Dynamic
location of phone call clusters, Motorola
Problem description (interim version):
A common problem in cellular systems is identifying the locations of
users on the system. Although mobile handsets incorporating GPS systems
do exist, their use is far from widespread and likely to remain so until
battery-life problems are solved. In addition, GPS does not function
well inside buildings or in heavily built-up areas, and may only provide
a crude estimate of location.
Mobile handsets (MS) are usually in contact with one or more base
station (BS) during and between calls. In wideband CDMA systems
such as UMTS, the mobile measures the received signal strength from
nearby base stations and attempts to access the best-received BS when
a call is to be established. As the user moves around the system, the
varying signal strengths received from neighbouring BSs are recorded
by the mobile and reported to the serving BS. If the user moves out
of the coverage area of the serving BS, a handover can be performed
which allocates the MS to a new serving BS. In UMTS systems, the MS
can optionally send and receive data to and from other BS as well as
the serving BS, in an attempt to improve call quality and data rates;
this is termed soft handover.
The MS sends the RSSI (Received Signal Strength Indication) information
back to the serving BS in the form of periodic measurement reports (MR),
in common with other cellular systems such as GSM. The primary function
of the MRs is for handover and mobility control; however it is possible
to sample and store the MRs by analysing the communication links from
the BS to its controller (RNC), or by call-trace techniques at the BS
itself. In this way a large quantity of MRs can be captured from the
entire population of MS in an area being served by a group of cells.
Many techniques have been proposed and implemented which attempt to
derive the MS location from the MRs, commonly based around a triangulation
approach or TDOA (time difference of arrival). Although these techniques
can be effective in some situations, there are a number of problems to
be solved. Firstly, there may only be RSSI information from the serving
cell (and no other neighbours) for many MS. Secondly, the BS in a UMTS
system are unsynchronised and their timing references may drift relative
to each other, introducing errors in the timing measurements reported by
the MS. Thirdly, the signal received by the MS is attenuated by other
factors such as Rayleigh and Rician fading caused by obstructions,
multipath scattering, and also Doppler effects caused by mobility.
The combined effects of all these factors need to be considered when
producing a location estimate for a specific MS.
Position estimation is often performed to provide individual users with
location-specific information — for example proximity to shops
or stations — or to provide contextual advertising or mapping
assistance. In this study group problem we wish to analyse the traffic
distributions and densities rather than the locations of individual
subscribers: is the traffic evenly spread over the serving area, or are
there localised clusters of heavy traffic (for example at station or
theatre exits). Are these clusters static, or do they change over time?
What is the size of these clusters, and how accurate are the estimates of
the cluster location and size? Furthermore, can the distribution of the
subscribers be classified as in-building or outdoor by observation
of the MR data?
Additional classification of the traffic would be to cluster the users in
terms of their mobility (static/pedestrian/vehicular) and distribution
in the vertical. The information gathered from the clustering analysis
would be invaluable for network operators wishing to determine where they
should be integrating additional network capacity, for example through
the introduction of picocells, femtocells, and WLAN access points.
Combining the cluster information with call models and sample tariffs
can provide detailed business plans to support analysis of likely return
on investment.
Problem presenter: Mike Ratford, Motorola.