IEEE 802.11 Distance Estimation Based on RTS/CTS Two-Frame Exchange Mechanism A. Bahillo ∗ , J. Prieto ∗ , S. Mazuelas ∗ , R. M. Lorenzo † , J. Blas ∗ and P. Fern´ andez † ∗ CEDETEL (Center for the Development of Telecommunications) Edificio Solar. Parque Tecnol´ ogico de Boecillo. 47151. Boecillo (Valladolid). SPAIN † Department of Signal Theory and Communications and Telematic Engineering University of Valladolid. Camino del Cementerio s/n. 47011. Valladolid. SPAIN Email: abahillo@cedetel.es, patfer@tel.uva.es Abstract—The addition of positioning capabilities to wide- spread communications such as IEEE 802.11 compliant networks could open up interesting markets, in particular if a low-cost hardware has to be added to the existing one and standard RTS/CTS control frames exchange could be used for this purpose. In this paper various statistical estimators of the delay profile observed, derived from Round-Trip Time measurements, are analyzed and a linear regression of the statistical estimators is applied in order to improve the accuracy of distance estimation between a Mobile User and an Access Point. A precision in distance estimation with errors in the range of a meter is achieved. I. I NTRODUCTION Mobile location is a growing practice in current communi- cation systems and many applications are already forecasted, such as provision of location-based services (LBS) in cellular networks, warehouses and hospitals. The world-wide spread of IEEE 802.11 WLANs allows the design of low cost and easy-to-deploy location systems. In this paper it is proposed an accurate distance estimation method with IEEE 802.11 devices based on the RADAR location model. As several methods to mitigate the Non Line-Of-Sight (Non-LOS) effect are proposed [1], [2], [3], in this paper, different environments with LOS between transmitter and receiver are assumed. The distance between two IEEE 802.11 WLAN nodes can be mainly estimated using a Time Of Arrival (TOA) or Received Signal Strength (RSS) based methods. Since propagation delays are more stable and they correlate more closely with the distance than RSS-based method, TOA-based method is more accurate [4]. In order to avoid need for time synchronization between WLAN nodes, which would entail a major increase in the complexity of system deployment, TOA is obtained by performing Round-Trip Time (RTT) measurements. The ranging technique presented in this pa- per is based on the estimation of the distance between a Mobile User (MU) and an Access Point (AP) through the RTT measurements as RADAR location model. The main characteristic that makes a RADAR location model possible is the RTS (Request-To-Sent)/CTS (Clear-To-Sent) IEEE 802.11 protection’s requirement for network APs to send a CTS frame This research is partially supported by the Directorate General of Telecom- munications of the Regional Ministry of Public Works from Castilla y Le´ on (Spain). of each RTS frame that is addressed to them, much like the return of RADAR pulses off targets within the RADAR’s LOS and range. Therefore, RTT is obtained by measuring the latency of a series of layer two CTS frames sent by and in response to a corresponding series of RTS frames initiated by the MU. On one hand, the method proposed takes advantage of the benefit of the RADAR location approach model which is easy to implement in an IEEE 802.11 WLAN by using the RTS/CTS mechanism, on the other hand, due to the use of an IEEE 802.11 infrastructure, the location capabilities would be an added value to the existing connectivity ones. In this paper, the measuring system is integrated in a Printed Circuit Board (PCB), which is used as additional hardware to the WLAN adapter from which appropriate signals, such as transmission and receiver pulses, are extracted to manage the measuring system [5]. In [6] a method to determine the distance between WLAN nodes based on RTT measurements without additional hardware is proposed, but the accuracy reached is lower than the one in this paper. According to [7], the range resolution is determined by the bandwidth of the transmitted signal, 22 MHz in the IEEE 802.11b standard. Therefore, the range resolution is about 6.8 m in that standard. High-precision location would require large transmission bandwidths and thus the use of multiple frequency-channels. Furthermore, with a 44 MHz clock as a reference of the measuring system, the accuracy of RTT is hampered by a discrete time resolution. To overcome low time resolution of the given hardware clocks, and the range resolution, various statistical estimators of the delay profile observed are analyzed. Afterwards, a linear regression of the estimators at each distance is applied to improve the accuracy of the distance estimation between the MU and the AP. In section III it is shown that high precision distance estimation with errors in the range of a meter is achieved. The method proposed in this paper is compared with [8] where the RTT measurements are performed in a similar way. But, in [8], the distance estimation is obtained by halving the ΔRTT, which corresponds to the pure propagation portion of the RTT, supposing the RTT at distance 0 m the AP processing time. In this paper the AP processing time is measured directly through the PCB and the accuracy achieved is higher than the one presented in [8]. 978-1-4244-2517-4/09/$20.00 ©2009 IEEE