As public safety organizations deploy band class 14 Long Term Evolution (LTE) networks throughout the country, a major challenge will be providing economical coverage in rural areas. Sixty-eight percent of the landmass in the U.S. is rural, and providing cost-effective coverage in these areas is crucial and a challenge due to budget limitations and implementation trade-offs. A potential solution is to extend the size of each cell beyond the typical radius of 6-8 miles in rural areas - where user density is lower. By using a smaller number of towers with increased cell sizes, deployment costs can be reduced. The Public Safety Communications Research (PSCR) program deployed an extended cell test network to evaluate extended cell coverage and performance. PSCR used eNodeB infrastructure that implemented standardized LTE protocols for extended cell operation - measurements were performed that demonstrated cell coverage can extend out to 62 miles.
The Middle Class Tax Relief and Job Creation Act of 2012 (H.R. 3630) appropriated $7 billion for the establishment of a nationwide interoperable public safety broadband network under FirstNet, an independent authority in the National Telecommunications and Information Administration (NTIA). FirstNet will provide first responders with dedicated and prioritized access to high-speed data, voice, and other mission critical applications. This network will use the LTE cellular communication standards developed by the 3rd Generation Partnership Projects (3GPP). Although LTE communications have existed commercially for years, this nationwide public safety broadband network utilizing band class 14 will be the first of its kind. PSCR built a band class 14 LTE Demonstration Network in Boulder, CO, that provides a central and independent test laboratory to help public safety understand LTE. The Demonstration Network offers an objective forum for public safety to test and verify new capabilities.
To increase the cell radius beyond the typical 6-8 mile range, a typical LTE system will need modifications. PSCR implemented and deployed two modifications to the LTE network to extend LTE coverage up to 62 miles. The first modification deployed the eNodeB and antenna at a higher height to increase the coverage area. Secondly, the LTE Physical Random Access Channel (PRACH) preamble was modified. The LTE 3GPP standards specify four preamble formats to accommodate longer round trip propagation delays in large cells. PRACH preamble format 1 supports a cell radius of up to 48 miles and preamble format 3 allows for cells up to a 62-mile radius. The extended cell testing project consisted of two phases that employed different PRACH implementations, eNodeB antenna deployment heights, antenna configurations, and UE transmit power levels. Phase I implemented PRACH preamble format 1 with an eNodeB antenna height of 280 feet, while Phase II implemented PRACH preamble format 3 with an eNodeB antenna height of 920 feet. A Power Class 3 UE capable of +23 dBm transmit power was used for testing, with a +31 dBm high power UE added for Phase II. Performance and coverage differences were measured to demonstrate the improvements achieved over both phases of the testing.
Public safety’s ability to test and evaluate advanced LTE technologies at PSCR will help integrate new features and capabilities to meet public safety’s unique requirements. Testing new features on a live network allows public safety organizations to evaluate new features in advance of actual deployment. The result of the extended cell project was the implementation of a first-of-its-kind band class 14 extended cell network capable of a 62 mile range that provides public safety with a viable solution for nationwide coverage.