# 5G Cognitive Radio Networks Using Reliable Hybrid Deep Learning Based on Spectrum Sensing

Authors: Vinodkumar Mohanakurup; Vishwadeepak Singh Baghela; Sarvesh Kumar; Prabhat Kumar Srivastava; Nitika Vats Doohan; Mukesh Soni; Halifa Awal
Journal: Wireless Communications and Mobile Computing
 (2022)

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## Abstract

Spectrum sensing is critical in allowing the cognitive radio network, which will be used in the next generation of wireless communication systems. Several approaches, including cyclostationary process, energy detectors, and matching filters, have been suggested over the course of several decades. These strategies, on the other hand, have a number of disadvantages. Energy detectors have poor performance when the signal-to-noise ratio (SNR) is changing, cyclostationary detectors are very complicated, and matching filters need previous knowledge of the main user (PU) signals. Additionally, these strategies rely on thresholds under particular signal-noise model assumptions in addition to the thresholds, and as a result, the detection effectiveness of these techniques is wholly dependent on the accuracy of the sensor. In this way, one of the most sought-after difficulties among wireless researchers continues to be the development of a reliable and intelligent spectrum sensing technology. In contrast, multilayer learning models are not ideal for dealing with time-series data because of the large computational cost and high rate of misclassification associated with them. For this reason, the authors propose a hybrid combination of long short-term memory (LSTM) and extreme learning machines (ELM) to learn temporal features from spectral data and to exploit other environmental activity statistics such as energy, distance, and duty cycle duration for the improvement of sensing performance. The suggested system has been tested on a Raspberry Pi Model B+ and the GNU-radio experimental testbed, among other platforms.


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## Body

## 1. Introduction

As wireless communication technology advances at a fast pace, and as new technologies such as 5G and the Internet of things (IoT) emerge, radio spectrum is becoming more scarce [1]. Because of the huge difference in overall band utilization indicated by the spectrum allocation and occupancy campaign [2–4], which ranges from 7 percent to 35 percent, it is clear that spectrum resources are underused. Cognitive radio has resolved the trade-off between spectrum availability and exponential growth, which was previously an issue (CR). It is feasible for these radios to detect and alter their settings to deliver the highest possible performance while causing no interference to the signals of other licensed users [5].The licensed user is referred to as the principal user (PU) in the CR, while the unlicensed user is referred to as the secondary user (SU). The fundamental function of CR is to let SUs get access to vacant licensed bands in a probabilistic and noninterfering way while minimizing interference. Spectrum sensing solutions that are both efficient and reliable are required to address this issue [6].A number of sensing algorithms that have been developed for a variety of contexts have been proposed. There are several scenarios proposed, including the semiblind maximum eigenvalue detector (MED) [7], the generalized likelihood ratio test (GLRT)-based signal subspace eigenvalues detector (SSE) [8], which works on the known noise power, and the total blind scenario, which includes the maximum to average eigenvalue ratio (MAER) detector [9] and the arithmetic to geometric mean (AGM) [10] detector, which excludes the effect of noise power. These detectors are ineffective because they only detect samples from the live sensing timeslots, which are insufficient for determining the amount of PU [11]. These processes operate on the basis of the signal noise model, which assumes that the signal is spatially unequally distributed. Deep learning (DL) plays an important part in illuminating the study by eliminating the need for signal noise model conjecture; instead, it learns from the sensing data intelligently and precisely through quick computing, allowing for the detection of a trustworthy spectrum. Better spectrum sensing approaches are made possible via the use of deep learning models such as convolutional neural networks (CNN) and long short-term memory (LSTM). A hybrid mix of CNN and LSTM [12–18] is also presented as a solution for the signal categorization difficulties [19]. Despite the fact that DL-based spectrum sensing algorithms have higher performance, improvisation is still necessary for improving the spectrum sensing performance even in low SNR conditions, as shown in this paper. To overcome this constraint, the authors suggest a unique hybrid deep learning model, which combines the best features of both LSTM and extreme learning machines (ELM), for improving the spectrum sensing approach. The following is the paper’s most significant contribution:We suggested a hybrid deep learning model consisting of LSTM combined with ELM, in which the prior events are supplied together with the current events as a starting point for learning. High-speed training and behavior of the suggested method have resulted in significant performance improvements in terms of likelihood of detection and sensing accuracy, even while operating in low signal-to-noise ratio (SNR) regimes [20].
(1)
Second, to provide a flexible and robust empirical hardware testbed to assess multiple learning models and to produce unbiased training datasets that comprise varied quantities of data under diverse signal-to-noise ratio (SNR) circumstances(2)
Finally, statistical aspects of PU activity such as duty cycle, distance, timestamps, and power being used as input data to improve the performance of the proposed model by including them into the modelA brief summary of the study is included in the following sections: Section2 highlights relevant studies by other authors. Section 3 discusses the preliminary perspectives on long short-term memory and extreme learning machines that have been developed. After discussing the system model in Section 4, we will go on to Section 5, which will go into the dataset description, feature extraction, and operating principles of the proposed model in further depth. Detailed descriptions of empirical testbeds, as well as results of experiments and comparative analyses, are offered in Section 6. Finally, in Section 7, the study concludes with a discussion of possible future improvements.


## 2. Works Which Are Connected

Using DNN for spectrum sensing, Surendra and colleagues came up with a novel method of detection. This paper proposes “DLSenseNet,” a DL-based model for spectrum sensing in which the structure information of received modulated signals is used for the purpose of spectrum sensing. The performance of the convolutional neural network (CNN) has been enhanced in order to recognize false alarms in CR users and lower the error rate. The suggested DNN-based spectrum detector [21] has a disadvantage in that it requires a large amount of training.Using SVM, CNN, and reinforcement learning algorithms for cooperative spectrum sensing, Wang and Liu [22] examined the supervised and unsupervised learning techniques for cooperative spectrum sensing [23]. Similarly, Sundous and Halawani [24] investigated the difficulties associated with implementing machine learning algorithms in real time for spectrum sensing applications. The study examined a number of supervised and unsupervised reinforcement models for the extraction of energy detection-based features, cyclostationary-based feature extraction, and signal processing-based feature extraction [25].Artificial neural network (ANN), support vector machine (SVM), decision trees (TREE), and KNN learning models” are all used in the detection of signals, according to Saber et al. (2020). According to [26], the performance of the classifiers was evaluated in order to establish which approach for spectrum sensing was the most effective among the three ways.Those severe challenges were addressed by Cheng et al. [27], who created a stacked autoencoder-based spectrum sensing technology to overcome them (SAE-SS). This architecture is very effective in sifting through incoming signals to extract the most crucial and least obvious pieces of information possible. Furthermore, it is more resistant to temporal delays in noise than previous sensing systems. The suggested methodology does not need the usage of past knowledge or specific characteristics of present users [28]. Furthermore, it does not need the usage of any third-party feature extraction techniques.Data-driven detectors with test statistics that are automatically generated from signal samples have been suggested by Xie et al. (2020) and are described in detail below. DL-based detectors that are currently available always need a substantial quantity of labelled training data in order to achieve good detection performance. UDSS is a “unsupervised deep spectrum sensing technique (UDSS)” created by the author to solve this problem. It is based on unsupervised deep learning (DL). Neither previous knowledge of the signal’s noise level nor its statistical covariance matrix was necessary for this strategy to be successful [29]. Furthermore, in the absence of PU signals, it simply necessitates a modest number of samples to be gathered [30]. Since semiautomatic algorithms require both labelled and unlabeled data in order to train, this approach has the disadvantage of lowering automation while also lowering the overall performance [31].When determining the state of PU transmission, Cheng et al. (2019) used an SAE (stacked autoencoder) in the time domain to preprocess the raw signal samples and a logistic regression classifier to determine the status of PU transmission. While other DL spectrum sensing algorithms achieve great detection performance, the SAE outperforms them due to its remarkable ability to learn critical aspects of signals [32].This work, conducted by Shah and Koo [33], describes a reliable spectrum sensing system that makes use of the K-nearest neighbor machine learning technique to detect interference. As part of the training phase, the fusion centre pools the local choices of CR users and a global decision is provided to each CR user by means of a majority vote. In the classification phase, each CR user compares their current sensing report to existing sensing classes and distance vectors are calculated as a result of this comparison [34]. In order to determine the quantitative variables that will be utilized in computing the posterior probability, the K-nearest neighbor technique is used. This collection of local choices is then combined at the fusion centre using a new decision combining strategy that takes into consideration the dependability of each CR user. The suggested KNN classifier has a drawback that makes it inefficient for a large number of users [22].Due to the fact that the great majority of contemporary spectrum sensing detectors are designed using specific signal-noise model assumptions, the detection performance of these detectors is heavily reliant on the validity of the assumed models [35]. The upshot is that much of the present research in the area of spectrum sensing has been concentrated on deep learning, which is not restricted by any model assumptions and is thus more flexible. Be mindful of the fact that deep learning techniques such as convolutional neural networks (CNN) and long short-term memory (LSTM) networks, both of which are employed in deep learning, are very effective in extracting spatial and temporal properties from their respective input datasets [36]. Specifically, in this paper, we propose a CNNLSTM detector that first employs a CNN to extract energy correlation features from the covariance matrices generated by the sensing data and then feeds a series of energy correlation features corresponding to multiple sensing periods into an LSTM to learn the pattern of PU activity. In order to maximize the possibility of detection in the future, it is necessary to understand PU activity patterns in order to do so. The superiority of the CNN-LSTM detector in both conditions with and without noise uncertainty has been shown by a large number of simulations in both situations [33]. The suggested study, on the other hand, explains that the methods rely on thresholds under certain signal-noise model assumptions in addition to the thresholds, and as a consequence, the detection efficacy of these approaches is completely reliant on the accuracy of the sensor in question. As a result, the development of a dependable and intelligent spectrum sensing technology continues to be one of the most sought-after challenges among wireless researchers today [37]. ML and DL algorithms, which are used in the construction of extremely accurate spectrum sensing models for wireless communications, have recently been forced into the limelight of the study in the field of machine and deep learning. A multilayer learning model, on the other hand, is not appropriate for dealing with time-series data because of its enormous computational cost and high rate of misclassification, which makes it unsuitable for dealing with time-series data [38]. Consequently, to improve sensing performance, the authors propose a hybrid combination of long short-term memory (LSTM) and extreme learning machines (ELM) to learn temporal features from spectral data and to exploit other environmental activity statistics such as energy, distance, and duty cycle duration. The proposed system has been tested on a variety of systems, including a Raspberry Pi Model B+ and the GNU-radio experimental testbed. A comparison is then conducted to assess the performance of the proposed LSTM-ELM-based spectrum sensing strategy in comparison to that of other already existing approaches. It has been discovered via experiments that the proposed spectrum sensing strategy surpasses other strategies in terms of detection time and classification accuracy, with the proposed technique requiring less time to detect and more time to classify than the other techniques [39].


## 3. Preliminary Overview

Here, we will go through the long short-term memory (LSTM) and the extreme learning machines (ELM) frameworks, which are both very important.


### 3.1. Long Short-Term Memory: An Overview

In contrast to conventional neural networks, which are described in [14], artificial neural networks have no memory components and are thus unable to retain data. Furthermore, when the size of the datasets grows, it has certain gradient difficulties. Thus, it is necessary for the structural alteration to receive feedback between subsequent timestamps in order for it to work. This widely used learning model is known as LSTM, and its flexibility in memory and ability to handle large databases make it a good fit for a variety of diverse applications. Figure 1 depicts the proposed LSTM model, which is made up of LSTM cells.Figure 1
LSTM structure.LSTM is a “memory-based NN” which comprises 4 gates, namely, “input gate (IG), output gate (OG), forget gate (FG), and cell input (CI)”. For each iteration, the LSTM network has the ability to remember the values. LetXt be the input, then, the hidden layer output is represented as ht, and its former output is represented as ht−1. The CI state is ➔Ct, then, the cell output state is represented as Gt, and its former state is represented as Gt−1. The 3 gate states are jt,Tf, and T0.The LSTM can resemble bothGt and ht which are able to communicate in RNN. In the LSTM network, the current input state is merged with the output of the previous unit. The OG and FG play important role to update the memory.The following mathematical expressions are utilized to determineGt and ht,
(1)I.G:jt=θGli.Ot+Ghi.et−1+si,F.G:Tf=θGlf.Ot+Ghf.et−1+sf,O.G:To=θGl0.Ot+Gho.et−1+so,C.I:TC~=tanhGlC.Ot+GhC.et−1+sC,where Gl0,Glf,Gli,GlC indicates the weight matrices between IG and output layers and Ghi,Ghf,Gho,GhC denotes the weight conditions generated between hidden and input layers. The si,sf,so,sC are the bias vectors and tanh is considered to be hyperbolic function. The following mathematical expression is utilized to determine the cell output state.
(2)Tc=kt∗TC~+Tf∗Tt−1,et=To∗tanhTC.The final output score is obtained using equation (2). This final output score is used to predict the presence of PU signals based on the power levels.


### 3.2. Extreme Learning Machines: An Overview

The traditional fully connected classification layers are designed based on the principle of extreme learning machines. Extreme learning machines are a category of the neural network proposed by Huang et al. [36]. This kind of neural network utilizes the single hidden layers in which the hidden layers do not require the tuning mandatorily. Compared with the other learning algorithms such as “support vector machines (SVM) and random forest (RF),” ELM exhibits the better performance, high speed, and less computational overhead [40].Based on machine learning, we developed a trustworthy spectrum sensing method, in which the FC applies a weighted decision combination rule to make decisions about which channels to use. Spectrum sensing is carried out by CR users during the training phase, and the sensing report is assigned to a sensing class based on the receipt of an acknowledgment signal (ACK) and the outcome of an overall decision. A CR user’s behavior in a changing environment is triggered by the changing activity of a PU, which is represented by the sensing class, and is referred to as “adaptive response.” Both the activity of the PU and the behavior of the CR user in response to that activity are correctly represented by these sensing classes. The categorization process may be initiated once adequate information about the surrounding environment has been gathered. As part of the training process, users of the CR may choose between two options on a local level. The local decision is made in the quantized-hard form, which is a quantized-hard form. Users’ local selections are sent to the FC, which then makes a global decision based on the information received. CR users may choose whether to stay silent or broadcast in line with the general decision made by the CR committee. If wireless communications and mobile computing (Wi-Fi and mobile computing).The detailed working mechanism of ELM is discussed in [27, 41]. In ELM, kernel function is utilized to obtain high accuracy. The ELM has minimum training error and better approximation. ELM is utilized for both classification as well as prediction because it uses nonzero activation function and autotuning of weight biases. In ELM, “L” neurons in the hidden layer are required to work with an activation function, so the output layer is “linear.” It is unnecessary to tweak these hidden layers consistently [42]. Generally, hidden layer nodes are arbitrarily appointed. In ELM, the single hidden layer output is expressed in equation (3) as follows:
(3)fLx=∑i=1Lβihix=hxβ,where x➔input, β➔output weight vector, and it is mathematically expressed as follows:
(4)β=β1,β2,⋯,βLT.Hx➔output hidden layer is mathematically expressed as follows:
(5)hx=h1x,h2x,⋯,hLx.To determine “output vectorO” which is called as the “target vector,” the hidden layers are mathematically expressed in equation (6) as follows:
(6)H=hx1hx2⋮hxN.The minimal nonlinear least square methods are significantly utilized for the basic implementation of the ELM and it is represented in equation (7) as follows:
(7)β′=H∗O=HTHHT−1O,where H∗➔inverse of H known as Moore−Penrose generalized inverse.The above expression is also written as follows:(8)β′=HT1CHHT−1O.By using the above expression, the output function is given as follows:(9)fLx=hxβ=hxHT1CHHT−1O.Equation (9) is used for better classification of signals based on the PU characteristics.


## 3.1. Long Short-Term Memory: An Overview

In contrast to conventional neural networks, which are described in [14], artificial neural networks have no memory components and are thus unable to retain data. Furthermore, when the size of the datasets grows, it has certain gradient difficulties. Thus, it is necessary for the structural alteration to receive feedback between subsequent timestamps in order for it to work. This widely used learning model is known as LSTM, and its flexibility in memory and ability to handle large databases make it a good fit for a variety of diverse applications. Figure 1 depicts the proposed LSTM model, which is made up of LSTM cells.Figure 1
LSTM structure.LSTM is a “memory-based NN” which comprises 4 gates, namely, “input gate (IG), output gate (OG), forget gate (FG), and cell input (CI)”. For each iteration, the LSTM network has the ability to remember the values. LetXt be the input, then, the hidden layer output is represented as ht, and its former output is represented as ht−1. The CI state is ➔Ct, then, the cell output state is represented as Gt, and its former state is represented as Gt−1. The 3 gate states are jt,Tf, and T0.The LSTM can resemble bothGt and ht which are able to communicate in RNN. In the LSTM network, the current input state is merged with the output of the previous unit. The OG and FG play important role to update the memory.The following mathematical expressions are utilized to determineGt and ht,
(1)I.G:jt=θGli.Ot+Ghi.et−1+si,F.G:Tf=θGlf.Ot+Ghf.et−1+sf,O.G:To=θGl0.Ot+Gho.et−1+so,C.I:TC~=tanhGlC.Ot+GhC.et−1+sC,where Gl0,Glf,Gli,GlC indicates the weight matrices between IG and output layers and Ghi,Ghf,Gho,GhC denotes the weight conditions generated between hidden and input layers. The si,sf,so,sC are the bias vectors and tanh is considered to be hyperbolic function. The following mathematical expression is utilized to determine the cell output state.
(2)Tc=kt∗TC~+Tf∗Tt−1,et=To∗tanhTC.The final output score is obtained using equation (2). This final output score is used to predict the presence of PU signals based on the power levels.


## 3.2. Extreme Learning Machines: An Overview

The traditional fully connected classification layers are designed based on the principle of extreme learning machines. Extreme learning machines are a category of the neural network proposed by Huang et al. [36]. This kind of neural network utilizes the single hidden layers in which the hidden layers do not require the tuning mandatorily. Compared with the other learning algorithms such as “support vector machines (SVM) and random forest (RF),” ELM exhibits the better performance, high speed, and less computational overhead [40].Based on machine learning, we developed a trustworthy spectrum sensing method, in which the FC applies a weighted decision combination rule to make decisions about which channels to use. Spectrum sensing is carried out by CR users during the training phase, and the sensing report is assigned to a sensing class based on the receipt of an acknowledgment signal (ACK) and the outcome of an overall decision. A CR user’s behavior in a changing environment is triggered by the changing activity of a PU, which is represented by the sensing class, and is referred to as “adaptive response.” Both the activity of the PU and the behavior of the CR user in response to that activity are correctly represented by these sensing classes. The categorization process may be initiated once adequate information about the surrounding environment has been gathered. As part of the training process, users of the CR may choose between two options on a local level. The local decision is made in the quantized-hard form, which is a quantized-hard form. Users’ local selections are sent to the FC, which then makes a global decision based on the information received. CR users may choose whether to stay silent or broadcast in line with the general decision made by the CR committee. If wireless communications and mobile computing (Wi-Fi and mobile computing).The detailed working mechanism of ELM is discussed in [27, 41]. In ELM, kernel function is utilized to obtain high accuracy. The ELM has minimum training error and better approximation. ELM is utilized for both classification as well as prediction because it uses nonzero activation function and autotuning of weight biases. In ELM, “L” neurons in the hidden layer are required to work with an activation function, so the output layer is “linear.” It is unnecessary to tweak these hidden layers consistently [42]. Generally, hidden layer nodes are arbitrarily appointed. In ELM, the single hidden layer output is expressed in equation (3) as follows:
(3)fLx=∑i=1Lβihix=hxβ,where x➔input, β➔output weight vector, and it is mathematically expressed as follows:
(4)β=β1,β2,⋯,βLT.Hx➔output hidden layer is mathematically expressed as follows:
(5)hx=h1x,h2x,⋯,hLx.To determine “output vectorO” which is called as the “target vector,” the hidden layers are mathematically expressed in equation (6) as follows:
(6)H=hx1hx2⋮hxN.The minimal nonlinear least square methods are significantly utilized for the basic implementation of the ELM and it is represented in equation (7) as follows:
(7)β′=H∗O=HTHHT−1O,where H∗➔inverse of H known as Moore−Penrose generalized inverse.The above expression is also written as follows:(8)β′=HT1CHHT−1O.By using the above expression, the output function is given as follows:(9)fLx=hxβ=hxHT1CHHT−1O.Equation (9) is used for better classification of signals based on the PU characteristics.


## 4. System Model

The system model considered the multiuser cognitive radio scenario. Figure1 represents the proposed dynamic resource allocation network (DRAIN-NETS) for an effective spectrum sensing. A PU transmitter is used for transmitting the PU signals. The primary signal users are collected and sampled [43]. These sampled signals are used to train and test the proposed model in such a way that the architecture can take the decision to determine the unknown samples in the network.ConsiderXn=Xx1n,x2n,x3n,⋯,xknt, where k represents the number of user and n denotes the received signals from k users. Xn indicates the discrete time sample present at kth users. The paper uses the binary hypothesis testing process for spectrum sensing as mentioned in [42].
(10)H1:Xn=RNn+Yn,H0:Xn=Yn.Here,R_Nn indicates the signal vectors which suffers from channel fading and path loss. Yn indicates the different noise vector with zero mean. Hence, by [42], hypothesis H1 indicates the presence of PU and H0 indicates its absence. These signal parameters are separated into real and imaginary components used to train and test the proposed architecture.


## 5. Proposed Framework

As in the proposed work, 5G communication indicates that it is possible to avoid interfering with other users by using a variety of conventional multiple-access techniques, such as time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), and code division multiple access (CDMA). However, because of the fast increase in the number of mobile devices, these approaches may not be sufficient to meet the needs of users who demand access to wireless communication networks. NOMA is becoming more important in 5G networks for building multiaccess schemes as a result, since it enables several users to simultaneously use the same frequency resources. There are two basic kinds of NOMA techniques: code domain and power domain, which are both discussed here. In this study, we concentrate on the power domain, in which numerous users are allocated to utilize the same frequency and time resources for their data transmissions, as described before. Specific to this method of transmission, the signals of several users are superposed to transmit over the same resources and successive interference cancellation (SIC) is performed to decode the users’ intended signals and eliminate interference at the receiver. In many various communication systems, such as the industrial Internet of things, machine-to-machine communications, and cooperative communications, several research studies on NOMA schemes have been conducted.


### 5.1. Dataset

It comprises over-the-air observations of authentic radio signals modulated with 11 different modulations derived from real-world radio broadcasts, which were collected from this dataset. The signals were generated via a USRP B210, which was connected to a PC running GNU Radio in order to do so, according to the authors. The numerous transmitters had to be implemented using the same source code and data sources as those used in the production of RadioML2016.10a; therefore, it was essential to utilize the same data sources and source code as before. As an additional point of clarification, it should be noted that the RadioML dataset had an inconsistency with AM modulations that was corrected in later versions of the RadioML dataset.On the receiver side, we captured the signals using the MIGOU platform, which we designed and built from the ground up. When combined with software-defined radio (SDR) capabilities, it is designed to overcome the hardware architectural constraints that now impede cognitive radio (CR) research and experimentation with low-power end-devices from being conducted successfully. There was a communication channel detected, and the rawI/Q samples were sent to a computer, which was designed to store the samples in the proper database [31].We took all of our measurements indoors, in a controlled environment like a lab or an office. Specific measurements were taken at two different distances from the transmitter: one meter and six meters. The average signal-to-noise ratios (SNR) at the two distances from the transmitter were 37 dB and 22 dB, respectively, at the two distances from the transmitter at the two distances from the transmitter. For the final result, all of the collectedI/Q signals were divided and processed into 128 bit vectors, which were then each individually normalized to get the final result. Adding 400,000 normalized vectors for each modulation-signal strength ratio (MOD-SNR) combination in the dataset was the last stage, resulting in an overall total of 8.8 million vectors in the final dataset.Figure2 shows the proposed architecture for the spectrum sensing. It consists of spectrum data collection and proposed hybrid spectrum sensing unit using LSTM-ELM architectures. As mentioned in Section 5, this research work acquired spectrum data by utilizing the empirical testbed. The data are captured using the measurement phase, and then, only the PU signal is obtained and measured in terms of η2X. To validate the model under noisy environments, additive white Gaussian noise is generated [43] and added to the raw PU signal. The signal Y is represented as 2N samples which is represented as follows:
(11)X=x1,x2,x3,x4,⋯,x2N.Figure 2
System model for the proposed architecture.Each sample is taken as the inputs to the proposed architecture. In this research work, nearly 350,200 samples are utilized [39] for the normal PU signal (SNR range of −20 dB to +20), and again, 350,200 samples are collected for AWGN signals. The datasets which are required for training the proposed architecture are shown in Algorithm 1.Algorithm 1: Data Collection Process for training the proposed learning model.
01 Procedure Create Datasets (Energy, Distance, Duty-Cycles, Time Period)02  For SNR =-20 dB to +20 Db03    Create PU_SNR_Signal= AWGN+ data signal04 End05 For i=1 to N do//where N = size of the data06    PU_Signal(i) = Data07 End for08 Return (PU_signal(i)


### 5.2. Statistical Feature Extraction

Once the data is acquired from the testbed, different features such as duty cycles, on-off time, and distances are calculated. The mathematical expression for calculating the statistical features is shown in Table1. We compare these measured characteristics to noise thresholds to establish ground truth for future investigation [38]. If the measured levels are greater than the threshold, the data is labelled as one, otherwise zero. These one- and zero-labelled power levels are used to train the LSTM model, and predicted power levels, distance, and duty cycles are used to train the ELM for the better classification of PUs.Table 1
Statistical features and its mathematical expression.
Sl. no.Statistical featuresMathematical expressionDescriptions01Power levelsETPUdata+TxExPUThe power levels are calculated by adding the power required for transmitting the signals, and the energy of the detector is used02Duty cycleD=Ton/Ton+ToffTon: on-time intervals; Toff: off-time intervals03Distance10ρo−σm−ρr−10nlogf+30n−32.4410nDistanceiscalculated;′ρo′indicates power of the signal (dBm) in the zero distance, ′ρr′ indicates signal power (dBm) in the distance d, ′f′ indicates the signal frequency (MHz), ′σm′ indicates fade margin, and ′n′ refers to the path-loss exponent


### 5.3. Proposed Hybrid Model Training and Hyperparameter Tuning

The hybrid learning model has been formulated for an efficient classification of the PUs under different SNR scenarios. Figure3 shows the proposed model in which LSTM is normally used for the prediction of the PUs using the power levels where the ELM is used to classify the PU based on the user activity statistical features. The input power level of the raw data is compared to the threshold, and therefore, information is assigned for practical LSTM training. Iterative trials provide the basis of the LSTM model’s construction. Table 2 provides the hyperparameters chosen for the network. The training methods for evaluating the LSTM model is presented in Algorithm 2. ELM takes the predicted output along with statistical features for the better classification of PUs. The working of the proposed model is summarized in Algorithm 2.Algorithm 2: Overall training method for the Proposed Framework.
1 Procedure Evaluate the model (Energy, Duty Cycle, Distance, Time Period)2 Output: Presence of PUs3 Feature Extraction Phase (Energy, Duty-cycle, Distance, Time period)4 For i =1 to N// where N refers to data size5      Predicted output (P(o)) is calculated using equation(4) &(6)6    ELM _Output=ELM(P(o), Distance, Duty-cycle, Tn) using Equation (9)7  PU presence = Elm_Output8    End9 End10 EndFigure 3
Schematic diagram for the proposed architecture.Table 2
Learning parameters.
Sl. no.HyperparametersValues1Learning rate0.0012Batch size303Number of epochs2504Hidden layers45OptimizerAdam6Activation function usedReLU7Loss functionMSE


## 5.1. Dataset

It comprises over-the-air observations of authentic radio signals modulated with 11 different modulations derived from real-world radio broadcasts, which were collected from this dataset. The signals were generated via a USRP B210, which was connected to a PC running GNU Radio in order to do so, according to the authors. The numerous transmitters had to be implemented using the same source code and data sources as those used in the production of RadioML2016.10a; therefore, it was essential to utilize the same data sources and source code as before. As an additional point of clarification, it should be noted that the RadioML dataset had an inconsistency with AM modulations that was corrected in later versions of the RadioML dataset.On the receiver side, we captured the signals using the MIGOU platform, which we designed and built from the ground up. When combined with software-defined radio (SDR) capabilities, it is designed to overcome the hardware architectural constraints that now impede cognitive radio (CR) research and experimentation with low-power end-devices from being conducted successfully. There was a communication channel detected, and the rawI/Q samples were sent to a computer, which was designed to store the samples in the proper database [31].We took all of our measurements indoors, in a controlled environment like a lab or an office. Specific measurements were taken at two different distances from the transmitter: one meter and six meters. The average signal-to-noise ratios (SNR) at the two distances from the transmitter were 37 dB and 22 dB, respectively, at the two distances from the transmitter at the two distances from the transmitter. For the final result, all of the collectedI/Q signals were divided and processed into 128 bit vectors, which were then each individually normalized to get the final result. Adding 400,000 normalized vectors for each modulation-signal strength ratio (MOD-SNR) combination in the dataset was the last stage, resulting in an overall total of 8.8 million vectors in the final dataset.Figure2 shows the proposed architecture for the spectrum sensing. It consists of spectrum data collection and proposed hybrid spectrum sensing unit using LSTM-ELM architectures. As mentioned in Section 5, this research work acquired spectrum data by utilizing the empirical testbed. The data are captured using the measurement phase, and then, only the PU signal is obtained and measured in terms of η2X. To validate the model under noisy environments, additive white Gaussian noise is generated [43] and added to the raw PU signal. The signal Y is represented as 2N samples which is represented as follows:
(11)X=x1,x2,x3,x4,⋯,x2N.Figure 2
System model for the proposed architecture.Each sample is taken as the inputs to the proposed architecture. In this research work, nearly 350,200 samples are utilized [39] for the normal PU signal (SNR range of −20 dB to +20), and again, 350,200 samples are collected for AWGN signals. The datasets which are required for training the proposed architecture are shown in Algorithm 1.Algorithm 1: Data Collection Process for training the proposed learning model.
01 Procedure Create Datasets (Energy, Distance, Duty-Cycles, Time Period)02  For SNR =-20 dB to +20 Db03    Create PU_SNR_Signal= AWGN+ data signal04 End05 For i=1 to N do//where N = size of the data06    PU_Signal(i) = Data07 End for08 Return (PU_signal(i)


## 5.2. Statistical Feature Extraction

Once the data is acquired from the testbed, different features such as duty cycles, on-off time, and distances are calculated. The mathematical expression for calculating the statistical features is shown in Table1. We compare these measured characteristics to noise thresholds to establish ground truth for future investigation [38]. If the measured levels are greater than the threshold, the data is labelled as one, otherwise zero. These one- and zero-labelled power levels are used to train the LSTM model, and predicted power levels, distance, and duty cycles are used to train the ELM for the better classification of PUs.Table 1
Statistical features and its mathematical expression.
Sl. no.Statistical featuresMathematical expressionDescriptions01Power levelsETPUdata+TxExPUThe power levels are calculated by adding the power required for transmitting the signals, and the energy of the detector is used02Duty cycleD=Ton/Ton+ToffTon: on-time intervals; Toff: off-time intervals03Distance10ρo−σm−ρr−10nlogf+30n−32.4410nDistanceiscalculated;′ρo′indicates power of the signal (dBm) in the zero distance, ′ρr′ indicates signal power (dBm) in the distance d, ′f′ indicates the signal frequency (MHz), ′σm′ indicates fade margin, and ′n′ refers to the path-loss exponent


## 5.3. Proposed Hybrid Model Training and Hyperparameter Tuning

The hybrid learning model has been formulated for an efficient classification of the PUs under different SNR scenarios. Figure3 shows the proposed model in which LSTM is normally used for the prediction of the PUs using the power levels where the ELM is used to classify the PU based on the user activity statistical features. The input power level of the raw data is compared to the threshold, and therefore, information is assigned for practical LSTM training. Iterative trials provide the basis of the LSTM model’s construction. Table 2 provides the hyperparameters chosen for the network. The training methods for evaluating the LSTM model is presented in Algorithm 2. ELM takes the predicted output along with statistical features for the better classification of PUs. The working of the proposed model is summarized in Algorithm 2.Algorithm 2: Overall training method for the Proposed Framework.
1 Procedure Evaluate the model (Energy, Duty Cycle, Distance, Time Period)2 Output: Presence of PUs3 Feature Extraction Phase (Energy, Duty-cycle, Distance, Time period)4 For i =1 to N// where N refers to data size5      Predicted output (P(o)) is calculated using equation(4) &(6)6    ELM _Output=ELM(P(o), Distance, Duty-cycle, Tn) using Equation (9)7  PU presence = Elm_Output8    End9 End10 EndFigure 3
Schematic diagram for the proposed architecture.Table 2
Learning parameters.
Sl. no.HyperparametersValues1Learning rate0.0012Batch size303Number of epochs2504Hidden layers45OptimizerAdam6Activation function usedReLU7Loss functionMSE


## 6. Empirical Experimental Testbed

Figure4 illustrates the empirical measurement setup. From this setup, the spectrum data are acquired for validation of the proposed E-LSTM0-SS technique. The hardware consists of RTL-SDR dongle interfaced with Raspberry Pi Model 3 and the Windows 10-based computer system for running the software. The software includes GNURADIO and Python 3.8. The different technologies of RTL-SDR configuration on raspberry pi 3 are shown in Table 3. Analyzers built with Python collect high SNR signals that are then analyzed to find PU signals. Analysis results are utilized in offline mode to verify suggested spectrum sensing methods.Figure 4
Empirical experimentation setup for validating the proposed sensing algorithm.Table 3
Specifications for the RTL_SDR for different frequency bands.
TunerFrequency rangeElonics E400052–2200 MHz with a gap from 1100 MHz to 1250 MHz (varies)Rafael Micro R820T24–1766 MHzRafael Micro R828D24–1766 MHzFitipower FC001322–1100 MHz (FC0013B/C, FC0013G has a separate L-band input, which is unconnected on most sticks)Fitipower FC001222–948.6 MHzFCI FC2580146–308 MHz and 438–924 MHz


### 6.1. Experimental Results and Discussion

This section presents the proposed algorithm validation. The proposed algorithm is developed using Keras libraries and TensorFlow backend to train and test the models. The datasets were collected and divided into two categories such as high SNR (−5 db to +5 db) and low SNR (−5 db to −20 db) [37]. Nearly 7,00,102 datasets were collected and used for training and testing the datasets. The performance metrics such as the performance metrics considered for evaluation are the prediction accuracy (Pa), precision (P), recall (R), probability of detection (Pd), probability of false alarm (Pf), and probability of miss detection (Pm) [33].Pd indicates the “probability of declaring the PU presence when it really occupies the spectrum.”Pf indicates the “probability of declaring that PU is present when the spectrum is really vacant.”Pm indicates the “probability of declaring that the spectrum is vacant when actually the PU is present.”ThePd and Pf are measured for different SNR values of the received signals [36].The mathematical expression used for calculating the above performance metrics is given in Table4.Table 4
Mathematical expression for calculating the different performance metrics.
SL. no.Performance metricsMathematical expression01Prediction accuracy (Pa)TP+TNTP+TN+FP+FN02RecallTPTP+FN×10003PrecisionTNTP+FP04Probability of detection (Pd)Total number of PU/total number of users (PU + noise signals)05Probability of missing ratio (Pm)1−Pd06Probability of false alarm (Pf)Number of noise signals diagnosed/total number of users (PU + noise signals)


### 6.2. Model Validation

To validate the proposed model, 70% of the total sample is utilized for training which is fed in batches to the proposed hybrid model and output functions are calculated as mentioned in Algorithm2 [35]. The training and validation accuracies of the proposed model are evaluated and validated as shown in Table 5. In Table 5, it is noticed that the number of epochs increases and training and validation accuracy also increases. It is evident in Table 5 that the LSTM model which is chosen has overcame the overfitting problem and is suitable for obtaining the better classification performance [34].Table 5
Validation and training accuracy for the proposed algorithm.
Sl. no.No. of epochsTraining accuracy (%)Validation accuracy (%)012096.5%95.0%024096.5%95.0%036096.75%96.50%048096.90%96.95%0510097.25%97.5%0612097.45%97.5%0714097.8%98.0%0816098.5%98.5%0918098.5%98.5%1020098.7%98.5%For the testing purpose, training datasets with different compositions were created by changing the ratio of the no. of samples in low SNR to the no. of samples in high SNR. The performance metrics used in Table6 is calculated for the different sample compositions.Table 6
Performance metrics of the proposed framework using the different composition of data samples.
CompositionLow SNR compositionHigh SNR compositionPerformance metricsSensing accuracyRecallPrecision90 : 1090100.9870.9820.98380 : 2080200.9860.9810.982570 : 3070300.9850.9790.978460 : 4060400.9850.9780.978350 : 5050500.98450.9780.978140 : 6040600.98420.97720.977230 : 7030700.98410.97630.973420 : 8020800.98400.97620.973210 : 9010900.98400.97610.9730Figure5 shows the proposed model performance in low SNR ranges from −20 db to 0 db for the different composition of data. It is evident in Figures 5(a)–5(d) that the performance of the proposed model has shown the highest performances at low SNR using 90 : 10 data composition and least performances at the same range using 10 : 90 and 20 : 80 data composition, respectively.Figure 5
Performance evaluation proposed model for the different composition of data. (a) Low SNR at −20 db, (b) at −15 db, (c) at −10 db, and (d) at −5 db.
(a)(b)(c)(d)Figure6 shows the performance of the proposed model in high SNR ranges from 0 db to 20 db for the different composition of data. It is evident in Figures 5(a)–5(d) that the performance of the proposed model has shown the highest performances at high SNR using 90 : 10 data composition and optimal performance at the same range using 10 : 90 and 20 : 80 data composition, respectively.Figure 6
Performance evaluation proposed model for the different composition of data. (a) High SNR from 0 db to 10 db. (b) High SNR from 15 db to 20 db.
(a)(b)Figure7 and Table 7 shows the impact of data compositions of training sets on the probability of detection (Pd) at different SNR thresholds. It is evident in Figure 7 that there is a significant impact on Pd at different SNR rates for various compositions of the training set. The low SNR range needs to be raised for the Pd to rise. The magnitudes of the PU signals are pretty comparable to the noise in this situation. So, the LSTM network struggles to differentiate PU signal and noise. But still, integration of ELM in LSTM has produced the optimal performances even at low SNR.Figure 7
Probability of detection (Pd) vs SNR performance curves.Table 7
Performance metrics of the proposed framework using the different composition of data samples.
Composition (%)Low SNR composition (%)High SNR composition (%)Performance metricsPdPfPm90 : 1090100.10150.10120.00180 : 2080200.09780.08890.02270 : 3070300.09230.08780.027860 : 4060400.09060.08680.032050 : 5050500.08920.085470.033040 : 6040600.08820.084230.034230 : 7030700.08800.08320.035220 : 8020800.085460.08240.040010 : 9010900.082010.08100.04014The developed hybrid method was validated on several radio frequencies, as shown in Table3, and compared to other sensing approaches, such as LSTM-SS [12], hybrid ANN-SS [14], PALM-SS [16], and regular energy detectors [18], as well as conventional energy detectors.Figure8 represents the comparative analysis. In Figures 8(a)–8(f), the proposed algorithm proves that it has better performance when compared with the other learning-based spectrum sensing techniques at different SNR. The ELM learning-based LSTM network is integral to the proposed spectrum sensing technique’s improved performance. It has been shown that the suggested method outperforms the other learning models because of its looping structure, information flow regulation, feedforward, and more error-prone ELM training methodology. Since these features are absent in other learning models, these existing spectrum sensing models fail to understand that the hidden features of the spectrum data tend to exhibit the lower performances than the proposed scheme.Figure 8
Probability of detection (Pd) metrics (a) @52–2,200 MHz, *(b) @24–1,766 MHz, (c) 22–110 MHz, (d) 22–9,486 MHz, (e) 146–308 MHz, and (f) 438–921 MHz.
(a)(b)(c)(d)(e)(f)The experiments are performed at different SNR with varying time intervals. Using the extreme learning machine approach, which is comprised of a collection of randomly selected hidden units and analytically set output weights, we hope to overcome these restrictions.Tables8–11 show the comparative analysis of different algorithms with respective to the different frequencies and SNR.Table 8
Performance analysis of the different algorithms at 52–2,200 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.9650.9550.9500.10120.100202PALM-SS0.9420.9300.9320.0930.092303Hybrid ANN-SS0.9210.9100.9230.0890.08704Normal energy detectors-SS0.7820.7620.7800.0860.086405RF-SS0.800.820.8120.0870.08506Proposed scheme0.9820.9730.9700.10120.1010Table 9
Performance analysis of the different algorithms at 24–1766 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.8630.8330.830.10120.100202PALM-SS03Hybrid ANN-SS0.8420.830.8320.0830.082304Normal energy detectors-SS0.8210.810.8230.0880.08705RF-SS0.7820.7620.780.0860.086406Proposed scheme0.80.820.8120.0870.083Table 10
Performance analysis of the different algorithms at 22–1100 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.7610.7110.710.10120.100202PALM-SS03Hybrid ANN-SS0.7420.730.7320.0730.072304Normal energy detectors-SS0.7210.710.7230.0870.08705RF-SS0.7820.7620.780.0860.086406Proposed scheme0.80.820.8120.0870.081Table 11
Performance analysis of the different algorithms at 22–9486 MHz and high SNR (0 db to 20 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.6620.6220.620.10120.100202PALM-SS03Hybrid ANN-SS0.6220.630.6320.0630.062304Normal energy detectors-SS0.6210.610.6230.0860.08705RF-SS0.7820.7620.780.0860.086206Proposed scheme0.80.820.8120.0870.082In Tables8–12, it is clear that the proposed algorithm has outperformed the other learning-based spectrum sensing techniques. Since the proposed model has learned the statistical features in the better way, performance remains to be optimal even in low SNR whereas the other existing models has shown the degraded performance as SNR decreases in different frequency measurements. Additionally, ELM learns not only the predicted output from LSTM but also other statistical features which makes the proposed model to detect effectively in different SNR.Table 12
Performance analysis of the different algorithms at 146–308 MHz and high SNR (0 db to 20 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.8810.8110.810.10120.100202PALM-SS03Hybrid ANN-SS0.8420.830.8320.0830.082304Normal energy detectors-SS0.8210.810.8230.0880.08705RF-SS0.7820.7820.780.0880.088406Proposed scheme0.80.820.8120.0870.081In proportion to the rise in Eb/No, the average energy consumption per sensor increases. It can be shown that the decrease in energy consumption is enhanced in both conventional and new approaches when we compare them to a noncooperative situation. For example, when Eb/No is equal to 5 dB, the suggested approach decreases the energy usage by 60%, compared to the conventional way.The rationale for this improvement is because the CUs will exchange the data regarding the PU’s presence, which will raise the overall detection probability overall. Additionally, it can be observed in Figure8 that the suggested approach provides a considerable increase in detection performance when compared to the previous way of detection. This is due to the employment of two phases in the detection process, which improves the detection performance, particularly in fine sensing, by increasing the detection threshold.The network animator (NAM) diagram for random topology is shown in the figure below. The total number of packets received is fewer than the total number of packets sent. Because wireless communication is the medium of choice, it is possible that some packets may be lost. The packet delivery ratio, on the other hand, is 87.17 percent, which is an excellent indication of packet delivery even in the case of wireless communication. In order to compensate for a lost packet, only retransmission of that packet may be used; nevertheless, this strategy will incur some latency. Depending on the kind of communication being sent, the delay may be characterized as bearable or nontolerable depending on the length of time it takes. For example, teleprotection, PMU (class A), control messages, smart meters, and other similar packets are admissible since the minimum latency requirement is 8 ms or less. The average delay of the simulation is 7 milliseconds, which is within the theoretical limit of the system.


## 6.1. Experimental Results and Discussion

This section presents the proposed algorithm validation. The proposed algorithm is developed using Keras libraries and TensorFlow backend to train and test the models. The datasets were collected and divided into two categories such as high SNR (−5 db to +5 db) and low SNR (−5 db to −20 db) [37]. Nearly 7,00,102 datasets were collected and used for training and testing the datasets. The performance metrics such as the performance metrics considered for evaluation are the prediction accuracy (Pa), precision (P), recall (R), probability of detection (Pd), probability of false alarm (Pf), and probability of miss detection (Pm) [33].Pd indicates the “probability of declaring the PU presence when it really occupies the spectrum.”Pf indicates the “probability of declaring that PU is present when the spectrum is really vacant.”Pm indicates the “probability of declaring that the spectrum is vacant when actually the PU is present.”ThePd and Pf are measured for different SNR values of the received signals [36].The mathematical expression used for calculating the above performance metrics is given in Table4.Table 4
Mathematical expression for calculating the different performance metrics.
SL. no.Performance metricsMathematical expression01Prediction accuracy (Pa)TP+TNTP+TN+FP+FN02RecallTPTP+FN×10003PrecisionTNTP+FP04Probability of detection (Pd)Total number of PU/total number of users (PU + noise signals)05Probability of missing ratio (Pm)1−Pd06Probability of false alarm (Pf)Number of noise signals diagnosed/total number of users (PU + noise signals)


## 6.2. Model Validation

To validate the proposed model, 70% of the total sample is utilized for training which is fed in batches to the proposed hybrid model and output functions are calculated as mentioned in Algorithm2 [35]. The training and validation accuracies of the proposed model are evaluated and validated as shown in Table 5. In Table 5, it is noticed that the number of epochs increases and training and validation accuracy also increases. It is evident in Table 5 that the LSTM model which is chosen has overcame the overfitting problem and is suitable for obtaining the better classification performance [34].Table 5
Validation and training accuracy for the proposed algorithm.
Sl. no.No. of epochsTraining accuracy (%)Validation accuracy (%)012096.5%95.0%024096.5%95.0%036096.75%96.50%048096.90%96.95%0510097.25%97.5%0612097.45%97.5%0714097.8%98.0%0816098.5%98.5%0918098.5%98.5%1020098.7%98.5%For the testing purpose, training datasets with different compositions were created by changing the ratio of the no. of samples in low SNR to the no. of samples in high SNR. The performance metrics used in Table6 is calculated for the different sample compositions.Table 6
Performance metrics of the proposed framework using the different composition of data samples.
CompositionLow SNR compositionHigh SNR compositionPerformance metricsSensing accuracyRecallPrecision90 : 1090100.9870.9820.98380 : 2080200.9860.9810.982570 : 3070300.9850.9790.978460 : 4060400.9850.9780.978350 : 5050500.98450.9780.978140 : 6040600.98420.97720.977230 : 7030700.98410.97630.973420 : 8020800.98400.97620.973210 : 9010900.98400.97610.9730Figure5 shows the proposed model performance in low SNR ranges from −20 db to 0 db for the different composition of data. It is evident in Figures 5(a)–5(d) that the performance of the proposed model has shown the highest performances at low SNR using 90 : 10 data composition and least performances at the same range using 10 : 90 and 20 : 80 data composition, respectively.Figure 5
Performance evaluation proposed model for the different composition of data. (a) Low SNR at −20 db, (b) at −15 db, (c) at −10 db, and (d) at −5 db.
(a)(b)(c)(d)Figure6 shows the performance of the proposed model in high SNR ranges from 0 db to 20 db for the different composition of data. It is evident in Figures 5(a)–5(d) that the performance of the proposed model has shown the highest performances at high SNR using 90 : 10 data composition and optimal performance at the same range using 10 : 90 and 20 : 80 data composition, respectively.Figure 6
Performance evaluation proposed model for the different composition of data. (a) High SNR from 0 db to 10 db. (b) High SNR from 15 db to 20 db.
(a)(b)Figure7 and Table 7 shows the impact of data compositions of training sets on the probability of detection (Pd) at different SNR thresholds. It is evident in Figure 7 that there is a significant impact on Pd at different SNR rates for various compositions of the training set. The low SNR range needs to be raised for the Pd to rise. The magnitudes of the PU signals are pretty comparable to the noise in this situation. So, the LSTM network struggles to differentiate PU signal and noise. But still, integration of ELM in LSTM has produced the optimal performances even at low SNR.Figure 7
Probability of detection (Pd) vs SNR performance curves.Table 7
Performance metrics of the proposed framework using the different composition of data samples.
Composition (%)Low SNR composition (%)High SNR composition (%)Performance metricsPdPfPm90 : 1090100.10150.10120.00180 : 2080200.09780.08890.02270 : 3070300.09230.08780.027860 : 4060400.09060.08680.032050 : 5050500.08920.085470.033040 : 6040600.08820.084230.034230 : 7030700.08800.08320.035220 : 8020800.085460.08240.040010 : 9010900.082010.08100.04014The developed hybrid method was validated on several radio frequencies, as shown in Table3, and compared to other sensing approaches, such as LSTM-SS [12], hybrid ANN-SS [14], PALM-SS [16], and regular energy detectors [18], as well as conventional energy detectors.Figure8 represents the comparative analysis. In Figures 8(a)–8(f), the proposed algorithm proves that it has better performance when compared with the other learning-based spectrum sensing techniques at different SNR. The ELM learning-based LSTM network is integral to the proposed spectrum sensing technique’s improved performance. It has been shown that the suggested method outperforms the other learning models because of its looping structure, information flow regulation, feedforward, and more error-prone ELM training methodology. Since these features are absent in other learning models, these existing spectrum sensing models fail to understand that the hidden features of the spectrum data tend to exhibit the lower performances than the proposed scheme.Figure 8
Probability of detection (Pd) metrics (a) @52–2,200 MHz, *(b) @24–1,766 MHz, (c) 22–110 MHz, (d) 22–9,486 MHz, (e) 146–308 MHz, and (f) 438–921 MHz.
(a)(b)(c)(d)(e)(f)The experiments are performed at different SNR with varying time intervals. Using the extreme learning machine approach, which is comprised of a collection of randomly selected hidden units and analytically set output weights, we hope to overcome these restrictions.Tables8–11 show the comparative analysis of different algorithms with respective to the different frequencies and SNR.Table 8
Performance analysis of the different algorithms at 52–2,200 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.9650.9550.9500.10120.100202PALM-SS0.9420.9300.9320.0930.092303Hybrid ANN-SS0.9210.9100.9230.0890.08704Normal energy detectors-SS0.7820.7620.7800.0860.086405RF-SS0.800.820.8120.0870.08506Proposed scheme0.9820.9730.9700.10120.1010Table 9
Performance analysis of the different algorithms at 24–1766 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.8630.8330.830.10120.100202PALM-SS03Hybrid ANN-SS0.8420.830.8320.0830.082304Normal energy detectors-SS0.8210.810.8230.0880.08705RF-SS0.7820.7620.780.0860.086406Proposed scheme0.80.820.8120.0870.083Table 10
Performance analysis of the different algorithms at 22–1100 MHz and low SNR (−20 db to 0 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.7610.7110.710.10120.100202PALM-SS03Hybrid ANN-SS0.7420.730.7320.0730.072304Normal energy detectors-SS0.7210.710.7230.0870.08705RF-SS0.7820.7620.780.0860.086406Proposed scheme0.80.820.8120.0870.081Table 11
Performance analysis of the different algorithms at 22–9486 MHz and high SNR (0 db to 20 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.6620.6220.620.10120.100202PALM-SS03Hybrid ANN-SS0.6220.630.6320.0630.062304Normal energy detectors-SS0.6210.610.6230.0860.08705RF-SS0.7820.7620.780.0860.086206Proposed scheme0.80.820.8120.0870.082In Tables8–12, it is clear that the proposed algorithm has outperformed the other learning-based spectrum sensing techniques. Since the proposed model has learned the statistical features in the better way, performance remains to be optimal even in low SNR whereas the other existing models has shown the degraded performance as SNR decreases in different frequency measurements. Additionally, ELM learns not only the predicted output from LSTM but also other statistical features which makes the proposed model to detect effectively in different SNR.Table 12
Performance analysis of the different algorithms at 146–308 MHz and high SNR (0 db to 20 db).
Sl. no.Algorithm detailsSamples (N=testing samples)Performance metricsSensing accuracyPrecisionRecallPdPf01LSTM-SSN=2000.8810.8110.810.10120.100202PALM-SS03Hybrid ANN-SS0.8420.830.8320.0830.082304Normal energy detectors-SS0.8210.810.8230.0880.08705RF-SS0.7820.7820.780.0880.088406Proposed scheme0.80.820.8120.0870.081In proportion to the rise in Eb/No, the average energy consumption per sensor increases. It can be shown that the decrease in energy consumption is enhanced in both conventional and new approaches when we compare them to a noncooperative situation. For example, when Eb/No is equal to 5 dB, the suggested approach decreases the energy usage by 60%, compared to the conventional way.The rationale for this improvement is because the CUs will exchange the data regarding the PU’s presence, which will raise the overall detection probability overall. Additionally, it can be observed in Figure8 that the suggested approach provides a considerable increase in detection performance when compared to the previous way of detection. This is due to the employment of two phases in the detection process, which improves the detection performance, particularly in fine sensing, by increasing the detection threshold.The network animator (NAM) diagram for random topology is shown in the figure below. The total number of packets received is fewer than the total number of packets sent. Because wireless communication is the medium of choice, it is possible that some packets may be lost. The packet delivery ratio, on the other hand, is 87.17 percent, which is an excellent indication of packet delivery even in the case of wireless communication. In order to compensate for a lost packet, only retransmission of that packet may be used; nevertheless, this strategy will incur some latency. Depending on the kind of communication being sent, the delay may be characterized as bearable or nontolerable depending on the length of time it takes. For example, teleprotection, PMU (class A), control messages, smart meters, and other similar packets are admissible since the minimum latency requirement is 8 ms or less. The average delay of the simulation is 7 milliseconds, which is within the theoretical limit of the system.


## 7. Conclusion

In this research work, hybrid DL-based spectrum sensing was proposed which significantly learns the statistical time series spectrum data. The novel testbed which comprises of Raspberry Pi 3 Model B++ interfaced with RTL-SDR dongle has been constructed to collect the raw spectral data under varying frequency technologies and different SNR conditions. The performance metrics such as sensing accuracy, precision, recall,Pd, and Pf are calculated and compared with the other existing learning model-based spectrum sensing techniques. From the experimentation, it is clear that the proposed framework has outperformed the other algorithm in terms of high sensing accuracy and high detection ratio even under low SNR. Additionally, the proposed scheme has shown the significant performance of detection using the statistical features and hybrid combination of LSTM and ELM techniques. However, the enhanced performance of the proposed algorithm is obtained at the cost of long training time and high computational overhead. The improvisation is required for the proposed algorithm in terms of handling the multiple PU and SU.The key criteria used by SU to make the PU signal detection are described and explored in detail in this section. For the situation of fullduplex operation, several modes of operation are detailed in detail. The use of learning approaches is also examined at the local and cooperative levels, among other things. Following that, the possibility for using spectrum sensing in WSN/IoT networks is researched, as well as the critical role played by IoT/WSN in the provision of spectrum sensing as a service is explored. In addition, we address the usage of cognitive radio in 5G and B5G networks from the viewpoints of spectrum allocation and frequency efficiency. Based on an in-depth examination of the current state of the art, we identify various problems and astounding issues that need additional investigation.



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*Source: 1830497-2022-04-11.xml*