Marco Pausini

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.

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.

Statistical Analysis of UWB Channel Correlation Functions

Various performance metrics of impulse-radio (IR) ultrawideband (UWB) receivers are closely connected to the correlation functions of the multipath channel responses to UWB pulses. Interpulse interference is related to the autocorrelation function (ACF) of the received pulse (RP), the RP energy and its fading correspond to the ACF at zero lag, and multiple access interference is connected to the cross-correlation function (CCF) between two channel pulse responses. Each realization of the multipath channel shows different correlation functions due to the ldquorandomnessrdquo of the UWB propagation environment. This paper derives the first- and second-order statistics of the ACF and CCF, capturing this randomness. Such results are useful for incorporating the multipath channel into the performance and design optimization studies of UWB systems. The analysis is based on a model of the received UWB pulse. The model describes the random channel response by two continuous functions of the excess delay time-one expresses the power, the other expresses power variations-and by a prototype pulse shape representing all linear system components including the band limitation of the RP and antenna effects. The analytical results are validated through the analysis of simulated and measured channel responses.

Performance enhancement of differential UWB autocorrelation receivers under ISI

The autocorrelation receiver (AcR) is a suboptimum, low-complexity receiver architecture particularly suited to ultra-wideband (UWB) communication systems. As the bit rate increases, interference among pulses due to multipath propagation causes serious impairments of the AcRs performance. To mitigate this effect, we propose an appropriate design of the chip code and of the delay hopping (DH) code. We provide conditions to be satisfied by the DH code in order to reduce the nonlinear intersymbol interference (ISI) and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. By extending the length of the chip code, we show that N/sub p/ transmitted pulses per symbol can be employed to suppress the average linear ISI of N/sub p/-1 previous symbols. Simulated results confirm the performance improvement in terms of bit-error rate. However, in previous work it has been shown that the noise power linearly increases with N/sub p/. Although a large number of pulses per symbol is favorable for ISI mitigation, we show that the transmission of a single pulse minimizes the probability of error, for bit rates lower than an upper bound depending on the channel root mean-square delay spread and on the noise power.

Analysis and comparison of autocorrelation receivers for IR-UWB signals based on differential detection

In this paper we propose two innovative detection schemes for IR-UWB communication systems based on the autocorrelation receiver and differential detection. The proposed schemes avoid the need for a complex analog multiplier by employing: (i) a limiter in the reference branch only; or (ii) employing also a limiter in the signal branch. Analytical models for the statistics of the decision variables are presented for the receivers, and the analytical and simulated BER exhibit an excellent agreement. We point out the dependence of the performance on some important system parameters, that shows the system design trade-off between receiver complexity and performance.

On the narrowband interference in transmitted reference UWB receivers

In this paper, we study the performance of a transmitted reference (TR) system corrupted by a single tone interferer in a multipath noise-free channel. It is shown that the auto-correlation of the narrowband interferer, arising from the dirty template adopted in the signal demodulation, can be modelled as a bias term. The magnitude of this term depends on the frequency of the tone interferer, and it can enormously affect the receiver performance. However this kind of interference can be completely suppressed by designing a simple delay and chip code, without requiring any information on the frequency of the interferer. The cross-correlation between the UWB and the narrowband signals can be modelled as a random variable, for which we derive the analytical probability density function. The statistic of this term also depends on the carrier frequency of the tone interferer, and a method is proposed to mitigate its effects.

Performance Analysis of UWB Autocorrelation Receivers Over Nakagami-Fading Channels

We study the performance of the autocorrelation receiver (AcR) with the transmitted-reference (TR) signaling scheme in the multipath UWB channel, where the fading of each resolvable multipath component is characterized by: 1) Nakagami distribution; 2) arbitrary fading parameter; and 3) arbitrary average power. The analysis, compared to conventional Rake receivers, is complicated by the fact that the instantaneous signal-to-noise ratio (SNR) at the AcR output is a nonlinear function of the instantaneous SNR at the receiver input, the latter given by the sum of the SNRs of each resolvable multipath component. Based on the derivation of exact and approximated expressions for the probability density function (pdf) of the output SNR, we evaluate several indicators of the receiver performance, such as: 1) average SNR; 2) amount of fading; 3) outage probability; and 4) average bit-error probability (BEP); all are given in a closed-form. Remarkably, we show that the pdf of the output SNR can be modeled as a gamma distributed random variable, which allows us to write the average BEP in terms of the well-known hypergeometrical Gaussian function. An approximate, yet accurate, expression of the integration time minimizing the average BEP is also derived.

Equivalent system model of ISI in a frame-differential IR-UWB receiver

A discrete-time equivalent system model is derived for the intersymbol interference in a differential ultra wideband (UWB) impulse radio (IR) system operating in a multipath propagation environment. Data is transmitted using differential binary modulation on a frame-level (i.e., among UWB pulses), which is a variant of the well-known transmitted reference UWB schemes, proposed for higher data rates. It is shown that the UWB system is accurately modeled as a second-order discrete time Volterra system. Equations are presented to determine the coefficients of this non-linear model. The impact of noise is characterized. The proposed model can be used to develop signal processing algorithms for UWB-receivers in order to enhance the data rate or multiuser capabilities, for instance.

Impact of multipath propagation on impulse radio UWB autocorrelation receivers

The autocorrelation function of the channels response to an ultra wideband (UWB) pulse determines the interframe-interference (IFI) in transmitted reference UWB systems employing autocorrelation receiver (AcR) front-ends. The statistical properties of this autocorrelation function were previously derived by the authors, i.e., expressions have been given for the mean and covariance of the correlator outputs in terms of channel parameters. In this paper, these previous results are validated by means of channel measurement data, which allows conclusions on the importance of the various components of a UWB channel model for an AcR in presence of inter-symbol-interference or multiple-access-interference. It is demonstrated that a model representing the received prototype pulse-shape, an exponentially decaying power delay profile plus a line-of-sight component, and a homogeneous ray arrival process can largely represent the impact of multipath propagation on the AcR.