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Texture featuresbased lightweight passive multistate crowd counting algorithm
EURASIP Journal on Wireless Communications and Networking volumeÂ 2023, ArticleÂ number:Â 79 (2023)
Abstract
Passive crowd counting using channel state information (CSI) is a promising technology for applications in fields such as smart cities and commerce. However, the most existing algorithms can only recognize the total number of people in the monitoring area and cannot simultaneously recognize the number and states of people and ignore the realtime performance of the algorithms. Therefore, they cannot be applied to the scenarios of multistate crowd counting requiring high realtime performance. To address this issue, a lightweight passive multistate crowd counting algorithm called TFLPMCC is proposed. This algorithm constructs CSI amplitude data into amplitude and timeâ€“frequency images, extracts texture features using the graylevel cooccurrence matrix (GLCM) and graylevel difference statistic (GLDS) methods, and uses the linear discriminant analysis (LDA) algorithm to count the crowd in multistates. Experiments show that the TFLPMCC algorithm not only has low time complexity but also achieves an average recognition accuracy of 98.27% for crowd counting.
1 Introduction
With the advancement of science and technology, humanâ€“computer interactions, including indoor crowd counting, indoor localization, and activity recognition, have become a new trend in the development of intelligent society. Crowd counting is the process of determining the number of people in a specific environment. As the urban population grows, various problems arise, such as the unreasonable allocation of public resources and declining service quality [1]. Consequently, the application requirements for crowd counting are also increasing. By utilizing effective crowd counting schemes, relevant departments or enterprises can obtain realtime information on the number of people in a specific area, thereby allocating public resources more reasonably, reducing resource waste, and improving service quality [2, 3]. For example, by counting the number of people applying for different businesses, more staff can be allocated to the business departments with larger queues. By counting the number of people in different exhibition halls of museums or exhibition centers, the manager can provide more air conditioning for the exhibition halls with larger number of people. By counting the number of people near different shelves in a supermarket, managers can place products with higher attention levels closer to the entrance or more easily visible to customers [4]. Therefore, crowd counting research has become an important research field in humanâ€“computer interaction, and it is of great significance for achieving smart cities, architectures, commerce, and homes.
Currently, crowd counting algorithms are categorized into four main categories: based on the video, based on the special sensors, based on the received signal strength (RSS), and based on the channel state information (CSI). Videobased crowd counting algorithms acquire data through cameras, extract features, and perform crowd counting using machine learning algorithms. For instance, Wu et al. [5] proposed a videobased spatialâ€“temporal graph network that fuses multiscale features from both temporal and spatial perspectives to achieve efficient crowd counting in videos. The method is mature and has high accuracy. However, it is unsuitable for nolineofsight environments or environments with smoke, may breach privacy, and has high deployment costs. Special sensorbased crowd counting methods use RFID, infrared sensors, and other technologies to obtain data for crowd counting. For example, Ding et al. [6] proposed a system called R# that estimates the number of people using passive RFID tags. These methods have good environmental adaptability and high accuracy but are costly and not suitable for mass application. RSSbased crowd counting methods count the number of people using the obtained RSS. For example, Denis et al. [7] designed and tested their crowd estimation systems which are wireless sensor networks using RSS information to estimate visitors. These methods have high device popularity, nonlineofsight, and protect privacy but are susceptible to environmental factors that cause unstable RSS. With the popularity of commercial WiFi, CSIbased crowd counting methods have become the research focus of scholars. CSI is a finegrained physical layer information that enables passive sensing using amplitude and phase information [8]. The advantages of this method are that no additional equipment needs to be deployed, it is not affected by light and occlusion, the signals are stable, and it can protect privacy.
CSIbased wireless sensing technology has been developed for over a decade, and numerous CSIbased crowd counting algorithms have emerged [2,3,4, 9,10,11,12,13,14,15,16]. In 2014, Xi et al. [9] proposed the Electronic Frog Eye system, the first to use CSI information for crowd counting. The system utilized gray theory for crowd prediction and proposed the dilatationbased crowed profiling algorithm, which was based on the positive correlation between the change of CSI and the number of people. Since then, numerous CSIbased crowd counting research papers have been published. In 2018, Zou et al. [10] proposed an indoor crowd counting system that achieved 96% recognition accuracy using a feature selection scheme based on information theory. However, the system had a high learning cost. Liu et al. [11] proposed the WiCount system in 2017, the first to use a neural network for crowd counting with an accuracy of 82.3%. They designed an online learning mechanism [12] to determine whether someone enters/leaves the room by using an activity recognition model, finetuning the deep learning model with an average accuracy of 87% for up to 5 people. Ma et al. [13] proposed a devicefree crowd density estimation system called Wisual, which predicted crowd density with 98% accuracy and accurately displayed the spectrum of mobile people based on CSI. Zhang et al. [2] proposed a queueing crowd counting system based on CSI and deep learning networks, called QueeFi system, which used a static model based on fully connected neural networks with convolutional long shortterm memory for queueing crowd counting. Zhang et al. [3] proposed a WiFibased crossenvironment crowd counting system with the ability to estimate walking directions and perform crowd counting over only one link, called WiCrowd. Liu et al. [4] proposed a CSIbased devicefree crowd counting scheme, which utilized the intuition that different numbers of people wandering in the environment would have different effects on WiFi signals. The scheme achieved an experimental accuracy of 87.2%. Guo et al. [14] proposed a wallpiercing crowd counting system using ambient WiFi signals, called TWCC, which took the phase difference data of channel state information (CSI) and fed it into a BP neural network after preprocessing, with an average recognition accuracy of 90%. Alizadeh et al. [15] proposed a HARC algorithm that simultaneously recognized human activity and counted the number of people at bus stops. The algorithm used a LSTMRNN model as a classifier with 94% recognition accuracy. Choi et al. [16] proposed a simultaneous recognition system for headcount and localization using CSI and machine learning, achieving a counting error of 0.35 MAE (89.8% of 1person internal error) and localization accuracy of 91.4%.
After analyzing the existing CSIbased crowd counting algorithms, we discovered that most previous studies focused on counting crowds with the same activity state, such as stationary or walking in sequence (known as singlestate crowd counting in this paper). However, in realworld scenarios, crowds can exhibit different states, such as stationary, walking in sequence, raising one hand, or running. Some applications not only need to count the number of people in the monitoring area, but also need to recognize the activity states of the crowd. For example, in fitness venues, in order to count the number of people who are exercising and the items that people are training, the total number of people in the venues and the number of people for each training item need to be recognized, so that the managers can adjust fitness equipment and improve business strategies. In nursing homes or kindergartens, the managers need to master the number and activity state data of the elderly or children to understand their living habits and provide better services. This type of application requires simultaneous recognition of multiple activity states and a total number of people (referred to as multistate crowd counting in this paper), which is an important problem faced by CSIbased crowd counting algorithms. Additionally, previous studies ignored the realtime performance of algorithms and often used algorithms with high time complexity to recognize the number of people, such as deep learning algorithms, which made them not suitable for applications requiring high realtime performance. This is another important problem currently faced by CSIbased crowd counting algorithms. To address the two problems, we propose a texture featuresbased lightweight passive multistate crowd counting algorithm, referred to as TFLPMCC, which can recognize both the number and activity state of volunteers at the same time by utilizing texture features of CSI images. The specific contributions of this paper are summarized as follows:

(1)
The existing research results have shown that the temporal stability of CSI can ensure capturing abnormal entities and their activities that cause environmental changes, and the frequency diversity of CSI can reflect the multipath reflection of wireless signals [17]. Therefore, in order to ensure the high recognition accuracy of multistate crowd counting algorithms, we construct CSI amplitude data into the form of amplitude images (utilizing the temporal stability of CSI) and timeâ€“frequency images (utilizing the frequency diversity of CSI). Then, two texture analysis methods of digital images, the graylevel cooccurrence matrix (GLCM) method and the graylevel difference statistics (GLDS) method, are used to extract the features of two types of images, which can characterize the local changes and spatial distribution of image pixels. The extracted features form feature vectors for recognizing the states and quantity of crowds. This above novel method extracts the features of CSI amplitude changes caused by different numbers and states of people from both timedomain and frequencydomain aspects, thereby achieving highprecision and more complex multistate crowd counting.

(2)
To reduce the time complexity of the algorithm, we construct CSI amplitude data of multiple subcarriers into CSI amplitude images and timeâ€“frequency images with matrix form, and extract CSI data features using the faster matrix operations. In addition, the linear discriminant analysis (LDA) algorithm with lower time complexity is used to recognize the state and number of people.

(3)
Numerous experiment results demonstrate that, compared with the other two stateoftheart algorithms, the proposed TFLPMCC algorithm achieved an average recognition accuracy of 98.27%, which increased by 4.04% and 4.42%, respectively. The running time was 0.068Â s, which decreased by 46.88% and 65.48%, respectively.
The remaining sections of this paper are structured as follows: Sect. 2 details the TFLPMCC algorithm; Sect. 3 gives the experimental results, and Sect.Â 4 discusses the limitations of the algorithm; Sect. 5 concludes the paper and presents future work.
2 TFLPMCC algorithm
2.1 Algorithm framework
The TFLPMCC algorithm proposed in this paper consists of four primary modules: data acquisition and preprocessing, image construction, texture feature extraction, and crowd counting. In the data acquisition and preprocessing module, two computers equipped with Intel 5300 network cards are utilized as transceiver devices to collect and preprocess CSI data at the receiving end. The image construction module involves using the preprocessed CSI information to construct amplitudesubcarrier images (also known as amplitude images) and frequencytime images (also known as timeâ€“frequency images). In the texture feature extraction module, texture features are extracted from the amplitude images and timeâ€“frequency images using GLCM and GLDS methods. Finally, in the crowd counting module, the feature vectors obtained from the texture feature extraction module are input to the LDA algorithm for crowd counting. The framework of the TFLPMCC algorithm proposed in this paper is shown in Fig.Â 1.
2.2 Data acquisition and preprocessing
The \(P_{{\text{t}}}\) and \(P_{{\text{r}}}\) antennas are connected to the wireless network card of the transmitting and the receiving device, respectively. This setup allows for simultaneous reception of data from \(P\) links (\(P = P_{{\text{t}}} \times P_{{\text{r}}}\)) at the receiving end. The measurement value matrix of CSI for the ith data packet can be expressed as follows:
\(P\) and \(Q\) represent the total number of wireless links and subcarriers, respectively. The variable \(h_{p,q} (i)\) is a complex number that includes amplitude and phase, representing a CSI value. Given a certain data packet sending rate, the CSI data of a sample can be expressed as follows:
The matrix \({\mathbf{H}}\) is threedimensional, with dimensions of \(P \times Q \times I\). Here, \(I\) represents the total number of CSI data packets for a sample, which includes all the data received by the wireless network card within a fixed period.
To reduce the amount of data processed in this study and the time complexity of the algorithm, we employ a setup where one antenna transmits and one antenna receives, with the transmitting link containing 30 subcarriers. As a result, each experimental sample contains 30 sets of CSI data. The CSI phase information is typically affected by factors such as carrier frequency offset and sampling frequency offset, so it needs to be corrected before use, which increases the algorithm's time complexity. However, the CSI amplitude information is generally more stable [18] and is used for multistate crowd counting in this paper. Due to the influence of transceiver hardware and the surrounding environment, each set of CSI data contains a significant amount of noise. FigureÂ 2 illustrates a set of CSI amplitude data without any processing, showing that raw CSI amplitude data cannot be directly used for crowd counting. Therefore, we needed to preprocess the collected CSI amplitude data to maximize the accuracy and stability of multistate crowd counting.
To handle outliers in the raw CSI amplitude data that deviate from the normal range, we use the PauTa Criterion to remove them [19]. This involves selecting a sliding window of data with a fixed length and calculating the mean and standard deviation of the data within the window. Then, we subtract the mean value from each data within the window. If the difference is greater than three times the standard deviation, we consider it an outlier and replace it with the mean value. This method was applied to the data in Fig.Â 2, and the results are shown in Fig.Â 3, which depicts the removal of outliers in Fig.Â 2.
The raw CSI amplitude data not only contains outliers affecting crowd counting, but also contains highfrequency noise caused by the surrounding environments and multipath effects. CSI amplitude changes caused by human bodies and their activities are mainly concentrated in the lowfrequency part of CSI amplitude data, so CSIbased wireless sensing algorithms generally use lowpass filters to filter CSI amplitude data [20], such as moving average filter, Gaussian filter [8], and wavelet threshold method [21]. This paper focuses on reducing the time complexity of the algorithm, so we use the moving average filter with lower time complexity [22] to filter the highfrequency noise of CSI amplitude data. Moreover, this paper aims to use CSI amplitude for recognizing the number and states of people, which belongs to coarsegrained information recognition. Therefore, the TFLPMCC algorithm using the moving average filter can already achieve high enough recognition accuracy for the coarsegrained crowd counting algorithm, which can be verified by the experimental results in Sect. 3.
The moving average filter is expressed as follows:
where \(i\) is the serial number of the data packet, \(N\) is the length of the sliding window and is set as 5 in the paper, and \(q\) is the serial number of the subcarrier. We applied the moving average filter to the CSI amplitude data shown in Fig.Â 3, and the resulting filtered data is presented in Fig.Â 4. As can be seen from the figure, the filter effectively reduced the amount of noise present in the data.
2.3 Image construction
In this paper, each link contains 30 subcarriers, and each subcarrier contains a few seconds of CSI data. The sampling frequency of CSI date is 1000Â Hz. Therefore, one sample of raw CSI data includes a large amount of CSI amplitude data. If raw CSI amplitude data is directly used, the algorithm's running speed will be very slow. To reduce the time complexity of the algorithm, we construct raw CSI amplitude data for each sample into the matrix form of amplitude image. On the one hand, the optimized matrix operation can greatly reduce the time complexity of the algorithm, such as the matrix operation in MATLAB. On the other hand, mature digital image processing technology can be used to extract the features that characterize differences between amplitude images such as texture and color. After experimental verification in this paper, these features can ensure that the multistate crowd counting algorithm achieves high accuracy.
To leverage the data correlation between CSI subcarrier and the temporal features of the CSI amplitude data, we represent the preprocessed CSI amplitude data as an amplitude image, using the following approach:
where \({\text{Am}}\) is a twodimensional matrix representing the amplitude image, \(q\) is the subcarrier serial number \(\left( {q = 1, \ldots ,Q} \right)\), \(i\) represents the serial number of data packets \(\left( {i = 1, \ldots ,I} \right)\), and \(\overline{{\left {h_{1,q} (i)} \right}}\) represents the amplitude of the ith data packet in the qth subcarrier. According to the Am matrixes of the samples, \(i\) is set as the xaxis and \(q\) is set as the yaxis, and the examples of amplitude images are drawn as shown in Fig.Â 5. From Fig.Â 5, the texture features of CSI amplitude images corresponding to the different states of 2 people and 4 people are also different, which shows that it is rationality to recognize the number and states of people by using the texture features of images.
While the amplitude image provides information on the trend of CSI amplitude changes over time, it does not capture the frequencydomain variations caused by different human actions. To address this limitation and enable to recognize the number of people in multistates, we apply the Morlet wavelet transform to the preprocessed CSI amplitude data. Thus, we can construct a timeâ€“frequency image of CSI amplitude, using the calculated wavelet coefficients as follows:
where \(i\) denotes the ith time component \(\left( {i = 1, \ldots ,I} \right)\), \(j\) denotes the jth frequency component \(\left( {j = 1, \ldots ,J} \right)\), and \(x_{ji}\) denotes the wavelet coefficient. As the action frequencies of humans are low, the number of frequency components \(J\) can be set to a fixed value. For example, in this paper, \(J\) is set to 60, which can fully represent the change frequency of human action. Using the TF matrixes of the samples, we set \(i\) as the xaxis and \(j\) as the yaxis, and draw the examples of timeâ€“frequency images as shown in Fig.Â 6. From Fig.Â 6, it can also be seen that the texture features of CSI timeâ€“frequency images affected by different number and states of people are also different. Therefore, using the texture features of CSI amplitude images and timeâ€“frequency images can more accurately count multistate crowds.
2.4 Texture feature extraction
The TFLPMCC algorithm aims to classify the texture features of the constructed CSI amplitude and timeâ€“frequency images to recognize the number of people in different states. The texture is a feature that reveals the local variations and spatial distribution of image pixels. In this paper, the GLCM method [23,24,25] is utilized to extract the texture features of the CSI amplitude and timeâ€“frequency images to achieve efficient crowd counting. GLCM can capture the amplitude change of images in different directions and neighboring intervals, and analyze the relevant features of spatial distribution and arrangement of image pixels. However, the CSI amplitude images also contain a significant number of local difference texture features. To improve the accuracy of crowd counting, the GLDS method is also used to extract texture features of the CSI amplitude images, and both sets of texture features are combined to form the feature vector for crowd counting.
2.4.1 GLCM
GLCM is a statistical method proposed by Haralick et al. [26] in 1973, which is used to represent the joint probability density between pixels of a certain distance and orientation. Its mathematical expression is shown below.
where \(\# \left\{ X \right\}\) represents the number of elements in the set \(X\), \(a,b = 0,1,2, \cdots ,K  1\), \(K\) is the total number of gray levels of the pixel values, \(p1,p2\) are two arbitrary gray levels, \(x,y\) are the column and row numbers of the pixels in the image, \(c\) and \(d\) denote the relative distances of two pixels in the \(x\) and \(y\) directions, respectively, and \(f(x,y)\) denotes the pixel value in the xth column and yth row.
To reduce the time complexity of the algorithm, we select four mutually uncorrelated statistics of energy, entropy, contrast, and correlation as the texture features of GLCM. Although Haralick et al. [26] proposed 14 kinds of statistics calculated according to GLCM, most of these statistics are correlated.

(1)
The feature known as energy, or angular secondorder moment, is used to measure the uniformity of image texture. It is calculated using the following equation:
$${\text{ASM}}(c,d){ = }\sum\limits_{a} {\sum\limits_{b} {P(\left. {a,b} \rightc,d)^{2} } } ,$$(7)where \(\sum\nolimits_{a} \cdot\) and \(\sum\nolimits_{b} \cdot\) denote \(\sum\limits_{a = 0}^{K  1} \cdot\) and \(\sum\limits_{b = 0}^{K  1} \cdot\), respectively.

(2)
Entropy is a feature that characterizes the level of confusion, complexity, and randomness in an image. It is calculated using the following equation:
$${\text{ENT}}(c,d) =  \sum\limits_{a} {\sum\limits_{b} {P(\left. {a,b} \rightc,d)\log (P(\left. {a,b} \rightc,d))} } .$$(8) 
(3)
Contrast is a feature that characterizes the sharpness and intensity of the transitions between neighboring pixel values in an image, indicating the presence of edges or boundaries, and is calculated as follows:
$${\text{CON}}(c,d) = \sum\limits_{a} {\sum\limits_{b} {(a  b)^{2} P(\left. {a,b} \rightc,d)} } .$$(9) 
(4)
Correlation is a feature that measures the degree of linear dependence between local pixels in an image and is calculated as follows:
$${\text{COR}}(c,d) = \frac{{\sum\nolimits_{a} {\sum\nolimits_{b} {abP(\left. {a,b} \rightc,d)  \mu_{1} (c,d)\mu_{2} (c,d)} } }}{{\sigma_{1}^{2} (c,d)\sigma_{2}^{2} (c,d)}},$$(10)where
$$\left\{ \begin{gathered} \mu_{1} (c,d) = \sum\limits_{a} a \sum\limits_{b} {P(\left. {a,b} \rightc,d)} \hfill \\ \mu_{2} (c,d) = \sum\limits_{b} b \sum\limits_{a} {P(\left. {a,b} \rightc,d)} \hfill \\ \sigma_{1}^{2} (c,d) = \sum\limits_{a} {\left( {a  \mu_{1} (c,d)} \right)^{2} \sum\limits_{b} {P(\left. {a,b} \rightc,d)} } \hfill \\ \sigma_{2}^{2} (c,d) = \sum\limits_{b} {\left( {b  \mu_{2} (c,d)} \right)^{2} \sum\limits_{a} {P(\left. {a,b} \rightc,d)} } \hfill \\ \end{gathered} \right..$$(11)
The above four features are all functions of \(c\) and \(d\). To comprehensively characterize the features of multiple directions of pixels and reduce the time complexity of the algorithm, in this paper, we set \(c\) and \(d\) as (1,0) and (1,1), respectively, and realize that the orientation angle \(\theta\) of the pixel is \(0^{ \circ }\) and \(45^{ \circ }\), respectively. Then, we calculate the mean and standard deviation of the above two texture features, respectively, which constitute the feature vector extracted according to GLCM as follows:
where \(\mu_{{{\text{ASM}}}}\), \(\sigma_{{{\text{ASM}}}}\), \(\mu_{{{\text{ENT}}}}\), \(\sigma_{{{\text{ENT}}}}\), \(\mu_{{{\text{CON}}}}\), \(\sigma_{{{\text{CON}}}}\), \(\mu_{{{\text{COR}}}}\) and \(\sigma_{{{\text{COR}}}}\) represent the mean and standard deviation of \({\text{ASM}}(c,d)\), \({\text{ENT}}(c,d)\), \({\text{CON}}(c,d)\), and \({\text{COR}}(c,d)\) of the GLCM, respectively.
Since the GLCM of the CSI amplitude and timeâ€“frequency images are calculated separately and the texture features are extracted, the GLCMbased texture features can be represented as follows:
where \(F_{{{\text{GLCM}}1}}\) and \(F_{{{\text{GLCM}}2}}\) denote the texture feature vectors extracted from the GLCM of the amplitude and timeâ€“frequency images, respectively.
2.4.2 GLDS
GLDS is a statistical technique that characterizes the variation of grayscale values among adjacent image pixels, allowing for the analysis of differences and fluctuations in localized regions of the image.
If the position of a pixel is \(\left( {x,y} \right)\) and the position of a neighboring pixel is \(\left( {x + \Delta x,y + \Delta y} \right)\), then the grayscale difference between the two pixels can be expressed as:
The grayscale difference, denoted as \(f_{\Delta } \left( {x,y} \right)\), represents the variation between adjacent pixel values in an image. Typically, both \(\Delta x\) and \(\Delta y\) are small deviations, and for the purposes of this paper, both \(\Delta x\) and \(\Delta y\) have been fixed at a value of 1.
Given \(M\) possible levels for grayscale differences, a histogram of \(f_{\Delta } \left( {x,y} \right)\) can be constructed to compute the probability, \(P_{\Delta } \left( m \right)\), for each value of \(f_{\Delta } \left( {x,y} \right)\) using the histogram, where \(m = 1,2, \cdots ,M\). In this paper, we utilize the grayscale difference probability distribution, \(P_{\Delta } \left( m \right)\), to extract four texture features from the amplitude images, namely contrast, angular secondorder moment, entropy, and mean. The following equations are employed for their computation:
The texture features of the amplitude image, which are based on the four aforementioned statistics (i.e., contrast, angular secondorder moment, entropy, and mean), can be expressed using the following equations:
The feature vector for multistate crowd counting consists of the texture features extracted from amplitude and timeâ€“frequency images using both GLCM and GLDS methods.
2.5 LDA algorithm
To increase the running speed of the algorithm, we utilize the LDA algorithm, which has low time complexity, to recognize the number of people. The LDA algorithm transforms the highdimensional classification problem into a onedimensional classification problem using the projection method. The specific algorithm is outlined below.
The intraclass dispersion matrix for the same category samples is calculated as follows:
where \(u\) denotes the serial number of the category (\(u = 1,2, \cdots ,U\), where \(U\) is the number of categories for multistate crowd counting.), \(v\) denotes the serial number of the sample (\(v = 1,2, \cdots ,V\), where \(V\) is the number of samples collected for each category.), \(F(u,v)\) denotes the eigenvector of the vth sample of the uth category, \(\mu_{u}\) is the mean of the \(V\) samples for the uth category, and \(T\) denotes the transpose of the matrix.
The interclass dispersion matrix is calculated as follows:
where \(\mu\) denotes the mean value of all \(\mu_{u}\).
To achieve the minimum intraclass dispersion and the maximum interclass dispersion, the following objective function must be optimized:
where \(\prod\limits_{diag}\) denotes the product of the main diagonal elements of the matrix, \(W = \left[ {w_{1} ,w_{2} , \cdots ,w_{r} } \right]\) is a lowdimensional matrix in the projection direction with dimension \(R \times r\), \(R\) denotes the length of the eigenvector \(F\), and \(r\) denotes the total number of basis vectors in \(W\). Generally, \(r\) is taken as the largest integer that is smaller than \(U\). EquationÂ (23) can become:
The maximum value of the objective function is the product of the largest \(r\) eigenvalues of the matrix \(S_{ic}^{  1} S_{bc}\). The eigenvectors corresponding to the largest \(r\) eigenvalues are \(w_{1} ,w_{2} , \cdots ,w_{r}\).
3 Results
3.1 Experimental setup and data acquisition
In this study, experiments were conducted in a \(3.5\,{\text{m}} \times 5\,{\text{m}}\) laboratory containing tables, chairs, cabinets, and experimental equipments. The experimental scenario is depicted in Fig.Â 7. Two computers with Intel 5300 network cards and Ubuntu 12.04 operating systems were used. One computer was connected to one antenna as the transmitter, while the other computer was connected to three antennas as the receiver. The antennas were placed 0.5 m from the ground, and the distance between the transmitting and receiving devices was 3 m. The channel bandwidth of WiFi was set to 20 MHz, and the operating frequency was 2.4 GHz. Data was transmitted through three channels, each with 30 subcarriers, resulting in each data packet containing \(1 \times 3 \times 30\) groups of CSI data. The experiment involved four volunteers, including two males and two females. Thirteen different experimental cases were conducted, as shown in Table 1. The crowd counting in these states has potential applications in smart education, such as calculating the number of students in different states in a classroom. The experimental setup required recognizing a total of \(3 \times 4 + 1\)â€‰=â€‰13 categories, including the case of no people. The device had a sending packet frequency of 1000 Hz, and 4 s of data were collected for each sample, consisting of 4000 data packets. Seventy samples were collected for each category, with 40 randomly selected for training and the remaining 30 for testing during algorithm simulation.
3.2 Parameter analysis
3.2.1 Analysis of parameter \(J\)
The TFLPMCC algorithm's resolution of frequency components varies with the number of frequency components \(J\), which may affect the recognition accuracy of action and crowd counting. To evaluate the impact of \(J\), we tested the algorithm's performance with \(J\) values of 20, 40, 60, 80, and 100, and compared the average recognition accuracy. The results are presented in Fig.Â 8, which shows that the algorithm achieves the highest average recognition accuracy of 98.27% when \(J = 60\). However, for all other \(J\) values, the average recognition accuracy was above 96%, indicating that \(J\) has a minor impact on the algorithm's performance. This is because human action frequency is relatively slow, and all tested \(J\) values can adequately capture the changes in human action frequency. In conclusion, when applying the TFLPMCC algorithm, the parameter \(J\) can be set to 20 for high operation speed or 60 for high average recognition accuracy.
3.2.2 Analysis of parameters \(c\) and \(d\)
The TLLPMCC algorithm uses texture features calculated according to the GLCM, where the texture features are functions of parameters \(c\) and \(d\). To analyze the effect of the values of parameters \(c\) and \(d\) on the performance of the algorithm, the authors set the parameters \(c\) and \(d\) according to Table 2 and compared the average recognition accuracy of the TLLPMCC algorithm. The experimental results, as shown in Fig.Â 9, demonstrate that the TLLPMCC algorithm achieves the highest average recognition accuracy when the parameters \(c\) and \(d\) are set to (1,0) and (1,1). As the parameters \(c\) and \(d\) vary from Cd1 to Cd6, the average recognition accuracy of the TLLPMCC algorithm gradually decreases. The authors note that the algorithm complexity is significantly lower when \(c\) and \(d\) are set to Cd1 and Cd2 than when other values are used. Therefore, the values of parameters \(c\) and \(d\) can be set to (1,0) and (1,1) when using the TLLPMCC algorithm.
3.2.3 Analysis of transmitting and receiving antennas
In the experiment, the transmitter is connected to 1 antenna, and the receiver is connected to 3 antennas. To analyze the impact of the number of transmitting and receiving antennas on the accuracy of the TFLPMCC algorithm, we number the receiving antennas as 1, 2, and 3. 1 Antenna, 2 Antenna, and 3 Antenna represent the experiments using CSI data received by antennas 1, 2, and 3 for multistate crowd counting, respectively, and 1â€‰+â€‰2 Antennas represent the experiments using CSI data received by antennas 1 and 2, and so on. FigureÂ 10 shows the average recognition accuracy of the TFLPMCC algorithm under different combinations of transmitting and receiving antennas. From Fig.Â 10, the average recognition accuracies obtained by using CSI data from different receiving antennas are also different. When only one antenna is used, the algorithm has the highest accuracy. The more antennas are used, the lower the accuracy of the algorithm, and the higher the time complexity of the algorithm. This is because the CSI data received by different antennas are affected differently by multipath interference effects. Therefore, we evaluate the performance of the TFLPMCC algorithm using CSI data from the first receiving antenna in the paper.
3.2.4 Analysis of subcarriers and bandwidth
To analyze the impact of the number of subcarriers on the accuracy of the TFLPMCC algorithm, we compare the average recognition accuracy of the TFLPMCC algorithm when using CSI data of 1, 5, 10, 20, and 30 subcarriers from the first link. The experimental results are shown in Fig.Â 11. From Fig.Â 11, the average recognition accuracy of the TFLPMCC algorithm shows an upward trend as the number of subcarriers increases. When using 30 subcarriers, the recognition accuracy of the algorithm is the highest. Therefore, we use 30 subcarriers in the subsequent experiments.
It is necessary to analyze the impact of channel bandwidth on the accuracy of the TFLPMCC algorithm, as signals with different frequencies are subject to different environmental interference and different impact of multipath effects. We set the bandwidth to 10, 20, 30, and 40Â MHz, respectively, and then evaluate the average recognition accuracy of the TFLPMCC algorithm. The experimental results are shown in Fig.Â 12. From Fig.Â 12, the average recognition accuracy of the TFLPMCC algorithm increases with the increase in bandwidth, because the greater the frequency difference between subcarriers, the less interference between them. However, when the bandwidth changes from 10 to 40Â MHz, the accuracy of the algorithm does not change much.
3.3 Performance of TFLPMCC algorithm
3.3.1 Ablation study
In the TFLPMCC algorithm, amplitude and timeâ€“frequency images of CSI are constructed and texture features are extracted using the GLCM method for both images. Additionally, texture features are extracted from the amplitude images using the GLDS method. The composed feature vectors are then input to the LDA algorithm for classification. To evaluate the contribution of each step in the TFLPMCC algorithm, we compare its performance with five other algorithms: (i) TFLPMCC(1) which uses only the GLCM method for texture feature extraction, (ii) TFLPMCC(2) which uses only the GLDS method for texture feature extraction, (iii) TFLPMCC(3) which uses only the CSI amplitude image, (iv) TFLPMCC(4) which uses only the GLCM method for texture feature extraction of the CSI amplitude image without using the GLDS method or CSI timeâ€“frequency image, and (v) TFLPMCCR(5) which extracts texture features only from the CSI timeâ€“frequency image without using the CSI amplitude image.
FigureÂ 13 shows the simulation results of the TFLPMCC algorithm and the five other algorithms. The TFLPMCC algorithm achieves the highest average recognition accuracy of 98.27%. In contrast, the TFLPMCC(2) and TFLPMCC(5) algorithms have significantly lower average recognition accuracies, suggesting that using only the CSI timeâ€“frequency images or the GLDS methods results in poorer performance. The average recognition accuracies of the TFLPMCCR(1), TFLPMCCR(3), and TFLPMCCR(4) algorithms are all above 93%, indicating that the CSI amplitude image and the GLCM method contribute much more to the recognition accuracy of the TFLPMCC algorithm than the CSI timeâ€“frequency image and the GLDS method. However, the CSI timeâ€“frequency image and the GLDS method can further improve the average recognition accuracy. Therefore, the TFLPMCC algorithm can adjust the composition of the algorithm based on the application's requirements. If high average recognition accuracy is required, the TFLPMCC algorithm can be used. If less running time is required, the TFLPMCC(1), TFLPMCC(3), or TFLPMCC(4) algorithms can be used.
3.3.2 Comparing different algorithms
Currently, there are fewer studies on CSIbased crowd counting compared to CSIbased activity recognition. Two existing works [14] and [10] achieved good crowd counting results using the SVM algorithm (referred to as PNRSVM) and naive Bayesian classification algorithm (referred to as PNRNB), respectively. In this paper, we compare the average recognition accuracy of TFLPMCC, PNRSVM, and PNRNB algorithms, as shown in Fig.Â 14. The results demonstrate that the TFLPMCC algorithm achieves the highest average recognition accuracy of 98.27%, which is 4.04% and 4.42% higher than that of PNRSVM and PNRNB algorithms, respectively. In TFLPMCC algorithm, the amplitude image of CSI fully utilizes the temporal stability of CSI, and the timeâ€“frequency image of CSI fully utilizes the frequency diversity characteristics of CSI. The GLCM and GLDS methods extract the local changes and spatial distribution features of CSI data. Therefore, the superior performance of the TFLPMCC algorithm is attributed to its ability to extract more finegrained features of multistate crowd information contained in the CSI amplitude. In addition, we compare the confusion matrices of the three algorithms, as shown in Figs.Â 15, 16 and 17, respectively, where the meanings of the serial numbers in the confusion matrix are shown in Table 1. The results indicate that the recognition accuracy of TFLPMCC algorithm is above 90% for all categories, while the recognition accuracy of PNRSVM algorithm is below 85% for categories 5 and 6, and the recognition accuracy of PNRNB algorithm is below 80% for categories 5 and 11. This demonstrates that the TFLPMCC algorithm not only has a high average recognition accuracy but also has a high recognition accuracy of all categories.
To evaluate the computational efficiency of the TFLPMCC algorithm, we measured the running times of the three algorithms on a laptop computer equipped with an Intel I57200U 2.5 GHz CPU and 8 GB RAM. The TFLPMCC, PNRSVM, and PNRNB algorithms took 0.068 s, 0.128 s, and 0.197 s, respectively, to recognize one sample. Notably, the TFLPMCC algorithm had the shortest running time compared to the PNRSVM and PNRNB algorithms, with a reduction of 46.88% and 65.48%, respectively. These results demonstrate that the TFLPMCC algorithm exhibits both high accuracy and low time complexity.
3.3.3 Analysis of algorithm scalability
In the previous analysis and experiments, we have verified that the TFLPMCC algorithm has low time complexity and high recognition accuracy in the experimental scenario described in Sect.Â 3.1. However, the scalability of this algorithm in other experimental scenarios and different states of volunteers still needs further analysis. For this purpose, we conducted an experiment in another laboratory that was different from the experimental scenario in Fig.Â 7, as shown in Fig.Â 18. In this experimental scenario, we still collect CSI data for 1â€“4 people, but the volunteers also add three states of chestexpanding exercise (State 4), bending and lifting both arms in front of the chest (State 5), and arm pulling from high to low (State 6), in addition to the three states of sitting (State 1), raising one hand (State 2), and walking in sequence (State 3). Therefore, the number and states of people are classified into 24 categories to recognize. The experimental setup is the same as Sect.Â 3.1. Fifty samples are collected for each category, where 30 samples are randomly selected for training and the remaining 20 samples are tested (Fig. 19). The experimental results are shown in Table 3.
Table 3 shows that in the new experimental scenario, the average recognition accuracy of the TFLPMCC algorithm for State 1, 2, and 3 is 98.75%, which is consistent with the experimental results in the scenario shown in Fig.Â 7. After adding State 4, 5, and 6, the average recognition accuracy of the algorithm slightly decreased, but it can still reach as high as 97.29%. However, in the case that there are four people who are in State 4, the recognition accuracy of the algorithm is only 80%, which shows that the number of people and the complexity of the state have a certain impact on the recognition accuracy of the algorithm. From Table 3, the more the number of people and the more complex the state of people, the lower the recognition accuracy of the algorithm. In summary, the TFLPMCC algorithm can still achieve high recognition accuracy of crowd size and state in different experimental scenarios and more states, which shows that this algorithm has good scalability. Although the number and states of people are limited, the average recognition accuracy of the algorithm can already meet the needs of most applications.
4 Discussion and limitation
Although the TFLPMCC algorithm performs well, there are still some issues that require further discussion.

(1)
In this paper, we conducted experiments on thirteen cases as shown in Table 1, collecting 70 samples for each case. Consequently, a total of 910 samples were collected, making the sample collection process laborintensive. As the number and states of crowds to be recognized by the TFLPMCC algorithm increase, so does the workload of collecting training samples. This hinders the applicability and scope of the TFLPMCC algorithm.

(2)
The TFLPMCC algorithm is capable of accurately recognizing the crowd size when all individuals perform the same action. However, in realworld applications, people may perform different actions. For example, in a room with four people, two may be sitting while the other two are walking. To test the performance of the TFLPMCC algorithm in such scenarios, we followed the same experimental setup as in Sect.Â 4.1 and conducted additional experiments in six new scenarios: 1 person walking with 1, 2, and 3 people sitting, respectively, 2 people walking with 1 and 2 people sitting, respectively, and 3 people walking with 1 person sitting. Therefore, the TFLPMCC algorithm needed to classify a total of 19 scenes. The experimental results show that the algorithm's average recognition accuracy can still reach 96.58%, indicating that the TFLPMCC algorithm can still perform well in counting crowds in arbitrary states. However, the confusion matrix shown in Fig.Â 15 reveals that the recognition accuracy of the algorithm decreased to less than 90% in the 11th, 14th, and 17th cases, indicating that while the algorithm's average recognition accuracy decreases less, the recognition accuracy of the algorithm decreases significantly for a few categories after adding six scenarios where the crowd is in arbitrary states.

(3)
If a human and an object, such as a robot or a chair, enter the monitoring area at the same time, the TFLPMCC algorithm cannot distinguish between the human and the object, and the object is also recognized as a human. Therefore, the TFLPMCC algorithm is only applicable to the scenarios where only humans are dynamically changing in the monitoring area.
5 Conclusion
As artificial intelligence continues to advance, the demand for crowd counting applications is increasing. However, existing studies cannot still count crowds in different states, and the accuracy and time complexity of crowd counting algorithms need further improvement. In response to this need, we propose the TFLPMCC algorithm, which constructs CSI data into amplitude images and timeâ€“frequency images, and extracts texture features from the two images using the GLCM method. To enhance the algorithm's recognition accuracy, we also extract texture features from the amplitude images using the GLDS method. The features extracted from both methods form the input feature vector of the LDA classification algorithm. We conducted extensive experiments to analyze the effects of the parameters and on the recognition accuracy of the TFLPMCC algorithm. Through an ablation study, we illustrated the contribution of each method of the TFLPMCC algorithm to recognition accuracy. Results compared with existing algorithms demonstrate that the TFLPMCC algorithm not only achieves a higher average recognition accuracy of up to 98.27%, but also has a lower algorithm running time of 0.068Â s.
Moving forward, we will focus on two aspects related to our work: (i) The TFLPMCC algorithm currently requires a large and expensive workload for testing training samples to recognize the number of people in multistates. To address this issue, we will explore algorithms that can achieve high recognition accuracy using smaller samples and also aim to enhance the crossdomain performance of the algorithm when adapting to new application environments. (ii) As the number of people increases, the stability of the TFLPMCC algorithm decreases when counting crowds in arbitrary states. This not only increases the human and financial cost of collecting training samples but also reduces the algorithm's performance. We will work toward developing algorithms that can effectively recognize the number of people in arbitrary states, even when counting more people.
6 Methods/experimental
The existing crowd counting algorithms struggle with low counting accuracy and high algorithm complexity when counting humans in multiple states. For this problem, we construct CSI amplitude data into amplitude and timeâ€“frequency images, and then extract texture features using the graylevel cooccurrence matrix (GLCM) and graylevel difference statistic (GLDS) methods, and finally use the linear discriminant analysis (LDA) algorithm to count the crowd in three states. To verify the TFLPMCC algorithm proposed in this paper, we conducted experiments in a laboratory. The layout of the laboratory, the devices and settings used in the experiments, the volunteers in the experiments, the activity design of the volunteers, and the data collection are all described in detail in Sect.Â 3.1. Using the collected data and a large number of simulations, we analyzed the performance of the proposed algorithm from many aspects and verified the accuracy and robustness of the proposed algorithm for multistate crowd counting.
Availability of data and material
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Abbreviations
 CSI:

Channel state information
 TFLPMCC:

Texture featuresbased lightweight passive multistate crowd counting
 GLCM:

Graylevel cooccurrence matrix
 GLDS:

Graylevel difference statistic
 LDA:

Linear discriminant analysis
 RSS:

Received signal strength
 SVM:

Support vector machine
 NB:

Naive Bayesian
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The authors would like to acknowledge the editors and reviewers and all the participants for the paper.
Funding
This work was supported in part by the National Natural Science Foundation of China under grant numbers 62076114, 71874025; the Applied Basic Research Program Project of Liaoning Province under grant number 2023JH2/101300189; the Humanities and Social Sciences Research Planning Foundation of the Ministry of Education of China under grant number 20YJA630058.
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YT was involved in methodology, supervision, writingâ€”review, investigation, software, writingâ€”original draft. JL helped in data collection, software, investigation. FG contributed to data preprocessing, software. CZ was involved in investigation, writingâ€”original draft. XD helped in resources, writingâ€”review & editing. All authors read and approved the final manuscript.
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Tian, Y., Li, Y., Li, J. et al. Texture featuresbased lightweight passive multistate crowd counting algorithm. J Wireless Com Network 2023, 79 (2023). https://doi.org/10.1186/s13638023022896
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DOI: https://doi.org/10.1186/s13638023022896