  Acoustic local positioning systems (ALPSs) are an interesting alternative for
indoor positioning due to certain advantages over other approaches, including
their relatively high accuracy, low cost, and room-level signal propagation.
Centimeter-level or fine-grained indoor positioning can be an asset for robot
navigation, guiding a person to, for instance, a particular piece in a museum
or to a specific product in a shop, targeted advertising, or augmented reality.
In airborne system applications, acoustic positioning can be based on using
opportunistic signals or sounds produced by the person or object to be located
(e.g., noise from appliances or the speech from a speaker) or from encoded
emission beacons (or anchors) specifically designed for this purpose. This work
presents a review of the different challenges that designers of systems based
on encoded emission beacons must address in order to achieve suitable
performance. At low-level processing, the waveform design (coding and
modulation) and the processing of the received signal are key factors to
address such drawbacks as multipath propagation, multiple-access interference,
nearfar effect, or Doppler shifting. With regards to high-level system design,
the issues to be addressed are related to the distribution of beacons, ease of
deployment, and calibration and positioning algorithms, including the possible
fusion of information. Apart from theoretical discussions, this work also
includes the description of an ALPS that was implemented, installed in a large
area and tested for mobile robot navigation. In addition to practical interest
for real applications, airborne ALPSs can also be used as an excellent platform
to test complex algorithms, which can be subsequently adapted for other
positioning systems, such as underwater acoustic systems or ultrawideband
radiofrequency (UWB RF) systems.
