The rapid increase of the number of mobile subscribers as well as the deployment of 3G technologies are putting strain on mobile backhaul operational expenditures (OPEX) which amount to 20-40% of total mobile operator's OPEX due to their reliance on T1/E1 copper lines. For these reasons, the current backhaul systems, a term commonly used to describe connectivity between base stations and radio controllers, are increasingly integrating more cost-effective, packet switched technologies, especially Ethernet/Internet technologies. In addition, Wi-Fi and WiMAX are emerging as promising backhaul solutions and initial findings have demonstrated their feasibility. However, the notion of network migration unavoidably raises new technical challenges relevant to aspects of TDM and packet network timing synchronization, QoS, and packet efficiency. This survey aims to provide a comprehensive study of state-of-the-art circuit switched and emerging packet switched backhaul technologies based on research articles and standard documents. For packet switched backhaul, we focus on the practically important Pseudowire approaches which are used to transport TDM services over packet switched networks. We also discuss the features and research findings on the use of Wi-Fi and WiMAX technologies which illustrate their potential for rapid and cost-efficient backhaul deployment. Finally, we highlight some open issues relevant to timing synchronization in wireless mesh backhaul and femtocells deployments, which offer a rich ground for further research.

The Evolution of Cellular Backhaul Technologies: Current Issues and Future Trends

A device for optically scanning and measuring an environment is provided. The device includes a movable scanner having at least one first projector for producing at least one uncoded first pattern on an object in the environment. The scanner includes at least one camera for recording images of the object provided with the pattern and a controller coupled to the first projector and the camera. The device further includes at least one second projector which projects a stationary uncoded second pattern on the object while the scanner is moved. Wherein the controller has a processor configured to determine a set of three-dimensional coordinates of points on a surface of the object from a set of images acquired by the camera based at least in part on the first pattern. The controller is further configured to register the set of images relative based in part on the stationary second pattern.

Device for optically scanning and measuring an environment

A portable articulated arm coordinate measuring machine for measuring the coordinates of an object in space is provided. The AACMM includes a base and an arm portion having an opposed first and second ends. The arm portion including a plurality of connected arm segments that each includes at least one position transducer for producing a position signal. An electronic circuit is provided that receives the position signal from the at least one position transducer and provides data corresponding to a position of the measurement device. A noncontact three-dimensional measuring device is coupled to the first end, the device having an electromagnetic radiation transmitter and is configured to determine a distance to an object based at least in part on the speed of light in air. A processor is configured to determine the three-dimensional coordinates of a point on the object in response to receiving the position signals and the distance to the object.

Coordinate measurement machines with removable accessories

Apparatus includes at least two devices that communicate with each other, wherein a first one of the at least two devices having an IEEE 1588 precision time protocol interface, the interface including one or more components configured for communications in both a wired manner and a wireless manner with a second one of the at least two devices. The second one of the at least two devices having an IEEE 1588 precision time protocol interface, the interface including one or more components configured for communications in both a wired manner and a wireless manner with the first one of the at least two devices. Wherein one of the at least two devices includes a master clock and the other one of the at least two devices includes a slave clock, wherein the master clock communicates a time to the slave clock and the slave clock is responsive to the communicated time from the master clock to adjust a time of the slave clock if necessary to substantially correspond to the time of the master clock, thereby time synchronizing the at least two devices together.

Method and apparatus for synchronizing measurements taken by multiple metrology devices

Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment toward real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.

Clock Synchronization Over IEEE 802.11—A Survey of Methodologies and Protocols

IEEE 1588 is a new standard to synchronize independent clocks running on separate nodes of a distributed measurement and control system. It is intended for high-accuracy implementations on compact systems such as a single subnet. This paper examines potential accuracy limitations introduced by the physical layer of the IEEE 802.11b wireless local area network. Experimental results are presented that show that these limitations do not preclude clock-synchronization accuracy of several hundred nanoseconds.

J.C. Eidson

Papers

An Implementation of IEEE 1588 Over IEEE 802.11b for Synchronization of Wireless Local Area Network Nodes