## 3 System Modeling  
In a wireless communication system, the signal power not only decreases with increasing propagation distance, but it is also affected by the additional fading caused by a variety of obstacles or complex terrain. At the same time, this additional fading will change randomly due to the movement of the mobile station and the change of the channel environment. The time-varying characteristic of this wireless channel will cause the received signal to be distorted, with the waveform broadened and overlapping, which affects the communication quality of the channel [18]. In severe cases, it may lead to communication abnormalities.  
The traditional adaptive silence management algorithm uses the shared data band to transmit feedback information. When the data band is affected by the environment and causes a sharp decline in the signal, the system performance of the scheme deteriorates significantly due to the fact that the SU cannot obtain the feedback information reliably.  
Considering that the small-scale fading in a wireless channel usually has frequency selectivity, we made a decision to construct a cognitive wireless communication system with an additional signaling channel (CWCS-ASC). The corresponding system model is shown in Figure 2. By selecting the specific frequency band outside the spectrum of the data channel as the signaling channel, adding the signaling channel not only avoids the influence of the PU on the signal transmission, but also enables the SU to divide the time into non-overlapping frames, and then divide the frames into non-overlapping channels. Additionally, the users will have one-to-one correspondence according to the time slot, in order to distinguish users from different addresses of the signal, thus completing the multiple access. Furthermore, opening additional signaling channels can increase the additional transmission power and through the error correction coding to reduce the noise interference, the system can still guarantee the reliable transmission of feedback information even when the data band suffers a great fading.  
Figure 1: The slot division scheme of the SPM algorithm (**a**) Traditional silence period; (**b**) Adaptive silence period.  
The information interaction sequence diagram for the CWCS-ASC system is shown in Figure 3. The SU decides whether to transmit data by the data band based on the perceived results of the current silent period and their own communication needs. The time window used to transmit data is defined as the data transmission period, whose length of time needs to be less than the sensing interval \(t_{i}\). The difference between the sensing interval and the data transmission period is defined as the free period. The longer the silence period, the higher the reliability of the detection, the lower the utilization rate of the band, and the smaller the throughput of the cognitive user [19]. The presence of the free period ensures that the data transmission from the SU does not interfere with the spectrum sensing in the next sensing period.  
In order to avoid communication conflicts, after obtaining the sensing result, the SU will use the signaling slot in the signaling band to transmit interactive information which is used to coordinate the allocation algorithm among SUs. In addition, after a certain secondary user (SU1) sends the business data, the other SUs in the region will generate feedback information based on the result of the received signals. Then they send the feedback information to the SU1 via the feedback slot in the signaling band. The SU1 can estimate the channel characteristics based on the feedback information obtained, thereby reasonably adjusting the configuration parameters for the next silent period.