\)-th SU \(h_{i}\) obeys the Rayleigh distribution with mean \(\overline{h}=-10\) dB, the channel gain among SU \(g=0\) dB. For the novel CSS scheme, the frame duration is \(T=200\) ms, in which the single reporting duration \(t_{r}=1\) ms when the information transmission rate from SU to FC is assumed to be 1 kbps. The chosen LEO height is \(h=1000\) km, and the feedback slot is approximately given as \(t_{0}=10\) ms taking space propagation delay and final decision-making delay into account. Figure 6 shows SU spectrum utilization efficiency versus the compressed sensing time allocated to each SU. The simulation results validate that SU spectral efficiency \(\mathcal{C}\) is the unimodal convex function with regard to compressed sensing time \(\tau\). There indeed exits one optimal point \(\tau_{opt}\) standing in the curve that maximizes the SU spectral efficiency. We can see that the spectral efficiency increases as \(\tau\) increases from 0 to \(\tau_{opt}\), whereas decreases when \(\tau\) is increased further. This is because little sensing time \(\tau\) degrades sensing performance, while long sensing time \(\tau\) results in short transmission duration. For a given SNR, \(\mathcal{C}\) decreases with increased targeted detection probability. Particularly, for SNR \(=-6\) dB, the maximal spectrum efficiency is 1.4 Bits-s\({}^{-1}\).Hz\({}^{-1}\) when target detection probability is 0.8, while it is only 0.8 Bits-s\({}^{-1}\).Hz\({}^{-1}\) and 0.5 Bits-s\({}^{-1}\).Hz\({}^{-1}\) when target detection probability is 0.9 and 0.95, respectively, which denotes that SU spectrum utilization decreases with debasement of the interference to PU. It can also be seen that optimal sensing time \(\tau_{opt}\) increases with the increase of SNR values. Taking \(p_{d}^{th}=0.9\) for example, when SNR \(=-6\) dB, \(\tau_{opt}=30\)\(ms\) to achieve the maximal spectral efficiency; when SNR \(=0\) dB, \(\tau_{opt}=60\)\(ms\) to achieve the maximal spectral efficiency. In order to achieve a tradeoff between interference to PU and spectral efficiency, the target cooperative detection probability \(\hat{Q}_{d}^{th}\) is set to be 0.9. In Figure 7, we plot SU spectral efficiency of the novel CSS with different reporting scheduling strategy, when the optimal sensing time is employed to cases where \(n\) is fixed at 1 (OFR), \(n\) is fixed at 2 (MFR), and \(n\) is fixed at 4 (AFR). The SNR \(\gamma_{i}\) at the \(i\)-th SU is set to be \(\{-20\) dB, \(-16\) dB, \(-12\) dB, \(-8\) dB}. The first scheduling strategy is the descending sort of received \(\gamma_{i}\), while the last is the ascending sort of \(\gamma_{i}\) among the total 24 cases of scheduling strategy. From Figure 7, it shows the novel CSS scheme outperforms the general CSS in the frequency reuse. In fact, the spectral efficiency of the novel CSS is closely related to scheduling strategy closely, while it has no effect on the general CSS. The optimal scheduling strategy for AFR is reporting the  
Figure 6: Spectral efficiency \(\mathcal{C}\) versus sensing time \(\tau\) under different levels of SNR and \(p_{d}^{th}\). sensing results in the ascending sort of SNR, and reporting the sensing results in the descending sort of SNR can the unoccupied spectrum been fully utilized for OFR and MFR. This is consistent with the aforementioned theoretical proof.