Manish Gupta, Symmetricom’s vice president of marketing and business development, answered questions posed by Directions Magazine’s editor in chief, Joe Francica, about potential GPS signal interference from LightSquared’s broadband initiative.
Directions Magazine (DM): Can you explain from your perspective some of the fact and hyperbole surrounding LightSquared’s planned deployment?
Manish Gupta (MG): The FCC approved use of satellite L-band spectrum (1525-1559 MHz and 1626.5-1660.5 MHz) for terrestrial mobile services for LightSquared. However, due to the proximity of the L-band spectrum to the GPS band (1560-1610 MHz), questions have been raised regarding potential interference to existing GPS antenna installations. In the U.S., most base stations have GPS receivers to ensure accurate frequency and time synchronization, which is required for voice switching, video streaming and any LBS (location- based services, with E911 being the most important).
DM: Please explain Symmetricom’s value proposition of its timing and synchronization products in non-GPS/GNSS environments.
MG: As networks transition from time-division multiplexing (TDM) to Ethernet systems, the synchronization chain in the network is broken when the underlying TDM layer is removed (T1/E1). For 3G and more importantly, 4G networks, service providers need to design in requirements for frequency and time (phase) synchronization to their networks. Every network needs precise timing and synchronization to function effectively, and derives time from one of the following primary reference sources: cesium, GPS, packet-based primary reference clock (PRS). The TimeProvider 1500 was developed to support packet-based systems (3G / 4G networks) and provide an alternative to GPS-based systems that have known vulnerabilities to jamming and spoofing. LTE architecture requirements for small cell deployments also make it cost prohibitive to deploy GPS receivers at every cell site.
DM: Given the situation regarding LightSquared and possible interference issues with the existing constellation of GPS satellites, please explain Symmetricom’s position in offering alternative solutions as suggested below:
1. Locating the GPS antenna so that it is not in the direct beam of the LightSquared L-band transmitter
MG: Symmetricom offers a number of options to facilitate antenna placement in a location not in the direct beam of the LightSquared transmitter: roof top antenna, wall-mount antenna or window antenna. However this is not a permanent solution as interference from other sources such as jamming and spoofing still remain.
2. Replacing the GPS antenna with a model incorporating enhanced narrow band filtering
MG: GPS antenna manufacturers are offering special L1 antennas with enhanced filtering to mitigate potential LightSquared L-Band interference. It must be noted that the filtering option may reduce, but not necessarily eliminate, potential interference with GPS signals as a result of close proximity to a LightSquared L-Band antenna.
3. Installing packet primary reference source synchronization solutions
MG: Symmetricom recommends the use of packet-based PRS synchronization solutions such as the TimeProvider 1500, which utilizes IEEE 1588 PTP to deliver a stratum-1 frequency and timing source without requiring a GPS antenna. TimeProvider 1500 is ideal for situations where antenna relocation or filtering is not sufficient to eliminate the threat of GPS signal interference.
DM: From your perspective, what do you think the outcome of the FCC’s ruling might be?
MG: The FCC’s deadline is June 15th for its working group to report test results and make recommendations for potential interference to GPS. While Symmetricom supports all three migration options, the company highly recommends the use of packet-based primary reference source synchronization solutions such as the TimeProvider 1500. Packet PRS solutions not only mitigate but totally eliminate potential GPS interference from LightSquared antenna sources since there is no GPS required at packet PRS locations. At the end of the day this is not just a LightSquared issue, but an industry issue associated with known vulnerabilities to GPS interference with jamming and spoofing and with network topology changing as the result of the transition to 4G/LTE.