Transmit sector selection with link correlation for SDM/OFDM

More spectrum-efficient techniques are required for wireless communications with a limited amount of bandwidth. Space division multiplexing (SDM) is one of the most promising techniques for achieving more efficient bandwidth utilisation, since it enables the transmission rate over multiple-input multiple-output chennels to be increased by using multiple antennas on both the transmitter and receiver sides. Recently, since the cost of RF transmitters is much higher than that of antennas, there is a growing interest in techniques that use a larger number of antennas than RF transmitters. These methods rely on selecting the optimal transmitter antennas and connecting them to the respective RF. In this case, feedback information (FBI) is required to select the optimal transmitter antenna elements. However, the transmission of FBI through a feedback channel is limited. Moreover, a multiple antenna system requires an antenna separation of five to ten wavelengths to keep the correlation coefficient below 0.7 to achieve diversity gain. In this case, the base station requires a large space to set up multiple antennas. To reduce these problems, a transmit sector antenna selection while considering the link correlation for SDM/OFDM without FBI is proposed and analysed.     

Outage probability of multiuser transmit antenna selection/maximal-ratio combining systems over arbitrary nakagami-m fading channels

The outage probability of multiuser diversity (MUD) in transmit antenna selection/maximal-ratio combining (TAS/MRC) systems is derived as an exact closed expression in independent and identically distributed (i.i.d.) Nakagami-m channels with an integer fading parameter. For a non-integer fading parameter, the exact outage probability is derived as a single infinite series of incomplete Gamma function. At high signal-to-noise ratios (SNRs), the analytical results deduce that the multiuser TAS/MRC systems can achieve a full diversity order equal to the product of the fading parameter, number of users, number of transmit antennas and number of receive antennas. The advantage of the total diversity gain becomes more pronounced on a severe fading channel. The achieved results provide an analytical framework for the assessment of multiuser TAS/MRC systems. All the derived expressions are verified by Monte Carlo simulations.     

Receive maximal-ratio combining with outdated arbitrary transmit antenna selection in Nakagami-m fading

The use of transmit antenna selection algorithms in multiple-antenna systems enables significant reduction in implementation cost and complexity while maintaining acceptable performance. An attractive and quite flexible selection algorithm is to allow the receiver to pick any of the transmit antennas that can satisfy a predetermined performance target. Such an algorithm is referred to as the arbitrarily ordered transmit antenna selection algorithm. However, the effectiveness of transmit antenna selection is decreased by several propagation impairments over the feedback channel from the receiver to the transmitter. Of these impairments, the feedback channel time delay may impose a significant impact on the achieved performance. This paper aims to investigate the impact of this time delay on the performance of receive maximal-ratio combining (MRC) diversity employing the arbitrarily ordered transmit antenna selection algorithm. In order to obtain quantitative measures for this impact, new expressions for various performance criteria are obtained by using the new derived formulas for the probability density function (pdf) and the moment generating function (MGF) of the combined signal-to-noise ratio (SNR). Numerical and simulation results are presented to illustrate the effect of delayed (i.e. outdated) feedback information on the system performance for various transmit antenna selection scenarios.     

Handheld direction of arrival finder with electronically steerable parasitic array radiator using the reactancedomain MUltiple SIgnal Classification algorithm

Reactance-domain multiple signal classification (RD-MUSIC) is a promising scheme for enabling ESPAR (electronically steerable parasitic array radiator) antennas to accurately estimate the DOAs (directions-of-arrival) of radio waves. The ESPAR antenna is a variable directional antenna designed for battery-operated mobile terminals, and is utilised for handheld DOA finder applications as well as for wireless communication systems. We present some experimental results showing that the RD-MUSIC with a conventional calibration obtains a sharp spectrum for accurate DOA estimation, although the ESPAR antenna is equipped with only a single-port output unlike a conventional array antenna. The results also demonstrate that RD-MUSIC works well even when exploiting fewer beam patterns than the number of antenna elements. Moreover, we have verified that a human body hardly generates any degradation of DOA estimation even holding the DOA finder close to the body. The degradation of DOA estimation because of a human body is kept to less than about 6deg, and removing beam patterns towards directions of reflections from the human body is found to be useful in decreasing the errors. Although the reduction in the number of beams results in reducing the total number of estimated waves, it can mitigate the influence of a human body for actual DOA estimation with the handheld DOA finder.     

On chip plasmonic monopole nano-antennas and circuits

Analogues of many radio frequency (RF) antenna designs such as the half-wave dipole and Yagi-Uda have been successfully adapted to the optical frequency regime, opening the door for important advances in biosensing, photodetection, and emitter control. Examples of monopole antennas, however, are conspicuously rare given the element's extensive use in RF applications. Monopole antennas are attractive as they represent an easy to engineer, compact geometry and are well isolated from interference due the ground plane. Typically, however, the need to orient the antenna element perpendicular to a semi-infinite ground plane requires a three-dimensional structure and is incompatible with chip-based fabrication techniques. We propose and demonstrate here for the first time that monopole antenna elements can be fashioned out of single element nanoparticles fabricated in conventional planar geometries by using a small nanorod as a wire reflector. The structure offers a compact geometry and the reflector element provides a measure of isolation analogous to the RF counterpart. This isolation persists in the conductive coupling regime, allowing multiple monopoles to be combined into a single nanoparticle, yet still operate independently. This contrasts with several previous studies that observed dramatic variations in the spectral response of conductively coupled particles. We are able to account for these effects by modeling the system using circuit equations from standard RF antenna theory. Our model accurately describes this behavior as well as the detailed resonance tuning of the structure. As a specific practical application, the monopole resonances are precisely tuned to desired protein absorption bands, thereby enhancing their spectroscopic signatures. Furthermore, the accurate modeling of conductive coupling and demonstrated electronic isolation should be of general interest to the design of complex plasmonic circuits incorporating multiple antennas and other current carrying elements.     

Solution‐Processed Ti3C2Tx MXene Antennas for Radio‐Frequency Communication

Highly integrated, flexible, and ultrathin wireless communication components are in significant demand due to the explosive growth of portable and wearable electronic devices in the fifth‐generation (5G) network era, but only conventional metals meet the requirements for emerging radio‐frequency (RF) devices so far. Here, it is reported on Ti₃C₂T_x MXene microstrip transmission lines with low‐energy attenuation and patch antennas with high‐power radiation at frequencies from 5.6 to 16.4 GHz. The radiation efficiency of a 5.5 µm thick MXene patch antenna manufactured by spray‐coating from aqueous solution reaches 99% at 16.4 GHz, which is about the same as that of a standard 35 µm thick copper patch antenna at about 15% of its thickness and 7% of the copper weight. MXene outperforms all other materials evaluated for patch antennas to date. Moreover, it is demonstrated that an MXene patch antenna array with integrated feeding circuits on a conformal surface has comparable performance with that of a copper antenna array at 28 GHz, which is a target frequency in practical 5G applications. The versatility of MXene antennas in wide frequency ranges coupled with the flexibility, scalability, and ease of solution processing makes MXene promising for integrated RF components in various flexible electronic devices.     

Exploiting multiple-input multiple-output in the personal sphere

An analysis of the propagation characteristics of communication links in the personal sphere is presented. The results of two wideband measurement campaigns in the 5.2 GHz band involving on-body multiple-input multiple-output (MIMO) arrays are reported. In the first measurement, transmission takes place between on-body antennas and a uniform linear array, positioned at close range and with line-of-sight (LOS) propagation conditions. Despite the LOS operation, MIMO is shown to offer a significant increase in the information theoretic capacity of the system when compared with a conventional single-input single-output (SISO) antenna system. Although this appears counter-intuitive to the well-known fact that uncorrelated scattering results in high capacity, two previously less well-known mechanisms are highlighted to explain this result. This analysis reveals the potential use of MIMO in personal area networks. The second measurement campaign uses two on-body MIMO arrays to focus on the subject of body area network propagation. Analysis of the SISO characteristics of the links highlights the influence of the user on the channel. Body shadowing and user motion are shown to lead to multiple rapid changes in the channel characteristics. Again, MIMO is shown to be able to offer performance enhancement. In the measured channels, polarisation diversity is shown to outperform spatial diversity.     

Differentially coherent code acquisition in the MIMO-aided multi-carrier DS-CDMA downlink

Both differentially coherent and non-coherent code acquisition schemes designed for the multiple-input multiple-output (MIMO)-aided multi-carrier (MC)-DS-CDMA downlink are analysed, when communicating over uncorrelated Rayleigh channels. The attainable mean acquisition time (MAT) performance is studied as a function of both the number of multiple transmit/multiple receive antennas and that of the number of subcarriers. It is demonstrated that in contrast to the expectations, when the number of multiple transmit antennas and/or that of the subcarriers is increased in both the differentially coherent and the non-coherent code acquisition scenarios, the achievable MAT deteriorates over the entire signal-to-interference plus noise ratio (SINR) per chip (E_c/I_o) range considered, except for the scenario of single-carrier (SC)-DS-CDMA using P = 2 transmit antennas and R = 1 receive antenna. As expected, the degree of performance degradation depends upon the specific scheme and the E_c/I_o ratio considered, although paradoxically, the correctly synchronised MIMO-aided system is capable of attaining its target bit error ratio performance at reduced SINR values.     

Energy-Efficient Low-Complexity Algorithm in 5G Massive MIMO Systems

Energy efficiency (EE) is a critical design when taking into account circuit power consumption (CPC) in fifth-generation cellular networks. These problems arise because of the increasing number of antennas in massive multiple-input multiple-output (MIMO) systems, attributable to inter-cell interference for channel state information. Apart from that, a higher number of radio frequency (RF) chains at the base station and active users consume more power due to the processing activities in digital-to-analogue converters and power amplifiers. Therefore, antenna selection, user selection, optimal transmission power, and pilot reuse power are important aspects in improving energy efficiency in massive MIMO systems. This work aims to investigate joint antenna selection, optimal transmit power and joint user selection based on deriving the closed-form of the maximal EE, with complete knowledge of large-scale fading with maximum ratio transmission. It also accounts for channel estimation and eliminating pilot contamination as antennas M → ∞. This formulates the optimization problem of joint optimal antenna selection, transmits power allocation and joint user selection to mitigate inter-cell-interference in downlink multi-cell massive MIMO systems under minimized reuse of pilot sequences based on a novel iterative low-complexity algorithm (LCA) for Newton’s methods and Lagrange multipliers. To analyze the precise power consumption, a novel power consumption scheme is proposed for each individual antenna, based on the transmit power amplifier and CPC. Simulation results demonstrate that the maximal EE was achieved using the iterative LCA based on reasonable maximum transmit power, in the case the noise power is less than the received power pilot. The maximum EE was achieved with the desired maximum transmit power threshold by minimizing pilot reuse, in the case the transmit power allocation ρ_d = 40 dBm, and the optimal EE = 71.232 Mb/j.     

Effects of intrinsic radiation ϕ on mismatch factor of three types of small antennas: single-resonance, gradual-transition and cascaded-resonance types

Resonators are traditionally characterised by their quality factor Q, which is inversely proportional to the relative bandwidth. Small antennas are often resonant, so they can be characterised by a Q, except for the fact that the correct quality measure of an antenna is the inverse Q, that is the relative bandwidth, rather than Q. Still, it has become common to study fundamental size limitations of small antennas in terms of a so-called radiation Q (or antenna Q). This study explains how this intrinsic radiation Q relates to: (i) the bandwidth-efficiency product of small single-resonance-type antennas, (ii) the gradual cut-off of spherical waves for wideband gradual-transition-type small antennas and (iii) the number of resonances needed to cover a certain frequency band for wideband cascaded-resonance-type small antennas. The study also introduces one intrinsic radiation Q for basic single TE and TM spherical mode sources, and another for combined TE and TM sources.     

Cross-entropy optimisation of multiple-input multiple-output capacity by transmit antenna selection

Radio channel capacity can be increased dramatically using a multiple-input multiple-output (MIMO) scheme, but at the expense of hardware complexity. An efficient approach for complexity reduction is antenna subset selection at the transmitter and/or receiver. A novel transmit antenna selection algorithm is presented using the cross-entropy optimisation method to maximise channel capacity. In contrast with the existing work, the proposed algorithm guarantees a result to within 99% of the true optimum (i.e. the maximal capacity with selected transmit antennas) with substantially low complexity. The simulation results indicate that the proposed algorithm is independent of the relationship between the selected transmit array size and receive array size. The proposed scheme has the potential to make practical MIMO systems with high performance simpler to implement.     

Virtual multiple input multiple output in multiple high-altitude platform constellations

Multiple-input multiple-output (MIMO) technology, which makes use of the spatial dimension by utilising multiple antennas at the transmitter and receiver, has proved an efficient solution for providing higher data throughput and/or link reliability in wireless systems. In this study, the authors investigate a virtual MIMO (V-MIMO) technique, based on a constellation of multiple high-altitude platforms (HAPs), providing broadband wireless access to high-speed trains under predominantly line-of-sight (LOS) propagation conditions. They analyse the performance of transmit diversity based on space?time block coding (STBC), in particular Alamouti and extended Alamouti schemes, using fixed wide-lobe receive antennas, and compare it to the reference receive diversity scheme based on best HAP selection that requires highly directional and steerable antennas. Simulation results for different diversity schemes are based on the calculation of carrier to interference-plus-noise ratio (CINR) along representative railway lines in a realistic terrain configuration. The CINR levels obtained are mapped onto transmission modes specified by standards developed for two wireless systems operating at distinct frequency bands, the DVB-S2 assumed for operation in mm-wave bands and the IEEE 802.16e (mobile WiMAX) for operation at 3.5 GHz. Simulation results show that the use of transmit diversity schemes in a multiple HAP constellation decreases the link outage and also increases the average spectral efficiency, but requires a marked increase in transmit power, especially for the system operating in mm-wave bands.     

Mean effective gain of antennas in a wireless channel

The mean effective gain (MEG) is one of the most important parameters for the characterisation of antennas in wireless channels. An analysis of some fundamental properties of the MEG is provided and corresponding physical interpretations are given. Three points are analysed in detail: (i) closed-form expressions for MEG in a mixed environment with both stochastic and deterministic components are provided, showing that the MEG can be written as a sum of gains for the deterministic and stochastic components, (ii) it is shown that under some assumptions, the propagation channel and the antenna are equivalent in the sense that the impact of the channel cross-polarisation ratio (XPR) and the antenna effective cross-polar discrimination on the MEG are symmetrical, (iii) based on the fact that MEG depends on random variables, such as the XPR and antenna rotations because of user?s movements, the average, the minimum and maximum MEG of antennas are defined, respectively. Finally, the maximum effective gain of antennas is derived and shown that it is bounded by 4phrad, where hrad is the radiation efficiency of the antenna.     

Adaptive opportunistic fair scheduling in power-controlled code division multiple access systems

The authors treat the multiuser scheduling problem for practical power-controlled code division multiple access (CDMA) systems under the opportunistic fair scheduling (OFS) framework. OFS is an important technique in wireless networks to achieve fair and efficient resource allocation. Power control is an effective resource management technique in CDMA systems. Given a certain user subset, the optimal power control scheme can be derived. Then the multiuser scheduling problem refers to the optimal user subset selection at each scheduling interval to maximise certain metric subject to some specific physical-layer constraints. The authors propose discrete stochastic approximation algorithms to adaptively select the user subset to maximise the instantaneous total throughput or a general utility. Both uplink and downlink scenarios are considered. They also consider the time-varying channels where the algorithm can track the time-varying optimal user subset. Simulation results to show the performance of the proposed algorithms in terms of the throughput/ utility maximisation, the fairness, the fast convergence and the tracking capability in time-varying environments are presented.     

Fast antenna subset selection algorithms for multiple-input multiple-output relay systems

The antenna subset selection technique balances the performance and hardware cost in the multipleinput multiple-output (MIMO) systems, and the problems on the antenna selection in MIMO relay systems have not been fully solved. This paper considers antenna selection on amplify-and-forward (AF) and decode-andforward (DF) MIMO relay systems to maximise capacity. Since the optimal antenna selection algorithm has high complexity, two fast algorithms are proposed. The selection criterion of the algorithm for AF relay is to maximise a lower bound of the capacity, but not the exact capacity. This criterion reduces algorithmic complexity. The algorithm for DF relay is an extension of an existing antenna subset selection algorithm for one-hop MIMO systems. The authors show the derivations of the algorithms in detail, and analyse their complexity in terms of numbers of complex multiplications. Simulation results show that the proposed algorithms for both cases achieve comparable performance to the optimal algorithm under various conditions, and have decreased complexity.     

A Novel Indirect Method to Determine the Radiation Impedance of a Handheld Antenna Structure

A novel indirect measurement technique is developed in which the input impedance of an unknown antenna structure is determined by measuring the change in the measured input impedance of a second probe antenna, while the unknown antenna is loaded at its input by a set of known impedances. The feasibility of the approach is explored using method-of-moment boundary element simulations of the interaction between two dipole antennas in free space. Examples of the measurement of the helical antenna of a terrestrial trunked radio (TETRA) handheld radio and a small portable device with an electrically small antenna are presented. The significant advantage of the technique in the accurate measurement of electrically small antennas is confirmed.      

Intrinsic measure of diversity gains in generalised distributed antenna systems with cooperative users

The diversity gains achievable in the generalised distributed antenna system with cooperative users (GDAS-CU) are considered. A GDAS-CU is comprised of M largely separated access points (APs) at one side of the link, and N geographically closed user terminals (UTs) at the other side. The UTs are collaborating together to enhance the system performance, where an idealised message sharing among the UTs is assumed. First, geometry-based network models are proposed to describe the topology of a GDAS-CU. The mean crosscorrelation coefficients of signals received from non-collocated APs and UTs are calculated based on the network topology and the correlation models derived from the empirical data. The analysis is also extendable to more general scenarios where the APs are placed in a clustered form due to the constraints of street layout or building structure. Subsequently, a generalised signal attenuation model derived from several stochastic ray-tracing-based pathloss models is applied to describe the power-decaying pattern in urban builtup areas, where the GDAS-CU may be deployed. Armed with the cross-correlation and pathloss model preliminaries, an intrinsic measure of cooperative diversity obtainable from a GDAS-CU is then derived, which is the number of independent fading channels that can be averaged over to detect symbols. The proposed analytical framework would provide critical insight into the degree of possible performance improvement when combining multiple copies of the received signal in such systems.     

Rainbow radiating single-crystal Ag nanowire nanoantenna

Optical antennas interface an object with optical radiation and boost the absorption and emission of light by the objects through the antenna modes. It has been much desired to enhance both excitation and emission processes of the quantum emitters as well as to interface multiwavelength channels for many nano-optical applications. Here we report the experimental implementation of an optical antenna operating in the full visible range via surface plasmon currents induced in a defect-free single-crystalline Ag nanowire (NW). With its atomically flat surface, the long Ag NW reliably establishes multiple plasmonic resonances and produces a unique rainbow antenna radiation in the Fresnel region. Detailed antenna radiation properties, such as radiating near-field patterns and polarization states, were experimentally examined and precisely analyzed by numerical simulations and antenna theory. The multiresonant Ag NW nanoantenna will find superb applications in nano-optical spectroscopy, high-resolution nanoimaging, photovoltaics, and nonlinear signal conversion.     

Semi-deterministic channel model for MIMO systems Part-II: results

This article presents the results of applying `single interaction scattering reflecting' (SISTER) model to different multiple-input multiple-output (MIMO) channels. The SISTER model is a semi-deterministic channel model which is described in details in Part-I of this article. MIMO performance is investigated for outdoor and indoor environments considering both line of sight (LOS) and non-line of sight (NLOS) cases. Different results are obtained by changing antenna array parameters such as spacing and number of elements and antenna half power beam width (HPBW), as well as environment parameters such as ground`s and walls` conductivity, and number and distribution of scatterers. In addition, space and angle diversity methods are compared for most of the studied cases. As the results show for most scenarios, the MIMO performance may be further improved by using multi-beams (angle diversity) rather than multi-antenna elements (space diversity).     

Non-line-of-sight on-body ultra wideband (1?6 GHz) channel characterisation using different antenna polarisations

Ultra wideband (UWB) is a promising technology for wireless body area networks (WBANs). The authors investigate the wave propagation in the frequency range of 1-6-GHz for non-line-of-sight (NLOS) channels from the front to back of a human body by considering different wave polarisations: perpendicular and tangential with respect to the body. Time domain electromagnetic (EM) simulation using the anatomical model of a human body assuming frequency-dependent tissue properties is conducted. Radiographs of the energy density and delay spread of the NLOS channels are provided. The energy fading of UWB signals for the on-body channel suggests that the optimisation of the receiving antenna position for efficient signal transmission can be useful. A significant enhancement of the on-body communication link is identified by exploiting the transceiver antennas at the perpendicular to body polarisations. The impacts of body environment reflections on the channel characteristics of different polarisations are studied. The results show that the perpendicular polarisation channel characteristics are less influenced by the body environment reflections and are more stable regarding energy density and delay spread than tangential polarisation. The frequency-dependent propagation loss of on-body UWB channels is derived, where the efficient communication bandwidths for both perpendicular and tangential wave polarisations are given.