“If timing ain’t everything, it’s damn close.” – Tom Peters, author of In Search of Excellence
Although Peters wasn’t referring specifically to wireless communications technology when he coined this quote, he certainly would have been on target given the role of precision timing as an essential ingredient for keeping wireless networks functioning without chaos. Since satellite navigation systems such as GPS provide reliable signals outdoors over wide geographies, they’ve long been used to provide the accurate time reference required by Time Division Multiplex (TDM) wireless access methods, most recently LTE and LTE-Advanced. However, GPS does have its limitations when applied to network timing, chief among them being its lack of indoor signal coverage.
This shortcoming is becoming critical as small cell base stations are deployed in increasing numbers to augment the coverage and capacity of outdoor macro cells. Although timing solutions are available to supplement GPS indoors, they require custom venue installation approaches, making them too expensive to be widely deployed and raising the total cost of ownership for small cells to levels that prevent their long-promised mass market presence. A different approach developed by NextNav can eliminate these limitations with minimal equipment cost impact to wireless carriers by enabling them to leverage their existing small cell supply chain while providing extremely high timing precision and carrier-grade system reliability.
“Timing as a Service (TaaS)” is delivered over NextNav’s Metropolitan Beacon System (“MBS”) network, a wide-area location and timing network developed to provide GPS-like services where satellite-based signals are not available. The network has several advantages including high signal strength based on its terrestrial proximity to the small cells and its use of building-penetrating 900 MHz spectrum licensed for provision of location-related services. Current NextNav licenses cover 98% of the urbanized US population.
Challenge and Solution
Wireless carriers have long benefited from GPS, and it is widely recognized that the timing signals that are inherent in GPS technology were necessary as the industry advanced from its analog beginnings to the current proliferation of CDMA and OFDM digital modulation schemes. That said; while GPS is unquestionably invaluable, it was never intended to provide coverage indoors and doesn’t perform well in urban canyons and other places where satellite signals are blocked. The currently available primary solutions for remedying the indoor timing situation are “grandmaster” clocks or dedicated time servers that rely on a remote antenna mounted where GPS signals are accessible. They route the signals throughout a specific area using Precision Time Protocol (PTP) over an Ethernet network or other wired methods. The problem with this scenario is that it is inherently local and not scalable. That is, the systems must be deployed on a single venue basis, and since a typical installation costs between $15,000 and $150,000 per building, deploying these solutions in large numbers is an expensive proposition. Along with the cost impact of the venue-based systems, there is an increasing reduction in the quality of timing accuracy delivered to the small cells as the scale of the network is increased. This limitation often necessitates the deployment, installation and maintenance of multiple grandmasters in a building, with associated costs. In addition, timing systems that rely purely on GPS rather than incorporating more expensive rubidium “atomic” clocks are subject to GPS vulnerabilities such as spoofing, jamming and solar events, which are known to cause timing outages today. While the dedicated venue approach works well in some situations, it’s not well suited for serving hundreds or thousands of small cells distributed throughout a metropolitan area. However, it is precisely this situation that is developing today and which will accelerate as the industry moves to the fifth generation cellular technology that showcases high capacity indoor small cells using high frequency spectrum.
In contrast to other approaches, the NextNav network transmits timing signals from a network of broadcast beacons (similar to GPS satellites on the ground)located throughout a large metropolitan area with sufficient signal strength to penetrate buildings and other “RF challenged” environments. Consequently, there is no need for timing infrastructure beyond the MBS receiver located at or integrated within the small cell, and the timing signals from the network are infinitely scalable within a coverage area given their broadcast nature.
Another issue with GPS is that, while its timing accuracy is very good in the open (less than 100 ns at its source), GPS reliability, availability, and accuracy will be impaired when the signal path from the satellite to the receiver is blocked or when the signals get distorted by strong reflections for example from adjacent buildings. This becomes a serious concern with the installation of small cells with increasingly more stringent timing performance requirements in urban locations. LTE-Advanced, for example, requires an absolute timing error of less than 0.5 μs (0.000005 seconds). NaxtNav’s MBS provides both timing and frequency accuracy that meets or exceeds the most stringent current LTE and LTE-A small cell requirements as well as those expected for future 5G basestations. The MBS network is inherently resilient as each of the system’s transmitters incorporate atomic clocks that provide backup timing capability throughout their coverage areas even if there is a GPS outage that might occur across a large area. Each small cell located within the MBS network receives signals from multiple beacons, ensuring a level of redundancy appropriate for critical timing.
Timing as a Service Summary
The wireless industry is undergoing unprecedented change that, if all goes even fairly close to plan, will dwarf the capabilities it can provide today and the applications it can serve. One of the most significant changes is the use of radio frequencies much higher than those employed today, and as signals travel much shorter distances at these frequencies, a large number of small base stations will be needed to complement macro and micro cells. A significant percentage of these base stations will be located indoors where essential timing synchronization signals provided by GPS cannot be received.
Current methods of making GPS timing signals, predominantly grandmaster clocks and dedicated time servers, are inherently local solutions and are not financially viable for covering larger metropolitan areas. NextNav’s Timing as a Service (TaaS) capabilities delivered by its MBS network can provide wide-area coverage at minimal cost per each small cell served. It requires none of the expensive infrastructure needed by wired GPS-complement solutions, has availability and reliability greater than that of urban GPS installations, has accuracies meeting the requirements of 5G, is backed up by atomic clock references, and provides carrier-grade reliability.
Positioning via the MBS network
NextNav developed the revolutionary Metropolitan Beacon System (MBS) positioning and timing technology to allow mobile phones and other devices to reliably determine their location and timing in indoor and urban environments where GPS signals can’t be received. NextNav’s three-dimensional location system provides accurate horizontal positioning with floor-level height precision. Delivered over a managed network on licensed spectrum, with carrier-grade dependability and metropolitan-wide coverage, NextNav’s positioning and timing services are designed for E911, public safety, critical infrastructure and emergency response as well as the multitude of consumer and commercial applications that require precise indoor timing and 3D location accuracy.
