3.2 Indoor Coverage from the Macro Layer
Why not just use the macro coverage to provide the needed indoor coverage? When designing a cellular network, especially in the first phase of rollout, many radio designers initially try to cover as many buildings from the macro layer as possible. This is despite knowing that most of the traffic originates from inside buildings.
To some extent, and in certain areas, this strategy makes sense. In many cases you are able to provide reasonably good overall indoor coverage from the macro base stations, but it is a fine balance and a compromise.
In a typical suburban environment you need to rely on a very tight macro grid with an inter-site distance of no more than 1–2 km, depending on the services that are offered. In urban environments the inter-site distance can be down to 300–500 m to provide the deep indoor penetration needed for GSM and UMTS. In many cases even this tight a site grid is not sufficient to provide the higher-data-rate EDGE coverage on GSM, and is not sufficient for providing higher data rates on UMTS (64–384 kps). On UMTS especially, HSPA will be a major concern, when covering from the outside network into the buildings.
As you can see from traffic data from many real-life examples like the one shown in Figure 3.1, even a tight macro grid will in many cases be insufficient to service the indoor users, and certainly not the data users on the higher data rates in particular, but also even voice users on GSM and 3G.