Trends in Small Cell Enhancements in LTE Advanced

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1 LTE TECHNOLOGY UPDATE: PART 2 Trends in Small Cell Enhancements in LTE Advanced Takehiro Nakamura, Satoshi Nagata, Anass Benjebbour, and Yoshihisa Kishiyama, NTT DOCOMO, INC Tang Hai, Shen Xiaodong, Yang Ning, and Li Nan, China Mobile Research Institute ABSTRACT 3GPP LTE, or Long Term Evolution, the fourth generation wireless access technology, is being rolled out by many operators worldwide. Since LTE Release 10, network densification using small cells has been an important evolution direction in 3GPP to provide the necessary means to accommodate the anticipated huge traffic growth, especially for hotspot areas. Recently, LTE Release 12 has been started with more focus on small cell enhancements. This article provides the design principles and introduces the ongoing discussions on small cell enhancements in LTE Release 12, and provides views from two active operators in this area, CMCC and NTT DOCOMO. INTRODUCTION Explosive demands for mobile data are driving changes in how mobile operators will need to respond to the challenging requirements of higher capacity and improved quality of user experience (QoE). Currently, fourth generation wireless access systems using Long Term Evolution (LTE) [1] are being deployed by many operators worldwide in order to offer faster access with lower latency and more efficiency than 3G/3.5G. Nevertheless, the anticipated future traffic growth is so tremendous that there is a vastly increased need for further network densification using small cells to handle the capacity requirements, particularly in high traffic areas (hot spot areas) that generate the highest volume of traffic. To optimize performance and provide cost/energy-efficient operation, small cells require further enhancements and in many cases need to interact with or complement existing macrocells. In this regard, a number of solutions have been specified in recent releases of LTE (i.e., Release [Rel]-10/11, and more solutions are to be studied in coming releases (Rel- 12 and beyond). Network densification using small cells has been of great interest in 3GPP since Rel-10, with techniques such as coordinated multipoint (CoMP) transmission/reception and enhanced intercell interference coordination (eicic) being introduced [2]. This article discusses the recent trends and the state-of-the-art technologies related to the design of small cells. First, a brief review of the main features related to small cells in LTE up to Rel-11 is provided. Then the status of the ongoing discussions and the agreements reached so far in Rel-12 are presented. Finally, the operators views of CMCC and NTT DOCOMO on small cell enhancements are provided. RECENT TRENDS IN MOBILE DATA USAGE AND THE RISE OF SMALL CELLS In recent years the proliferation of high-specification handsets, in particular smartphones, has led to unprecedented market trends being observed. Image transfer and video streaming, as well as innovative cloud services are reaching an increasing number of customers. In 2011 alone, the volume of mobile data traffic grew 2.3 times with a nearly threefold increase in the average smartphone usage rate [3]. In the future, the amount data traffic will grow at a pace never seen before. Many recent forecasts project mobile data traffic to grow more than 24-fold between 2010 and 2015, and thus beyond 500-fold in 10 years ( ), assuming that the same pace of growth is maintained. Thus, the capacity of future systems needs to be increased significantly so that it can accommodate such growth in the traffic volume. Revenue growth is becoming more challenging after many operators worldwide introduced flat rate tariffs. Further reduction in the deployment cost of small cells will therefore be a necessity in the future. 3GPP STATUS TOWARD REL-12 AND BEYOND In order to continue to ensure the sustainability of 3GPP radio access technologies over the coming decade, 3GPP standardization will need to identify and provide new solutions that can respond to future challenges. To this end, 3GPP initiated a workshop on further steps in the evolution of LTE toward the future (i.e., Rel-12 and on) in June There were 42 presentations from 3GPP member organizations, including /13/$ IEEE IEEE Communications Magazine February 2013

2 F1 F2 Figure 1. Deployment scenarios for enhanced small cells:. F1 and F2 are the carrier frequencies for the macro layer and small cell layers, respectively. network operators [e.g., 4, 5], considering future requirements and candidate technologies. The key areas of enhancement that were identified included capacity increase to cope with the traffic explosion, energy savings, cost efficiency, support for diverse application and traffic types, higher user experience/data rate, and backhaul enhancement. As a potential technology to meet these requirements, a great majority of companies showed interest in enhanced small cells. The Rel-12 specifications are expected to be completed around June A summary of the workshop can be found in [6]. 3GPP AGREEMENTS ON REQUIREMENTS AND SCENARIOS OF SMALL CELL ENHANCEMENTS As a follow-on to the workshop, 3GPP decided in September 2012 to start a study on the scenarios and requirements of small cell enhancements. This study was completed successfully in December 2012 and the agreed deployment scenarios and relevant technical requirements are captured in a technical report [7]; these are briefly introduced hereinafter. DEPLOYMENT SCENARIOS IDENTIFIED IN THE STUDY Enhanced small cells can be deployed both with macro coverage and standalone, both indoor and outdoor, and support both ideal and non-ideal backhauls. Enhanced small cells can also be deployed sparsely or densely. An illustration of possible deployment scenarios is shown in Fig. 1 [7]. With and Without Macro Coverage An enhanced small cell may benefit from the presence of overlaid macro cells, and it should also work without macro coverage, for example, in deep indoor situations. Cooperative mechanisms between macro cell and small cell, as well as among small cells, may be beneficial for the abovementioned usage cases. Outdoor and Indoor A key differentiator between indoor and outdoor scenarios is mobility support. In indoor scenarios, users normally stay stationary or move at very low speeds. In outdoor scenarios, operators may deploy small cell nodes to cover certain busy streets where relatively higher terminal speeds can be expected. 3GPP has decided to focus on low terminal speeds (0 3 km/h) for indoor and medium terminal speeds (up to 30 km/h and potentially higher) for outdoor scenarios. Backhaul The backhaul, which generally means the link connecting the radio access network and core network, is another important aspect for enhanced small cells, especially when considering the potentially large number of small cell nodes to be deployed. 3GPP has decided both the ideal backhaul (i.e., very high throughput and very low latency backhaul, e.g., dedicated point-to-point connection using optical fiber or line of sight [LOS] microwave) and non-ideal backhaul (e.g., typical backhaul widely deployed today, e.g., xdsl, non-los [NLOS] microwave) should be studied. Examples of nonideal backhaul are listed in Table 1: Distribution of Small Cells In some scenarios (e.g., hotspot indoor/outdoor locations), a single or a few small cell node(s) is/are sparsely deployed, for example, to cover the traffic hotspot(s). In some other scenarios (e.g., dense urban or large shopping malls), a large number of small cell nodes are densely deployed to support a huge amount of traffic over a relatively wide area. Synchronization Both synchronized and unsynchronized scenarios should be considered between small cells as well as between small cells and macrocell(s). IEEE Communications Magazine February

3 Backhaul technology Latency (one way) Throughput Fiber ms 10 Mb/s 10 Gb/s DSL ms Mb/s Wireless backhaul (typically NLOS) 5 35 ms Table 1. Examples of non-ideal backhaul Mb/s typical, maybe up to 1 Gb/s range Spectrum Small cell enhancement should address a deployment scenario in which different frequency bands are separately assigned to the macro layer and small cell layers. Co-channel deployment scenarios where the macro and small cell layers share the same carrier should be considered as well. Small cell enhancements should be applicable to all existing as well as future cellular bands, with special focus on higher frequency bands such as the 3.5 GHz band, to exploit wider bandwidths. Small cell enhancements should also take into account the possibility of frequency bands that, at least locally, are only used for small cell deployments. Traffic Patterns In a small cell deployment, it is likely that the traffic will fluctuate greatly since the number of users per small cell node is typically not large due to the small coverage area; it is also likely that the user distribution is very non-uniform and fluctuates between the small cell nodes. It is also expected that the traffic could be highly asymmetrical, either downlink- or uplink-centric. Traffic load distribution in the time domain and spatial domain could be uniform or non-uniform. Backward Compatibility Backward compatibility, that is, the possibility for legacy (pre-rel- 12) user equipment (UE) to access a small-cell node/carrier, shall be guaranteed (except for features studied for small cells using the new carrier type, which is the subject of a separate article in this magazine) and the ability for legacy (pre- Rel-12) UE to benefit from small-cell enhancements can be considered, which shall be taken into account in the evaluation of the different proposed enhancements. The introduction of non-backward-compatible features should be justified by sufficient gains. TECHNICAL REQUIREMENTS FOR SMALL CELL ENHANCEMENTS Based on the deployment scenarios identified, deployment-related requirements, capability/performance requirements, and operational requirements for enhanced small cells can be determined as outlined below. Deployment-Related Requirements Enhanced small cells can be deployed by either operators or independent users such as an organization in an office building. Automatic mechanisms such as plug-and-play provisioning to support flexible configuration and lower cost for operation and maintenance could be considered. Small-cell enhancements should minimize signaling load (e.g., caused by mobility) to the core network as well as increase of backhaul traffic due to increasing numbers of small-cell nodes. Capability and Performance Requirements Small cell enhancements should: 1 Significantly support increased user throughput for both downlink and uplink with the main focus being on typical user throughput given a reasonable system complexity. 2 Keep a fair distribution of user throughput for both downlink and uplink in a scenario where the user distribution changes dynamically. 3 Target the capacity per unit area (e.g., bits per second per square kilometer) to be as high as possible, for a given user and small cell distribution, with typical traffic types and reasonable system complexity. 4 Provide improved system performance with realistic backhaul delays. Further aspects were also identified: For UE being served on a macro layer and for targeted mobile speeds up to 30 km/h, small cell nodes need to be discovered, and potential mobility to a small cell node performed, in a timely manner and with low UE power consumption in a situation when the UE moves into the coverage area of the small cell layer. Mobility across densely deployed small cell nodes, and between macro and small cells on the same frequency layer, should be targeted with good performance for mobile speeds up to 30 km/h. The benefits of allowing high-speed UE in small cells should be evaluated (e.g., UE throughput gain, improved robustness of mobility, improved UE power efficiency, and up to which speed offloading is beneficial). Real-time services should be supported by small-cell enhancements. The impact of mobility between small cell nodes and between small cell and overlaid macro nodes on quality (e.g., interruption time, packet loss) should be less than or equal to that provided by LTE Rel-10/11. Small-cell enhancements should consider techniques and mechanisms to reduce control (C)-plane/user data (U)-plane latency and packet loss during mobility between macro and small cell nodes, as well as between small cell nodes compared to LTE Rel-10/11. Operational Requirements Small-cell enhancements should allow for low network cost by: Allowing for solutions aiming at different backhauls Allowing for low-cost deployment, and low operation and maintenance tasks (e.g., by means of self-organizing network [SON] functionality and minimization of drive tests) 100 IEEE Communications Magazine February 2013

4 Existing cellular bands (high power density for coverage) Wide area Figure 2. Combined use of lower and higher frequency bands. Higher frequency bands (wider bandwidth for high data rate) Very wide (ex. > 3 GHz) Super wide (ex. > 10 GHz) Local area Frequency Placing small cells in a dormant mode could be supported considering the increased likelihood of small cells not serving any active users at certain times. High UE energy efficiency should be targeted taking into account the small cell s short range transmission path. Allowing for reduced base station implementation cost, considering, say, relaxation of radio frequency (RF) requirements in small cell scenarios Different UE capabilities should be considered for small-cell enhancements, especially with respect to features related to UE RF complexity such as the possibility for simultaneous transmission to and reception from the macro and small cell layers. Placing small cells in a dormant mode could be supported considering the increased likelihood of small cells not serving any active users at certain times. High UE energy efficiency should be targeted, taking into account the small cell s short-range transmission path. OPERATOR VIEWS ON SMALL CELL ENHANCEMENTS AND POSSIBLE FUTURE IMPLICATIONS TO MOBILE INDUSTRY In this section we provide examples of some network operator insights into the potential deployment considerations of enhanced LTE small cells. NTT DOCOMO S VIEW From the spectrum utilization point of view, spectrum in the lower frequency bands is becoming scarce. Thus, it is crucial to explore and utilize higher frequency bands in the development of techniques for future radio access. However, higher frequency bands are difficult to accommodate in wide areas in macrocells because of either space limitations on the enb side, for example, in terms of RF equipment and antenna size, coverage limitations (e.g., higher path loss), or cost issues due to the need to alter the already established network infrastructure. Therefore, NTT DOCOMO s intention is to use lower frequency bands such as existing cellular bands in macrocells to provide basic coverage and mobility, and to use separate higher frequency bands in local areas for small cells to provide high-speed data transmission, as shown in Fig. 2 [8]. Such combined use of lower and higher frequency bands will make higher frequency bands useful and beneficial for cellular operators. There will no longer be a coverage issue, and we can provide very high throughput performance using local area access technologies with a wider spectrum bandwidth in the higher frequency bands while obtaining a significant offloading gain from the existing cellular bands. From another point of view, the market size for utilizing higher frequency bands needs to be sufficiently large. It is therefore desirable from the operator perspective that higher frequency bands can be used by many UE devices and for many service areas as much as possible. In this sense, higher frequency bands need to be utilized in various deployments, not only for indoor but also for outdoor deployments. Phantom Cell Concept In the current deployments, there are a number of capacity solutions for indoor environments such as WiFi, femtocells, and in-building cells using distributed antenna systems (DAS). However, there is a lack of capacity solutions for high-traffic outdoor environments that can also support good mobility and connectivity. Thus, we propose the concept of macro-assisted small cells, called the Phantom Cell [9], as a capacity solution that offers good mobility support while capitalizing on the existing LTE network. In the Phantom Cell concept, the C-plane/U-plane are split as shown in Fig. 3. The C-plane of UE in small cells is provided by a macrocell in a lower frequency band, while for UE in macrocells both the C-plane and U-plane are provided by the serving macrocell in the same way as in the conventional system. On the other hand, the U- plane of UE in small cells is provided by a small cell using a higher frequency band. Hence, these macro-assisted small cells are called Phantom Cells as they are intended to transmit UE-specific signals only, and the radio resource control (RRC) connection procedures between the UE IEEE Communications Magazine February

5 2 GHz (example) 3.5 GHz (example) Phantom cell Split!! U-plane C-plane (RRC) Macrocell Figure 3. Phantom Cell concept with C/U plane split. and the Phantom Cell, such as channel establishment and release, are managed by the macrocell. The Phantom Cells are not conventional cells in the sense that they are not configured with cellspecific signals and channels such as cell-id-specific synchronization signals, cell-specific reference signals (CRS), and broadcast system information. Their visibility to the UE relies on macrocell signaling. The Phantom Cell concept comes with a range of benefits. One important benefit of macro assistance of small cells is that control signaling due to frequent handover between small cells and macrocells and among small cells can be significantly reduced, and connectivity can be maintained even when using small cells and higher frequency bands. In addition, by applying the new carrier type (NCT) that contains no or reduced legacy cell-specific signals (see separate article in this magazine), the Phantom Cell is able to provide further benefits such as efficient energy savings, lower interference and hence higher spectral efficiency, and reduction in cellplanning effort for dense small cell deployments. To establish a network architecture that supports the C/U-plane split, and interworking between the macrocell and Phantom Cell is required. A straightforward solution to achieve this is to support Phantom Cells by using remote radio heads (RRHs) belonging to a single macro enb. This approach can be referred to as intraenb carrier aggregation (CA) using RRHs [10]. However, such a tight CA-based architecture has some drawbacks as it requires single-node operation with low-latency connections (e.g., optical fibers) between the macro and Phantom Cells. Therefore, more flexible network architectures should be investigated to allow for relaxed backhaul requirements between macro and Phantom Cells and to support a distributed node deployment with separated network nodes for each (i.e., inter-enb CA). Other Technical Considerations Enhanced Discovery and Mobility In frequency-separated deployments, efficient discovery and mobility for small cells in higher frequency bands is an important technical issue. To achieve efficient discovery (e.g., for UE battery and network energy savings), we propose utilizing a macro-assisted property in the Phantom Cell concept and introduce newly defined discovery signals, which are transmitted by small cells in a time-synchronized manner with macro downlink signals [9]. Assisted by the C- plane provided by the macrocell, small cells simultaneously transmit the discovery signals with a relatively long transmission interval (e.g., longer than 100 ms), and the UE attempts the detection of discovery signals from small cells only in a short time interval. This discovery signal should be designed to satisfy some important requirements for small cell deployment such as robustness against intercell interference including user-deployed or closed subscriber group (CSG) cells, and support for a large number of sequence (e.g., more than the 504 currently provided by LTE) to reduce cell planning efforts. Furthermore, it can potentially be used for purposes such as time/frequency synchronization and path loss estimation for power control. Dynamic Time-Division Duplex and Interference Coordination for Dense Small Cells In small cell deployments, it may be expected that the number of UE devices per small cell will not be large, and the traffic pattern in each small cell will change widely over time depending on user applications and user locations. As a result, dynamic spectrum sharing between the uplink and downlink [11] would provide gains in terms of spectral efficiency compared to semistatic partitioning of the uplink and downlink. However, when small cells are densely deployed, uplink-to-downlink and downlink-to-uplink interference will become more problematic. Therefore, interference coordination and management schemes for dense small cells need to be established if dynamic time-division duplex (TDD) is used. Massive MIMO A Future Topic Massive multiple-input multiple-output (MIMO) with an active antenna system (AAS) but very large numbers of antennas (e.g., more than 100 antenna elements) is a potential technology for small cells in future higher frequency bands (e.g., beyond 10 GHz) [12]. For high frequency bands, antenna elements can be miniaturized, and more elements can be placed in the same space; thus, very narrow beams can be formed. We expect such very narrow beamforming will be essential in higher frequency bands in order to support practical coverage areas for small cells by compensating for the increased path loss. Assuming ideal beamforming gain, a two-dimensional mapping of antenna elements can compensate for the path loss with a frequency factor of 20 db/decade. However, there are several technical issues toward massive-antenna technologies that need to be resolved, such as how to achieve accurate beamforming or how to support control signaling for mobility and connectivity over highly directive links. One possibility to address the control signaling issue is to apply macro-assisted small cells (i.e., the Phantom Cell). CMCC S VIEW In the foreseeable future, the percentage of voice and data traffic that occurs indoors is expected to increase to 90 percent. This trend is the motivation for a collection of enhancements 102 IEEE Communications Magazine February 2013

6 LTE baseline CRS reduction Multi-TTI scheduling Traffic adaptation 256QAM Carrier aggregation, 5CC Peak data rate improvement breakdown for small cell enhancements 100% 70M bps (100%) 2x2M IMO, conf.1, CFI=3, DwPTS=10 80M bps 109.5% (+9.5%) Reduced 2-port CRS overhead 126% 92M bps (+15%) Only 1 OFDM reserved every 5 ms 173% 126M bps (+37%) Maximum 4 DL subframe every 5 ms 230% 168M bps (+33%) Improved 33%SE compared to 64QAM 1150%, estimated peak rate with CA support 838M bps (+400%) carrier aggregation with 5 CCs To reduce cost and provide maximum coverage, enhanced small cells should be able to utilize various existing and future backhauls, including optical fiber, Ethernet and microwave systems for indoor and outdoor hotspots, as well as xdsl, FTTH or Cable TV for home use. Figure 4. Example of peak data rate improvement breakdown for small cell enhancements. we refer to as LTE enhancements for hotspot and indoor (LTE-Hi) to ensure the future competitiveness of 3GPP technologies, where denser networks, easy-to-deploy low-power nodes, new techniques to improve the spectrum efficiency and throughput, and large bandwidth support are the four main areas for technical enhancement. LTE-Hi Concept LTE-Hi targets local access enhancements for hotspot and indoor scenarios considering the above-mentioned technical areas. Denser network deployment together with easy-to-deploy low-power nodes play important roles to cope with the mobile data explosion, and from the operator perspective it would be beneficial that a large number of small cells can be deployed wherever necessary. A series of technical innovations will be needed to meet this requirement. Interference and mobility coordination are key technologies in this area. Autonomous configuration acquisition/coordination including synchronization establishment among multiple neighboring small cells are essential for the operators to reduce the complexity of network planning, especially for the case of independent user deployment of small cells. Coordination with overlaid macrocells could help to ease the difficulties of mobility and service continuity when UE devices are moving between potentially discontinuous coverage of small cells, as the targeted cell radius is small (e.g., 50 m or less) due to low transmission power. One example is outdoor hotspots, where the UE speed can be up to 30 km/h, at which speed the time needed for UE to pass through a cell with 50 m will be just 6 s; thus, it may be more appropriate for such UE to reside in macrocells in order to avoid unnecessary hand - overs. For low-speed UE, the residence time will be long enough for LTE-Hi cells to serve the UE with high-speed data. When macrocells are not present (e.g., deep indoors), small cells must be able to work as standalones. In this case coordination among small cells is essential to ensure mobility performance. After identifying the necessary information/signaling to be exchanged between macro and small cells as well as among small cells, it can be decided whether to reuse the LTE X2 interface or introduce new interfaces. The support of any backhaul is another cornerstone for denser network deployment with a large number of low-power nodes. To reduce cost and provide maximum coverage, enhanced small cells should be able to utilize various existing and future backhauls, including optical fiber, Ethernet, and microwave systems for indoor and outdoor hotspots, as well as digital subscriber line (xdsl), fiber to the home (FTTH), or cable TV for home use. Consequently, enhanced small cells must be able to accommodate a relatively large range of delay budgets and throughput performance, which will have a substantial impact on the system design. New Techniques to Improve the Spectral Efficiency and Throughput Link-Level Improvements Some room for improvement is still present for optimization on top of current LTE technology, especially in small cell scenarios. For example, 256- quadrature amplitude modulation (QAM) takes advantage of the greater probability of higher signal-to-interference-plus-noise ratio (SINR) in small cells. Also, further overhead reduction is possible for control channel and demodulation reference signals. In TDD deployments, flexible uplink and downlink timeslot allocations yield the possibility to match uplink and downlink resources to traffic demand, thus maximizing the system throughput. IEEE Communications Magazine February

7 CMCC strongly believes that it is essential that WRC-15 achieves a positive outcome on new spectrum allocation to IMT. A globally aligned spectrum allocation will play a key role in the development of the mobile ecosystem. Performance Gain Provided by Cell Densification Fast cell discovery: When macro assistance is unavailable, UE may need to search exhaustively all possible neighboring cells. Thus, a well designed cell discovery reference signal (RS) would not only improve cell-level energy efficiency, but also facilitate neighbor cell identification and synchronization. A serving small cell could also provide information to assist detection of neighboring cells. Handling of macro-layer/small cell information exchange: In practice, coordination between small cells, and between small cells and macro cells is necessary to provide sufficient robustness of mobility, joint transmission, and efficient resource allocation. Therefore, it is important to study mechanisms for how to handle information exchange between cells. One approach may be an air interface listening mechanism since it is not sensitive to backhaul restrictions; it would then be of interest to study how to configure transmission gaps for a cell in order to enable efficient listening between cells without altering the physical layer transmission operation more than necessary. SUPPORT OF LARGE BANDWIDTHS Allocating a large amount of spectrum to small cells is a straightforward way to increase system capacity, but unfortunately, spectrum is a limited and scarce resource, especially in the existing frequency bands. Seeking new spectrum resources in higher bands may be a natural choice for small cell deployment. In 2015, the International Telecommunication Union (ITU) World Radiocommunications Conference (WRC-15) will identify new spectrum for international mobile telecommunications (IMT). According to estimations in China, the total spectrum requirement is about MHz of the spectrum for IMT in At least 1000 MHz of additional spectrum needs to be introduced by From CMCC s point of view, the main target bands are 1.5 GHz, GHz, GHz, and above 5 GHz. These candidate bands could provide a sufficient quantity of spectrum resource. CMCC strongly believes that it is essential that WRC-15 achieves a positive outcome on new spectrum allocation to IMT. A globally aligned spectrum allocation will play a key role in the development of the mobile ecosystem. Taking all the above possibilities into account, an example of the combined peak data rate improvement for an LTE TDD system that may be brought about by small cell enhancements is shown in Fig. 4. CONCLUSION 3GPP has recognized that small cells are a promising approach to meet future mobile service requirements, especially for indoor and outdoor hotspots. 3GPP has already extensively discussed the relevant deployment scenarios and technology requirements for LTE small cell enhancements. Detailed technical studies following the requirements and scenario definition are proceeding from the start of 2013, with specifications expected to be completed for Release 12 by mid In this article, two active LTE operators, CMCC and NTT DOCOMO, have described the current status of the small cell enhancement studies in 3GPP, as well as the two companies views on future technology evolution for small cells. REFERENCES [1] S. Sesia, I. Toufik, and M. Baker, LTE The UMTS Long Term Evolution from Theory to Practice, Wiley, [2] T. Abe et al., Radio Interface Technologies for Cooperative Transmission in 3GPP LTE-Advanced, IEICE Trans. Commun., vol. E94-B, no. 12, Dec. 2011, pp [3] Cisco whitepaper, Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, , 15 Feb [4] 3GPP, RWS , CMCC, Views on LTE Rel-12 & Beyond, June [5] 3GPP, RWS , NTT DOCOMO, Requirements, Candidate Solutions & Technology Roadmap for LTE Rel-12 Onward, June [6] 3GPP, RWS , Summary of 3GPP TSG-RAN Workshop on Release 12 and Onward, June [7] 3GPP, TR (V12.0.0), Scenarios and Requirements for Small Cell Enhancements for E-URTA and E-UTRAN, Dec [8] Y. Kishiyama et al., Evolution Concept and Candidate Technologies for Future Steps of LTE-A, Proc. IEEE ICCS 12, Nov [9] H. Ishii, Y. Kishiyama, and H. Takahashi, A Novel Architecture for LTE-B, C-Plane/U-Plane Split and Phantom Cell Concept, IEEE GLOBECOM 2012 Wksp., Dec [10] M. Iwamura et al., Carrier Aggregation Framework in 3GPP LTE-Advanced, IEEE Commun. Mag., vol. 48, no. 8, Aug. 2010, pp [11] J. Li et al., Dynamic TDD and Fixed Cellular Networks, IEEE Commun. Letters, vol. 4, no. 7, July 2000, pp [12] T. L. Marzetta, Non-Cooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas, IEEE Trans. Wireless Commun., vol. 9, no. 11, Nov BIOGRAPHIES TAKEHIRO NAKAMURA (nakamurata@nttdocomo.com) received his B.E. and M.E. degrees in electrical communication engineering from Yokohama National University, Japan, in 1988 and 1990, respectively. He joined NTT Laboratories in In 1992, he transferred to NTT DOCOMO, Inc. He is now director of the Radio Access System Group of NTT DOCOMO, Inc. He has been working on research and development of W-CDMA. He has been engaged in the W- CDMA standardization activity at ARIB in Japan since 1997 and is currently the leader of the Mobile-Partnership Group in ARIB since March He has been contributing to standardization activities in 3GPP since1999. He has been the 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 during March 2005 to March He is currently a chairman of 3GPP TSG-RAN since April SATOSHI NAGATA received his B.E. and M.E. degrees from Tokyo Institute of Technology, Japan, in 2001 and 2003, respectively. In 2003, he joined NTT DOCOMO, Inc. He worked on the research and development for wireless access technologies for LTE and LTE-Advanced, and has been involved in the 3GPP standardization activities since He was a recipient of the IEICE Young Researcher s Award in Since November 2011, he has been serving as a vice chairman of 3GPP RAN WG1. ANASS BENJEBBOUR [SM] received his B.E., M.E., and Dr. Eng. degrees in communications and computer engineering in 1999, 2001, and 2004, respectively, all from Kyoto University, Japan. Since 2004 he has been with NTT DOCOMO Inc. R&D Japan, where he has been actively involved in the development of system concepts and the design of technologies for future radio access. He was also involved in 104 IEEE Communications Magazine February 2013

8 3GPP standardization for LTE Release 10 and 11. He received the best paper award at IEEE PIMRC in 2002 and the young researcher s award from IEICE in He is a senior member of IEICE. YOSHIHISA KISHIYAMA received his B.E., M.E., and Dr. Eng. degrees from Hokkaido University, Sapporo, Japan, in 1998, 2000, and 2010, respectively. In 2000, he joined NTT DOCOMO, Inc. He is currently manager for a team of the Radio Access System Group of NTT DOCOMO. He has been involved in the LTE and LTE-Advanced standardization activities in 3GPP since His current research interests include advanced multiple access technologies and future network concepts for efficient deployments and spectrum utilization. He was a recipient of the IEICE Young Engineer Award in 2004, and a recipient of the ITU Association of Japan Award in He is a member of the IEICIE. HAI TANG received his B.E. from Zhejiang University, China, in 1997, and his M.E. from China Academy of Telecommunications in He joined Datang Telecommunication Group in 2000 for research and development of the TD- SCDMA, and transferred to Datang Mobile Communications Equipment Co. Ltd in He has been engaged in the LTE standardization activity since He joined China Mobile Communications Corporation and is currently a Vice Chairman of TSG RAN since March NING YANG received his Ph.D in circuits and systems from Beijing University of Posts and Telecommunications, P.R.China, in He is currently working in the Wireless Technology Department of China Mobile Research Institute as a senior engineer. Since 2008 he has been actively involved in 3GPP specification work, including architecture and interface design, and signaling and protocol research of high layers. His research interests are in many areas, such as embms, relay technology, HeNB, and SON. XIAODONG SHEN received his B.S. in physics from Peking University in 2004, and his M.S. in electrical engineering from Beijing University of Posts and Telecommunications in He has been a project manager in China Mobile Research Institute since His current interests include LTE, LTE-Advanced, and IMT-Advanced research and standardization for 3GPP, ITU, and NGMN. NAN LI received his M.S. in signal and information processing from Beijing University of Posts and Telecommunications in He is currently working in the Wireless Technology Department of China Mobile Research Institute as a senior engineer. Since 2007 he has been actively involved in 3GPP, CCSA, NGMN, and ITU standardization work, including LTE and LTE-Advanced RF requirements, performance evaluation, and spectrum regulations. IEEE Communications Magazine February