This paper presents FreqMAE, a novel self-supervised learning framework that synergizes masked autoencoding (MAE) with physics-informed insights to capture feature patterns in multi-modal IoT sensor data. FreqMAE enhances latent space representation of sensor data, reducing reliance on data labeling and improving accuracy for AI tasks. Differing from data augmentation-based methods like contrastive learning, FreqMAE's approach eliminates the need for handcrafted transformations. Adapting MAE for IoT sensing signals, we present three contributions from frequency domain insights: First, a Temporal-Shifting Transformer (TS-T) encoder that enables temporal interactions while distinguishing different frequency bands; Second, a factorized multi-modal fusion mechanism for leveraging cross-modal correlations and preserving unique modality features; Third, a hierarchically weighted loss function that emphasizes important frequency components and high Signal-to-Noise Ratio (SNR) samples. Comprehensive evaluations on two sensing applications validate FreqMAE's proficiency in reducing labeling needs and enhancing resilience against domain shifts.

This paper proposes a novel contrastive learning framework, called FOCAL, for extracting comprehensive features from multimodal time-series sensing signals through self-supervised training. Existing multimodal contrastive frameworks mostly rely on the shared information between sensory modalities, but do not explicitly consider the exclusive modality information that could be critical to understanding the underlying sensing physics. Besides, contrastive frameworks for time series have not handled the temporal information locality appropriately. FOCAL solves these challenges by making the following contributions: First, given multimodal time series, it encodes each modality into a factorized latent space consisting of shared features and private features that are orthogonal to each other. The shared space emphasizes feature patterns consistent across sensory modalities through a modal-matching objective. In contrast, the private space extracts modality-exclusive information through a transformation-invariant objective. Second, we propose a temporal structural constraint for modality features, such that the average distance between temporally neighboring samples is no larger than that of temporally distant samples. Extensive evaluations are performed on four multimodal sensing datasets with two backbone encoders and two classifiers to demonstrate the superiority of FOCAL. It consistently outperforms the state-of-the-art baselines in downstream tasks with a clear margin, under different ratios of available labels. The code and self-collected dataset are available at https://github.com/tomoyoshki/focal.

DNS has encountered complex and diversified attacks over the years due to its special status on the Internet. The concept of DNS-over-HTTPS (DoH) has been proposed to protect user privacy by encapsulating DNS into HTTPS, which increases the difficulty of DNS tunnel detection but also faces some new attacks. In recent years, many researchers have discussed the detection methods of DoH tunnel. However, most of them need large-scale labeled datasets and extract statistical features, which is time-consuming and costs immense manpower, so it is impractical to be used in the real-world. In this paper, we developed a system called AADDS: an Autoencoder-based Anomaly Detection for the DoH traffic System consists of Traffic Capture module and Anomaly Detection module. The Traffic Capture module is developed based on nff-go, which can collect features stably in a high-speed Ethernet environment and greatly reduce the workload. For the Anomaly Detection module, we used bidirectional Long and Short-Term Memory (Bi-LSTM) to build an autoencoder network. Several essential experiments proved that our method has fewer parameters while ensuring higher accuracy, and it outperforms the state-of-the-art methods.

Recent concepts in defense herald an increasing degree of automation of future military systems, with an emphasis on accelerating sensing-to-decision loops at the tactical edge, reducing their network communication footprint, and improving the inference quality of intelligent components in the loop. These requirements pose resource management challenges, calling for operating-system-like constructs that optimize the use of limited computational resources at the tactical edge. This paper describes these challenges and presents IoBT-OS, an operating system for the Internet of Battlefield Things that aims to optimize decision latency, improve decision accuracy, and reduce corresponding resource demands on computational and network components. A simple case-study with initial evaluation results is shown from a target tracking application scenario.

Recent developments in deep learning have motivated the use of deep neural networks in mobile sensing applications. Human Activity Recognition (HAR), as one of the most important mobile sensing applications, has enjoyed great success due to the utilization of deep neural networks. Motivated by the success of self-supervised learning frameworks in computer vision and natural language processing, self-supervised models have been proposed to efficiently leverage massive unlabeled data and reduce the labeling burden of HAR applications. Current approaches use self-supervised pre-training (with unlabeled data) followed by downstream training (with labeled data). However, we claim that labeled data can still help in the pre-training process and propose SemiC-HAR, a Semi-supervised Contrastive learning framework for HAR. SemiC-HAR efficiently uses both of the labeled and unlabeled data during the pre-training process and combines the advantages of supervised and self-supervised contrastive learning frameworks. We evaluate SemiC-HAR on six HAR datasets with multiple sensing signals and show comparable performance to previous supervised and semi-supervised models seen at much lower fractions of labeled data.

This paper presents a contrastive self-supervised representation learning framework that is new in being designed specifically for deep learning from frequency domain data. Contrastive self-supervised representation learning trains neural networks using mostly unlabeled data. It is motivated by the need to reduce the labeling burden of deep learning. In this paper, we are specifically interested in applying this approach to physical sensing scenarios, such as those arising in Internet-of-Things (IoT) applications. Deep neural networks have been widely utilized in IoT applications, but the performance of such models largely depends on the availability of large labeled datasets, which in turn entails significant training costs. Motivated by the success of contrastive self-supervised representation learning at substantially reducing the need for labeled data (mostly in areas of computer vision and natural language processing), there is growing interest in customizing the contrastive learning framework to IoT applications. Most existing work in that space approaches the problem from a time-domain perspective. However, IoT applications often measure physical phenomena, where the underlying processes (such as acceleration, vibration, or wireless signal propagation) are fundamentally a function of signal frequencies and thus have sparser and more compact representations in the frequency domain. Recently, this observation motivated the development of Short-Time Fourier Neural Networks (STFNets) that learn directly in the frequency domain, and were shown to offer large performance gains compared to Convolutional Neural Networks (CNNs) when designing supervised learning models for IoT tasks. Hence, in this paper, we introduce an STFNet-based Contrastive Self-supervised representation Learning framework (STF-CSL). STF-CSL takes both time-domain and frequency-domain features into consideration. We build the encoder using STFNet as the fundamental building block. We also apply both time-domain data augmentation and frequency-domain data augmentation during the self-supervised training process. We evaluate the resulting performance of STF-CSL on various human activity recognition tasks. The evaluation results demonstrate that STF-CSL significantly outperforms the time-domain based self-supervised approaches thereby substantially enhancing our ability to train deep neural networks from unlabeled data in IoT contexts.

This paper describes a novel diffusion model, DyDiff-VAE, for information diffusion prediction on social media. Given the initial content and a sequence of forwarding users, DyDiff-VAE aims to estimate the propagation likelihood for other potential users and predict the corresponding user rankings. Inferring user interests from diffusion data lies the foundation of diffusion prediction, because users often forward the information in which they are interested or the information from those who share similar interests. Their interests also evolve over time as the result of the dynamic social influence from neighbors and the time-sensitive information gained inside/outside the social media. Existing works fail to model users' intrinsic interests from the diffusion data and assume user interests remain static along the time. DyDiff-VAE advances the state of the art in two directions: (i) We propose a dynamic encoder to infer the evolution of user interests from observed diffusion data. (ii) We propose a dual attentive decoder to estimate the propagation likelihood by integrating information from both the initial cascade content and the forwarding user sequence. Extensive experiments on four real-world datasets from Twitter and Youtube demonstrate the advantages of the proposed model; we show that it achieves 43.3%relative gains over the best baseline on average. Moreover, it has the lowest run-time compared with recurrent neural network based models.

This article proposes unsupervised truth-finding algorithms that combine consideration of multi-modal content features with analysis of propagation patterns to evaluate the veracity of observations in social sensing applications. A key social sensing challenge is to develop effective algorithms for estimating both the reliability of sources and the veracity of their observations without prior knowledge. In contrast to prior solutions that use labeled examples to learn content features that are correlated with veracity, our approach is entirely unsupervised. Hence, given no prior training data, we jointly learn the importance of different content features together with the veracity of observations using propagation patterns as an indicator of perceived content reliability. A novel penalized expectation maximization (PEM) algorithm is proposed to improve the quality of estimation results for observations bolstered by multiple features. In addition, we develop a constrained expectation maximum likelihood with multiple features (CEM-MultiF) that introduces a novel constraint to boost the probability of correctness of some claims. Finally, we evaluate the performance of the proposed algorithms, called EM-Multi, CEM-Multi and PEM-MultiF, respectively, on real-world data sets collected from Twitter. The evaluation results demonstrate that the proposed algorithms outperform the existing fact-finding approaches, and offer tunable knobs for controlling robustness/performance trade-offs in the presence of malicious sources.

Much work on social media opinion polarization focuses on a flat categorization of stances (or orthogonal beliefs) of different communities from media traces. We extend in this work in two important respects. First, we detect not only points of disagreement between communities, but also points of agreement. In other words, we estimate community beliefs in the presence of overlap. Second, in lieu of flat categorization, we consider hierarchical belief estimation, where communities might be hierarchically divided. For example, two opposing parties might disagree on core issues, but within a party, despite agreement on fundamentals, disagreement might occur on further details. We call the resulting combined problem a hierarchical overlapping belief estimation problem. To solve it, this paper develops a new class of unsupervised Non-negative Matrix Factorization (NMF) algorithms, we call Belief Structured Matrix Factorization (BSMF). Our proposed unsupervised algorithm captures both the latent belief intersections and dissimilarities, as well as hierarchical structure. We discuss properties of the algorithm and evaluate it on both synthetic and real-world datasets. In the synthetic dataset, our model reduces error by 40%. In real Twitter traces, it improves accuracy by around 10%. The model also achieves 96.08% self-consistency in a sanity check.

This paper develops a novel detection system of possibly fake accounts on public social media, called FADE, that uses features based on group behaviors to identify suspicious groups. The work is motivated by the prospect of mitigating misinformation campaigns on social media, where malicious entities on directional social networks pose as credible sources and coordinate the spreading of highly corroborated false information. Instead of account-level detection, this paper aims to detect the very group activity that underlies misinformation campaigns; namely, the coordinated spreading of messages to boost (misinformation) visibility. The existing group detection methods group users into two clusters (fake or not) and directly produce clusters of fake accounts. Conversely, we group users into many clusters based on information propagation patterns and user features and then classify them. The benefit of multiple clusters is that we can detect suspicious behavior more easily from cluster-wide statistics. In order to improve clustering accuracy, we analyze and select the most important features for clustering based on Bayesian optimization instead of using all the features. Accordingly, similarity metrics are defined that allow clustering of individually plausible accounts in a manner that enables one to detect suspicious clusters of activity. Cluster-level features are then used to decide if the cluster is benign. We further explore the cost of adversarial attacks on our detection model. Evaluation results on Twitter data sets demonstrate that our proposed approach outperforms state-of-the-art baselines in detecting accounts created for information manipulation campaigns. In addition, we show that the cost of subverting detection (without reducing the effectiveness of the attacker's campaign) is high.

Variational Autoencoders (VAE) and their variants have been widely used in a variety of applications, such as dialog generation, image generation and disentangled representation learning. However, the existing VAE models may suffer from KL vanishing in language modeling and low reconstruction quality for disentangling. To address these issues, we propose a novel controllable variational autoencoder framework, ControlVAE, that combines a controller, inspired by automatic control theory, with the basic VAE to improve the performance of resulting generative models. Specifically, we design a new non-linear PI controller, a variant of the proportional-integral-derivative (PID) control, to automatically tune the hyperparameter (weight) added in the VAE objective using the output KL-divergence as feedback during model training. The framework is evaluated using three applications; namely, language modeling, disentangled representation learning, and image generation. The results show that ControlVAE can achieve much better reconstruction quality than the competitive methods for the comparable disentanglement performance. For language modeling, it not only averts the KL-vanishing, but also improves the diversity of generated text. Finally, we also demonstrate that ControlVAE improves the reconstruction quality for image generation compared to the original VAE.

The paper presents a real-time computing framework for intelligent real-time edge services, on behalf of local embedded devices that are themselves unable to support extensive computations. The work contributes to a new direction in realtime computing that develops scheduling algorithms for machine intelligence tasks that enable anytime prediction. We show that deep neural network workflows can be cast as imprecise computations, each with a mandatory part and (several) optional parts whose execution utility depends on input data. With our design, deep neural networks can be preempted before their completion and support anytime inference. The goal of the realtime scheduler is to maximize the average accuracy of deep neural network outputs while meeting task deadlines, thanks to opportunistic shedding of the least necessary optional parts. The work is motivated by the proliferation of increasingly ubiquitous but resource-constrained embedded devices (for applications ranging from autonomous cars to the Internet of Things) and the desire to develop services that endow them with intelligence. Experiments on recent GPU hardware and a state of the art deep neural network for machine vision illustrate that our scheme can increase the overall accuracy by 10% ~ 20% while incurring (nearly) no deadline misses.

With recent advances, neural networks have become a crucial building block in intelligent IoT systems and sensing applications. However, the excessive computational demand remains a serious impediment to their deployments on low-end IoT devices. With the emergence of edge computing, offloading grows into a promising technique to circumvent end-device limitations. However, transferring data between local and edge devices takes up a large proportion of time in existing offloading frameworks, creating a bottleneck for low-latency intelligent services. In this work, we propose a general framework, called deep compressive offloading. By integrating compressive sensing theory and deep learning, our framework can encode data for offloading into tiny sizes with negligible overhead on local devices and decode the data on the edge server, while offering theoretical guarantees on perfect reconstruction and lossless inference. By trading edge computing resources for data transmission time, our design can significantly reduce offloading latency with almost no accuracy loss. We build a deep compressive offloading system to serve state-of-the-art computer vision and speech recognition services. With comprehensive evaluations, our system can consistently reduce end-to-end latency by 2X to 4X with 1% accuracy loss, compared to state-of-the-art neural network offloading systems. In conditions of limited network bandwidth or intensive background traffic, our system can further speed up the neural network inference by up to 35X 1.

With the exponential growth of the demand for wireless communications, people are faced with the shortage of spectrum resources. To tackle this problem, researchers consider about the further usage of TV channels with low occupancy, referred as white spaces. In this paper, we propose a mobile platform based indoor white space exploration method, namely MISEN, to profile the white space distribution inside a building. In this method, we design a tensor completion based algorithm based on the framework of Alternating Direction Method of Multipliers (ADMM) to recover a complete spectrum map, where we consider the linear dependency of white space information. Moreover, we build a prototype of MISEN and evaluate the performance in real scene. It is shown that MISEN senses on average 18.7% more white spaces with 20.0% less false alarm rate than the state of art methods.

This paper presents realistic traffic simulations in four representative regions in Chicago, using real map data and historical traffic statistics, to estimate the amount of fuel that can be saved by two types of systems, namely, a Green Light Optimal Speed Advisory (GLOSA) system and an Eco-Routing system. In particular, two previous systems called GreenDrive and GreenRoute are selected to estimate how much fuel they can save in a year, assuming they enjoy the same popularity as Google Maps. Evaluating intelligent transportation systems in simulation has attracted significant research efforts as large scale real-world experiments are often too expensive to carry out. However, simulation-based evaluations pose serious questions on how real the simulated environments are. In this paper, we resort to several data sources from the city of Chicago to quantify the approximation in simulated traffic, so as to convincingly derive the amount of fuel savings on a larger scale. Specifically, we create SUMO simulations of four real regions of Chicago, and utilize traffic counts data, road speed data, and taxi trips data to validate each simulation scenario. In the end, we show that by using our previously proposed systems, the estimated fuel saved in a year can be 17.6 million gallons for the entire Chicago area.

Large volumes of event data are becoming increasingly available on online social networks. These events are usually causally dependent to each other, reflecting the interactions and collaborations among different parties. Learning and interpreting the temporal patterns and dynamics within these event streams plays an important role in many practical applications, such as trend prediction and anomaly detection. Since causal dependencies can be reflected in both event time (i.e., when) and event content (i.e., who and what), we thus develop a user community based generative model, called latent Hawkes process (LHP), taking into account both-side information to illustrate the generation of such inter-dependent event streams on GitHub repositories, where each attribute is assumed to be generated by interplays between correlated latent communities. Through learning of our model, two functionalities are fulfilled concurrently: event clustering (i.e., community discovery) and temporal dependency learning among these clusters (i.e., dependency profiling). To do so, we design an EM-based framework integrating sequential Monte Carlo sampling to estimate model parameters in an end-to-end manner. Through experiments on practical GitHub event data, we validate the effectiveness of LHP in extracting user community structures and learning their correlated temporal dynamics. Such knowledge further enables us to gain new insights into the development status of software, such as the project persistence and anomaly detection.

The paper discusses an emerging suite of machine intelligence services that are of increasing importance in the highly instrumented world of the Internet of Things (IoT). The suite, called Eugene, would offer a form of intelligent behavior (based on deep neural networks) to otherwise simple embedded devices; the clients of the service. These devices would benefit from service resources to learn from data and to perform intelligent inference, classification, prediction, and estimation tasks that they are too limited to carry out on their own. The paper discusses the taxonomy of such services and the state of implementation, as well as the various challenges entailed, including scheduling, caching (of intelligent functions), and cooperative learning.

Deep neural networks are becoming increasingly popular in Internet of Things (IoT) applications. Their capabilities of fusing multiple sensor inputs and extracting temporal relationships can enhance intelligence in a wide range of applications. However, one key problem is the missing of adaptation to heterogeneous on-device sensors. These low-end sensors on IoT devices possess different accuracies, granularities, and amounts of information, whose sensing qualities are heterogeneous and vary over time. The existing deep learning frameworks for IoT applications usually treat every sensor input equally over time or increase model capacity in an ad-hoc manner, lacking the ability to identify and exploit the sensor heterogeneities. In this work, we propose SADeepSense, a deep learning framework that can automatically balance the contributions of multiple sensor inputs over time by exploiting their sensing qualities. SADeepSense makes two key contributions. First, SADeepSense employs the self-attention mechanism to learn the correlations among different sensors over time with no additional supervision. The correlations are then applied to infer the sensing qualities and to reassign model concentrations in multiple sensors over time. Second, instead of directly learning the sensing qualities and contributions, SADeepSense generates the residual concentrations that are deviated from the equal contributions, which helps to stabilize the training process. We demonstrate the effectiveness of SADeepSense with two representative IoT sensing tasks: heterogeneous human activity recognition with motion sensors and gesture recognition with the wireless signal. SADeepSense consistently outperforms the state-of-the-art methods by a clear margin. In addition, we show that SADeepSense only imposes little additional resource-consumption burden on embedded devices compared to the corresponding state-of-the-art framework.

Recent advances in deep learning motivate the use of deep neural networks in Internet-of-Things (IoT) applications. These networks are modelled after signal processing in the human brain, thereby leading to significant advantages at perceptual tasks such as vision and speech recognition. IoT applications, however, often measure physical phenomena, where the underlying physics (such as inertia, wireless signal propagation, or the natural frequency of oscillation) are fundamentally a function of signal frequencies, offering better features in the frequency domain. This observation leads to a fundamental question: For IoT applications, can one develop a new brand of neural network structures that synthesize features inspired not only by the biology of human perception but also by the fundamental nature of physics? Hence, in this paper, instead of using conventional building blocks (e.g., convolutional and recurrent layers), we propose a new foundational neural network building block, the Short-Time Fourier Neural Network (STFNet). It integrates a widely-used time-frequency analysis method, the Short-Time Fourier Transform, into data processing to learn features directly in the frequency domain, where the physics of underlying phenomena leave better footprints. STFNets bring additional flexibility to time-frequency analysis by offering novel nonlinear learnable operations that are spectral-compatible. Moreover, STFNets show that transforming signals to a domain that is more connected to the underlying physics greatly simplifies the learning process. We demonstrate the effectiveness of STFNets with extensive experiments on a wide range of sensing inputs, including motion sensors, WiFi, ultrasound, and visible light. STFNets significantly outperform the state-of-the-art deep learning models in all experiments. A STFNet, therefore, demonstrates superior capability as the fundamental building block of deep neural networks for IoT applications for various sensor inputs 1.

Recent advances in deep-learning-based applications have attracted a growing attention from the IoT community. These highly capable learning models have shown significant improvements in expected accuracy of various sensory inference tasks. One important and yet overlooked direction remains to provide uncertainty estimates in deep learning outputs. Since robustness and reliability of sensory inference results are critical to IoT systems, uncertainty estimates are indispensable for IoT applications. To address this challenge, we develop ApDeepSense, an effective and efficient deep learning uncertainty estimation method for resource-constrained IoT devices. ApDeepSense leverages an implicit Bayesian approximation that links neural networks to deep Gaussian processes, allowing output uncertainty to be quantified. Our approach is shown to significantly reduce the execution time and energy consumption of uncertainty estimation thanks to a novel layer-wise approximation that replaces the traditional computationally intensive sampling-based uncertainty estimation methods. ApDeepSense is designed for neural net-works trained using dropout; one of the most widely used regularization methods in deep learning. No additional training is needed for uncertainty estimation purposes. We evaluate ApDeepSense using four IoT applications on Intel Edison devices. Results show that ApDeepSense can reduce around 88.9% of the execution time and 90.0% of the energy consumption, while producing more accurate uncertainty estimates compared with state-of-the-art methods.

Exploring and utilizing indoor white spaces (vacant VHF and UHF TV channels) have been recognized as an effective way to satisfy the rapid growth of the radio frequency (RF) demand. Although a few methods of exploring indoor white spaces have been proposed in recent years, they only focus on the exploration of the current indoor white spaces. However, due to the dynamic nature of the spectrum and the time delay in the process of exploration, users often cannot get accurate white space information in time, resulting in issues, such as spectrum utilization conflicts or inadequate white space utilization. To solve the problem, in this paper, we first perform an indoor TV spectrum measurement to study how the spectrum state changes over time and the spatio-temporal-spectral correlation of spectrum. Then, we propose a real-time aCcurate indoOR whiTe spacE predictioN mechanism, called CORTEN. CORTEN can predict the white space distribution for various time spans with high accuracy. Furthermore, we build a prototype of CORTEN and evaluate its performance based on the real-world measured data. The evaluation results show that CORTEN can predict accurately 38.7% more indoor white spaces with 51.3% less false alarms compared with the baseline approach when predicting the white spaces one hour ahead.

Deep neural networks show great potential as solutions to many sensing application problems, but their excessive resource demand slows down execution time, pausing a serious impediment to deployment on low-end devices. To address this challenge, recent literature focused on compressing neural network size to improve performance. We show that changing neural network size does not proportionally affect performance attributes of interest, such as execution time. Rather, extreme run-time nonlinearities exist over the network configuration space. Hence, we propose a novel framework, called FastDeepIoT, that uncovers the non-linear relation between neural network structure and execution time, then exploits that understanding to find network configurations that significantly improve the trade-off between execution time and accuracy on mobile and embedded devices. FastDeepIoT makes two key contributions. First, FastDeepIoT automatically learns an accurate and highly interpretable execution time model for deep neural networks on the target device. This is done without prior knowledge of either the hardware specifications or the detailed implementation of the used deep learning library. Second, FastDeepIoT informs a compression algorithm how to minimize execution time on the profiled device without impacting accuracy. We evaluate FastDeepIoT using three different sensing-related tasks on two mobile devices: Nexus 5 and Galaxy Nexus. FastDeepIoT further reduces the neural network execution time by 48% to 78% and energy consumption by 37% to 69% compared with the state-of-the-art compression algorithms.

Exploring the utilization of white spaces (vacant VHF and UHF TV channels) is a promising way to satisfy the rapidly growing radio frequency (RF) demand. Although a few white space exploration methods have been proposed in the past few years, they mainly focused on outdoor scenarios. In this paper, we propose a novel cost-efficient indoor white space exploration method by exploiting the location dependence and channel dependence of TV spectrum in indoor environments. We first measure the UHF TV channels in a building, and study the spatial and spectral features of indoor white spaces. Then, we design a cost-eFficient Indoor White space EXploration (FIWEX) mechanism based on the extracted features. Furthermore, we build a prototype of FIWEX and extensively evaluate its performance in real-world environments. The evaluation results show that FIWEX can identify 30.0% more indoor white spaces with 51.2% less false alarms compared with the best known existing solution.

Exploration of white spaces has been recognized as a promising way to improve the utilization of wireless spectrums. Especially, indoor white space exploration has been shown to be much more challenging than that in outdoor environment. Most of existing works on indoor white space exploration infer availabilities of TV channels based on the correlations among the channels and locations, which is learned from the training data measured beforehand. However, the process of training data collection normally requires considerable time and devices, as well as human power. In this paper, we perform a measurement of indoor white spaces and study their characteristics. Based on the in-depth understanding of the characteristics, we propose a Training-free Indoor white space exploration MEchanism (TIME). In TIME, we design an algorithm for channel state inference based on Bayesian compressive sensing, as well as an incremental method for deployment of spectrum detectors. Furthermore, we present an algorithm to determine a proper number of spectrum detectors in need. Extensive real-world experiments are conducted to evaluate TIME's performance. The evaluation results demonstrate that TIME achieves competitive performances against the state-of-the-art training-based mechanisms.

Exploring the utilization of white spaces (vacant VHF and UHF TV channels) is a promising way to satisfy the rapid growth of the radio frequency (RF) demand. Although a few outdoor white space exploration methods have been proposed in the past few years, researches that focus on indoor white space exploration just emerge recently. In this paper, we propose a novel cost-efficient indoor white space exploration method by exploiting the location dependence and channel dependence of TV channels' signal strength in indoor environments. We measure the UHF TV channels in a building, and study the temporal and spatial features of indoor white spaces. Based on the extracted features, we design a cost-eficient Indoor White space EXploration mechanism, namely FIWEX. Furthermore, we build a prototype of FIWEX and extensively evaluate its performance in real world environments. The evaluation results show that FIWEX can identify 47.8% more indoor white spaces with 38.4% less false alarms compared with the best known existing solution.