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Superconducting systems for communications

Objective

SUCOMS intends to exploit the scientific and technology base in High Temperature Superconductivity (HTS) to demonstrate new approaches to component and sub-system designs as required for UMTS Third Generation Mobile Telecommunication Systems. Superconducting sub-systems in a variety of architectures will be produced and trialed to demonstrate the benefits to mobile communication systems of the future.
In this context HTS offers:
- Selectivity: For a given maximum passband insertion loss (e.g. 0.3dB) much steeper filter skirts can be achieved, reducing interference.
- Size Reduction: A number of planar HTS filters can be integrated with one cooler leading to a reduction in size (including the cooler volume) compared to conventional waveguide technology.
- Sensitivity: Decreased surface resistance directly translates into higher conductor quality factors and reduced insertion loss of bandpass filters.

Technical Approach
Due to the low r.f. losses in High Temperature Superconductors, bandpass filters can be build in planar technology with a high degree of miniaturisation, high selectivity and low insertion loss. The surface resistance of HTS at 2GHz, 60K is about 3 orders of magnitude lower than copper. In addition, the loss tangent of the LaAlO3 substrate rapidly decreases with decreasing temperature. SUCOMS has designed and manufactured bandpass filters and duplexers using two HTS materials: GdBa2Cu3O7- and YBa2Cu3O7- . These thin film materials were grown on 2" diameter single crystal LaAlO3 and MgO substrates.
By cooling the LNAs, the HTS bandpass filters and duplexers, the receiver sub-system developed by SUCOMS will achieve an overall improved receiver noise figure. Cooling is achieved by a newly developed linear Stirling cooler operating two opposing cold fingers.

Summary of Trials
During an initial study phase of the project, "System Architecture and Specification", SUCOMS identified three key demonstrators to be tested in trials. These are specified as Actions 1, 2 and 3.

- To demonstrate increased sensitivity and selectivity in a basestation (BTS) receiver.
- To demonstrate improved blocking characteristics in a BTS receiver.
- To demonstrate a full transceiver with reduced power input and size.
Description of trial platforms

The demonstrators use the DCS 1800 system as a test vehicle. Action 1 comprises an HTS front end receiver with six pre-select bandpass filters, six cooled LNAs and three HTS duplexers containing all of the equipment for a 3-sector, 2-antenna-per-sector DCS-1800 BTS (see figure 1). Gains in sensitivity and selectivity are achieved due to the steeper filter skirts and lower passband insertion loss in the HTS filters and the reduced noise figure in the cooled LNAs. The frequency band of receiver is: 1770 - 1785 MHz.
Action 2 is an extension of Action 1 with down-conversion. It incorporates a cooled mixer and a low-noise local oscillator (both microstrip and disk resonator designs are investigated). Due to the reduced noise of the oscillator the blocking characteristics of the BTS receiver to the so-called "in-band signals" will be dramatically improved. This will allow the HTS-basestations to withstand stronger spurious signals closer to the channels in use. Action 3 comprises a full basestation r.f. transceiver sub-system with multiple HTS components mounted on top of the mast. In contrast to conventional transmitter-combiners using cavity resonators or hybrid couplers, the dimensions of the HTS-transceiver sub-system are suitable for tower mounting. Due to the low insertion loss of the transmitter-combiner, and because the feeder cable with its associated losses can be eliminated, the output of the power amplifier can be reduced by a factor of approximately 10 without affecting the BTS range. The frequency band of the transmitter will be: 1865 - 1880 MHz.
In addition SUCOMS will demonstrate the capability and reliability of newly developed, compact, cost-efficient cryo-packages during the field trials. These packages are required for all three actions and comprise maintenance-free closed cycle cryo-coolers and vacuum encapsulations able to cope with the extreme environmental conditions encountered on top of the antenna mast head (e.g. a temperature range from -40°C - +65°C) and a MTBF in excess of 3 years.
A representative and meaningful trials programme in operational networks has been negotiated with three DCS-1800 operators. The first trial will be supervised by Telecomunicações Móveis Nacionais and will take place during the EXPO'98 in Lisbon, Portugal. This will be followed by trials in collaboration with SFR in Strasbourg, France and SingTel Mobile in Singapore.
Achievements

The deliverables scheduled for the first two years have been completed. They included the System Architecture and Specification, the HTS-bandpass filters, duplexers and the cryogenic environment and cooler.
Novel HTS-bandpass filters based on lumped element and distributed designs have been manufactured and tested achieving the required specification. The filters have been built in planar technology with a high degree of miniaturisation (22.5 x 39.0mm). A measured unloaded Q of 50,000 at 60K and an input power of -10dBm was achieved for a resonator.
The cryogenic system provides the low temperature environment for the HTS components and LNAs and includes the vacuum encapsulation and the cooler. Both a Gifford McMahon cooler with a pulse tube cold head and a Stirling cooler with a displacer cold head have been developed and tested by SUCOMS. The Stirling cooler, which has been specially developed for this application, is very efficient and compact and delivers a heat lift of >3W at 60K. The coolers have been designed for a maintenance free operation in excess of three years, however the reliability has yet to be proven.

Expected Impact
Superconducting components are compact and light weight which creates new opportunities for novel system architectures at basestation level. The use of superconducting devices with low insertion loss and high Q-factors increases the sensitivity of basestations and enhances the Quality-of-Service.
The selectivity of HTS BPF will be useful for future UMTS systems. Advanced HTS-RF front end sub-systems can be used to increase the dynamic range and selectivity in W-CDMA multicarrier receivers leading to a better Quality-of-Service in future UMTS systems. A better blocking/jamming resistance between the adjacent channels ensures a more efficient use of the available spectrum. The HTS sub-system of figure 2 has a capacity of 12 RF locations with receive bandpass filters and LNAs or transmit/receive duplexers or a combination of both.
Lower losses and reduced noise will allow the output power of the handsets to be reduced bringing environmental and possible health benefits to the end user.
A capacity and revenue increase tohether with reductions in the infrastructure and maintenance costs are expected to be the main techno-economic benefits resulting from the incorporation of HTS technology. HTS sub-systems are expected to be the enabling technologies that will impact on future basestations involving "software radio" and "adaptive antenna" concepts.

Main contributions to the programme objectives:
Main deliverables
Development and demonstration in a real network of High Temperature Superconductivity (HTS) subsystems.
Contribution to the programme
In cellular systems, HTS offers increased selectivity and sensitivity (i.e., increase in capacity), as well as significant equipment size and power requirement reduction, allowing for pole mounting and thus resulting in the minimisation and simplification of wiring. Another advantage is the reduction of maintenance costs through maintenance free operation during the expected life time of cellular equipment.
Technical Approach
The project involves the design, fabrication and assessment of communications transceiver components and sub-systems, built to a predefined specification. Concurrent engineering of the cooler and vacuum dewar will be imperative to the overall strategy.
A strong technology base is available as the starting point: HTS planar filters, delay lines, low phase noise oscillators and antennas have already been developed as technology demonstrators, and hybrid systems comprising these HTS passive components and semiconductor devices have been demonstrated by the consortium partners. The initial phase of the work began with the top down definition of systems/sub-systems/specifications drawn from RACE/ACTS project outputs, the relevant standards (ETSI, CCIR, ITU) and close operator interaction while taking into account the technical enhancements provided by HTS components. This workpackage provided the input to define the component specifications in terms of, for example, cooler capacity and services including dewar requirements, filter response, stage gain, oscillator noise and stability. Successive workpackages will address the HTS and cooled semiconductor component design; manufacture and assessment; system assembly; cooler integration and laboratory trials and finally system trials in collaboration with an operator. Essential to the system is an integrated custom designed dewar and a closed-cycle cryocooler. Two companies are pro-actively collaborating with the project in these important areas.
Summary of Trial
The application is specifically targeted at the third generation mobile communication system (2GHz UMTS). A representative and meaningful trials programme is currently being negotiated with the DCS-1800 operator SFR. In addition SUCOMS has approached other operators in order to widen the trial aspect of the project.
Key Issues
* Identification of sub-system/system requirements drawn from RACE/ACTS project outputs.
* Identification of, and contributions to relevant standards: ETSI, CCIR, ITU, etc.
* Close operator interaction.
* Manufacture of superconducting components combined with cooled semiconductor devices.
* Integration of sub-system/system encapsulation with a compact closed cycle cryocooler with low power requirements suitable for battery back-up.
* Collaborative system trials programme with DCS1800 basestation operators.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

GEC-Marconi Research Centre
Address
Elstree Way
WD6 1RX Borehamwood
United Kingdom

Participants (9)

GEC-Marconi Infra Red
United Kingdom
GEC-Marconi Research Centre
United Kingdom
Leybold Vacuum
Germany
SingTel Mobile
Singapore
Telecomunicações Móveis Nacionais
Portugal
Thomson CSF Central Research Laboratory
France
Thomson CSF Communications
France
UNIVERSITY OF BIRMINGHAM
United Kingdom
Address
Edgbaston
B15 2TT Birmingham
University of Wuppertal
Germany