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 singlephoton 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 squarewave modulation. Simulations and successive experimental data reveal the proposed improvements compared to trivial squarewave 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.