Abstract—Attractive features of time-hopping spread-spectrum multiple-access systems employing impulse signal technology are outlined, and emerging design issues are described. Performance of such communications systems in terms of achievable transmission rate and multiple-access capability are estimated for both analog and digital data modulation formats under ideal multiple-access channel conditions. Index Terms—Impulse radio, ultra-wide bandwidth. I. INTRODUCTION TO IMPULSE RADIO SYSTEMS THE TERM wideband, as applied to communication systems, can have different meanings. In conventional systems, “wideband ” implies a large modulation bandwidth and thus a high data transmission rate. In this paper, a spread-spectrum

Abstract — Impulse radio, a form of ultra-wide bandwidth (UWB) spread-spectrum signaling, has properties that make it a viable candidate for short-range communications in dense multipath environments. This letter describes the characteristics of impulse radio using a modulation format that can be supported by currently available impulse signal technology and gives analytical estimates of its multiple-access capability under ideal multiple-access channel conditions. Index Terms—Impulse radio, spread-spectrum multiple access, time hopping, ultra-wideband radio. I. A RATIONALE FOR IMPULSE RADIO IMPULSE RADIO communicates with baseband pulses of very short duration, typically on the order of a nanosecond, thereby spreading the energy of the radio signal very thinly from near dc to a few gigahertz. When this pulse is applied to

This paper describes the results of an ultra-wideband (UWB) propagation study in which arrays of propagation measurements were made. After a description of the propagation measurement technique, an approach to the spatial and temporal decomposition of an array of measurements into wavefronts impinging on the receiving array is presented. Based on a modification of the CLEAN algorithm, this approach provides estimates of time-of-arrival, angle-of-arrival, and waveform shape. This technique is applied to 14 arrays of indoor propagation measurements made in an office/laboratory building. Statistical description of the results is presented, based on a clustering model for multipath effects. The parameters of these statistical models are compared to results derived for narrowband signal propagation in the indoor environment.

The performance of a single-user ultra-wideband (UWB) communication system employing binary block-coded pulse-position modulation (PPM) and suboptimal receivers in multipath channels is considered. The receivers examined include a rake receiver with various diversity combining schemes and an autocorrelation receiver, which is used in conjunction with transmitted reference (TR) signaling. A general framework is provided for deriving the performance of these receivers in multipath channels corrupted by additive white Gaussian noise (AWGN). By employing previous measurements of indoor UWB channels, we obtain numerical results for several cases which illustrate the tradeoff between performance and receiver complexity.

Realizing the great potential of impulse radio communications depends critically on the success of timing acquisition. To this end, optimum data-aided timing offset estimators are derived in this paper based on the maximum likelihood (ML) criterion. Specifically, generalized likelihood ratio tests (GLRTs) are employed to detect an ultra-wideband waveform propagating through dense multipath, as well as to estimate the associated timing and channel parameters in closed form. Capitalizing on the pulse repetition pattern, the GLRT boils down to an amplitude estimation problem, based on which closed-form timing acquisition estimates can be obtained without invoking any line search. The proposed algorithms only employ digital samples collected at a low symbol rate, thus reducing considerably the implementation complexity and acquisition time. Analytical acquisition performance bounds and corroborating simulations are also provided.

One of the features characterizing almost every multiple access (MA) communication system is the processing gain. Through the use of spreading sequences, the processing gain of random direct sequence code division multiple access (RCDMA) systems, or any other CDMA systems, is devoted to both bandwidth expansion and orthogonalization of the signals transmitted by different users. Another type of multiple access systems is Impulse Radio (IR). In many aspects, IR systems are similar to time division multiple access (TDMA) systems, and the processing gain of IR systems represents the ratio between the actual transmission time and the total time between two consecutive transmissions (on-plus-off to on ratio). While CDMA systems, which constantly excite the channel, rely on spreading sequences to orthogonalize the signals transmitted by different users, IR systems transmit a series of short pulses and the orthogonalization between the signals transmitted by different users is achieved by the fact that at the receiver most of the pulses do not collide with each other.

[A look at positioning aspects of future sensor networks] Ultra-wideband (UWB) radios have relative bandwidths larger than 20 % or absolute bandwidths of more than 500 MHz. Such wide bandwidths offer a wealth of advantages for both communications and radar applications. In both cases, a large bandwidth improves reliability, as the signal contains different frequency components, which increases the probability that at least some of them can go through or around obstacles. Furthermore, a large absolute bandwidth offers high resolution radars with improved ranging accuracy. For communications, both large relative and large absolute bandwidth alleviate small-scale fading [1], [2]; furthermore, spreading information over a very large bandwidth decreases the power spectral density, thus reducing interference to other systems, effecting spectrum overlay with legacy radio services, and lowering the probability of interception. UWB radars have been of long-standing interest, as they have been used in military applications for several decades [3], [4]. UWB communications-related applications were introduced only in the early 1990s [5]–[7], but have received wide interest after the U.S. Federal Communications Commission (FCC) allowed the use of unlicensed UWB communications [8].

Performance of Ultra-Wideband (UWB) communication systems can be enhanced by collecting multipath diversity gains, once the channels are acquired at the receiver. In this paper, we develop a novel pilot waveform assisted modulation (PWAM) scheme that is tailored for powerlimited UWB communications, and can be implemented in analog form. The PWAM parameters are designed to jointly optimize performance, and information rate. The resulting transmitter design also minimizes the meansquare error (MSE) of channel estimation, and thereby achieves the Cramer-Rao Lower Bound (CRLB).

Impulse radio (1R) is an ultra-wideband system wRh attractive features for baseband asynchronous multiple access (MA), multimedia services, tactical wireless communications and networking. Implemented with analog components, the continuous-time 1RMA model utilizes pulse-pesition modulation (PPM) and random time-hopping codes to alleviate multipath effects and suppress multiuser interference (MUD. We develop here a novel all-digital IRMA scheme and its discrete-time equivalent model that relies on pulse-amplitude modulation (PAM) and judiciously designed orthogonal user codes to eliminate MUI deterministically and account for frequency-selective multipath in the downlink. We also design a time-division-duplex access protocol and low-complexity linear multichannel receivers that we compare and test both analytically and by simulation.

This paper presents an overview of UWB propagation channels. It first demonstrates how the frequency selectivity of propagation processes causes fundamental differences between UWB channels and ¨conventional¨(narrowband) channels. The concept of pathloss has to be modified, and well-known WSSUS model is not applicable anymore. Next, describe deterministic and stochastic models for UWB channels and identify the key parameters for the description of delay dispersion, attenuation, and directional characterization, and we survey the typical values that have been measured. We also discuss measurement techniques, and methods for extracting model parameters showing that the concepts of narrowband channel parameter estimation (e.g., maximum-likelihood estimation) have to be modified. Finally we discuss the impact of channel models on various UWB systems.