# Assessment of Dynamic Swarm Heterogeneous Clustering in Cognitive Radio Sensor Networks

Authors: Ruby Bhatt; Edeh Michael Onyema; Khalid K. Almuzaini; Celestine Iwendi; Shahab S. Band; Tripti Sharma; Amir Mosavi
Journal: Wireless Communications and Mobile Computing
 (2022)

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## Abstract

Many optimization algorithms have been created to determine the most energy-efficient transmission mode, allowing for lower power consumption during transmission over shorter distances while minimising interference from primary users (PUs). The improved cooperative clustering algorithm (ICCA) performs superior spectrum sensing across groups of multiusers compared to any other method currently available in terms of sensing inaccuracy, power savings, and convergence time than any other method currently available. The proposed ICCA algorithm is employed in this research study to find the optimal numbers of clusters based on its connectivity and the most energy-efficient distributed cluster-based sensing technique available. In this research, many randomly chosen secondary users (SUs) and primary users (PUs) are investigated for potential implementation opportunities. Therefore, as compared to the current optimization strategies, the proposed ICCA algorithm enhanced the convergence speed by integrating the multiuser clustered communication into a single communication channel. Experimental results revealed that the new ICCA algorithm reduced node power by 9.646 percent compared to traditional ways when comparing the novel algorithm to conventional approaches. In a similar vein, as compared to the prior methodologies, the ICCA algorithm reduced the average node power of SUs by 24.23 percent on average. When the SNR is decreased to values below 2 dB, the likelihood of detection improves dramatically, as seen in the figure. ICCA has a low false alarm rate when matched to other optimization algorithms for direct detection, and the proposed method outperforms them all. Following the findings of the simulations, the proposed ICCA technique effectively addresses multimodal optimization difficulties and optimizes network capacity performance in wireless networks. A detailed discussion of SS applications for the IoT and wireless sensor networks, both based on CR, is provided. There is also a thorough discussion of the most recent advancements in spectrum sensing as a facility. IoT or WSN may be essential in feeding the CR networks with spectrum sensing data and the future of spectrum sensing. The use of CR for fifth generation and afar its potential application in frequency allocation are discussed. To stay up with the advancement of communication technology, SS should give additional features to remain competitive, like the capacity to investigate various available channels and accessible places for transmission. Based on present and prospective methods in wireless communications, we highlight the crucial upcoming study paths and difficulty spots in signal processing for cognitive radio and potential solutions (SS-CR).


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## Body

## 1. Introduction

Improvements in the precision of spectrum sensing in CR are a crucial problem in today’s communication scenarios [1, 2]. For the objective of selecting the best spectrum nodes, a number of diverse methods have been given. As a result of cooperative sensing, which involves the participation of all secondary users, reliable sensing information with the maximum system dependability is obtained while also decreasing the sensing time [2, 3]. Because secondary users are responsible for communicating the detected data, a centralized control unit is not required. It is possible for each user to identify and utilise the main channel via distributed cooperative sensing, which reduces the latency and transmission burden associated with making choices about the presence or absence of a major signal [4–6]. Clustering algorithms have been created that include groupwise constrained agglomerative, k-means, distributed spectrum aware clustering (DSAC), and k-neighborhood clustering processes, to name a few. Particle swarm optimization (PSO) and firefly algorithm (FA), to mention a few examples, are often used to fine-tune the clustering process. The jumper firefly algorithm (JFA) is one such optimization approach. A significant amount of energy would be used since the sink station from cluster head (CH) is placed at a large distance. The PSO algorithm is included in this protocol in order to address the aforementioned issue while also increasing the protocol’s lifetime and energy efficiency. Groupings of nodes are generated via optimization algorithms, and the cluster head is selected according to the amount of energy contained inside the cluster. Afterwards, data transmission for the sensor nodes commences, with the shortest path feasible being used to do this. Several researchers, including Kaliki et al. [7], have developed a firefly approach for node clustering, which can be used to two distinct types of clustering: partitioned clustering and hierarchical clustering. With the hierarchical technique, a big numbers of hierarchical cluster are grouped together into a lesser numbers of cluster with surrounding centroids, resulting in a reduced number of clusters. Both of the following approaches may be used to implement it: (i) the agglomerative approach repeatedly merges two more similar clusters until a single big cluster comprising all of the data is created. (ii) The divisive method iteratively splits the most different groups until each item belonging to a single cluster is formed. (iii) The partitioned clustering approach creates discontinuous groups of data without creating a hierarchical structure in the process. To represent the data contained inside each cluster, prototypes are employed. The authors [8] were able to determine the optimal solution in the status table by using JFA in the base station, which they subsequently put into effect after testing it. In order to identify an acceptable scenario-based function for the jumping process, the present situation of the table is observed and utilised to determine an appropriate scenario-based function. It is necessary to perform this method on the search agents in order to finish the decision-making procedure.Adaptive spectrum access techniques are essential due to inefficient use of the limited spectrum, which serves users that do not have valid spectrum licenses, and in order to accommodate these users. The users of the CR have been given permission to temporarily utilise the licensed spectrum that has been left unused. The spectrum management features of CR enable SUs to choose the best suitable frequency from a number of available bands based on their specific requirements and capabilities. Because of the constantly changing channel characteristics, the quality of service (QoS) changes over time. Four difficulties must be resolved before effective spectrum management can be achieved spectrum decision-making, spectrum sharing, spectrum sensing, and spectrum mobility. The common control channel issue, the power management technique, the spectrum sensing and scheduling strategy, the hidden and exposed terminal problems, and mobility are all addressed in this study. A high-level summary of the technical and regulatory concerns connected with opportunistic channel access is provided in this report (OSA). According to the OSA standard, it is feasible to effectively cohabit with both licensed and unlicensed users in the same frequency band while maintaining signal integrity.In order to maximise the capacity of SUs, many optimization methodologies, including convex optimization, have been suggested in the scientific literature. The authors of an existing research proposed an optimal time-sharing technique as well as a power-allocation plan for CR broadcast channels. Because the average transmit and interference powers are limited, the optimal power optimization techniques given in the existing research are intended to function within these constraints, respectively.The use of a dual-path FSO system for long-distance communication is suggested, with each route having two hops and each path having two hops with the aid of the SSC diversity system; the optical signals arrive at destination D where they are combined. In the DSSC system, one of the two paths is selected depending on how optimum it is for the situation. Whenever the current SNR goes below a certain threshold, the other route is activated. Because the relays on both channels operate in automatic failover mode, it is predicted that the distributions for the two branches will not be identical to one another (AF). The Malaga (‘M) distribution is assumed to be used for the hops of the S-RA-D connection, and the GG distribution is assumed to be used for the hops of the S-RB-D connection, as shown in the diagram. A lengthy amount of time has passed since the Malaga distribution was well acknowledged for its ability to replicate mild to high turbulence, while the GG distribution has been widely recognized for its ability to simulate FSO channels. There are two nonregenerative nodes on paths A and B, which are represented by the relays RA and RB, respectively, on the network. They initially enhance the signal before transmitting it to the next step of transmission. Each FSO route is configured with one of two modulation schemes: an IM/DD modulation scheme and an OOK modulation scheme. CSI access should be available at both the relays (RA and RB) as well as the destination node, according to expectations (D)The chance of a given signal being detected by a signal detector is referred to as detection probability in the field of signal detection. Consider the case in which a detector reports the presence of PU even though the PU is really absent. In such a circumstance, the chance of the detector reporting the presence of PU is one hundred percent. First and foremost, the likelihood that a detector would signal the presence of PU when, in fact, PU is absent is referred to as the false alarm probability (FAP) (FAP). It is three times more likely than not that a missed detection will occur: it is defined as the likelihood that the detector will claim PU to be missing while in fact PU is present when talking about probability. When it comes to computing, the time takes for a secondary user to notice the existence of a main user channel and to decide whether or not the channel is open is known as the detection time. When the sensing period is excessively lengthy, the throughput of the SUs suffers a decrease. It is possible for the sensing time to be too lengthy, and the data transmission period to be too short, resulting in reduced throughput (sensor units). The SNR may be described as the fraction of signal intensity to noise. SNR of 1 is achieved at the CR receiver due to the intensity of the delivered PU signal and the propagation environment. The sensing time, signal-to-noise ratio, and detection threshold values, among other things, are used to link the two error probabilities together. It has been shown that refining the SNR may increase the performance of detecting systems. When it comes to communication channels, the number of successful messages that are transmitted over them at any particular time is described as the number of messages that are sent over them. The term “throughput” refers to the amount of data that can be processed by the algorithms that are used to gather and analyse data. There are additional considerations to take into account, such as the overhead for collaboration and the length of sensing. In order to determine the modulation type of a PU signal, one of the following methods is employed: Because in certain situations, it is vital to know what sort of modulation is being used in a signal in order to appropriately adjust the receiver settings, the ability to identify modulation type in a PU transmission is a desired quality of that signal. Complex sensing algorithms that are difficult to implement are preferred over simpler and more implementable sensing algorithms that are equivalent in terms of complexity and implementation but are less energy-efficient in terms of energy efficiency.Machine learning algorithms are being utilised to improve the performance of statistical sampling, which is an important instrument in the field of critical care (CR) (SS). To illustrate, take the binary classification problem that occurs when PU is present in the system: SS in contrast to normal SS, learning techniques may be able to remove the requirement for previous knowledge of the statistical features of the channel or the PU signal, which may be advantageous in certain situations. As an added bonus, these methodologies are presented for forecasting PU activities, which may increase the secondary network’s spectral efficiency while also protecting the main transmission from secondary interference by boosting the PU activity prediction [9].With the introduction of the Internet of Things and wide area networks, the scope of the use of CR has been broadened. This was triggered by the necessity to offer additional frequency resources [10] due to the high numbers of new IoT or WSN devices that are using more frequency resources [11].A lot more study is needed on the application of CR for WSN and IoT. For example, the design of exchange protocol and the managing of access rights are two areas that need to be explored more thoroughly.As previously mentioned, cognitive radio of the 5G is probable to have a vital role in addressing needs of an increasing numbers of data-hungry gadgets in the future. Given that 5G will broaden the spectrum range to encompass mm-wave frequencies, it is possible that CR might be used to maximise spectrum utilisation while simultaneously providing greater security to coexisting users in the same area. Aspects of CR that are applicable to the space, frequency, and time domains include the ability to cope with interference problems that emerge in these domains. Considering that one of the key aspects of 5G networks is expected to be the usage of spatial reuse of spectrum, this is essential information. 5G networks are expected to be more efficient than current networks in a number of ways. A number of challenges must be solved before CR may be integrated into 5G networks, according to the authors [12].Because it has shown to be one of the most efficient approaches for ensuring fair and flexible frequency allocation, CR is suggested for a variety of wireless communication technologies. Cognition realizes the benefits of the rise and growth of learning methods that are used to wireless communication networks. As a result, in order to stay up with the most recent technical breakthroughs, SS needs to be updated on a regular basis [13]. A number of challenges arise in this environment, including the necessity for large amounts of frequency resources as well as a sense of geographical availability, intelligent spectrum sensing, and the creation of energy-efficient protocol designs.SS has been the subject of many papers in the literature that study its use in the setting of CR. The authors examined a broad variety of features of spectrum sensing from the perspective of cognitive radio in their paper titled. However, this research was published more than a decade ago and does not include any of the most recent applications or paradigms in this subject, which makes it out of date. By emphasising on the concurrent transmit-sense mode, this review provides an overview of the FDCR technique; alternative methods, such as transmit-receive, are not included in this survey. Specifically, reference covers in detail the issues connected with implementing CR in IoT networks, with a special focus on the concerns related with SS. Reference is a good source of information on IoT networks. A general introduction of the techniques employed in SS is provided, with a specific attention on wideband and compressive sensing [14], as well as examples of their application.When it comes to SS, the authors present an overview of contemporary techniques while highlighting the mathematical models that are utilised to produce the SS measurements (detection and false alarm probabilities). It is incapable of supporting new communication paradigms like full-duplex communication or new applications like those based on the IoT. The research is only concerned with the technological issues associated with the deployment of CR in the Internet of Things. After that, the authors of [15] examine the use of CR for fifth generation communication deprived of offering any more information on recent advancements in SS.The purpose of this study is to give an in-depth review and evaluation of current research accomplishments and possible application in the fields of statistical computing for critical care research. It is advised that readers contact in order to get a numerical evaluation of SS techniques. We present an overview of the current state in the context of SS for CR [16], as well as an explanation of the fundamental concepts of SS.It has been developed a revolutionary clustering approach which would extend the life of CR sensor network by grouping them together in an energy-efficient way. This novel clustering technique is being introduced in the present research activity. According to the recommendations, the study’s efforts should be directed on cooperative sensing among secondary users, which was eventually accomplished. According to the objectives of this research, correct sensing information should be collected to decrease the number of false alarms and enhance system dependability. This will decrease the amount of time spent sensing and enhance the detection rate of system. In the end, heuristic algorithm, for example, distributed groupwise constrained clustering (DGCC), distributed global search clustering (DGSC), distributed clustering firefly groupwise constrained (DCFGC), distributed clustering jumper firefly groupwise constrained (DCJFGC), and adaptive swarm distributed intelligent-based clustering (ICCA), techniques have been used to compare the algorithms [17].According to the DGSC algorithm, each cluster node advances in the direction of the finest swarm particle with the smallest neighborhood distance [18], which is the finest swarm particle with the shortest neighborhood distance. Next, every velocity of particles and location are estimated based on objective function, with the stop criteria being the place with the greatest probability of occurrence in the whole universe. In the findings, it has been shown that, when compared to other genetic algorithms, the DGSC algorithm has a convergence rate that is comparable to that of the genetic algorithms being compared. A significant disadvantage of the DGSC approach is its weak convergence rate as well as its restricted ability to do local searches [19].In response to these restrictions, DCFGC was developed, and it is capable of grouping the nodes that are the most cognitively effective [20]. As part of the processing, the DCFGC algorithm guides all of the cognitive nodes toward the brighter firefly, resulting in an organized cluster in the least period of time feasible. As seen in the DCFGC technique, the firefly disappears at critical points, and when clustering without a status table, the algorithm does not recall the history of prior events in the same way that it does when clustering with a status table. As a consequence, the DCJFGC algorithm [20], a third technique, has been proposed as an alternative. They are nonlinear optimization algorithms inspired by the attraction and intensity behaviour of fireflies. The DCJFGC and DCFGC algorithms are two examples of such algorithms. It is important to use the DCJFGC approach in order to compile a comprehensive picture of the current record situation, with any updates being noted in the table’s status. DCJFGC algorithm improves spectrum access for SUs and PUs by a wider margin than the DCFGC approach, according to the researchers. If you compare it to the DCFGC and DGSC algorithms, the convergence speed of DCJFGC algorithms is much quicker. As a result of this method, a high probability of detection was achieved, as was an optimal number of cluster communications.As a final step, an optimization method called the adaptive swarm distributed intelligence-based clustering algorithm was developed, with the primary goal being to diminish the mean sensing time of the PUs by using cooperative SUs, because it was impossible to parallelize with all of the locations initially. The strength of light force is calculated by the application of objective functions, and as a consequence of this calculation, the whole population is separated into subswarms (see Figure1). The performance of many clustering algorithms, including DGSC, DCFGC, DCJFGC, and ICCA, is investigated in this work using performance analysis.Figure 1
An illustration for cooperative ICCA clustering structure.It is necessary to compare the following performance metrics so that a fair comparison may be made:Between the conservative design concepts are the following: conservative merge duration with CRSN-sizes, conservative node power for matching clusters number, conservative power for PU’s and SU’s nodes, and a conservative spectrum sensing detection approach, all of which are instances.Listed below is the general organization of this paper: it is discussed in Section2 what present state of cognitive radio network research is, as well as the difficulties that have been faced. A brief summary of the proposed ICCA algorithm is provided in Section 3, along with a description of the proposed cluster communication power reduction technique. Section 4 offers a thorough analysis of the findings as well as an explanation of the proposed technique. Finally, in Section 5, the final conclusions are made, which also contains suggestions for additional study.


## 2. Background

This algorithm was developed by Alnuaim et al. [6] to bring the spectrum sensing operation to a successful conclusion. It functions on the basis of three separate steps, which include, among other things, channel detection, beaconing, and coordination. Every available vacant channel was identified and evaluated in connection to the previously felt results that had been collected during the channel sensing stage. It was discovered that there were several vacant channels; therefore, the node used them to broadcast its information during the beaconing process. In this stage, the strength of surrounding beacon signals was determined, and pairwise distances were calculated based on the findings. Following that, during the intercluster coordination stage, the clusters and CHs were updated to reflect the most up-to-date information available at the time. The clustering procedure was also restarted from the beginning if the SUs or PUs moved or changed their positions. As a consequence of the network’s relocation, there was an increase in the amount of control required.A method known as the Gale–Shapley matching algorithm was developed by Lalama et al. [21] in order to build a channel allocation mechanism for the SUs of a cognitive radio network. SUs were assigned to a channel using the SPDA channel allocation mechanism, which was used to locate an appropriate channel for them among the available channels. Channel allocation in this situation was based on two distinct factors: the service needs for the SU class of services and the quality of the channels available for allocation. The newly developed SDPA was utilised to increase throughput on the secondary network while concurrently reducing congestion on the main network as a result of this. The established SPDA, on the other hand, just looked at SU satisfaction and did not take into consideration PU or SU satisfaction in its calculations.Alnuaim et al. [22] conducted an analysis of the average spectrum efficiency for MRCN network based on multislot statistical spectrum status in order to assess the network’s efficiency. A method called SMCA was developed to transform the nonconvex complex fractional MINLP characteristic into a successive convex characteristic by employing the iteration convex optimization strategy developed as a result of this. Cognitive radio networks (SMCA) are a bit more sophisticated in terms of data transport than traditional radio networks.Using a Markov-based logical model, the authors [23] estimated the gain for a nonswitching spectrum handoff technique including multiclass SUs, which was then used to compute the gain for a switching spectrum handoff approach. In CRN, several spectrum access approaches are linked via the use of hybrid interweaves. In order to underlay and interweave spectrum access approaches, the model-based approach is utilised. As long as an SU has a large number of prioritised classes, each prioritised class is taken into consideration in the traffic for the purpose of accessing the hybrid spectrum access method that offers sufficient quality of service in the delay-sensitive traffic. Whenever the primary traffic load on the cognitive radio network is higher, the SU must wait for each channel to become accessible for longer periods of time before it can use the channel that is available.Ganesh Babu and Amudha [24] developed semitensor product compressed spectrum sensing, which use the semitensor product for spectrum signal reconstruction and energy compression, as well as other techniques (STP-CSS). When developing the STP-CSS technique, a collection of wideband random filters were used in conjunction with one another. Because the perceptual reconstruction process was carried out in parallel with the spectrum sensing process, the speed of spectrum sensing was increased as a consequence of this. According to the findings, the CRN becomes less efficient when the compression ratio decreases and the amount of energy used increases.Rahim et al. [25] introduced a semisoft decision approach for cooperative spectrum sensing (CSS) to solve the tradeoff among band costs and sensing performances. The technique is both cost-effective and high-performing. The development of the sensing technology made use of three different components. In the beginning, the energy detection-based PU signal detection was accomplished via the use of a local detection module, and the final judgement findings were delivered as either 1-bit or 2-bit information, depending on the application. In addition, a data reconstruction module was used by the fusion centre to evaluate the decision data that had been gathered. The final conclusion was obtained at the global decision module, in part, by merging the results of the assessments and producing a single determination. In contrast, a huge numbers of sensors supplied data to the fusion centre, which necessitated the need for greater capacity on the cognitive radio network.In the study conducted by the authors [9, 10, 26], they developed an iterative strategy for determining the most optimal sensing and transmission time. According to the findings of the research, the reporting, detecting, and sending times should be adjusted at an ideal amount in order to improve throughput while minimising energy consumption for second-tier users.Almost all of the previous research articles have a number of shortcomings, which are discussed below. More overhead, more energy consumption, higher latency, and an accidental mismatch between the joy of the SU and the pleasure of the PU are just a few of the drawbacks. The ICCA is being developed in order to overcome the limitations mentioned above by enhancing the speed at which the solution is reached. In addition, the ICCA is advantageous in terms of strengthening the detection capacities of an organization [19].


## 3. Proposed Improved Cooperative Clustering Algorithm for Cognitive Radio Networks

Figure1 depicts a diagrammatic representation of the proposed ICCA technique. A dynamic spectrum environment divides cognitive radio sensor networks (CRSNs) based on the connectivity of clustered structures, represented by the letter “K,” and each of the CRSN nodes is in charge of identifying the channels that are accessible to them. Allotted to certain frequency bands, the PUs and SUs run sequentially and continuously within those bands. As a result, the CRSN avoids the reclustering process problem that has plagued earlier techniques, and nodes are grouped according to the swarm intelligence embedded into the clustering methodology [27], rather than according to the conventional clustering process, as previously described. It is possible to have several nodes in the sensor network, and an optimal cluster is created by merging the nodes that are in the ideal place. The following equation is used in order to determine the most optimal clusters [6].
(1)Kopt=Ndmax3ρ+0.5.The number of available nodes is represented by the equationN, the number of CRSNs available corresponding to the unit area is represented by the symbol, and the maximum transmission range between CRSN nodes is represented by the symbol.It is decided which CH will be picked at random from among the several clusters that are within transmission range of the base station. The information received from the base station is sent to the appropriate locations of the other CRSN nodes, using the list of CHs that has been given in the configuration. Each of the CRSN nodes functions is called into action in order to estimate the distance between themselves and the CHs nodes, which is a function of the distance between themselves and the CHs nodes. The ICCA that has been proposed ranks the CHs according to the fitness function at the lowest possible cost. It will find the CRSN node with the highest cost fitness function among those available if there are a big number of CRSN nodes available, and it will converge more quickly if there are a large number of CRSN nodes available [28]. The lifetime of CRSN energy is prolonged in the case of efficient clusters that are employed to construct the dynamic channel allocation system, as shown in Figure 1. The channels become inaccessible as soon as they have been deployed by the SUs under the transmission range of the PUs. The spectrum is intended to be used in an opportunistic way to access dynamic spectrum techniques that are being used for next-generation networks in order to optimize its use. It is necessary to transport information collected locally between each SU and between other SUs that do not have access to a central control unit. All of the cognitive users in the distributed cooperative sensing environment have been programmed to find the unused main channel in the way that they have indicated in their configuration files.The CR was selected for communication since it was the most available spectrum band at the time of the incident. Spectrum management is responsible for determining the physical attributes of the spectrum, the reconfiguration operations of the CR, and the selection of users’ needs for the spectrum in question. To improve the overall criteria in terms of overall quality of service spectrum, the most appropriate spectral band from the next-generation network is selected and used to increase the overall criteria [28]. This is done to increase the overall criteria in terms of overall quality of service spectrum. A channel identification scheme is used in accessible channels to identify channels that are not being used. This helps to prevent interfering with other channels. Each of the channels that make up the unit of measurement is represented by a distinct colour, which is shown on the display. By using PU nodes that are available in both CRSN and the same cluster, the common channel is not conquered by the CRSN nodes or the same cluster, resulting in a more stable network. Each CH detects the channels that are available and selects one channel to which they assign one of the channel members that have been made available to them by the other CHs. While channel selection is occurring, the condition checks for a match with the cluster head, which selects the channel depending on how close the PUs are to the channel that is being picked [29]. Different clustering algorithms are used to communicate the best CHs, and channel members are assigned based on the power of the least powerful clustering node, which aids in the solution of multimodal optimization issues by addressing their many aspects. The hypothesis model examines the following criteria in order to determine whether or not PUs exist.
(2)rt=ntH0,hst+ntH1.It can be seen in the equation thatrt represents the received signal from the SUs, st represents the signal broadcast from the PUs, and nt represents the zero mean of the AWGN. The channel’s amplitude gain represented by letter “h.” It is also known as the “null hypothesis” when there is no evidence of PU across a certain spectrum band channel represented by the value “H0.” As an example, the hypothesis known as “H1” is known as “PU presence over the channel,” and it is sometimes referred to as the “presence hypothesis.”


### 3.1. Calculations of Power Using ICCA Algorithm

The CRSN nodes are linked to each CH in order to compute the shortest distance between the base station and the CH and to pick the CH. The cluster must adhere to the following two requirements:In order for the CH to be effective, the base station’s communication range must be confined inside it. Additionally, the residual energy of the CH must be high.The entire amount of transmission power utilised for communication is shown by the symbol:(3)Ptx=∑k=1K∑i=1NDistminnik,CenterkCH.The minimum distance between the two points of pair of coordinates such asx0,y0 and x1,y1 is calculated by using the following:
(4)Distmin=x0−x12+y0−y12.It is made out of over-the-air readings of authentic radio transmissions that have been altered with 11 different modulations taken on genuine radio broadcasts. According to the authors, the signals were created using USRP-B210 that linked to computer running GNU-Radio. The various transmitters needed to be developed by utilising the same source code and data providers as RadioML 2016.10A; hence, it remained critical to utilise same data providers and source code as previously. It should also be mentioned that an irregularity with AM modulation, which was modified in subsequent version of the RadioML datasets, was also modified for the sake of generating our dataset.When dealing with constrained optimization, the goal is to maximise or minimise a certain objective/cost function while taking into consideration a range of constraints. Because linearity characterises both the objective function and the restrictions, the problem is turned into linear programming, and it becomes fairly straightforward to locate the global optimum in this situation [30]. When the problem is convex, on the other hand, different techniques are applied to find the global optimal solution.When it comes to addressing an unconstrained convex issue, the gradient technique and Newton’s method are the two most often utilised methodologies. As illustrated in the picture, after each iteration of the gradient method, the initial feasible point moves in the direction of the gradient in the direction of the optimal value towards the optimum value. Newton’s technique is more rapid in terms of convergence than the gradient approach; nevertheless, it necessitates the calculation of the objective function’s Hessian as part of the convergence process, which is more time-consuming than the gradient approach.When solving constrained convex problems, the interior point technique, the ellipsoid method, and the projected gradient algorithm are all employed to achieve success. An objective function is penalised when it crosses the boundary of the search point; a technique known as the inner point approach is used to do this. It is used to construct a sequence of ellipses in the feasible set, which are then assessed using the ellipsoid approach. With each iteration, the volume of the ellipse decreases, resulting in the convex function being enclosed in the maximum feasible enclosure.Nonlinear programming is used to describe optimization problems in which one or more of the constraints or the objective function are nonlinear (NLP). For global optimization of NLP in practise today, the interior point approach, genetic algorithms, and simulated annealing are the three most often used techniques for global optimization of NLP. The maximum and minimum values of a multivariable function are defined by the Lagrange multiplier, which is a mathematical constant. The Lagrange function has been enhanced in this version.We collected the signals on the receiver side with the MIGOU platforms that we created and constructed from the ground up. It is meant to circumvent the hardware-architectonic restrictions which then prevent CR study and testing with low-power end-device as of being undertaken magnificently when paired with SDR capabilities. When a communication channel was detected, the platform was configured to transport the raw I/Q sample to a computer, which would then stock them in proper database for later retrieval.All of these observations were taken inside in an atmosphere that should be in control like the labs or offices. Specific computations were performed at one-metre and six-metre distances from the transmitter. At two spans from transmitter, the average SNR were 37 dB and 22 dB, correspondingly [31]. To obtain concluding outcome, every recorded I/Q signal was split and treated into 128-bit vector that was then separately normalised. The closing stage was to add 400,000 normalised vectors for each MOD-SNR mixture in the dataset, for an overall of 8.8 million vectors in last dataset.Internal cluster communication between cluster members may transmit information from the source node to the CH centre location over the completely available channel if it is carried out via the shortest distance possible for node communication between cluster members [32]. The k-th cluster’s minimum distance from the i-th node is represented by the symbol, while the k-th cluster’s centre position of CH is represented by the symbol.
(5)PintraTot=∑k=1KDistminnik,CenterCHCk=∑k=1KDistminnik,njkck.Similarly, in the intercluster communications, the CH position is at the centre, gathers the information from the source node, compressively communicates with the CHs that are nearer, and finds the shortest communicating power among the clustered centres [33]. The minimum distance for the k-th cluster with the j-th node is represented as “ck” and CH central position for the k-th cluster is indicated by ck represented as
(6)PinterTot=∑k=1K∑i=1N∑j=1MDistminCenterkCHi,CenterkCHjck=∑k=1K∑i≠jDistminCenterkCHi,CenterkCHjck,where CenterkCHi is the i-th CH’s centre position in the k-th cluster (ck) and CenterkCHj is the j-th cluster centre position for the k-th cluster is ck.The coordinates are used for calculating the minimum distance, which is expressed as shown in the below equation.PTot is represented as the summation between the power for interclustering and intraclustering communication, where Pinter is the power for interclustering communication and Pintra is the power obtained for clustering communication.
(7)PTot=∑i=1NPintra+Pinterni.


### 3.2. Proposed ICCA Algorithm

From Figure2, the proposed ICCA sensor network is made up of “N” nodes with preset clusters of “N” indicated as “K” which is composed of the following:
(1)
“S” is the number of element sets for comprising “K” CHs chosen arbitrarily among all the CH suitable candidates. At every node points, ni(i=1,2,..,N) determines the distances dni,CHp,k among the nodes of all CHs’ positioned points allocated in the CH point at each nodeni. Here, CHp,k is the k-th CH of the particle p, and dni,CHp,k=mindni,CHp,k for k=1,2,..,K(2)
The centralized algorithm [34] uses the clustering process criteria to acknowledge the base station. FA finalizes the CHs at base station to identify best optimal location for the cost function.
(8)Cost=f1×β+f2×1−β,(9)f1=maxk=1,2,⋯K∑∀ni∈Cp,kdni,CHp,kCp,k,(10)f2=∑i=1NEni∑k=1KECHp,k,where β is known as the user-defined constant. Let us consider β=0.5 for best match fitness coefficient. The CHs associated with K nodes for the cluster particle p give the maximum average distance associated as f1. The function f2 is the ratio for the average energy for the nodes of CHs. Eni is known as the energy for i-th node, and the energy of the k-th node for particle p is represented as ECHp,k(3)
The CHs choose the available channels in their range(4)
The channel with high quality is selected based on the condition that selects the channel using the nearby PUs(5)
The data from the cluster members of CHs are aggregated through the available local common channel(6)
Lastly, from the base station, the collected information is transmitted to CHsFigure 2
ICCA flow chart.


## 3.1. Calculations of Power Using ICCA Algorithm

The CRSN nodes are linked to each CH in order to compute the shortest distance between the base station and the CH and to pick the CH. The cluster must adhere to the following two requirements:In order for the CH to be effective, the base station’s communication range must be confined inside it. Additionally, the residual energy of the CH must be high.The entire amount of transmission power utilised for communication is shown by the symbol:(3)Ptx=∑k=1K∑i=1NDistminnik,CenterkCH.The minimum distance between the two points of pair of coordinates such asx0,y0 and x1,y1 is calculated by using the following:
(4)Distmin=x0−x12+y0−y12.It is made out of over-the-air readings of authentic radio transmissions that have been altered with 11 different modulations taken on genuine radio broadcasts. According to the authors, the signals were created using USRP-B210 that linked to computer running GNU-Radio. The various transmitters needed to be developed by utilising the same source code and data providers as RadioML 2016.10A; hence, it remained critical to utilise same data providers and source code as previously. It should also be mentioned that an irregularity with AM modulation, which was modified in subsequent version of the RadioML datasets, was also modified for the sake of generating our dataset.When dealing with constrained optimization, the goal is to maximise or minimise a certain objective/cost function while taking into consideration a range of constraints. Because linearity characterises both the objective function and the restrictions, the problem is turned into linear programming, and it becomes fairly straightforward to locate the global optimum in this situation [30]. When the problem is convex, on the other hand, different techniques are applied to find the global optimal solution.When it comes to addressing an unconstrained convex issue, the gradient technique and Newton’s method are the two most often utilised methodologies. As illustrated in the picture, after each iteration of the gradient method, the initial feasible point moves in the direction of the gradient in the direction of the optimal value towards the optimum value. Newton’s technique is more rapid in terms of convergence than the gradient approach; nevertheless, it necessitates the calculation of the objective function’s Hessian as part of the convergence process, which is more time-consuming than the gradient approach.When solving constrained convex problems, the interior point technique, the ellipsoid method, and the projected gradient algorithm are all employed to achieve success. An objective function is penalised when it crosses the boundary of the search point; a technique known as the inner point approach is used to do this. It is used to construct a sequence of ellipses in the feasible set, which are then assessed using the ellipsoid approach. With each iteration, the volume of the ellipse decreases, resulting in the convex function being enclosed in the maximum feasible enclosure.Nonlinear programming is used to describe optimization problems in which one or more of the constraints or the objective function are nonlinear (NLP). For global optimization of NLP in practise today, the interior point approach, genetic algorithms, and simulated annealing are the three most often used techniques for global optimization of NLP. The maximum and minimum values of a multivariable function are defined by the Lagrange multiplier, which is a mathematical constant. The Lagrange function has been enhanced in this version.We collected the signals on the receiver side with the MIGOU platforms that we created and constructed from the ground up. It is meant to circumvent the hardware-architectonic restrictions which then prevent CR study and testing with low-power end-device as of being undertaken magnificently when paired with SDR capabilities. When a communication channel was detected, the platform was configured to transport the raw I/Q sample to a computer, which would then stock them in proper database for later retrieval.All of these observations were taken inside in an atmosphere that should be in control like the labs or offices. Specific computations were performed at one-metre and six-metre distances from the transmitter. At two spans from transmitter, the average SNR were 37 dB and 22 dB, correspondingly [31]. To obtain concluding outcome, every recorded I/Q signal was split and treated into 128-bit vector that was then separately normalised. The closing stage was to add 400,000 normalised vectors for each MOD-SNR mixture in the dataset, for an overall of 8.8 million vectors in last dataset.Internal cluster communication between cluster members may transmit information from the source node to the CH centre location over the completely available channel if it is carried out via the shortest distance possible for node communication between cluster members [32]. The k-th cluster’s minimum distance from the i-th node is represented by the symbol, while the k-th cluster’s centre position of CH is represented by the symbol.
(5)PintraTot=∑k=1KDistminnik,CenterCHCk=∑k=1KDistminnik,njkck.Similarly, in the intercluster communications, the CH position is at the centre, gathers the information from the source node, compressively communicates with the CHs that are nearer, and finds the shortest communicating power among the clustered centres [33]. The minimum distance for the k-th cluster with the j-th node is represented as “ck” and CH central position for the k-th cluster is indicated by ck represented as
(6)PinterTot=∑k=1K∑i=1N∑j=1MDistminCenterkCHi,CenterkCHjck=∑k=1K∑i≠jDistminCenterkCHi,CenterkCHjck,where CenterkCHi is the i-th CH’s centre position in the k-th cluster (ck) and CenterkCHj is the j-th cluster centre position for the k-th cluster is ck.The coordinates are used for calculating the minimum distance, which is expressed as shown in the below equation.PTot is represented as the summation between the power for interclustering and intraclustering communication, where Pinter is the power for interclustering communication and Pintra is the power obtained for clustering communication.
(7)PTot=∑i=1NPintra+Pinterni.


## 3.2. Proposed ICCA Algorithm

From Figure2, the proposed ICCA sensor network is made up of “N” nodes with preset clusters of “N” indicated as “K” which is composed of the following:
(1)
“S” is the number of element sets for comprising “K” CHs chosen arbitrarily among all the CH suitable candidates. At every node points, ni(i=1,2,..,N) determines the distances dni,CHp,k among the nodes of all CHs’ positioned points allocated in the CH point at each nodeni. Here, CHp,k is the k-th CH of the particle p, and dni,CHp,k=mindni,CHp,k for k=1,2,..,K(2)
The centralized algorithm [34] uses the clustering process criteria to acknowledge the base station. FA finalizes the CHs at base station to identify best optimal location for the cost function.
(8)Cost=f1×β+f2×1−β,(9)f1=maxk=1,2,⋯K∑∀ni∈Cp,kdni,CHp,kCp,k,(10)f2=∑i=1NEni∑k=1KECHp,k,where β is known as the user-defined constant. Let us consider β=0.5 for best match fitness coefficient. The CHs associated with K nodes for the cluster particle p give the maximum average distance associated as f1. The function f2 is the ratio for the average energy for the nodes of CHs. Eni is known as the energy for i-th node, and the energy of the k-th node for particle p is represented as ECHp,k(3)
The CHs choose the available channels in their range(4)
The channel with high quality is selected based on the condition that selects the channel using the nearby PUs(5)
The data from the cluster members of CHs are aggregated through the available local common channel(6)
Lastly, from the base station, the collected information is transmitted to CHsFigure 2
ICCA flow chart.


## 4. Simulation Results and Discussion

It is described in this part how Network Simulator 2 (NS2) was used to evaluate the proposed ICCA’s performance on the CR network utilising the Network Simulator 2. Figure3 depicts whole number of PUs and SUs required for access channels based on the ideal number of clustered structure connections (the “K” number). When the simulation is running, the 10 PUs and 90 SU nodes are being used to assess the proposed model’s performance in terms of dynamic spectrum access [35]. It is determined by chance where the user nodes will be placed on the 1000×1000 metre fields, and each of the ten channels available will be marked with a different colour from the others. These colours include pink, deep pink, blue cyan, sky blue, yellow-green, maroon, yellow, violet-red, and green. When running the NS2 simulation (Version 2.34), the channels occupied by SUs are represented by the numbers 0, 1, and 9, and the channels filled by PUs are represented by the numbers C0, C1, and so on. Each PU picks the same ten channels, and the fortification range is around 200 metres, beyond which it is not possible to reach the channels that have already been taken by the CRSN neighbor who has stayed. The study is carried out over the course of 131 seconds of simulation time, with packet sizes remaining constant at around 512 bytes.Figure 3
The process of channel distribution for SUs and PUs.In this section, execution correlation is investigated for the existing DGCC strategy with different proposed advanced grouping strategies by utilizing the measurements of conservative merging time for several CRSN sizes, conservative node power for various group numbers, conservative node power for PUs and SUs, detection probability and missed recognition with different estimations of PFA, and detection probability with different estimations of SNR. Figure4 shows the general examinations of conservative merge duration for various CRSN sizes in various improved grouping strategies. From Figure 4, it is seen that conservative merge time for proposed ICCA is less contrasted with other enhanced grouping strategies. For various CRSN sizes, the proposed ICCA conservative process time is 47.2 sec less than the existing DGCC.Figure 4
Correlation of conservative merge duration in various improved grouping strategies.From Figure5, it is noticed that compared to previous advanced grouping algorithms, the suggested ICCA has a lower conservative node power. The proposed ICCA conservative node power is 2585.77 μW lower than the present DGCC for several clusters. Essentially, it is noticed that the conservative node power of main and secondary users for proposed ICCA is less contrasted with other improved grouping strategies. ICCA Conservative node power is 75.3 μW lower than DGCC for the primary users. The secondary users’ ICCA conservative node power is 281.978 μW lower than DGCC. Figure 5 demonstrates the general examinations of node power for several cluster numbers, PUs, and SUs in various improved grouping strategies.Figure 5
Correlation of conservative node power in various improved grouping strategies.In order to determine the average convergence time (average) for different CRSN sizes, the suggested method is used to analyse the mean node power for both PUs and SUs. For performance detection, the average power of each node in a given cluster is under consideration. To communicate with base station, the CRSN’s size is critical.The average convergence time for ICCA is lesser than the existing optimizing clustering methods. The suggested ICCA showed a mean convergence time of 47.2 sec smaller when compared with the existing DGCC method having distinct CRSN size obtained in the performance measure as 74.98%.Figure6 describes the comparative analysis for an average convergence time for different CRSN sizes with the various optimized clustering approaches. When the size of CRSN increases, the average convergence time maximises linearly [35]. The convergence time is required to evaluate the speed factors in the proposed ICCA method. The maximum of 280 is the CRSN size considered in this work.Figure 6
The comparison chart of average convergence time for different CRSN sizes.From Figure6, it was examined that the proposed ICCA obtained power for average node lesser compared to the existing clustering methods. An average node power of 2585.774 μW is obtained for the proposed ICCA which is lesser than the existing DGCC for many number of clusters obtained performance as 89.44%. Figure 7 shows the comparative graph for the power for average nodes concerning any number of clusters as compared with the various optimized clustering approaches. The chart depicts the power witnessed across a range of cluster sizes from 2 to 28. The ratio of the overall energy of SUs and PUs to overall nodes in the clusters indicated in watt (W) is the mean node power for CRSN.
(11)AverageNodePower=∑i=1NPUiEnergy+∑i=1NSUiEnergyNumberofnodesinclusters.Figure 7
The comparison graph of average node power for different cluster numbers of clusters.Figure7 describes that the power remains the same if the dimension of clusters is higher than 28, and time simulation would be ended. The figure shows several clusters against the average node power values for the existing DGCC, DGSC, DCFGC, DCJFGC, and proposed ICCA techniques. During the analysis, combining the PUs with SUs in the network indicates the lower and higher power at the nodes using the ICCA algorithm. The PU’s average node power is the ratio of overall energy of PUs to number of PUs given in watt (W).
(12)PUsNodePower=∑i=1NPUiEnergyNumberofPUs.Average power increases linearly with the number of PUs. Figure8 describes PU’s number versus average node power values with existing optimized clustering approaches for 10 PU’s node. The mean node power of ICCA is 75.3 μW for the primary user, which is lesser than the DGCC with the “9.646%.”Figure 8
PU’s average node power with existing optimized clustering approaches.Figure8 demonstrates the comparison of mean node power in PUs with existing optimized clustering algorithms. When more number of PUs is in the range of transmission, the clustering process needs more spectrum resources. So, the outcomes of clustering are affected by energy consumption [6]. Thus, 10 nodes of primary user are considered with 200 metres of fortification range during the simulation. The ICCA is influential among existing optimized clustering approaches by enhancing utilization of spectrum within the PUs for lesser power of mean node.Correspondingly, the mean node power of SUs is estimated by using the equation.(13)SUsNodePower=∑i=1NSUiEnergyTotalNumberofSUs.The numbers of SUs would increase linearly as the node power also increases. Figure9 shows the results obtained for the optimized clustering process for node 90 SUs. The node power average for ICCA for the secondary user is obtained as 281.978 μW showing a lesser percentage compared to the existing DGCC for SU average node power. The comparison for an average node in terms of power for distinct SUs with the existing optimized clustering techniques is shown. During simulation, the 90 nodes of secondary user are utilised within 150 metres of the fortification range. The proposed ICCA approach is effective than the existing optimized clustering approaches by enhancing the utilization spectrum with the minimum average node power of secondary users.Figure 9
The comparison graph of SU average node power.In order to determine the average convergence time (average) for different CRSN sizes, the suggested method is used to analyse the mean node power for both PUs and SUs. For performance detection, the average power of each node in a given cluster is under consideration. To connect with the base station, CRSN’s size is critical [36]. The SNR estimation for detecting a received signal and determining the response from detection results using spectrum sensing methods.The threshold’s value ofλ=4 dB is according to the observations and experimental outcomes [37].
(a)
When the value of thresholdλ is higher than the SNR (the overchannel of PU is incorrectly identified “H1”), the hypothesis is carried out by probability of false alarm rate approach. Such as if λ>SNR, accept H=H1∣H0(b)
When the value of thresholdλ is higher than the SNR (the PU for the overchannel has been accurately identified as “H1”), the probability of detection approach is used to test the hypothesis. Such as if λ>SNR, accept H=H1∣H1(c)
When the value of thresholdλ shows lesser SNR when compared over the channels of PU, which is not detected “H0.” The hypothesis is known as the probability, which is missed for detection approach need to satisfy the condition as λ≤SNR, accept H=H0∣H1The implementation of the proposed method is performed using the MATLAB R2021a. To determine when a channel is being used in PUs, detection of the statistical performance output ofY is compared and defined as a threshold λ. The probability of false alarm (PFA) obtains the chance of dealing with hypothesis test that selects H1 when it is in fact of H0.
(14)PFA=PY>λH0=Γm,λ/2Γm.The probability of detection (PD) indicates the correctly identified chanceH1 when it is H1.
(15)PD=PY>λH1=Qm2γavg,λ.Figure9 shows the comparison graph and Figure 10 shows the PFA versus probability rate. When the detection threshold is attained, and the full gamma function, the incomplete gamma function, the average SNR, and the general Marcum Q-function are all equal to one, then the detection threshold is met. Where m=TW is the amount of time, it takes for the bandwidth product to complete its computation. The ICCA takes the value of m=5 into consideration when computing the ICCA.Figure 10
PFA versus probability of detectionFormula for calculating PD is provided in (15). This function is referred to as the “marcumq()” function in the MATLAB programming language.The goal of the probability of missed detection (PMD) is to lower the likelihood of false alarms (PFA) while simultaneously improving the probability of detection (PD). The probability that PUs are present in the channel but are unable to identify the signal of primary transmission with the theory is indicated by the probability of detection (PD).PFA detection performance withSNR=4dB is shown in Figure 10 when different levels of the PFA are employed, and the results are shown in the table below. SNR=4dB is somewhat higher than SNR=3dB throughout a broad range of sensing spectrum [37], indicating that the signal-to-noise ratio is higher. It is probable that the chance of SUs misidentifying the fact that PUs may access the channels in the spectrum bands will be underestimated [38]. As a consequence, the SUs miss out on chances to maximise the usage of available channels. Compared to greater false alarm rates, the detection probability of the current DGCC technique is lower for lower false alarm rates, such as 0.1, than for larger false alarm rates. When the PFA value is more than 0.1, the proposed ICCA model surpasses the presently existing optimal clustering techniques, which is a significant improvement [39].Using the assumptions that SNR values vary between 0 and 30 dB and that the false alarm probability is represented by the number 0.1, it is feasible to get the detection of performance. In proportion to the increase in SNR, the likelihood of detection increases linearly and finally reaches a constant value of “1.” Figure11 demonstrates the performance of detection at different levels of signal-to-noise ratio (SNR) (see text for more information). A decrease in interferences with the PUs is followed by an increase in the chance of detection, and vice versa [40]. With respect to performance, the present DGCC achieved a substantially superior result (0.937) at zero decibels of SNR when compared to the ICCA’s PD. The ICCA curve converges more rapidly to “1” when the SNR is greater than 4 dB, as seen in Figure 1, compared to existing optimum clustering algorithms, which is a result of the higher SNR [41].Figure 11
SNR versus PD with optimized clustering from the existing techniques.We devised a reliable spectrum sensing approach based on machine learning, when deciding whichever channels to use, and the FC uses a weighted decision combination algorithm. During the training phase, CR users perform spectrum sensing; also, sensing report is allocated to the sensing class according to reception of acknowledgement signals which are also the result of total judgement. A CR behaviour of the user in a varying atmosphere is caused through altering action of PU, that is specified as sensing class also stated as follows: these sensing classes accurately capture both the PU’s activity and the CR user’s behaviour in reaction to that activity. Once sufficient data about the neighboring atmosphere has been acquired, the classification procedure may begin. When it comes to making decisions at the local level, CR users make them during the training phase. The quantized-hard form is used to make the local choice. The FC receives the users’ local options and builds an overall decision according to the data collected. In accordance with the CR committee’s general decision, CR users might opt to remain quiet or broadcast.Figure11 depicts the performance that was missing for the detection of distinct PFAs in terms of the ROC curve. As the rate of PFA increases, the likelihood of a fall in the missed detection rate reduces slowly for the ROC curve as well. The PUs would continue to function, but they would be unable to discriminate between the primary and secondary broadcasts of the signal in question. As the rate of PFA rises, the rate probability for the ROC curve lowers, and the rate probability for missed detection reduces as the rate of missed detection increases. With a PFA of 0.6, the current DGCC model was able to create a PMD value of up to 1, and when the point 1 was moved to the left, a linear graph was formed. PMD was received as 1 when the value of PFA was 0.1, as predicted by the indicated ICCA. As a result of reducing the value of PFA to zero, the suggested ICCA achieves PMD as 1. When compared to the current optimization technique, the ICCA achieves excellent performance when it comes to identifying the major transmissions with the highest false alarm rate (e.g., PD=0.372, PFA=0.7, and PMD=0.628), among other things.On the basis of this research, it is shown that the ICCA approach beats four other optimal clustering strategies that are used to save transmission power over shorter distances and enhance cluster energy efficiency while minimising primary user disturbance in Figure12.Figure 12
PFA versus missed detection probability.


## 5. Conclusion

The results of the simulations indicate that ICCA is capable of attaining consistency and scalability in huge datasets. According to the performance analysis, power consumption has been lowered by 89.440 percent when compared to previous versions. Although the proposed ICCA used CRSN nodes of size 280, it provided enhanced CHs and achieved a convergence speed of 32.09 seconds while using the CRSN nodes. Compared to the DGCC, the ICCA outperformed it by 9.646 percent in terms of average node power from processors, whereas the DGCC surpassed the ICCA by 9.646 percent in terms of processor performance. When it came to average node power from supercomputers, the ICCA outperformed the DGCC by a factor of 24.231 when compared to the DGCC. A lesser number of clusters using the reclustering approach that produces stable stability displayed less control within the minimum average node power than the process that achieves maximum control within the minimum mean node power for same number of clusters.The simulation results reveal that the ICCA algorithm is capable of properly detecting a range of white spaces in a given environment. According to the simulation, as compared to the optimal clustering algorithms, the ICCA has a lower PFA and a greater possibility of reducing sensing errors and detection. Data clustering will be improved by using the convolutional neural network (CNN) in future investigations, according to the researchers.



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*Source: 7359210-2022-06-23.xml*