In this paper, we compare the performance between two multi-user processing methods in vibrational radar backscatter communications (VRBC). VRBC is a low-latency, scalable communications solution that uses millimeter-wave radars as vibrometers while also allowing them to perform sensing as usual. It allows for vibrational modulation on nearby surfaces to be decoded from the phase of its radar backscatter without requiring RF bandwidth sharing or changes to the radar waveform. One advantage of VRBC is that a single receiver can simultaneously decode messages from multiple users provided they are spatially separable from the radar’s perspective. This paper discusses both range-Doppler and direct de-chirped signal processing for multi-user VRBC. We present simulation results that show better performance can be achieved using direct dechirped signal processing. Additionally, we demonstrate that this method continues to allow for transponder separation in range just as the range-Doppler based approach traditionally does.

In various practical systems, certain data patterns are prone to errors if written or transmitted. In magnetic recording and communication over transmission lines, data patterns causing consecutive transitions that are not sufficiently separated are prone to errors. In Flash memory with two levels per cell, data patterns causing high–low–high charge levels on adjacent cells are prone to errors. Constrained codes are used to eliminate error-prone patterns, and they can also achieve other goals. Recently, we introduced efficient binary symmetric lexicographically-ordered constrained (LOCO) codes and asymmetric LOCO (A-LOCO) codes to increase density in magnetic recording systems and lifetime in Flash systems by eliminating the relevant detrimental patterns. Due to their application, LOCO and A-LOCO codes are associated with level-based signaling. Studying the power spectrum of a random signal with certain properties is principal for any storage or transmission system. It reveals important properties such as the average signal power at DC, the bandwidth of the signal, and whether there are discrete power components at certain frequencies. In this paper, we first modify a framework from the literature in order to introduce a method to derive the power spectrum of a sequence of constrained data associated with level-based signaling. We apply our method to infinitely long sequences satisfying symmetric and asymmetric constraints. Next, we show how to generalize the method such that it works for a stream of finite-length codewords as well, thus demonstrating how to overcome the associated finite-length challenges. We use the generalized method to devise closed forms for the spectra of finite-length LOCO and A-LOCO codes from their transition diagrams. Our LOCO and A-LOCO spectral derivations can be performed for any code length and can be extended to other constrained codes. We plot these power spectra, and discuss various important spectral properties for both LOCO and A-LOCO codes. We also briefly discuss an alternative method for deriving the power spectrum and introduce an idea towards reaching the spectra of self-clocked codes.