Experimental Calculation of Rssi for Jennic Wireless Sensor Network Platform

Wireless Sensor Network received signal strength and link quality Indicators, RSSI and LQI respectively, of Wireless Sensor Network are considered as a crucial indication for channel characterization and thus network communication success. Most of researcher used LQI as an estimation of RSS, although it is not accurate since it depend on approximation estimation. This research introduces an experimental method to calculate the RSS value of Jennic wireless network transceiver in dBm. The work has done indoor and outdoor environments to validate the results. The results can be used by researchers who adopt this platform based propagation and network communication prospective. Keyword: RSSI, LQI, wireless sensor network, ZigBee, propagation path loss. INTRODUCTION The emerging and advancing of nowadays significant technologies that encompass the facility to modernize the exploitation of physical environment data, enabling Wireless Sensor Networks (WSNs) to be used with various applications that were difficult or impossible to be conducted [1-3]. Applications WSNs for both Indoor and Outdoor, currently, are widely adopted within civilian, military, industry and space fields. The extensive observing of the earth’s environment, animal habitats, industrial sensing and analytics, civil structures, significant infrastructure safety, health monitoring and collecting of monitored data in hospitable locations counting biochemical hazard detection and tracking, as well as precision agriculture applications in agriculture fields beside military applications uses [1, 4, 5, 6]. Figure-1 shows the structure of WSN. It depicts the composing elements of the network which composed of plentiful end nodes that equipped with sensors and transceiver unit. These nodes are communicated for a coordinator node directly in single hope strategy or using multi-hope techniques. WSNs may use one of many communication technologies as a communication medium [7]. Figure-1. The structure of JN5148 wireless sensor nodes. One of popular technology used with WSN is ZigBee. It’s a 2400 MHz with 250 Kbps and less power consumption wireless platform technology. The radio unit of ZigBee is adopting signal quality gauges represented by the strength of the received wave indicator (RSSI) and quality of the communication link indicator (LQI). The received signal energy is considered as a worth indication of the wireless channel connection quality where the energy received within the radio unit is used as a computation factor. On the other hand, the link quality indicator is computed with reference to the ratio of the signal to the noise of the channel operating frequency, thus it offers for each received packet a mean correlation value [8]. Practically, it is difficult to compute the RSSI value since no instructions anywhere as how to compute it in dBm [9] even though, hardware manufacturing are not so clearly in providing a way to measure the received energy value within their manufactured chips and kits. The author in [10] uses the datasheet with the aids of protocol stack to specify the RSS value. They found that RSSI value is attached as the last byte within the internal data packet, following the LQI byte [9-10]. They are successfully read VOL. 10, NO. 18, OCTOBER 2015 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 8044 RSSI and LQI values easily, but when comparing the RSSI value to real RSS value (dBm); it became complicated due to mismatching. RSSI value is defined in datasheets but never explained as to how to calculate real RSS value in dBm. Although that, for Jennic WSN processor equipped in wireless sensor nodes [11], the signal strength (above the receiver sensitivity level) is displayed on the Master board using LEDs D1, D2, D3 and D4, as follows:  When all 4 LEDs are on; Very strong signal (greater than 60 dB).  When 3 LEDs are on; Good signal (greater than 40 dB).  When 2 LEDs are on; Acceptable signal (greater than 20 dB).  When 1 LEDs is on; Weak signal (less than 20 dB).  When 0 LEDs on; No packets are being received. The Link Quality Indication (LQI) value is output to the serial port to be read by a host computer. Figure-2 shows the record of serial data captured. This research, introduce an experimental method to calculate the (RSS LQI) of Jennic Wireless Sensor Network processor and clear the ambiguity for other researchers that used same hardware. Figure-2. Jennic wireless network node serial data captured. RSSI AND LQI INDICATORS There are two popular practical approaches are used to define the energy of wireless received packets; the received-power and the estimated received link-quality indicator, RSSI and LQI respectively. The calculated power of a received broadcasting signal is defined as RSSI. RSSI is applied and extensively used in 802.11 standards while received power can be calculated from RSSI. On the other hand, the estimation of how simply the received signal can be modulated when considering noise existing in the communication channel is defined by LQI [12]. RECEIVED SIGNAL ENERGY STRENGTH (RSS) The idea behind RSS is that the configured transmission power at the transmitting device (Pt) directly affects the receiving power at the receiving device (Pr). According to Friis’ free space transmission equation [12], the detected signal strength decreases quadratically with the distance to the sender [12]. = ∙ � ∙ � � � Where: Pt, Pr : The Power of the Transmission and Receive signals. Gt, Gr : Transmitter and Receiver gains. � : Signal wave length. d : Separation distance (sender and receiver). The received signal strength indicator (RSSI) is derived from the received signal strength, equation 2, and thus, within embedded devices, it is defined as a ratio of the received power to the reference power (Pref). Typically, the reference power represents an absolute value of PRef =1mWatt [12]. �[�� ] = ∙ �� ( � � ) VOL. 10, NO. 18, OCTOBER 2015 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 8045 Hence, increasing of RSSI value indicating of an increasing of received power. In practical scenarios, the ideal distribution of Pr is not applicable, because the propagation of the radio signal is interfered with a lot of influencing effects such as reflections on metallic objects, superposition of electromagnetic fields, diffraction at edges, and refraction by media with different propagation velocity, polarizations of electromagnetic fields and etc. These effects degrade the quality of the determined RSSI significantly [12-13]. Link quality indicator The abovementioned affects during broadcasting packets will lower the quality of RSSI tremendously. The definition of LQI in the 802.15.4 standard is presented but its context of use is not specified in this standard. Some works on the LQI, few of which are field experiments, have shown that the LQI decreases as the distance increases. The differentiation between RSSI and LQI is shown in Table-1 [12]: