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Software Defined Radio (Enabling Technologies)

Software Defined Radio (Enabling Technologies). Ch5. Superconductor Microelectronics : A Digital RF Technology for Software Radios. 마이크로파공학 연구실 김준철. Commercialization of superconductor microelectronics technology promise to be a key enable of ‘pure’ software radio architectures

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Software Defined Radio (Enabling Technologies)

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  1. Software Defined Radio (Enabling Technologies) Ch5. Superconductor Microelectronics : A Digital RF Technology for Software Radios 마이크로파공학 연구실 김준철

  2. Commercialization of superconductor microelectronics technology promise to be a key enable of ‘pure’ software radio architectures • This chapter provides a description of the underlying technology and its potential in both commercial and defense wireless systems

  3. 5.1 Introduction • The speed and flexibility enabled by superconductor microelectronics seems well matched to the goals of proposed software radio architectures • LTS ( Low Temperature Superconductors ) -- Nb ( niobium ) Tc : 9.23K, operate : 4.2K - 5K • Digital RF -- superconductor logic gates will directly process digital signals at RF or multi- GHz frequencies. • RSFQ ( Rapid Single Flux Quantum ) -- flexible, high data rate applications (for 3G, 4G wireless services)

  4. 5.1.1 Superconductivity and the Josephson Effect - Phenomenon of Superconductor 1) zero resistance (when cooled below a critical transition temperature (Tc) 2) superconductor can contain magnetic flux only in certain discrete quantities  called ‘flux quantization’

  5. - In order to create digital circuits, an active superconductor component is needed : the Josephson junction (JJ) * I < Ic : device exhibits no resistance * I > Ic : JJ becomes briefly resistive

  6. - Design consideration for Josephson junctions in RSFQ circuits is that they be sufficiently damped to prevent hysteresis upon exceeding the critical current, so that the junction quickly return to the Zero voltage - Rapid voltage pulse corresponds to a single flux quantum

  7. 5.1.3 Emerging Applications – Software Defined Radio - as the 'cryophobia' associated with superconductor microelectronics is overcome, the range of possible applications continues to widen - in communications, dispersion-free, ultra-high Q superconductor microwave filters are used in cellular base stations - the use of superconductor material allows the very high Qs to be maintained, while microminiaturizing the overall filter size - the ultra-sharp filter 'skirts' that result enable increased channel selectivity and, with a cooled LNA, yield increased sensitivity as well - an increasingly embraced solution to surmount these obstacles and lies in the concepts of software radio - realization of software radio systems presents a host of challenges - chief among them the unprecedented requirement on analog-to-digital converter (ADC) performance - this is the area where superconductor microelectronics represents an emerging solution - with demonstrated ADC, DAC, and DSP components, this technology may well become a key enabling technology for software radio

  8. (table 5.1)  summarizes the performance already achieved with such superconducting • devices to date • - military radio requirements are far more demanding than those for commercial systems

  9. 5.2 Rapid Single Flux Quantum (RSFQ) Digital Logic • Rather than relying directly on quantized bundles of magnetic flux as bits, many effort attempted to use the voltage state of the JJ as a ‘1’ and the superconducting state as a ‘0’ • Power consumption : Typical latching gate : dissipated about 3 pW RSFQ technology : dissipated 0.3 pW

  10. 5.2.1 Circuit Characteristics • In RSFQ circuits, it in not a static voltage level, but the presence or absence of quantized • magnetic flux (fluxons) that represents information bits • The basic RSFQ structure is a superconducting ring that contains one Josephson junction • plus a resistive shunt outside it

  11. 5.2.2 Example RSFQ Logic Gate – RS Flip Flop

  12. 5.2.3 RSFQ Data Converter 5.2.3.1 Analog to Digital Converters - This superconductor ADC design is especially linear, because the quantization thresholds are set by a ratio of fundamental physical constants ( ) in the SQUID in the front end - Common performance netrics for ADCS are the SINAD and SFDR SINAD : signal-to-noise and distortion measurement  represents the dynamic range of the signal

  13. The circuit consists of two major parts •  front end quantizer, digital decimation low pass filter • * front end 1) phase modulator – consist of a signle-junction SQUID • 2) phase demodulator • - consist of a time-in terleaved bank of race arbiters(SYNC) followed by a • thermometer to binary encoder (DEC) • ENOB (effective number of bits)

  14. 5.2.3.2 Digital-to-Analog Converters

  15. 5.2.4 RSFQ scaling Theory • Considerations of the junction fabrication process • : thickness of the tunneling barrier, critical current • density (Jc), thermal fluctuation. • For RSFQ circuits, the high frequency performance • is determined primarily by the width of the SFQ pulse • Although JJs are the central elements of RSFQ • circuits equally important are inductors and resistors

  16. 5.3 Cryogenic Aspects • Superconductor RSFQ circuitry must be cooled for operation (critical temperature : Tc) • The temperature of operation is about one-half Tc suitable platform for a commercial product : a closed cycle refrigerator (CCR or cryocooler) • Carnot efficiency • MTBF (mean time between failures)

  17. current, developing, and forecasted cryocooler

  18. A cost-reliability goal for commercial cryocooler is US $ 1000-10000, 2~10 year lifetime • RSFQ chips heat < heat leaks of radiation and conduction from the surrounding interface • electronics • one of the main concerns is reliability • reality of reliability • * using oil lubricated air-conditioning compressor (increasing life time) • * using noncontacting suspension methods for the piston in the cylinder (increasing MTBF)

  19. 5.4 Superconductor SDR for commercial Application 5.4.1 Superconductor in wireless communications • Growth of commercial wireless communication technology evloving forward a ‘software radio’ • implementation •  Because of spectrum regulation • The only way to fit in more callers or higher rate data transfer or both is to increase their • ‘spectral efficiency’ • due to the competition of many providers need the cost/performance ratio • so, CCR is needed, in practice, cryocooled high temperature superconductor (HTS) filter are • now being installed in cell base station receiver with good success • cryocooled LTS (low temperature superconductor) mixed signal electronics is probably the • only technology with the speed, linearity, and sensitivity to allow direct RF digitization and • subsequent fast digital signal processing • Beside, there are now reliable 5K cryocoolers with an appropriate market, further • improvements in efficiency, size, and costs are expected

  20. 5.4.2 Advantages of superconductor Receivers • Using LTS -- possibility of digital RF •  replacing analog components with digital components • Advantages • * far less distortion • * digitizing a wider band of the spectrum • * the high sensitivity 5.4.2.1 Receiver noise Temperature calculation • Using superconducting receiver •  reducing noise temperature ( from 1000 ~ 2000K to 250K )

  21. For conventional Receiver • Digital RF superconductor receiver is used, • directly digitizing the signal carrier • We conclude that by employing a digital RF approach with a superconductor ADC, we move • from a system limited by the noise figure of its components to one limited by its environment.

  22. 5.4.3 Trends in Spread Spectrum Communications 5.4.3.1 Interference Limited System - the Myth and the Reality • The Myth : low noise receivers are not useful in interference limited systems, because • thermal noise compared to interference from other users • The Reality : low noise receivers are useful in interference limited systems, because they • allow a reduction in power for all users, permitting higher information capacity 5.4.3.2 Benefits form Lower Receiver Noise • Receiver noise power density N0=KBTS • From Viterbi • benefit • 1) the lower Pr must be, the lower our transmitted power, leading to less drain on our • battery • 2) callers can use a higher required Eb/No to meet a lower bit error rate requirement or • support higher symbol constellation • 3) the number of simultaneous users (M+1) increase • 4) using a higher data rate (Rb)

  23. 5.4.4 High Power Amplifier Linearization • The high power amplifiers (HPAs) of any base station transmitter system consume a • significant part of the cost and power budget • overcome inherent nonlinearities in the HPA transfer function • 1) extends the bandwidth of operation • 2) fewer total HPAs •  HPAs of lower quality (lower cost) , to enable the system to still function correctly • superconductor ICs may have the ability to allow real time digital adaptive linearization at RF

  24. 5.4.5 Digital RF Transceiver • How almost all the functions required by • a digital RF transceiver could be • integrated monolithically into a ‘radio on a • chip’ • analog designer of a bandpass ADC • circuits faces two large problem • 1) linearity of the down- conversion over • a wide frequency and dynamic range • 2) matching of the anti-alias low pass filter • and digitizers •  digital RF bandpass receiver does not • suffer from these problems

  25. 5.5 Superconductor SDR for Military Application 5.5.1 Co-site interference (CSI) • CSI stems from the simultaneous use of multiple radio protocols/frequencies/systems in • close proximity • CSI ‘s three main manifestations • 1) trying to receive a weak spread spectrum signal in large interfering signals • 2) multipath confusion • 3) impulsive interference from hopping signals • Superconductor electronics and the high dynamic range, wideband ADCs and DACs can • overcome these problems by employing brute force fidelity and speed to resolve the issues.

  26. 5.5.2 Digitally dehopping spread spectrum signals • need complexity of the dedicated analog systems to access spread spectrum wave forms • however it is possible by using a high SFDR receiver • ex) digitally dehopping, sharp skirt HTS analog filter using superconductor 5.5.3 Satellite Communications • Narrowband (5Khz) DAVA system’s poor link margin • Why? Transponder’s nonlinearities, SOQPSK demodulator’s legacy implementation • However, by using a low noise cryogenic SQUID- based front end, reduce loss • A digital RF approach is possible that it is processing increasing throughput at once, sorting • out the relevant channels, and synchronization • Because of the possible sensitivity of RSFQ circuits, one can actually reduce the noise floor • of its fundamental limit

  27. 5.5.4 Accommodating New Wave forms • Hallmarks of the SDR approach • * compatibility for future waveforms • * accessing hop rate • * complete denial of access to the wave form • * RSFQ circuits computational speed • * fundamental accuracy of RSFQ DACs • * allowing the flexibility to add new algorithms 5.5.5 Massive Time Multiplexing • Speed of RSFQ circuits can produce independent lulti – TeraOPS • reducing both capitalization and operational costs • offering better utilization of resources

  28. 5.5.6 Conclusions • Using the voltage state of a Josephson junction to suing quanta of magnetic flux to represent • information bits, has radically transformed the capability limits of superconducting technology. • The advances in reliability of cryocoolers has further meant that superconducting devices • (HTS filters) have begun to be deployed in commercial wireless networks • evolution of digital RF technology, coming as it does at the same time as the emerging • commercial acceptance of the concepts software radio, promises to revolutionize wireless • systems in the coming decades, with initial applications anticipated in the relatively near future • for base station and defense applications

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