Fully passive sensors (FPS) are widely used as a simple and inexpensive alternative of their active counterparts in many applications. However, measurement of FPSs relies on extremely bulky and complicated reader circuitry. In some cases the readout method depends on measurement distance. In this paper, we propose a readout technique based on a two-mode LC-oscillator formed with two coupled resonators, i.e., the FPS and the LC tank in the reader. We show that such a configuration can offer robust measurement of fully passive resistive and capacitive sensors provided that the system operates under strong coupling. We examine measurement errors due to non-idealities of the system (finite coil$Q$and frequency mismatch) and provide design guidelines to ameliorate these unwanted effects. Although applied to sensing in this work, this analysis can be extended to wireless power transfer and impedance transformation.

A millimeter (mm) wave radio is presented in this work to support wireless MRI data transmission. High path loss and availability of wide bandwidth make mm-waves an ideal candidate for short range, high data rata communication required for wireless MRI. The proposed system uses a custom designed integrated chip (IC) mm-wave radio with 60 GHz as radio frequency carrier. In this work, we assess performance in a 1.5 T MRI field, with the addition of optical links between the console room and magnet. The system uses ON-OFF keying (OOK) modulation for data transmission and supports data rates from 200 Mb/s to 2.5 Gb/s for distances up-to 65 cm. The presence of highly directional, linearly polarized, on-chip dipole antennas on the mm-wave radio along with the time division multiplexing (TDM) circuitry allows multiple wireless links to be created simultaneously with minimal inter-channel interference. This leads to a highly scalable solution for wireless MRI.

Superdirectivity has been a controversial idea to increase the resolving power of an antenna aperture and hence increase the number of spatial degrees of freedom. In this paper, we take into account both radiated power and reactive power, and show that superdirectivity does not have a drastic impact on the total number of spatial-temporal degrees of freedom in wireless channels.

Efficient wireless power transfer across tissue is highly desirable for removing bulky energy storage components. Most existing power transfer systems are conceptually based on coils linked by slowly varying magnetic fields (less than 10 MHz). These systems have many important capabilities, but are poorly suited for tiny, millimeter-scale implants where extreme asymmetry between the source and the receiver results in weak coupling. This paper first surveys the analysis of near-field power transfer and associated strategies to optimize efficiency. It then reviews analytical models that show that significantly higher efficiencies can be obtained in the electromagnetic midfield. The performance limits of such systems are explored through optimization of the source, and a numerical example of a cardiac implant demonstrates that millimeter-sized devices are feasible.

Antenna arrays have been demonstrated as a very promising technique for high-speed wireless networks, and are anticipated to be indispensable components in future wireless communication systems. This review paper presents an overview of the circuit techniques on combining signals from different receive array elements as well as splitting signals to different transmit array elements with emphasis on silicon-based solutions. It also reviews circuit techniques to control the phase and the amplitude of signals from/to each array element such that signals from those array elements can be combined and split in a desired manner for achieving high data rate communications as well as interference management.

The mmWave communication system is operating at a regime with high number of antennas to extend the communication range and to recover the high path loss. Before any communication starts, devices are needed to align their beam pointing angles towards each other. The best beam pointing angles are obtained through BeamForming (BF) training protocol. If the communication environment and devices are stationary, the training results can stay valid for a long period of time. However, the environment is very dynamic. Device movement and human blockage in the link may invalidate the BF training results. Prior work focuses on improving training time when a specific type of errors happens. For example, switching to alternative paths is suggested when human blockage occurs. However, solutions that tackle a type of error are not compatible with others. Therefore, it is important to identify the type of occurring beam errors before applying any remedies. In this paper, we propose a linear dynamical system model to demonstrate the dynamics of the occurring errors due to link blockage and device movement. By using the model, we show that it is hard to estimate the errors purely based on the observable quantity provided. To solve this, we propose two probing protocols that consume very few network resources but are effective in identifying the beam errors as well as correcting them. The simulations show that it effectively handles the beam errors and only consumes 3% of network resources while more than 30% resources are required in the prior work for a fast moving device experiencing occasional link blockage.

The mmWave communication system is operating at a regime with high number of antennas and very limited number of RF analog chains. Large number of antennas are used to extend the communication range for recovering the high path loss while fewer RF analog chains are designed to reduce transmit and processing power and hardware complexity. In this regime, typical MIMO algorithms are not applicable. Before any communication starts, devices are needed to align their beam pointing angles towards each other. An efficient searching protocol to obtain the best beam angle pair is therefore needed. It is called BeamForming (BF) training protocol. This paper presents a new BF training technique called beam coding. Each beam angle is assigned unique signature code. By coding multiple beam angles and steering at their angles simultaneously in a training packet, the best beam angle pair can be obtained in a few packets. The proposed BF training technique not only shows the robustness in non-line- of-sight environment, but also provides very flat power variation within a packet in contrast to the IEEE 802.11ad standard whose scheme may lead to large dynamic range of signals due to beam angles varying across a training packet.

Polarization could be the last resource to be exploited for space-limited devices. Over the past years, theoretical studies and experimental work present different conclusions on the potential increase in the number of degrees of freedom from polarization. This paper attempts to unify the different conclusions and provide a mathematical framework that can be applied to any array geometry and channel scattering condition. It shows that the degree-of-freedom gain from using polarimetric antenna elements ranges from 2 to 6 and the gain depends on the array geometry and the channel scattering condition. Sampling techniques and vector multipole decomposition are applied to derive the results.

Recent attention on correlated multi-input multi-output systems has centered around the case of imperfect channel or statistical information at the transmitter. The focus of this work is on correlated channels with arbitrary antenna array geometry and spacings, a coherent receiver, and imperfect statistical knowledge at the transmitter. Leveraging a recently proposed channel modeling paradigm that exploits processing in the angular domain, we first elucidate the structure of the optimal schemes when 'genie-aided' perfect statistical information is available at the transmitter. In the low-S N R case, we then show that the beamforming scheme that is optimal when perfect statistical information is available does not degrade smoothly with imperfections in the statistical information. We then go on to show that there exists a certain low-complexity beamforming approach which, while being sub-optimal in the genie-aided case, is robust to statistical feedback as well as the dynamics of its evolution.

This paper unifies the different conclusions on the polarization degrees of freedom in multiple-antenna channels. It shows that the multiplicative gain in the degrees of freedom from polarization depends on the array geometry and the scattering condition.

Most existing techniques for reconfigurable processors focus on the computation model. This paper focuses on increasing the granularity of configurable units without compromising flexibility. This is carried out by matching the granularity to the degree-of-freedom processing in most wireless systems. A design flow that accelerates the exploration of tradeoffs among various architectures for the configurable unit is discussed. A prototype processor is implemented using the Intel 0.13-mum CMOS standard cell library. The estimated energy efficiency is in the same order as dedicated hardware implementations

The impact of scattering condition and array configuration on performances are inseparable in early analyses of multiple-antenna systems. An array-independent scattering model is introduced where three basic scattering mechanisms are modeled. Performance results become more intrinsic property of the scattering channel itself. For linear arrays of length L in an environment of total angle spread |Omega|, the ergodic capacity is shown to increase linearly with L|Omega| for large arrays. When antenna arrays reduce to practical sizes, the capacity scaling depends on the signal-to-noise ratio (SNR) as well. This implies that the number of antennas used should also depend on the SNR. In terms of outage capacity, the tradeoff between spatial multiplexing gain and diversity gain is shown to be very sensitive to the underlying scattering mechanisms. Finally, as |Omega| varies with the propagation range, the tradeoff among multiplexing gain, diversity gain, and propagation range is studied

Multiple-antenna systems that are limited by the area and geometry of antenna arrays, are considered. Given these physical constraints, the limit on the available number of spatial degrees of freedom is derived. The commonly used statistical multiple-input multiple-output (MIMO) model is inadequate. Antenna theory is applied to take into account the area and geometry constraints, and to define the spatial signal space so as to interpret experimental channel measurements in an array-independent but manageable description of the physical environment. Based on these modeling strategies, for a spherical array of effective aperture A in a physical environment of angular spread |/spl Omega/| in solid angle, the number of spatial degrees of freedom is shown to be A|/spl Omega/| for uni-polarized antennas and 2A|/spl Omega/| for tri-polarized antennas. Together with the 2WT degrees of freedom for a system of bandwidth W transmitting in an interval T, the total degrees of freedom of a multiple-antenna channel is therefore 4WTA|/spl Omega/|.

This paper develops a physical channel model for multiple-antenna systems. The model appropriately abstracts the scattering intensity of physical environments, geometry of antenna arrays and degrees of polarization. Then, we use the model to demonstrate that the "space" in wireless systems is composed of three signal dimensions: wavevector, array and polarization. The impact of physical environment, array geometry and polarization are captured separately in the wavevector, array and polarization dimensions respectively. Thus, we solidify the "space" dimensions in complement to the time and frequency dimensions, and unify the available signal space in wireless channels.

In multiple-antenna channels, the optimality of a transmission scheme hinges on a detailed characterization of the channel. In this paper, we will present measurement results from 2 to 8 GHz in both LOS and NLOS scenarios, at both office and residential environments. Data processing methods are briefly outlined and a statistical characterization of the channel is presented as well.

The paper proposes an adaptive multiantenna transceiver for narrowband reception. Blind channel tracking algorithms are developed to track the eigen directions of the channel directly instead of the channel itself. Two algorithms are proposed to track the column space of the channel at the receiver, based on the received data. One of the algorithms is free of any division operation, which is more favorable in practice. For the row space of the channel, two approaches are proposed as well. The first approach requires periodic feedback of the demodulated signal from the receiver back to the transmitter where it can make use of its knowledge on the prior transmitted symbols to estimate the row space. In the second approach, the estimation is done at the receiver based on the detected symbols, and the estimated row space is sent back to the transmitter. Adaptive resource allocation is also incorporated into the design.

The performance and computational complexity of five multi-access receivers are compared. A methodology is then presented for making area and power estimates of these algorithms for both software programmable DSP and dedicated direct mapped architectures. With this methodology and by using experimental data from previous designs, the feasibility of implementation of the multi-access receivers can be determined.