  Research in location determination for GSM phones has gained interest
recently as it enables a wide set of location based services. RSSI-based
techniques have been the preferred method for GSM localization on the handset
as RSSI information is available in all cell phones. Although the GSM standard
allows for a cell phone to receive signal strength information from up to seven
cell towers, many of today's cell phones are low-end phones, with limited API
support, that gives only information about the associated cell tower. In
addition, in many places in the world, the density of cell towers is very small
and therefore, the available cell tower information for localization is very
limited. This raises the challenge of accurately determining the cell phone
location with very limited information, mainly the RSSI of the associated cell
tower. In this paper we propose a Hidden Markov Model based solution that
leverages the signal strength history from only the associated cell tower to
achieve accurate GSM localization. We discuss the challenges of implementing
our system and present the details of our system and how it addresses the
challenges. To evaluate our proposed system, we implemented it on Androidbased
phones. Results for two different testbeds, representing urban and rural
environments, show that our system provides at least 156% enhancement in median
error in rural areas and at least 68% enhancement in median error in urban
areas compared to current RSSI-based GSM localization systems
