Clock Reference

Several clock reference and synchronization systems are available for various devices within the ORBIT testbeds.

GPS & PPS/10MHz Clock Reference Distribution System

A rooftop-mounted GPS antenna provides the necessary reference signal to the WiMAX and LTE basestations within the testbed domains (namely SB4 and Outdoor). This signal also drives the Octoclock based PPS/10MHz distribution system used within the Grid domain. An Octoclock-G unit containing a GPS disciplined oscillator (GPSDO) generates the PPS and 10MHz reference signals which are distributed via equal length cables (75ft + 7.5ft) to 8 other Octoclock units which do not have GPSDOs. Four (4) of these Octoclock units are installed in the corners of the Grid testbed to split the reference signals using equal length cables (10ft) among the node connected SDRs/USRPs (see Table 1). The other four (4) Octoclocks are installed in the massive-MIMO mini racks and split the reference signals using equal length cables (10ft) among 8 USRPs in each rack. The use of equal length cables and a symmetric topology ensures that all connected devices will see the same reference signals with very little deviation in phase and time.

Figure 1: GPS & PPS/10MHz Distribution System

Location	Node
"1-1 Corner"	1-1, 1-2, 2-1, 2-2, 3-1, 3-2
"1-20 Corner"	1-19, 1-20, 2-19, 2-20, 3-19, 3-20, 4-19, 4-20
"20-1 Corner"	17-1, 17-2, 18-1, 18-2, 19-1,19-2, 20-1, 20-2
"20-20 Corner"	18-19, 18-20, 19-19, 19-20, 20-19, 20-20
Massive-MIMO Mini-Rack	23-1, 23-2, 23-3, 23-4, 23-5, 23-6, 23-7, 23-8
Table 1: Grid domain synchronized USRPs/SDRs

PTP and NTP Clock Synchronization

Any network connected device (node, server, etc.) within the testbed domains that supports PTP or NTP can be synchronized with their respective domain console. The consoles are synchronized to ORBIT's DHCP servers which currently synchronize with the Rutgers Time Server. In the future, dedicated GPSDO hardware will be installed to provide a stratum 0 reference which will further improve the clock synchronization of higher strata devices.