R. Nilavalan

Brunel University, अक्सब्रिज, England, United Kingdom

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Publications (87)31.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Long term evolution (LTE)-advanced was introduced as real fourth generation (4G) with its new features and additional functions, satisfying the growing demands of quality and network coverage for the network operators’ subscribers. The term muti-tier has also been recently used with respect to the heterogeneity of the network by applying the various sub-network cooperative systems and functionalities with self-organising capabilities. Using indoor short-range low-power cellular base stations, for example, femtocells, in cooperation with existing long-range macrocells are considered as the key technical challenge of this multi-tier configuration. Furthermore, shortage of network spectrum is a major concern for network operators which forces them to spend additional attentions to overcome the degradation in performance and quality of services in 4G HetNets. This study investigates handover between the different layers of a heterogeneous LTE-advanced system, as a critical attribute to plan the best way of interactive coordination within the network for the proposed HetNet. The proposed comprehensive handover algorithm takes multiple factors in both handover sensing and decision stages, based on signal power reception, resource availability and handover optimisation, as well as prioritisation among macro and femto stations, to obtain maximum signal quality while avoiding unnecessary handovers.
    IET Science, Measurement and Technology 09/2014;
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    Christos Chousidis, Rajagopal Nilavalan, Laurentiu Lipan
    International Wireless Communications and Mobile Computing Conference (IWCMC 2014); 08/2014
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    ABSTRACT: A novel transparent ultra-wideband antenna for photovoltaic solar-panel integration and RF energy harvesting is proposed in this paper. Since the approval by the Federal Communications Committee (FCC) in 2002, much research has been undertaken on UWB technology, especially for wireless communications. However, in the last decade, UWB has also been proposed as a power harvester. In this paper, a transparent cone-top-tapered slot antenna covering the frequency range from 2.2 to 12.1 GHz is designed and fabricated to provide UWB communications whilst integrated onto solar panels as well as harvest electromagnetic waves from free space and convert them into electrical energy. The antenna when sandwiched between an a-Si solar panel and glass is able to demonstrate a quasi omni-directional pattern that is characteristic of a UWB. The antenna when connected to a 2.55-GHz rectifier is able to produce 18-mV dc in free space and 4.4-mV dc on glass for an input power of 10 dBm at a distance of 5 cm. Although the antenna presented in this paper is a UWB antenna, only an operating range of 2.49 to 2.58 GHz for power scavenging is possible due to the limitation of the narrowband rectifier used for the study.
    IEEE Transactions on Antennas and Propagation 03/2014; 62(4). · 2.33 Impact Factor
  • Christos Chousidis, Rajagopal Nilavalan
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    ABSTRACT: The increasing use of multimedia streaming applications in addition with advent of internet television and radio, demands from today's wireless networks to handle with reliability multiple broadcasting and multicasting sources. However, the way that 802.11 standard, which is the primary technology in wireless networking, handle this type of traffic raises a series of problems mainly related to the lack of an effective feedback mechanism [1]. This lack in turn, limits the capability of random backoff process to eliminate collisions and reduce reliability and fairness. This inherited drawback of the standard is affecting the way broadcast [2] and multicast traffic is transmitted as well as the overall performance of the network. In this paper initially we are highlighting the drawback of the IEEE 802.11 MAC algorithm in handling multiple stations “media type” data broadcasting in an ad-hoc wireless network [3]. Then, we propose two different approaches in alleviating these problems. The first approach is the simple linear increase of the contention window (CW) while the second propose a linear increase of the CW implementing an exclusive backoff number allocation (EBNA) algorithm. In addition we are modifying the 802.11 medium access control (MAC) algorithm to use the clear to send to self (CTS-to-Self) protection mechanism prior to every transmission. Both the above techniques are simulated and compared with the classic 802.11 MAC. The result sows that the overall performance of the network can be improved using these alternative MAC methods.
    Science and Information (SAI) Conference 2013, London, UK; 10/2013
  • 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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    Christos Chousidis, Rajagopal Nilavalan
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    ABSTRACT: Multimedia applications over wireless networks have dramatically increased over the past years. Numerous new devices and applications that distribute audio and video over wireless networks are introduced every day and all of them demands a reliable and efficient wireless standard. Either operating as independent ad-hoc networks or as terminal parts of wired networks or the internet, wireless networks are frequently facing the need to broadcast multimedia data from multiple sources to multiple users. IEEE 802.11 standard (Wi-Fi) is the primary technology in wireless networking today. However, it has some inherited problems when it comes to broadcasting caused mainly by the lack of an acknowledgment mechanism. These problems do not allow the standard to take full advantage of the bandwidth offered by its latest amendments. In this paper two independent modifications of the medium access control (MAC) mechanism of the standard, are proposed along with the expanded use of the CTS-to-Self protection mechanism. The main objective of this study is to explore the ability of the modified MAC mechanisms to improve broadcasting performance while are operating in conjunction with a regular wireless network, and also to define the cases were the use of CTS-to-Self-protection mechanism can improve the overall performance of the network. The results show that the overall performance can be improved using these alternative MAC methods. Also, the cases where the CTS-to-Self technique can additionally contribute to the network performance are defined and analyzed.
    International Journal of Computer Science and Applications 01/2013;
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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.
    01/2011;

Publication Stats

313 Citations
31.02 Total Impact Points

Institutions

  • 2008–2014
    • Brunel University
      • • School of Engineering and Design
      • • Department of Electronic and Computer Engineering
      अक्सब्रिज, England, United Kingdom
  • 2011
    • The University of Hong Kong
      • Department of Electrical and Electronic Engineering
      Hong Kong, Hong Kong
  • 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