LTE Goes Unlicensed: How the 5G Era Changes the Way We Use Spectrum

by Shahid Mumtaz, Instituto de Telecomunicações, Portugal, Jonathan Rodriguez , Instituto de Telecomunicações, Portugal, and Zhenyu Zhou, North China Electric Power University, Beijing

IEEE 5G Tech Focus: Volume 1, Number 3, September 2017 

1. Introduction

Spectrum extension beyond the current licensed charts is one of the most important endeavors for mobile operators to meet the ever-increasing traffic demand. From a 3GPP perspective, the network capacity can be increased by improving the spectral efficiency of the LTE-based cellular system through higher order modulation, massive MIMO, mmWave and carrier aggregation, to name a few. However, the current amount of spectrum allocated by ITU to LTE-based cellular systems is insufficient until new spectrum is available in WRC-19. For example, it is estimated that there is a spectrum shortage of at least 63 MHz in China [1], and a similar problem can be seen worldwide. Considering GHz unlicensed band, telecom carriers have already made their decision to exploit this underutilized bulk of spectrum using an additional interface integrated to the LTE-based cellular system. This provides an additional 500 MHz of spectrum on a global basis [2] in a band that has so far been occupied only by WiFi. In 3GPP LTE Release 13, the Licensed-Assisted Access (LAA) [2] proposed to enable LTE operators to offload their traffic into the 5GHz band and leverage their bitrates by aggregating licensed/unlicensed carriers while still offering seamless mobility support. Early attempts to use LTE in unlicensed spectrum were not standardized and included the use of new algorithms and methods that did not consider regional regulatory restrictions for each given country. One such pre-standards solution is LTE-U (LTE in the unlicensed band) which received limited acceptance. LAA specification intends to produce a single global solution framework for licensed assisted access to the unlicensed spectrum (5 GHz). Such a solution considers regional power limits while enabling LTE to use low power secondary cells in the unlicensed spectrum using carrier aggregations [3].

Enabling LTE in unlicensed band raises the coexisting issue with WiFi. To solveoexistence issue, there are mainly two methods: 1) Listen before Talk (LBT) and 2) duty cycle. LBT is cited by Sony, Intel, Huawei, Qualcomm, ETSI, Nokia, and many other companies as the preferable technology to provide coexistence between WiFi and LTE in unlicensed band. LBT needs to be supported to meet the spectrum regulations at some geographical regions, such as EU and Japan. This CCA (Clear Channel Assessment) requirement is also specified in [6]. The duty cycle approach is also adopted by some companies like LBT approach. Duty cycle divides the channel time equally to the small cells (LTE-U, WiFi APs) so that the WiFi transmission time is guaranteed and fairness is achieved. However, the disadvantages are the fact that channel access of non-LBT LTE becomes inflexible and cannot access WiFi´s share immediately when WiFi devices are in low channel utilization. Furthermore, the long duty cycle period can directly affect the latency of each technology.

2. Evolution of LTE-Unlicensed Standard

Although LTE-U is a very attractive technology for 5G, the difference in Physical/MAC layer design principles makes coexistence of these technologies challenging, as shown in Table 1. Thus, there is a huge debate between 5G developers on the use cases to enable a fair spectrum management between LTE-U and WiFi in shared bands [3-11].

A. Carrier Wi-Fi

The first step towards unlicensed band is the carrier WiFi as shown in Figure 1(a). The carrier WiFi access points are deployed by the operators in their network to offload traffic from LTE to the WiFi network. Carrier WiFi helps the operator to reduce the congestion in the network and provide better Internet access for customers. However, carrier WiFi adopts different access and management mechanisms from the LTE network, such as control and authentication. This leads to a visible reduction in typical WiFi flexibility to adapt between channels.

Due to above disadvantages of carrier WiFi, operators are now looking to access the 5GHz unlicensed band using new unified LTE radio interfaces. The unified LTE radio interfaces will solve all authentication, O&M and QoS issues which were initially presented in carrier WiFi. To this end, a different solution has been designed, such as LTE-U, LAA, and LTE-WiFi link aggregation.

B. LTE-Unlicensed

LTE-U was developed by the LTE-U forum [4] based on 3GPP Release 10/11/12. This forum was formed by top industrial players in 2014 and targets the early deployment in countries (USA, China, and Korea) without LBT requirements. LTE-U uses Carrier Sense Adaptive Transmission (CSAT) protocol for fair coexistence with non-IMT devices in the 5GHz band. In CSAT, LTE devices sense the 5GHz channel for a longer duration (200ms) as compared to LBT to see the activities of other non-IMT devices. Based on sensing observation, LTE devices then adaptively define the certain percentage of ON/OFF duty cycle to access the 5GHz channel. In the ON period LTE can transmit, and in the OFF period it keeps silent, while WiFi can transmit. The LTE-U base station can be deployed as collocated/non-collocated with ideal and non-ideal backhaul in indoor/outdoor scenarios as shown in Figure 1(a-e).

C. License Assisted Access (LAA)

LAA is approved by 3GPP as a Release 13 Work Item in June 2015. It specifies the utilization of unlicensed spectrum to boost downlink through a process of carrier aggregation via Supplemental Downlink (SDL). At the same time, 3GPP approved and enhanced LAA (eLAA) Work Item in Release 14, which aims at identifying uplink transmission in the 5GHz band. Unlike TE-U, which cannot be applied in the countries (Europe, Japan) demanding LBT, LAA is the 3GPP effort to standardize the operation of the unlicensed band as a single global framework to be adopted in all regions with or without LBT requirement. In LBT, the transmitter needs to sense the channel before sending a frame, no less than which complies with the regulations specified by ETSI [5]. If the channel is ideal and the interference level is below the threshold, it transmits. The channel occupancy time requirement for the transmitter is between 1ms and 10ms. In addition to the above rules, some other compliance rules also defined by regulators in different regions is described in [5-6]. For example, transmission power for indoor and outdoor bands and Dynamic Frequency Selection (DFS) which is targeted for the semi-static sensing and avoiding interference to non- IMT systems (such as radar systems and Power Control (TPC)) [12].

D. LTE and WI-FI Link Aggregation (LWA)

Another approach to offload data is LTE-WiFi integration [7] as shown in Figure 1(f). This approach is mostly used to improve the indoor coverage and capacity. In LWA, LTE runs on the licensed band without impacting the unlicensed band. LTE and WiFi aggregate at PDCP layer and then split payload into two streams: one stream of payload transmits on the licensed band (LTE) and another stream transmits on the unlicensed band (WiFi). WiFi acts as the second carrier managed by the operator. LWA can leverage existing WiFi APs without changes to the LTE and WiFi RATs. LWAto implement by software updating WiFi to support LWA. However, the operator needs to manage two different networks, which increases the complexity.

References: 

1. Wang et al., “5G spectrum: Is China Ready?” IEEE communication Mag. Vol.53, no.7, July 2015, pp.58-65
2. 3GPP, “Technical Specification Group Radio Access Network; Study on Licensed-Assisted Access to Unlicensed Spectrum;” tech. rep. 3GPP-36889-d00, 2015.
3. 5G America, “Inside 3GPP Release 13 - 5gamericas”, 2016 (update)
4. Mumtaz, A. Al-Dulaimi, K. M. S. Huq, F. B. Saghezchi and J. Rodriguez, "WiFi in Licensed Band," in IEEE Communications Letters, vol. 20, no. 8, pp. 1655-1658, Aug. 2016.
5. http://www.lteuforum.org/
6. ESTI EN 301893, “Broadband Radio Access Networks (BRAN); 5GHz High-Performance RALN; Harmonized EN Covering the Essential Requirements of Article 3.2 of the R&TTE Directive,” July 2014
7. Study on Licensed-Assisted Access to Unlicensed Spectrum, 3GPP TR 36.889, May. 2015
8. Qualcomm, “LTE-U/LAA, MuLTEfire, and Wi-Fi; Making Best Use of Unlicensed Spectrum,” tech. rep., 2015
9. Rahman, M.I.; Behravan, A.; Koorapaty, H.; Sachs, J.; Balachandran, K., "License-exempt LTE systems for secondary spectrum usage: Scenarios and first assessment," New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 2011 IEEE Symposium on , vol., no., pp.349,358, 3-6 May 2011.
10. Ratasuk, R.; Uusitalo, M.A.; Mangalvedhe, N.; Sorri, A.; Iraji, S.; Wijting, C.; Ghosh, A., "License-exempt LTE deployment in heterogeneous network," Wireless Communication Systems (ISWCS), 2012 Int. Symposium on, vol., no., pp.246,250, 28-31 Aug. 2012
11. Qualcomm Whitepaper, “Extending LTE Advanced to unlicensed spectrum,” December 2013.
12. H. Zhang, X. Chu, W. Guo, and S. Wang, "Coexistence of Wi-Fi and Heterogeneous Small Cell Networks Sharing Unlicensed Spectrum," IEEE Communications Magazine, vol. 53, no. 3, pp. 158-164, March 2015

 

 Shahid Mumtaz (This email address is being protected from spambots. You need JavaScript enabled to view it. ) has more than 10 years of wireless industry experience and is currently working as Senior Research Scientist and Technical Manager at Instituto de Telecomunicações Aveiro, Portugal. Prior to his current position, he worked as Research Intern at Ericsson and Huawei Research Labs in 2005 at Karlskrona, Sweden. He received his MSc and Ph.D. degrees in Electrical & Electronic Engineering from Blekinge Institute of Technology (BTH) Karlskrona, Sweden and University of Aveiro, Portugal in 2006 and 2011, respectively. Dr. Shahid has several years of experience in 3GPP radio systems research with experience in HSPA/LTE/LTE-A and strong track-record in relevant technology field, especially physical layer technologies, LTE cell planning and optimization, protocol stack and system architecture. Dr. Shahid has more than 90 publications in international conferences, journal papers, and book chapters. He is serving as a Vice-Chair of IEEE 5G Standardization. In 2012, Shahid was awarded an "Alain Bensoussan" fellowship by the European Research Consortium for Informatics and Mathematics (ERCIM) to pursue research in communication networks for one year at the VTT Technical Research Centre of Finland. He is also an editor of three books and served as guest editor for a special issue of IEEE Wireless Communications Magazine and IEEE Communication Magazine. Recently, he is appointed as a permanent associate technical editor for IEEE Communication Magazine, IEEE Journal of IoT and Elsevier Journal of Digital Communication and Network. He has been on the technical program committee of different IEEE conferences, including Globecom, ICC, and VTC, and chaired some of their symposia. He was the workshop chair in many conferences and recipient of the 2006 IITA Scholarship, South Korea. Dr. Shahid is a Senior IEEE member.

 

Jonathan Rodriguez (M’04-SM’13) (This email address is being protected from spambots. You need JavaScript enabled to view it. ) received his Master and Ph.D degree in Electronic and Electrical Engineering from the University of Surrey (UK), in 1998 and 2004 respectively. In 2005, he became a researcher at the Institutõ de Telecomunicações and Senior Researcher in the same institution in 2008 where he established the 4TELL Research Group (http://www.av.it.pt/4TELL/) targeting the next generation mobile networks with key interests on energy efficient design, cooperative strategies, security and electronic circuit design. He has served as project coordinator for major international research projects (Eureka LOOP, FP7 C2POWER), whilst acting as the technical manager for FP7 COGEU and FP7 SALUS. He is currently leading the H2020-ETN SECRET project, a European Training Network on 5G communications. Since 2009, he has been invited as the Assistant Professor at the Universidade de Aveiro, and granted as the Associate Professor in 2015. He has authored of more than 350 scientific works that include 9 book editorials. His professional affiliations include: Senior Member of the IEEE and Chartered Engineer (CEng) since 2013, and Fellow of the IET (2015).

 

Zhenyu Zhou (M'11) (This email address is being protected from spambots. You need JavaScript enabled to view it.) received his M.E. and Ph.D degree from Waseda University, Tokyo, Japan in 2008 and 2011 respectively. From April 2012 to March 2013, he was the chief researcher at Department of Technology, KDDI, Tokyo, Japan. From March 2013 to now, he is an Associate Professor at School of Electrical and Electronic Engineering, North China Electric Power University, China. He is also a visiting scholar with Tsinghua-Hitachi Joint Lab on Environment-Harmonious ICT at University of Tsinghua, Beijing from 2014 to now. He served as workshop co-chair for IEEE ISADS 2015, session chair for IEEE Globecom 2014, and TPC member for IEEE Globecom 2015, ACM Mobimedia 2015, IEEE Africon 2015, etc. He received the "Young Researcher Encouragement Award" from IEEE Vehicular Technology Society in 2009. His research interests include green communications and smart grid. He is a member of IEEE, IEICE, and CSEE. 

Editor: Haijun Zhang