Ultrawideband (UWB) technology can provide short-range, high bandwidth communications at a very low energy consumption level, which is quite attractive for wireless body area networks (WBAN). In such a system, the presence of the human body brings huge challenges for both the design of the wearable antenna and the propagation model. First of all, the coupling between the wearable antenna and the human body should be considered even in the initial steps of the design, in order to be able to deal with both the possibly deteriorating performance of the antenna as a result of the body and the exposure risk for the body. Second, the propagation channel in WBAN is influenced by the continuous movement of the human body, due to the time-varying scattering of the electromagnetic waves. Lots of researchers have been active in this area and some significant progress has been achieved recently. This paper retrospects the latest results in the field of wearable ultrawideband antennas, propagation channels, and their respective application in WBAN systems.

Wearable Ultrawideband Technology—A Review of Ultrawideband Antennas, Propagation Channels, and Applications in Wireless Body Area Networks

This paper reports a millimeter-wave radio frequency (RF) microelectromechanical systems (MEMS)-based variable attenuator implemented by monolithically integrating CPW-based hybrid couplers with lateral MEMS varactors on a silicon-on-insulator (SOI) substrate. The MEMS varactor features a Chevron-type electrothermal actuator that controls the lateral movement of a thick plate allowing precise change of capacitive loading on a CPW line leading to a change in isolation between input and output. The proposed variable attenuator is successfully fabricated on an SOI substrate with a device footprint of 3.8 $\text {mm}\times 3.1$ mm. The fabrication process provides flexibility to extend this module and implement more complex RF signal conditioning functions, thus making it more appealing to realize a wide range of reconfigurable RF devices. The measured RF performance shows that the device exhibits attenuation levels ( $S_{{21}}$ ) ranging from 10 to 25 dB, at the center frequency of 60 GHz with a bandwidth of 4 GHz and a return loss of better than 20 dB. The device involves laterally moving 20- $\mu \text{m}$ -thick structures, hence offers reliable operation and eliminates MEMS problems like stiction, dielectric charging, and microwelding observed in surface micromachined thin membranes.

Monolithically Integrated RF MEMS-Based Variable Attenuator for Millimeter-Wave Applications

A new antenna structure is presented in this paper. The antenna is operating at the Ultra-Wideband (UWB) band, 3.75-4.25 GHz, defined originally in Body Area Networks (BAN) part of IEEE 802.15.6 standard. The antenna size is 89 mm × 60 × 21 mm. The antenna is a directive with a measured gain of 8 dBi at 4 GHz center frequency. Since the proposed antenna is designed to meet the aspects of a receiving antenna for wireless capsule endoscopy localization, the antenna behavior in close proximity to a human body, in particular the small intestine area, is performed. For this end, initial on-body simulations were carried out by means of a tissue-layer model emulating the dielectric properties of the human body tissues at 4 GHz center frequency. This was followed by voxel model investigations. The human body impact on the antenna characteristics was analyzed firstly, followed by the examination of the power flow propagation inside the tissues. These analyses are consistent to evaluate the antenna ability to communicate with a capsule placed at the small intestine. Later, the antenna free-space propagation was assessed and validated by measurements. These results are followed by a measured on-body investigation conducted on male and female persons. Simulation results were obtained by CST Microwave Studio. Results were confirmed by measurements, conducted in an anechoic chamber at University of Oulu, Finland. The results were measured in frequency domain and later post-processed to a time domain. Consequently, measured results converge to the simulation ones. It is concluded that, the antenna could be used for Wireless Capsule Endoscopy communications with UWB signaling complying with IEEE 802.15.6 standard.

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Directive Low-Band UWB Antenna for In-body Medical Communications

The paper presents a novel transmitter architecture for short-range asynchronous wireless communication, applicable to simultaneous multi-user wireless acquisition of biological signals. The analog signal, provided from an analog biosensor, is transformed to time information using an Integral Pulse Frequency Modulator (IPFM) as a Time-Encoding Machine. The IPFM generates a time-encoded unipolar pulse train, maintaining the linear dependence of the output pulse distance on analog input voltage. The system enables continuous acquisition of the signals from multiple sensors in which each transmitter has unique feedback loop delay used for multi-user coding. IPFM pulses trigger the Impulse Radio Ultra-Wideband pulse generator directly, providing two ultra-wideband (UWB) pulses per each IPFM pulse. Due to the lack of internal clock signal and microprocessor-free multi-user coding, the circuitry satisfies the requirements of multi-user coding energy efficiency and size reduction, which are crucial demands in biomedical applications. The proposed Time-Encoded UWB (TE-UWB) transmitter is implemented in 0.18 $\mu{\text{m}}$ CMOS technology. Measurement results of the IPFM transfer function for input voltage ranging from 0.15 to 1.5 ${\rm V}$ are presented, providing the dependence of the IPFM pulse time distance on analog input voltage and power consumption dependence on the input voltage level. For continuous monitoring operation, total power consumption of the transmitter circuitry for the maximum input voltage is 10.8 $\mu{\text{W}}$ , while for the lowest input voltage it increases to 40.48  $\mu{\text{W}}$ . The circuit occupies 0.14 ${\text{mm}}^2$ .

An Energy Efficient Multi-User Asynchronous Wireless Transmitter for Biomedical Signal Acquisition

Parallel-plates based micro-tunable capacitors are known to have low travel ranges, which deteriorate as going even lower in terms of their initials gap sizes. Such conditions have put strict requirements on the operation of such designs and hence hindering their use in numerous practical applications requiring high tunability. This work is proposed to examine the possibility to implement a closed-loop control strategy to increase the maximum capacitance and therefore tunability of micro tunable capacitors. The suggested control strategy is implemented on an electrostatically actuated parallel-plates (one stationary and one movable) based micro-capacitor and had an objective to stabilize the movable electrode when it is close to the fixed one for the sake of maximizing its maximum capacitance and possibly improving its overall tunability. Robustness of the micro-capacitor to the so-called pull-in phenomenon (short-circuit instability) when using the closed loop control scheme is studied. Indeed, an adaptive sliding mode controller is designed to compensate the effects of uncertainty, disturbance and eliminate any possibility for chattering phenomenon. The controller proficiencies in terms of stabilizing the micro-capacitor and its robustness to uncertainty as well as disturbance have been thoroughly examined. Furthermore, the effects of the control parameters on the behavior of micro-capacitor, such as overshoot, settling time, steady state error, robustness to uncertainty, external disturbances and to the chattering phenomenon, have been completely inspected. The obtained results indicated satisfactory proficiency and trustworthiness of the proposed control strategy to achieve high level of tunability and maximum capacitance.

https://radar.brookes.ac.uk/radar/file/4fcd7157-db9b-42ab-a477-dddd93aa8892/1/Controller in stabilization of micro-tunable capacitor - 2020 - Mobki Sabegh Azizi Ouakad.pdf

On the implementation of adaptive sliding mode robust controller in the stabilization of electrically actuated micro-tunable capacitor

Performance of impulse radio-based ultrawideband (IR-UWB) communications in wireless body area networks is investigated using the dedicated IEEE 802.15.6 standard. An IR-UWB transceiver system is implemented for both on-off keying and differential binary phase-shift keying modulations. Bit error rates are determined from measurements for different on-body links with different data rates. It is shown that using a 25-dB-gain low noise amplifier (LNA) at the receiver, reaching an uncoded bit error rate BER of ${10^{ - 3}}$ was not possible for some links operating at higher data rates. Power and energy consumption issues are then addressed, and results in terms of required pJ/bit to achieve a certain quality of communication are given and discussed.

/pdf/performance-assessment-of-ir-uwb-body-area-network-ban-based-22eosj0pzb.pdf

Performance Assessment of IR-UWB Body Area Network (BAN) Based on IEEE 802.15.6 Standard

A novel micro-electro-mechanical system variable capacitor with a nonplanar top plate and a side leverage electrode structure is proposed to achieve a large tuning ratio and excellent linearity. A theoretical analysis of the structure is performed, including both a mechanical analysis and an electrical analysis. The effects of the structural parameters on the linearity were analyzed and a design method was derived to optimize the linearity. A simulation using CoventorWare was carried out to verify the capacitor performance with particular emphasis on the capacitance versus voltage (C–V) response. The results show a high linearity factor of 99.92% between 0.077 and 0.238 pF (the linear C–V response range) that corresponds to a linear capacitance tuning ratio of 309%.

Optimization of a MEMS variable capacitor with high linearity and large tuning ratio

An ultra-broadband RF MEMS (radio frequency micro-electro-mechanical system) switch for 15-110GHz applications is presented in this paper. A contact-capacitive structure is used to realize the low insertion loss and stable isolation in the broad band. Owing to the contact-capacitive topology, the ON-state and OFF-state can be designed separately. An RLC (resistor-inductor-capacitor) model is established to design the switch. The elements in the RLC model are designed carefully to obtain the ultra-broadband performance. The device is fabricated by using surface manufacturing process. The S-parameters of the switch are tested. The insertion loss and return loss at ON-state are better than -0.5dB and -15dB, respectively, from 15GHz to 110GHz. The isolation at OFF-state is better than -15dB.

An Ultra-Broadband Contact-Capacitive RF MEMS Switch for 15-110GHZ Applications

Invited) Tunable RF MEMS and Its Applications

This paper presents an RF MEMS switch with distributed multi-contact structure for power handling capability improvement. The main advantages include: (a) the use of distributed contacts structure which can result in different resistances among different contacts to suppress the uneven current distribution caused by skin effect of the RF signal. (b) the proposed current uniformity factor (α) to illustrate the uneven distribution of the current. CoventorWare and HFSS are used to model the structure and simulate the performance. The simulated results indicate that the current uniformity factor of proposed switch is improved from 0.503 to 0.688, which results in a higher theoretical power handling capability. Measurement results show that the switch with distributed contacts can bear 2 W RF power @ 10 GHz for 2 hours under prolonged pull-down condition, which is higher than the switch with parallel contacts.

Zewen Liu

Papers

Performance Assessment of IR-UWB Body Area Network (BAN) Based on IEEE 802.15.6 Standard

Optimization of a MEMS variable capacitor with high linearity and large tuning ratio

An Ultra-Broadband Contact-Capacitive RF MEMS Switch for 15-110GHZ Applications

Invited) Tunable RF MEMS and Its Applications

Distributed Multicontact RF MEMS Switch for Power Handling Capability Improvement