Design and analysis approaches to compact directional antennas for cognitive radio

Cognitive radio (CR) ist eine neuartige Technologie, die es erlaubt die spektralen Funkressourcen intelligent und effektiv zu nutzen. Jüngste Messkampagnen beweisen, dass die zugewiesenen Frequenzbänder der lizenzierenden Benutzer nicht effizient genutzt werden. Außerdem benötigen moderne Funktechnologien mehr Spektrum, um wachsenden DatenÃijbertragungsrate- und Quality-of- Service-Anforderungen gerecht zu werden. Cognitive radio erlaubt die Sekundärnutzung von nicht vollständig genutzten Frequenzbereichen, wobei die Primärnutzung durch Lizenzinhaber nicht gestört werden darf.Seit der ersten Erwähnung von Cognitive radio im Jahr 1999 lag der Fokus auf Frequenz- bzw. spektralen Ressourcen. Allerdings ist dies für die Anforderungen von zukunftsweisenden Funktechnologien nicht ausreichend. Eine Kombination aus der Betrachtung von Frequenz, Raum/Richtung und Zeit ermöglicht eine noch effizientere Nutzung des Funkspektrums. Dabei kommen Technologien wie beispielsweise die Schätzung der Empfangsrichtung und die Interferenzunterdrückung zum Einsatz. In dieser Arbeit werden Methoden des Entwurfs und der Analyse von direktiven Multibandantennen zur Bereitstellung richtungs- und frequenzabhängiger Funktionalitäten vorgestellt. Dies geschieht mit Hilfe orthogonal angeordneter Multibandantennen und mit kompakten Multibandantennenarrays.Die entworfenen Antennen wurden mit Hilfe von Simulationen, Messungen und durch die Emulation von channel-sounder-Messungen analysiert. Als Referenzantennensystem dient eine konzentrische Anordnung aus Monopolantennenarrays und Absorberplatten zwischen den Antennenelementen. Dieses Referenzantennensystem wurde für die Durchführung von Machbarkeitsstudien in Messkampagnen eingesetzt. Mit einem aus neun Elementen bestehenden Array können entsprechend neun Freiheitsgrade erzielt werden. Diese setzen sich aus drei wählbaren Frequenzbändern (GSM 900 MHz, GSM 1800 MHz, und IEEE 802.11b/g) und drei Richtungen pro Frequenzband zusammen. Das Referenzantennensystem ist in der Lage, Frequenzbänder und Signaleinfallsrichtungen mit einem Signal-zu-Interferenz-Verhältnis von 20 dB unter den reflexionsarmen Bedingungen in einer Absorberkammer aufzulösen. Für das Band des GSM 1800 wurde eine Feldmessung in der Umgebung von vier Basisstationen durchgeführt.Das spektrale Sensing erfolgte nach dem Prinzip der Leistungsdetektion. Möglichkeiten zur richtungsselektiven Kommunikation konnten in einer Vielzahl von GSM-Kanälen für ca. 50 % der Beobachtungszeit detektiert werden. Durch die Reduzierung der Zwischenelementabstände konnte eine kompakte Antenne des konzentrischen Antennenarrays konstruiert werden. Dies führt zu einer gegenseitigen Verkopplung der Antennenelemente und damit zu einer Beeinflussung der Stromverteilung und schließlich der Antennenrichtdiagramme. Um diese Effekte zu minimieren, wurde ein multibandfähiges Anpassungs- und Entkopplungsnetzwerk entworfen, welches die Entkopplung und Anpassung der Antennenelemente mit Modenspezifischen Lasten ermöglicht. Die Rekonfigurierbarkeit in jedem Frequenzband wird durch kapazitive Justierung mit Hilfe von Varaktordioden erreicht. Das multibandfähige Anpassungs- und Entkopplungsnetzwerk und das rekonfigurierbare Netzwerk für GSM 900 wurden auf einer Leiterplatte realisiert und im Hinblick auf Entkopplung, Anpassung, und Strahlungsdiagramme der Ports getestet. Die 10 dB-Bandbreite für Anpassung und Entkopplung der statischen Netzwerke ist ca. 30 MHz. Das rekonfigurierbare Netzwerk stellt eine Bandbreite von mehr als 100 MHz bereit, die mit insgesamt 5 Stufen erreicht wird.Die Richtdiagramme waren in verschiedenen Richtungen mit einem Korrelationskoeffizient kleiner als 30 % orthogonal, und in verschiedenen Frequenzbereichen mit einer Korrelation besser als 70 % selbstähnlich. Schließlich wurde das Verhalten von Richtantennen in heterogenen Ausbreitungsszenarien durch Simulation und Emulation untersucht. Dies beinhalteten Kanalmodelle für Simulation von statischen Szenarien und vorhandenen channel-sounder-Messungen zur Emulation der Mobilitätsszenarien. Verschiedene gemessene und analytisch bestimmte Richtdiagramme wurden verwendet, um die Verfügbarkeit von richtungsabhängigen Kommunikationsressourcen für Cognitive radio zu untersuchen.Simulationen mit analytischen Richtdiagrammen von uniform zirkularer Arrays zeigten, dass die Empfangssignalstärke über die Einfallsrichtungen proportional zum Nebenkeulenpegel der direktiven Richtdiagramme ist. Ein Nebenkeulenpegelvon 20 dB eines Antennenarrays mit 6 Elementen wurde als Optimum gefunden. Die richtungsabhängigen Sendemöglichkeiten von ca. 50 % wurden mit einem Sensing-Schwellwert kleiner -120 dB für mobile Szenarien ermittelt. Die Verfügbarkeit richtungsabhängiger Ressourcen ist abhängig von dem Schwellwert des gewählten Algorithmus für das spektrale Sensing.Zusammenfassend lässt sich sagen, dass sorgfältig konstruierte direktive Antennen die Existenz richtungsabhängiger Ressourcen für Cognitive radio aufspüren können. Anpassungs- und Entkopplungsnetzwerke für kompakte Antennenarrays können mittels kommerziell verfügbaren konzentrierten Bauelementen mit engen Toleranzen hergestellt werden. Die Rekonfigurierbarkeit solcher Netzwerke kann mittels Varaktordioden erreicht werden. Richtungsabhängige Kommunikation ist mit den vorgeschlagenen Antennen sowohl in statischen als auch mobilen Szenarien möglich.Cognitive radio is an emerging radio technology, promising intelligent and effective use of spectrum resources. State-of-the-art measurement campaigns show that the allocated spectrum is not efficiently used by the licensed users. On the other hand, future radio technologies require more spectrum to meet high capacity and quality of service requirements. Cognitive radio proposes secondary usage of the under-utilised spectrum resources while preserving the access-rights of the licensed (primary) users.Since the introduction of cognitive radio, in 1999, the focus of cognitive radio communications has been on frequency resources. However, frequency resourcealone may not be sufficient to fulfil the needs of future communication systems. A combination of frequency, space/direction, and time can ensure a more efficient use of the spectrum, by employing techniques like direction-of-arrival estimation, interference mitigation, etcetera. Approaches to design and analyse compact multi-band directional antennas, required to support directional as well as frequency resources, are proposed in this thesis. Design of such antennas was accomplished with orthogonal arrangement of multi-band antennas, and with compact multi-band antenna arrays. Analysis of directional antennas was carried out with simulations, measurement campaigns, and emulation of channel sounder measurements. A concentric arrangement of monopole antenna arrays was used as a reference antenna system, where directional patterns were obtained using metallic/absorber walls between antenna elements. This reference antenna system was used to perform proof-of-principle measurement campaigns. With an antenna array of nine elements, nine degrees-of-freedom (frequency-directional resources) were obtained at the antenna ports. These consist of three selectable frequency bands, namely GSM 900 MHz, GSM 1800 MHz, and IEEE 802.11b/g, and three directions per frequency band. The reference antenna system was capable of separating frequency and directions with a signal-to-interference-ratio of 20 dB, inside an anechoic chamber. An outdoor measurement of such an antenna system was carried out for GSM 1800 MHz, at a location surrounded by four base-stations. Power detection was used as the spectrum sensing algorithm. The opportunity to communicate in a certain direction using the occupied frequency channels was observed for about 50 % of the sensing time for various GSM channels.This concentric arrangement was made compact by reducing the inter-element spacing. The reduction of inter-element spacing results in mutual coupling between the antenna elements, which disturbs the current distribution and hence the beam patterns of the antenna arrays. To reduce this negative effect, a multiband decoupling and matching network was designed to mitigate the element coupling and to match the elements with mode-specific loads. Reconfigurable networks were designed with the help of the capacitive tuning of varactor diodes. The multi-band decoupling and matching network, and the reconfigurable network for GSM 900 MHz were manufactured on a printed circuit board, and tested in terms of decoupling, matching, and resulting port beam patterns. The 10 dB bandwidth for matching and decoupling by the fixed network, for compact antenna arrangement with an inter-element spacing of lambda/6, was about 30 MHz. Reconfigurable network provided a bandwidth above 100 MHz, achievable with five reconfigurable states. The patterns were orthogonal in different directions with correlation coefficients less than 30 % and self-similar at different frequency bands with a correlation better than 70 %.Finally, the behaviour of directional antennas under heterogeneous propagation scenarios was studied using simulation and emulation. This involved channel models for statistical simulation of static scenarios, and existing channel sounder measurements for emulation of mobility scenarios. Various measured and analytical beam patterns were used to study the availability of directional communications resources for cognitive radio. Simulations with analytical patterns of uniform circular arrays indicated that the received signal strength is directly proportional to the side-lobe level of the directional patterns. A side-lobe level of 20 dB, achievable with an array of 6 elements, was found to be optimum. The opportunity to communicate in certain directions using the occupied frequency channels (directional opportunity) was obtained for 50% of the total snapshots for a threshold level lower than -120 dB, in mobility scenarios. The availability of directional resources was dependent on the threshold level chosen for the spectrum sensing algorithm.It is concluded that well-designed directional antennas can identify the existence of directional resources for cognitive radio communications. Exploitation of unexplored antenna strategies for cognitive radio empowers a cognitive node with significant additional degrees-of-freedom. However, angular distribution of multipath, mobility of primary or secondary user, and speed of detection influence the usability of directional resources for cognitive radio. Decoupling and matching networks for compact arrays can be fabricated with off-the-shelf lumped elements with tight tolerances. Such networks can be made reconfigurable using varactor diodes. The work presented in the thesis is expected to facilitate the design of future directional antennas for cognitive radios resulting in more efficient utilisation of the spectrum

Balanced antennas for mobile handset applications. Simulation and Measurement of Balanced Antennas for Mobile Handsets, investigating Specific Absorption Rate when operated near the human body, and a Coplanar Waveguide alternative to the Balanced Feed.

The main objectives of this research are to investigate and design low profile antennas for mobile handsets applications using the balanced concept. These antennas are considered to cover a wide range of wireless standards such as: DCS (1710¿1880 MHz), PCS (1850¿1990 MHz), UMTS (1920¿2170 MHz), WLAN (2400¿2500 MHz and 5000 ¿ 5800 MHz) and UWB frequency bands. Various antennas are implemented based on built-in planar dipole with a folded arm structure. The performance of several designed antennas in terms of input return loss, radiation patterns, radiation efficiency and power gain are presented and several remarkable results are obtained. The measurements confirm the theoretical design concept and show reasonable agreement with computations. The stability performance of the proposed antenna is also evaluated by analysing the current distribution on the mobile phone ground plane. The specific absorption rate (SAR) performance of the antenna is also studied experimentally by measuring antenna near field exposure. The measurement results are correlated with the calculated ones. A new dual-band balanced antenna using coplanar waveguide structure is also proposed, discussed and tested; this is intended to eliminate the balanced feed network. The predicted and measured results show good agreement, confirming good impedance bandwidth characteristics and excellent dual-band performance. In addition, a hybrid method to model the human body interaction with a dual band balanced antenna structure covering the 2.4 GHz and 5.2 GHz bands is presented. Results for several test cases of antenna locations on the body are presented and discussed. The near and far fields were incorporated to provide a full understanding of the impact on human tissue. The cumulative distribution function of the radiation efficiency and absorbed power are also evaluated.UK Engineering and Physical Sciences Research Council (EPSRC

Recent Advances in Antenna Design for 5G Heterogeneous Networks

The aim of this book is to highlight up to date exploited technologies and approaches in terms of antenna designs and requirements. In this regard, this book targets a broad range of subjects, including the microstrip antenna and the dipole and printed monopole antenna. The varieties of antenna designs, along with several different approaches to improve their overall performance, have given this book a great value, in which makes this book is deemed as a good reference for practicing engineers and under/postgraduate students working in this field. The key technology trends in antenna design as part of the mobile communication evolution have mainly focused on multiband, wideband, and MIMO antennas, and all have been clearly presented, studied and implemented within this book. The forthcoming 5G systems consider a truly mobile multimedia platform that constitutes a converged networking arena that not only includes legacy heterogeneous mobile networks but advanced radio interfaces and the possibility to operate at mm wave frequencies to capitalize on the large swathes of available bandwidth. This provides the impetus for a new breed of antenna design that, in principle, should be multimode in nature, energy efficient, and, above all, able to operate at the mm wave band, placing new design drivers on the antenna design. Thus, this book proposes to investigate advanced 5G antennas for heterogeneous applications that can operate in the range of 5G spectrums and to meet the essential requirements of 5G systems such as low latency, large bandwidth, and high gains and efficiencies

Implantable antennas for biomedical applications

Recently, the interest in implantable devices for biomedical telemetry has significantly increased. Amongst the different components of the implantable device, the antenna plays the most significant role in the wireless data transmission. However, the human body around the antenna alters its overall characteristics and absorbs most of its radiation. Therefore, this thesis is mainly focused on improving the antenna characteristics (bandwidth and radiation efficiency) to overcome the human body effect and investigating new structures that reduce the power absorption by the human body tissues. A novel antenna design methodology is developed and used to design new flexible implantable antennas of much lighter weight, larger radiation efficiency, and wider bandwidth than existing embedded antennas. These antennas work for multiple ((401-406 MHz) MedRadio, 433 MHz and 2.45 GHz ISM) bands which satisfy the requirements of low power consumption and wireless power transfer. This has been combined with thorough investigations of the antenna performance in the anatomical human body. New effective evaluation parameters such as the antenna orientation are investigated for the first time. New structures inspired by complementary and multiple split ring resonators (CSRRs and MSRRs) are designed. The structures are found to reduce the electric near field and hence the absorbed power which increases the radiated power accordingly. This new promising function of metamaterial based structures for implantable applications is investigated for the first time. The path loss (between pacemaker and glucose monitoring implantable antennas inside the anatomical body model) and (between an implantable and external antennas for a wireless power channel at 433 MHz) are estimated. Moreover, the optimum antenna type for on-in body communication is investigated. Loop antennas are found to outperform patch antennas in close proximity to the human body

Wearable metamaterial dual-polarized high isolation UWB MIMO Vivaldi antenna for 5G and satellite communications

A low-profile Multiple Input Multiple Output (MIMO) antenna showing dual polarization, low mutual coupling, and acceptable diversity gain is presented by this paper. The antenna introduces the requirements of fifth generation (5G) and the satellite communications. A horizontally (4.8–31 GHz) and vertically polarized (7.6–37 GHz) modified antipodal Vivaldi antennas are simulated, fabricated, and integrated, and then their characteristics are examined. An ultra-wideband (UWB) at working bandwidths of 3.7–3.85 GHz and 5–40 GHz are achieved. Low mutual coupling of less than −22 dB is achieved after loading the antenna with cross-curves, staircase meander line, and integration of the metamaterial elements. The antennas are designed on a denim textile substrate with = 1.4 and h= 0.5 mm. A conductive textile called ShieldIt is utilized as conductor with conductivity of 1.8 × 10⁴. After optimizing the proposed UWB-MIMO antenna’s characteristics, it is increased to four elements positioned at the four corners of a denim textile substrate to be employed as a UWB-MIMO antenna for handset communications, 5G, Ka and Ku band, and satellite communications (X-band). The proposed eight port UWB-MIMO antenna has a maximum gain of 10.7 dBi, 98% radiation efficiency, less than 0.01 ECC, and acceptable diversity gain. Afterwards, the eight-ports antenna performance is examined on a simulated real voxel hand and chest. Then, it is evaluated and compared on physical hand and chest of body. Evidently, the simulated and measured results show good agreement between them. The proposed UWB-MIMO antenna offers a compact and flexible design, which is suitably wearable for 5G and satellite communications applications