Ultra-dense networks will arise not only from an expanded user base and shorter links, but also from an enriched topology, including different technologies and operating in different spectral bands. Their purpose is to provide more capacity through offloading where needed, such as in large sporting events (on an as-needed basis, since there are of relatively short duration), in public places with a lot of users such as airports, university campuses, or malls, or indoors, where the absorption loss due to walls can considerably reduce link margin and, therefore, throughput. With the advent of ultra-dense networks combined with the diminished role of the legacy macrocell, handsets in close vicinity of each other can communicate directly through device-to-device links. The many advantages extend from reduced power emission and interference, to group broadcast and mesh networking, to enhanced security provided from the absence of intermediate network routing. However, ultra-dense networks pose their own set of challenges that must be addressed in order for them to help achieve the 1000x increase in capacity required for 5G networks. The challenges which fall within the scope of this project are described below.
Propagation and system modeling: New propagation models are needed for ultra-dense networks. Driven by legacy cellular systems, most models currently available are tower-to-ground with base stations typically at 30 m and above. Device-to-device links, however, will be ground-to-ground: picocells on lightpoles and indoor femtocells placed on tabletops or mounted on walls or ceilings will be < 10 m above ground. The problem is that few ground-to-ground channel models to date exist in the literature. In addition, most channel models assume independent fading between links. In ultra-dense networks in particular, this assumption does not hold because there will be significant correlation between links, necessitating the study of joint links. NIST also has expertise in this area, but much more work needs to be done for sufficient characterization. In addition, the introduction of carrier aggregation to provide additional bandwidth will require developing ground-to-ground and indoor channel models for new frequency bands. In addition to channel propagation models, detailed system models are needed to accurately characterize and develop signal processing algorithms at the physical (PHY) layer, as well as Medium Access Control (MAC) layer and routing protocols. A big issue that needs to be dealt with is interference mitigation:
Mobility management and self-organization: Mobility management for hyper-dense networks is an important problem that needs to be addressed. A typical mobile device will be within coverage of multiple small cells. Thus, determining which standard and spectrum to utilize and which base station to associate becomes a truly complex task. While high-mobility nodes can be safely restricted to macrocells, leaving the low-mobility cells to exclusively use the small cells, there is still an issue involving rapid handovers and ping-pongs for UEs moving through very small coverage areas. Possible solutions to be examined include:
Also, small cells in ultra-dense deployments that are not serving UEs may go into sleep mode to conserve power, resulting in a network populated by base stations that turn on and off in response to UE migration. Additionally, future networks may include small cells that are deployed by customers without direct setup by the service provider. This means that base stations in ultra-dense deployments must be able to autonomously perform self-organizing functions such as discovery, synchronization, authentication, and power control, and selection of appropriate common carriers in networks that use carrier aggregation.