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Adaptive RF front-end for 4G communication systems and beyond

Final Report Summary - ARTISAN (Adaptive RF front-end for 4G communication systems and beyond)

Emerging wireless cellular systems are needed in order to support densely deployed small cellular cells. Combining operational agility and adaptability with stringent performance requirements is an extremely challenging task for existing RF technologies. Innovations are therefore required that range from basic research through to the development of enabling technologies to create the next generation of adaptive Wireless front-ends. To meet these new technological challenges, the ARTISAN project trained early stage researchers (ESRs) with a set of skills in order to enable them to explore and develop new concepts in the context of real-life industry.

The alliance of Bell Labs, Nokia, Dublin (RoI) and QUB Centre for Wireless Innovation, Belfast (UK) creates an excellent environment for these ESR researchers with QUB acting as the coordinator of ARTISAN project. The main scientific and technological objectives of the ARTISAN research training programme are to investigate, develop and implement innovative architectures, circuit designs and technical solutions for high-performance RF front ends for the next generations of wireless communications systems. The expertise and experience acquired in the course of this programme expanded ESR horizons and enriched their technical capabilities.

The primary objectives of the ARTISAN programme were: (I) develop ESR skill base and expertise in tackling complex technical problems of system-oriented RF technologies and their applications in academic research and industrial design, (ii) investigate, develop and implement innovative technologies and circuit designs for adaptive RF front ends which can address challenges related to future wireless communications.

The research activities primarily focus on four core topics which provide enabling technologies for adaptive RF front-ends:

WP1 multiband transmitter and power amplifier (PA) architectures; WP2 electrically tunable materials and substrates; WP3 adaptive multiband antennas with tailored radiation characteristics; WP4 passive intermodulation (PIM) mitigation in tunable components and devices.

All ESRs started their projects with extensive literature reviews in order to familiarise themselves with the state-of-the art in their research area and to expand their background knowledge. Major progress made by ESRs in their research activities included;

WP1: New types of Third- and Fifth-Harmonic Peaking Class-EF power amplifiers were developed and published. A new generic analysis method of the Doherty power amplifier (DPA) was presented and verified through physically realisation of specified broadband designs, 2.1-2.7 GHz. In addition, a study on the effect of the second harmonic components on the power amplifier inter-modulation products was conducted. Additionally the research work extended state of the art by significantly improving the efficiency of the Fifth-Harmonic Peaking Class-EF power amplifier through the use of fundamental load pulling. This novel approach was then employed in a new type of Doherty power amplifier architecture, wherein the Class-EF design was employed, for the first time, in both Carrier and Peaking cells of the Doherty amplifier architecture at 2.4 GHz and producing a drain efficiency of 80% for 43-dBm output power. A PhD thesis entitled Doherty Power Amplifier Efficiency and Bandwidth Enhancement Based on a Generic Design Approach has been submitted on these topics, and a number of peer reviewed conference and Journal papers published.
WP2: ElectroChromic (EC) materials were studied in depth and test cells based on liquid crystal (LC) counterparts developed in order to characterize the unknown dielectric properties of the material under RF/micowave excitation. Here new dispersion models were developed and the concept of an equivalent parallel plate waveguide introduced. The corresponding results were compared with full wave simulations as well as similar third party circuits available in the literature. Several test circuits were fabricated and the analytical studies compared to full wave simulation. The corresponding results were compared against experiments conducted on custom fabricated EC test cells in order to obtain the unknown dielectric permittivity. Extensive characterisation was conducted over temperature, the first time that this has been done, and the results reported. An industrial internship and a PhD studentship were created in order to allow this work to be extended beyond the duration of Artisan and a number of peer reviewed conference and Journal papers published.
WP3: An eigenmode decomposition scheme was applied to a four-port monopole-based antenna achieving simultaneously high port decoupling and polarization agility. This included near-field parasitic element decoupling theory based on Z-parameters modelling that, for the first time, include the use of parasitic elements. The mathematical model demonstrated the possibility of performance improvement in terms of port isolation and input impedance. A novel balun structure for dipole-based dual-polarized antennas was developed which allowed the antenna to be fed in a differential manner. This feature generates superior performance with respect to prior art antenna performance in terms of port isolation and polarization purity. A review of mutual coupling effect on channel performance was carried out, and a simulation tool developed which in contrast to the vast majority of those in the open literature can tie operational channel characteristics to far field patterns as well as antenna terminal characteristics and mutual coupling. The results of these activities have been reported in the open literature. An industrial internship and a PhD studentship were created in order to allow this work to be extended beyond the duration of Artisan, and a number of peer reviewed conference and Journal papers published.
WP4: Through behavioural modelling of passive RF circuits with weak distributed and localised nonlinearities it was demonstrated that discontinuities can cause additional PIM products which alter the nonlinear response of the distributed circuit due to the interference with other sources. PIM distortions of analogue and digitally modulated signals in the canonical passive components were simulated. The effect of PIM products on the two-carrier digitally modulated BPSK and QPSK signals in microstrip with distributed nonlinearity was assessed. A Memoryless polynomial behavioural model was retrieved. It was experimentally and theoretically demonstrated that the retrieved model parameters are scalable with line length and applicable to the qualitative estimations of the higher order PIM products. Closed-form polynomial transfer functions were obtained to map an input two-carrier modulated signal onto the output waveform. A PhD based on these investigations was submitted and a number of peer reviewed conference and Journal papers published.

Across WP1-4 around 12 conference papers and 6 journal papers have been published/accepted over the reporting period with a further 2 under review and 6 in preparation and up to three patents applications are under development.

The ARTISAN training programme has equipped each of the ESRs with a set of advanced technical and transferable skills that has enabled them to conduct research and development at the frontiers of microwave wireless communications. Each ESR was enrolled as a fulltime PhD candidate at QUB and was assigned an individual research project. In order to develop ESR abilities, the extensive ARTISAN training programme amalgamated courses on RF techniques covering the advanced topics, complemented by the transferable skill and career development activities. These were open to interested parties at QUB, Bell Labs and outside of the consortium. External personnel provided much the core delivery of these training events. these activities have added considerable value to the ESRs equipping them for future leadership roles in future advanced technology programmes.

Throughout the project the Supervisory Board (SB) consisting of industrial and academic ESR supervisors had oversight of all the various aspects of the project, including project management, planning and delivery of the training, research, dissemination and outreach activities. An external Advisory Board (AB) composed of world leading domain experts was established to critically assess ARTISAN activities in the technical, industrial and commercial aspects of the research. The ARTISAN website (http://www.artisan-itn.net) was maintained throughout by QUB in order to promote the project activities, disseminate outcomes and support multi-site operation. Attendance at national and international conferences, presentations at the Marie Curie Actions dedicated events, University Open Days and public lectures were also captured by the site.

The results of the technical investigations conducted under the ARTISAN project have lead to a number of innovations, in relation to the basic RF through microwave properties of ElectroChromic materials, high performance compact antennas for small base stations, low d.c power consumption amplifiers for greener base stations, and new methods for reduction of electromagnetic intermodulation within passive structures.

Each of these advances has potential impact and direct use in future wireless communication systems that will allow greater connectivity levels than previously possible with attendant socio-economic impact for the EU and the wider societal implications for example in the use of digital media and E-healthcare.

For further details contact Prof V Fusco (v.fusco@qub.ac.uk) or visit (www.artisan-itn.net)