# An Optical Interference Suppression Scheme for TCSPC Flash LiDAR Imagers
## 3 Related Work  
Several contributions have been published on topics that are closely related to LiDAR optical interference suppression. It is challenging, however, to pinpoint one main line of research on the topic, due to the variety of technologies and techniques available for 3D imaging and range finding. Signal-to-noise ratio (SNR) improvement is often seen as a necessary step towards proper rejection of extraneous interference. Matched filter approaches [16] and customized digital post-processors [17] have been proposed to achieve high SNR of the LiDAR signal in avalanche photo-diode (APD)-based sensors. One special case of LiDAR interference that has been studied extensively is the multi-path interference (MPI) [18; 19; 20; 21], which is a LiDAR system interfering with itself through a plurality of  
Figure 2: Illustrative example of a TCSPC histogram. The position of the peak correlates to the distance of the imaged object. The level of the background gives a measure of the intensity of ambient light and of internal noise in the detector. optical paths, either as a consequence of the complex geometry of the imaged scene or the presence of semi-transparent or specular objects. Published approaches to MPI suppression include the use of multiple modulation frequencies in indirect time-of-flight (i-ToF) [22], sometimes in conjunction with compressive sensing to reconstruct multi-path reflections [23]. Proper multi-camera interference occurs when multiple LiDAR sensors operate in the same environment. Several techniques borrowed from telecommunications technology and signal processing theory are known to minimize the interference between multiple LiDAR cameras sharing the same communication channel: space-division multiple access (SDMA), frequency-division multiple access (FDMA), wavelength-division multiple access (WDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). The SDMA method requires spatial separation of the LiDAR cameras so that their fields of view do not overlap. Each camera monitors a well-defined portion of space, and its emission never overlaps with the field of view of any other camera in the same environment. SDMA is scalable to an arbitrary number of cameras but is unsuitable for use in uncontrolled environments without considerable coordination between all the cameras. In FDMA, each LiDAR camera must be assigned a unique operating frequency, which is represented by the pulse repetition frequency (PRF) in d-ToF. When operating at different PRFs, two LiDAR systems will not interfere with one another and will appear to each other as uncorrelated noise. The LiDAR PRFs must be separated enough to guarantee the minimization of systematic errors at short integration times, thereby limiting the maximum number of parallel channels. It should be noted that LiDAR imagers based on FDMA require coordination in uncontrolled environments in order to reassign the PRFs in the case of collision. A popular option for optical devices such as LiDAR imagers is WDMA. WDMA mandates the use of multiple, non-overlapping optical channels centered on different wavelengths, separated on the receiver side by means of appropriate narrow-band optical filters. Such systems must handle the relatively low number of available channels in the operating band, which is limited by eye safety limits in the visible range, the bandwidth of the optical filter, the spectral purity of the emitter, and by the spectral sensitivity of the detector. In TDMA, concurrent imagers are assigned unique, non-overlapping time intervals in which they are allowed to emit. TDMA side-steps the issue of interference by guaranteeing that at most one camera is active at any given time. The trade-off is of course the need to centrally coordinate all cameras (although peer-to-peer approaches are also possible). The CDMA technique achieves interference suppression by borrowing alternative modulation schemes from spread-spectrum telecommunication systems.