Regulators have rarely allocated spectrum specifically for indoor usage, though they sometimes set different rules, such as power limits, for that environment. In the 2G era, when most networks were in low band spectrum, indoor coverage was not a major issue, but since then, it has steadily become a major barrier to full adoption of mobile services, especially in the enterprise, and is starting to feature in policy-making. Macro networks in higher bands such as 2.1 GHz find it hard to penetrate indoors; but for MNOs, there is often a poor business case for building specific networks inside, unless the enterprise or building owner will share the cost. But good indoor signals are essential to many of the industrial use…