Recently mobile location based services (LBS) are provided via a mobile terminal such as a personal digital assistant (PDA), a cellular phone and so on. The location method that searches where the mobile terminal of a user is accurately is a key factor for providing convenient and useful LBS. The most well known method associated with positioning system is GPS. The signal of GPS satellites can be always acquired outdoors and this system provides comparatively accurate location information. However, there are difficulties in applying GPS directly to indoor positioning because of the weakness of signal. And the positioning using the mobile communication signal between a base station and a cellular phone does not provide adequate accuracy due to some technical limitations of communication systems when applied to navigation. In addition, severe multipaths are present in an indoor environment. So a new wireless communication technology is required for indoor positioning to achieve a better and appropriate accuracy. Wireless local area network (LAN) has been installed in a number of indoor areas such as office, terminal, campus, and park with interests in mobile Internet. Therefore, it is expected that a wireless LAN signal would be easily acquired for indoor positioning. Various positioning measurements like time of arrival (TOA), time difference of arrival (TDOA), and received signal strength may be employed for indoor positioning. Employing TOA or TDOA measurements requires a time synchronization between a transmitter and a receiver. But, a time synchronization between a transmitter and a receiver is very difficult if not impossible in a wireless LAN. Therefore, it is more appropriate to use signal strength for indoor positioning with a wireless LAN.

In the aftermath of the terrorist attacks on New York and Washington, finding people and bodies in the rubble was of utmost importance. This task was directed at finding location bearing devices incorporated in commonplace instruments such as mobile phones, personal digital assistants (PDAs) and electronic pagers. By polling these devices electronically using a system of triangulation points it was thought that persons and bodies could be found. Polling is simply sending electronic impulses to receivers to ascertain where these devices are located. However, and sadly, such devices have a limited range and may have no effect if buried in more than a meter of rubble. Also, some such devices incorporate global positioning systems (GPS) and these require access to the sky and to the constellation of satellites orbiting the Earth. Even the sniffer dogs gave up this mammoth task.