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We present new results relating far-field and near-field superdirectivity for a dimer endfire array in which only one of the meta-atoms is excited. We deduce conditions for achieving superdirectivity in terms of structure parameters (distances between meta-atoms, their quality factors and the values of mutual coupling) and show that the required sets of parameters are different for far-field and near-field superdirectivity. Our results are relevant to medical imaging applications such as MRI and associated technologies.

Sparse ultrawideband antenna arrays suppress the grating lobes due to the fact that the duration of pulses processed by individual antenna elements is too short to constructively superpose them in the time domain. Such arrays represent an attractive option for design of many low-comlexity low-cost ultrawideband systems, where narrow beamwidth is achieved by increase of element spacing. This paper empirically explores the minimum bandwidth required for suppression of the grating lobes. It is found that the relative bandwidth required to suppress grating lobes is independent on the centre frequency, element spacing, and number of elements. There is a weak dependency of the bandwidth on the type of the pulse used, but the general conclusion is that the relative bandwidth has to exceed 100% in order to suppress the grating lobes. It is, however, noted that whilst sparse array with a low number of antenna elements can suppress the grating lobes and posses a narrow beam, the compromise for this feature is a reduction of the level of spatial filtering between the main lobe and other directions, when compared to arrays where grating lobes are suppressed by dense spacing. © 2012 IEEE.

This work discusses the standard algorithm for measurement of free-space antenna reflection coefficient in a reverberation chamber. This algorithm is explored in the time domain and it is shown that the bandwidth of ultrawideband antennas allows a significant reduction of the number of measurements required. This is because the ultrawideband bandwidth allows us differentiating between the free space antenna reflection coefficient and the backscattering of the environment. The limits on the minimum bandwidth, the distance to the nearest object, and the number of measurement points are discussed and experimental results confirm the methodology. The proposed methodology significantly reduces the duration of the measurement and it is believed to be of interest for industrial production and testing of ultrawideband antennas. © 2012 IEEE.

The use of multiple antennas is an attractive option for ultrawideband communication and positioning systems. Despite intensive research interest, the framework for analysing ultrawideband arrays is ambiguous. For OFDM based systems which use many narrowband subcarriers, the preferred choice is the use of the classical array description by means of the array factor. For impulse radio systems, array factors cannot be used and new beam pattern descriptors have been developed. In this paper we explore and quantify the transition between narrowband and ultrawideband systems. We define the upper bandwidth for which it is still acceptable to use the description by the means of classical array factors. This limit enables a rule of thumb decision on the selection of a suitable UWB MIMO strategy

Myography aims to detect muscular activity. This paper presents the concept of a novel myographic method that fundamentally differs from standard methods that aim to detect a signal produced by muscle contraction. Our method is based on the measurement of muscle thickness by the means electromagnetic reflectometry. For a layered medium, the problem is shown to be simplified to a detection of phase change of the reflection coefficient. A feasibility study shows that a change of thickness of the muscle tissue causes sufficient change of the phase of reflection coefficient to be detectable by commercial phase detectors or phase locked loops. Unlike conventional methods, this paves the way to non-invasive myography for deep core muscles, which would be useful for back pain treatment. Furthermore, the issues of non-ideal detectors are discussed and an algorithm removing the systematic error is introduced.

Cognitive radio (CR) is an enabling technology that allows the unlicensed users to opportunistically access the licensed spectrum bands, without any modifications or updates for the licensed systems. In this paper, we first propose a new spectrum sharing policy, which achieves efficient spectrum sharing between two licensed networks. Then, a Markov chain model is developed to analyze the proposed spectrum sharing policy in the cases with and without spectrum handoff. Numerical results show that the proposed policy achieves a higher overall throughput of the two networks. Extensive simulations verify the analytical model. © 2011 IEEE.

A wireless sensor network consists of hundreds or thousands of low cost, small wireless sensor nodes which allow users to monitor a variety of remote environments. In this paper, a novel cluster-head selection algorithm is presented and analyzed which uses the minimum mean distance between sensor nodes as a selection parameter. The proposed algorithm has clear advantages and takes 1.2 times longer to reach the point where 50% sensor nodes remain alive than the Low Energy Adaptive Clustering Hierarchy algorithm (LEACH) while maintaining information throughput at a high level. Furthermore, the new algorithm takes 1.2 times longer than the LEACH to reach the point where the number of cluster-heads and the throughput from the cluster-heads to the base station are reduced by 50%. In the example 100 sensor network presented, the new algorithm ensures that there are always five cluster-heads during each round throughout the network lifetime, which is longer than the LEACH protocol and delivers 2.1 times more data to the base station compared to the LEACH protocol. All simulation results show that this new algorithm can significantly improve the overall wireless sensor network performance compared with the LEACH.

This paper presents an experimental characterization of the ultra-wide bandwidth (UWB) indoor channel based on an extensive measurement in a PC tower case environment to identify the optimum channel performance. The investigation is based on the analysis of the statistical properties of the multipath profiles, measured over a finely spaced measurement grid. The small-scale statistics of the received path energy follow Normal distributions with better time dispersion performance to be achieved with the existence of the Main Board (MB) on the measured channel. The Ricean K factor is examined to identify the multipath behavior of the channel. Finally, the channel capacity is calculated for three different transmitter levels with two different antenna polarization modes. A maximum channel capacity of 6.24 Gbps is achieved using the whole UWB band with a signal to noise ratio (SNR) of 10 dB with the main board present. Results show a significant reduction in overall channel capacity when antennas parallel to the main board plane are used. © 2009 IEEE.

This paper reports the design of a new resonator system to enhance the identification of buried underground assets. Results from theoretical calculation, simulation, laboratory measurement, and field test are presented and practical considerations are highlighted. The proposed system has the potential to benefit future civil engineering installations. ©2008 IEEE.

In this paper, we clarify inter-link interference in wireless ad-hoc networks by using link-directional interference graphs (I-graph). Most of the interference graphs in the literature simply model the DATA and ACK traffic of a link by a single vertex. They fail to capture the link-directionalities. In fact, some instances of directional traffic can actually transmit simultaneously but are prohibited by the interference graphs. Thus, in our link-directional interference graph, a link is represented by two vertices, one for DATA traffic and the other for ACK traffic. We then apply a colouring algorithm in the I-graphs. The colouring results provide insights in order to boost network capacities in TDMA or FDMA ad-hoc networks. We show that the network capacities can be improved by 100% in a triangular topology and 33% in a lattice topology. Simulations also show that a distributed dual channel protocol assigning channels according to link-directionalities can boost the capacities by 70% in large-scale random networks. We believe this is a first paper in the literature to take into account link-directionalities in interference graphs.

A typical ultrawideband (UWB) antenna acts as a space-frequency filter. As a result, the UWB signal transmitted or received by such an antenna undergoes joint angular-spectral distortion, and consequently the system suffers serious performance degradation. We discuss a technique to compensate for this distortion and recover the original signal using multiple antenna arrays. To evaluate its performance, the proposed technique is applied to UWB signal propagation in an indoor multipath environment simulated using a specialized ray-tracing approach. The results demonstrate that our antenna equalization scheme significantly reduces the effect of small-scale fading introduced by the antennas, and can therefore improve the performance of a UWB communications link as well as facilitate the extraction of antenna-independent channel measurements. © 2007 IEEE.

In this paper, we propose a novel and efficient procedure to extract the UWB propagation channel parameters, that employs a RT simulation carried out at different frequencies and followed by a proper processing of the simulated data. A parallel ray approximation, usually adopted in array analysis, is successfully used to dramatically reduce the computational time of the procedure. An excellent agreement between simulation results and measured data is achieved, showing that the RT-based tool can be successfully used for accurate and reliable site- planning in UWB systems.

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2 - C. However, most of the multichannel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% of the 2 ldr C targeted capacity. Simulations show that our proposed scheme can achieve more than 106% of our targeted capacities, 1.06*2 ldr C = 2.12 ldr C. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link on different channels, a key step that yields significant potential for multiplying the network capacities of ad-hoc networks.

Correlation between diversity branches is the key parameter in determining the diversity performance of wireless systems. In this paper, measurements in a temporally and spatially stationary indoor environment are carried out to obtain the channel impulse responses for the UWB impulse radio using receivers with different positions or different polarisation states. A two-dimension (2D) RAKE receiver with post-detection equal gain combining (EGC) diversity is assumed to be used. Three definitions of correlation coefficients of the diversity branches are considered: correlation coefficient, normalized correlation and channel profile correlation, will be analysed in this paper. The results show that the normalised correlation is more effective to assertain the degree of similarity of the correlated diversity branches where medium scale fading dominates.

155 Mb/s operation of an optical wireless link is achieved by using the spectral characteristics and angular emission spectra of a 7-element tracking array of 980 nm RC-LEDs. Preliminary results show extension to 200 Mb/s/channel.

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 radar's 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).

The frequency variance of antenna characteristics may result in severe signal distortion in an ultra wideband communications system. We identify the antenna parameters important in the context of propagation and channel modelling, and analyse their variation with frequency. The group delay, VSWR, impedance and radiation pattern of omni-directional UWB antennas are studied over a range of frequencies. We propose a spatio-spectral normalisation scheme to isolate the effect of the antenna from the channel, which can be useful in channel measurements. We anticipate that higher SNR can be obtained with the proposed angular compensation technique.

This paper presents a comparative analysis of three Multiple-Input Multiple-Output (MIMO) techniques. It is observed that the spatial MIMO technique slightly out-performs both the polarisation and joint spatial and polarisation MIMO Systems. But, owing to the fact that the orthogonal polarisation MIMO antennas are co-located, the resulting compact set makes the polarisation MIMO technique attractive for future designs of high-speed high-capacity mobile communications systems. © 2004 IEEE.

Relatively simple approximate formulas are proposed for describing the transmission characteristics of HTS (high temperature superconductor) microstrip resonator structures with a monocrystalline ferrite component and for describing the microwave losses in such structures caused by magnetostatic waves. It is shown that excitation of magnetostatic waves leads to a decrease of the quality factor and to a "distortion" of the transmission characteristics of the structure.

Currently, there is much research activity in the area of satellite on-board signal processing. This interest stems from the potential to use the space segment as more than just a transponder. On-board processing would improve the overall link budget and allow such features as signal re-routing to be exploited. Typically, most multiple access schemes to the satellite use either time or frequency division techniques. However, spread spectrum multiple access techniques are gaining in popularity and can, in certain instances, provide a viable alternative to the above methods. As with any spread spectrum system, the implementation of the correlator dictates, to an extent, the overall system performance. This paper describes a novel implementation of the correlation function which would be suited to space payloads and in addition considers the various design constraints and benefits. It then reports on the experimental results obtained using a prototype device operating in the C-band region with a spreading code rate of up to 500 Mbit/s. Implementation of the design in high temperature superconducting (HTS) technology is then discussed and results for an HTS delay line fabricated on a 50 mm diameter wafer presented. >

A novel approach to the evaluation of bit error rate performance for directive optical wireless links, which takes into account shot noise, fluorescent lighting and intersymbol interference (ISI) is presented. The evaluation technique is based on nonclassical Gauss Quadrature Rules, with the fluorescent light and the ISI from high pass filtering being modeled mathematically. As an application of this method the BER performance of NRZ-OOK and Manchester modulation has been evaluated and numerical results are presented. It is shown that Manchester modulation is quite robust in combating fluorescent lighting interference and its performance is not critical on the selection of the HPF cutoff frequency.

Currently, there is considerable interest in infrared as an alternative medium to radio for indoor wireless communications, particularly for computer networking applications. This paper describes a novel IR-LAN link topology that employs an optical spatial-angular mapping technique which minimizes link losses and facilitates high link rates, whilst maintaining a useful coverage area. Blocking probability effects and overall system performance are examined in terms of aggregate transmission rates.

The effects of planning parameter uncertainties in designing a cellular network are presented. Potential errors are quantified and consequences in cellular planning are shown. The analytical model employed is based upon novel general expressions, derived for the exact calculation of cochannel interference probability for nonstationary terminals in combined short and long term fading conditions. In particular the required quality of information about the propagation environment, for service quality and cell capacity estimation, is analyzed.

A strategy leading to development of tunable microwave planar devices is described. The characterisation of composite material systems is outlined and initial results are reported. Material systems based on SU-8 epoxy resist offer a viable route to ferrite loaded elements suitable for planar device fabrication.