Then the location of the reference node that reports the highest RSSI is considered the estimated location of the tracked node. The simplest method to determine the location of a tracked node is to request that the tracked node transmit a signal. The averaging time is eight symbol periods, which is required by IEEE 802.15.4 if the RSSI is going to be used to generate LQI. The received signal strength is measured over a period of time and then averaged to generate RSSI. The RSSI linearity indicates the maximum deviation of the plot of RSSI from a straight line versus the actual received signal power (in logarithmic scale). A typical commercially available transceiver is expected to be capable of providing ±4 dB or better typical RSSI accuracy. The RSSI accuracy indicates the average error associated with each received signal strength measurement. Also, the maximum signal energy that this receiver can report as RSSI is 4 dBm. For example, if the RSSI provided by a receiver has dynamic range of 92 dB (from −88 dBm to +4 dBm), the minimum signal energy the receiver can measure is −88 dBm. The RSSI dynamic range is specified in dB and indicates the minimum and maximum received signal energy that the receiver is capable of measuring. There are four parameters associated with RSSI: dynamic range, accuracy, linearity, and averaging period. Availability of RSSI means that a location-estimation system can be implemented without the need for any additional hardware for the individual nodes in the network. The RSSI can also be used to develop a coarse but simple method of location estimation. For example, the simplest way to generate the link quality indicator (LQI) is to use the RSSI as an indication of link quality. The RSSI and the time at which the packet was received (timestamp) are available to MAC, NWK, and APL layers for any type of analysis. The measured signal energy is quantized to form the received signal strength indicator (RSSI). The received signal strength (energy) can be measured for each received packet. Shahin Farahani, in ZigBee Wireless Networks and Transceivers, 2008 7.2 Received Signal Strength-Based Locationing Algorithms
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