Active and Reconfigurable Antennas and Microwave Devices

Radio Frequency (RF) communications have existed for centuries, and the frequency spectrum for these communications is an increasingly congested resource, particularly below 30GHz. As a result of this, researchers and designers in both industry and academia are looking at multiple ways to manage this congestion, whilst still maintaining the high performance demanded by consumers. Different approaches are being taken to find novel methods around the spectrum congestion. These include reconfigurable systems such as those used in Cognitive Radio, which locate available spectrum space, and reconfigure the system to operate at the available frequencies. Devices, which can intelligently shift parameters of their operation in this way, to provide optimal performance, are actively being researched.

Another approach is to move away from the congested areas of the spectrum, into the millimetre wave, Extremely High Frequency (EHF) bands. These bands not only offer less congested spectrum space, but also the opportunity for significantly more compact systems due to the short wavelengths. However, the short wavelengths present a challenge to designers, due to the increased absorption and reduced propagation range at these high frequencies. The interposition of objects, particularly those that are not stationary, is one of the predominant problems in communication systems that operate at millimetre wave frequencies. To add to this, the increased absorption by most building materials leads to multipath interference. To alleviate the propagation problems, research is being carried out into reconfigurable systems which have high gain, and are able to intelligently alter the propagation direction around obstacles.

Reconfigurable Designs

Reconfigurable antennas have been in use for a number of decades, first being developed in the early 1930s. Throughout the 20th Century, designs focused around large antennas, using a range of reconfiguration techniques. Both mechanical and electronic based reconfiguration techniques were explored and this is still the focus of a lot of research. The rise in the demand for smaller, ‘connected’ devices from the 1990s onwards has led to the development of planar reconfigurable designs. Many of these designs focus on the use of semiconductor devices such as Field Effect Transistor (FET) switches and PIN diode switches. Others use an alternative switching technique based on Micro Electro-Mechanical System (MEMS) switches. Mechanical reconfiguration also remains, but research is moving towards smart materials to provide the reconfiguration. Examples include Ferro-electric materials and Piezo-electric materials, which change their properties based on an externally applied excitation.

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Millimetre Wave reconfiguration

Millimetre wave communication is gaining a large amount of attention, from both Universities and Industry. One of the general reasons that the spectrum from 30GHz to 300GHz is attractive is due to the small size a transceiver system can have, due to the short wavelengths. Adding to this, the availability of spectrum space in this band is desirable. The focus of antenna and system reconfiguration in the millimetre wave bands moves towards the shaping of the beam and the ability to alter the direction of the main beam. Beamforming, also known as spatial filtering, can offer significant improvement over a traditional omnidirectional single element antenna. A beam can be formed by a linear array of antenna elements in any direction between broadside and endfire, by progressively phasing the elements of the array. This can be further developed into beam-steerable systems, which can direct the beam over a range of angles.

Applying Reconfigurable Millimetre wave technology

Short range, High data rate communications

Specific bands in the millimetre wave regions are gaining large amounts of interest. In the United Kingdom, the frequency band between 57.1 and 63.9 GHz has been allocated for Fixed Wireless Systems (FWS). Since nearly 7GHz has been reserved, this band will be able to support multi-gbps communications such as those implemented in WirelessHD and 802.11ad (WiGig) protocols. Similar frequency allocations have taken place internationally, meaning that this band will be heavily exploited in the near future. Although the wide bandwidth available is useful, system operation in this band requires the use of antenna arrays and beamforming systems to provide enough propagation range for the system to be useful. Both switched beam and adaptive arrays have been designed for use at 60GHz. This can be extended to the use of adaptive arrays allowing multiple individual users to simultaneously make use of the high data rate systems. However, there is still scope for improvement in terms of power consumption, fabrication technology and overall array performance.

Millimetre-wave imaging

Another application of reconfigurable millimetre wave technology is that of imaging systems. Various applications for this technology exist, including medical, defence, security and safety purposes. The frequency of operation depends on the application, but the typical frequencies used are 35GHz, 94GHz, 140GHz and 220 GHz. These frequencies are used due to the comparably lower attenuation by air and moisture in these bands when compared to the other millimetre wave frequencies. This makes these frequencies applicable to imaging systems used in commercial aircraft for landing in low visibility. Similarly, since these frequencies do not cause ionisation, they can be used for security imaging systems at airports to detect potentially malicious objects.

Whitespace Machine Communication Lab

Some of the research in this theme is organised under the Whitespace Machine Communication (WMC) Lab led by Dr Yue Gao. The lab focuses on developing theoretical research into practice in the interdisciplinary area among antennas, signal processing and white space spectrum for Cyber Physical System (CPS), Machine-to-Machine (M2M) and Internet-of-Things (IoT) applications. Areas of research include:

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