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San Diego Gas and Electric is currently in the process of rolling out an ambitious Advanced Metering Infrastructure (AMI) Home Area Network (HAN) to meet the region's demand response (DR) and energy efficiency (EE) programs. It is working closely with two local universities " San Diego State University and University of California San Diego, in addition to the University of California Berkeley. The purpose of the partnerships is to develop an efficient, reliable, and cost effective AMI HAN wireless system for managing San Diego's growing energy needs.
An important attribute to consider in the implementation of this type of system is wireless device battery optimization. The objective is to design power efficient communication nodes to optimize battery lifetime. One possible medium by which it is intended to implement the HAN system is by utilizing ZigBee wireless technology.
ZigBee is a wireless technology software protocol that is stacked on top of the 802.15.4 physical PHY and medium access control MAC layer radio that encompasses Wireless Personal Area Networks (WPAN) working in the license free 868/915/2.4 GHz frequency bands. Due to its wireless nature, and the likelihood that the end devices/sensors will be battery powered, it is imperative that one strives for ways to optimize battery efficiency and longetivity. Therefore, increasing battery lifetime longetivity and optimization is essential and paramount in wireless sensor nodes. It is a topic worth pursuing and investigating when designing ZigBee wireless sensor networks.
When dealing with wireless sensors, there are three main broad schemes by which to optimize battery optimization; including battery management, transmission power, and system power. This paper focuses on transmission power management, in particular from the standpoint of antenna design, power calibration, and temporal control.
To increase range, one can optimize energy efficiency by increasing the antenna's directive gain (unidirectional as opposed to omnidirectional where antenna radiation power is radiated in all directions or bidirectional where power is radiated in two opposite directions " 180 degrees apart " known as lobes " other radiation patterns are possible) and thereby enabling the lowering of the power requirement for each transmission.
By definition, directive gain of an antenna is "a measure of the concentration of the radiated power in a particular direction." The idea is to direct as much of the energy used during transmissions into one direction as opposed to transmitting the energy in all directions and thus strengthening the power intensity of the transmitted signal in only one direction.
A good analogy is with the comparison between a light bulb and a flash light. Both have the same illumination, but because the flash light has its light focused in only one direction, its light intensity is strongest in that direction. The drawback of course is that of compromising coverage in other directions. This might not be bad considering that one can manipulate the HAN physical topology in such a way so as to minimize this shortfall.
However, this might be a bit difficult to do using a single antenna element. Therefore, to achieve greater directivity, antenna arrays might be needed. Antenna arrays are basically "radiating elements arranged so as to produce some particular radiation characteristic." However, although this adds complexity to the design of the antenna, and initial start up costs, if one is willing to comprise design complexity for battery optimization, this could be a design route to consider.
The motivation behind this is the basic assumption that technicians and installation personnel will exploit this feature so as to best configure the physical network topology and individual setup of nodes and sensors. Furthermore, with the current technology capability of implementing antennas in microstrip form, makes this all feasible.
With respect to power programming, the TI CC2431 SoC, allows for sixteen different output power settings. The idea is to optimize the battery lifetime by setting the correct output power setting and not overcompensating or under-compensating the power requirements needed for communication with other ZigBee devices.
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