A Hybrid TDoA/RSSI Model for Mitigating NLOS Errors in FMCW Based Indoor Positioning Systems

In this paper, the received signal strength indicator (RSSI) is combined with the time-difference-of-arrival (TDoA) technique as a hybrid approach to mitigate positioning errors due to non-line-of-sight (NLOS) propagation in indoor positioning systems. In order to study the effect of NLOS propagation on frequency modulated continuous wave (FMCW) radars, a simple model based on ray tracing was developed to calculate the positioning error profiles inside an area of interest. The model also calculates the RSSI values inside the area. The model showed a correlation between positioning errors and low RSSI values for some base stations (BSs). Therefore, an algorithm is proposed that uses the RSSI values of each BS as a weighting factor to rule out the NLOS BSs and rely only on the line-of-sight (LOS) BSs. Applying this algorithm on a test scenario with NLOS conditions, an improvement of more than a factor of 3 in the maximum positioning error was achieved. Also the average positioning error was improved by more than a factor of 5.