Indoor Multipath Environment Research Articles

Modeling of the Passive UHF RFID Multipath Channel for Tag Read Region Estimation

The estimation of tag read and no-read zones is the most important preliminary planning step in radio-frequency identification (RFID) system installations. Today, in the ultra-high frequency band (UHF: 860–960 MHz), this estimation is based on either a free-space or a simplified multipath channel model for the signal propagation in the wireless link between the interrogator and the tag. Using measurements in a standard multipath indoor environment, this paper proves that recent approaches are not able to estimate tag read regions in a given passive UHF RFID system setup with high reliability. A modified multipath channel model is presented, that considers environmental and setup specific properties as well as an arbitrary number of signal paths to achieve a more accurate estimation. It includes the orientation of the tag antenna and the interrogator antenna, their polarizations and their three-dimensional gain patterns, as well as complex reflection coefficients for the reflected signal paths. The presented model equations are ready to use and are implemented into a flexible, easy-to-use, and easy-to-setup simulation environment with low computing times. It predicts the tag read regions separately for the downlink and the uplink and then combines the results for an overall estimation. The comparison of read region simulations and tag readability measurements shows that the our model delivers a 87.2 % reliability in the prediction of the tag read regions. The results of the estimation can be used to optimize RFID system setups in a way that read regions are maximized.

High-Resolution AoA Estimation for Hybrid Antenna Arrays

Communication with unlicenced 60-GHz band is attractive for wireless networks since a free-of-charge bandwidth of 7 GHz is available. The antenna array, conducting transmitter/receiver beamforming, is often used to enhance the transmission range. However, to conduct beamforming, the transceivers have to know the angles of signal arrival. Many angle-of-arrival (AoA) estimation algorithms are well known. Unfortunately, most of them are developed for digital arrays, requiring an analog-to-digital-converter (ADC) and a digital-to-analog converter (DAC) for each antenna element. For an array with a large number of antennas, the implementation cost will be very high. Recently, hybrid antenna array, in which multiple antennas share an ADC/DAC, was proposed as a low-cost solution. However, the existing AoA estimation algorithms for hybrid arrays can only support one AoA; their applications are then limited in multipath indoor environments. In this paper, we propose new AoA estimation algorithms to solve the problem. Using an augmented beam-space approach, we extend the use of the well-known AoA estimation algorithms, MUSIC and ESPRIT developed for digital arrays, to the scenario of hybrid antenna arrays. Simulations show that the proposed algorithms, although developed for hybrid arrays, can have similar performance as MUSIC and ESPRIT in digital arrays.

The application of ZigBee phase shift measurement in ranging

This paper describes the initial assessment of the phase shift measurement unit embedded in a ZigBee communication device. Such a device can provide a very good base for a range-based indoor positioning or proximity detection system. The paper gives a description of the device and shows the results of test measurements. Tests were performed in various conditions: multipath free, outdoor environment, severe multipath indoor environment and dynamic environment with moving transceiver. The examples of results are depicted and conclusions are drawn. The phase shift measurement technique in the radio frequency (RF) network can increase the use of this networks in the area of localization based services. ARTICLE INFO

Frequency Hopping for Indoor Fading Channels With Varying Level of Environmental Mobility

The operation range and reliability of single-carrier wireless transmission systems are limited due to effects of the indoor multipath propagation environment. Multichannel systems can exploit frequency diversity and increase the reliability. Common frequency-hopping protocols change the channel continuously without consideration of the actual channel quality and require regular frame exchange for synchronization. However, in residential application scenarios, control data traffic occurs only very sporadic. In this letter, we describe an asynchronous wireless multichannel system, which utilizes good channels and avoids unnecessary overhead, but at the same time is able to change the frequency in case of a deep fade. The optimal protocol for the initial connection setup is extended for efficient long-term operation. The hopping sequences are modified based on successful transmissions in the past. The method is evaluated and simulated.

An Indoor AOA Estimation Algorithm for IEEE 802.11ac Wi-Fi Signal Using Single Access Point

We propose an indoor angle-of-arrival (AOA) estimation algorithm for IEEE 802.11ac Wi-Fi signal using single access point (AP) with multiple antennas. The auto-focusing method is first used to obtain a coherently combined matrix among different subcarrier frequencies. After that, we implement Toeplitz processing on the coherently combined matrix to decorrelate the received multipath signals in spatial domain. Simulation and experiment results in an indoor multipath environment show that our proposed method outperforms the conventional auto focusing method by a significant margin, when the multipath signals are highly correlated. For the case with low correlations, both methods can obtain the AOAs with high accuracy. The observations are validated by both simulations and indoor multipath experiments.

Precise Indoor Positioning and Attitude Determination using Terrestrial Ranging Signals

This paper presents the results of a new multipath mitigating antenna “V-Ray” for use with terrestrial ranging signals in severe multipath indoor environments. The V-Ray antenna – as used in the Locata positioning system – forms tight beams that provide line-of-sight range measurements as well as azimuth measurements. To take advantage of these two types of measurements a new navigation algorithm – Position and Attitude Modelling System (PAMS) – is proposed for processing carrier phase and azimuth measurements via an unscented Kalman filter. The PAMS can output the complete navigation parameters of position, velocity, acceleration and attitude simultaneously. The indoor test was conducted in a metal warehouse and the results confirmed that the horizontal positioning solutions had an accuracy of better than four centimetres and an orientation accuracy of better than 1°.

Narrow-Band Interference Suppression in Impulse-Radio Ultrawideband Systems

For impulse-radio ultrawideband (IR-UWB) communications, pulse overlap often occurs in indoor dense multipath environments, where the time separation between adjacent received multipaths is typically less than one pulsewidth. Moreover, due to its low power spectral density (PSD) and its overlay usage model, the IR-UWB signal is also vulnerable to narrow-band interference (NBI) of a high power level. Taking the effect of pulse overlap into account, this paper presents an optimal prerake/postrake UWB transceiver design to counteract NBI. The prerake/postrake uses rake structures at both the transmitter and the receiver. The optimum weights for the prerake and postrake are derived by maximizing the signal-to-inference-plus-noise ratio (SINR). Furthermore, an adaptive scheme is proposed to achieve the optimal solution iteratively. Simulations show that the proposed prerake/postrake and the adaptive scheme can significantly eliminate the adverse effect of NBI and thus perform well even in the presence of severe NBI.

Virtual Reference Device-Based NLOS Localization in Multipath Environment

This letter presents a novel non-line-of-sight (NLOS) localization scheme based on the concept of virtual reference device. The position of virtual reference device for each NLOS path can be determined if the initial mobile device position can be estimated or when the mobile device transits from LOS to NLOS region. With the position of virtual reference devices, the subsequent localization of mobile device requires only one signal path. Simulation and experiment conducted in indoor multipath environment show that the performance of our proposed localization scheme surpasses the existing localization schemes by a significant margin at all simulated and measured locations.

ADAM: An Adaptive Beamforming System for Multicasting in Wireless LANs

We present the design and implementation of ADAM, the first adaptive beamforming-based multicast system and experimental framework for indoor wireless environments. ADAM addresses the joint problem of adaptive beamformer design at the PHY layer and client scheduling at the MAC layer by proposing efficient algorithms that are amenable to practical implementation. ADAM is implemented on a field programmable gate array (FPGA) platform, and its performance is compared against that of omnidirectional and switched beamforming based multicast. Our experimental results reveal that: 1) switched multicast beamforming has limited gains in indoor multipath environments, whose deficiencies can be effectively overcome by ADAM to yield an average gain of threefold; 2) the higher the dynamic range of the discrete transmission rates employed by the MAC hardware, the higher the gains in ADAM's performance, yielding up to ninefold improvement over omni with the 802.11 rate table; and 3) finally, ADAM's performance is susceptible to channel variations due to user mobility and infrequent channel information feedback. However, we show that training ADAM's signal-to-noise ratio (SNR)-rate mapping to incorporate feedback rate and coherence time significantly increases its robustness to channel dynamics.

Implementation of Dynamic System Approach for Radio Location Fingerprinting in WLANS

Wireless indoor positioning systems have become very popular in recent years. These systems have been successfully used in many applications such as asset tracking and inventory management. Three typical location estimation schemes of triangulation, scene analysis, and proximity are analyzed. And also location fingerprinting in detail since it is used in most current system or solutions. This project focuses on the localization using Received Signal Strength (RSS) in dense multipath indoor environments. A dynamic system approach is proposed in the fingerprinting module, where the location is estimated from the state instead from RSS directly. The state is reconstructed from a temporal sequence of RSS samples by incorporating a proper memory structure based on Taken’s embedded theory. Then, a more accurate statelocation correlation is estimated because the impact of the temporal variation due to multipath is considered. General Terms Triangulation, scene analysis, proximity

A Spatial Diversity Technique for IEEE 802.11ad WLAN in 60 GHz Band

This paper shows that a spatial diversity technique can maintain a stable link quality for a 60 GHz WLAN system in an indoor multi-path environment, which suffers from the human-induced shadowing. The 60 GHz spatial diversity technique is validated by simulations and experiments.

Diversity Gain through Antenna Blocking

As part of the typical usage mode, interaction between a handheld receiver antenna and the operator's RF absorbing body and nearby objects is known to generate variability in antenna radiation characteristics through blocking and pattern changes. It is counterintuitive that random variations in blocking can result in diversity gain of practical applicability. This diversity gain is quantified from a theoretical and experimental perspective. Measurements carried out at 1947.5 MHz verify the theoretical predictions, and a diversity gain of 3.1 dB was measured through antenna blocking and based on the utilized measurement setup. The diversity gain can be exploited to enhance signal detectability of handheld receivers based on a single antenna in indoor multipath environments.

Automated linear regression tools improve RSSI WSN localization in multipath indoor environment

Received signal strength indication (RSSI)-based localization is emerging in wireless sensor networks (WSNs). Localization algorithms need to include the physical and hardware limitations of RSSI measurements in order to give more accurate results in dynamic real-life indoor environments. In this study, we use the Interdisciplinary Institute for Broadband Technology real-life test bed and present an automated method to optimize and calibrate the experimental data before offering them to a positioning engine. In a preprocessing localization step, we introduce a new method to provide bounds for the range, thereby further improving the accuracy of our simple and fast 2D localization algorithm based on corrected distance circles. A maximum likelihood algorithm with a mean square error cost function has a higher position error median than our algorithm. Our experiments further show that the complete proposed algorithm eliminates outliers and avoids any manual calibration procedure.

Performance Bound on Ergodic Capacity of MIMO Beam-Forming in Indoor Multi-Path Channels

This paper aims to characterize the ergodic capacity of multiple input multiple output (MIMO) antenna beam-forming in indoor multi-path propagation environments. Using Double-Directional Cluster-Ray (DDCR) channel models, the second order statistics of the strongest eigen-mode of the channel is analyzed. An upper bound on the ergodic capacity of the MIMO beam-forming is derived and the impact of channel propagation parameters, such as cluster/ray arrival rates and power decay profiles, on this upper bound is investigated. The theoretical results are subsequently applied to estimate the capacity of the next generation very high throughput WLAN at the 60 GHz frequency band.

Performance Evaluation of a Four-Element Antenna Array with Selection Circuits for Adaptive MIMO Systems

Reconfigurable antenna arrays increase the flexibility of adaptive MIMO systems. At present, most designs have adopted antenna arrays with reconfigurable elements. However, antenna selection is also an effective method, which has not been fully investigated. In this paper, the potential benefits of a four-element antenna array with selection circuits in the UMTS band (1920–2170 MHz) are explored. The array has eight pin-diodes embedded in the feeding network to select any sub-set of elements. For evaluation, an adaptive MIMO system was set up and a measurement campaign was taken in an indoor multi-path environment. The measurements were performed over a 300 MHz bandwidth centered at 2.05 GHz, covering the UMTS band. The results show that different channel conditions prefer different antenna array configurations. Therefore, in varying channel conditions the antenna array can support antenna selection algorithms to select the best sub-set of elements to increase channel capacity.

Signal detection performance in Rayleigh fading environments with a moving antenna

The performances of a single-antenna handheld receiver in detecting a narrowband signal in a Rayleigh fading environment that is temporally static but decorrelates spatially are analysed. Of interest is comparing the detection performance of a static antenna with that of a moving antenna subject to constant processing time. It is shown that the net processing gain resulting from randomly moving the antenna relative to keeping it static can be large, namely over 11 dB in some cases, which is significant for numerous indoor applications. It is further demonstrated that, for a given utilisation scenario, there is an optimum number of spatial samples that maximise the processing gain advantage of the moving antenna. Generally, if the spatial trajectory of the antenna becomes too large, then the loss associated with the signal decorrelation dominates and undermines the gains achieved by the increased spatial diversity. Practical implementation issues including the sensitivity of the proposed method to trajectory estimation are investigated. An extensive set of measurements based on CDMA 2000 signals propagated from outdoor terrestrial base stations and captured in indoor multipath environments using static and moving antennas are utilised to experimentally substantiate these theoretical findings.

A dynamic system approach for radio location fingerprinting in wireless local area networks

This study focuses on the localization using Received Signal Strength (RSS) in dense multipath indoor environments. A dynamic system approach is proposed in the fingerprinting module, where the location is estimated from the state instead from RSS directly. The state is reconstructed from a temporal sequence of RSS samples by incorporating a proper memory structure based on Taken's embedded theory. Then, a more accurate state-location correlation is estimated because the impact of the temporal variation due to multipath is considered. An indoor experiment in Wireless Local Area Networks (WLAN) shows the effectiveness of our approach.

Frequency Offset Compensation for OFDM Systems Using a Combined Autocorrelation and Wiener Filtering Scheme

One of the orthogonal frequency division multiplexing (OFDM) system disadvantages is its sensitivity to frequency offset and phase noise, which lead to losing the orthogonality between the subcarriers and thereby degrade the system performance. In this paper a joint scheme for frequency offset and pilot-based channel estimation is introduced in which the frequency offset is first estimated using an autocorrelation method, and then is fined further by applying an iterative phase correction by means of pilot-based Wiener filtering method. In order to verify the capability of the estimation algorithm, the scheme has been implemented and tested using a real measurement system in a multipath indoor environment. The results show the algorithm capability of compensating for the frequency offset with different transmission and channel conditions.

Combined Spatial-Polarization Correlation Function for Indoor Multipath Environments

The combined spatial-polarization correlation formulation for a multipath environment based on the uniform sphere of scatters model is developed. The closed-form expressions developed are useful for quantifying the potential diversity gain in antenna diversity systems. An extensive set of measurements were carried out at 1947.5 MHz to verify the theoretical results. The proposed model can be utilized for characterizing spatial and polarization diversity systems for indoor multipath environments.

Performance and capacity analysis of 60 GHz WPAN channel

AbstractIn this work, the performance and capacity of both 60 GHz ultrawide‐band (UWB) and multiple‐in multiple‐out (MIMO) channels are analyzed based on experimental channel models and specifications for millimeter‐wave wireless personal area networks (WPANs). In the analysis of 60 GHz UWB channel, the radio link budget is estimated by applying two experimental channel models where blocking effect is considered and fading margin (FM) is modeled as a function of channel bandwidth. Results show that high gain antennas are required in 60 GHz UWB channel. The high gain antenna systems suffer from poor flexibility and the directional signals can be easily blocked in multipath indoor environments. This indicates that gigabit communications is quite difficult in 60 GHz UWB channel. To overcome this problem, adaptive antenna arrays (or MIMO systems) are required as they can search and direct the available signals to improve radio link budget. The 60‐GHz MIMO channel capacity is analyzed by modeling FM as a function of number of antenna elements. The proposed NLOS channel model is employed in rich scattering or multipath environments for achieving high capacity in MIMO channel. Tests were also performed for determining gigabit data rate, operation range and configuration in the 60‐GHz MIMO case. The goal of this study is to provide useful information for the design of future 60 GHz WPAN systems. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2671–2675, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24698