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In this paper, we propose a new two-way time synchronization (TS) method for orthogonal frequency division multiplexing (OFDM)-based wireless systems. It relies on the time-reversal (TR) technique to remove the effect of the channel phase after the signal round-trip. Thus, the method is named TR-TS. TR technique yields a linear phase rotation across subcarriers, regardless whether the channel is minimum, maximum, or mixed phase. This phase is proportional to the difference of the OFDM symbol-timing synchronization errors at the two receivers. Thus, the clock-offset between the radios is determined using the local reception times and the estimated linear phase resulting from the TR technique. A reliable low-complexity algorithm called fast Fourier transform-weighted least-squares is proposed to estimate the linear phase slope, and its meansquare error is compared with the Cramér-Rao Lower Bound (CRLB). The results show that the proposed algorithm attains the CRLB at low SNR, even when a single OFDM symbol is used. The OFDM packets are time-stamped at the medium access control layer, which allows eliminating errors caused by different and varying delays in the internal lower level processing of the data. Hence, an accurate estimate of the clock-offset is obtained. The impact of various nonidealities on the proposed algorithm is also studied. In addition, we propose a ranging method that employs the novel TS method and a first-path delay estimation technique. The performance of the proposed methods is studied in simulations, as well as using real-world measured channels. The results show that the proposed methods can be successfully applied in low to moderate mobility scenarios such as indoors despite harsh multipath, since they rely on channel reciprocity.

With the development of wireless positioning and popularity of OFDM (orthogonal frequency division multiplexing) as a key technology in wireless systems, such as 3GPP LTE, WLAN, and WiMAX, precise OFDM-based ranging has become of particular interest. In this paper, we propose a ranging method measuring the round-trip time-of-flight of an OFDM symbol. Timing measurements are vulnerable to symbol-timing synchronization error. We propose a novel approach to overcome such errors. This approach is based on the time-reversal technique in which multipath effect appears as an advantage. In frequency selective channels, where different OFDM subcarriers experience different delays, time-reversal method conveniently eliminates the effect of the channel phase response. Thus, phase rotation of the signal caused by the timing offset in the OFDM system can be determined and removed from the measurements. We derive an approximate maximum likelihood method for estimating the phase rotation. Simulation studies demonstrate that the method provides high-quality estimates even for noisy frequency selective channels.

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