فایل ورد کامل RF-MAC : پروتکل کنترل دسترسی متوسط برای شبکه های حسگر با قابلیت شارژ مجدد با قدرت برداشت انرژی بی سیم

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در این مقاله، پروتکل Rf-MAC را پیشنهاد کردیم که مخصوصاً مسائل انتخاب مشترک فرستنده های انرژی و فرکانس های آنها را براساس تاثیر جمعی زمان شارژ و تداخل انرژی خطاب قرار داده، حداکثر آستانه شارژ انرژی را تعیین نموده، انرژی درخواست و اعطا نموده و اولویت دسترسی آگاه از انرژی تعیین می نماید. گروه بندی ET ها به دو مجموعه با فرکانس های انتقال و ارسال متغیر، و حداقل سربار کنترل نیازمندیهای سخت افزاری را در حد ساده نگه داشته و پیاده سازی پروتکل راحت تر می شود. پروتکل پیشنهادی مسئله مهم چگونگی تعیین رابطه جانشینی انتقال انرژی و داده ها را کشف می کند، به ویژه از آنجایی که یکی با هزینه دیگری رخ می دهد. بالاخره نتایج بستر تست و شبیه سازی نشان می دهد RF-MAC از لحاظ متوسط انرژی برداشت شده و متوسط کارایی شبکه، برتر از CSMA تغییریافته عمل می کند.

عنوان انگلیسی:RF-MAC: A Medium Access Control Protocol for Re-Chargeable Sensor Networks Powered by Wireless Energy Harvesting~~en~~

WIRELESS SENSOR NETWORKs (WSNs) are being increasingly used in a wide variety of applications including industrial and infrastructure monitoring, smart home, smart grid, medical systems, and so on. One of the main challenges and performance bottleneck in these systems is the limited lifetime of the sensor nodes due to their energy supply. Recent advances in the area of wireless energy transfer allow sensors to recharge during network operation, thereby extending their lifetimes and minimizing application downtime. Our recent research on powering Mica2 sensor motes by harvesting the energy contained in radio frequency (RF) electromagnetic waves in [1] indicated the potential for large scale deployment of this technology. However, at the protocol level, this form of in-band energy replenishment is fraught with several challenges on: (i) how and when should the energy transfer occur, (ii) its priority over, and the resulting impact on the process of data communication, (iii) the challenges in aggregating the charging action of multiple transmitters, and (iv) impact of the choice of frequency. Thus, the act of energy transfer becomes a complex medium access problem, which must embrace a crossdisciplinary approach incorporating wave propagation effects and device characteristics, apart from the classical link layer problem of achieving fairness in accessing the channel. We focus on in-band transmission since multiple separate frequencies for data and energy transfer increases the complexity of the sensors, brings in numerous antennas and transceiver related hardware requirements, and imposes additional spectrum availability needs. This paper is concerned with the design of a CSMA/CA based MAC protocol for such RF energy harvesting sensors, inspired by experimental evaluations on our testbed. Our MAC protocol that works with RF energy harvesting, called as RF-MAC, allows a node to broadcast its request for energy (RFE) packet containing its ID, and then waits to hear from the energy transmitters (ETs) in the neighborhood. These responses from ETs are called cleared for energy (CFE) pulses, which are simple, time-separated energy beacons. These pulses maybe transmitted by more than one ET concurrently as overlapping CFEs need not be distinguished. Rather, the concurrent emission of the CFEs increases the received energy level at the sensor, and this indicates a higher number of potential transmitters from the energy requesting sensor. The responding ETs are then classified into two sets, based on rough estimates of their separation distance from the energy requesting node to minimize the impact of destructive interference as much as possible. Each set of ETs is assigned a slightly different peak transmission frequency (separated by only few KHz, hence, still called in-band as the channel separation is typically 5 MHz for 802.11) so that each set of ETs contributes constructively to the level of RF energy received at the node. While we retain the essential concepts of the CSMA/CA mechanism for the data access mechanism [3], there are several points of departure from the classical implementation. We separately select and dynamically vary the slot time, the inter-frame spacing, and the contention window size for both energy transfer and data communication. The core contributions of our work can be summarized as follows: • We experimentally identify the operating constraints of the RF energy transferring MAC protocol using actual wireless energy harvesting circuits interfaced with Mica2 motes. We demonstrate how two slightly separated energy transfer frequencies can be assigned to ETs to improve the constructive interference of their collective action. • We design a MAC protocol to balance the needs of ef- ficient wireless energy delivery and data exchange. We bridge these dissimilar concepts by establishing the importance of a node in the data communication, which in turn quantifies how much should the node charge. • We analytically establish optimality conditions for the energy transfer, and create a strongly coupled protocol that operates on link layer metrics with the awareness of both the underlying hardware and fundamental limits of RF energy harvesting. The rest of this paper is organized as follows: In Section II, we give the related work. The key design challenges are described in Section III with experimental studies. A brief overview of our RF-MAC protocol in given in Section IV, with a comprehensive detailed description in Section V. The simulation and experimental results are presented in Sections VI and VII, respectively. Finally, Section VIII concludes our work. II. RELATED WORK MAC protocols that aim for energy conservation have been extensively explored in the recent past, with a comprehensive classification and survey on this topic presented in [4], [14], [16], [19]. In this section, we review the related MAC protocols for energy harvesting sensor networks.

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