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
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 , 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  in a band that has so far been occupied only by WiFi. In 3GPP LTE Release 13, the Licensed-Assisted Access (LAA)  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 .
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 . 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.
LTE-U was developed by the LTE-U forum  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 . 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)) .
D. LTE and WI-FI Link Aggregation (LWA)
Another approach to offload data is LTE-WiFi integration  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.
- Wang et al., “5G spectrum: Is China Ready?” IEEE communication Mag. Vol.53, no.7, July 2015, pp.58-65
- 3GPP, “Technical Specification Group Radio Access Network; Study on Licensed-Assisted Access to Unlicensed Spectrum;” tech. rep. 3GPP-36889-d00, 2015.
- 5G America, “Inside 3GPP Release 13 - 5gamericas”, 2016 (update)
- 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.
- 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
- Study on Licensed-Assisted Access to Unlicensed Spectrum, 3GPP TR 36.889, May. 2015
- Qualcomm, “LTE-U/LAA, MuLTEfire, and Wi-Fi; Making Best Use of Unlicensed Spectrum,” tech. rep., 2015
- 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.
- 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
- Qualcomm Whitepaper, “Extending LTE Advanced to unlicensed spectrum,” December 2013.
- 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
Editor: Haijun Zhang
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