Time-of-Flight (ToF) cameras acquire 3-D images where each pixel relates to the distance in the observed scene. These images are acquired by emitting modulated light and measuring the phase of its reflection from an object. Improvements in ToF camera technology make it a promising alternative to current 3-D imaging sensors despite possessing comparatively lower resolution. Overcoming lower resolution, of single ToF camera images, is achieved using a multi-camera setup. However, the simultaneous operation of multiple ToF cameras causes interference resulting in erroneous depth measurements. An option to mitigate interference is the arrangement of multiple cameras in a consecutive order sharing the same frequency in assigned time slots, such that each camera illuminates the scene exclusively, a procedure known as time division multiple access (TDMA). The assignment of acquisition times is usually performed by a master synchronizing device implementing a time synchronization protocol that requires an additional communication channel. This paper presents a different approach requiring no additional infrastructure to utilize TDMA by adding a synchronization software procedure to the camera's microprocessor which effectively enables a camera to sense the acquisition process of other cameras and rapidly synchronize itself autonomously to operate without interference. Experimental results with two ToF cameras provide substantive evidence of the efficacy of this approach. The operation of the standalone cameras achieved the desired synchronization without illumination interference. The proposed synchronization procedure takes advantages of the zero interference of TDMA and overcomes traditional hard-wired synchronization setups, offering an autonomous and potentially mobile operation of ToF camera deployment.

The last decade bore witness to increased development of time-of-flight (ToF) camera hardware, algorithms, and applications, particularly 3-D measurement in the fields of human-robot collaboration. Their advantage is in faster digital 3-D image target representation from efficient signal computations. ToF cameras emit modulated light, measuring the round-trip time from its illumination source to its sensor, deriving a distance image. A disadvantage of using multiple ToF cameras is light interference with other cameras' 3-D measurements. Commonly, adopting an access protocol called frequency division multiple access (FDMA) minimizes interference-related errors but does not eliminate them. The quantification of this distance error has remained a challenging research problem. This paper presents an interference model for two time-of-flight cameras, quantitative analysis, and mathematical modeling to determine the interference-related distance error when using FDMA.

Time-of-flight (ToF) cameras are an evolving technology, finding applications in a wide range of fields, including machine vision, automotive applications, face or hand recognition, and human-robot collaboration. A setup consisting of multiple ToF cameras can provide a 3-D model of the recorded scene with a larger field of view and avoid occlusion. The operating principle of ToF cameras' requires the emission of light to measure its round-trip time to the objects in the field of view and the reflection back to the camera. If multiple ToF cameras operate simultaneously, their light waves may interfere, resulting in erroneous depth measurements. In this paper, we propose a multi-camera time-of-flight system based on dynamic time-division multiple access (dynamic TDMA) to avoid any interference. The time synchronization is implemented optically, so that no further time synchronization, external to the cameras, is required. To achieve a high frame rate, every camera is extended with a photodiode, signaling a free channel. A synchronization protocol is implemented to allow for an autonomous operation of the ToF cameras.

High-precision sensor networks and localization systems require precise time and frequency synchronization. In this paper, we present a novel high-precision frequency synchronization approach for wireless network devices. It adapts the local oscillator frequency of a receiver to the frequency of a transmitter and can be integrated into existing wireless communication systems. The measurement of frequency differences as well as the frequency adjustment is realized in Field Programmable Gate Arrays (FPGAs). Using a 60 GHz wireless experimental setup, the receiver clock is aligned to the transmitter clock with a precision of 37 picoseconds.

Time-of-Flight (ToF) cameras are a promising alternative to current 3-D imaging systems. A setup of multiple ToF cameras is used to overcome their comparatively lower resolution, increase the field of view, and to reduce occlusion. However, the simultaneous operation introduces the possibility of interference resulting in erroneous depth measurements. One option to mitigate interference is the consecutive arrangement of multiple cameras in time slots, such that each camera illuminates the scene exclusively, a procedure known as time division multiple access (TDMA). Instead of requiring a master synchronizing device, an approach is presented requiring no additional hardware or infrastructure to utilize TDMA by adding a synchronization software procedure to the camera's firmware. It effectively enables a camera to sense the acquisition process of other cameras and rapidly synchronize its acquisition times to operate without interference. During the experimental verification with three ToF cameras no interference occurred. The cameras each acquired 29 depth images per second while synchronizing themselves every 10 s. The proposed synchronization procedure implements an interference-free TDMA operation and overcomes traditional hard-wired synchronization setups, offering an easy setup and a methodology to upgrade existing systems without hardware modifications.