Cellular-Solar is the name of a technology initiative promoted by Nicolae Carpathia before his rise to power, and ultimately implemented by the Global Community's custodial arrangement of telecommunication systems.
It consists of the mass production of prefabricated cell phone towers that can be powered either from the existing electrical grid or, should it fail, with solar panels and batteries mounted on the tower itself.
Since Cellular-Solar pylons are intended to operate without ground infrastructure, they communicate with each other via satellite, or via mesh networking. Power requirements for the relay pylons are sufficiently low that the satellites in orbit can actually provide some of the energy necessary for them, sent along with the data stream.
The system is later upgraded to carry digital data as well as voice.
In The Regime, James Corona Jr. pitches the technology, which is in an early prototype stage, to Carpathia who then promises to raise 102 million dollars in capital to further develop it.
In Soul Harvest, Cameron "Buck" Williams and Tsion Ben-Judah see CATS workers replace a downed utility pole with a Cellular-Solar tower in minutes, as part of the recovery and rescue efforts immediately after the Wrath Of The Lamb earthquake.
Shortly after, Nicolae Carpathia explains the technology to Rayford Steele, presenting it as part of a coordinated plan to unify communication capabilities under Global Community ownership. Despite the clear benefits of the technology for disaster recovery efforts, and the fact that CellSol is a mesh networking system, Rayford is disgusted by the centralization of control that he thinks that it implies.
In Apollyon, Carpathia cites the system (shortened as CellSol) as one of the chief accomplishments of his administration, along with the swift reconstruction of cities after World War III and the global earthquake.
In Assassins, David Hassid mentions using CellSol sidebands as a secure channel for Tribulation Force members, indicating that CellSol pylons are generally spread too far apart to allow for precise triangulation of a user.
While the technology is not referenced explicitly past this point, the fact that phone and data links continue to operate throughout the Tribulation despite natural and supernatural disasters indicates that CATS workers construct additional pylons right up to the Glorious Appearing.
Since in the Millennial Kingdom the sun shines at all hours, and Cellular-Solar pylons were designed explicitly to survive any disaster, it is reasonable to assume that the technology remains in use in the Millennium, thus marking it as the longest-lasting legacy of the Carpathia administration, at least until the destruction of the old earth at the end of the Millennium.
Real life
In November 2020, the CellSol project was released to the public as a way to maintain basic communication (text messaging) in case of a natural disaster that would disable cell phone infrastructure. The design is open source and may be freely copied and distributed.