Technical view

There are a number of factors to consider when designing mobile communication coverage in an In-building environment.

Due to the construction of some buildings (i.e. reinforced concrete, small windows) penetration from external base stations can be poor especially on lower levels (i.e. Ground to 4th Level) and almost all cases coverage below ground level will be non existent. Provision of an In-building scheme will provide dedicated coverage to the subscribers within the building.

In areas where there is a large external public presence external macro site can suffer from capacity issues. This is referred to as congestion or blocking and results in network busy messages on mobiles trying to set up calls. If a new building occupation has hundreds of additional subscribers then surrounding macros will experience congestion at peak periods. Limitations in frequency allocation and or building capability may prevent enlargement of existing macro sites to increase capacity. An in-building solution will alleviate this type of issue.

In high rise buildings once you get above the Clutter level the floors, which can start as low as the 5th floor, start to receive signals from multiple macro sites and due to these sites having clear Radio Line Of Sight (LOS) into the building, what can occur is areas in the upper floors can have similar or the same levels of RF signal from multiple external sites. Under normal circumstances the mobile device will register on the strongest macro site offering the best quality, however, if it can see multiple macros of equal strength and because these signals will vary with time the device will constantly try and change the site it is registered on to. This results in inability to make calls, despite full strength signal bars on the device, and also dropped calls due to registering on sites that are not defined as neighbours of closer marco cells. This effect is known as Non-dominance and an In-building scheme will provide a dominant cell for the phones to register on to, overcoming the external sites.

Due to the limited frequency spectrum that is available to mobile communication operators the usage of these frequencies can lead to macro sites, which normally service a smaller area, being allocated the same frequencies. At lower levels the building clutter will separate these sites sufficiently to ensure that these do not overlap and thus cause any interference causing the QoS to drop. However, once again in high rise buildings where the site can see multiple macro sites the possibility of interference from multiple sites on the same frequencies can cause interference with significant affect on QoS of calls. As In-building schemes are designed to have limited leakage outdoors these frequencies can be reused to overcome this type of issue without causing further interference problems.

These are the four primary factors when considering whether to deploy In-building schemes. These picocells are expensive to install and maintain and so operators deploy them generally where the client is a significant income. It now becoming increasingly common that the operators, especially in the non corporate areas, are looking to share infrastructure costs to reduce their capital expenditure.