## 8 Future Challenges for the Enhancement of ED Limitations
### Research Challenges Related to Noise Uncertainty
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Major ED Disadvantages** & **Challenges in Future ED Research** & **Possible Enhancements** \\ \hline Reduction of detection precision as a consequence of variations in noise power (NU) [3; 8; 10; 16; 45; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64] & Implementation of new algorithms for optimal selection of DT and its assessment & Improvement of ED accuracy in environments impaired with noise fluctuations (NU) \\ \hline Degradation of detection accuracy at low SNRs [55; 56; 57; 58; 59; 60; 61; 62; 63; 64] & Development of new noise estimation techniques & Improvement of ED accuracy at lower SNRs \\ \hline Reduction of detection accuracy for a low sample quantity [72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82] & Optimising detection time (sample quantity) for exact detection & Increase in throughput of SU, reduction in SU energy consumption (in CWSN) \\ \hline The existence of more accurate local SS methods [7; 77; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124] & Combining the ED approach with other approaches for SS at the same SU node & Ensuring desired sensing performance or accurate detection of specific idle bands \\ \hline Degradation of detection accuracy due to fading channels and hidden node problem [66; 68; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124] & Novel collaboration approaches among SUs for reducing the impact of fading or hidden node problem [66; 68; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124] & Improvement of ED robustness in fading channels and elimination of any hidden node problems \\ \hline Inability to differentiate between interference, PU and SU signals [125; 126; 127; 128; 129] & Collaboration among distributed SUs based on combining ED with other detection methods in collaborative sensing & Elimination of the problem of PU, SU and interference distinction, and contribution to the improvement of wideband sensing \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of main ED drawbacks and expected improvements. the adaptive threshold has a low false alarm rate when the standard deviation coefficient of the noise is selected properly. This approach can satisfy the detection requirements of multi-channel CRs for either narrow or wideband SS. The adaptive threshold control for ED was implemented in Refs. [60; 63] with a linear adaption of the threshold based on SINR (Signal to Interference plus Noise Ratio) [61]. This approach achieves a much higher SU throughput than the energy detectors with a fixed threshold, while maintaining the good stability of false alarm and missed detection probabilities. The proposed approach is based on the concept of estimating channel noise in which only noise is received by the SU. In Ref. [62], the usage of DT computed based on minimising the probability of error while keeping the transmission rate was proposed. The result indicated that the probability of an error occurring can be minimal when the DT scheme is used, while the noise variance can be constant or variable in the channel. In Ref. [49], an estimated noise variance was used to calculate the DT for the SS based on ED. In Ref. [10], a new adaptive threshold algorithm was proposed for the discrete wavelet packet transform and [PERSON]'s ED methods to assess the trade-off between detection and false alarm probability. The proposed adaptive algorithm demonstrated that the target performance requirements can be achieved for very low SNRs (\(-18\) dB). According to the results presented, the constant change in noise power over time makes the dynamic ED threshold adaptation a demanding activity and no optimal algorithm has been deduced so far. Thus, the enhancements in the algorithms which will set the iteratively optimal DT in any moment of ED process remain an open research issue for SS based on the ED method.