R. Nilavalan

Brunel University, अक्सब्रिज, England, United Kingdom

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Publications (81)28.52 Total impact

  • 03/2014; 62(4).
  • M. Spasos, R. Nilavalan
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    ABSTRACT: This paper presents in detail the entire procedure of calculating the bias resistance of an ohmic RF-MEMS switch, controlled under resistive damping (charge drive technique). In case of a very stiff device, like the North Eastern University switch, the actuation control under resistive damping is the only way to achieve controllability. Due to the short switching time as well as the high actuation voltage, it is not practical to apply a tailored control pulse (voltage drive control technique). Implementing a bias resistor of 33 MΩ in series with the voltage source, the impact velocity of the cantilever has been reduced 80 % (13.2 from 65.9 cm/s), eliminating bouncing and high initial impact force during the pull-down phase. However, this results in an affordable cost of switching time increase from 2.38 to 4.34 μs. During the release phase the amplitude of bouncing has also been reduced 34 % (174 from 255 nm), providing significant improvement in both switching operation phases of the switch.
    Microsystem Technologies 01/2013; · 0.83 Impact Factor
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    ABSTRACT: The analysis, design and simulation of a novel easy to control all-metal in-line-series ohmic RF MEMS switch is presented, for applications where the operating frequency ranges from DC to 4GHz. The proposed switch, due to its unique shape and size, assures high isolation and great linearity fulfilling the necessary requirements as concerns loss, power handling and power consumption. Simplicity has been set as the key success factor implying robustness and high fabrication yield. On the other hand, the specially designed cantilever-shape (hammerhead) allows distributed actuation force ensuring high controllability as well as reliability making the presented RF MEMS switch one of its kind. KeywordsOhmic RF MEMS switch–Simplicity–Controllability–Reliability
    Analog Integrated Circuits and Signal Processing 04/2012; 71(1):21-28. · 0.55 Impact Factor
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    ABSTRACT: A reconfigurable wideband and multiband C-Slot patch antenna with dual-patch elements is proposed and studied. It occupies a compact volume of 50 × 50 × 1.57 (3925 mm<sup>3</sup>), including the ground plane. The antenna can operate in two dual-band modes and a wideband mode from 5 to 7 GHz. Two parallel C-Slots on the patch elements are employed to perturb the surface current paths for excitation of the dual-band and the wideband modes. Two switches, implemented using PIN diodes, are placed on the connecting lines of a simple feed network to the patch elements. Dual-band modes are achieved by switching “ON” either one of the two patch elements, while the wideband mode with an impedance bandwidth of 33.52% is obtained by switching “ON” both patch elements. The frequencies in the dual-band modes can be independently controlled using positions and dimensions of the C-Slots without affecting the wideband mode. The advantage of the proposed antenna is that two dual-band operations and one wideband operation can be achieved using the same dimensions. This overcomes the need for increasing the surface area normally incurred when designing wideband patch antennas. Simulation results are validated experimentally through prototypes. The measured radiation patterns and peak gains show stable responses and are in good agreements. Coupling between the two patch elements plays a major role for achieving the wide bandwidth and the effects of mutual coupling between the patch elements are also studied.
    IEEE Transactions on Antennas and Propagation 02/2012; · 2.33 Impact Factor
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    ABSTRACT: This paper presents the design of a low-profile compact printed antenna for fixed frequency and reconfigurable frequency bands. The antenna consists of a main patch, four sub-patches, and a ground plane to generate five frequency bands, at 0.92, 1.73, 1.98, 2.4, and 2.9 GHz, for different wireless systems. For the fixed-frequency design, the five individual frequency bands can be adjusted and set independently over the wide ranges of 18.78%, 22.75%, 4.51%, 11%, and 8.21%, respectively, using just one parameter of the antenna. By putting a varactor (diode) at each of the sub-patch inputs, four of the frequency bands can be controlled independently over wide ranges and the antenna has a reconfigurable design. The tunability ranges for the four bands of 0.92, 1.73, 1.98, and 2.9 GHz are 23.5%, 10.30%, 13.5%, and 3%, respectively. The fixed and reconfigurable designs are studied using computer simulation. For verification of simulation results, the two designs are fabricated and the prototypes are measured. The results show a good agreement between simulated and measured results.
    IEEE Transactions on Antennas and Propagation 01/2012; 60(8):3867-3874. · 2.33 Impact Factor
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    ABSTRACT: In this letter, the design of a compact monopole antenna for multiband and wideband operations is proposed. The antenna has three distinct frequency bands, centered at 0.94, 2.7, and 4.75 GHz. The antenna has a compact size of only 30$\,\times \,$ 40$\,\times \,$ 1.57 mm$^{3}$ including the ground plane. The multiband and wideband operations are achieved by using an E-shaped slot on the ground plane. The design procedure is also discussed. The frequency bands can be independently controlled by using the parameters of the E-slot. The impedance bandwidth, current distributions, radiation patterns, gain, and efficiency of the antenna are studied by computer simulation and measurements.
    IEEE Antennas and Wireless Propagation Letters 01/2012; 11:539-542. · 1.67 Impact Factor
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    ABSTRACT: AbstractA small ultra‐thin planar inverted‐F antenna (PIFA) which can be switchable or tunable using PIN or varactor diodes, respectively, is proposed. The antenna consists of two radiators and connected together using two PIN or varactors diodes. Frequency reconfigurability is obtained by (i) tuning using varactor diodes or (ii) switching using PIN diodes. In the study, when varactors are used, the PIFA is optimized to operate in five frequency bands centered at 0.72, 1.35, 1.98, 3.6, and 5 GHz. Varying the voltages across the varactors can tune these five bands over the range of 45.33% (0.58–0.92 GHz), 17.77% (1.23–1.47 GHz), 26% (1.57–2.04 GHz), 6.66% (3.48–3.72 GHz), and 0.97% (4.99–5.04 MHz), covering the DVB‐H, GSM 800/900, GPS, PCS, DCS, UMTS, Wibro phase III, WiMAX, WLAN systems, etc. If PIN diode switches are used instead, the frequency bands can be switched among different standards, including the DVB‐H, GSM, UMTS, DCS, GPS, WiMAX, and WLAN systems, depending on the switching states. The two radiators are supported using a shorting wall and a shorting pin connected to a small ground plane. Results of intensive investigations using computer simulations and measurements show that the ground plane size has little effects on the performance of the antenna. The effects of user's hand on the S11, radiation patterns and efficiency are also characterized. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:725–729, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26640
    Microwave and Optical Technology Letters 01/2012; 54(3). · 0.59 Impact Factor
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    ABSTRACT: Radio over Fiber (RoF) technology uses optical fiber links to distribute RF signals from a central station to multiple remote access units due to its ability to provide simple antenna front ends, increased capacity, and multi radio wireless access coverage. In this paper, an All-photonic Digital Radio over Fiber (DRoF) architecture is proposed and its performance is individually investigated in a dedicated and an integrated fiber link and compared with an Analog Radio over Fiber (ARoF) and Baseband over Fiber communication systems. Link's performance is investigated in the presence of other channels interference and Mode-locked Laser Diode pulse trains with about overall 1 picoseconds timing jitter. Simulation results proved that the proposed All-Photonic DROF is a suitable candidate for very high sampling rate of the digital optical links and is more robust against channel interferences than Analog Radio over Fiber (ARoF).
    Communication Technologies Workshop (Swe-CTW), 2011 IEEE Swedish; 11/2011
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    ABSTRACT: Radio over fibre (RoF) technology is currently receiving large attention due to its ability to provide simple antenna front ends, increased capacity, and multi radio wireless access coverage. In this paper, an all-photonic Digital Radio over Fibre (DRoF) link is proposed. By using this DRoF system, performance of an integrated transportation system, for transporting multi-types analogue, digital and baseband signals over single mode fibre is investigated. Simulation results proved that bit error rate (BER) of DRoF communication link is remarkably better than the analogue RoF (ARoF).
    Wireless Communications and Mobile Computing Conference (IWCMC), 2011 7th International; 08/2011
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    ABSTRACT: Reliability and longevity comprise two of the most important concerns when designing micro-electro-mechanical-systems (MEMS) switches. Forcing the switch to perform close to its operating limits underlies a trade-off between response bandwidth and fatigue life due to the impact force of the cantilever touching its corresponding contact point. This paper presents for first time an actuation pulse optimization technique based on Taguchi’s optimization method to optimize the shape of the actuation pulse of an ohmic RF-MEMS switch in order to achieve better control and switching conditions. Simulation results show significant reduction in impact velocity (which results in less than 5 times impact force than nominal step pulse conditions) and settling time maintaining good switching speed for the pull down phase and almost elimination of the high bouncing phenomena during the release phase of the switch.
    Microsystem Technologies 08/2011; 17(8):1351-1359. · 0.83 Impact Factor
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    ABSTRACT: The design of a small ultra-thin printed inverted-F antenna (PIFA) with independent control on the resonant frequency bands is proposed. The antenna consists of a slotted radiator supported by shorting walls and a small ground plane. The structure is designed and optimized to operate at 2.09, 3.74 and 5 GHz with achievable bandwidths of 11%, 8.84% and 10%, respectively. These three bands cover the existing wireless communication frequency bands from 1.5-6.8 GHz. Each of the three bands can be controlled independently without affecting the other two bands. The 2.09 GHz band can be controlled to operate between 1.5-2.09 GHz (33.33%), the 3.74 GHz band can be controlled over the range of 3.57-4.18 GHz (15.76%) and the 5 GHz band can be controlled to cover the band from 5.00-6.80 GHz (30.50%). Results of intensive investigations using computer simulations and measurements show that the ground plane and the feed locations of the antenna have marginal effects on the performance of the antenna. The effects of the user's hand and mobile phone housing on the return loss, radiation patterns, gains and efficiency are characterized. The measured peak gains of the prototype antenna at 2.09, 3.74 and 5 GHz are 2.05, 2.32 and 3.47 dBi, respectively. The measured radiation efficiencies for the corresponding three bands are 70.12, 60.29 and 66.24% respectively.
    IEEE Transactions on Antennas and Propagation 08/2011; · 2.33 Impact Factor
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    ABSTRACT: Wideband electronic analogue to digital conversion (ADC) systems have critical problems encountered in high-frequency broadband communication systems that the recent electronic ADCs (EADC) have experienced those such as uncertainty of sampling time. In this paper, an all photonic sampling and quantization ADC and photonic digital to analogue conversion system with six effective number of bits (ENOB) is designed. By using this photonic ADC (PADC), a novel digital radio over fibre link for wireless radio frequency (RF) signal transportation over 20 Km single mode fibre has been designed whose performance is investigated in this paper. In the digital radio over fibre, the dynamic range is independent of the fibre length.
    Kaleidoscope: Beyond the Internet? - Innovations for Future Networks and Services, 2010 ITU-T; 01/2011
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    ABSTRACT: Four-wave mixing (FWM) is a nonlinear phenomenon in optical communication system that introduces undesired interferences on wavelength division multiplexing (WDM) application for radio over fibre (RoF) broadband access networks. This impairment remarkably degrades the RoF system's performance. In this paper, the proposed fully-photonic Digitizaed RoF (DRoF) system's performance is investigated with utilizing different uniform and non-uniform WDM schemes. The simulation results demonstrated that uniform WDM application increases power penalty at central station about 2 dB higher than non-uniform wavelength allocation plan.
    14th International Symposium on Wireless Personal Multimedia Communications, WPMC 2011, Brest, France, October 3-7, 2011; 01/2011
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    Michalis Spasos, Rajagopal Nilavalan
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    ABSTRACT: Efficient control of RF MEMS switches is a very important issue as it is correlated to main failure mechanisms/modes such as the impact force and bouncing phenomena, which degrade their dynamic performance and longevity. This paper presents the control of specific ohmic RF MEMS switches under three different actuation modes, a tailored pulse optimization method based on Taguchi's technique (voltage mode actuation control), resistive damping (charge mode actuation control) and finally the Hybrid actuation mode, which is a combination of the tailored pulse, the resistive damping and Taguchi's optimization technique. Coventorware simulations indicate that under optimized Tailored pulse and Hybrid actuation modes, the impact velocity is reduced by around 90%, the initial impact force by around 75% and the maximum bouncing displacement during the release phase by around 95%, while the switching speed is increased by around 20% compared with the step pulse control mode. The resistive damping control mode is inappropriate for this type of switch and only partial improvement during the pull-down phase has been achieved. Finally, a comparison between Hybrid and optimized tailored modes shows that Hybrid actuation mode excels with better switching characteristics and most importantly offers immunity to manufacturing and operation tolerances.
    Microelectronics Journal. 01/2011; 42:1239-1251.
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    ABSTRACT: Photonic analogue-to-digital converters (ADCs) by using a Mode-Locked Laser Diode (MLLD) scale the sampling timing jitter to femtosecond. Pipeline ADC's architecture has good performance in terms of speed and conversion latency. In this paper, an eight-bit pipeline time-interleaved all-photonic sampling and quantization ADC with a MLLD's sampling pulse period duration conversion latency is proposed and its performance is investigated.
    3rd International Congress on Ultra Modern Telecommunications and Control Systems and Workshops, ICUMT 2011, Budapest, Hungary, October 5-7, 2011; 01/2011
  • T. Peter, T. I. Yuk, R. Nilavalan, S. W. Cheung
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    ABSTRACT: A novel technique to improve the performance of AgHT-8 transparent polymer antennas is proposed in this paper. A spit-ring resonator is introduced on the radiating patch to enhance gain. The resonator basically concentrates the radiating energy to the central area of the patch thus improving gain. The designed antenna demonstrates good gain while maintaining the original transparency of the material. Such an antenna inscribed on the commercially available AgHT-8 sun shielding film material makes it a viable option for wireless applications like in-house base stations and applications requiring fast data rate transfers which can be mounted on windows and glass panels.
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    ABSTRACT: This study presents an H-shaped reconfigurable antenna for wireless applications. The antenna consists of an H-shape radiator and a CPW printed on a circuit board and a varactor diode connecting the upper and lower arms of the H-shape radiator for reconfigurability. The uniqueness of the antenna lies on the ability to select the operating mode and frequencies electronically using a varactor diode. By selecting the DC-bias voltages of 11.5, 10 and 8 V across the varactor diode, which in turn selects the corresponding varactor capacitances of 2, 4 and 6 pF, the antenna can be controlled to operate in three different modes, namely a single-band mode to cover the GSM1900, a dual-band mode at 1.88 and 2.4 GHz to cover the GSM1900 and Bluetooth/WLAN, respectively and a tri-band mode at 1.57, 1.88 and 2.4 GHz to cover the GSM1900, WLAN and GPS, respectively. Furthermore, by varying the varactor capacitance from 7 to 13 pF, the GPS and WLAN bands can be tuned by 11.44% (1.57-1.4 GHz) and 6.46% (2.4-2.25 GHz), respectively, yet keeping the 1.88-GHz band unchanged. Detailed studies on the antenna's performance are carried out to investigate the behaviour of the antenna at each resonant frequency in each operating mode.
    IET Microwaves Antennas & Propagation 01/2011; 5(14):1675-1682. · 0.84 Impact Factor
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    ABSTRACT: In this paper, the design of a UWB antenna using a transparent AgHT-8 material is proposed for green wireless applications. Computer simulation is used for studies. For verification of the design, the proposed antenna is fabricated on an AgHT-8 film and measured. Results show that the antenna has better radiation efficiency relative to its size than the previous designs, good omni-directional radiation patterns throughout the FCC bandwidth of 3.1-10.6 GHz and a comparable gain. To filter out the unwanted signals in the WLAN band, two vertical slots are introduced to produce a tuning notch in the 5 GHz frequency band. For demonstration of green wireless applications, the transparent antenna is incorporated with a solar panel for harnessing solar energy. Results show that the transparency of the antenna makes it a good candidate for future green wireless applications.
    Antenna Technology (iWAT), 2011 International Workshop on; 01/2011
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    ABSTRACT: This paper presents Taguchi's optimization method implemented in the design of a single feed (without any matching network) 5×5 microstrip antenna array operating around 12.5 GHz. The proposed optimization method is statistical and is widely used for quality assurance in many fields such as mechanical and chemical production, consumer electronics, services; however it has been underused in the field of electromagnetics. It allows optimization of multiparameter, multitarget complex designs in a very short time in conjunction with advanced simulation tools. The proposed antenna has been fully evaluated under Taghuchi's and PSO's optimization methods, and the experimental results show total Gain of 15 dB, and good matching with S11 better than −20 dB, in the frequency range 12.3 to 12.8 GHz.
    International Journal of Antennas and Propagation 01/2011; 2011. · 0.68 Impact Factor
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    ABSTRACT: A non-thermal soldering technique using cold solder or electrically conductive epoxy for connecting SMA connectors to polymer based antennas is proposed in this paper. The proposed technique prevents damage to the polymer due to the solder iron heat and also the loss of efficiency through the use of indirect connections of the coaxial feed via copper pads glued to the antenna. The direct connection of the feed points via SMA connectors on to a transparent antenna designed on AgHT-8 material has been demonstrated. The method can also be applied to solder the coaxial feed points directly to the antenna instead of using copper pads which will introduce additional reflection losses. The technique involves the use of colder soldering instead of hot soldering so as to not damage the polymer based antenna as well as improve the efficiency of the antenna.
    Microwave Conference (EuMC), 2010 European; 10/2010

Publication Stats

281 Citations
28.52 Total Impact Points


  • 2008–2013
    • Brunel University
      • • School of Engineering and Design
      • • Department of Electronic and Computer Engineering
      अक्सब्रिज, England, United Kingdom
  • 2010
    • University of Surrey
      • Faculty of Engineering and Physical Sciences
      Guildford, ENG, United Kingdom
    • University of Tehran
      • College of Engineering
      Tehrān, Ostan-e Tehran, Iran
    • Alexander Technological Educational Institute of Thessaloniki
      • Department of Electronics
      Thessaloníki, Kentriki Makedonia, Greece
  • 1999–2007
    • University of Bristol
      • Department of Electrical and Electronic Engineering
      Bristol, ENG, United Kingdom
  • 2005
    • Technical University of Denmark
      København, Capital Region, Denmark