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Study of Realistic Antenna Patterns in 5G Mmwave Cellular Scenarios

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Study of Realistic Antenna Patterns in 5G Mmwave Cellular Scenarios M. Rebato, L. Resteghini, C. Mazzucco, and M. Zorzi, “Study of realistic antenna patterns in 5G mmwave cellular scenarios”, in IEEE ICC Communications QoS, Reliability, and Modeling Symposium (ICC18 CQRM), Kansas City, USA, May 2018. Study of Realistic Antenna Patterns in 5G mmWave Cellular Scenarios Mattia Rebato∗, Laura Resteghiniy, Christian Mazzuccoy, Michele Zorzi∗ ∗University of Padova, Italy and yHUAWEI Technologies, Milan, Italy frebatoma, [email protected] flaura.resteghini, [email protected] Abstract—Large antenna arrays and millimeter-wave 3 arrays (mmWave) frequencies have been attracting growing attention serving in 3 sectors undesired lobes few as possible candidates to meet the high requirements of future 120° undesired lobes 5G mobile networks. In view of the large path loss attenuation TX in these bands, beamforming techniques that create a beam in TX RX RX the direction of the user equipment are essential to perform the transmission. For this purpose, in this paper, we aim at characterizing realistic antenna radiation patterns, motivated by desired steering desired steering A B direction C direction the need to properly capture mmWave propagation behaviors and understand the achievable performance in 5G cellular scenarios. Figure 1: Illustration of the different antenna configurations. Scheme A: In particular, we highlight how the performance changes with Configuration with three arrays serving three sectors with the single-element the radiation pattern used. Consequently, we conclude that it 3GPP antenna radiation pattern [3]. Scheme B: Antenna radiation pattern is crucial to use an accurate and realistic radiation model for using beamforming with isotropic radiation elements. Scheme C: Antenna proper performance assessment and system dimensioning. radiation pattern using beamforming with 3GPP radiation elements. Index Terms—Millimeter-wave, antenna radiation pattern, 5G, 3GPP antenna characterization, performance analysis sectors already used in traditional 4G LTE systems seems to be appropriate also for future cellular scenarios, and permits a better control in the design of both desired and undesired I. INTRODUCTION lobes (e.g., using arrays of patches that perform the steering Due to high path loss attenuations, multiple-input multiple- within the interval [−60◦; +60◦]). output (MIMO) systems with beamforming techniques are In this paper, motivated by the need to properly capture essential to ensure an acceptable range of communication in mmWave propagation behaviors and understand the achievable millimeter-wave (mmWave) networks [1]. In particular, the use performance (e.g., capacity or interference studies) in 5G of antenna arrays for future mobile scenarios is fundamental in cellular scenarios, we aim at accurately characterizing the order to create a beam in the direction of the user equipment antenna radiation pattern. In addition to a realistic antenna (UE), thus increasing the gain of the transmission. Among the pattern, we also incorporate mmWave channel characteristics possible antenna array designs, the most suitable approach is as derived from a measurement-based mmWave channel model the use of uniform planar arrays (UPA) where the antenna operating at 28 GHz provided by the New York University elements are evenly spaced on a two-dimensional plane and a (NYU) Wireless Group. This model is described in [4], [5], 3D beam can be synthesized [2]. and was adopted in our previous works [6], [7]. In order to precisely evaluate 5G mmWave cellular sce- For tractability of analysis or ease of computation, most of narios, it is important to consider realistic and accurate the works in the literature approximate the actual beamforming radiation models. Related works in the literature character- arXiv:1802.01316v1 [cs.NI] 5 Feb 2018 patterns by a sector model. An example of this approximation ize the antenna array either over-simplifying its gain with can be found in [8], where a piece-wise beamforming gain piece-wise functions, or modeling it as an array of isotropic function is used to characterize key features of an antenna transmitting sources. At high frequencies (e.g., mmWave pattern such as directivity gain, half-power beamwidth, and bands), where high attenuations are present, quantifying the front-back ratio. This model is too generic and cannot be actual antenna gain obtained due to the radiation pattern is compared with a realistic pattern because design parameters fundamental in order to precisely evaluate any mmWave sys- like gain, beamwidth and front-back ratio change according to tem. The radiation model proposed by the 3GPP in [3] can the steering direction. It is therefore challenging to compare be used to address this issue. This model precisely simulates this model with a realistic pattern since it uses fixed parameters the radiation pattern of a patch antenna element assuming large for all the steerable directions. attenuation for lobes in the opposite plane of transmission. A more precise antenna pattern can be obtained combining In addition, as illustrated in Scheme A of Fig. 1, the 3GPP together the array factor expression, which provides informa- specifications suggest modeling each base station (BS) site tion on the directivity equation of an antenna array, with the with three sectors, thus three arrays placed with central an- single element radiation pattern [9]. In [6], the array directivity ◦ gles shifted by 120 each. The configuration with multiple equation with isotropic antenna elements is applied. As a The authors would like to thank Dr. Danilo De Donno for useful discussions result, each transmitting source (i.e., antenna element) radiates about this work. equal power in all directions, while a detailed lobe-shaped radiation pattern is used to capture the beamforming gain. realistic. It assumes that any antenna element irradiates an (ISO) When considering beamforming with isotropic transmitting equal amount of power in each direction, thus: AE = sources, undesired lobes are generated as shown in Scheme 0 dB; 8θ 2 [0; π]; 8φ 2 [−π; π]. B of Fig. 1. Since BSs in real cellular systems do not transmit 2) 3GPP: The 3GPP model is realized following the spec- omnidirectionally but, instead, in sectors, it appears essential ifications in [3], [10], [11]. First, differently from the ISO to consider antenna radiation models that avoid the generation configuration, it implies the use of three sectors, thus three of undesired lobes. In this respect, a more realistic antenna arrays, placed as in traditional mobile networks. Second, the pattern (e.g., the one shown in Scheme C of Fig. 1) should be single element radiation pattern presents a high directivity with used when modeling UPA radiation. maximum gain in the main-lobe direction of about 8 dBi. The Our study leads to the following observations. First, we 3GPP AE of each single antenna element is composed of highlight how the radiation pattern significantly influences the horizontal and vertical radiation patterns. Specifically, this last performance evaluation of cellular scenarios. Results obtained pattern AE;V (θ) is obtained as with a simplified pattern appear to be significantly different ( 2 ) from those obtained using realistic radiation models. Specif- θ − 90 AE;V (θ) = − min 12 ; SLAV ; (1) ically, we try to quantify the interference perceived by a θ3dB generic user in a way to identify the working regime (e.g., where θ = 65◦ is the vertical 3 dB beamwidth, and noise or interference limited) of mmWave networks. Second, 3dB SLA = 30 dB is the side-lobe level limit. Similarly, the we compare different realistic antenna patterns in order to V horizontal pattern is computed as evaluate how the design choices influence the entire network ( ) performance. Finally, we note that a proper estimation of φ 2 the element radiation pattern should consider also the error AE;H (φ) = − min 12 ;Am ; (2) φ3dB introduced by the use of analog phase shifters. In fact, although ◦ the beam steering performance depends on the number of bits where φ3dB = 65 is the horizontal 3 dB beamwidth, and used to quantize the phase shifter, the current state of silicon Am = 30 dB is the front-back ratio. Bringing together the technologies makes the design of high-resolution mmWave previously computed vertical and horizontal patterns we can phase shifters costly or even impractical [2]. To this end, we obtain the 3D antenna element gain for each pair of angles as analyze the performance loss when considering an error in the A(3GPP)(θ; φ) = G −min {− [A (θ) + A (φ)] ;A g ; synthesis of transmitter and receiver beams. E max E;V E;H m (3) II. ANTENNA ARRAY RADIATION PATTERNS where Gmax = 8 dBi is the maximum directional gain of the In this section we describe how to compute the realistic antenna element [3]. The expression in (3) provides the dB antenna array patterns considered in this study and report gain experienced by a ray with angle pair (θ; φ) due to the expressions for the diverse field factors. The radiation pattern effect of the element radiation pattern. of the entire array (called array radiation pattern) is obtained 3) HFSS: In addition to the 3GPP pattern, we consider by the superposition of its array factor, which provides in- also a realistic pattern obtained reproducing a real patch formation on the directivity equation of an antenna array, and antenna with a finite element simulator called High-Frequency the element radiation pattern. This last term takes into account Structural Simulator (HFSS) [12]. This last model is the most how power is radiated by each single antenna element. realistic among the models studied, but is also the most We precisely describe in the following sub-sections how to computationally intensive. As in the 3GPP model, it considers obtain the pattern expression for the different antenna radiation a three-sector cell, while the element radiation pattern is models used in this comparison. modeled as a real patch antenna element working at 29.5 GHz A. Element radiation pattern: The element radiation with horizontal and vertical spacing and sizes equal to 0.55λ and 0.77λ respectively.
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