“We’re aiming to design a wireless network of sensors which are IP addressable”, says Nihesh, a researcher from the Department of Electrical Communication Engineering, IISc.
Over the past few decades, scientists have sought to develop a system where machines can talk to other machines without any human interaction. In 1999, they finally conceptualized a system called the Internet-of-Things (IoT) – a network wherein objects are given unique identifiers (IP addresses) and are provided with a platform for the transfer of information. Researchers from the Indian Institute of Science (IISc) seek to capitalize on this concept and have identified appropriate technologies for the employment of an IoT network in the campus.
In order to come up with a feasible network plan, the researchers had to compare existing wireless devices. Energy efficiency of the devices was the first parameter considered. Three types of sub-GHz devices and three types of GPRS/GSM modules were considered for the analysis. Zigbee and WiFi devices, being short ranged were excluded from the study. The team found that sub-GHz devices outperformed the GSM devices with the latter requiring 3.15 times more power than the sub-GHz devices. Semtech LoRa iM880A and TI CC1200-DK, two sub-GHz based devices were found to consume the least power. Sensitivity of these two devices was then measured. The antenna model with the least varying sensitivity was considered for field experiments. Accurate reading of the devices is an important aspect of the experiments.
Performance of the two devices was tested in different environments such as open grounds, straight roads, moderately and heavily wooded areas, inside buildings, and building roof tops. This is important because the signal strength of the device varies with the environment. Three performance indicators were considered for the experiments, namely, Received Signal Strength Indicator (RSSI), Packet Error Rate (PER) and Bit Error Rate (BER). A higher RSSI value with a lower PER is considered favourable. “RSSI indicates the power of the signal at the receiver end. It determines the rate of degradation of a signal in a medium”, says Nihesh, “PER measurements are useful in determining how reliably information can be sent over a wireless medium. Information is usually transmitted in the form of packets with each packet consisting of a fixed number of information bits (0 or 1). So while the BER specifies the percentage of received bits that are in error, the PER specifies the percentage of packets in error”.
The researchers found that CC1200-DK had better PER than iM880A when there was a clear line-of-sight (LoS) i.e. when there were no obstructions such as trees or buildings. In non-LoS iM880A performed slightly better than CC1200-DK, owing to its spread spectrum modulation which allows it to tolerate interference. CC1200-DK was also found to have better RSSI and header efficiency than iM880A.
The researchers are developing a tool that can predict the performance of sub-GHz devices using the RSSI values they have obtained from the study. “It will decrease the time required for testing thus saving time and money for sensor deployment”, says Nihesh.
The researchers plan to develop an IoT network for water management that is reliable and energy efficient. Selection of an appropriate device for the network depends on whether the header inefficiencies of iMM880A can be removed. “Based on our study, we have proposed a network plan for water level monitoring inside IISc campus. Sensor deployment and network setup will be the next step”, says Nihesh.
About the authors
Dr. Rajesh Sundaresan is an Associate Professor of Department of Electrical Communication Engineering, IISc. Nihesh Rathod is a student at the Department of Electrical Communication Engineering.
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