# Effect of Randomness in Element Position on Performance of Communication Array Antennas in Internet of Things

Authors: Congsi Wang; Yan Wang; Xiaodong Yang; Wei Gao; Chao Jiang; Lu Wang; Yiqun Zhang; Meng Wang
Journal: Wireless Communications and Mobile Computing
 (2018)

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## Abstract

As a critical component for wireless communication, active phased array antennas face the restrictions of creating effective performance with the effect of randomness in the position of the array element, which are inevitably produced in the manufacturing and operating process of antenna. A new method for efficiently and effectively evaluating the statistic performance of antenna is presented, with consideration of randomness in element position. A coupled structural-electromagnetic statistic model for array antenna is proposed from the viewpoint of electromechanical coupling. Lastly, a12×12 planar array is illustrated to evaluate the performance of antenna with the saddle-shaped distortion and random position error. The results show that the presented model can obtain the antenna performance quickly and effectively, providing an advantageous guidance for structural design and performance optimization for array antennas in wireless application.


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## Body

## 1. Introduction

The application of wireless communication promotes the realization and development of physical things in our daily life exchanging information from a network, which is called the Internet of Things (IoT) [1–3]. In the wireless communication, the choice of antenna is a critical component. Active phased array antennas have such significant advantages, including rapid reconfiguration or revisit rate, multibeams, shaped beams, sidelobe control, and high reliability over other types of antennas, and there has been an increased interest in their use for a wide variety of communication and remote-sensing applications, such as serving as the ground station terminal to track the satellite for IOT data connection [4]. In the working process, however, the manufacturing and processing of the antenna, any movement of the carrier platform, and the external environment load could lead to the structure errors, including both the random position error and systematic error. The combination of the random position error and systematic distortion finally results in the randomness in element position. Finally, the electromagnetic performance of array antenna could be degraded, such as the gain loss, sidelobe level (SLL) rising, beam width broadening, and pointing error [5–9]. As a result, the communication distance will be shortened and the resistance to interference will be reduced, which seriously restricts the realization of high performance of array antenna. The antenna faces the restrictions of creating effective performance under structure errors. Therefore, it is necessary to explore deeply the coupling relationship between structure error and electromagnetic performance for active phased array antenna with the effect of randomness in element position [10–12].The present works for analyzing the effect of structure error on electromagnetic performance of array antenna are carried out mainly from the following three aspects. Firstly, assuming that the systematic distortion of antenna is a certain shape and that the influence of only systematic error on performance is analyzed, [13] studied the effect of symmetrical and unsymmetrical bend of array plane on the performance of planar antenna. Reference [14] discussed the influence of four array plane distortions including the sag, potato chip, and sinusoidal and Bessel character for array antenna. The above works in [13, 14] consider only the systematic error without considering the random position errors which are also produced inevitably during the manufacturing and working process. Secondly, some works are focusing on the analysis of the effect of random error uncertainty. Reference [15] explored the influence of random errors on the performance of hexagonal active phased array antenna and achieved some instructive conclusions. But there are no discussions on the systematic distortion. Furthermore, there is research indicating that the effect of random errors could be equivalent to the influence of excitation current errors, and the performances of antenna with random current errors are studied [16, 17]. However, there is lack of direct relationship of random position error with the antenna performance. In addition, [18] investigated the subarray position error and its influence on antenna performance by using the probability statistical theory. But the method requires lots of repeated calculations to get the statistical performance of antenna. Thirdly, some works introduced the linear combination of random error and systematic distortion to analyze their effect on antenna performance. In [19, 20], the sum of random error and systematic error was regarded as the structure error item, used to analyze the performance of distorted array antenna. However, in practice the degrees of random error and systematic error show great differences; the linear combination could mitigate the effect of random error when there is big difference between the degrees of random and systematic errors. Moreover, systematic error is supposed to be deterministic but random error is stochastic; the combination errors could result in the randomness of electromagnetic performance. So it is more accurate to estimate the electromagnetic performance from the perspective of statistic property when random error exists.Therefore, this paper presents a new method for efficiently and effectively evaluating the statistic performance of active phased array antenna, with consideration of randomness in the position of the array element. A coupled structural-electromagnetic statistic model is proposed, from the viewpoint of electromechanical coupling. The method provides an advantageous guidance for structural design and performance optimization for array antennas in wireless application.


## 2. Coupled Structure-Electromagnetic Statistic Model with Randomness in Element Position

As shown in Figure1, the array radiation elements are assembled with an equal interval, whose numbers are M×N. The intervals of the array elements along x and y directions are dx and dy, respectively.Figure 1
Element configuration of planar array antenna.θ , ϕ is the direction of the far-field target relative to the coordinate system Oxyz as shown in Figure 2, whose direction cosine is cos⁡αx,cos⁡αy,cos⁡αz [21].Figure 2
Space geometrical relationship of far-field target.The manufacturing and working process of antenna lead to the structure errors, including the random error and systematic distortion. Suppose the random error of elementm,n0≤m≤M-1,0≤n≤N-1 is Δxmnr,Δymnr,Δzmnr; the phase difference ΔΦmnr in regard to the coordinate origin in Figure 1 is given as follows.(1)ΔΦmnr=kΔxmnrsin⁡θcos⁡ϕ+Δymnrsin⁡θsin⁡ϕ+Δzmnrcos⁡θ.Next, suppose the systematic distortion of elementm,n is Δxmns,Δymns,Δzmns; the phase difference ΔΦmns in regard to the coordinate origin in Figure 1 is given as follows.(2)ΔΦmns=kΔxmnssin⁡θcos⁡ϕ+Δymnssin⁡θsin⁡ϕ+Δzmnscos⁡θ.According to the superposition principle of the array antenna without element coupling, the filed density pattern function for planar rectangular active phased array antenna with systematic distortion and random position error is developed as follows.(3)Esrθ,ϕ=feθ,ϕ∑m=0M-1∑n=0N-1Imn·ejΔΦmn+ΔΦmnr+ΔΦmns+φmnB,where feθ,ϕ is the pattern function of element in free space, Imn is the amplitude of excitation current of element m,n, φmnB is the array’s inherent phase difference determined by phase shifter, and ΔΦmn=kmdxsin⁡θcos⁡ϕ+kndysin⁡θsin⁡ϕ is the initial spatial phase distribution.In practice, random position errors of elements are probabilistic variables; it is necessary to analyze the statistic property of electromagnetic performance for array antenna with random error included. Suppose the random position errors alongx, y, and z directions are Δxmnr, Δymnr, and Δzmnr, which are subjected to the normal distribution, with means of zero and variances of σx2, σy2, and σz2, respectively. Then phase difference ΔΦmnr in (1) is also normally distributed, and the variance is obtained as follows.(4)σΦr2=k2σx2sin⁡θcos⁡ϕ2+σy2sin⁡θsin⁡ϕ2+σz2cos⁡θ2.For any random variable with normal distribution, expressed asx~N0,σx, the function relations cos⁡x¯ and sin⁡x¯ can be obtained from [22]. Applying the function relations, the mean of exponential function is ejx¯=e-σx2/2. Therefore, the mean of the field density pattern function Esr(θ,ϕ) in (3) is deduced as follows.(5)Esrθ,ϕ¯=feθ,ϕ∑m=0M-1∑n=0N-1Imn·ejΔΦmn+ΔΦmns+φmnB·ejΔΦmnr¯=Esθ,ϕ·e-1/2σΦr2,where Es(θ,ϕ) is the field density pattern function with only the systematic distortion existing.Then the mean of the power pattern function for array antenna is deduced according to the property of varianceσEsr2=Esr(θ,ϕ)·Esr∗(θ,ϕ)¯-Esr(θ,ϕ)¯·Esr(θ,ϕ)¯∗ as follows.(6)Psrθ,ϕ¯=Esrθ,ϕ·Esr∗θ,ϕ¯=σEsr2+Esrθ,ϕ¯·Esrθ,ϕ¯∗.Substituting the functionEsr(θ,ϕ)¯ into (6), the mean of power pattern function is expressed as follows.(7)Psrθ,ϕ¯=σEsr2+Esθ,ϕ2·e-σΦr2,where σEsr2 is the variance of function Esr(θ,ϕ). The expression of the variance σEsr2 is presented as follows.Firstly, as a vector quantity,Esr(θ,ϕ) can be represented as the combination of the real part and imaginary part [23]. Assume X=∑m=0M-1∑n=0N-1Xmn and Y=∑m=0M-1∑n=0N-1Ymn are the real and imaginary parts, respectively. The expressions of Xmn and Ymn are Xmn=feθ,ϕ·Imn·cos⁡ΔΦmn+ΔΦmnr+ΔΦmns+φmnB and Ymn=feθ,ϕ·Imn·sin⁡ΔΦmn+ΔΦmnr+ΔΦmns+φmnB, respectively.Next, the varianceσEsr2 is deduced as follows.(8)σEsr2=σX2+σY2=∑m=0M-1∑n=0N-1σXmn2+∑m=0M-1∑n=0N-1σYmn2,where σXmn2=Xmn2¯-Xmn¯2 and σYmn2=Ymn2¯-Ymn¯2.Since the functioncos⁡ΔΦmn+ΔΦmnr+ΔΦmns+φmnB¯ is equal to cos⁡ΔΦmn+ΔΦmns+φmnBe-1/2σΦr2, then(9)Xmn¯=feθ,ϕ·Imn·cos⁡ΔΦmn+ΔΦmns+φmnBe-1/2σΦr2Xmn2¯=feθ,ϕ2·Imn2·1+cos⁡2ΔΦmn+2ΔΦmns+2φmnBe-2σΦr22.Therefore, the varianceσXmn2 is deduced as follows.(10)σXmn2=feθ,ϕ2Imn2·1+cos⁡2ΔΦmn+2ΔΦmns+2φmnBe-2σΦr22-1+cos⁡2ΔΦmn+2ΔΦmns+2φmnB2e-σΦr2.Since the functionsin⁡Δφmn+Δφmnr+Δφmns+φmnB¯ is equal to sin⁡Δφmn+Δφmns+φmnBe-1/2σΦr2, then(11)Ymn¯=feθ,ϕ·Imn·sin⁡2ΔΦmn+2ΔΦmns+2φmnBe-1/2σΦr2(12)Ymn2¯=feθ,ϕ2·Imn2·1-cos⁡2ΔΦmn+2ΔΦmns+2φmnBe-2σΦ22(13)σYmn2=feθ,ϕ2Imn2·1-cos⁡2ΔΦmn+2ΔΦmns+2φmnBe-2σΦr22-1-cos2ΔΦmn+2ΔΦmns+2φmnB2e-σΦr2.The varianceσEsr2 then is deduced by substituting (10) and (13) into (8) as follows.(14)σEsr2=∑m=0M-1∑n=0N-1σXmn2+∑m=0M-1∑n=0N-1σYmn2=feθ,ϕ21-e-σΦr2∑m=0M-1∑n=0N-1Imn2.Finally, the mean of power pattern function with randomness in element position, which is also called the coupled structure-electromagnetic statistic model, is developed as follows.(15)Psrθ,ϕ¯=Esθ,ϕ2e-σΦr2+feθ,ϕ21-e-σΦr2∑m=0M-1∑n=0N-1Imn2=Esθ,ϕ2e-k2σx2sin⁡θcos⁡ϕ2+σy2sin⁡θsin⁡ϕ2+σz2cos⁡θ2+feθ,ϕ21-e-k2σx2sin⁡θcos⁡ϕ2+σy2sin⁡θsin⁡ϕ2+σz2cos⁡θ2∑m=0M-1∑n=0N-1Imn2.


## 3. Verification of Coupled Structure-Electromagnetic Statistic Model

In order to illustrate the effectiveness of the developed model, the coupled structure-electromagnetic statistic model is verified by simulations with HFSS software, which can provide stable and accurate solutions at even high frequencies and has been widely used by engineers to design reliable products [24].In consideration of the array antennas in wireless applications, an8×8 patch array antenna is illustrated [25], with the intervals along x and y directions being both 0.75λ, as shown in Figure 3. The frequency of the patch array antenna is 30 GHz. The physical size of the patch array antenna is 60 mm × 60 mm. For each patch antenna, the length and width are L=3.953mm and W=3.160mm, respectively. The initial excitation current is equal in amplitude and phase. Saddle-shaped distortion with the maximum displacement in z direction of λ/6 is assumed as the systematic distortion, and the random position errors in x, y, and z directions are all assumed to satisfy the normal distribution with mean of 0 and variance of λ/30.Figure 3
Patch array antenna model.Firstly, random samples of structure errors are generated by adding the specific saddle-shaped distortion with each random position error sample produced according to the normal distribution. Then the antenna performance with every structure error sample is simulated separately with HFSS software without consideration of element coupling. Lastly the mean of the antenna performance is calculated by averaging the sum of the performances obtained from all the structure error samples. Here simulations with 1000 structure error samples are taken because the mean of antenna performance with greater than 1000 samples results in no further change. The comparison with the performance calculated by the developed model is shown in Figure4 and the corresponding parameters are listed in Table 1.Table 1
Parameters of HFSS-based and coupled model-based results.
Performance HFSS-based result Coupled model-based result Gain/dB 60.12 60.12 First SLL/dB inϕ=0° plane 50.17 50.17 inϕ=90° plane 49.87 49.96 Second SLL/dB inϕ=0° plane 44.15 44.52 inϕ=90° plane 43.88 44.07 Third SLL/dB inϕ=0° plane 41.34 41.61 inϕ=90° plane 40.28 40.61 Fourth SLL/dB inϕ=0° plane 39.23 39.66 inϕ=90° plane 37.46 37.85 Fifth SLL/dB inϕ=0° plane 37.44 37.84 inϕ=90° plane 33.71 34.19 Beam width/° inϕ=0° plane 8.80 8.80 inϕ=90° plane 8.79 8.79 Boresight pointing/° inϕ=0° plane 0.15 0.15 inϕ=90° plane 0.13 0.13 Note. Every SLL is the right SLL.Figure 4
HFSS-based and coupled model-based results comparison.
(a)
ϕ = 0 ° plane (b)
ϕ = 90 ° planeAs shown in Figure4 and Table 1, the coupled model-based and HFSS-based results show good consistency in both the main lobe area and sidelobe area. The gain, beam width, and boresight pointing are the same in both ϕ=0∘ and ϕ=90∘ planes, which indicate that the main lobe areas are the same obtained by the coupled model and HFSS, respectively. The differences of the first sidelobes in absolute values are 0 dB and 0.09 dB in ϕ=0∘ and ϕ=90∘ planes, respectively. For the second, third, fourth, and fifth sidelobes, the maximum differences in absolute values are 0.43 dB and 0.48 dB in ϕ=0∘ and ϕ=90∘ planes, respectively. Therefore, the results above prove that the proposed coupled model is effective for analyzing the effect of randomness in element position on electromagnetic performance for array antennas.


## 4. Simulation and Discussion

For active phased array antenna, both the random position error and systematic distortion consist of the structure deformation for antenna in practice. Random position error appears as the random variable.In engineering, the antenna array region is generally distorted into a representative saddle shape. Based on the mathematical features of the surface, thez-coordinate of the phase centre of array element (m,n) in saddle shape is given as follows.(16)Δzmn=Δzmaxxmn2xmax2-ymn2ymax2,where xmax and ymax are the half-length and width of the array aperture size, respectively, and Δzmax is the maximum displacement of the array elements in z direction that belongs to the systematic error.Therefore, the planar array antenna with both the saddle-shaped distortion and element random position error is discussed as follows. A12×12 planar rectangular array is illustrated, with the intervals of array elements in x and y directions being both λ/2. The frequency is 9.375 GHz and the initial excitation current is equal in amplitude and phase. After analysis suppose the random errors along x, y, and z directions are subjected to the normal distribution, with mean of zero and the same variance of λ/20, and the maximum displacement of the saddle shape in z direction is selected as λ/5. The presented statistic model is applied to evaluate the statistic performance of distorted array antenna. In addition, the comparison with the existing linearly combination of systematic distortion and different random distributions of random errors is also discussed, where the used power pattern function is Psr(θ,ϕ)=Esr(θ,ϕ)·Esr∗(θ,ϕ) and Esr(θ,ϕ) is the field density in (3). The systematic distortion is taken as the same saddle shape. Meanwhile, four random samples from sample 1 to sample 4 are generated according to the same normal distribution, taken as the random errors along x, y, and z directions. The comparison results are shown in Figure 5 and the corresponding parameters are listed in Table 2.Table 2
Parameters variations with systematic distortion and different random errors.
Performance Random sample 1 Random sample 2 Random sample 3 Random sample 4 Statistic random error Gain loss/dB - 2.00 - 2.53 - 1.89 - 2.22 - 2.23 First SLL change/dB inϕ=0° plane + 0.40 + 0.41 + 0.56 + 0.72 + 0.53 inϕ=90° plane + 1.65 + 2.09 + 0.64 + 2.10 + 1.53 Second SLL change/dB inϕ=0° plane - 1.32 - 0.14 - 0.52 - 0.39 - 0.01 inϕ=90° plane - 0.16 - 1.47 + 0.93 - 1.40 - 0.01 Third SLL change/dB inϕ=0° plane - 4.91 + 1.26 + 0.63 + 0.79 - 0.31 inϕ=90° plane + 0.28 + 0.66 - 1.40 + 0.29 - 0.31 Fourth SLL change/dB inϕ=0° plane - 0.34 - 5.45 + 1.74 + 0.59 - 0.25 inϕ=90° plane + 1.26 - 0.52 - 0.92 - 1.25 - 0.25 Fifth SLL change/dB inϕ=0° plane - 0.54 + 0.56 - 0.87 - 2.36 - 0.09 inϕ=90° plane - 2.89 + 1.21 - 2.81 - 0.28 - 0.09 Beam width change/° inϕ=0° plane + 0.30 + 0.34 + 0.23 + 0.28 + 0.29 inϕ=90° plane + 0.40 + 0.34 + 0.28 + 0.34 + 0.35 Boresight pointing/° inϕ=0° plane + 0.34 - 0.31 + 0.12 - 0.14 + 0.28 inϕ=90° plane + 0.33 + 0.30 - 0.31 + 0.16 + 0.19 Note. Every SLL is the right SLL, + indicates upgrade and right side of θ=0°, and − indicates decrease and left side of θ=0°.Figure 5
Performance with systematic distortion and random error.
(a)
ϕ = 0 ∘ plane (b)
ϕ = 90 ∘ planeAs shown in Figure5 and Table 2, it follows that( 1 ) the gain decreases greatly when saddle shape and random error coexist, with the maximum gain loss being −2.53 dB. The gain loss obtained from the statistic random error, which is directly substituting the variances of the random errors and systematic distortion into the presented statistic model, shows a little difference from the values calculated by the four random samples, which are obtained by, respectively, adding each random sample with the systematic distortion as the whole structure error. The maximum difference is 0.34 dB, which indicates that the change of gain depends mainly on the systematic distortion.( 2 ) the sidelobe levels change differently with the four random samples in both ϕ=0∘ and ϕ=90∘ planes. From the first to the fifth sidelobe level, the maximum change is −5.45 dB, which indicates that the sidelobe level is closely related to the distribution of random error. Large performance errors will be produced when evaluating from any random distribution. Thus the effect of statistics of random error must be considered. The sidelobe levels from the statistic model could be regarded as the evaluation values for they are given from the mean of power pattern function.( 3 ) the beam width changes in small scales with different random samples, and the values are almost the same with the statistic random error. The maximum variations in ϕ=0∘ and ϕ=90∘ planes are 0.06∘ and 0.07∘, respectively. It shows that the beam width is influenced mainly by the systematic distortion.( 4 ) for the saddle shape is symmetry without influence on the pointing direction, the boresight pointing changes mainly from the different random error distribution. So it is more precise to extract from the statistic performance than from the performance calculated by adding a random sample with systematic distortion.In addition, the performance obtained from the presented statistic model is further verified especially on the sidelobe level and boresight pointing. Firstly the above four random samples are increased to one thousand samples with the same normal distribution. Next the mean of power pattern function is calculated by averaging the sum of performances obtained from all the samples. After one thousand times of calculations for all the samples, the average gain loss is −2.23 dB, which is the same value from the statistic model. The first sidelobe levels increase by 0.52 dB and 1.54 dB inϕ=0∘ and ϕ=90∘ planes, respectively. The absolute errors are only 0.01 dB in both the two planes. The beam width changes by 0.29∘ and 0.34∘, respectively, and the boresight pointing varies by 0.28∘ and 0.19∘, respectively, which are almost the same values as obtained from the presented static model.Therefore, the presented coupled structure-electromagnetic statistic model can evaluate the antenna performance quickly and effectively with the effect of randomness in element position.


## 5. Conclusion

Aimed at the restrictions of creating effective performance under the effect of randomness in the position of the array element, a coupled structural-electromagnetic statistic model for active phased array antenna is presented. The effect of randomness in element position on antenna performance is analyzed, compared with the electrical parameters obtained from the random sample errors combined with systematic distortion. The results show that random errors lead to the randomness of electromechanical performance of antenna, especially on the sidelobe levels compared to the gain and the beam width. The electrical parameters obtained from the statistic model are more accurate than from any random sample errors. The performance got from the statistic model is almost the same with the average of the sum of performances from a large amount of random samples. Thus the presented method can evaluate the antenna performance quickly and effectively. It provides an advantageous guidance for structural design and performance optimization for array antennas in wireless application. In addition, it can effectively reduce the repeated design time of antennas to achieve the objective of lower development cost and time.



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*Source: 6492143-2018-03-21.xml*