Time reversal for ultra-wideband communications: Architecture and test-bed

This chapter presents a wideband radio system design concept that takes advantage of rich multipath propagation. Depending upon the channel information, each pair of transmitter and receiver in the system chooses a transmit waveform that is optimal in some sense. The transmitters are capable of waveform level preprocessing, while the receivers can be rather simple since they do not need special means (like a RAKE combiner) to capture dispersed energy over time, and even equalizers may not be necessary. In particular, the channel impulse response (CIR) can be utilized to provide physical-layer security enhancement thanks to the spatial focusing property of time reversal preprocessing. This design concept is especially attractive to wireless sensor networks (WSN) because (1) low-complexity and low-cost receivers are strongly demanded for any WSN, (2) security is vulnerable in WSNs and enhancement at physical layer is extremely desirable, and (3) deploying WSNs in harsh and scattering radio frequency (RF) environments is quite meaningful and necessary. A few realistic radio channels are studied based on experimental data, aiming at best designing the preprocessing systems. In addition, design and implementation work of an experimental time reversal radio test-bed is reported, verifying the concept of preprocessing plus simple receivers.