Direct-sequence-CDMA in highly sensitive indirect time-of-flight distance sensor

Abstract

This work presents the application of Direct-Sequence Code-Division-Multiple-Access (DS-CDMA) to tackle the primary problem of simultaneous multi-access of a shared light channel in an indirect Time-of-Flight measurement system based on on-off-keyed (OOK) lasers and highly sensitive single-photon avalanche diodes (SPADs). A theoretical model of the generalized sensor system, possibly suffering from multi-path propagation, is devised for the general application of DS-CDMA. The particular choice of Gold codes is argued to not only enable multi-access but even allow for multi-path resolution down to the sampling time of the utilized digital correlator. It also increases the unambiguously detectable range by three orders of magnitude in comparison to trivial continuous square-wave modulation. Simulations and successive experimental data reveal the proposed improvements compared to trivial square-wave modulation, whilst showing an expected degradation in precision. However, still a precision below 1.5 mm is reached. A novel interleaving procedure of the codes is postulated and verified to recover the precision originally experienced upon trivial modulation.