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  1. 1. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® 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 First, a brief review of the main features related to small cells in LTE up to Rel-11 is provided. 3GPP LTE, or Long Term Evolution, the Then the status of the ongoing discussions and fourth generation wireless access technology, is the agreements reached so far in Rel-12 are pre- being rolled out by many operators worldwide. sented. Finally, the operators’ views of CMCC Since LTE Release 10, network densification and NTT DOCOMO on small cell enhance- using small cells has been an important evolu- ments are provided. tion direction in 3GPP to provide the necessary y means to accommodate the anticipated huge traffic growth, especially for hotspot areas. RECENT TRENDS IN Recently, LTE Release 12 has been started with more focus on small cell enhancements. This MOBILE DATA USAGE AND THE article provides the design principles and intro- RISE OF SMALL CELLS duces the ongoing discussions on small cell enhancements in LTE Release 12, and provides In recent years the proliferation of high-specifi- v iews from two active operators in this area, cation handsets, in particular smartphones, has CMCC and NTT DOCOMO. led to unprecedented market trends being observed. Image transfer and video streaming, INTRODUCTION as well as innovative cloud services are reaching an increasing number of customers. In 2011 Explosive demands for mobile data are driving alone, the volume of mobile data traffic grew changes in how mobile operators will need to 2.3 times with a nearly threefold increase in the respond to the challenging requirements of high- average smartphone usage rate [3]. In the er capacity and improved quality of user experi- future, the amount data traffic will grow at a ence (QoE). Currently, fourth generation pace never seen before. Many recent forecasts wireless access systems using Long Term Evolu- project mobile data traffic to grow more than tion (LTE) [1] are being deployed by many oper- 24-fold between 2010 and 2015, and thus ators worldwide in order to offer faster access beyond 500-fold in 10 years (2010–2020), w ith lower latency and more efficiency than assuming that the same pace of growth is main- 3G/3.5G. Nevertheless, the anticipated future tained. Thus, the capacity of future systems traffic growth is so tremendous that there is a needs to be increased significantly so that it can vastly increased need for further network densi- accommodate such growth in the traffic vol- fication using small cells to handle the capacity ume. Revenue growth is becoming more chal- requirements, particularly in high traffic areas lenging after many operators worldwide (hot spot areas) that generate the highest vol- introduced flat rate tariffs. Further reduction in ume of traffic. To optimize performance and the deployment cost of small cells will therefore provide cost/energy-efficient operation, small be a necessity in the future. cells require further enhancements and in many cases need to interact with or complement exist- ing macrocells. In this regard, a number of solu- 3GPP STATUS TOWARD tions have been specified in recent releases of LTE (i.e., Release [Rel]-10/11, and more solu- REL-12 AND BEYOND tions are to be studied in coming releases (Rel- In order to continue to ensure the sustainability y 12 and beyond). Network densification using of 3GPP radio access technologies over the com- small cells has been of great interest in 3GPP ing decade, 3GPP standardization will need to since Rel-10, with techniques such as coordinat- identify and provide new solutions that can ed multipoint (CoMP) transmission/reception respond to future challenges. To this end, 3GPP and enhanced intercell interference coordination initiated a workshop on further steps in the evo- (eICIC) being introduced [2]. This article dis- lution of LTE toward the future (i.e., Rel-12 and cusses the recent trends and the state-of-the-art on) in June 2012. There were 42 presentations technologies related to the design of small cells. from 3GPP member organizations, including 98 0163-6804/13/$25.00 © 2013 IEEE $ IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  2. 2. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® F1 F2 Figure 1. Deployment scenarios for enhanced small cells:. F1 and F2 are the carrier frequencies for the macr layer and small cell lay- ro ers, respectively. network operators [e.g., 4, 5], considering future between macro cell and small cell, as well as requirements and candidate technologies. The among small cells, may be beneficial for the key areas of enhancement that were identified abovementioned usage cases. included capacity increase to cope with the traf- fic explosion, energy savings, cost efficiency, sup- Outdoor and Indoor — A key differentiator port for diverse application and traffic types, between indoor and outdoor scenarios is mobili- higher user experience/data rate, and backhaul ty support. In indoor scenarios, users normallyy enhancement. As a potential technology to meet stay stationary or move at very low speeds. In these requirements, a great majority of compa- outdoor scenarios, operators may deploy small nies showed interest in enhanced small cells. The cell nodes to cover certain busy streets where Rel-12 specifications are expected to be com- relatively higher terminal speeds can be expect- pleted around June 2014. A summary of the ed. 3GPP has decided to focus on low terminal workshop can be found in [6]. speeds (0–3 km/h) for indoor and medium termi- nal speeds (up to 30 km/h and potentially high- er) for outdoor scenarios. 3GPP AGREEMENTS ON Backhaul — The backhaul, which generally y REQUIREMENTS AND SCENARIOS OF means the link connecting the radio access net- SMALL CELL ENHANCEMENTS work and core network, is another important aspect for enhanced small cells, especially when As a follow-on to the workshop, 3GPP decided considering the potentially large number of f in September 2012 to start a study on the sce- small cell nodes to be deployed. 3GPP has decid- narios and requirements of small cell enhance- ed both the ideal backhaul (i.e., very high ments. This study was completed successfully in throughput and very low latency backhaul, e.g., December 2012 and the agreed deployment sce- dedicated point-to-point connection using opti- narios and relevant technical requirements are cal fiber or line of sight [LOS] microwave) and captured in a technical report [7]; these are non-ideal backhaul (e.g., typical backhaul widely y briefly introduced hereinafter. deployed today, e.g., xDSL, non-LOS [NLOS] microwave) should be studied. Examples of non- DEPLOYMENT SCENARIOS ideal backhaul are listed in Table 1: IDENTIFIED IN THE STUDY Distribution of Small Cells — In some scenar- Enhanced small cells can be deployed both with ios (e.g., hotspot indoor/outdoor locations), a macro coverage and standalone, both indoor and single or a few small cell node(s) is/are sparsely outdoor, and support both ideal and non-ideal deployed, for example, to cover the traffic backhauls. Enhanced small cells can also be hotspot(s). In some other scenarios (e.g., dense deployed sparsely or densely. An illustration of urban or large shopping malls), a large number possible deployment scenarios is shown in Fig. 1 of small cell nodes are densely deployed to sup- [7]. port a huge amount of traffic over a relatively wide area. With and Without Macro Coverage — An enhanced small cell may benefit from the pres- Synchronization — Both synchronized and un- ence of overlaid macro cells, and it should also synchronized scenarios should be considered w ork without macro coverage, for example, in between small cells as well as between small cells deep indoor situations. Cooperative mechanisms and macrocell(s). IEEE Communications Magazine • February 2013 99C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  3. 3. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® nisms such as plug-and-play provisioning to sup- Backhaul technology Latency (one way) Throughput port flexible configuration and lower cost for operation and maintenance could be considered. Fiber 10–30 ms 10 Mb/s–10 Gb/s Small-cell enhancements should minimize sig- naling load (e.g., caused by mobility) to the core DSL 15–60 ms 10–100 Mb/s network as well as increase of backhaul traffic due to increasing numbers of small-cell nodes. Wireless backhaul 10–100 Mb/s typical, maybe 5–35 ms (typically NLOS) up to 1 Gb/s range Capability and Performance Requirements — Small cell enhancements should: Table 1. Examples of non-ideal backhaul. d 1 Significantly support increased user throughput for both downlink and uplink with the main focus being on typical user S pectrum — Small cell enhancement should throughput given a reasonable system com- address a deployment scenario in which different plexity. frequency bands are separately assigned to the 2 Keep a fair distribution of user throughput macro layer and small cell layers. Co-channel for both downlink and uplink in a scenario deployment scenarios where the macro and where the user distribution changes dynam- small cell layers share the same carrier should be ically. considered as well. 3 Target the capacity per unit area (e.g., bits Small cell enhancements should be applicable per second per square kilometer) to be as to all existing as well as future cellular bands, high as possible, for a given user and small w ith special focus on higher frequency bands cell distribution, with typical traffic types such as the 3.5 GHz band, to exploit wider band- and reasonable system complexity. widths. 4 Provide improved system performance with Small cell enhancements should also take realistic backhaul delays. into account the possibility of frequency bands Further aspects were also identified: that, at least locally, are only used for small cell • For UE being served on a macro layer and deployments. for targeted mobile speeds up to 30 km/h, small cell nodes need to be discovered, and Traffic Patterns — In a small cell deployment, potential mobility to a small cell node per- it is likely that the traffic will fluctuate greatly formed, in a timely manner and with low since the number of users per small cell node is UE power consumption in a situation when typically not large due to the small coverage the UE moves into the coverage area of the area; it is also likely that the user distribution is small cell layer. v ery non-uniform and fluctuates between the • Mobility across densely deployed small cell small cell nodes. It is also expected that the traf- nodes, and between macro and small cells fic could be highly asymmetrical, either down- on the same frequency layer, should be tar- link- or uplink-centric. geted with good performance for mobile Traffic load distribution in the time domain speeds up to 30 km/h. and spatial domain could be uniform or non-uni- • The benefits of allowing high-speed UE in form. small cells should be evaluated (e.g., UE throughput gain, improved robustness of Backward Compatibility — Backward compat- mobility, improved UE power efficiency, ibility, that is, the possibility for legacy (pre-Rel- and up to which speed offloading is benefi- 12) user equipment (UE) to access a small-cell cial). node/carrier, shall be guaranteed (except for fea- • Real-time services should be supported by tures studied for small cells using the new carrier small-cell enhancements. The impact of type, which is the subject of a separate article in mobility between small cell nodes and this magazine) and the ability for legacy (pre- between small cell and overlaid macro Rel-12) UE to benefit from small-cell enhance- nodes on quality (e.g., interruption time, ments can be considered, which shall be taken packet loss) should be less than or equal to into account in the evaluation of the different that provided by LTE Rel-10/11. proposed enhancements. The introduction of • Small-cell enhancements should consider non-backward-compatible features should be jus- techniques and mechanisms to reduce con- tified by sufficient gains. trol (C)-plane/user data (U)-plane latency and packet loss during mobility between TECHNICAL REQUIREMENTS FOR macro and small cell nodes, as well as SMALL CELL ENHANCEMENTS between small cell nodes compared to LTE Rel-10/11. Based on the deployment scenarios identified, deployment-related requirements, capability/per- Operational Requirements — Small-cell enhance- formance requirements, and operational require- ments should allow for low network cost by: ments for enhanced small cells can be • Allowing for solutions aiming at different determined as outlined below. backhauls • Allowing for low-cost deployment, and low Deployment-Related Requirements — operation and maintenance tasks (e.g., by Enhanced small cells can be deployed by either means of self-organizing network [SON] operators or independent users such as an orga- functionality and minimization of drive nization in an office building. Automatic mecha- tests) 100 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  4. 4. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Existing cellular bands Higher frequency bands Placing small cells in (high power density for coverage) (wider bandwidth for high data rate) a dormant mode Very wide Super wide could be supported (ex. > 3 GHz) (ex. > 10 GHz) considering the increased likelihood Frequency of small cells not serving any active users at certain times. High UE energy efficiency should be targeted taking into account Wide area Local area the small cell’s short range Figure 2. Combined use of lower and higher frequency bands. transmission path. • Allowing for reduced base station imple- local areas for small cells to provide high-speed mentation cost, considering, say, relaxation data transmission, as shown in Fig. 2 [8]. Such of radio frequency (RF) requirements in combined use of lower and higher frequency y small cell scenarios bands will make higher frequency bands useful Different UE capabilities should be considered and beneficial for cellular operators. There will for small-cell enhancements, especially with no longer be a coverage issue, and we can pro- respect to features related to UE RF complexity vide very high throughput performance using such as the possibility for simultaneous transmis- local area access technologies with a wider spec- sion to and reception from the macro and small trum bandwidth in the higher frequency bands cell layers. while obtaining a significant offloading gain Placing small cells in a dormant mode could from the existing cellular bands. be supported considering the increased likeli- From another point of view, the market size hood of small cells not serving any active users for utilizing higher frequency bands needs to be at certain times. High UE energy efficiency sufficiently large. It is therefore desirable from should be targeted, taking into account the small the operator perspective that higher frequency cell’s short-range transmission path. 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 uti- OPERATOR VIEWS ON SMALL CELL lized in various deployments, not only for indoor but also for outdoor deployments. ENHANCEMENTS AND POSSIBLE FUTURE IMPLICATIONS TO Phantom Cell Concept — In the current deployments, there are a number of capacity MOBILE INDUSTRY solutions for indoor environments such as WiFi, In this section we provide examples of some net- femtocells, and in-building cells using distributed work operator insights into the potential deploy- antenna systems (DAS). However, there is a lack k ment considerations of enhanced LTE small of capacity solutions for high-traffic outdoor cells. environments that can also support good mobili- ty and connectivity. Thus, we propose the con- NTT DOCOMO’S VIEW cept of macro-assisted small cells, called the From the spectrum utilization point of view, Phantom Cell [9], as a capacity solution that spectrum in the lower frequency bands is becom- offers good mobility support while capitalizing ing scarce. Thus, it is crucial to explore and uti- on the existing LTE network. In the Phantom lize higher frequency bands in the development Cell concept, the C-plane/U-plane are split as of techniques for future radio access. However, shown in Fig. 3. The C-plane of UE in small higher frequency bands are difficult to accom- cells is provided by a macrocell in a lower fre- modate in wide areas in macrocells because of quency band, while for UE in macrocells both either space limitations on the eNB side, for the C-plane and U-plane are provided by the example, in terms of RF equipment and antenna serving macrocell in the same way as in the con- size, coverage limitations (e.g., higher path loss), ventional system. On the other hand, the U- or cost issues due to the need to alter the already plane of UE in small cells is provided by a small established network infrastructure. Therefore, cell using a higher frequency band. Hence, these NTT DOCOMO’s intention is to use lower fre- macro-assisted small cells are called Phantom quency bands such as existing cellular bands in Cells as they are intended to transmit UE-specif- macrocells to provide basic coverage and mobili- ic signals only, and the radio resource control ty, and to use separate higher frequency bands in (RRC) connection procedures between the UE IEEE Communications Magazine • February 2013 101C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  5. 5. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® small cells in a time-synchronized manner with Macrocell macro downlink signals [9]. Assisted by the C- plane provided by the macrocell, small cells Split!! simultaneously transmit the discovery signals 2 GHz with a relatively long transmission interval (e.g., (RR lane (example) longer than 100 ms), and the UE attempts the C) detection of discovery signals from small cells C -p U- pl only in a short time interval. This discovery sig- Phantom cell an e nal should be designed to satisfy some impor- 3.5 GHz tant requirements for small cell deployment (example) such as robustness against intercell interference including user-deployed or closed subscriber group (CSG) cells, and support for a large Figure 3. Phantom Cell concept with C/U plane split. n number of sequence (e.g., more than the 504 currently provided by LTE) to reduce cell plan- ning efforts. Furthermore, it can potentially be and the Phantom Cell, such as channel establish- used for purposes such as time/frequency syn- ment and release, are managed by the macrocell. chronization and path loss estimation for power The Phantom Cells are not conventional cells in control. the sense that they are not configured with cell- specific signals and channels such as cell-ID-spe- Dynamic Time-Division Duplex and Interfer- cific synchronization signals, cell-specific ence Coordination for Dense Small Cells — reference signals (CRS), and broadcast system In small cell deployments, it may be expected information. Their visibility to the UE relies on that the number of UE devices per small cell will macrocell signaling. not be large, and the traffic pattern in each The Phantom Cell concept comes with a small cell will change widely over time depend- range of benefits. One important benefit of ing on user applications and user locations. As a macro assistance of small cells is that control sig- result, dynamic spectrum sharing between the naling due to frequent handover between small uplink and downlink [11] would provide gains in cells and macrocells and among small cells can terms of spectral efficiency compared to semi- be significantly reduced, and connectivity can be static partitioning of the uplink and downlink. maintained even when using small cells and However, when small cells are densely deployed, higher frequency bands. In addition, by applying uplink-to-downlink and downlink-to-uplink inter- the new carrier type (NCT) that contains no or ference will become more problematic. There- reduced legacy cell-specific signals (see separate fore, interference coordination and management article in this magazine), the Phantom Cell is schemes for dense small cells need to be estab- able to provide further benefits such as efficient lished if dynamic time-division duplex (TDD) is energy savings, lower interference and hence used. higher spectral efficiency, and reduction in cell- planning effort for dense small cell deployments. Massive MIMO — A Future Topic — Massive To establish a network architecture that sup- multiple-input multiple-output (MIMO) with an ports the C/U-plane split, and interworking active antenna system (AAS) but very large num- between the macrocell and Phantom Cell is bers of antennas (e.g., more than 100 antenna required. A straightforward solution to achieve elements) is a potential technology for small cells this is to support Phantom Cells by using remote in future higher frequency bands (e.g., beyond 10 radio heads (RRHs) belonging to a single macro GHz) [12]. For high frequency bands, antenna eNB. This approach can be referred to as intra- elements can be miniaturized, and more ele- eNB carrier aggregation (CA) using RRHs [10]. ments can be placed in the same space; thus, very However, such a tight CA-based architecture has narrow beams can be formed. We expect such some drawbacks as it requires single-node opera- very narrow beamforming will be essential in tion with low-latency connections (e.g., optical higher frequency bands in order to support prac- fibers) between the macro and Phantom Cells. tical coverage areas for small cells by compensat- Therefore, more flexible network architectures ing for the increased path loss. Assuming ideal should be investigated to allow for relaxed back- beamforming gain, a two-dimensional mapping haul requirements between macro and Phantom of antenna elements can compensate for the path Cells and to support a distributed node deploy- loss with a frequency factor of 20 dB/decade. ment with separated network nodes for each However, there are several technical issues (i.e., inter-eNB CA). toward massive-antenna technologies that need to be resolved, such as how to achieve accurate Other Technical Considerations beamforming or how to support control signaling for mobility and connectivity over highly directive Enhanced Discovery and Mobility — In fre- links. One possibility to address the control sig- quency-separated deployments, efficient discov- naling issue is to apply macro-assisted small cells ery and mobility for small cells in higher (i.e., the Phantom Cell). frequency bands is an important technical issue. To achieve efficient discovery (e.g., for UE bat- CMCC’S VIEW tery and network energy savings), we propose In the foreseeable future, the percentage of f utilizing a macro-assisted property in the Phan- voice and data traffic that occurs indoors is tom Cell concept and introduce newly defined expected to increase to 90 percent. This trend is discovery signals, which are transmitted by the motivation for a collection of enhancements 102 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  6. 6. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® To reduce cost and Peak data rate improvement breakdown for small cell enhancements provide maximum coverage, enhanced 100% 70M bps ( (100%) ine 2x2M IMO, conf.1, CFI=3, DwPTS=10 small cells should be sel ba able to utilize various L TE 80M bps (+9.5%) 109.5% ion Reduced 2-port CRS overhead existing and future uct ed Sr backhauls, including CR 92M bps (+15%) ling 126% Only 1 OFDM reserved every 5 ms du optical fiber, Ether- sche TTI lti- 126M bps (+37%) net and microwave Mu tio n 173% Maximum 4 DL subframe every 5 ms pta systems for indoor a da ffic 168M bps (+33%) Tra 230% Improved 33%SE compared to 64QAM and outdoor AM 6Q hotspots, as well as 25 C 1150%, estimated peak rate with CA support xDSL, FTTH or Cable 5C n, tio ga TV for home use. gre 838M bps (+400%) r ag rrie carrier aggregation with 5 CCs Ca Figure 4. Example of peak data rate improvement breakdown for small cell enhancements. w e refer to as LTE enhancements for hotspot When macrocells are not present (e.g., and indoor (LTE-Hi) to ensure the future com- deep indoors), small cells must be able to petitiveness of 3GPP technologies, where denser work as standalones. In this case coordination networks, easy-to-deploy low-power nodes, new among small cells is essential to ensure mobil- techniques to improve the spectrum efficiency ity performance. After identifying the neces- and throughput, and large bandwidth support sary information/signaling to be exchanged are the four main areas for technical enhance- b etw e en m ac ro a nd s m a l l c el l s a s w el l a s ment. among small cells, it can be decided whether to reuse the LTE X2 interface or introduce LTE-Hi Concept — LTE-Hi targets local access new interfaces. enhancements for hotspot and indoor scenarios The support of “any backhaul” is another considering the above-mentioned technical cornerstone for denser network deployment areas. with a large number of low-power nodes. To Denser network deployment together with reduce cost and provide maximum coverage, easy-to-deploy low-power nodes play important enhanced small cells should be able to utilize roles to cope with the mobile data explosion, various existing and future backhauls, including and from the operator perspective it would be optical fiber, Ethernet, and microwave systems beneficial that a large number of small cells can for indoor and outdoor hotspots, as well as dig- be deployed wherever necessary. A series of ital subscriber line (xDSL), fiber to the home technical innovations will be needed to meet this (FTTH), or cable TV for home use. Conse- requirement. quently, enhanced small cells must be able to Interference and mobility coordination are accommodate a relatively large range of delay key technologies in this area. Autonomous con- budgets and throughput performance, which figuration acquisition/coordination including syn- will have a substantial impact on the system chronization establishment among multiple design. neighboring small cells are essential for the operators to reduce the complexity of network New Techniques to Improve the Spectral l planning, especially for the case of independent Efficiency and Throughput user deployment of small cells. Coordination with overlaid macrocells could Link-Level Improvements — Some room for help to ease the difficulties of mobility and ser- improvement is still present for optimization v ice continuity when UE devices are moving on top of current LTE technology, especially between potentially discontinuous coverage of in small cell scenarios. For example, 256- small cells, as the targeted cell radius is small quadrature amplitude modulation (QAM) (e.g., 50 m or less) due to low transmission takes advantage of the greater probability off power. One example is outdoor hotspots, where higher signal-to-interference-plus-noise ratio the UE speed can be up to 30 km/h, at which (SINR) in small cells. Also, further overhead speed the time needed for UE to pass through a reduction is possible for control channel and cell with 50 m will be just 6 s; thus, it may be demodulation reference signals. In TDD more appropriate for such UE to reside in deployments, flexible uplink and downlink macrocells in order to avoid unnecessary hand- timeslot allocations yield the possibility to overs. For low-speed UE, the residence time will match uplink and downlink resources to traffic be long enough for LTE-Hi cells to serve the demand, thus maximizing the system through- UE with high-speed data. put. IEEE Communications Magazine • February 2013 103C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  7. 7. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Performance Gain Provided by Cell Densifi- enhancements. Detailed technical studies follow- CMCC strongly cation ing the requirements and scenario definition are • Fast cell discovery: When macro assistance proceeding from the start of 2013, with specifica- believes that it is is unavailable, UE may need to search tions expected to be completed for Release 12 essential that exhaustively all possible neighboring cells. by mid-2014. In this article, two active LTE WRC-15 achieves a Thus, a well designed cell discovery refer- operators, CMCC and NTT DOCOMO, have ence signal (RS) would not only improve described the current status of the small cell positive outcome on cell-level energy efficiency, but also facili- enhancement studies in 3GPP, as well as the two new spectrum tate neighbor cell identification and syn- companies’ views on future technology evolution chronization. A serving small cell could also for small cells. allocation to IMT. provide information to assist detection of A globally aligned neighboring cells. REFERENCES spectrum allocation • Handling of macro-layer/small cell informa- [1] S. Sesia, I. Toufik, and M. Baker, LTE — The UMTS Long tion exchange: In practice, coordination Term Evolution from Theory to Practice, Wiley, 2009. will play a key role between small cells, and between small cells [2] T. Abe et al., “Radio Interface Technologies for Cooper- and macro cells is necessary to provide suf- ative Transmission in 3GPP LTE-Advanced,” IEICE Trans. in the development Commun., vol. E94-B, no. 12, Dec. 2011, pp. 3202–10. ficient robustness of mobility, joint trans- [3] Cisco whitepaper, “Cisco Visual Networking Index: of the mobile mission, and efficient resource allocation. Global Mobile Data Traffic Forecast Update, 2011- ecosystem. Therefore, it is important to study mecha- 2016,” 15 Feb 2012. nisms for how to handle information [4] 3GPP, RWS-120029, CMCC, “Views on LTE Rel-12 & Beyond,” June 2012. exchange between cells. One approach may [5] 3GPP, RWS-120010, NTT DOCOMO, “Requirements, be an air interface listening mechanism Candidate Solutions & Technology Roadmap for LTE since it is not sensitive to backhaul restric- Rel-12 Onward,” June 2012. tions; it would then be of interest to study [6] 3GPP, RWS-120045, “Summary of 3GPP TSG-RAN Work- shop on Release 12 and Onward,” June 2012. how to configure transmission gaps for a [7] 3GPP, TR36.932 (V12.0.0), “Scenarios and Requirements cell in order to enable efficient listening for Small Cell Enhancements for E-URTA and E-UTRAN,” between cells without altering the physical Dec. 2012 layer transmission operation more than [8] Y. Kishiyama et al., “Evolution Concept and Candidate Technologies for Future Steps of LTE-A,” Proc. IEEE ICCS necessary. ’12, Nov. 2012. [9] H. Ishii, Y. Kishiyama, and H. Takahashi, “A Novel Architec- SUPPORT OF LARGE BANDWIDTHS ture for LTE-B, C-Plane/U-Plane Split and Phantom Cell Con- Allocating a large amount of spectrum to small cept,” IEEE GLOBECOM 2012 Wksp., Dec. 2012. [10] M. Iwamura et al., “Carrier Aggregation Framework in cells is a straightforward way to increase system 3GPP LTE-Advanced,” IEEE Commun. Mag., vol. 48, no. capacity, but unfortunately, spectrum is a limited 8, Aug. 2010, pp. 60–67. and scarce resource, especially in the existing [11] J. Li et al., “Dynamic TDD and Fixed Cellular Net- frequency bands. Seeking new spectrum works,” IEEE Commun. Letters, vol. 4, no. 7, July 2000, pp. 218–20. resources in higher bands may be a natural [12] T. L. Marzetta, “Non-Cooperative Cellular Wireless with choice for small cell deployment. Unlimited Numbers of Base Station Antennas,” IEEE E In 2015, the International Telecommunica- Trans. Wireless Commun., vol. 9, no. 11, Nov. 2010. tion Union (ITU) World Radiocommunications Conference (WRC-15) will identify new spec- trum for international mobile telecommunica- BIOGRAPHIES tions (IMT). According to estimations in China, TAKEHIRO NAKAMURA ( received ________________ his B.E. and M.E. degrees in electrical communication engi- the total spectrum requirement is about neering from Yokohama National University, Japan, in 1700–2100 MHz of the spectrum for IMT in 1988 and 1990, respectively. He joined NTT Laboratories in 2020. At least 1000 MHz of additional spectrum 1990. In 1992, he transferred to NTT DOCOMO, Inc. He is needs to be introduced by 2020. From CMCC’s now director of the Radio Access System Group of NTT DOCOMO, Inc. He has been working on research and point of view, the main target bands are 1.5 development of W-CDMA. He has been engaged in the W- GHz, 3.3–3.4 GHz, 3.4–3.6 GHz, and above 5 CDMA standardization activity at ARIB in Japan since 1997 GHz. These candidate bands could provide a and is currently the leader of the Mobile-Partnership Group sufficient quantity of spectrum resource. CMCC in ARIB since March 2006. He has been contributing to standardization activities in 3GPP since1999. He has been strongly believes that it is essential that WRC-15 the rapporteur for LTE and LTE-Advanced in 3GPP TSG-RAN achieves a positive outcome on new spectrum since December 2004 and March 2008, respectively. He allocation to IMT. A globally aligned spectrum contributed to 3GPP TSG-RAN as a vice chairman during allocation will play a key role in the develop- March 2005 to March 2009. He is currently a chairman of 3GPP TSG-RAN since April 2009. ment of the mobile ecosystem. Taking all the above possibilities into account, S ATOSHI N AGATA received his B.E. and M.E. degrees from an example of the combined peak data rate Tokyo Institute of Technology, Japan, in 2001 and 2003, improvement for an LTE TDD system that may respectively. In 2003, he joined NTT DOCOMO, Inc. He worked on the research and development for wireless be brought about by small cell enhancements is access technologies for LTE and LTE-Advanced, and has shown in Fig. 4. been involved in the 3GPP standardization activities since 2005. He was a recipient of the IEICE Young Researcher’s Award in 2008. Since November 2011, he has been serving CONCLUSION as a vice chairman of 3GPP RAN WG1. 3GPP has recognized that small cells are a ANASS BENJEBBOUR [SM] received his B.E., M.E., and Dr. Eng. promising approach to meet future mobile ser- degrees in communications and computer engineering in vice requirements, especially for indoor and out- 1999, 2001, and 2004, respectively, all from Kyoto Univer- sity, Japan. Since 2004 he has been with NTT DOCOMO door hotspots. 3GPP has already extensively Inc. R&D Japan, where he has been actively involved in the discussed the relevant deployment scenarios and development of system concepts and the design of tech- technology requirements for LTE small cell nologies for future radio access. He was also involved in 104 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  8. 8. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® 3GPP standardization f for LTE Release 10 and 11. He NING YANG received his Ph.D in circuits and systems f from received the best paper award at IEEE PIMRC in 2002 and Beijing University of Posts and Telecommunications, the young researcher’s award from IEICE in 2006. He is a P.R.China, in 2008. He is currently working in the Wire- senior member of IEICE. less Technology Department of China Mobile Research Institute as a senior engineer. Since 2008 he has been Y OSHIHISA K ISHIYAMA received his B.E., M.E., and Dr. Eng. actively involved in 3GPP specification work, including degrees from Hokkaido University, Sapporo, Japan, in architecture and interface design, and signaling and pro- 1998, 2000, and 2010, respectively. In 2000, he joined NTT tocol research of high layers. His research interests are in DOCOMO, Inc. He is currently manager for a team of the many areas, such as eMBMS, relay technology, HeNB, Radio Access System Group of NTT DOCOMO. He has been and SON. involved in the LTE and LTE-Advanced standardization activities in 3GPP since 2005. His current research interests XIAODONG SHEN received his B.S. in physics from Peking Uni- include advanced multiple access technologies and future versity in 2004, and his M.S. in electrical engineering from network concepts for efficient deployments and spectrum Beijing University of Posts and Telecommunications in utilization. He was a recipient of the IEICE Young Engineer 2007. He has been a project manager in China Mobile Award in 2004, and a recipient of the ITU Association of Research Institute since 2007. His current interests include Japan Award in 2012. He is a member of the IEICIE. LTE, LTE-Advanced, and IMT-Advanced research and stan- dardization for 3GPP, ITU, and NGMN. HAI TANG received his B.E. from Zhejiang University, China, in 1997, and his M.E. from China Academy of Telecommu- NAN LI received his M.S. in signal and information process- nications in 2000. He joined Datang Telecommunication ing from Beijing University of Posts and Telecommunica- Group in 2000 for research and development of the TD- tions in 2007. He is currently working in the Wireless SCDMA, and transferred to Datang Mobile Communica- Technology Department of China Mobile Research Institute tions Equipment Co. Ltd in 2002. He has been engaged in as a senior engineer. Since 2007 he has been actively y the LTE standardization activity since 2005. He joined involved in 3GPP, CCSA, NGMN, and ITU standardization China Mobile Communications Corporation and is currently work, including LTE and LTE-Advanced RF requirements, a Vice Chairman of TSG RAN since March 2009. performance evaluation, and spectrum regulations. IEEE Communications Magazine • February 2013 105C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®