Millimeter Wave Channel Modeling and Cellular Capacity Evaluation Mustafa Riza Akdeniz, Student Member, IEEE, Yuanpeng Liu, Student Member, IEEE, Mathew K
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1 Millimeter Wave Channel Modeling and Cellular Capacity Evaluation Mustafa Riza Akdeniz, Student Member, IEEE, Yuanpeng Liu, Student Member, IEEE, Mathew K. Samimi, Student Member, IEEE, Shu Sun, Student Member, IEEE, Sundeep Rangan, Senior Member, IEEE, Theodore S. Rappaport, Fellow, IEEE, Elza Erkip, Fellow, IEEE Abstract—With the severe spectrum shortage in conventional currently largely constrained to the prime RF real estate under cellular bands, millimeter wave (mmW) frequencies between 30 3 GHz [5]. Moreover, the very small wavelengths of mmW and 300 GHz have been attracting growing attention as a possible signals combined with advances in low-power CMOS RF candidate for next-generation micro- and picocellular wireless networks. The mmW bands offer orders of magnitude greater circuits enable large numbers (≥ 32 elements) of miniaturized spectrum than current cellular allocations and enable very high- antennas to be placed in small dimensions. These multiple an- dimensional antenna arrays for further gains via beamforming tenna systems can be used to form very high gain, electrically and spatial multiplexing. This paper uses recent real-world steerable arrays, fabricated at the base station, in the skin of measurements at 28 and 73 GHz in New York City to derive a cellphone, or even within a chip [6], [10]–[17]. Given the detailed spatial statistical models of the channels and uses these models to provide a realistic assessment of mmW micro- and very wide bandwidths and large numbers of spatial degrees of picocellular networks in a dense urban deployment. Statistical freedom, it has been speculated that mmW bands will play a models are derived for key channel parameters including the path significant role in Beyond 4G and 5G cellular systems [8]. loss, number of spatial clusters, angular dispersion and outage. However, the development of cellular networks in the mmW It is found that, even in highly non-line-of-sight environments, bands faces significant technical obstacles and the precise strong signals can be detected 100m to 200m from potential cell sites, potentially with multiple clusters to support spatial value of mmW systems needs careful assessment. The increase multiplexing. Moreover, a system simulation based on the models in omnidirectional free space path loss with higher frequencies predicts that mmW systems can offer an order of magnitude due to Friis’ Law [18], can be more than compensated by a increase in capacity over current state-of-the-art 4G cellular proportional increase in antenna gain with appropriate beam- networks with no increase in cell density from current urban forming. We will, in fact, confirm this property experimentally deployments. below. However, a more significant concern is that mmW Index Terms—millimeter wave radio, 3GPP LTE, cellular sys- signals can be severely vulnerable to shadowing resulting in tems, wireless propagation, 28 GHz, 73 GHz, urban deployments. outages, rapidly varying channel conditions and intermittent connectivity. This issue is particularly concerning in cluttered, urban deployments where coverage frequently requires non- I. INTRODUCTION line-of-sight (NLOS) links. The remarkable success of cellular wireless technologies In this paper, we use the measurements of mmW outdoor have led to an insatiable demand for mobile data [1], [2]. cellular propagation [19]–[24] in 28 and 73 GHz in New York The UMTS traffic forecasts [3], for example, predicts that by City to derive detailed the first statistical channel models that 2020, daily mobile traffic will exceed 800 MB per subscriber can be used for proper mmW system evaluation. The models leading to 130 exabits (1018) of data per year for some are used to provide an initial assessment of the potential arXiv:1312.4921v3 [cs.NI] 25 Apr 2014 operators. Keeping pace with this demand will require new system capacity and outage. The NYC location was selected technologies that can offer orders of magnitude increases in since it is representative of likely initial deployments of mmW cellular capacity. cellular systems due to the high user density. In addition, the To address this challenge, there has been growing interest urban canyon environment provides a challenging test case for in cellular systems based in the so-called millimeter-wave these systems due to the difficulty in establishing line-of-sight (mmW) bands, between 30 and 300 GHz, where the available (LOS) links – a key concern for mmW cellular. bandwidths are much wider than today’s cellular networks [4]– Although our earlier work has presented some initial analy- [9]. The available spectrum at these frequencies can be easily sis of the data in [19]–[23], this work provides much more 200 times greater than all cellular allocations today that are detailed modeling necessary for cellular system evaluation. In particular, we develop detailed models for the spatial This material is based upon work supported by the National Science Foun- characteristics of the channel and outage probabilities. To dation under Grants No. 1116589 and 1237821 as well as generous support obtain these models, several we present new data analysis from Samsung, Nokia Siemens Networks and InterDigital Communications. M. Akdeniz (email:[email protected]), techniques. In particular, we propose a clustering algorithm Y. Liu (email:[email protected]), M. Samimi that identifies the group of paths in the angular domain from (email:[email protected]), S. Sun (email:[email protected]), S. subsampled spatial measurements. The clustering algorithm Rangan (email: [email protected]), T. S. Rappaport (email: [email protected]) and E. Erkip (email: [email protected]) are with NYU WIRELESS Center, is based on a K-means method with additional heuristics to Polytechnic Institute of New York University, Brooklyn, NY. determine the number of clusters. Statistical models are then 2 derived for key cluster parameters including the number of clusters, cluster angular spread and path loss. For the inter- cluster power fractions, we propose a probabilistic model with maximum likelihood (ML) parameter estimation. In addition, while standard 3GPP models such as [25], [26] use proba- bilistic LOS-NLOS models, we propose to add a third state to explicitly model the models the possibility of outages. The key findings from these models are as follows: • The omnidirectional path loss is approximately 20 to 25 dB higher in the mmW frequencies relative to current Fig. 1: Image from [19] showing typical measurement locations cellular frequencies in distances relevant for small cells. in NYC at 28 GHz. Similar locations were used for 73 GHz. However, due to the reduced wavelength, this loss can be completely compensated by a proportional increase in antenna gain with no increase in physical antenna size. indoor environments [28]–[34]. However, the propagation of Thus, with appropriate beamforming, locations that are mmW signals in outdoor settings for micro- and picocellular not in outage will not experience any effective increase networks is relatively less understood. Due to the lack of actual in path loss and, in fact, the path loss may be decreased. measured channel data, many earlier studies [4], [7], [35], [36] • Our measurements indicate that at many locations, energy have thus relied on either analytic models or commercial ray arrives in clusters from multiple distinct angular direc- tracing software with various reflection assumptions. Below, tions, presumably through different macro-level scattering we will compare our experimental results with some of these or reflection paths. Locations had up to four clusters, models. with an average of approximately two. The presence of Also, measurements in Local Multipoint Distribution Sys- multiple clusters of paths implies that the the possibility tems at 28 GHz – the prior system most close to mmW cellular of both spatial multiplexing and diversity gains. – have been inconclusive: For example, a study [37] found • Applying the derived channel models to a standard cel- 80% coverage at ranges up to 1–2 km, while [38] claimed lular evaluation framework such as [25], we predict that that LOS connectivity would be required. Our own previ- mmW systems can offer at least an order of magnitude ous studies at 38 GHz [39]–[43] found that relatively long- increase in system capacity under reasonable assumptions range links (> 300 m) could be established. However, these on bandwidth and beamforming. For example, we show measurements were performed in an outdoor campus setting that a hypothetical 1GHz bandwidth TDD mmW system with much lower building density and greater opportunities with a 100 m cell radii can provide 25 times greater for LOS connectivity than would be found in a typical urban cell throughout than industry reported numbers for a deployment. 20+20 MHz FDD LTE system with similar cell density. Moreover, while the LTE capacity numbers included both II. MEASUREMENT METHODOLOGY single and multi-user multi-input multi-output (MIMO), our mmW capacity analysis did not include any spatial To assess of mmW propagation in urban environments, our multiplexing gains. We provide strong evidence that these team conducted extensive measurements of 28 and 73 GHz spatial multiplexing gains would be significant. channels in New York City. Details of the measurements can • The system performance appears to be robust to outages be found in [19]–[21]. Both the 28 and 73 GHz are natural can- provided they are at levels similar or even a little worse didates for early mmW deployments. The 28 GHz bands were than the outages we observed in the NYC measure- previously targeted for Local Multipoint Distribution Systems ments. This robustness to outage is very encouraging (LMDS) systems and are now an attractive opportunity for ini- since outages is one of the key concerns with mmW tial deployments of mmW cellular given their relatively lower cellular. However, we also show that should outages be frequency within the mmW range. The E-Band (71-76 GHz significantly worse than what we observed, the system and 81-86 GHz) [44] has abundant spectrum and adaptable for performance, particularly the cell edge rate, can be greatly dense deployment, and could accommodate further expansion impacted.