Accurate wireless timing synchronization has been an extremely important topic in wireless sensor networks, required in applications ranging from distributed beam forming to precision localization and navigation. However, it is very challenging to realize, in particular when the required accuracy should be better than the runtime between the nodes. This work presents, to our knowledge for the first time, an experimental timing synchronization scheme that achieves a timing accuracy better than 5-ns rms in a network with 4 nodes. The experimental hardware is built from commercially available components and based on software-defined ultra-wideband transceivers. The protocol for establishing the synchronization is based on our recently developed “blink” protocol that can scale from the small network demonstrated here to larger networks of hundreds or thousands of nodes.

Ultra-wideband (UWB) technology is a good candidate to provide accurate position information indoors and in dense urban environments where Global Positioning System (GPS) is usually not reliable. This paper provides the results of a UWB ranging measurement campaign carried out in a dense urban environment. Measurements were taken with two different antenna heights in line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. It is observed that the ranging errors increases when the antennas are closer to the ground, and is more significant in NLOS conditions. Also, in NLOS conditions ranging errors of more than 10 meters were observed when the LOS component is completely blocked by a building. Errors of such magnitude are typically not captured by IEEE 802.15.4a CM6 (outdoor NLOS) channel model. Since the conventional thresholding schemes provide bad ranging accuracy in presence of multiuser interference (MUI), we propose a nonlinear processing scheme of time-hopping impulse radio (TH-IR) and apply it to our measurements to show that it gives much better ranging accuracy.

Roundtrip time-of-arrival (ToA) measurements employing ultra-wideband (UWB) signals can provide high-precision ranging information. However, the accuracy is degraded by multiuser interference (MUI), in particular in the presence of multipath propagation. While the processing gain of time-hopping impulse radio (TH-IR) can be used to suppress the MUI, this is often insufficient. We propose instead a nonlinear processing scheme of TH-IR that effectively suppresses MUI without requiring knowledge of the time-hopping sequences of the interfering users. The principle is that multipath components (MPCs) of interferers do not align closely, for the majority of transmission frames, with the MPCs of the desired signal. Through a judicious choice of algorithm parameters we show that our algorithm is superior to existing (realizable) thresholding and median filter algorithms, and in some cases can even beat genie-aided thresholding algorithms. The performance is robust to both strength and number of the interferers. The results are validated with both standardized 802.15.4a channel models and measured outdoor UWB channels.

Ultra-wide bandwidth communication systems are known to suffer from non-Gaussian interference. In general, wireless ad-hoc networks, due to the lack of perfect power control and the random placement of nodes, are very likely to experience non-Gaussian multiple access interference. In systems with additive non-Gaussian noise, non-Gaussian signal detection and nonlinear signal processing can play important roles, especially in suppressing the multiple access interference. Practical ultra-wide bandwidth receiver designs based on the α-stable non-Gaussian additive impairment model are proposed and evaluated. It is demonstrated that the α-stable model together with a fractile based parameter estimation technique constitute a robust technique for adaptive receiver designs. The proposed receivers based on the α-stable model suppress multiple access interference, adapt to the noise and the interference in the environment, and are robust to estimation errors in the receiver parameters. The proposed receivers provide much better bit error rates than linear detectors and previously known nonlinear detectors.

Wireless communication systems operating in the presence of additive Gaussian noise have been well studied in the literature. The additive thermal and ambient noise in the system can be modeled by a Gaussian distribution. This model makes the analysis tractable due to the desirable analytic properties of the Gaussian distribution. However, in many situations the additive noise contains another impulsive noise component which cannot be modeled by a Gaussian approximation. The Laplacian distribution is one of the widely used distributions for modeling non-Gaussian and impulsive noise. The performance of linear diversity combining schemes corrupted by additive Gaussian-plus-Laplacian noise in a Rayleigh fading channel is analyzed.

A more appropriate statistical model for the multiple access interference than the generally used Gaussian approximation is proposed to reflect the heavy-tailed nature of the multiple access interference in ultra-wide bandwidth systems. Novel receiver structures which surpass the performance of the conventional matched filter receiver are studied for ultrawide bandwidth multiple access communications in both AWGN and fading multipath channels. A proposed Rake receiver is advantageous over the conventional Rake receiver when multiuser interference dominates ambient Gaussian noise and originates from a small to moderate number of interferers. Explanation is provided for the inaccuracy of a Gaussian approximation with regard to the multiple access interference, and the heavy-tailed nature of the probability density function of the multiple access interference is also discussed.

The maximum likelihood optimal combiner for signal detection in alpha-stable noise is not known in general, except for some special values of the characteristic exponent ?. A linear Rake combiner receiver is simple and easy to realize. The optimal linear Rake receiver, in the sense of minimizing the bit error rate, for the detection of signals contaminated by symmetric alpha stable noise is derived for values of ?, 0 < ? ? 1. Interestingly, for this range of ?, the optimal combiner is found to be a selection combiner which selects the channel (finger) with the largest signal amplitude and suppresses all other channels (fingers). This interesting result is valid over the range 0 < ? ? 1 and allows one to implement effective signal detection without having to know the actual value of the parameter ?. Therefore, the result yields a very simple form of diversity combiner for signal detection in symmetric alpha-stable noise for 0 < ? ? 1. Comparisons with the widely used maximal ratio combining and equal gain combining schemes are made in terms of signal-to-noise ratio advantage defined in the bit error rate sense.

An integrated cross-noise component frequently arises in communications and in signal processing. One typical example is the autocorrelation receiver. Integrated cross-noise generally appears when a noisy signal is recovered (detected) using correlation techniques with a noisy signal as the reference. The statistics of the integrated cross-noise component are analyzed in general for both the cases of uncorrelated and correlated noise terms in the integrand. The exact 2ndorder statistics of the integrated cross-noise are derived, and new improved expressions are derived for an approximate probability density function. Accurate computer simulations are performed to evaluate the actual probability density function of the integrated cross-noise component and these results are compared with the approximations. The convergence of the approximation to a Gaussian distribution is also discussed.

The optimal linear Rake receiver for the detection of signals contaminated by symmetric alpha stable noise is derived for 1 < ¿ ¿ 2. Although this structure is suboptimal, it is optimal among the class of linear Rake receivers in the sense of minimizing the bit error rate. The proposed receiver is a very simple form of diversity combiner for signal detection in symmetric alpha-stable noise. The bit error rate improvements of the optimal linear Rake receiver over the maximal ratio combiner and the equal gain combiner are also derived in the form of signal-to-noise ratio advantage.

UWB multiple access interference is known to be non-Gaussian. Work has attempted to design better UWB detectors by approximating the distribution of the multiple access interference by a non-Gaussian noise model. In this paper, a non-linear detector is proposed, which is inspired by the empirical finding that the UWB multiple access interference can be better modeled by a symmetric-alpha-stable distribution than the Laplacian, generalized Gaussian, Cauchy and Gaussian distributions. The new detector which is adaptive to the current interference-plus-noise condition is found to outperform many other non-Gaussian (non-linear) detectors proposed for UWB multiuser communication.

The optimal linear Rake receiver for the detection of binary signals contaminated by symmetric alpha-stable noise is derived for 1 < alpha les 2. The bit error rate improvements of the optimal linear Rake receiver over the maximal ratio combiner and the equal gain combiner are also derived in the form of signal-to-noise ratio advantage. The proposed receiver is a very simple form of diversity combiner for signal detection in alpha- stable noise.

A new analytical method for evaluating the bit error rate performance of a transmitted reference ultra-wide bandwidth system with an arbitrary front-end filter is derived. The method is based on expanding the received signal using the Karhunen-Loeve expansion and then evaluating the characteristic function of the receiver decision metric. This new method puts no restriction on the front-end filter type, is suitable for any arbitrary value of receiver time-bandwidth product and is highly accurate for evaluating bit error rates as small as 10-9 in both additive white Gaussian noise and fading multipath channels. The method is also extended to the case of correlated message and reference noises. The results indicate that the effect of this cross-correlation is not negligible in evaluating the bit error rate. It is found that differences in the front-end filter can lead to significant differences in the system bit error rate.

Two novel receiver structures which surpass the performance of the conventional matched filter receiver are proposed for ultra-wide bandwidth multiple access communications. The proposed receiver structures are derived based on a more appropriate statistical model for the multiple access interference than the generally used Gaussian approximation. Reasons are provided for the slow convergence of a central limit theorem with regard to the multiple access interference, and the heavy tailed nature of the probability density function of the multiple access interference is also discussed.

The sampling expansion approach for computing the performance of TR UWB systems has restricted validity and is very inaccurate in some cases. A new analytical method to evaluate the BER performance of a TR UWB system is derived. The method is based on expanding the received signal using the Karhunen-Loeve expansion and then evaluating the characteristic function of the receiver decision metric. This new method puts no restriction on the front-end filter type and is accurate for any arbitrary value of receiver time-bandwidth product. The analytical results are validated with simulation results, and it is found that the method is highly accurate and can evaluate BER rates down to 10-9.

An integrated cross-noise component frequently arises in communications and in signal processing. One typical example in communication is the autocorrelation receiver. Integrated cross-noise generally appears when a noisy signal is recovered (detected) using correlation techniques with a noisy signal as the reference. The statistics of the integrated cross- noise component are analyzed in general for both the cases of uncorrelated and correlated noise terms in the integrand. The statistics are characterized theoretically by an approximate probability density function. This theoretical probability density function is verified by simulation and its convergence towards a Gaussian distribution is discussed.

A novel analytical method is introduced for exact statistical modeling of multiple access interference (MAI), in time hopping pulse position modulation and pulse amplitude modulation (TH-PPM and TH-PAM) ultra wideband (UWB) systems operating in additive white Gaussian noise (AWGN) channels. Based on this method, exact bit error rates (BER) are expressed in simple formulas. In a similar fashion, the exact BER of direct sequence (DS) UWB is also derived for PAM modulation. The proposed modeling of MAI considers complete asynchronism in user access, and is also suitable for accurately modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables. We further extend this method to derive general expressions for the BER performance in log-normal fading multi-path channels. In the course of these derivations, we also introduce a more accurate numerical approach to evaluate the characteristic function (CF) of a lognormal random variable

A new analytical method is introduced for the exact statistical modeling of multiple access interference (MAI), in time hopping pulse position modulation (TH-PPM) ultra wideband (UWB) systems. Based on this model the exact bit error rate (BER) is expressed in simple formulas. The proposed modeling of MAI considers complete asynchronism, and is also suitable for modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables.

In this paper, a new analytical method based on exact statistical modeling of multiple access interference (MAI) is proposed for exact bit error rate (BER) computation of time hopping pulse position modulation (TH-PPM) ultra wideband (UWB) systems. The proposed modeling considers full asynchronism. Furthermore, it is also suitable for modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables.