Lorenzo Taponecco

Joint TOA and AOA Estimation for UWB Localization Applications

A joint TOA/AOA estimator is proposed for UWB indoor ranging under LOS operating conditions. The estimator employs an array of antennas, each feeding a demodulator consisting in a squarer and a low-pass filter. Signal samples taken at Nyquist rate at the filter outputs are processed to produce TOA and AOA estimates. Performance is assessed with transmitted pulses with a bandwidth of either 1.5 GHz (type-1 pulses) or 0.5 GHz (type-2 pulses), which correspond to sampling rates of 3 GHz and 1 GHz, respectively. As expected, the estimation accuracy decreases with the pulse bandwidth. Ranging errors of about 10 cm and angular errors of about 1° are achieved at SNR of practical interest with type-1 pulses and two antennas at a distance of 50 cm. With type-2 pulses the errors increase to 35 cm and 3°. Comparisons are made with other schemes discussed in literature.

TOA Estimation with the IEEE 802.15.4a Standard

This paper discusses a ranging algorithm appropriate for impulse radio ultra-wideband signals compliant with the IEEE 802.15.4a standard. Three steps are indicated to locate the so-called ranging marker in the received signal. The first is to find the highest peaks in the channel responses to the transmitted pulses. The second consists in measuring the distance of each peak to the starting time of the response. The last one involves the identification of the first symbol of the physical-layer header. Algorithms to accomplish all these tasks are indicated. They are implemented in digital form and are in keeping with a non-coherent receiver structure. The achievable accuracy depends on the sampling rate. For example, at SNR = 20 dB and with a rate of 1 GHz the ranging errors are on the order of 20 cm.

A differential receiver for uwb systems

We consider a UWB communication system operating in a typical indoor environment and we propose a novel differential receiver in which encoding is performed symbol-by-symbol rather than frame-by-frame, as is normally done. The detector structure is derived in a single-user scenario using a generalized likelihood ratio test approach. Its performance is assessed by simulation in the presence of multiple-access interference. The proposed receiver has a simple structure and outperforms other differential schemes of comparable complexity

Interference-free code design for MC-CDMA uplink transmissions

In a recent study, solutions have been proposed for completely suppressing the multiple access interference (MAI) arising in the uplink of a quasi-synchronous multicarrier code-division multiple-access network as a consequence of multipath distortions and carrier frequency offsets. This result is achieved by employing exponential orthogonal codes or selecting a particular subset of the Walsh-Hadamard code family without the need for any channel state information at the transmitter side. In the present letter, we revisit this problem and show that MAI suppression can be achieved by following a different line of reasoning which leads to a transmission scheme exhibiting a lower peak-to-average power ratio.

Synchronization for Differential Transmitted Reference UWB Receivers

This paper investigates timing recovery for differential transmitted reference (DTR) ultra-wideband receivers. In DTR systems the symbol energy is distributed over several time slots (frames) and a single pulse is transmitted per frame. The information is conveyed by differentially encoding the pulse polarities. Equal polarities correspond to a bit +1 while alternate polarities correspond to a bit -1. The detector computes the decision statistic by correlating pulses from adjacent frames and summing the contributions from all the frames of a symbol. Synchronization consists in providing accurate timing for the correlation computations. A timing algorithm is proposed requiring a limited amount of circuitry in addition to that needed for detection purposes. No training sequences are needed as the synchronizer operates in a blind mode. Two possible receiver architectures are envisioned, analog or digital. The effects of the timing errors on the receiver performance are investigated by simulation. In general, they are found to be negligible, except when the implementation is digital and the analog-to-digital converter resolution is limited to 1 bit.

Ultra-Wideband TOA Estimation in the Presence of Clock Frequency Offset

The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receivers and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.

Impact of MAI and channel estimation errors on the performance of Rake receivers in UWB communications

The performance of Rake receivers for ultrawide-bandwidth communications is discussed, taking into account the effects of multiple access interference (MAI) and channel estimation errors. Two alternative signaling formats are considered: time-hopping pulse-position modulation (TH-PPM) and TH pulse-amplitude modulation (TH-PAM). The channel exhibits multipath propagation and its impulse response is either assumed known or is estimated with least squares methods. Computer simulations show that, even with perfect channel knowledge (PCK), TH-PAM is superior to TH-PPM. The superiority increases with the number of users and becomes substantial in the presence of channel estimation errors. An intuitive explanation of this fact is provided.

Performance comparisons between two signaling formats for UWB communications

Two signaling formats for ultrawide bandwidth indoor communications are compared in terms of bit error rate performance in the presence of multiple access interference. The channel exhibits multipath propagation and its impulse response is either assumed known or is estimated with least squares methods. Computer simulations show that, even with perfect channel knowledge, time-hopping pulse-amplitude modulation is superior to time-hopping pulse-position modulation. The superiority increases with the number of users and becomes substantial in the presence of channel estimation errors. An intuitive explanation of this fact is provided.

On the Feasibility of Overshadow Enlargement Attack on IEEE 802.15.4a Distance Bounding

Distance-bounding protocols are able to measure a secure upper bound to the distance between two devices. They are designed to resist to reduction attacks, whose objective is reducing the measured distance. In this paper we focus on the opposite problem, the enlargement attack, which is aimed at enlarging the measured distance. We analyze the feasibility of enlargement attacks through overshadow strategies on 802.15.4a UWB distance-bounding protocols. We show that the overshadow strategies, generally considered feasible by the existing literature, are actually difficult to carry out. Depending on the delay introduced by the adversary, there are cases in which they have no effect or their effect is not controllable.

Modeling Enlargement Attacks Against UWB Distance Bounding Protocols

Distance bounding protocols make it possible to determine a trusted upper bound on the distance between two devices. Their key property is to resist reduction attacks, i.e., attacks aimed at reducing the distance measured by the protocol. Recently, researchers have also focused on enlargement attacks, aimed at enlarging the measured distance. Providing security against such attacks is important for secure positioning techniques. The contribution of this paper is to provide a probabilistic model for the success of an enlargement attack against a distance bounding protocol realized with the IEEE 802.15.4a ultra-wideband standard. The model captures several variables, such as the propagation environment, the signal-to-noise ratio, and the time-of-arrival estimation algorithm. We focus on non-coherent receivers, which can be used in low-cost low-power applications. We validate our model by comparison with physical-layer simulations and goodness-of-fit tests. The results show that our probabilistic model is sufficiently realistic to replace physical-layer simulations. Our model can be used to evaluate the security of the ranging/positioning solutions that can be subject to enlargement attacks. We expect that it will significantly facilitate future research on secure ranging and secure positioning.