Ultra-wideband Regime Research Articles

On Numerical Simulations of Ultra-Wideband Long-Haul Optical Communication Systems

This paper discusses the key issues in reliable modeling and numerical simulation of ultra-wideband (UWB) long-haul fiber-optic systems, such as those spanning the C+L band. We analyze the complexity of the split-step Fourier method (SSFM), including modern scenarios based on graphical processing units (GPU), and its numerical setup for accurate simulations. We explain why using the Manakov equation in the nonlinear step of the SSFM remains a good solution also in the UWB scenario, even though the underlying assumptions may be violated. We next propose a technique, called virtual channel grouping, to speed up SSFM simulations in the UWB regime. Importance sampling-based techniques are proposed to speed up the computation of the enhanced Gaussian noise (EGN) model, with computational times of the order of seconds. With such novel solutions, accurate simulations are feasible even for UWB systems.

Capacity of Sparse Multipath Channels in the Ultra-Wideband Regime

This paper studies the ergodic capacity of time-and frequency-selective multipath fading channels in the ultrawide-band (UWB) regime when training signals are used for channel estimation at the receiver. Motivated by recent measurement results on UWB channels, we propose a model for sparse multipath channels. A key implication of sparsity is that the independent degrees of freedom in the channel scale sublinearly with the signal space dimension (product of signaling duration and bandwidth). Sparsity is captured by the number of resolvable paths in delay and Doppler. Our analysis is based on a training and communication scheme that employs signaling over orthogonal short-time Fourier (STF) basis functions. STF signaling naturally relates sparsity in delay-Doppler to coherence in time and frequency. We study the impact of multipath sparsity on two fundamental metrics of spectral efficiency in the wideband/low-SNR limit introduced by Verdu: first- and second-order optimality conditions. Recent results by Zheng et al. have underscored the large gap in spectral efficiency between coherent and noncoherent extremes and the importance of channel learning in bridging the gap. Building on these results, our results lead to the following implications of multipath sparsity: (1) the coherence requirements are shared in both time and frequency, thereby significantly relaxing the required scaling in coherence time with SNR; (2) sparse multipath channels are asymptotically coherent - for a given but large bandwidth, the channel can be learned perfectly and the coherence requirements for first- and second-order optimality met through sufficiently large signaling duration; and (3) the requirement of peaky signals in attaining capacity is eliminated or relaxed in sparse environments.

A Fully Electronic System for the Time Magnification of Ultra-Wideband Signals

We present the first experimental demonstration of a fully electronic system for the temporal magnification of signals in the ultra-wideband regime. The system employs a broadband analog multiplier and uses chirped electromagnetic-bandgap structures in microstrip technology to provide the required signal dispersion. The demonstrated system achieves a time-magnification factor of five in operation on a 0.6-ns time-windowed input signal with up to 8-GHz bandwidth. We discuss the advantages and limitations of this technique in comparison to recent demonstrations involving optical components

Achievable rates of transmitted-reference ultra-wideband radio with PPM

In this letter, we study the achievable rates of practical ultra-wideband (UWB) systems using pulse position modulation (PPM) and transmitted-reference (TR) transceivers. TR obviates the need for complex channel estimation, which is particularly challenging in the context of UWB communications. Based on an upper bound we derive for the error probability with random coding, we establish that for signal-to-noise ratio values of practical interest, PPM-UWB with TR can achieve rates on the order of C(/spl infin/)=P/N/sub 0/ (nats/s), where C(/spl infin/) denotes the capacity of an additive white Gaussian noise channel in the UWB regime for average received power P and noise power spectrum density N/sub 0/.