Robust Multi-Carrier Frame Synchronization for Localization Systems with Ultrasound

Time of arrival (TOA) based localization extracts the positions from the signal delay between senders and receivers in a network and puts high requirements on synchronization, in particular synchronization on the line-of-sight (LOS) signal. We propose an ultrasound based system with pure LOS synchronization and data transmission for sender identification. We use two carriers with 38 respectively 40 kHz in a baseband OFDM scheme and by comparison of the phase shift of both carriers we estimate with the Cramér-Rao lower bound a distance error between sender and receiver of 0.047 mm for a transmission range of 20 m. We transmit 8 bit in 1.5 ms where no intersymbol interferences are expected in most practical scenarios for robust localization. When transmitting data in a communication system, the properties transmission delay and signal attenuation available as RSSI values can additionally be used for distance measurements between sender and receiver, which is the input for a localization system. However, the measured distance can be imprecise due to multipath propagation, since the signal strength has strong local fluctuations due to the wave characteristics and the reception time of the multipath signal is a mixture of all paths. In contrast, the line-of-sight (LOS) signal only represents the shortest path between sender and receiver where the signal attenuation and signal delay can be uniquely mapped onto the distance between sender and receiver. Thus, separating the LOS signal from the remaining signal paths can drastically improve the localization accuracy. We present a novel data transmission system for localization which transmits a short message in such a short time (1.5 ms) that the LOS path and the other paths do not overlap in time in practical scenarios. Thus, we can process the pure LOS signal at the receiver and ignore the signal of the remaining paths giving false information. Our ultrasound system uses two carriers in a baseband OFDM scheme to increase the data rate and keep the transmission time short. We focus additionally in this paper on precise frame synchronization at the receiver not only for a low bit error rate (BER) at low signal-to-noise ratio (SNR) and long transmission ranges but also to measure a precise signal delay1. Table I shows a comparison of related and this work. 1The signal delay can be computed with the sending and receiving time of the message and clock synchronization between sender and receiver. For localization systems based on Time Difference of arrival (TDOA), the reception times of several transmissions without sending time are sufficient and clock synchronization can also be determined with enough transmissions. Table I COMPARISON OF ULTRASOUND LOCALIZATION SYSTEMS. (NOT AVAILABLE DATA IS DENOTED WITH NA) Range Precision Bandw. Pulse Modulation Data Ref [m] [mm] [kHz] [ms] [bit] 4 NA 47-52 5.10 BPSK, CDMA [?] 4 10 35-65 0.87 Chirp [?] 4 7 32-48 20.46 BPSK, CDMA 64 [?] 3 70 35-45 0.18 BPSK, CDMA [?] 5 0.62 39-41 1.00 NA [?] 10 0.4 38-42 1.50 π/4-DQPSK 8 this Our system can outperform other systems in transmission range, and precision for localization whereas providing data transmission for sender identification at the same time. Our sender consumes 2.4 mW with sending rate 1 Hz compared to 360 mW in [?]. I. SYSTEM Figure 1 shows the ultrasound baseband transmission system with the two carrier frequencies 38.8 kHz and 40.8 kHz. On both, we use π/4-DQPSK modulation for data transmission. The low power design of the sender enables photovoltaic powering. Further, the communication works with acceptable bit error rate up to 20 m.