Round-trip delay indoor ranging experiments with OFDM signals

Ranging accuracy and reliability is crucial for any cooperative- and non-cooperative positioning system indoors. State of the art systems exploit received signal strength (RSS) from WiFi devices, or ultra-wide bandwidth (UWB) timing based range estimation. RSS proved to be unreliable in rich multipath environments. UWB combats multipath at the cost of high bandwidth and specialized hardware which is not integrated in state of the art mobile radio transceivers, e.g., in mobile phones. We tackle this problem with round-trip delay (RTD) based range estimation with orthogonal frequency division multiplex (OFDM) modulated signals which are widely used in WiFi and 3GPP-LTE. In this paper, we present our measurement setup, specifically the developed prototype and two different indoor environments with varying multipath conditions. Measurements along ground truth points are collected for each environment and post-processed. We apply four different estimators, two based on cross-correlation and interpolation, and two based on Maximum-likelihood (ML) multipath estimation. Furthermore, we analyze differences for each estimator in each measurement environment. This analysis reveals that a simple correlation based estimator with interpolation and smart thresholding is sufficient and a very good trade-off between computational complexity and accuracy compared to ML multipath estimators. These indoor experiments show that our simple ranging technique is suitable for indoor distance estimation with low bandwidth, low transmit power, and can be exploited for LTE Direct with low complexity.