Christopher J. Stevens

Optimizing Polyphase Sequences for Orthogonal Netted Radar

Orthogonal netted radar transmitter signals require a very low aperiodic autocorrelation peak sidelobe level (PSL), low aperiodic cross-correlation, and a good resilience to small Doppler shifts. A new set of polyphase sequences is presented with good correlation properties as well as resilience to Doppler shifts. These sequences are built using numerical optimization based on correlation properties. A structural constraint is imposed on the optimized polyphase sequences, which maintains Doppler tolerance. Cross entropy (CE) technique is used to optimize the sequences. Correlation and Doppler results are compared with best-known sequences on various merit factors and shown to be superior

Ultra wideband multiple-input multiple-output radar

The utilization of ultra wideband (UWB) signals enables the radar designer to solve the most important problems of radar target observation. The extremely wide bandwidth enables greater information to be obtained due to high time resolution and the frequency dependence of the scattering centers over this large bandwidth. Increase in the radars signal bandwidth can improve radar performance by providing better range measurement accuracy, improving the target identification and tracking capability, improving radar immunity to passive interference, and enhancing radar countermeasure against narrowband electromagnetic signal interference. Recently there have been many advances in multiple-input multiple-output (MIMO) antenna systems in communications. These diversity systems have been shown to have the potential to dramatically improve the performance of the communications systems. Unlike the traditional beamforming approach, which uses highly correlated signals of an array of transmitting or receiving antenna elements to collimate a beam towards a certain direction in space, MIMO capitalizes on the independence between signals from different transmitters and on the diversity of target scattering to improve the information received from the response, Motivated by the advances and benefits of MIMO in communications and advantages of using UWB signals, this paper presents the experimental investigation of UWB-MIMO radars. The analysis of such radars has been carried out to demonstrate its promising features in terms of better target identification and improved signal to noise ratio (SNR).

Magnetic Metamaterials as 1-D Data Transfer Channels: An Application for Magneto-Inductive Waves

Magneto-inductive waves are a form of propagation that only exists in certain types of magnetic metamaterials formed from inductively coupled resonant circuits. These waves are candidates for forming a contactless data channel between suitably designed devices. This paper reports on an investigation into the data carrying capacity of these novel channels in one dimension from both an analytical and experimental perspective. The derivation of a simple analytical model for both channel and resonating terminals is presented based on simple circuit theory. Resonating terminals are found to have potential as efficient transducers of magneto-inductive waves. A simple demonstration device is constructed from copper rings and lumped-element capacitors. Experimental data capacities, determined from experimental data, are of the order of 60 Mb/s for signal-to-noise levels (SNR = 104) measured in the experiments. It is found that an optimum coupling strength exists for the most efficient excitation magneto-inductive waves in the device, and this leads to an understanding of the limits on the coupling and the eventual data capacity in terms of the interresonator coupling. The conclusion is that the optimal coupling for a resonating terminal will be approximately twice the interresonators coupling in the waveguide for 1-D structures and four times for 2-D devices.

Mechanism of subwavelength imaging with bilayered magnetic metamaterials: Theory and experiment

We present a theoretical and experimental study of a bilayered metamaterial structure for subwavelength imaging of magnetic field. The simplest version of such a structure consists of one or two linear arrays of capacitively loaded split pipe resonators. Its subwavelength physics is governed by strongly anisotropic magnetic coupling between individual resonators and by propagation of magnetoinductive waves with wavelength much shorter than the wavelength of the electromagnetic radiation in free space. It is shown that magnetoinductive waves propagating in the lateral direction are undesirable because they spread the image. Good subwavelength imaging is achieved when, due to the strong interlayer coupling, a stop band in the vicinity of the resonant frequency appears in the dispersion characteristics. The imaging properties of the single and double lens are compared and it is shown that the double lens has a superior performance. Excellent agreement is obtained between experimental and theoretical results ...

Magnetoinductive Waves and Wireless Power Transfer

Recent research in wireless power transfer has highlighted the potential benefits for relaying power from source to receiver by a number of resonating relay coils coupled via mutual inductance. A number of researchers have reported experimental systems based on relay coils and have noted that power transfer efficiency to loads located at different points on the structure can vary widely. Such structures, often known as magnetoinductive waveguides are well known to carry signals known as magnetoinductive waves (MIW) when excited with a frequency in their passband. This paper presents an investigation into their impact on wireless power systems and methods by which negative effects may be minimized. Using the physics of magnetoinductive waves it becomes possible to understand the behavior of relay coil systems and to model them in a closed form. The effects of reflections and standing waves on a one-dimensional system are considered and their effect on the input impedance and the variation of matching conditions determined. An optimum receiver load is proposed based on the results and tested experimentally. A simple experimental demonstrator is used as a model for study, which achieves 58% efficient power transfer to a single load at any point on its length.

Spatio-temporal ultrawideband indoor propagation modelling by reduced complexity geometric optics

A simple and efficient virtual-source ray-tracing technique for the simulation of indoor wideband radio and optical propagation channels is proposed. The parametric deterministic model considers the room geometry, transceiver locations, material properties and probe signal types. It is applied to the indoor ultrawideband channel in the FCC-allocated 3.1-10.6 GHz band, and a range of novel results are presented to illustrate several possible applications. The channel small-scale fading statistics and spatial variability are examined by synthesising a densely sampled aperture. Multiple-antenna array systems are simulated to evaluate multiple-input multiple-output performance. The multipath angular characteristics are analysed from the simulated azimuth-delay profile. The simulation results closely match previous channel measurement studies and statistical models, validating the proposed technique. It is shown that specular reflection is dominant, and power convergence is achieved with three reflections in a typical indoor environment. Thus, it is demonstrated that despite its simplicity, the model yields reliable and accurate results, and can therefore be a useful tool for indoor wireless network planning and performance prediction.

Evaluation of coverage area for a wide line-of-sight indoor optical free-space communication system employing coherent detection

The application of coherent detection to indoor optical free-space communications is considered here. Analytical expressions are derived for a wide line-of-sight (W-LOS) optical link which determine the coverage area and the required photo-detection area given different binary digital transmission schemes and a given bit rate. The coverage area is maximised for systems using both metal-semiconductor-metal (MSM) and PIN photo-detectors. Results show that MSM photo-detectors, due to their inherently lower capacitance per unit photo-detection area, result in a greater coverage area than their PIN counterparts. To demonstrate the viability of implementing coherent detection indoors, a 200 Mb/s LOS optical link which uses coherent detection is also reported.

Frequency Dependence of Fading Statistics for Ultrawideband Systems

The variation of channel fading and dispersion statistics between the ultrawideband (UWB) channel subbands is characterized experimentally. Within the FCC-allocated UWB frequency range (3.1-10.6 GHz), the multipath characteristics in various subbands are found to be highly dissimilar. It is demonstrated that the higher subbands suffer greater attenuation, and the rms delay spread varies significantly with the center frequency. The impact of this variation on multiband-OFDM UWB system performance is evaluated, and a large performance variation across the band is reported

Multipath Effects in Ultrawideband Rake Reception

Rake reception can improve system performance significantly in wideband multipath channels. Its practical implementation, however, becomes prohibitively expensive in channels with dense multipath, such as the ultrawideband (UWB) channel. This paper investigates the effect of various system and environment parameters on rake performance, with emphasis on the amount of multipath and channel bandwidth. The treatment includes hybrid selection/maximal-ratio combining (H-S/MRC) and unordered, partial combining rake architectures, and is based on indoor channel measurements in the FCC-allocated UWB frequency range (3.1-10.6 GHz). The diversity gain is shown to follow the law of diminishing returns with the rake complexity. It is demonstrated that the rake can extract most of the incident signal power by combining only a subset of the resolved multipath components. The required number of rake fingers increases linearly with the number of resolved paths but sublinearly with the channel bandwidth. The characterization of the interplay of bandwidth, amount of scattering and rake complexity will facilitate efficient implementation of UWB systems.

Experimental evaluation of RAKE receiver performance in a line-of-sight ultra-wideband channel

This paper assesses the utility of signal processing techniques for multipath mitigation in an ultra-wideband (UWB) indoor radio channel. UWB systems have the ability to resolve rapidly arriving multipaths. RAKE receivers can provide multipath diversity and integrate the energy from various multipaths, helping overcome fading and effectively increasing the received power. We report experimental studies of multipath propagation in an indoor UWB environment, based on which we predict the performance of RAKE receivers with a variety of configurations and find that incremental gain is high only for low order RAKEs.