Evolving to ever-smaller cells requires eversmaller,
lower-power and cheaper BSs, and there is no fundamental
reason a BS needs to be more expensive than a user
device or a WiFi node [26]. Nevertheless, obtaining permits,
ensuring fast and reliable backhaul connections, and paying
large monthly site rental fees for operator-controlled smallcell
placements have proven a major hindrance to the growth
of picocell, distributed antennas, and other enterprise-quality
small cell deployments. Of these, only the backhaul is primarily
a technical challenge. Regulatory reforms and infrastructure
sharing (cf. Section IV-C) may help address the other
challenges.