Principles and Limitations of Ultra-Wideband FM Communications Systems

TLDR

Multipath may limit the useful subcarrier bandwidth to one octave. The paper proposes a constant‑envelope UWB system based on a low‑modulation digital FSK followed by a high‑modulation analog FM, and studies its performance under AWGN, multipath, narrowband interference, and simultaneous detection of multiple FM signals at the same carrier. The system employs FDMA subcarrier techniques, targets low to medium bit rates for short‑range WPANs, and uses a wideband delay‑line FM demodulator without a preceding limiting amplifier as its key receiver component. The approach allows multiple users to share the same RF bandwidth, and simulations and a hardware demonstrator confirm its principles and limitations.

Abstract

This paper presents a novel UWB communications system using double FM: a low-modulation index digital FSK followed by a high-modulation index analog FM to create a constant-envelope UWB signal. FDMA techniques at the subcarrier level are exploited to accommodate multiple users. The system is intended for low (1–10 kbps) and medium (100–1000 kbps) bit rate, and short-range WPAN systems. A wideband delay-line FM demodulator that is not preceded by any limiting amplifier constitutes the key component of the UWBFM receiver. This unusual approach permits multiple users to share the same RF bandwidth. Multipath, however, may limit the useful subcarrier bandwidth to one octave. This paper addresses the performance with AWGN and multipath, the resistance to narrowband interference, as well as the simultaneous detection of multiple FM signals at the same carrier frequency. SPICE and Matlab simulation results illustrate the principles and limitations of this new technology. A hardware demonstrator has been realized and has allowed the confirmation of theory with practical results.