Optimizing Signal Propagation for 2.4 GHz In-field WSN Systems over Outdoor Line-of-Sight ChannelsZhen Li, Tiansheng Hong*College of Engineering South China Agricultural University Guangzhou, China, 510642E-mail: {lizhen, tshong}@Ning WangDept. of Biosystems and Agricultural Engineering Oklahoma State UniversityStillwater, OK, USA, 74078E-mail: ning.wang@Abstract—The objective of this study was to evaluate in-field radio frequency signal propagation at 2.4GHz band wireless sensor network (WSN) links. Commercial wireless sensor motes using this band as transmitter was wirelessly connected to its corresponding receiver and a hand-held spectrum analyzer. Indexed packets transmitted from the transmitter were captured by a spectrum analyzer to measure path-loss and synchronously received by a receiver using equal mote model to calculate packet delivery rate. Experiments were implemented in a research field of Oklahoma State University where wheat was planted. Impact factors considered were: plant canopy height, transmitter height, receiver height, carrier frequency, transmitter-to-receiver distance (T-R distance). Results indicated that RF signal was subject to plant canopy height. Univariate ANOVA results indicated in-field RF signal path-loss was subject to system configurations and plant height as well as their interactions. Estimated marginal means plots indicated that the best performance in general, if taking all plant canopy heights into consideration, might happen when the transmitter was installed at a height of 2 m and the receiver was mounted at the height of 3 m.Keywords-path-loss; wireless sensor networks; signal propagation; precision agricultureI.I NTRODUCTIONThe current state-of-the-art wireless sensor network (WSN) technology is a promising solution for remote, large-scale, real-time, and continuous environmental data acquisition [1,2]. It offers the advantages of simplified wiring and harnessing thus has particular benefits for agricultural applications such as field physical property monitoring. Successful WSN implementations have already demonstrated improved precision agricultural management and operation in a real-time fashion and continue to progress [3-6].ZigBee technology is a low cost and low power consumption solution for WSN applications which need long battery lives but does not require high data transfer rates [7]. Four physical layers (PHYs) in the industrial, scientific and medical (ISM) bands were specified for the low-rate wireless personal area networks (IEEE 802.15.4/LR-WPANs) in *Corresponding author which three of them were in the 868/915MHz bands and one in the 2.4GHz band [8]. Advantages of using 2.4GHz high frequency band include 250kbps data rate compared to 20kbps or 40kbps of using 868MHz or 915MHz bands, usability of small size antennas, frequency reuse, and low power consumption. However, the low frequency bands achieve longer signal propagation range and larger cover area due to receiver sensitivity reached -92dBm for 868/915MHz bands at lower data rate compared to -85dBm for 2.4GHz band [9].Quantified relationships between path-loss and the impact factors during data transmission and radio propagation coverage will be useful to design and deploy a reliable, high-performance WSN. However, research on radio performance as affected by agricultural configuration and environment was limited and no standard tests were available. This research aimed to carry out quantitative studies on the effects of the pass-loss introduced by various system configurations and vegetations including transmitter height, receiver height and plant canopy height on the in-field radio-wave path-loss in 2.4GHz band. The specific objectives include:1.To find and compare the major factors that affectRF path-loss at 2.4GHz band for in-field WSNthroughout the wheat growing stages.2.To evaluate and compare impact factor effects onRF signal path-loss and packet delivery rate at theselected RF band.3.To determine the optimal configurations for reliabledata communications in a field WSN 2.4GHz RFband.II.M EHOD AND M ATERIALSA.Definition of In-field Path-lossPath-loss measures the average radio frequency (RF) attenuation along the path of radio propagation imposed on a transmitted signal when it arrives at the receiver. In this study, the experiment relied on the narrow-band measurements of continuous radio wave (CW) signals at 2.4GHz. Path-loss within a distance d was calculated using in-field measured transmitted and received power, respectively[10]. Logarithmic transformations on path-loss,transmitted and received power were carried out as described in [11].The logarithmic transformed equation for path-loss calculation does not hold when the measured received power equals the measured transmitted power. The result is that the actual power coming out of the antenna is inaccessible. In this study, a representation for a close-in distance (d 0) and the received power reference point were introduced into the path-loss calculations [12]. Here, d 0 was set to 1 m during the experiments. The path-loss within distance d was calculated as in0()rmd rmd PL d P P =- (1)where P rmd 0 is the power of the received signal at d 0 = 1 m (dBm), P rmd is the measured signal strength at d (dBm), and PL (d ) is the path-loss within d (dB). Further analysis of path-loss at 2.4GHz band was generalized using Eq.1. It was assumed that there was no significant difference between signal attenuation at a distance of 1 m during the experiments. B. Selection of Impact FactorsThe impact factors considered in the experiments included the distance between a transmitter and a receiver (T-R distance, d ), the height of a transmitter (h t ), the height of a receiver (h r ), the height of plant canopy (h p ), carrier frequency (f ), antenna gain (G ) of both transmitter and receiver (G t and G r ). The height of plant canopy served as a blocking factor. Values of the impact factors used in the experiments are shown in Table 1. The maximum separation distance tested was 130m, the maximum values of the heights of the transmitter and receiver were set at 3 m. These values were determined based on the results from previous tests [11].TABLE I. S ELECTED IMPACT FACTORS AND THEIR LEVELSFactors d (m) h t (m) h r (m) f (MHz) G (dBi) Values20~130 with 10 interval1 2 31 2 324700 (G t ) 2.15 (G r )C. Measurement DevicesTransmitters : One type of node developed by Crossbow Technology [11] named IRIS was applied as transmitters. Once a node received a predefined request from a paired controller using equal model of node, it transmitted indexed packets to a receiver which measured both received signal strength and packet delivery rate (PDR) at the base station. A tri-pole was built to fix the transmitter at the heights of 1, 2, and 3 m (Fig. 1a). A plastic mounting pad was fixed at each height. The transmitter was attached to the pads using Velcro, as shown in Fig.1b. The tri-pole could be carried to any source spots, and it introduced minimum interference to the antenna polarization pattern.The IRIS node used Atmel’s AT86RF230 (Atmel Crop., San Jose, CA, USA) as the IEEE 802.15.4 compliant RF transceiver. The transmission power was set to be 3.2dBm and the selected carrier frequency was 2470MHz (Channel 24) for not disturbing the working nodes in the same field. The modulation technique was the direct-sequence spread-spectrum and quadratic phase shift keying (DSSS/QPSK). A 1/4 wave dipole antenna with 0dBi gain was used.Receivers : The receivers were located at the receiving spot for different measurements. It was composed of four major components: a Laptop computer (D630, Dell, USA) for displaying and restoring experimental results including spectrum and packets in real-time, a handheld RF spectrum analyzer (N9340B, Agilent Technology, USA) for path-loss and signal attenuation analysis, an IRIS mote attached to a MIB510 mother board, and a cattle mote attached to a CC1010 development kit mother board both as base stations to receive indexed packets from different transmitters for packet PDR calculation.A frame was built to hold the spectrum analyzer while the two mother boards were mounted on the side for retrieving similar heights. The frame was fixed on a flag-pole which was placed on the edge of the field. The height of the base station (Laptop not included) was adjustable. Key configurations of the spectrum analyzer were: pre-amplifier on, frequency span: 5MHz, resolution bandwidth (RBW): 100kHz, video bandwidth: 30kHz, attenuation: -10dB, reference level: -30dBm. By using the configurations above, the DANL was -100dBm in the experiments.(a) (b)Figure 1. Transmitter fixtures. (a) Tri-pole overview. (b) Cattle node onmounting pad.D. Measurement SoftwareTwo software were used during the field experiments: Agilent N9340 PC Software (Version A.01.04, Agilent Technologies, USA) and Realterm (Version 2.0.0.43, open source). The N9340 PC Software could (1) display real-time graphical RF power spectrum in span of certain carrier frequency, (2) export power spectrum to spread sheets, and (3) make basic configurations of the analyzer. The XSniffer and Realterm were used to display and restore the received indexed data for PDR calculations.E. Criterion of Using Wheat Canopy Heights as Blocks In the field, wheat canopy is the major attenuation source along transmission paths. In this study, canopy height was used as a blocking factor to make signal attenuation pattern similar within each block. The height thresholds for different blocks were calculated based on the principles of the Rayleigh roughness [13] and the Fresnel zone clearance [14]. These thresholds blocked the wheat growth into three stages [11].Eq. 2 was derived from the Rayleigh roughness criterion but took the transmitter and receiver heights into1t rH（2）where H1 was the first threshold of crop height in m, h t and h r were the heights of a transmitter and a receiver in m, respectively, d was the separation distance in m and λwas the wavelength in m. If the wheat height was lower than H1, the reflected waves from both the ground and plant were in phase and led to the situation of specular reflection. If the wheat height is higher than H1, more diffusion reflection is introduced which means waves are not in phase. The minimum H1is 0.14 m for 915MHz carrier frequency and 0.05 m for 2470MHz. As a result, H1=0.05m was considered as the first plant height threshold.The second principle applied for the height threshold determination was the Fresnel zone clearance. It was commonly used to analyze interference introduced by obstacles near the path of a radio beam for line-of-sight communications [15]. The first Fresnel zone ellipsoid is the highest in the center of the line-of-sight RF transmission. It must be kept largely free from obstructions to avoid interference with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated, as a rule of thumb, the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less. The second plant height threshold (H2) was calculated for using both 915MHz and 2470MHz carrier frequencies in previous studies as 0.40 m [11].Using the two calculated thresholds, the growth of wheat was divided into three stages (blocks) based on plant canopy height (h p). Thresholds and signal attenuation patterns within each stage were explained in Table 2. H1 equaled 0.05 m as the threshold for dividing specular and diffusion reflection and H2 equaled 0.4 m for dividing whether or not the Fresnel zone was clear when both transmitter and receiver were at the height of 3 m.TABLE II. T HRESHOLDS AND SIGNAL ATTENUATION PATTERNS OFDIFFERENT PLANT HEIGHT BLOCKSPlant height blocks Canopy heightrange*Radio-wave attenuationpattern1 0 ≤ h p < H1Specular reflection, Fresnel zone clear2 H1≤h p < H2Diffusion reflection, Fresnel zone clear3 H2≤h p < 0.80mDiffusion reflection, Fresnel zone not clear* H1 = 0.05 m, H2 = 0.40 mThe maximum plant height for the third stage was measured during field experiments valued around 0.8 m. In the following sections, the three stages of wheat growth were referred to as plant height blocks 1, 2, and 3 with plant height intervals of [0m, 0.05m], [0.05m, 0.4m] and [0.4m, 0.8m], respectively. F.Experimental Field SetupField experiments started on 6 January 2009 and endedon 22 May 2009, which covered a complete wheat growthseason in Stillwater, Oklahoma. The testing plots werelocated in the experimental field where a WSN for soilproperty monitoring was deployed [9,10]. Crop heights andreceived signal strength data were collected during theexperiment period.A receiving spot was located at one edge of the field.Twelve spots with different T-R distances from Table 1, namely “source spots”, were marke d inside the field. Theseparation distance between the first source spot and the basestation was 20 m ± 0.5 m. The following source spots werelocated in series with a 10 m ±0.5 m interval toward theother end of the field. A tri-pole with the node acting astransmitter was placed at each source spot during the experiment. The strengths of the received signal and amount indexed packets, when the transmitters were at different T-R distances and under different system configurations, were recorded by a hand-held spectrum analyzer and two base stations corresponding to the two transmitters separately at the receiving spot.G.Evaluation of Signal Propagation PerformanceThree dimensional (3D) surface curves were plottedusing Matlab (Version 2008a, The MathWorks, Natick, MA,USA) to evaluate the effects of T-R distance andtransmitter/receiver elevation differences on the performanceof the RF signal propagation under field conditions blocked in different plant growth stages with ling-of-sight communication.H.Evaluation of Impact Factors’ Influence on Path-LossUnivariate analysis of variance (ANOVA) was carriedout to identify and compare which impact factor(s) hadsignificant influences on the path-loss using 2.4GHz carrier frequency. Independent variables included transmitter height, receiver height, plant canopy height, and T-R distance. Treatment levels were as displayed in Table 1. Evaluated interactions included transmitter height and plant canopy height, receiver height and plant canopy height, and transmitter height and receiver height. A significance level of 0.05 was used. Estimated marginal means plots were generated to find the optimal transmitter height and receiver height when using 2.4GHz carrier frequency.Marginal analysis was carried out and estimated marginalmeans were plotted to evaluate and compare transmitter or receiver height affecting RF signal performance when taking all plant canopy heights into consideration. Two pairs of optimized transmitter and receiver heights were obtained for using both carrier frequencies based on estimated marginal means plots when the at which the marginal means reached the lowest point.III.R ESULTS AND D ISCUSSIONA.Signal Strength at the Received-Power Reference PointTo determine the signal strength at the received-powerreference point, both base station and motes were kept at 1 min height and 1 m apart from each other. The transmission power was 4.0 dBm for the cattle mote. The effective radiated power as measured signal strength at the received-power reference point was -18.76 dBm. The path-loss from using 2.4GHz carrier frequency used in the following analysis were calculated as inPL(d) = -18.76 - P rmd (3) where P rmd was the measured signal strength at d in dBm, PL(d) was the path-loss within d in dB.B.Signal Propagation Performance Evaluation3D surface curves of path-loss versus T-R distance and transmitter/receiver elevation difference were depicted in Fig.2. In general, there were combined effects of transmitter/receiver height and T-R distance on path-loss. The surface areas colored from green to orange extended from Fig. 2a to Fig. 2c, indicating that the transition from low path-loss (dark blue) to high path-loss (dark red) went smoothly along with plant growth for 2.4GHz carrier frequency.(a)(b)(c)Figure 2. Measured path-loss along the T-R distance under different transmitter/receiver heights. (a) to (c) Path-loss blocked in plant growthstages 1 to 3 at 2.4GHz band. C.Evaluation of Impact Factors’ Influence on Path-lossUnivariate ANOVA results of the impact factors’influences to the path-loss were shown in Table 2. Results from using 2.4GHz carrier frequency indicated that: (1) all the single impact factors had significant influence to path-loss (Sig < 0.01); (2) all the impact factor interactions had significant influences to path-loss (Sig < 0.01).TABLE III. U NIVARIATE ANOVA RESULTS OF THE IMPACT FACTORS’INFLUENCES ON THE PATH-LOSSSource df F Sig.Corrected Model 125 15.731 0.000*TH 2 23.998 0.000*RH 2 42.895 0.000*Dist 11 120.112 0.000*PH 2 100.681 0.000*TH * PH 4 12.629 0.000*RH * PH 4 11.142 0.000*TH * RH 4 6.652 0.000*Total 320Corrected Total 319*Factor influence was significant at the 0.01 levelTwo out of seven analyzed affecting sources had significant influences to path-loss from using 2.4GHz carrier frequency. It can be concluded that in-field RF signal path-loss was more subject to system configurations and plant height as well as their interactions for using higher carrier frequency.(a)(b)Figure 3. Estimated marginal means of path-loss. (a) and (b), Transmitter height or receiver height as influencing source using 2.4GHz carrierfrequency.Estimated marginal means of the path-loss to transmitter height or receiver height from using 2.4GHz carrier frequency were depicted in Fig. 3 (a) and (b), respectively. InFig.3 a, the minimum path-loss marginal mean was achieved at the transmitter height of 2 m. However, even it was at the valley of the curve, there was no significant difference between marginal means achieved at the transmitter heights of 2 m and 3 m. In Fig.3 b, the minimum path-loss marginal mean was achieved at the receiver height of 3 m. The trend of the curve in Fig.3 b was suggesting that lower marginal means of path-loss may be achieved at a higher receiver elevation. As a result, the best performance in general, if taking all plant canopy heights into consideration, might happen when the transmitter was installed at the height of 2 m and the receiver at the height of 3 m using 2.4GHz carrier frequency in this study.D.Reliable Communication Distance Based on PDRDuring the field experiments, the scenario when both transmitter and receiver heights were at one meter height at plant stage 3 was considered to be the worst case when the lowest PDR was found using the maximum transmission power. Another extreme case was that both transmitter and receiver heights were 3 meter at plant stage 1 which was considered to be the best case. As a result, a reliable communication distance for in-field WSN application using 2.4GMHz carrier frequency was considered to be 70 m, since the one-time PDR values for both the best and worst cases were higher than 80% at this distance.IV.C ONCLUSIONIn this study, wireless sensor motes using 2.4GHz carrier frequency as transmitters were wirelessly connected to their corresponding receivers and a hand-held spectrum analyzer. Plant canopy height, transmitter height, receiver height and transmitter-to-receiver distance were considered as impact factors on the radio propagation.Univariate ANOVA results indicated that all seven analyzed affecting sources had significant influences to path-loss from using 2.4GHz carrier frequency. It can be concluded that in-field RF signal path-loss was more subject to system configurations and plant height as well as their interactions for using higher carrier frequency.Estimated marginal means plots indicated that the best performance might happen when the transmitter was installed at a height of 2 m and the receiver was mounted at the height of 3 m for using both kinds of motes. However, it was possible to further lower the path-loss marginal means thus to achieve better communication with higher receiver elevations for using 2.4GHz carrier frequency.Environmental factors such as humidity, temperature, wind and solar radiation are still unclear factors for in-field radio signal propagation. 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