Mingming Gan

On the use of ray tracing for performance prediction of UWB indoor localization systems

The most important factors impairing the performance of radio-based indoor localization systems are propagation effects like strong reflections or diffuse scattering. To the full extent, these effects can be captured only by time-consuming measurement campaigns. Ray tracing (RT) offers the possibility to predict the radio channel for a certain environment, avoiding the need for measurements. However, it is crucial to include all relevant propagation mechanisms in the RT as well as to validate the obtained results. In this paper, we analyze if sub-band divided RT can yield realistic ultra-wideband channel impulse responses. We use the RT results for performance analysis of multipath-assisted localization, which depends directly upon the above mentioned propagation effects. In particular, it has been shown that the ratio of the signal energies of deterministically reflected paths to interfering diffuse components quantifies the amount of position-related information of deterministic multipath components. Comparison of this ratio to measurement data is thus useful to validate the sub-band divided RT. The results highlight the need for proper modeling of the diffuse multipath, as estimates of this energy ratio using RT are often overly optimistic. However, the obtained localization performance predictions using measurements and RT show general agreement.

Relaying for IEEE 802.11p at road intersection using a vehicular non-stationary channel model

Traffic s afety at road intersections can be improved by establishing reliable communications between vehicles. For vehicle-to-vehicle communications, this requires information exchange in non line-of-sight (NLOS) conditions due to the obstruction by buildings. In order to overcome the low receive signal-to-noise ratio (SNR) due to NLOS, we consider to place a relay at road intersections to enhance the reliability of communication links. In this paper, we implement a vehicular non-stationary geometry based stochastic channel model for road intersections, which is an extension of an existing highway channel model. The model is verified by comparison with vehicular channel measurements. Using the proposed channel model, we present link level simulation results for IEEE 802.11p relaying at varying transmitter/receiver locations using different channel estimation techniques. The results show that a relay at the intersection is able to greatly extend the reliable communications region. Besides, in the high SNR regime with moderate or high mobility transmitter and receiver, the block type least square channel estimator is the bottleneck that limits the relaying performance. An advanced iterative channel estimator is also simulated, which exhibits robustness against increased vehicle velocities.

Modeling Time-Variant Fast Fading Statistics of Mobile Peer-to-Peer Radio Channels

The radio channels between nodes of an indoor peer-to-peer network show specific fast fading characteristics. Depending on the mobility and on the scattering properties of the environment, different kinds of fading distributions can occur: Ricean fading between static nodes, but also Rayleigh or even double-Rayleigh fading between mobile nodes. We investigate fast fading in indoor peer-to-peer networks based on radio channel measurements. It turns out that the fading statistics change over time. While the predominant fading mechanism is a combination of Rayleigh and double-Rayleigh fading, Ricean fading also occasionally occurs. On top of that, indoors, the statistics of the fast fading change over time even for small-motions of the nodes, since the propagation environment is inhomogeneous. We comprehensively model these effects using a hidden Markov model, parameterized from our measurements. The model is validated, revealing a convincing fit between the model and the measurements.

Low-complexity sub-band divided ray tracing for UWB indoor channels

Ray tracing has been extensively used to simulate indoor channel characteristics. For an ultra-wideband system, the channel characteristics vary significantly over the entire bandwidth. To cope with this, sub-band divided RT has been proposed by dividing the frequency of interest into multiple subbands and superposing the RT results at the individual center frequency of each subband. Thus, the computational complexity is directly proportional to the number of subbands. In this paper, we propose a mathematical method to significantly reduce the computational complexity of the sub-band divided RT, making it almost independent of the number of subbands. It is important to note that, based on our approach, not only the determination of the rays reaching a give location is made only once, but also the electromagnetic calculation of the received signal is not needed to perform repeatedly. The accuracy of low-complexity subband divided RT algorithm is verified through a measurement campaign.

A ray tracing algorithm using the discrete prolate spheroidal subspace

Ray tracing (RT) is an accurate propagation prediction tool that has been widely used to simulate channel characteristics in indoor environments. To date, the developed RT tool includes not only specular reflection, penetration through dielectric blocks and diffraction, but also diffuse scattering mechanisms. The accuracy, provided by a detailed modeling of the environment, comes at the cost of a high computational complexity, which directly scales with the number of propagation paths considered. We are interested in simulating the radio propagation conditions for a mobile terminal, communicating in a frame based communication system indoors with several fixed nodes. This communication shall be used to obtain the position of the mobile terminal in indoor scenario. Therefore, the correlated temporal and spatial evolution of the channel impulse response is of utmost concern. In this paper, we propose a method to significantly reduce the computational complexity of RT by using a projection of all propagation paths on a subspace spanned by two-dimensional discrete prolate spheroidal (DPS) sequences. With this method the computational complexity can be reduced by more than one order of magnitude for indoor scenarios. The accuracy of our low-complexity DPS subspace based RT algorithm is verified by numeric simulations.

Hybrid Model For Reverberant Indoor Radio Channels Using Rays and Graphs

Ray tracing tools allow for deterministic simulation of the channel impulse response. Studies show that these tools work well when the impulse response consists only of a few distinct components. However, measurements of the channel impulse response in indoor environments reveal a diffuse tail. This diffuse tail is difficult to include in ray tracing due to the computational complexity. We propose a hybrid model to include deterministic components and the diffuse tail by combining ray tracing with a propagation graph. The recursive structure of the propagation graph allows for a computationally efficient calculation of the channel transfer function considering infinitely many components. We use ray tracing and the theory of room electromagnetics to obtain the parameter settings for the propagation graph. Thus, the proposed hybrid model does not require new or additional parameters in comparison to ray tracing. Simulation results show good agreement with measurements with respect to the inclusion of the diffuse tail in both the delay power spectrum and the azimuth-delay power spectrum.

Threshold-based selective feedback for opportunistic interference alignment

Opportunistic interference alignment (OIA) exploits channel randomness and multiuser diversity by user selection. In this paper, we address the major disadvantage of OIA which requires the feedback of the locally measured interference alignment from all users. We propose a selective feedback scheme for OIA by thresholding, where only a subset of users are required to send feedback to the transmitter. The proposed scheme can reduce the amount of feedback and still achieve the optimal degrees of freedom (DoF). We characterize the threshold and the corresponding feedback load to achieve the full DoF for a given signal-to-noise ratio. Both theoretical analysis and simulation results show that the amount of feedback can be dramatically reduced (by one order of magnitude at 20dB SNR and two orders of magnitude at 30dB SNR), while still preserving the essential DoF promised by conventional OIA with full feedback.

Calibration of indoor UWB sub-band divided ray tracing using multiobjective simulated annealing

Sub-band divided ray tracing (RT) has been widely used to reproduce as reliably as possible the ultra-wideband (UWB) radio wave propagation channel in realistic indoor environments. However, its accuracy is strictly limited by the available description of the environment. Moreover, its computational complexity scales with the number of selected subbands and the number of propagation paths. In the present work, our RT tool considers not only deterministic propagation paths but also diffuse scattering components. Based on a low-complexity sub-band divided RT implementation, we propose a calibration method for indoor UWB sub-band divided RT. The method estimates the optimal material parameters, including the dielectric parameters and the scattering parameters, using channel measurements and multiobjective simulated annealing (MOSA). This calibration can improve the accuracy of sub-band divided RT in terms of the power delay profile (PDP) and the root mean square (RMS) delay spread for all test locations including those not considered by the calibration. A measurement campaign is used to verify the calibration technique.

Opportunistic interference alignment with 1-bit feedback in 3-cell interference channels

Opportunistic interference alignment (OIA) exploits channel randomness and multiuser diversity by user selection. The transmitter needs channel state information (CSI), which is usually measured on the receiver side and sent to the transmitter side via a feedback channel. Lee and Choi show that d degrees of freedom (DoF) per transmitter are achievable in a 3-cell MIMO interference channel assuming a fully informed network, where every user feeds back a real-valued variable to their own transmitter. This paper investigates the achievable DoF using only 1-bit feedback per user. We prove that 1-bit feedback is sufficient to achieve the optimal DoF d. Most importantly, the required number of users for OIA with 1-bit feedback remains the same as with real-valued feedback. Moreover, for a given system configuration, we provide an optimal choice of the 1-bit quantizer, which captures most of the capacity provided by a system with real-valued feedback.

A ray tracing algorithm for intelligent transport systems in tunnels

It is well-known that the radio wave propagation mechanisms inside a tunnel are different from the typical outdoor and indoor situations. Since the tunnels represent a significant type of vehicular environments, understanding the channel characteristics for the in-tunnel scenario is crucial for intelligent transport systems design. A widely used tool for simulating channel characteristics for outdoor and indoor scenarios is a deterministic propagation prediction tool, known as ray tracing (RT). However, RT applied for tunnel scenarios has not been studied adequately. In this paper, we first evaluate the real-world in-tunnel vehicle-to-vehicle radio channel measurements on the basis of time-varying power delay profile analysis. Secondly we introduce a RT tool that includes influence of the moving objects, to predict wave propagation mechanisms in the tunnel. In order to reduce computational complexity of RT, we suggest to combine an approximate algorithm for the higher-order reflection components with conventional RT and use a novel subdivision algorithm for modeling the diffuse scattering. Combining the higher-order reflection algorithm with conventional RT allows us to obtain more accurate delay spread results. The numerical simulations show that contribution of both the higher-order reflection and the diffuse components are equally important for the in-tunnel scenarios.