Special Articles on LTE-Advanced Technology —Ongoing Evolution of LTE Toward IMT-Advanced— Overview of LTE-Advanced and Standardization Trends

Overview of LTE-Advanced and Standardization Trends

LTE-Advanced IMT-Advanced Standardization

Special Articles on LTE-Advanced Technology —Ongoing Evolution of LTE toward IMT-Advanced— Overview of LTE-Advanced and Standardization Trends

† The standardization of LTE-Advanced, which is the develop- Radio Access Network Development Department Takehiro Nakamura ment radio interface for LTE, is currently progressing at the Sadayuki Abeta† 3GPP. LTE-Advanced maintains backward compatibility Mikio Iwamura† †

Journal with LTE, while achieving higher system performance than Tetsushi Abe LTE and satisfying the minimum requirements for IMT- Motohiro Tanno† Advanced, which is in the process of being standardized by the ITU-R. In order to achieve these goals, radio interface technologies such as support of wider transmission band- width, enhancement of MIMO technology used and relay, are being studied on a base of LTE technology. Technical

er system capacity and cell edge user in the future. LTE Rel. 9, which was 1. Introduction throughput than High Speed Packet completed in spring, 2010, provides Currently, NTT DOCOMO is Access (HSPA)*3. It also significantly various new functions including Closed developing a commercial mobile com- reduces transmission and connection Subscriber Group (CSG) functions, munications system to increase the data set up delays, making major improve- extensions to network self-optimiza- rate, sophistication and economy of our ments on system performance[3]. tion, location information services and radio network, based on the LTE*1 NTT DOCOMO is playing a central role Multimedia Broadcast and Multicast

DOCOMO Release 8 (hereinafter referred to as in advancing the standardization of Services (MBMS) [4]. “LTE Rel. 8”) specifications [1][2] LTE in the 3GPP, from proposing the NTT DOCOMO is also promoting which completed standardization by the basic concepts to completing the specifi- the standardization of LTE-Advanced 3GPP in the spring of 2009. LTE Rel. 8 cations. NTT DOCOMO is also support- (LTE Rel. 10 and beyond), which will NTT adopts various high-level radio inter- ing the standardization of LTE Rel. 9, further increase the system performance face technologies such as orthogonal which will further increase the sophisti- of radio access networks. This is in con- multi-access, frequency-domain sched- cation of LTE, enabling the economical sideration of the need to respond quick- uling and Multiple Input Multiple Out- introduction of many different services, ly to the demands of traffic which is put (MIMO)*2, and provides even high- and meeting the demands of LTE users expected to increase dramatically as

† Currently Network Department *1 LTE: Extended standard for the 3G mobile multiple transmit/receive antennas. communication system studied by 3GPP. Achieves faster speeds and lower delay than HSPA (see *3). *2 MIMO: Wireless communications technology for expanding transmission capacity by using

4 NTT DOCOMO Technical Journal Vol. 12 No.2 high-volume content services such International Mobile Telecommunica- as video continue to spread [5]. tions 2000 (IMT-2000)*4 system, and 2. Standardization NTT DOCOMO acted as rapporteur LTE-Advanced is also a radio interface Schedule for the LTE-Advanced Study Item candidate for this new system. Because The schedules for standardization of (SI)[6], which was approved at the of this, it is very important for LTE- IMT-Advanced with the ITU-R and 3GPP in March, 2008, and study is now Advanced, that the minimum require- LTE-Advanced with the 3GPP are advancing on the detailed specifications ments for IMT-Advanced are realized shown in Figure 1. At the ITU-R, new for LTE Rel. 10. within the IMT-Advanced standardiza- frequency bands for IMT were identi- In addition to realizing major sys- tion schedule [7]. fied at the World Radiocommunication tem performance increases over LTE In this article, we provide an Conference 2007 (WRC-07)*5 held in Rel. 8, maintaining backward compati- overview of standardization trends with November 2007, and a circular letter*6 bility with LTE Rel. 8 is an important LTE-Advanced, the main radio inter- soliciting radio interface proposals for requirement of LTE-Advanced, to face technologies involved, and the IMT-Advanced was issued in March enable smooth development of the sys- results of 3GPP system performance 2008 [11][12]. In the same year, the Journal tem [7]. On the other hand, standardiza- evaluation. Please refer to other articles requirements and evaluation conditions tion of IMT-Advanced is progressing at for details on the radio interface tech- for IMT-Advanced were specified the International Telecommunications nologies [8]-[10]. [13][14], and by the end of the proposal Union-Radio communications sector submission period in October 2009, two (ITU-R), as the successor to the proposals, based on LTE-Advanced and

2007 2008 2009 2010 2011 Technical ITU-R WP5D No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9 No.10

Development of radio interface candidate technologies Issue circular letter Solicit proposals WRC-07 Evaluation

Building consensus Solicit proposals IMT-Advanced proposal for IMT-Advanced Create radio-interface recommendations

Submit radio interface specifications DOCOMO 3GPP RAN #38 #39 #40 #41 #42 #43 #44 #45 #46 #47 #48 #49 #50 #51 #52 #53 #54

Workshop 2nd Workshop

LTE-Advanced SI WI NTT

LTE Stabilization

*3GPP logo : European Telecommunications Standards Institute (ETSI) trademark. *LTE logo : ETSI trademark. Figure 1 Standardization schedules for IMT-Advanced (ITU) and LTE-Advanced (3GPP)

*3 HSPA: A specification for increasing packet- (HSUPA), which increases speed from the ter- tions, including W-CDMA. data rates in W-CDMA, and a general term minal to the base station. encompassing High Speed Downlink Packet *4 IMT-2000: The 3G mobile communications Access (HSDPA), which increases the speed systems for increasing the speed of communi- from the base station to the mobile terminal, cation. These are summarized in ITU-R recom- and High Speed Uplink Packet Access mendations, and there are currently six varia-

NTT DOCOMO Technical Journal Vol. 12 No.2 5 Overview of LTE-Advanced and Standardization Trends

IEEE 802.16m*7 respectively, were ied, with LTE Rel. 8 as the base, as radio relay transmission for the back- received. After that, external evalua- described below. haul is being introduced as a low-cost tions by evaluation groups registered LTE Rel. 8 supports bandwidths to means to expand coverage in environ- with the ITU-R began, and completion a maximum of 20 MHz, but support for ments where wired transmission is par- of recommendations for the radio inter- peak data rates of up to 1 Gbit/s on the ticularly expensive [10]. face specifications is expected at the downlink and 500 Mbit/s on the uplink beginning of 2011, after final agree- is a goal of LTE-Advanced, so wider 4. System Perfor- ment on IMT-Advanced has been bandwidths are needed. On the other mance Evaluation reached. hand, LTE-Advanced must also main- Results Relative to At the 3GPP, an SI for the LTE- tain backward compatibility with LTE ITU-R Requirements Advanced radio interface was started in Rel. 8. Accordingly, wider bandwidths ITU-R has specified eight items as March 2008[6], prompted by issue of are supported by combining multiple the minimum requirements for IMT- the circular letter by the ITU-R, and the frequency blocks of bandwidth support- Advanced. requirements and evaluation conditions ed by LTE Rel. 8, called Component ¥ Peak spectral efficiency Journal for LTE-Advanced were specified Carrier (CC). This technique is called ¥ Cell spectral efficiency [7][15]. Then, technical study of the Carrier Aggregation (CA) [8]. ¥ Cell edge user spectral efficiency LTE-Advanced radio interface proceed- With LTE Rel. 8, MIMO multiplex- ¥ Bandwidth ed, and a proposal for LTE-Advanced ing up to 4 layers is supported on the ¥ Latency was submitted to the ITU-R in October downlink, but MIMO multiplexing is ¥ Mobility 2009 [16]. Doing so showed that all of not supported on the uplink. Converse- ¥ Handover interruption time the requirements for IMT-Advanced ly, LTE-Advanced supports single-user ¥ VoIP capacity *8 Technical were satisfied through the 3GPP’s self- MIMO multiplexing with up to 8 lay- evaluation results, as described below. ers on the downlink and 4 layers on the Next, we will describe some of the A Work Item (WI) to study the detailed uplink in order to satisfy the peak spec- results of the self-evaluation done at the specifications for the LTE-Advanced tral efficiency*9 requirements of 30 3GPP. radio interface was started in December bit/s/Hz on the downlink and 15 The peak spectral efficiency results 2009, and approval of this specification bit/s/Hz on the uplink. Multi-user for the uplink and downlink are shown is planned for December 2010, to MIMO*10 has also been improved in in Table 1. For details on overhead and match the IMT-Advanced standardiza- order to increase system capacity. We are other assumptions, refer to [17]. The

DOCOMO tion schedule. also studying the introduction of Coor- table shows that ITU-R requirements dinated Multi-point transmission/recep- can be satisfied by using 4-layer MIMO 3. Major Radio tion (CoMP), which coordinates com- multiplexing, which can implement Interface munication among multiple cells, in LTE Rel. 8 single-user MIMO, for the NTT Technologies for order to improve throughput for cell downlink and 2-layer MIMO multi- LTE-Advanced edge users in particular [9]. plexing, which is able to implement To satisfy the requirements for Another important requirement for LTE-Advanced single-user MIMO, for LTE-Advanced, improvements to the LTE-Advanced is to reduce the cost of the uplink. As well, the requirements radio interface technologies were stud- the radio access network, so use of for LTE-Advanced specified by 3GPP

*5 WRC-07: A global wireless communications technical standards, and use of satellite orbits, *7 IEEE 802.16m: The IEEE candidate radio conference held in 2007. This conference is to ensure efficient use of radio frequencies. interface for IMT-Advanced. held every three to four years in order to revise *6 Circular letter: An official document used to *8 Single-user MIMO: Technology which uses international Radio Regulations (RR) for use communicate among participating national reg- MIMO transmission over the same time and of radio waves, including use of various fre- ulatory agencies, companies and organization frequency for a single user. quency bands, operation of radio base stations, members.

6 NTT DOCOMO Technical Journal Vol. 12 No.2 (30 bit/s/Hz downlink, 15 bit/s/Hz the indoor, microcellular (microcell)*11, the uplink and downlink in the ITU test uplink) can also be satisfied using the base coverage urban (macrocell)*12 and environments are shown in Figures 2 up-to-8-layer multiplexing on the high speed environments, respectively. and 3. The characteristics of the down- downlink and up-to-4-layer multiplex- For the three environments besides link were evaluated with four transmit *13 ing on the uplink. This can be achieved indoor, a wrap-around multi-cell and two receive antennas (4 × 2) for with LTE-Advanced single-user structure with 19 sites (3 cells/site) is LTE Rel. 8 single-user MIMO, and MIMO. used, with cell size, mobility speed and with four transmit and two receive The cell spectral efficiency and cell channel models differing according to antennas (4 × 2) for LTE-Advanced edge-user spectral efficiency require- environment. Refer to [17] for details multi-user MIMO. On the uplink, the ments [13] of the test environment on other characteristics and evaluation characteristics were evaluated with one specified by the ITU-R [14] are shown conditions. transmit and four receive antennas (1× in Table 2. For IMT-Advanced, four The cell spectral efficiencies and 4) for LTE Rel.8 single-antenna trans- test environments are specified, called cell edge user spectral efficiencies for mission, and with two transmit and four receive antennas (2 × 4) for LTE- Journal Table 1 Peak spectral efficiencies Advanced single-user MIMO. Downlink Uplink Fig. 2 shows that on the downlink, ITU-R requirement 15 6.75 the requirements are satisfied by LTE (bit/s/Hz) Rel. 8 single-user MIMO, in the indoor MIMO multiplexing 4-Layer 8-Layer 2-Layer 4-Layer and high speed environments, but not in Peak spectral efficiency 16.3 30.6 8.4 16.8 (bit/s/Hz) the microcellular and base coverage urban environments, and that these

Technical requirements can be satisfied by using Table 2 ITU test environment and requirements (a) ITU test environment LTE-Advanced multi-user MIMO. It Test environment Indoor Microcellular Base coverage urban High speed also shows that LTE-Advanced multi- Carrier frequency (GHz) 3.4 2.5 2.0 0.8 user MIMO provides a significant Rectangular Cell layout Hexagonal, 19-cell site, 3 cells/site improvement over LTE Rel. 8 single- (2-cell model) user MIMO. Accordingly, in addition Inter-site distance (m) 60 200 500 1,732 to single-user MIMO, which increases Mobility speed (km/h) 3 3 30 120 the peak rate by increasing the per-user Full-buffer Full-buffer Full-buffer Full-buffer Traffic model DOCOMO model model model model multiplexing factor, specifications are System bandwidth (MHz) 20 10 10 10 also being created at the 3GPP for Number of users (per cell) 10 10 10 10 multi-user MIMO as one of the core

(b) ITU requirements technologies of LTE-Advanced, NTT Cell spectral efficiency Downlink 3.0 2.6 2.2 1.1 increasing network capacity by increas- (bit/s/Hz/cell) Uplink 2.25 1.8 1.4 0.7 ing the number of users able to connect Cell edge user Downlink 0.1 0.075 0.06 0.04 simultaneously. spectral efficiency Figure 3 shows that on the uplink, (bit/s/Hz/cell/user) Uplink 0.07 0.05 0.03 0.015 LTE Rel. 8 single-antenna transmission

*9 Spectral efficiency: The number of data bits *11 Microcell: A communications area of radius *13 Wrap-around: In a system-level simulation that can be transmitted per unit time and unit from several tens to several hundreds of using a limited number of cells, an arrange- frequency band. meters, covered by a single base station. ment in which cells are placed surrounding a *10 Multi-user MIMO: Technology which uses *12 Macrocell: A communications area of radius central cell in order to simulate any interfer- MIMO transmission over the same time and from several hundreds to several tens of kilo- ence a cell receives from outside of the cell. frequency for a multiple users. meters, covered by a single base station.

NTT DOCOMO Technical Journal Vol. 12 No.2 7 Overview of LTE-Advanced and Standardization Trends

satisfies the requirements in all of the 6 5.5 5 environments. Additionally, LTE- 4.1 4 Advanced single-user MIMO provides 3.2 2.9 3 3.0 even further improvements to the char- 2.6 2.4 1.9 2.2 2 1.8 acteristics.

(bit/s/Hz/cell) 1.4 1.1 1 In addition to these results, the Cell spectral efficiency 0 3GPP is evaluating CoMP technology Indoor Microcellular Base coverage urban High-speed as part of the evaluation of ITU-R 0.3 requirements, and application of CoMP 0.22 has provided still further improvements 0.2 0.19 to characteristics [17]. In addition to spectral efficiency 0.1 0.087 0.09 0.1 0.06 0.075 0.041 0.066 0.06 0.057 evaluations, the 3GPP evaluations show (bit/s/Hz/cell/user) 0.04 that the IMT-Advanced requirements Journal 0 Indoor Microcellular Base coverage urban High-speed Cell edge user spectral efficiency are satisfied by LTE Rel. 8 and LTE-

Rel. 8 single-user MIMO (4×2) LTE-Advanced multi-user MIMO (4×2) Advanced in areas of bandwidth, laten- ITU-R requirement values cy, mobility, handover-interruption Figure 2 Spectral efficiency in the ITU test environment (downlink) time and in the number of VoIP users accommodated [17].

5 5. Conclusion 4.3

Technical 4 In this article we have given an 3.3 3 overview of standardization trends pro- 2.2 2.2 2.25 1.9 ceeding at the 3GPP for LTE- 2 1.7 1.8 1.8 1.5 (bit/s/Hz/cell) 1.4 Advanced, of the main radio interface 1 0.7 Cell spectral efficiency technologies and of system perfor- 0 Indoor Microcellular Base coverage urban High-speed mance evaluation results. Standardiza- 0.3 tion work for LTE-Advanced will con- 0.25 0.24 tinue, and approval of the standard

DOCOMO 0.2 specifications for the LTE Rel. 10 radio interface is planned for the end of 2010. 0.11 0.1 0.073 0.084 0.075 0.082 (bit/s/Hz/cell/user) 0.07 0.062 References 0.05

NTT 0.03 0.015 [1] 3GPP TS36.300 V8.11.0: “Evolved Uni- 0 Cell edge user spectral efficiency Indoor Microcellular Base coverage urban High-speed versal Terrestrial Radio Access (E-UTRA) Rel. 8 single-antenna transmission MIMO (1×4) and Evolved Universal Terrestrial Radio LTE-Advanced single-user MIMO (2×4) ITU-R requirement values Access Network (E-UTRAN); Overall Figure 3 Spectral efficiency in the ITU test environment (uplink) description; Stage 2,” 2009. [2] T. Nakamura et. al: “Activities and Contri-

8 NTT DOCOMO Technical Journal Vol. 12 No.2 butions for Completion of the 3GPP for further advancements for Evolved [13] ITU-R Report M.2134: “Requirements LTE/SAE Standard Specifications,” Universal Terrestrial Radio Access (E- related to technical performance for IMT- NTT DOCOMO Technical Journal, Vol. UTRA) (LTE-Advanced),” 2009. Advanced radio interface(s),” 2008. 11, No. 2, pp. 39-49, Sep. 2009. [8] N. Miki et. al: “CA for Bandwidth Exten- [14] ITU-R Report M.2135: “Guidelines for [3] 3GPP TS25.912 V8.0.0: “Feasibility study sion in LTE-Advanced,” NTT DOCOMO evaluation of radio interface technologies for evolved Universal Terrestrial Radio Technical Journal, Vol. 12, No. 2, pp. 10- for IMT-Advanced,” 2008. Access (UTRA) and Universal Terrestrial 19, Sep. 2010. [15] 3GPP TR36.814 V1.0.0: “Further Radio Access Network (UTRAN),” 2008. [9] H. Taoka et. al: “MIMO and CoMP in Advancements for E-UTRA Physical Layer [4] M. Iwamura et. al: “Further Enhance- LTE-Advanced,” NTT DOCOMO Technical Aspects,” 2009. ments of LTE—LTE Release 9—,” Journal, Vol. 12, No. 2, pp. 20-28, Sep. [16] ITU-R Document 5D/564: “Complete NTT DOCOMO Technical Journal, Vol. 2010. submission of 3GPP LTE Release 10 and 12, No. 1, pp. 45-53, Jun. 2010. [10] M. Iwamura et. al: “Relay Technology in beyond (LTE-Advanced) under Step 3 of [5] S. Abeta et. al: “Standardization Trends in LTE-Advanced,” NTT DOCOMO Technical the IMT-Advanced process,” 2009. LTE Enhancement,” NTT DOCOMO Tech- Journal, Vol. 12, No. 2, pp. 29-36, Sep. [17] 3GPP TR36.912 V.9.0.0: “Feasibility study nical Journal, Vol. 12, No. 1, pp. 42-44, 2010. for Further Advancements for E-UTRA Jun. 2010. [11] ITU-R Conference Publications: “Final (LTE-Advanced),” 2009. [6] 3GPP TD RP-080137: “Proposed SID on Acts WRC-07, Geneva,” 2007. Journal LTE-Advanced,” 2008. [12] ITU-R Document IMT-ADV/1: “Back- [7] 3GPP TR36.913 V8.0.1: “Requirements ground on IMT-Advanced,” 2008. Technical DOCOMO NTT

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