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Developments in wireless technology and sensors have resulted in a range of new applications for Wireless Sensor Networks (WSNs). One such type of WSNs is the Built Environment Networks (BENs) which can be deployed in a wide variety of applications such as environmental monitoring, surveillance and healthcare applications. WSNs are characterized by having a large number of devices (motes) with sensing capabilities, limited processing capability and wireless connectivity to other devices. The wireless capability allows the sensors to be deployed close to the phenomenon being observed while their limited memory and processing capabilities result in low cost; this allows the deployment of a large number of such devices. The motes used in WSNs usually contain a micro-controller, RF chip, sensors and often use low level operating systems such as TinyOS (Levis, Madden, Polastre, Szewczyk, Whitehouse, Woo, Gay, Hill, Welsh, Brewer, & Culler, 2005) or Contiki (Dunkels, Gr”onvall, & Voigt, 2004).
When deploying a wireless network within a building, it is vital to have an understanding of how the transmitted signals are affected through obstacles and with distance. The channel between transmitter and receiver may be a line of sight (LOS), but more likely the presence of objects such as office furniture, lab equipment and people will create obstructions and provide multiple paths (multipath) for the waves to reach the receiver. Diffraction, reflection and scattering are main causes of multipath and fading. Multipath and fading are the dominant effects on the channel as the transmitted signal propagates through the media to the receiver (Su & Alzagal, 2008). The ideal antenna radiation pattern for a mote is symmetric in all directions and would result in constant radio range and performance in a spherical radius of the mote (Su & Alzagal, 2008). In reality this uniform transmission pattern does not exist and failure to understand a mote antenna transmission pattern and how it can be affected by the environment can result in network reliability issues and/or inappropriate placement of motes. There are numerous papers that investigate the relationship between signal attenuation and building materials (Jang & Healy, 2009; Wilson, 2002; Masri, Chew, Wong, & Lias, 2005) and software packages that simulate the radio frequency (RF) attenuation within buildings using ray tracing techniques (Kim & Lee, 2009). Little attention has, however, been given to the effects that antenna pattern irregularity and human presence can have on the signal strength and reliability of even a small scale network when it has been deployed.
This paper provides measurements of Received Signal Strength (RSS) from within various locations of a typical office environment during a twenty four hour period and an understanding of how signal attenuation is affected by building materials, antenna selection, orientation and both human presence and movement.
The paper is divided into sections. First we describe the experimental setup and software used. The results of the experiments are provided and discussed following this. Finally, a conclusion is given by highlighting the main outcomes of the paper.