COMPASS time synchronization and dissemination—Toward centimetre positioning accuracy

In this paper we investigate methods to achieve highly accurate time synchronization among the satellites of the COMPASS global navigation satellite system(GNSS).Owing to the special design of COMPASS which implements several geo-stationary satellites(GEO),time synchronization can be highly accurate via microwave links between ground stations to the GEO satellites.Serving as space-borne relay stations,the GEO satellites can further disseminate time and frequency signals to other satellites such as the inclined geo-synchronous(IGSO)and mid-earth orbit(MEO)satellites within the system.It is shown that,because of the accuracy in clock synchronization,the theoretical accuracy of COMPASS positioning and navigation will surpass that of the GPS.In addition,the COMPASS system can function with its entire positioning,navigation,and time-dissemination services even without the ground link,thus making it much more robust and secure.We further show that time dissemination using the COMPASS-GEO satellites to earth-fixed stations can achieve very high accuracy,to reach 100 ps in time dissemination and 3 cm in positioning accuracy,respectively.In this paper,we also analyze two feasible synchronization plans.All special and general relativistic effects related to COMPASS clocks frequency and time shifts are given.We conclude that COMPASS can reach centimeter-level positioning accuracy and discuss potential applications.

Efficient Peer-to-peer File Sharing in 3G Networks

In 3G networks upgraded with high speed packet access(HSPA) technology,the high access bandwidth and advanced mobile devices make it applicable to share large files among mobile users by peer-to-peer applications.To receive files as quickly as possible is essential for mobile users in file sharing applications,since they are subject to unstable signal strength and battery failures.While many researches present peer-to-peer file sharing architectures in mobile environments,few works focus on decreasing the time spent in disseminating files among users.In this paper,we present an efficient peer-to-peer file sharing design for HSPA networks called efficient file sharing(EFS) for 3G networks.EFS can decrease the dissemination time by efficiently utilizing the upstream-bandwidth of mobile nodes.It uses an adaptive rearrangement of a node's concurrent uploading transfers,which causes the count of the node's concurrent uploading transfers to lower while ensuring that the node's upstream-bandwidth can be efficiently utilized.Our simulations show that,EFS achieves much less dissemination time than other protocols including Bullet Prime and a direct implementation of BitTorrent for mobile environments.