In this article, a novel analytical equation is proposed to determine the equivalent time of arrival (E-TOA) for achieving sub-ns resolution, with much reduced analog-to-digital converter sampling frequency (in the order of 2-3 MHz). The timing information is extracted from high resolution channel impulse response, which is obtained using an equivalent time sampling (ETS) technique. The proposed E-TOA equation is different from the conventional real-time sampling equation due to the presence of an additional transmitter clock drift, and thus sensitive to both the transmitter and receiver clock drift variations. The validation of the E-TOA equation is carried out numerically using simulations along with experimental validation. The effect of timing uncertainties relating to the transmitter clock start time and the receiver clock offset is analyzed with variations in the transmitter and receiver clock drifts. With E-TOA measurements, an equivalent differential time difference of arrival based one-way ranging scheme for unsynchronized anchors is further proposed. It is thus demonstrated, using in house designed sensor nodes, that high ranging accuracy, in the order of few centimeters, can be achieved by utilizing the proposed analytical E-TOA technique, even with low sampling rate.

We equip an ultra-wideband (UWB) node and a 2D LiDAR sensor a.k.a. 2D laser rangefinder on a mobile robot, and place UWB beacon nodes at unknown locations in an unknown environment. All UWB nodes can do ranging with each other thus forming a cooperative sensor network. We propose to fuse the peer-to-peer ranges measured between UWB nodes and laser scanning information, i.e., range measured between robot and nearby objects/obstacles, for simultaneous localization of the robot, all UWB beacons and LiDAR mapping. The fusion is inspired by two facts: 1) LiDAR may improve UWB-only localization accuracy as it gives a more precise and comprehensive picture of the surrounding environment; 2) on the other hand, UWB ranging measurements may remove the error accumulated in the LiDAR-based SLAM algorithm. Our experiments demonstrate that UWB/LiDAR fusion enables drift-free SLAM in real-time based on ranging measurements only.

Despite of the ultra-wideband (UWB) system's robustness against multipath in cluttered environments, a number of challenges remain before UWB localization can be implemented. In particular, non-line-of-sight (NLOS) propagation is especially critical for high-resolution localization systems because non-negligibly positive biases will be introduced in distance measurements, thus degrading the localization performance. Here, based on received and first path powers obtainable from channel impulse response (CIR), we propose a simple but very efficient method to distinguish between NLOS and LOS conditions. Our method needs neither the training data nor the prior knowledge about the environments, thus enabling realtime NLOS identification. Despite the simplicity of our method, the experimental results verify its speediness and highly correct detection rate.

This paper considers the problem of range-based decentralized localization in wireless sensor networks when the impulsive measurement noise is present. We develop a robust localization estimator requiring no a priori knowledge of the noise distribution. The approach to robust localization presented here follows the concept of M-estimation and is implemented in a decentralized manner thus suiting the dynamic nature of wireless networks. The performance of the proposed scheme is verified by simulations.

This paper considers the problem of decentralized, cooperative, and dynamic self-localization in wireless sensor networks. In particular, we are interested in a restrictive but very realistic scenario where few anchors are deployed and each anchor whose location is priori known may only communicate with very few agents (e.g. just one agent) whose location is unknown and to-be-estimated. The lack of agent-to-anchor communication links renders slow estimation convergence thereby demanding more message exchanges among the nodes i.e. agent-to-agent and agent-to-anchor. This urges us to propose an efficient localization method that needs less iterations (i.e. less message exchanges) to achieve a certain accuracy.

Ultra-wideband impulse radio (UWB-IR) exhibits good immunity to the multipath in dense cluttered environments because of its high temporal resolution. However, UWB-IR source direction finding is more challenging due to the frequency-selective distortion. Furthermore, the multipath interferences in both far-field and near-field regions affect the direct path resolution. This paper presents a robust solution for direction finding using UWB-IR in the presence of far-field and near-field multipath components overlapping with the direct path signal. An alternating projection (AP) method is proposed to separate the multiple interfering plane waves for better angle-of-arrival (AOA) estimation accuracy based on the signal subspace of UWB discrete Fourier transform (DFT). Detection of the number of plane waves is suggested before AOA estimation for fast computational convergence. Compared to the MUSIC algorithm based on IEEE 802.15.4a channel simulation, the AP has average root mean square error (RMSE) of less than 2.3° while the MUSIC has poorer RMSE of 22.6° in angular resolution when the signal-to-noise ratio (SNR) ranges from 41 dB to 15 dB. The AP is also found to perform well in the presence of strong multipath overlapping with the direct path with AOA average RMSE error of less than 3.4°.

In this paper, we present a highly efficient compact size three stage distributed power amplifier (DPA) for high power broad bandwidth applications. The DPA is fabricated using InGaP/GaAs hetero-junction bipolar transistor (HBT) technology. The design trade-off between power, efficiency and terminal matching is discussed. Reactive termination is used at the idle port of input and output transmission lines to improve efficiency. By compromising matching, the chip size is optimized to 1.8 mm2. Output power at 1dB gain compression (P1dB) is 30.5dBm at 3.5GHz and averages output power is around 28.5dBm across the (2.5 to 6) GHz band. The associated maximum and average Power Added Efficiency (PAE) at P1dB are 43% and 33%, respectively. Average large signal gain is 7 dB across the band. Small signal simulation results agree well with the measurements results.

This paper presents a method for positioning using ultra-wideband impulse radio (UWB-IR) signals that is robust in indoor environments characterized by dense multipath channel with path overlapping. Path overlapping effects arising from multipath in dense cluttered environments decrease the direct path resolution in time domain, and hence induce time-of-arrival (TOA) and angle-of-arrival (AOA) based positioning inaccuracy. To mitigate this problem, system design that is capable of resolving the closely-spaced multipath at low cost is of value. Our method yields the least-squares estimation of joint TOA and AOA with low computational cost. It is based on the spectral observation of beam forming, in which the path overlapping effect is mitigated using multipath-aided acquisition. The computational cost is reduced using in-band power spectrum and accurate initial estimations. The performance is verified both in IEEE 802.15.4a CM3 channel model and in a real environment. Simulation results in CM3 channel model show that the TOA and AOA estimation improvements on average are 2.4cm and 13.1° in the dense multipath scenario. Measurement in an indoor open hall environment shows that the AOA error decreases from 10.6° to 7.2°, and the TOA error decreases from 2.1cm to 1.9cm when the path overlapping effects are mitigated in the range of 21m.

Abstract To realize precise localization for UWB backscattering RFID system, the arrival time of the direct path in the received signal (referred as time of arrival (TOA)) needs to be accurately estimated. The background clutter is one of the major factors that deteriorates the TOA estimation accuracy in backscattering RFID system. To suppress the background clutter while maintain low‐complexity system design, we propose a novel UWB backscattering RFID system with its tag being implemented with dual pulse (DP) modulation. By transforming the mathematical model of received signal into a proper form, we discover the tag's sequence selection criteria which are able to mitigate the background clutter. For the proposed tag coded with dedicated sequences, a two‐stage TOA estimation algorithm able to effectively mitigate clutter is developed. Simulation results show that the proposed RFID system coupled with the algorithm is able to achieve accurate TOA estimation in the environment where the clutter overwhelms the tag response. Copyright © 2010 John Wiley & Sons, Ltd.

Location finding is a major application for indoor wireless sensor networks. Some of these applications include, but not limited to, logistic tracking for goods, monitoring of patients in hospitals, security application and location aware advertising. In practice, there are many factors such as noise, non-line of sight (NLOS) influence, device crystal clock time drift and accurate synchronisation among sensor nodes that can affect the ranging accuracy. In this study, the authors first propose how non-synchronised sensor nodes can apply a time difference of arrival localisation technique to compute the position of the tags. Next, the authors discuss the impact of crystal clock time drift on localisation accuracy and propose a scheme to compensate them among the sensor nodes in order to reduce its influence on localisation errors. This allows low-cost devices to operate in the network with low crystal clock qualities.

This paper introduces a modified complex to real amplitude least-squares (CRALS) algorithm for the estimation of dense multipath in an UWB-IR channel. The traditional CRALS is ineffective for UWB-IR signal due to its wide bandwidth and large off period. This algorithm is modified to resolve three closely spaced paths in a correlation window. One novelty of the modified algorithm is that region-based processing is used for the initial estimate in place of coordinate descent processing used in traditional algorithm, and so the long iteration due to large off period is saved. The other novelty is that the procedures are simplified because of no necessity to increase the penalty term to infinity in UWB-IR case. Its efficiency is compared to the generalized maximum-likelihood estimator (GML) by simulation in different signal to noise ratios (SNR). The reason for its efficiency improvement comes from fewer effective spectrum samples and region-based processing of correlation results. Real measurement is conducted to verify this algorithm's performance. Its estimation error of time difference of arrival (TDOA) is about 0.18ns.

In this paper, a Modified Phase-Only Correlator (MPOC) is proposed for resolving overlapped Ultra Wideband (UWB) radio pulses in order to achieve high-resolution multi-target UWB ranging. A Phase-Only Correlator (POC) is able to deconvolve the multi-target UWB channel from the received signal spectrum and its impulse-like output enables it to resolve UWB signals that are overlapping. However, the deconvolved channel is often corrupted by amplified noise arising from spectral nulls in the local template. To mitigate this problem, a MPOC which consists of a POC followed by a kurtosis-based noise suppression module is developed to suppress the amplified noise while maintaining the sharpness of the MPOC output. This enables the MPOC to achieve Time-of-Arrival (ToA) resolution much smaller than the UWB pulse duration. The excellent ToA detection performance of the proposed MPOC is verified by experimental measurement.

Ranging and localization are some of the key applications in a wireless sensor network. Good accuracy can be obtained if the signals captured are of good qualities. In a practical scenario, crystal clock timing drift in the devices in a wireless sensor network will greatly influence the ranging and localization accuracy. In this paper, we shall use Time Difference of Arrival technique to determine the locations of the devices. In particular, we will highlight the importance of time drift in affecting the accuracy of location determinations and proposed a scheme to overcome time drift when performing Time Difference of Arrival localization. In addition, we will also estimate the devices transmit time offset so that we can compensate this value from the range information. In this way, there is no requirement for the sensor nodes to be synchronized initially.

Based on the frequency-domain oversampling and minimum mean-square error (MMSE) principles, we propose three linear single-user detectors for downlink multicarrier codedivision multiple-access (MC-CDMA) systems. We begin with an optimal linear MMSE detector, which is computationally demanding. To reduce the complexity, a two-stage MMSE detector and a diagonal one-stage MMSE detector are developed subsequently. Simulation results show that the proposed detectors can efficiently suppress the multiple access interference (MAI) caused by frequency-selective fading, near-far effect, frequency offset, and nonlinear power amplification.

This paper studies the impact of crystal clock drift on the ranging accuracy. A novel technique in which a group of sensor devices, each having a different clock drift, can reduce this adverse effect by a 2-phase round robin fashion is presented. In comparison to the conventional way of point-to-point group ranging scheme, this technique has the desirable advantage of low energy consumption, which is a very important requirement in wireless sensor networks. The ability to reduce the effect of timing drift is illustrated in the simulation results where our proposed method outperforms prior works under the effect of clock drift on the ranging accuracy.

Abstract In this paper, we propose a receiver structure for transmitted‐reference ultra‐wideband (TR‐‐UWB) systems with both narrowband interference (NBI) and inter‐pulse interference (IPI) mitigation capabilities. The effects of additive white Gaussian noise (AWGN) and the IEEE 802.15.4a fading channel are also taken into consideration. We adopt band‐stop filtering to suppress the NBI. For IPI, it is statistically removed by a sum‐and‐average process. Theoretical analysis is carried out to study the lower bound of the bit‐error rate (BER) performance of the proposed receiver. Numerical results show that the proposed receiver is able to provide satisfactory performance and is robust to variations in the system design parameters. It is also observed that the proposed receiver is able to deliver good performance even when there is zero delay between the reference and data pulses, which can effectively double the system throughput. Copyright © 2008 John Wiley & Sons, Ltd.

In wireless sensor networks, it is very important to obtain an accurate position of each sensor node. In this paper, we consider the potential of Ultra Wideband (UWB) signals for positioning due to its special characteristics. Accurate angle of arrival (AOA) can be obtained using UWB antenna arrays. We propose a new cooperative positioning method based on the AOA's. The method takes advantages of the pair-wise AOA's information among all the sensor nodes while existing positioning method only utilizes the AOA's from the anchor nodes. Then, the Cramer-Rao Lower Bound (CRLB) for Cooperative AOA localization is analyzed. Through the simulation experiments, we show that our proposed method could achieve higher positioning accuracy than existing non-cooperative positioning methods and its accuracy increases with the number of blind nodes.

Indoor radio channels suffer multipath and non- line-of-sight (NLOS) fading, which is undesirable for accurate indoor positioning. High bandwidth of Ultra-wideband (UWB) implies high temporal resolution and hence high ranging accuracy. Furthermore, its robustness to multipath fading makes UWB perfect for indoor applications. This paper analyses the theoretical bounds of position estimation based on UWB time-difference-of-arrival (TDOA) measurements in multipath and NLOS environments. The TDOA range errors are modeled as range dependent, where the variance of the range errors is a function of the interested TDOA range. Cramer-Rao Lower Bound (CRLB) for distance- independent and distance-dependent TDOA range errors has been derived and compared.

This paper concerns the multiple access performance of spreading codes used in the downlink of multicar- rier code-division multiple-access (MC-CDMA). Complex-valued spreading codes, particularly polyphase codes, are investigated based on a system model in which modulation formats, equalization/combining schemes, and fading statistics are general. Different from previous approaches, we pay special attention to the influence of channel correlation on the performance of spreading codes and try to take advantage of the channel correlation in the design of spreading codes. We begin by considering a worst-case frequency-selective fading channel characterized by independent and identically distributed (i.i.d.) fading for all subcarriers. Unitary polyphase spreading codes are proven to be optimal among all polyphase codes for the i.i.d. fading channel. Next, optimal spreading codes for a two-user case are derived analytically. We proceed to consider a block-wise correlated fading channel model. We propose a class of channel-correlation- matched Kronecker-product unitary spreading codes, which can exploit the channel correlation, thereby reducing multiple access interference (MAI) significantly. Finally, we extend the Welch lower bound to the downlink of MC-CDMA over correlated fading channels and investigate the performance of Welch bound equality codes.

Space-time block codes are known to be a powerful tool to achieve full spatial diversity gain while maintaining a very simple receiver structure. In this paper, the authors analyze the asymptotic error performance of orthogonal space-time block codes over flat Nakagami-m fading channels. Both identical and non-identical m-distributions are considered. Based on the moment generation function (MGF) approach, the closed-form asymptotic symbol error probabilities over Nakagami-m fading channels are derived. Compared to the previous works in literature, our results are more accurate and more general. Simulation results are provided to verify our analytical results.

When power control is employed in a Ka-band LEO satellite system featured with a Markov channel, the long round-trip propagation delay will cause power control gaps when the channel transits from a good state to a bad state, resulting in long (large-scale) burst-errors. A novel hybrid ARQ (HARQ) scheme is proposed to combat this kind of burst error. Based on the iterative SOVA (soft-output Viterbi algorithm) decoder, the proposed scheme utilizes the side information of the turbo decoder to derive the average reliability of the desired packet. When the average reliability is below the specified threshold, a go-back-N ARQ protocol is launched. With this scheme, the large-scale burst errors are effectively detected and corrected by retransmission; the small-scale burst errors are filtered out for the interleaving and FEC coding to correct. The simulation results show that this scheme can achieve good BER performance with small cost in throughput and delay in a severe Markov satellite channel, which is especially suitable for traffic having strict delay requirement and moderate BER requirement.

In this paper, to meet the Ka-band hostile propagation environment and satisfy the need of high quality multimedia communication, a new ARQ based multistep power control scheme for the Ka-band LEO satellite system is proposed. With low complexity, high throughput and small extra delay, the proposed algorithm alleviates the problem of long control delay inherent in LEO satellite systems. It is suitable for the Ka-band LMS (land mobile satellite) channel where the slow fade has high dynamics (frequently experienced shadowing results in more pronounced channel state transition between shadowing and nonshadowing).