Theoretical Considerations Regarding the Application of Received Signal Strength within Heterogeneous Indoor Positioning Systems

Nowadays, there are a variety of different indoor positioning systems, where some of them use communication hardware taking advantage of the Received Signal Strength (RSS) such as Wireless Local Area Networks (WLAN) or Bluetooth. These variants are employed if low cost is of primary importance. However, the accuracy provided is in the meter range. The alternative are positioning-tailored approaches like Frequency Modulated Continuous Wave (FMCW) radar, Ultra-WideBand (UWB) radar or phase-based positioning, which offer superior accuracy in the low decimetre range. If there is such a system in use, the question arises whether there is any improvement, if utilizing additional RSS measurements, which are performed by most systems anyway. With the help of the Cram´er-Rao Lower Bound (CRLB), this paper demonstrates that these additional readings can improve accuracy significantly, thus widen the application field for RSS from a low-budget only technique to enabling enhanced accurate positioning. To demonstrate this statement we compare the CRLB for Time of Arrival (ToA) with hybrid ToA/RSS. Our evaluations show that in practice the CRLB is approximately divided by two, if incorporating the RSS for each base station.