Autonomous LTE Networking
An LTE network today consists of three main components: handsets (i.e. phones) wirelessly connect to towers (called eNodeBs in LTE parlance) which are managed by the carrier's core network, which is called the EPC. Each carrier has one logical EPC, which handles everything from network authentication and billing to routing and forwarding. Note that all communication, both within the carrier's network (known as a PLMN, for Public Land Mobile Network) as well as out to the Internet, is routed through the EPC.
An unfortunate side-effect of this design is that if the tower can't reach the EPC, all service is lost. To solve this problem, the EmergenCell comes packaged and integrated with an onboard EPC (branched from the OpenAirInterface project here). Thus, each EmergenCell can be considered an all-in-one independent LTE network, consisting of one tower and one EPC. This means that multiple EmergenCells in an area will not be able to talk to each other, but it also means that each EmergenCell is resilient, reliable, and fully self-contained.
The LTE attach procedure is bidirectionally authenticated. This means that for a handset to connect to a network, the network must authenticate the handset, and the handset must authenticate the network. This is important for security, but has the unfortunate side effect of requiring users to obtain a new SIM card to get on a new network. However, in an immediate disaster-response scenario, requiring emergency responders to distribute SIMs is an unrealistic expectation.
To resolve this challenge, we have to use a roaming attach procedure. Under the roaming attach procedure, a handset establishes a connection with the visted network's EPC (in this case, the EmergenCell), and then the visited network's EPC contacts the home network to perform network authentication. Once authentication succeeds, all data-plane communication from the handset is terminated at the visited network, and no further communication between the visited and home EPC is necessary.
Roaming attach provides many benefits for our use-case. First, it dramatically simplifies network operation and extends network support: instead of wasting valuable response time passing out SIM cards, or only supporting users who are safe and capable enough to travel and obtain a SIM, roaming enablesthe EmergenCell to automatically support all phones within range. Second, roaming allows the EmergenCell to coexist with existing infrastructure: users who still have carrier service will remain attached to their carrier's tower, leaving the EmergenCell to focus on users without coverage. Third, roaming allows for gradual and seamless restoration of service: as carriers restore coverage after the disaster, users will automatically leave the EmergenCell and return to their home networks, allowing for a natural and gradual wind-down of the emergency network.
Multiple studies have found that during crises, modern users place a high priority on using their phones to communicate with loved ones, inform others that they are safe, and obtain up-to-date information about resources such as food and shelter. We support each of these goals, and more, by way of several custom-made web applications hosted on the EmergenCell. When users' handsets complete the emergency attach procedure, the EmergenCell automatically texts them to navigate to "home.emergencell", where they can obtain access to the following webapps. By packaging these apps directly on the EmergenCell, we ensure that users get full functionality and support even without an active Internet connection.
Spectrum and Range
There currently exist over 40 LTE bands; these bands have varying levels of range, attenuation, licensing models, and handset-support, and are heavily influenced by the existing networks in the target communities. We are still investigating the many tradeoffs in this space, but plan to leverage one of the longer-range bands (such as 3, 5, or 8) that are best suited to wide-area macrocell deployments.
Electrical infrastructure is often damaged, disabled, or overtaxed during emergencies; it logically follows that any equipment for disaster response needs to be as power-efficient as possible. However, this approach does not consider that power access is fundamentally tied to geographic location: even during emergencies, certain locations (e.g. hospitals or responder headquarters) maintain power by various means, such as backup generators. This means that it wasn't enough to simply design a power efficient box, we also had to consider a disparity in access to power and conserve power in remote devices that are unable to recharge, even at the cost of increased power consumption at powered sites.
This realization heavily influenced our decision to build a centralized LTE network managed by the EmergenCell, as opposed to a wireless mesh or other such design. While the EmergenCell comes with a battery backup, we assume that the EmergenCell will be located near a reliable power source, likely a command center or emergency headquarters. This assumption goes hand in hand with our deployment model, and is made possible by the long range of LTE macrocells described above.
Cost-per-unit is an important consideration for any piece of equipment. Fortunately, our use of open source software (specifically the OpenAirInterface EPC stack) combined with a recent increase in cellular hardware manufacturing combine to keep our prices relatively low. The EmergenCell is comprised of a $150 Zotac Zbox, and a BaiCells commercial eNodeB that costs approximately $2200, for an out-the-door total of ̃$2500, including antennas and mounting hardware.