Klaus Witrisal

Noncoherent ultra-wideband systems

The need for low-complexity devices with low-power consumption motivates the application of suboptimal noncoherent ultra-wideband (UWB) receivers. This article provides an overview of the state of the art of recent research activities in this field. It introduces energy detection and autocorrelation receiver front ends with a focus on architectures that perform the initial signal processing tasks in the analog domain, such that the receiver does not need to sample the UWB received signals at Nyquist rate. Common signaling and multiple access schemes are reviewed for both front ends. An elaborate section illustrates various performance tradeoffs to highlight preferred system choices. Practical issues are discussed, including, for low-data-rate schemes, the allowed power allocation per pulse according to the regulators ruling and the estimated power consumption of a receiver chip. A large part is devoted to signal processing steps needed in a digital receiver. It starts with synchronization and time-of-arrival estimation schemes, introduces studies about the narrowband interference problem, and describes solutions for high-data-rate and multiple access communications. Drastic advantages concerning complexity and robustness justify the application of noncoherent UWB systems, particularly for low-data-rate systems.

Equivalent system model and equalization of differential impulse radio UWB systems

A discrete-time equivalent system model is derived for differential and transmitted reference (TR) ultra-wideband (UWB) impulse radio (IR) systems, operating under heavy intersymbol-interference (ISI) caused by multipath propagation. In the systems discussed, data is transmitted using differential modulation on a frame-level, i.e., among UWB pulses. Multiple pulses (frames) are used to convey a single bit. Time hopping and amplitude codes are applied for multi user communications, employing a receiver front-end that consists of a bank of pulse-pair correlators. It is shown that these UWB systems are accurately modeled by second-order discrete-time Volterra systems. This proposed nonlinear equivalent system model is the basis for developing optimal and suboptimal receivers for differential UWB communications systems under ISI. As an example, we describe a maximum likelihood sequence detector with decision feedback, to be applied at the output of the receiver front-end sampled at symbol rate, and an adaptive inverse modeling equalizer. Both methods significantly increase the robustness in presence of multipath interference at tractable complexity.

A new method to measure parameters of frequency-selective radio channels using power measurements

This paper describes a method of deriving channel parameters for a time-dispersive (=frequency-selective) radio channel from simple wide-band power measurements. A novel relationship is found allowing the estimation of the root-mean-square (RMS) delay spread from such data. Furthermore, we describe a set of equations that can be used for fitting the (measured) channel parameters to a mathematical model, the so-called frequency-domain (FD) model. We also show a simulation procedure, which directly implements the mathematical description of the channel. The output of this procedure-realizations of frequency-selective channel transfer functions-may be used for instance in the investigation of OFDM systems. The study is restricted to the small-scale modeling; Rayleigh and Ricean fading channels are considered. The estimation of the RMS delay spread is based on the frequency-domain level crossing rate (LCR/sub f/), which is derived from the FD-channel model. It is shown that the RMS delay spread is proportional to the LCR/sub f/. Because of the simple hardware required for finding the LCR/sub f/, the suggested measurement method is particularly interesting for the millimeter-wave frequency band (>30 GHz). However, it can be used at other frequencies as well, where standard laboratory equipment is sufficient for conducting the measurements. The accuracy of this technique depends on the bandwidth observed and can be increased further by combining multiple measurements performed within a small local area.

Transmitted-reference UWB systems using weighted autocorrelation receivers

The application of a fractionally sampled autocorrelation receiver (AcR) is proposed for the demodulation of transmitted-reference (TR) signals, to simplify synchronization and suppress nonlinear intra-symbol-interference and inter-symbol-interference (ISI) by means of linear weighting. A system model is presented for the TR system and its statistical properties are derived, taking into account both noise and ISI. The weighting coefficients are computed in closed form and a method is presented to compute the bit error probability. Multiple ultra-wideband TR systems are compared, which differ with respect to correlation lag, bandwidth, sampling rate, and bit rate. In the absence of ISI, a 250-MHz bandwidth system outperforms the larger bandwidth systems. On the other hand, the larger bandwidth systems are found to be inherently less sensitive to ISI. Furthermore, increasing the fractional sampling ratio is shown to enable the AcR to equalize more (nonlinear) ISI.

Analysis of a UWB Indoor Positioning System Based on Received Signal Strength

This paper explores the possibility to design an indoor ultra wideband (UWB) ranging and positioning system using the received signal strength (RSS). Due to the extremely large bandwidths, the effects of small scale fading are reduced to the level where the knowledge of the path loss model (PLM) can be employed for accurate and reliable distance estimation. This approach enables trilateration based position estimation while significantly reducing the synchronization effort. A limited number of measurements is necessary in order to calibrate the PLM parameters but no extensive database of measurements is required, such as in fingerprinting methods. Based on simulated UWB channels, the effects of uncertainties in the PLM parameters on the estimated distance are characterized. Data from a UWB measurement campaign in indoor line-of-sight (LOS) scenarios are used to verify the performance of such a system.

Bandwidth dependence of CW ranging to UHF RFID tags in severe multipath environments

In this paper the impact of the signal bandwidth on the performance of frequency modulated continuous wave (FMCW) radar based ranging to ultra high frequency (UHF) radio frequency identification (RFID) tags is investigated. The analyses are based on ultra-wideband (UWB) channel measurements performed in a warehouse portal, which is a severe multipath environment. It is illustrated that the available bandwidth of the usual ISM bands at 900 MHz, 2.5 GHz and 5.8 GHz is only sufficient for a precise RFID tag localization if moderate or low multipath conditions are given. However, in severe multipath channels the ISM bands are unsuited and UWB signals are needed. The results can be considered a lower bound for signal time of flight (TOF) based localization approaches that utilize Fourier or correlation methods for the signal travel time estimation.

Multifrequency Continuous-Wave Radar Approach to Ranging in Passive UHF RFID

In this paper, we present the extension of a recently published two-frequency continuous-wave (CW) ultra-high-frequency RF identification ranging technique to multiple carriers. The proposed system concept relies on exact phase information; hence, the passive tag cannot be accurately modeled as a frequency-flat linear device. A linearized model of the tags reflection coefficient is devised to bridge the gap between the nonlinear reality and the linear CW radar theory. Estimation error bounds are derived and effects caused by noise and multipath propagation are analyzed in detail. It has been found that systematic errors introduced by the tags reflection characteristic cannot be compensated by using multiple carriers due to large variations caused by detuning. Nonetheless the system, while being vulnerable to multipath propagation effects, still performs well under line-of-sight conditions; mean average errors below 15% of the true distance are possible in typical fading environments..

Multiuser interference and inter-frame interference in UWB transmitted reference systems

In transmitted reference ultra-wideband (TR-UWB) systems and differential transmitted reference (DTR-UWB) systems, the performance is, besides noise, determined by interference among pulses due to the dispersive multipath radio channel (interframe interference). The multiple access interference is also heavily influenced by multipath propagation. As a first step towards the analysis and optimization of such systems, this paper analyzes statistically the response of the pulse-pair correlators, including integrate and dump circuits which are the basic building blocks of TR-UWB receivers, to desired and un-desired pulse-pairs. Results are given in terms of basic channel parameters like RMS delay spread and Ricean K-factor. As an example for the application of our analysis, we present a novel multiuser DTR-UWB system.

High-accuracy localization for assisted living: 5G systems will turn multipath channels from foe to friend

Assisted living (AL) technologies, enabled by technical advances such as the advent of the Internet of Things, are increasingly gaining importance in our aging society. This article discusses the potential of future high-accuracy localization systems as a key component of AL applications. Accurate location information can be tremendously useful to realize, e.g., behavioral monitoring, fall detection, and real-time assistance. Such services are expected to provide older adults and people with disabilities with more independence and thus to reduce the cost of caretaking. Total cost of ownership and ease of installation are paramount to make sensor systems for AL viable. In case of a radio-based indoor localization system, this implies that a conventional solution is unlikely to gain widespread adoption because of its requirement to install multiple fixed nodes (anchors) in each room. This article therefore places its focus on 1) discussing radiolocalization methods that reduce the required infrastructure by exploiting information from reflected multipath components (MPCs) and 2) showing that knowledge about the propagation environment enables localization with high accuracy and robustness. It is demonstrated that new millimeter-wave (mm-wave) technology, under investigation for 5G communications systems, will be able to provide centimeter (cm)-accuracy indoor localization in a robust manner, ideally suited for AL.

Modeling and Mitigation of Narrowband Interference for Transmitted-Reference UWB Systems

Transmitted-reference ultra-wideband (TR-UWB) systems, in conjunction with autocorrelation-receivers (AcR), can collect all energy of the multipath channel response at low complexity. Unfortunately the AcR front-end also collects the energy of narrowband interference (NBI) signals, which severely degrades its performance. We present an elaborate statistical analysis of the NBI for a multichannel AcR front-end. Our results show specific correlation properties, which can be exploited to mitigate the NBI by post-processing the AcR output samples. This explains how linear signal processing algorithms can be used to efficiently suppress the cross-terms arising in the nonlinear AcR between all input signals: the UWB, NBI, and noise signals. Linear least squares (LS) and minimum mean squared error (MMSE) receivers are investigated. To support our analysis, computer simulations are conducted using an IEEE802.11a WLAN signal as NBI.