Pochiang Lin

Performance comparison of indoor positioning techniques based on location fingerprinting in wireless networks

Appropriate and correct indoor positioning in wireless networks could provide interesting services and applications in many domains. There are time of arrival (TOA), time difference of arrival (TDOA), angle of arrival (AOA), and location fingerprinting schemes that can he used for positioning. We focus on location fingerprinting in this paper since it is more applicable to complex indoor environments than other schemes. Location fingerprinting uses received signal strength to estimate locations of mobile nodes or users. Probabilistic method, k-nearest-neighbor, and neural networks are previously proposed positioning techniques based on location fingerprinting. However, most of these previous works only concentrate on accuracy, which means the average distance error. Actually, it is not enough to measure the performance of a positioning technique by the accuracy only. A comprehensive performance comparison is also critical and helpful in order to choose the most fitting algorithm in real environments. In this paper, we compare comprehensively various performance metrics including accuracy, precision, complexity, robustness, and scalability. Through our analysis and experiment results, k-nearest-neighbor reports the best overall performance for the indoor positioning purpose.

Location Fingerprinting In A Decorrelated Space

We present a novel approach to the problem of the indoor localization in wireless environments. The main contribution of this paper is fourfold: 1) we show that by projecting the measured signal into a decorrelated signal space, the positioning accuracy is improved, since the cross correlation between each AP is reduced, 2) we demonstrate that this novel approach achieves a more efficient information compaction and provides a better scheme to reduce online computation (the drawback of AP selection techniques is overcome, since we reduce the dimensionality by combing features, and each component in the decorrelated space is the linear combination of all APs; therefore, a more efficient mechanism is provided to utilize information of all APs while reducing the computational complexity), 3) experimental results show that the size of training samples can be greatly reduced in the decorrelated space; that is, fewer human efforts are required for developing the system, and 4) we carry out comparisons between RSS and three classical decorrelated spaces, including Discrete Cosine Transform (DCT), Principal Component Analysis (PCA), and Independent Component Analysis (ICA) in this paper. Two AP selection criteria proposed in the literature, MaxMean and InfoGain are also compared. Testing on a realistic WLAN environment, we find that PCA achieves the best performance on the location fingerprinting task.

Dynamic Search Algorithm in Unstructured Peer-to-Peer Networks

Designing efficient search algorithms is a key challenge in unstructured peer-to-peer networks. Flooding and random walk (RW) are two typical search algorithms. Flooding searches aggressively and covers the most nodes. However, it generates a large amount of query messages and, thus, does not scale. On the contrary, RW searches conservatively. It only generates a fixed amount of query messages at each hop but would take longer search time. We propose the dynamic search (DS) algorithm, which is a generalization of flooding and RW. DS takes advantage of various contexts under which each previous search algorithm performs well. It resembles flooding for short-term search and RW for long-term search. Moreover, DS could be further combined with knowledge-based search mechanisms to improve the search performance. We analyze the performance of DS based on some performance metrics including the success rate, search time, query hits, query messages, query efficiency, and search efficiency. Numerical results show that DS provides a good tradeoff between search performance and cost. On average, DS performs about 25 times better than flooding and 58 times better than RW in power-law graphs, and about 186 times better than flooding and 120 times better than RW in bimodal topologies.

Calibration-Free Approaches for Robust Wi-Fi Positioning against Device Diversity: A Performance Comparison

Received signal strength (RSS) in Wi-Fi networks is commonly employed in indoor positioning systems; however, device diversity is a fundamental problem in such systems. This problem becomes more important in recent years due to the tremendous growth of new Wi-Fi devices, which perform differently in respect to the RSS values and degrade localization performance significantly. Several studies have proposed methods to improve the robustness of positioning systems against device diversity. This paper is primarily concerned with the performance of calibration-free approaches, including signal strength difference (SSD), hyperbolic location fingerprinting (HLF), and DIFF. The performance comparison is based on two Wi-Fi positioning systems in a 3-D indoor building, including a zero-configuration and a fingerprinting-based system. The results show that these calibration-free techniques perform much better than the original RSS with heterogeneous devices. However, the improvement in robustness is gained at the expense of losing some discriminative information. When the testing and training data are both measured from the same device, the performance of HLF and SSD is clearly below that of RSS in both systems. Although DIFF performs the best, it has to suffer from dealing with a space of large dimensions.

Achieving airtime fairness of delay-sensitive applications in multirate IEEE 802.11 wireless LANs

In multirate IEEE 802.11 wireless LANs, performance anomaly is a well-studied problem. Most existing solutions try to solve this problem by achieving airtime fairness. However, these solutions tend to increase the frame delay of some stations. If these stations run delay-sensitive applications, e.g., VoIP and video conferences, their quality of service would be seriously degraded. In this article we introduce and classify the existing solutions. The frame delay of each type of approach is illustrated by analytical results. We also propose an adaptive approach to solve this delay issue. The proposed approach could accurately model the condition of each station and concurrently adjust multiple MAC parameters. Simulation results validate that the proposed approach not only achieves airtime fairness but also improves frame delay. Therefore, the proposed approach could be applied to improve the quality of service of multimedia applications over WLAN environments.

Indoor localization by a novel probabilistic approach

The main contribution of our work is to develop a novel localzation algorithm called transformation based probabilistic approach for building a robust and compact localization system. Instead of operating in the measured RSS domain, our algorithm projects the measured signal into a transformed signal space. This approach offers a more efficient mechanism to utilize information of all APs and overcomes the drawback of traditional AP selection techniques which unavoidably throws out the information of unselected APs. Furthermore, the transformation can be viewed as passing through different filters and these filtering processes help reduce the noisy components of the measured signal. Experimental results show that the proposed algorithm demonstrates outstanding performance while the coefficients implementation is based on principal component analysis (PCA) technique. Not only the information compaction, but also the robustness is achieved in our indoor WLAN positioning system. The numerical results show that the error mean is reduced by 42% and the error variance is reduced by 71% on the average.

Performance analysis of a cross-layer handoff ordering scheme in wireless networks

In this paper we propose a cross-layer handoff ordering scheme. The frame success rate (FSR) is adopted as the basis of prioritization. Different quality of service (QoS) requirements of various applications would result in different FSR requirements. In order to indicate how critical a handoff request is, both the FSR requirement from the application layer and the FSR measurement from the medium access control layer are taken into consideration in the proposed scheme. The prioritization of handoff requests follows the most-critical-first policy. Performance analysis shows that the proposed scheme effectively reduces the forced termination probabilities. Under the same forced termination probability requirements, it could provide 1.95% to 11.13% more arrival calls compared to previous works.

Optimal dynamic spectrum access in multi-channel multi-user cognitive radio networks

Wireless spectrum is a limited and valuable resource for communications. However, wireless spectrum is known to be underutilized in spacial, temporal, and spectral domains. The dynamic spectrum access (DSA) of cognitive radio networks provides the capability to improve the spectrum efficiency by allowing secondary users to access the spectrum opportunistically without interfering primary users. The dynamic spectrum access is a joint channel allocation and power control problem with the objective to maximize the aggregated throughput of all secondary users. This problem is especially difficult in multi-channel multi-user cognitive radio networks. In the literature it is often formulated as a mixed integer nonlinear programming (MINLP) problem which is NP-hard. Therefore, some approximation methods are proposed to solve this problem, which lead to suboptimal solutions. In this paper we carefully reexamine the DSA problem, and prove that the original MINLP problem formulation is over-parameterized. We show that the DSA problem could be formulated as a nonlinear programming (NLP) problem without losing globally optimal objective function values. Moreover, the optimal solution to this NLP problem could be obtained by an interior point DSA optimization algorithm in polynomial time. Simulation results show that the proposed method performs better than other approximation methods do.

Hndoff ordering using link quality estimator for multimedia communications in wireless networks

Traditional handoff ordering methods adopt the received signal strength (RSS) as the basis of prioritization. However, the RSS is not the only one metric to represent the user perceived quality of service, since many other factors, like the packet length, interference, and the modulation/codec schemes, would also affect it. In this paper, we propose a handoff ordering method based on packet success rate (PSR) for multimedia communications in wireless networks. The priority of a handoff request is based on its current PSR, the PSR degradation rate, and the minimum PSR requirement of its service class. The major contribution of our method is that we improve the user perceived QoS during the handoff process. Simulation results indicate that our method can effectively improve the handoff call dropping probability with little or no increase of the new call blocking probability.

Novel User-Placement Ushering Mechanism to Improve Quality-of-Service for Femtocell Networks

Today, it is essential to provide the sufficient quality-of-service (QoS) for real-time mobile applications over wireless channels. Researchers have proposed a variety of solutions, thereby, including resource allocation, data scheduling, and cross-layer design, etc. However, hardly any previous study has investigated the advantage of users’ mobility. This paper proposes a novel QoS-oriented navigation approach to usher the mobile device(s) to the appropriate location, which can enable a sufficient QoS. The proposed mechanism will establish a two-dimensional geographical packet-success-rate (PSR) map, which can help to guide the mobile user(s) to locate the spot providing the QoS sufficing the application(s). With the help of the indoor positioning system and this PSR map, an innovative ushering method can be facilitated for the appropriate mobile users’ placement to reach the satisfactory QoS. The corresponding performance for our proposed scheme has been evaluated through the experiments over the femtocell network. Experimental results demonstrate the effectiveness of our proposed method to provide the sufficient QoS by ushering the users placement for three networked multimedia applications.