Phase-based 3-D localization of radio-frequency (RF) markers has high sensitivity and accuracy. However, phase measurements suffer from oscillator phase noises, wavelength ambiguities, and multipath interferences. Additionally in the near field, antenna detuning and medium inhomogeneity render the phase-distance relation nonlinear and nonmonotonic and bring forth extra ambiguities, especially with obstructed Line-of-Sight (LoS). In this work, we present a novel precision localization framework which leverages spatially diverse redundant channels to resolve ambiguities without relying on broad bandwidth. First, measured differential phases were used to accurately retrieve differential distances from spline-fitted phase-distance curves. Then, distances from multiple channels were combined to generate 3-D location estimates. Finally, location ambiguities were removed by taking different channel subsets to identify one unambiguous location using spatial clustering. An experimental multiple-input–multiple-output (MIMO) network was implemented by a Universal Software Radio Peripheral (USRP) platform and harmonic RF markers to demonstrate millimeter-level 3-D localization at sub-1GHz carrier frequencies within heavy multipath ambient, simulating the condition inside building structures.

Users of wearable devices often hope to collect continuous personal vital signs such as heartrates, blood pressures, respiration rates and breath efforts with comfort and free movement. Current approaches are however hindered by their accuracy, comfort, convenience, power consumptions and sensing capabilities. The requirement of direct skin contacts in electrocardiogram (ECG) and acoustic methods limits their sensation of comfort, wearing convenience, body motion and hence long-term use. Photo-plethysmography (PPG) in wrist watches and finger clamps do not require immediate skin touch but its signal can be severely degraded by relative movement. In this paper, we introduce a new method modulating the external and internal motion of the wearer body directly onto multiplexed radio frequency (RF) signals by near-field coherent sensing (NCS). To minimize the deployment and maintenance cost of NCS vital-sign monitoring, passive (battery-free) RF identification (RFID) tags can be integrated into garments at the chest and wrist areas as laundry-ready wearable devices. NCS utilizes both amplitude and phase of the RF signals to sense and isolate the vital signs, which significantly increases the signal quality in comparison to methods based solely on signal strength. The high frequency components of the NCS signal is employed to mitigate the body movement interference to obtain more accurate heartrates.

Continuous monitoring of vital signs including heart rates, blood pressures, respiration rates and breath efforts is critical for eHealth. Current approaches are limited in accuracy, convenience and sensing capabilities. Some methods such as electrocardiogram (ECG) is difficult for long-term applications due to the requirement of direct skin contact, which is uncomfortable to wearers, restricts body motion and disrupts circadian rhythms. Meanwhile accurate measurement of the inter-heartbeat timing features is important for wellness monitoring and human emotion studies. In this paper, we introduce a new method to modulate the external and internal mechanical motion of the wearer body directly onto multiplexed radio frequency (RF) signals by near- field coherent sensing (NCS). To minimize the deployment and maintenance cost of NCS vital-sign monitoring, passive RF identification (RFID) tags can be integrated into garments at the chest and wrist areas. The unique ID of each tag helps discriminate its signal against interferences from ambient interferences and other tags. NCS utilizes both amplitude and phase of the electromagnetic field to couple the motion, which significantly increases the signal quality in comparison to methods based solely on signal strength. The tag backscattering signal is then processed with spectral equalization to amplify the high frequency components to recover not only the waveform details originally submerged in the low-frequency components but also the sharp peaks for precise heartbeat intervals with improved peak detection certainty. The derived heartbeat interval shows improved stability in comparison with synchronous ECG.

Extremely low-cost passive RFID tags have become key components for the Internet of Things (IoT), where ubiquitous object locating is one of the most important functions. Although the number of tags in indoor environment continues growing fast, objects cannot be always assumed tagged, whether intentionally or unintentionally. Compared to tagged objects, a tagless target which neither emits nor modulates signal is much more difficult to locate, especially when the size is small. In this paper, we achieve accurate tagless object locating through a dense and wide non-uniform sampling in the Fourier domain of target reflectivity with the help of ambient passive RFID tags as landmarks. Unlike conventional RFID systems, we leverage nonlinearity in passive tags to backscatter second harmonic signals. The frequency separation of uplink and downlink in harmonic RFID allows ready interference cancellation. We embrace spatial diversity enabled by ambient tags and frequency diversity by broadband harmonic backscattering to improve accuracy and robustness. We present the fundamental theory and a prototype system to verify the proposed approach.

We propose and demonstrate accurate 3D real-time indoor localization via broadband nonlinear backscatter in passive devices. The proposed method does not need any relative motion between a reader and a tag or the use of reference anchor nodes. In the conventional radio frequency identification (RFID) system, a passive tag responds to a reader by switching its antenna "on" and "off". The operation of such conventional backscatter is essentially "linear", since the reader-to-tag (downlink) and tag-to-reader (uplink) signals overlap on the same carrier frequency. Although linear backscatter is straightforward, the self-jamming problem caused by strong leakage signals from the transmitter to the receiver is notorious and poses many constraints on the received signal quality, operation bandwidth, modulation flexibility and system complexity. To enable high-accuracy real-time 3D indoor localization for passive devices, we show that nonlinear backscatter is more effective than linear backscatter. Nonlinear backscatter exploits nonlinear elements in passive devices to generate second or higher-order harmonics as the uplink response. Separation of downlink and uplink on different carriers allows immediate self-jamming cancellation and direct un-modulated carrier phase decoding, hence resulting in better received signal quality and broad bandwidth of operation, both of which are critical for the localization system. The broad bandwidth allows the design of an efficient phase-based ranging algorithm - heuristic multi-frequency continuous wave (HMFCW) ranging which resolves ambiguous phase cycles with heuristically optimized sparse carrier frequencies. HMFCW ranging can correctly pin down the phase cycle integer with 100% reliability as long as the phase error falls within ±90° × BW% (percentage bandwidth). In our present implementation, we achieved a median ranging error below 1 cm under phase error bounds of ±50°. We realized real-time 3D localization from differential ranging by nonlinear conjugate gradient (CG) search for hyperboloids intersection in a multi-static transceiving system with 1 Tx antenna and 4 Rx antennas. The measured 3D localization median error was 3.5 cm in the indoor environment. Presently, the measurement latency was less than 0.155 seconds. We will present system design, algorithms and a prototype with experimental evaluation.

Interference caused by unpredictable body motion is notorious in RF-based ranging and tracking. For passive tags, the strong self-leakage signal and large unwanted direct reflections encountered in conventional RFID transceivers make precision ranging even more difficult. To achieve fast, high accuracy ranging capability against body motion interference, we investigate a second harmonic backscattering solution in the opportunistic method of sequential-test heuristic multi-frequency continuous wave ranging (ST-HMFCW). We show that direct reflection and self-interference are greatly suppressed, so the received second harmonic phase variation is decoupled from the power-dependent phase distortion caused by large interference. ST-HMFCW further seeks opportunities in phase fluctuations caused by interférer motion with drastic performance improvement as bandwidth increases. Robust millimeter-accuracy ranging is achieved under strong body-motion interference with the broadband nonlinear transmission line (NLTL) backscatter tags. We present the theory, simulation and experimental prototypes to verify the effectiveness of the proposed ST-HMFCW ranging method.

Nonlinear transmission lines (NLTLs) are used in diverse applications such as edge-sharpening, pulse generation, and frequency conversion, however, length of a useful NLTL can require significant MMIC or RFIC real estate. We present an analytical model for the complex propagation constant of lossy, distributed NLTLs and fabricate several NLTLs in 0.25µm CMOS for verification. Space-saving layout techniques such as serpentine folding and floating-grid substrate shielding are fabricated and we examine their impact on the edge-sharpening performance of the NLTLs.

Nonlinear Transmission Lines (NLTLs) have been used in many applications for waveform shaping and impedance engineering. To minimize insertion loss and jitter, adaptive impedance is desirable. Here we propose to use switched varactors on NLTL to achieve impedance-tuning. We present an analytical model to examine the impact of switch parasitics on the performance of the NLTL. We find that for a narrow range of switch series resistance, the NLTL can be made to compress the opposite edge of the input pulse. We report the first observation of this reverse-edge sharpening phenomenon in NLTLs fabricated with 0.18µm SOI technology.