Zdenek Chaloupka

An Empirical Evaluation of a Probabilistic RF Signature for WLAN Location Fingerprinting

Localization for indoor environments has gained considerable attention over the last decade. The most popular technique is based on location fingerprinting using received signal strength (RSS) mainly due to the fact that it exploits the available wireless infrastructure and that RSS fingerprints are readily available using different wireless standards (IEEE 802.11, etc.). This simplicity however incurs a cost in accuracy and researchers focus on improving the performance from a pattern recognition perspective. Recently improvement in performance has been demonstrated using physical layer channel-based fingerprints such as the Channel Transfer Function (CTF) and Channel Impulse Response (CIR) at a cost of increased storage and computation requirements. In this paper we experimentally evaluate the performance of a probabilistic physical layer fingerprint that is based on entropy of the magnitude and phase of the CTF. We will show through extensive frequency domain channel measurements in an indoor office environment that entropy can be a practical alternative to RSS fingerprinting; where it shares the latters simplicity of structure (scalar) but outperforms RSS and complex CIR fingerprints. We further investigate the impact of realistic channel and system impairments such as small-scale fading (Doppler), Signal-to-noise ratio (SNR) and interference on the performance of the proposed fingerprint signature.

Real time evaluation of RF fingerprints in wireless LAN localization systems

RF location fingerprinting has received significant attention as a practical solution to the indoor localization problem due to its use of the available wireless infrastructure (WLANs) and the simplicity of measuring the Received Signal Strength (RSS) fingerprint. The improvement of RSS-based fingerprinting has been limited due to RSS being a weak fingerprint structure; where it has been reported in literature that using more complex pattern recognition algorithms provides diminishing gains. Recently channel-based RF fingerprints such as the channel impulse response (CIR), channel transfer function (CTF) and frequency coherence function (FCF) have been proposed to improve the accuracy at the physical layer. An empirical evaluation of the physical layer fingerprints does not exist in literature and there is a need to understand the advantages/limitations of these fingerprint structures/signatures. As a result in this paper we provide a comprehensive empirical performance evaluation of location fingerprinting with a focus on analytical comparison of the RSS, CIR, CTF and FCF -based fingerprints using the weighted k-nearest neighbor (kNN) pattern recognition technique. By conducting frequency domain channel measurements in the IEEE 802.11 band at the university campus we evaluate the accuracy of the fingerprints and their robustness to human induced motion perturbations to the channel. We also provide analysis on the impact of system parameters such as the number of access points and the number of nearest neighbors.

Entropy-based location fingerprinting for WLAN systems

Localization for indoor environments has gained considerable attention over the last decade due to the enormous potential in the technology and the significant challenges facing this area of research. One practical localization technique that relies on the available fixed wireless infrastructure is RF location fingerprinting. Received Signal Strength (RSS)-based location fingerprinting has been the dominant fingerprinting approach in the literature due to the simplicity and practicality of measuring the RSS in a variety of wireless technologies (such as IEEE 802.11 and UMTS). Recognizing the diminishing gains using the RSS-based techniques, researchers have recently shifted focus to proposing improvements at the physical layer by adopting the channel impulse response (CIR) as an alternate fingerprint. In this paper we propose a novel fingerprint structure that is based on the entropy estimation of the channel; which provides a more unique/robust fingerprint that is capable of distinguishing between locations more effectively. Through extensive frequency domain channel measurements and analysis in a typical indoor environment we further validate the proposed technique and compare it against RSS and CIR-based fingerprinting. We will show that the technique combines the advantage of RSS-based fingerprinting simplicity of structure (storage and pattern recognition requirements) and improves on the robustness of the CIR-based fingerprinting techniques. Finally we will illustrate that our entropy-based location fingerprinting can be practically integrated into the architecture of popular OFDM-based WLAN systems.

Clock Synchronization Over Communication Paths With Queue-Induced Delay Asymmetries

Timing protocols such as the IEEE 1588 Precision Time Protocol and the Network Time Protocol require an accurate measurement of the communication path delay between the time server (master) and the client (slave) in order to provide a precise timing synchronization. The precise time at the clients site is then estimated using an assumption that forward and backward delays due to physical propagation time through network are equal, or any difference between them is calibrated beforehand. Apart from physical link delays, a timing packet experiences queue-induced delay due to switching/routing devices on the path. This queuing delay is usually different in forward and backward directions, thus introducing the queue-induced asymmetry (QIA), which is a major contributor to the time error between master and slave clocks if physical asymmetries are calibrated. This letter proposes a new technique for QIA compensation that does not require any on-path timing support, thus is easily deployed with current network devices.

Efficient and precise simulation model of synchronization clocks in packet networks

As packet technologies like Ethernet and IP are becoming the dominant in modern telecommunication networks, clock frequency and time synchronization over packet networks is an active area of research. Packet networks are asynchronous by design so frequency and time synchronization with sub-microsecond precision is a demanding task due to packet delay variations. A design of proper algorithms, that meet the telecommunication requirements, usually involves simulations over long period of time with nanoseconds resolution. Such precise and long-term simulation of synchronization performance is challenging, because of the vast amount of computer resources involved. This paper describes a simulation model that is both, efficient and highly precise. The model is able to simulate algorithms behavior over long period of time (tens of hours) while the computation time remains within interval of few minutes.

Entropy-based non-line of sight identification for wireless positioning systems

Indoor positioning has gained considerable attention over the last decade. For time-of arrival based systems, operating in urban and indoor environments the availability of line-of-sight signals is not always guaranteed due to physical obstructions such as buildings, walls, elevator shafts, etc. Specifically non-line-of-sight (NLOS) conditions introduce a bias to the distance estimation which results in significant localization errors. It is therefore vital to be able to identify NLOS channels accurately in real-time and mitigate their impact on location estimation. Channel-based metrics such as RMS delay spread and kurtosis have been proposed for Ultra Wideband systems to identify NLOS channels. Their performance under different system bandwidths (e.g. WLAN), however, has not been investigated in the literature. In this paper we introduce a novel NLOS identification metric based on the entropy of the Channel Impulse Response (CIR) which exhibits distinct statistical properties in different channel conditions. Using frequency domain wireless propagation measurements in a typical indoor environment we show that the entropy metric has a higher detection/identification rate compared to RMS delay spread and kurtosis. We provide empirical evaluation of the test metrics under different system bandwidths and illustrate that entropy shows significant performance gains especially at lower bandwidths. The performance of NLOS identification depends on the accuracy of the estimated metric. Thus in order to estimate the entropy metric accurately we have selected the autoregressive (AR) modeling method which has shown accurate and consistent performance. Finally, we present an investigation of the impact of AR modeling parameters on the performance of entropy-based NLOS identification.

Packet selection technique for clock recovery over packet networks

The primary challenge in the design of a clock recovery system for packet networks is to deal with Packet Delay Variations (PDVs) - that is, the variation of the packet transit delays introduced by queuing process in Network Elements (NEs) such as switches and routers. In order to obtain clean signal from recovered clock various techniques are employed to mitigate the effect of the PDVs. One of such mechanisms, referred to as Packet Selection Algorithm (PSA), reduces recovered clock noise variance by filtering incoming timing packets in such a way that only packets complying with predefined selection criteria are passed to the clock recovery algorithm. Since it is a non-trivial task to find optimal PSA criteria analytically using output variance minimization, this paper introduces a probabilistic approach for the PSA optimality analysis. Based on this probabilistic analysis a new packet selection technique is proposed, which outperforms prior art when compared in simulation by means of Time Deviation (TDEV) standardized metric.

Transparent clock characterization using IEEE 1588 PTP timestamping probe

As new application areas like SmartGrids and 4G cellular mobile backhaul networks emerge, requirements on precision of the frequency and time synchronization increase. IEEE 1588v2 Precision Time Protocol (PTP) offers sub-microsecond synchronization precision using conventional Ethernet networks. In practical network scenarios the PTP performance is greatly reduced due to varying queuing latencies introduced by the networks switching and routing devices. This drawback has been overcome with the introduction of network devices with a Transparent Clock (TC) functionality defined in the IEEE 1588v2 PTP standard. As different vendors use proprietary implementations of the TC mechanism, it becomes essential to characterize TCs accuracy. Literature survey revealed that so far two measurement techniques have been proposed for that purpose, however, a comprehensive study of the measurements error with regards to hardware imprecisions have not yet been published. In this paper we provide a thorough mathematical analysis with respect to hardware limitations of the TC measurement techniques and based on this analysis measurement calibrations are introduced. Moreover, we propose a new measurement setup that requires less expensive hardware yet maintains similar precision to prior art techniques. The new setup is provided with a detailed proof of concept and an experimental verification through measurements.