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A Wireless sensor network (WSN) has important applications such as remote ecological monitoring and target tracking. This has been enabled by the availability, particularly in recent years, of sensors that are smaller in size and smart. These sensors are equipped with wireless interfaces with which they can communicate with one another to form a network. Wireless sensor networks consist of sensor nodes with sensing and communication capabilities. As sensor nodes are generally battery-powered devices, the critical aspects to face concern how to reduce the energy consumption of nodes, so that the network lifetime can be improved to reasonable times. In this paper we first describe the energy consumption for components of a typical sensor node, and discuss the main directions to energy saving methods in wireless sensor networks. Then we present a methodical and comprehensive taxonomy of the energy optimization methods in wireless sensor networks. The main goal of energy optimization methods is to collect and aggregate data in an energy efficient manner so that network life time is enhanced. We conclude with possible future research directions. KeywordsWireless sensor network (WSN), Multiple-input Multipleoutput, Energy efficiency, Alamouti diversity schemes.

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A dual-band quadrature branch line coupler (BLC) with wide frequency ratio has been proposed in this paper. A dual-band quarter wave (λ/4) transmission line structure with wide frequency ratio operation has also been presented here with closed-form design equations. An analysis for the BLC's circuit parameters variations versus frequency ratio in the view of the implementation has been carried out as well. In order to the experimental verification, a dual-band BLC has been designed, implemented, and analyzed then tested for long term evolution (LTE) 0.7GHz and public safety band (PSB) 4.9GHz. The novelty of the proposed design is the wide frequency ratio of its dual-band operation, which can reach up to 11.8. INDEX TERMS Branch line coupler, dual band, wide frequency ratio, coupled line, open stub.

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