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Global Standardization Activities

Vol. 10, No. 1, pp. 48–52, Jan. 2012. https://doi.org/10.53829/ntr201201gls

Overview and Standardization Trends of LTE-Advanced

Sadayuki Abeta^†, Tetsushi Abe, and Takehiro Nakamura

Abstract

This article explains the features of and standardization schedule for LTE-Advanced, which has been approved by ITU-R (International Telecommunication Union, Radio Communication Sector) as one of the IMT-Advanced systems (IMT: International Mobile Telecommunication).

†	NTT DOCOMO Chiyoda-ku, Tokyo, 100-6150 Japan

1. Introduction

NTT DOCOMO launched Xi™^* (read “crossy”), Japan’s first wireless connectivity service based on the extra-high-speed technology, Long Term Evolution (LTE), on December 24, 2010 to increase the data rate, sophistication, and economy of our radio network. It is based on the LTE Release 8 specifications (hereinafter referred to as LTE Rel. 8) [1], for which standardization was completed by 3GPP (3rd Generation Partnership Project) in the spring of 2009. LTE Rel. 8 adopts various high-level radio interface technologies such as orthogonal multi-access, frequency-domain scheduling, and multiple-input multiple-output (MIMO) and provides even higher system capacity and throughput for cell edge users than High Speed Packet Access (HSPA). It also significantly reduces transmission and connection delays, resulting in major improvements in system performance [2]. NTT DOCOMO is playing a central role in advancing the LTE standardization in 3GPP, from proposing the basic concepts to completing the specifications. It is also supporting the standardization of LTE Rel. 9, which will further increase the sophistication of LTE, enabling the economical introduction of many different services and meeting the demands of LTE users in the future. LTE Rel. 9, which was completed in March 2010, provides various new functions including functional enhancement of HeNB (Home eNodeB or LTE femtocell [3]), extensions to network self-optimization, location services, and multimedia broadcast and multicast services (MBMS) [4].

NTT DOCOMO is also promoting the standardization of LTE-Advanced (LTE Rel. 10 and beyond), which will further increase the system performance of radio access networks. This is in consideration of the need to respond quickly to the explosive growth of data traffic caused by new devices such as smartphones and tablet computers together with social network services, web browsing, video streaming, and so on. NTT DOCOMO acted as rapporteur for the LTE-Advanced Study Item [5], which was approved in 3GPP in March 2008, and detailed specifications for LTE Rel. 10 were completed in June 2011.

In addition to achieving major system performance increases over LTE Rel. 8, maintaining backward compatibility with LTE Rel. 8 is an important requirement of LTE-Advanced to enable smooth development of the system [6]. On the other hand, standardization of IMT-Advanced is progressing in ITU-R (International Telecommunications Union, Radio Communications Sector) as the successor to the International Mobile Telecommunications 2000 (IMT-2000) system, and LTE-Advanced is also a radio interface candidate for this new system. Because of this, it is very important for LTE-Advanced that the minimum requirements for IMT-Advanced are attained within the IMT-Advanced standardization schedule.

*	Xi™ is a trademark of NTT DOCOMO.

2. Standardization schedule

The schedules for standardization of IMT-Advanced in ITU-R and LTE-Advanced in 3GPP are shown in Fig. 1. At ITU-R, new frequency bands for IMT were identified at the World Radiocommunication Conference 2007 (WRC-07) held in November 2007, and a circular letter soliciting radio interface proposals for IMT-Advanced was issued in March 2008 [7], [8]. In the same year, the requirements and evaluation conditions for IMT-Advanced were specified [9], [10], and by the end of the proposal submission period in October 2009, two proposals, based on LTE-Advanced and IEEE802.16m, respectively, were received. After that, external evaluations by evaluation groups registered with ITU-R began, and recommendations for the radio interface specifications are expected to be completed at the beginning of 2012, after final agreement on IMT-Advanced has been reached.

Fig. 1. Schedules for IMT-Advanced (in ITU) and LTE-Advanced (in 3GPP).

At 3GPP, a Study Item for the LTE-Advanced radio interface was started in March 2008 [5] prompted by the issue of ITU-R’s circular letter, and the requirements and evaluation conditions for LTE-Advanced were specified. Then, technical study of the LTE-Advanced radio interface proceeded, and a proposal for LTE-Advanced was submitted to ITU-R in October 2009 [11]. Doing so showed that all of the requirements for IMT-Advanced were satisfied according to 3GPP’s self-evaluation results. After the evaluation results had been reviewed by an external evaluation group registered with ITU-R, LTE-Advanced was approved as one of the IMT-Advanced systems by ITU-R working party 5D in October 2010.

A Work Item to draft the detailed specifications for the LTE-Advanced radio interface was started in December 2009 and the technical specifications were completed in June 2011.

3. Requirements for LTE-Advanced in 3GPP

The requirements for LTE-Advanced are given in [6]. The following general requirements for LTE-Advanced were agreed upon. First, LTE-Advanced will be an evolution of LTE Rel. 8. Hence, distinctive performance gains from LTE Rel. 8 are required. Moreover, LTE-Advanced will satisfy all of the relevant requirements for LTE Rel. 8 [12]. Second, full backward compatibility with LTE Rel. 8 is required in LTE-Advanced. Thus, LTE-Advanced user equipment must be able to access LTE Rel. 8 networks, and LTE-Advanced networks must be able to support LTE Rel. 8 user equipment. Third, LTE-Advanced shall meet or exceed the IMT-Advanced requirements within the ITU-R time plan. The major performance requirements are listed in Table 1.

Table 1. Major performance requirements of LTE-Advanced in 3GPP.

4. Major radio interface technologies for LTE-Advanced

To satisfy the requirements for LTE-Advanced, improvements to the radio interface technologies were studied, with LTE Rel. 8 as the base. These are described below.

4.1 Carrier aggregation

LTE Rel. 8 supports bandwidths up to 20 MHz, but one goal of LTE-Advanced is to support peak data rates of up to 1 Gbit/s on the downlink and 500 Mbit/s on the uplink, so wider bandwidths are needed. On the other hand, LTE-Advanced must also maintain backward compatibility with LTE Rel. 8. Accordingly, wider bandwidths are supported by combining multiple frequency blocks of bandwidth supported by LTE Rel. 8, called the component carrier, as shown in Fig. 2.

Fig. 2. Carrier aggregation.

4.2 MIMO enhancement

With LTE Rel. 8, MIMO multiplexing of up to four layers is supported on the downlink, but MIMO multiplexing is not supported on the uplink. By contrast, LTE-Advanced supports single-user MIMO multiplexing with up to eight layers on the downlink and four layers on the uplink in order to satisfy the peak spectral efficiency requirements of 30 bit/s/Hz on the downlink and 15 bit/s/Hz on the uplink. Multi-user MIMO has also been improved in order to increase system capacity. For Rel. 11, we are also studying the introduction of coordinated multipoint transmission/reception (CoMP), which coordinates communication among multiple cells in order to improve throughput for cell edge users in particular, as shown in Fig. 3.

Fig. 3. CoMP.

4.3 Interference coordination in heterogeneous networks

Another important requirement for LTE-Advanced is to reduce the cost of the radio access network. The deployment scenario for a heterogeneous network, which deploys small cells of various different types such as picocells and femtocells in the macrocell’s area to increase the capacity in high traffic areas, is shown in Fig. 4. To further increase the system capacity and throughput for cell edge users in the area, interference coordination between the macrocell and the picocells is specified.

Fig. 4. Heterogeneous network.

4.4 Relay network

Another approach to reducing the cost of the radio access network is to reduce the cost of the backhaul potion of the network. Radio relay transmission for the backhaul is being introduced as a low-cost means of expanding coverage in environments where wired transmission is particularly expensive, as shown in Fig. 5.

Fig. 5. Relay network.

References

[1]	3GPP TS36.300, "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2".
[2]	3GPP TS25.912, "Feasibility Study for Evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN)," 2008.
[3]	3GPP TS25.367, "Mobility Procedures for Home Node B (HNB); Overall Description; Stage 2".
[4]	M. Iwamura, A. Umesh, and W. A. Hapsari, "Further Enhancements of LTE––LTE Release 9––," NTT DOCOMO Technical Journal, Vol. 12, No. 1, pp. 45–53, June 2010. http://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/rd/ technical_journal/bn/vol12_1/vol12_1_045en.pdf
[5]	3GPP TD RP-080137, "Proposed SID on LTE-Advanced," 2008.
[6]	3GPP TR36.913, "Requirements for Further Advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)".
[7]	ITU-R Conference Publications, "Final Acts WRC-07, Geneva," 2007.
[8]	ITU-R Document IMT-ADV/1, "Background on IMT-Advanced," 2008.
[9]	ITU-R Report M.2134, "Requirements Related to Technical Performance for IMT-Advanced Radio Interface(s)," 2008.
[10]	ITU-R Report M.2135, "Guidelines for Evaluation of Radio Interface Technologies for IMT-Advanced," 2008.
[11]	ITU-R Document 5D/564, "Complete Submission of 3GPP LTE Release 10 and beyond (LTE-Advanced) under Step 3 of the IMT-Advanced Process," 2009.
[12]	3GPP TR25.913, "Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)".

	Sadayuki Abeta Director of the Radio System Group, Radio Access Network Development Department, NTT DOCOMO. He received the B.E., M.E., and Ph.D. degrees in electrical communication engineering from Osaka University in 1993, 1995, and 1997, respectively. He has been with NTT DOCOMO since 1997, where has been working on research, standardization, and development of UMTS/W-CDMA, HPSA, LTE, and LTE-Advanced. From June 2000 to May 2001, he was a visiting researcher at Princeton University, NJ, USA. From 2005 to 2009, he was a vice chairman of 3GPP TSG-RAN WG1 and rapporteur of LTE and LTE-Advanced in 3GPP TSG RAN WG1.
	Tetsushi Abe Manager, Mitsubishi Corporation. He received the B.S. and M.S. degrees in electrical and electronic engineering and the Dr.Eng. degree in communications and integrated systems from Tokyo Institute of Technology in 1998, 2000, and 2007, respectively. During 1998–1999, he studied electrical and computer engineering at the University of Wisconsin-Madison, WI, USA, under a scholarship exchange student program offered by the Japanese Ministry of Education. He joined NTT DoCoMo in 2000. During 2005–2009, he was with DOCOMO Euro-Labs, Munich, Germany. From 2009 to 2011, he was a vice chairman of 3GPP TSG-RAN WG1. He moved to Mitsubishi Corporation in November 2011.
	Takehiro Nakamura Director, Radio System Design Group, NTT DOCOMO. He joined NTT in 1990 and moved to NTT DoCoMo in 1992. He has been working on R&D of W-CDMA. He has also been engaged in W-CDMA standardization at ARIB in Japan since 1997 and has been the leader of the IMT-Partnership Group in ARIB since March 2006. He has contributed to standardization activities in 3GPP since 1999. He has been a rapporteur for LTE and LTE-Advanced in 3GPP TSG-RAN since December 2004 and March 2008, respectively. He contributed to 3GPP TSG-RAN as a vice chairman from March 2005 to March 2009. He has been the chairman of 3GPP TSG-RAN since April 2009.

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