In-building

Wireless systems have numerous applications for in-building applications.
The applications include improving coverage for a convention center or
large client, disaster recovery, or a wireless PBX to mention a few.With the
advent of better transport for data services, the possibility exists that 3G
will find more uses for in-building systems.

The propagation of the radio-frequency energy, however, takes on unique
characteristics in an in-building application as compared to an outdoor
environment. The primary difference in propagation characteristics for inbuilding
versus outdoors is the fading, shadowing, and interference. The
fading situation for in-building results in deeper and has spatially closer
fades when a system is deployed in an in-building application. Shadowing
is also quite different in an in-building application due to the lower antenna heights and excessive losses through floors, walls, and cubicles. The shadowing
effects in an in-building application severally limit the effective coverage
area to almost line of site (LOS) for mobile communications. The
interference issue with in-building systems can actually benefit in-building
applications because the interference is primarily noise driven and not
interference. The reason the in-building systems are primarily noise-driven
is due to the attenuation experienced by external cell sites as they transverse
into the buildings and various structures.

There are some unique considerations that must be taken into account
regarding micro-cell system design for inside a building. Some of the design
considerations that need to be factored into an in-building design are
■ Base-to-mobile power
■ Mobile-to-base power
■ Link budget
■ Coverage area
■ Antenna system type and placement
■ Frequency planning

The base-to-mobile power needs to be carefully considered to ensure that
the desired coverage is met, deep fades are mitigated in the area of concern,
the amplifier is not being over or potentially under drive, and mobile overload
does not take place. The desired coverage that the in-building system
is to provide might require several transmitters because of the limited output
power available from the units themselves. For example if the desired
coverage area required 1W ERP to provide the desired result, a 10 W amplifier
would not be able to perform the task if you needed to deliver a total of
40 channels to that location, meaning only 25 mW of power per channel was
really available. The power limitation can, and often does, makes the limiting
path in the communication system for an in-building system the forward
link.

The forward link power problem is further complicated by the fact that
portable and potential mobile units will be operating in very close proximity
to the in-building systems antenna. If the forward energy is not properly
set, a subscriber unit could easily go into gain compression causing the
radio to be desensitized.
The mobile-to-base power also needs to be factored into the in-building
design. If the power windows and dynamic power control are not set properly,
then imbalances could exist in the talk out to talk back path. Usually
the reverse link in any in-building system is not the limiting factor but the mobile-to-base path should be set so that there is a balanced path between
the talkout and talk back paths.
Most in-building systems have the ability to utilize diversity receive but
do not utilize it for a variety of reasons. The primary reason for not utilizing
diversity receive in an in-building system is the need to place two distinct
antenna systems in the same area.

The link budget for the communication system needs to be calculated in
advance to ensure that both the forward and reverse links are set properly.
The link-budget analysis plays a very important role in determining where
to place the antenna system, distributed or leaky feeder, and the amount of
micro and pico-cell systems required to meet the coverage requirements.
The antenna system selected for the in-building application is directly
related to the uniformity of the coverage and quality of the system. The
antenna system, no diversity, primarily provides LOS coverage to most of
the areas desired in the defined coverage area. Based on the link budget
requirements, the antenna system can either be passive or active. The
antenna system for an in-building system may take on the role of having
passive and active components indifferent parts of the system to satisfy the
design requirement.

Typically a passive antenna system is made up of a single or distributed
antenna system; it can also utilize a leaky coaxial system. A leaky coaxial
system could also be deployed within the same building to provide coverage
for the elevator in the building. The advantage a leaky coaxial system has
over a distributed antenna is it provides a more uniform coverage to the
same area over a distributed antenna system. However, the leaky coaxial
system does not lend itself for an aesthetic installation in a building. The
use of a distributed antenna system for providing coverage in an in-building
system makes the communication system stealthy.

If the antenna system requires the use of active devices in the communication
path, the level of complexity increases. The complexity increases
for active devices because they require AC or DC power and introduce
another failure point in the communication system. However, the use of
active devices in the in-building system can untimely make the system
work in a more cost-effective fashion. The most common active device in an
in-building antenna system is a bi-direction amplifier.
The frequency planning for an indoor system needs to be coordinated
with the external network. Most in-building systems are designed to facilitate
hand-offs between the in-building and external cellular system. If the
in-building system is utilizing its own dedicated channels assigned to it, then it is imperative that the in-building system be integrated into the
external network.