GNSS over Fiber – Solving the Synchronisation Challenge for LTE
High data throughput over LTE requires more stringent timing and synchronisation than any previous cellular standards. RF over fiber allows timing signals to be distributed to base stations from GNSS antennas placed in optimal locations. A single GNSS signal (e.g. GPS) can be distributed to multiple base stations even over long distances or a single link combined with PTP over a local Ethernet network.
High data rates means accurate synchronisation
Unlike previous cellular standards, LTE-TDD and LTE-A require very stringent timing and synchronisation for small cells. IEEE1588 PTP with multiple switching via unknown routes leads to phase errors at the edge of the network. Moreover, synchronisation is also a challenge in LTE-FDD for applications such as video streaming where data is aggregated from multiple base stations to achieve a high throughput. The challenge is most acute where cell density is high such as cities. The simplest solution is to provide a precise timing source to each cell by co-locating a GNSS antenna and receiver e.g. GPS, GLONASS or Galileo. However, the cost of such integration is likely to be prohibitive – particularly for small cells. More fundamentally, micro and pico base stations are typically installed in locations which do not have a clear view of the sky, such as inside office buildings, shopping malls, and train stations. Furthermore, “urban canyons” and tunnels are common characteristics of a city environment that hinder reception of satellite signals.
Delivering GNSS signals via fiber
RF over fiber links allow a base station to be separated from the GNSS antenna. The propagation delay in fiber is approximately 5ns/m, meaning an RF over fiber link of up to 300m could be used and still meet a phase requirement of 1.5us. Furthermore, this is a consistent, known delay that can be calibrated out. Therefore, the real constraint to link length is the delay that can be incorporated by the network. The losses of fiber are so low – typically 0.6dB/km – that even a standard link can cover 10km and a long distance link up to 100km. Many metropolitan areas have existing optical fiber networks that can be leased to cover these distances. Using DWDM can minimize leasing costs by squeezing up to 80 links down a single fiber.
Solution 1 – RF over fiber links to each base station
RF over fiber links mean a single antenna can be mounted in an optimal location, e.g. on top of a building where there is a clear view of the sky, and fiber links used to distribute the GNSS signal to base stations within the building. Long link distances mean distribution of GNSS signals into tunnels and urban canyons is also not a problem.
Solution 2 – RF over fiber plus locally distributed PTP
An antenna remoting approach can be combined with locally-distributed PTP (IEEE-1588). Rather than distributing the GNSS timing signal to every cell in a building with individual fiber links, a single link can deliver the timing reference to the building, and an Ethernet network distributes the signal throughout the building over PTP via an Edge Grandmaster.
Solution 3 – Long distance links to many base stations
It is also possible to combine longer distance RF over fiber links, which use high power laser transmitters with optical splitters to be distribute a GNSS signal to hundreds of locations, reducing equipment and roof top rental costs for the antennas.
RF over fiber is a proven technology
“RF over fiber links are the default choice for antenna remoting because copper is not a good conductor of RF due to the skin effect”, says Neil Seager, President of ViaLite Communications, which has been manufacturing RF over fiber links for over 20 years. “Long link distances mean much more flexibility with antenna location as well as the ability to split the signal and redistribute it within a public fiber network, quite often owned by the cellular operator. This is a proven technology – ViaLiteHD links have already been used successfully in LTE-TDD trials in Asia. We have systems operating all over the world in a variety of applications including mission-critical installations such as satellite uplinks for live TV broadcast.”
“ViaLiteHD links have already been successful in LTE-TDD trials in Asia.” Neil Seager, ViaLite