Advanced Technologies Enabling Unlicensed Spectrum Utilization in Cellular Networks

As the rapid progress and pleasant experience of Internet-based services, there is an increasing demand for high data rate in wireless communications systems. Unlicensed spectrum utilization in Long Term Evolution (LTE) networks is a promising technique to meet the massive traffic demand. There are two effective methods to use unlicensed bands for delivering LTE traffic. One is offloading LTE traffic toWi-Fi. An alternative method is LTE-unlicensed (LTE-U), which aims to directly use LTE protocols and infrastructures over the unlicensed spectrum. It has also been pointed out that addressing the above two methods simultaneously could further improve the system performance. However, how to avoid severe performance degradation of the Wi-Fi network is a challenging issue of utilizing unlicensed spectrum in LTE networks. Specifically, first, the inter-system spectrum sharing, or, more specifically, the coexistence of LTE andWi-Fi in the same unlicensed spectrum is the major challenge of implementing LTE-U. Second, to use the LTE and Wi-Fi integration approach, mobile operators have to manage two disparate networks in licensed and unlicensed spectrum. Third, optimization for joint data offloading to Wi-Fi and LTE-U in multi- cell scenarios poses more challenges because inter-cell interference must be addressed. This thesis focuses on solving problems related to these challenges. First, the effect of bursty traffic in an LTE and Wi-Fi aggregation (LWA)-enabled network has been investigated. To enhance resource efficiency, the Wi-Fi access point (AP) is designed to operate in both the native mode and the LWA mode simultaneously. Specifically, the LWA-modeWi-Fi AP cooperates with the LTE base station (BS) to transmit bearers to the LWA user, which aggregates packets from both LTE and Wi-Fi. The native-mode Wi-Fi AP transmits Wi-Fi packets to those native Wi-Fi users that are not with LWA capability. This thesis proposes a priority-based Wi-Fi transmission scheme with congestion control and studied the throughput of the native Wi-Fi network, as well as the LWA user delay when the native Wi-Fi user is under heavy traffic conditions. The results provide fundamental insights in the throughput and delay behavior of the considered network. Second, the above work has been extended to larger topologies. A stochastic geometry model has been used to model and analyze the performance of an MPTCP Proxy-based LWA network with intra-tier and cross-tier dependence. Under the considered network model and the activation conditions of LWA-mode Wi-Fi, this thesis has obtained three approximations for the density of active LWA-mode Wi-Fi APs through different approaches. Tractable analysis is provided for the downlink (DL) performance evaluation of large-scale LWA networks. The impact of different parameters on the network performance have been analyzed, validating the significant gain of using LWA in terms of boosted data rate and improved spectrum reuse. Third, this thesis also takes a significant step of analyzing joint multi-cell LTE-U and Wi-Fi network, while taking into account different LTE-U and Wi-Fi inter-working schemes. In particular, two technologies enabling data offloading from LTE to Wi-Fi are considered, including LWA and Wi-Fi offloading in the context of the power gain-based user offloading scheme. The LTE cells in this work are subject to load-coupling due to inter-cell interference. New system frameworks for maximizing the demand scaling factor for all users in both Wi-Fi and multi-cell LTE networks have been proposed. The potential of networks is explored in achieving optimal capacity with arbitrary topologies, accounting for both resource limits and inter-cell interference. Theoretical analyses have been proposed for the proposed optimization problems, resulting in algorithms that achieve global optimality. Numerical results show the algorithms’ effectiveness and benefits of joint use of data offloading and the direct use of LTE over the unlicensed band. All the derived results in this thesis have been validated by Monte Carlo simulations in Matlab, and the conclusions observed from the results can provide guidelines for the future unlicensed spectrum utilization in LTE networks

Cellular and Wi-Fi technologies evolution: from complementarity to competition

This PhD thesis has the characteristic to span over a long time because while working on it, I was working as a research engineer at CTTC with highly demanding development duties. This has delayed the deposit more than I would have liked. On the other hand, this has given me the privilege of witnessing and studying how wireless technologies have been evolving over a decade from 4G to 5G and beyond. When I started my PhD thesis, IEEE and 3GPP were defining the two main wireless technologies at the time, Wi-Fi and LTE, for covering two substantially complementary market targets. Wi-Fi was designed to operate mostly indoor, in unlicensed spectrum, and was aimed to be a simple and cheap technology. Its primary technology for coexistence was based on the assumption that the spectrum on which it was operating was for free, and so it was designed with interference avoidance through the famous CSMA/CA protocol. On the other hand, 3GPP was designing technologies for licensed spectrum, a costly kind of spectrum. As a result, LTE was designed to take the best advantage of it while providing the best QoE in mainly outdoor scenarios. The PhD thesis starts in this context and evolves with these two technologies. In the first chapters, the thesis studies radio resource management solutions for standalone operation of Wi-Fi in unlicensed and LTE in licensed spectrum. We anticipated the now fundamental machine learning trend by working on machine learning-based radio resource management solutions to improve LTE and Wi-Fi operation in their respective spectrum. We pay particular attention to small cell deployments aimed at improving the spectrum efficiency in licensed spectrum, reproducing small range scenarios typical of Wi-Fi settings. IEEE and 3GPP followed evolving the technologies over the years: Wi-Fi has grown into a much more complex and sophisticated technology, incorporating the key features of cellular technologies, like HARQ, OFDMA, MU-MIMO, MAC scheduling and spatial reuse. On the other hand, since Release 13, cellular networks have also been designed for unlicensed spectrum. As a result, the two last chapters of this thesis focus on coexistence scenarios, in which LTE needs to be designed to coexist with Wi-Fi fairly, and NR, the radio access for 5G, with Wi-Fi in 5 GHz and WiGig in 60 GHz. Unlike LTE, which was adapted to operate in unlicensed spectrum, NR-U is natively designed with this feature, including its capability to operate in unlicensed in a complete standalone fashion, a fundamental new milestone for cellular. In this context, our focus of analysis changes. We consider that these two technological families are no longer targeting complementarity but are now competing, and we claim that this will be the trend for the years to come. To enable the research in these multi-RAT scenarios, another fundamental result of this PhD thesis, besides the scientific contributions, is the release of high fidelity models for LTE and NR and their coexistence with Wi-Fi and WiGig to the ns-3 open-source community. ns-3 is a popular open-source network simulator, with the characteristic to be multi-RAT and so naturally allows the evaluation of coexistence scenarios between different technologies. These models, for which I led the development, are by academic citations, the most used open-source simulation models for LTE and NR and havereceived fundings from industry (Ubiquisys, WFA, SpiderCloud, Interdigital, Facebook) and federal agencies (NIST, LLNL) over the years.Aquesta tesi doctoral té la característica d’allargar-se durant un llarg període de temps ja que mentre treballava en ella, treballava com a enginyera investigadora a CTTC amb tasques de desenvolupament molt exigents. Això ha endarrerit el dipositar-la més del que m’hagués agradat. D’altra banda, això m’ha donat el privilegi de ser testimoni i estudiar com han evolucionat les tecnologies sense fils durant més d’una dècada des del 4G fins al 5G i més enllà. Quan vaig començar la tesi doctoral, IEEE i 3GPP estaven definint les dues tecnologies sense fils principals en aquell moment, Wi-Fi i LTE, que cobreixen dos objectius de mercat substancialment complementaris. Wi-Fi va ser dissenyat per funcionar principalment en interiors, en espectre sense llicència, i pretenia ser una tecnologia senzilla i barata. La seva tecnologia primària per a la convivència es basava en el supòsit que l’espectre en el que estava operant era de franc, i, per tant, es va dissenyar simplement evitant interferències a través del famós protocol CSMA/CA. D’altra banda, 3GPP estava dissenyant tecnologies per a espectres amb llicència, un tipus d’espectre costós. Com a resultat, LTE està dissenyat per treure’n el màxim profit alhora que proporciona el millor QoE en escenaris principalment a l’aire lliure. La tesi doctoral comença amb aquest context i evoluciona amb aquestes dues tecnologies. En els primers capítols, estudiem solucions de gestió de recursos de radio per a operacions en espectre de Wi-Fi sense llicència i LTE amb llicència. Hem anticipat l’actual tendència fonamental d’aprenentatge automàtic treballant solucions de gestió de recursos de radio basades en l’aprenentatge automàtic per millorar l’LTE i Wi-Fi en el seu espectre respectiu. Prestem especial atenció als desplegaments de cèl·lules petites destinades a millorar la eficiència d’espectre llicenciat, reproduint escenaris de petit abast típics de la configuració Wi-Fi. IEEE i 3GPP van seguir evolucionant les tecnologies al llarg dels anys: El Wi-Fi s’ha convertit en una tecnologia molt més complexa i sofisticada, incorporant les característiques clau de les tecnologies cel·lulars, com ara HARQ i la reutilització espacial. D’altra banda, des de la versió 13, també s’han dissenyat xarxes cel·lulars per a espectre sense llicència. Com a resultat, els dos darrers capítols d’aquesta tesi es centren en aquests escenaris de convivència, on s’ha de dissenyar LTE per conviure amb la Wi-Fi de manera justa, i NR, l’accés a la radio per a 5G amb Wi-Fi a 5 GHz i WiGig a 60 GHz. A diferència de LTE, que es va adaptar per funcionar en espectre sense llicència, NR-U està dissenyat de forma nativa amb aquesta característica, inclosa la seva capacitat per operar sense llicència de forma autònoma completa, una nova fita fonamental per al mòbil. En aquest context, el nostre focus d’anàlisi canvia. Considerem que aquestes dues famílies de tecnologia ja no estan orientades cap a la complementarietat, sinó que ara competeixen, i afirmem que aquesta serà el tendència per als propers anys. Per permetre la investigació en aquests escenaris multi-RAT, un altre resultat fonamental d’aquesta tesi doctoral, a més de les aportacions científiques, és l’alliberament de models d’alta fidelitat per a LTE i NR i la seva coexistència amb Wi-Fi a la comunitat de codi obert ns-3. ns-3 és un popular simulador de xarxa de codi obert, amb la característica de ser multi-RAT i, per tant, permet l’avaluació de manera natural d’escenaris de convivència entre diferents tecnologies. Aquests models, pels quals he liderat el desenvolupament, són per cites acadèmiques, els models de simulació de codi obert més utilitzats per a LTE i NR i que han rebut finançament de la indústria (Ubiquisys, WFA, SpiderCloud, Interdigital, Facebook) i agències federals (NIST, LLNL) al llarg dels anys.Esta tesis doctoral tiene la característica de extenderse durante mucho tiempo porque mientras trabajaba en ella, trabajaba como ingeniera de investigación en CTTC con tareas de desarrollo muy exigentes. Esto ha retrasado el depósito más de lo que me hubiera gustado. Por otro lado, gracias a ello, he tenido el privilegio de presenciar y estudiar como las tecnologías inalámbricas han evolucionado durante una década, de 4G a 5G y más allá. Cuando comencé mi tesis doctoral, IEEE y 3GPP estaban definiendo las dos principales tecnologías inalámbricas en ese momento, Wi-Fi y LTE, cumpliendo dos objetivos de mercado sustancialmente complementarios. Wi-Fi fue diseñado para funcionar principalmente en interiores, en un espectro sin licencia, y estaba destinado a ser una tecnología simple y barata. Su tecnología primaria para la convivencia se basaba en el supuesto en que el espectro en el que estaba operando era gratis, y así fue diseñado simplemente evitando interferencias a través del famoso protocolo CSMA/CA. Por otro lado, 3GPP estaba diseñando tecnologías para espectro con licencia, un tipo de espectro costoso. Como resultado, LTE está diseñado para aprovechar el espectro al máximo proporcionando al mismo tiempo el mejor QoE en escenarios principalmente al aire libre. La tesis doctoral parte de este contexto y evoluciona con estas dos tecnologías. En los primeros capítulos, estudiamos las soluciones de gestión de recursos de radio para operación en espectro Wi-Fi sin licencia y LTE con licencia. Anticipamos la tendencia ahora fundamental de aprendizaje automático trabajando en soluciones de gestión de recursos de radio para mejorar LTE y funcionamiento deWi-Fi en su respectivo espectro. Prestamos especial atención a las implementaciones de células pequeñas destinadas a mejorar la eficiencia de espectro licenciado, reproduciendo los típicos escenarios de rango pequeño de la configuración Wi-Fi. IEEE y 3GPP siguieron evolucionando las tecnologías a lo largo de los años: Wi-Fi se ha convertido en una tecnología mucho más compleja y sofisticada, incorporando las características clave de las tecnologías celulares, como HARQ, OFDMA, MU-MIMO, MAC scheduling y la reutilización espacial. Por otro lado, desde la Release 13, también se han diseñado redes celulares para espectro sin licencia. Como resultado, los dos últimos capítulos de esta tesis se centran en estos escenarios de convivencia, donde LTE debe diseñarse para coexistir con Wi-Fi de manera justa, y NR, el acceso por radio para 5G con Wi-Fi en 5 GHz y WiGig en 60 GHz. A diferencia de LTE, que se adaptó para operar en espectro sin licencia, NR-U está diseñado de forma nativa con esta función, incluyendo su capacidad para operar sin licencia de forma completamente independiente, un nuevo hito fundamental para los celulares. En este contexto, cambia nuestro enfoque de análisis. Consideramos que estas dos familias tecnológicas ya no tienen como objetivo la complementariedad, sino que ahora están compitiendo, y afirmamos que esta será la tendencia para los próximos años. Para permitir la investigación en estos escenarios de múltiples RAT, otro resultado fundamental de esta tesis doctoral, además de los aportes científicos, es el lanzamiento de modelos de alta fidelidad para LTE y NR y su coexistencia con Wi-Fi y WiGig a la comunidad de código abierto de ns-3. ns-3 es un simulador popular de red de código abierto, con la característica de ser multi-RAT y así, naturalmente, permite la evaluación de escenarios de convivencia entre diferentes tecnologías. Estos modelos, para los cuales lideré el desarrollo, son por citas académicas, los modelos de simulación de código abierto más utilizados para LTE y NR y han recibido fondos de la industria (Ubiquisys, WFA, SpiderCloud, Interdigital, Facebook) y agencias federales (NIST, LLNL) a lo largo de los años.Postprint (published version

- Frequency Sharing of Wi-Fi/LTE-U -

학위논문 (석사) -- 서울대학교 대학원 : 공과대학 협동과정 기술경영·경제·정책전공, 2020. 8. Jorn Altmann .5세대(5G) 이동통신 확산을 위하여 정부는 '비면허 5G' 주파수 대역인 6GHz 대역폭에 최대 1.2㎓ 폭을 공급할 예정이다. 4차 산업에 대응하여 기업이 스마트 공장에 적합한 맞춤형 5G망을 구축하기 위한 목적이다. 과학기술정보통신부(MSIT)는 6㎓ 대역 대상으로 비면허 주파수 대역의 5G 연구반을 가동, 주파수 분배 방안 수립과 기술 기준 제정에 착수한 것으로 확인됐다. 비면허 5G(NR-U)는 정부가 개방한 비면허 주파수 대역에 5G 코어망과 기지국 등 표준 기술을 적용해 초저지연초고속 성능을 구현하는 기술로, 6월 국제민간표준화기구(3GPP) 상용화를 앞두고 있다. 그러나 5/6GHz 대역의 주파수는 와이파이와 LTE-LAA가 공존하게 될 영역으로 이해관계자들의 주장이 엇갈리고 있다. LBT (Listen-Before-Talk)는 비면허 주파수를 공유하는 방법으로 기술적/행정적 방안으로, 채널을 분석하여 데이터를 전송하기 가장 적절한 주파수대역을 선출하는 방법중에 하나이다. 이 대역을 선출하기 위한 기술 방법은 CCA (Clear Channel Assessment)라고 한다. 이 기술은 LTE 통신사업자들이 개발한 기술로 구체화된 기술표준을 요구하고 있다. 하지만, 이 기술을 와이파이 사업자 입장에서는 이 기술의 실제 실용가능 여부에 대해 의구심을 갖고 있다. 그 이유는 해당 기술이 지나치게 LTE 통신사업자들에게 유리하게 개발된 기술이라는 논쟁이 있기 때문이다. 이에 통신기술정책 결정자는 양측이 공존할 수 있는 법 제정방안을 강구해야 하며, 이 논문에서는 기존 와이파이 사용자들을 보호하며 통신 사업자들 역시 주파수를 공유할 수 있는 제도 정책의 방법을 정책자 입장에서 연구해보자 한다. 비면허 주파수를 개방하게 되었을 때 채널 사용자 비율과 라이선스 배부 비율 사이의 효율성에 대해 연구하였다. 그리고, 사용 가능한 채널을 비교하는 방법을 기존연구를 통해 비교하고 새로운 모델을 개발 했다. 또 기술 표준으로 이미 제정 된 노이즈 한계치가 실제 공유가 됐을 때 어떤식으로 작용되는 지를 알아보기 위해 시뮬레이션 분석을 통해 예측해 보았다.A spectrum-sharing policy is yet to be established for unlicensed bands. There are several controversies related to providing 5-GHz (5G) band service with the coexistence of Wi-Fi and LTE–LAA (License Assisted Access). LTE–LAA is a technical and political scheme for sharing spectrum bands. Using this technique, the channel can be evaluated, and a proper channel can be selected before data transmission. This mechanism is called listen before talk (LBT), and the channel selection procedure is called clear channel assessment (CCA). LTE service providers suggest standardizing this technology using an unlicensed band. However, existing Wi-Fi users are skeptical about the sharing process. In this paper, we describe proper spectrum-sharing mechanisms based on policy-based channel selection algorithms. Based on the results, proper regulation of the 5G LTE-U is required to avoid conflicts among service providers. This work uses a policy-based mechanism to understand the role of the government in managing these bands. The main idea is that stricter sharing-policy regulations and higher thresholds must be implemented to protect the rights of existing users.1. Introduction 1 1.1 Research Background 1 1.2 Problem Description 2 1.2.1 Controversies on Wi-Fi and LTE-U Coexistence 2 1.3 Research Question 4 1.3.1. Fairness and Efficiency of Sharing Policy 4 1.3.2. Evaluation and Simulation of Sharing Technology 5 1.4 Contribution 5 2. Literature Review 6 2.1 Spectrum-Sharing Policy 6 2.1.2 Case of South Korea 8 2.2 Spectrum-Sharing Technologies 10 2.2.1 Licensed Assisted Access (LAA) 11 2.2.2 Listen Before Talk (LBT) 13 2.2.2.1 Qualcomm Evaluation 13 2.2.2.2 3GPP Evaluation 16 2.3 Comparison with Previous Work 17 3. Modeling 20 3.1 General Assumption 22 3.2 Flow Chart 23 3.2.1 License Type Evaluation (Guard Interval Evaluation) 23 3.2.2 Energy Detection Evaluation 26 4. Simulation 29 4.1 Parameter setting 29 4.2 Simulation Result 31 5. Conclusion 36 5.1 Overall Implication 36 5.2 Limitation 37 5.3 Further study 37 Appendix 39 Bibliography 47 Abstract (Korean) 50Maste

An adaptive LTE listen-before-talk scheme towards a fair coexistence with Wi-Fi in unlicensed spectrum

The technological growth combined with the exponential increase of wireless traffic are pushing the wireless community to investigate solutions to maximally exploit the available spectrum. Among the proposed solutions, the operation of Long Term Evolution (LTE) in the unlicensed spectrum (LTE-U) has attracted significant attention. Recently, the 3rd Generation Partnership Project announced specifications that allow LTE to transmit in the unlicensed spectrum using a Listen Before Talk (LBT) procedure, respecting this way the regulator requirements worldwide. However, the proposed standards may cause coexistence issues between LTE and legacy Wi-Fi networks. In this article, it is discussed that a fair coexistence mechanism is needed to guarantee equal channel access opportunities for the co-located networks in a technology-agnostic way, taking into account potential traffic requirements. In order to enable harmonious coexistence and fair spectrum sharing among LTE-U and Wi-Fi, an adaptive LTE-U LBT scheme is presented. This scheme uses a variable LTE transmission opportunity (TXOP) followed by a variable muting period. This way, co-located Wi-Fi networks can exploit the muting period to gain access to the wireless medium. The scheme is studied and evaluated in different compelling scenarios using a simulation platform. The results show that by configuring the LTE-U with the appropriate TXOP and muting period values, the proposed scheme can significantly improve the coexistence among LTE-U and Wi-Fi in a fair manner. Finally, a preliminary algorithm is proposed on how the optimal configuration parameters can be selected towards harmonious and fair coexistence

A Q-learning scheme for fair coexistence between LTE and Wi-Fi in unlicensed spectrum

During the last years, the growth of wireless traffic pushed the wireless community to search for solutions that can assist in a more efficient management of the spectrum. Toward this direction, the operation of long term evolution (LTE) in unlicensed spectrum (LTE-U) has been proposed. Targeting a global solution that respects the regional regulations worldwide, 3GPP has published the LTE licensed assisted access (LAA) standard. According to LTE LAA, a listen before talk (LBT) procedure must precede any LTE transmission burst in the unlicensed spectrum. However, the proposed standard may cause coexistence issues between LTE and Wi-Fi, especially in the case that the latter does not use frame aggregation. Toward the provision of a balanced channel access, we have proposed mLTE-U that is an adaptive LTE LBT scheme. According to mLTE-U, LTE uses a variable transmission opportunity (TXOP), followed by a variable muting period. This muting period can be exploited by co-located Wi-Fi networks to gain access to the medium. In this paper, the system model of the mLTE-U scheme in coexistence with Wi-Fi is studied. In addition, mLTE-U is enhanced with a Q-learning technique that is used for autonomous selection of the appropriate combinations of TXOP and muting period that can provide fair coexistence between co-located mLTE-U and Wi-Fi networks. Simulation results showcase the performance of the proposed model and reveal the benefit of using Q-learning for self-adaptation of mLTE-U to the changes of the dynamic wireless environment, toward fair coexistence with Wi-Fi. Finally, the Q-learning mechanism is compared with conventional selection schemes showing the superior performance of the proposed model over less complex mechanisms

Hybrid Spectrum Sharing in mmWave Cellular Networks

While spectrum at millimeter wave (mmWave) frequencies is less scarce than at traditional frequencies below 6 GHz, still it is not unlimited, in particular if we consider the requirements from other services using the same band and the need to license mmWave bands to multiple mobile operators. Therefore, an efficient spectrum access scheme is critical to harvest the maximum benefit from emerging mmWave technologies. In this paper, we introduce a new hybrid spectrum access scheme for mmWave networks, where data is aggregated through two mmWave carriers with different characteristics. In particular, we consider the case of a hybrid spectrum scheme between a mmWave band with exclusive access and a mmWave band where spectrum is pooled between multiple operators. To the best of our knowledge, this is the first study proposing hybrid spectrum access for mmWave networks and providing a quantitative assessment of its benefits. Our results show that this approach provides major advantages with respect to traditional fully licensed or fully unlicensed spectrum access schemes, though further work is needed to achieve a more complete understanding of both technical and non technical implications