evolution concept and candidate technologies for...

Evolution Concept and Candidate Technologies for Future Steps of LTE-A

Yoshihisa Kishiyama†, Anass Benjebbour†, Hiroyuki Ishii‡, and Takehiro Nakamura†

†Radio Access Network Development Department NTT DOCOMO, INC.

3-5 Hikari-no-oka, Yokosuka, Kanagawa 239-8536 Japan

‡DOCOMO Innovations, Inc. 3240 Hillview Avenue Palo Alto, CA 94304-1201 USA

Abstract—This paper presents our views on the evolution concept and candidate technologies for future steps of 3GPP LTE-Advanced (LTE-A), which was the first major step in the continuous evolution of LTE. In the future steps of LTE-A, we will need to ensure the sustainability of 3GPP radio access technologies in order to respond to the anticipated challenging requirements in the future. Taking into account the ever-increasing importance of local area (small cells) and the need for further spectrum extension in particular, we present a common evolution concept for the future steps of LTE-A (referred to as LTE-B, C, and so on). This concept emphasizes integration of local area and wide area and focuses on frequency-separated deployment between local area and wide area as an important scenario for the efficient utilization of higher frequency bands. Furthermore, we identify key potential technologies for further spectrum efficiency enhancements, e.g., 3D/massive MIMO/beamforming, receiver interference cancellation, and dynamic TDD, and for integrating local area with wide area assuming the frequency-separated scenario using the proposed macro-assisted small cell that we refer to as a Phantom cell.

Keywords − LTE-B; enhanced Local Area; higher frequency bands; massive MIMO; Phantom cell

I. INTRODUCTION The 3rd Generation Partnership Project (3GPP) standardized the radio interface specifications for the next generation mobile communications system called the Long-Term Evolution (LTE) [1],[2]. The initial specifications for LTE, i.e., Release 8, were finalized in 2008, and commercial LTE service in Japan was launched in December 2010. There have been efforts towards establishing an enhanced LTE radio interface called LTE-Advanced (LTE Release 10 and beyond), and now specifications for LTE Release 11 are in their finalization phase.

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. Toward this end, the 3GPP initiated discussions on further steps in the evolution of LTE towards the future, i.e., Release 12 and onwards. This paper is based on NTT DOCOMO’s contribution to the 3GPP workshop for Release 12 and onwards held in June 2012 [3]. So far the general consensus in the 3GPP is that further standardization of LTE will continue to follow step-by-step evolutionary phases in the form of LTE Releases 12, 13, and so on. We refer to these new steps in the LTE evolution as “LTE-B,” “LTE-C,” and so on to indicate the steps after “LTE-A (Advanced)” as shown in Fig. 1. Here, LTE-B refers to the next set of releases, 12 and 13 [3].

This paper provides our views on future requirements, the evolution concept, and candidate solutions for the next steps after LTE-A. Regarding both the requirements and solutions, the proposed approach is to try not only to look into the next step, i.e., LTE-B, but also to keep in mind the long-term evolution path, i.e., LTE-C and

thereafter. Thus, we start by envisioning the long-term evolution concept, then develop a roadmap for the technology required to satisfy the future requirements for the next decade. The envisioned evolution concept consists of two key aspects: 1) integration of local area enhancements with general wide area enhancements and 2) support of efficient utilization of higher and wider frequency spectrum bands along with existing lower frequency spectrum bands.

To concretize this evolution concept, potential spectrum-efficiency enhancing candidate technologies on both transmitter and receiver sides are identified for both wide area and local area. In addition, frequency-separated deployment between wide and local areas is considered as an important scenario towards the efficient utilization of higher frequency bands. For this scenario, we propose a macro-assisted small cell, called “Phantom cell,” as a key solution for further network densification and spectrum extension in the future. The Phantom cell solution brings with it many important benefits ranging from system capacity and data rate boosting to network cost reduction, mobility robustness, and energy savings. Note again that the intention of the specific proposals is to ensure the sustainability of the proposed evolution concept not only for LTE-B, but also for other future steps, e.g., LTE-C, as indicated in Fig. 1.

The rest of the paper is organized as follows. In Section II, we first present high-level requirements envisioning the next decade. In Section III, we present the proposed evolution concept for the next steps after LTE-A. In Section IV, we discuss the overall candidate technologies along with a technology roadmap. Finally, Section V presents our conclusions.

LTE LTE-A LTE-B,C, ...

Rel. 8/9Rel. 8/9 Rel. 10/11Rel. 10/11 Rel. 12 and onwardsRel. 12 and onwards

Common concept toward efficient use of higher frequency bands

LTE LTE-A LTE-B,C, ...

Rel. 8/9Rel. 8/9 Rel. 10/11Rel. 10/11 Rel. 12 and onwardsRel. 12 and onwards

Common concept toward efficient use of higher frequency bands

Figure 1. Next steps for LTE-A, i.e., LTE-B and C, with a common evolution

concept toward efficient use of higher frequency bands.

II. FUTURE REQUIREMENTS In terms of future requirements, the most important one is higher system capacity [4]. Over the past few years, there has been significant growth in the volume of mobile data traffic following the proliferation of smartphones and new mobile devices that support a wide range of applications and services. Last year alone, the volume of mobile data traffic grew 2.3 fold with an increase in the average smartphone usage rate of nearly three fold [5]. There is now a general consensus that such growth will continue in the future. Many recent forecasts project mobile data traffic to grow beyond 24 fold between 2010 and 2015, and beyond 500 fold in 10 years (2010 – 2020) assuming the same rate of growth. System capacity needs to be

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increased so that it can support such an order of magnitude growth in the traffic volume. Another important requirement is the efficient support for a variety of traffic types such as a reduction in the impact of the ever-increasing volume of signaling traffic both on the network and hand-held device sides, the support of lower latency for the currently proliferating cloud applications, and the handling of simultaneous connections of large numbers of small data packets for machine-to-machine (M2M) traffic.

The next requirement is to improve both the achievable data rates and fairness of user throughput in terms of the user experience. The target here is to achieve a 10-fold improvement in the data rate over the next decade. We also aim to achieve gigabit-per-second-order user-experienced throughput in a wide area.

One more requirement is to design flexible and cost-efficient network deployments. This becomes of particular importance as we extend the spectrum utilization to higher and wider frequency spectrum bands, further increase the density of the network using small cells, and attempt to support diverse environments and deployment scenarios, i.e., wide and local areas.

We also have other requirements regarding energy/battery savings and system robustness against emergencies, e.g., tsunami and earthquakes. We believe that these requirements, however, are rather an implementation issue for which non-standardized solutions would be generally more effective. Thus, we do not see them as a primary target but rather an add-on to complement other requirements as part of the whole system design.

III. CONCEPTUAL VIEWS ON SOLUTIONS Prior to explaining in detail candidate solutions to satisfy future requirements, we describe the proposed concept and overall image for future evolution. In the future, we believe that wide area enhancement will continue to be important, and that local area enhancement will be becoming more important compared to that in the past. Here we use the term local area to refer to mainly outdoor dense deployments and hotspots where a high volume of traffic needs to be supported and better user-experienced throughput needs to be provided. The key challenge that we identified resides in how to enhance and integrate local area with wide area in such a way that future requirements are satisfied. In particular, we need to consider the differences in requirements between wide area and local area, as given in Table I, while maintaining reasonable complexity/cost levels from both the specification and network implementation points of view.

TABLE I. DIFFERENCE IN REQUIREMENTS BETWEEN WIDE AREA AND LOCAL AREA

Wide Area Local AreaMobility Medium-to-high Low-to-mediumCoverage Essential Wider is betterDL/UL radio link(Tx power & antenna gain) Asymmetric More symmetric

Traffic load More uniform More fluctuationCell planning Essential To be simplified

Wide Area Local AreaMobility Medium-to-high Low-to-mediumCoverage Essential Wider is betterDL/UL radio link(Tx power & antenna gain) Asymmetric More symmetric

Traffic load More uniform More fluctuationCell planning Essential To be simplified

Thus, the proposed concept for Release 12 onward consists of two

aspects: 1) Integration of wide and local area enhancements, and 2) efficient utilization of both lower and higher frequency bands through frequency-separated deployments between wide and local areas. ・ Integration of wide area and local area enhancements -

In Releases 8 to 11, some local area technologies were introduced to LTE and LTE-A. As we move toward Release 12 onward, the performance of LTE-B, C and so on, needs to be

improved further and local area technologies will continue to evolve and will play a more important role in the future. Thus, the performance of the local area should be further enhanced and specified. We refer to this as an enhanced Local Area (eLA) as shown in Fig. 2. An eLA should be an integrated part of LTE Release 12 onward along with general LTE enhancements. Thus, we should retain commonality between wide and local areas from the specification point of view and enhance it as a part of the general enhancements for wide area. Specific local area enhancements however, can be specified if a sufficient performance gain can be identified. Thus, the proposed way forward is to retain common specifications between wide and local areas in terms of a fundamental LTE radio interface and add eLA specific specifications, e.g., a new carrier type, only when necessary.

・ Efficient utilization of both lower and higher frequency bands

through frequency-separated deployments between wide and local areas - From the spectrum utilization point of view, the spectrum in the lower frequency bands is becoming scarce. Thus, it is crucial to explore and utilize higher frequency bands in the future. Higher frequency bands however, are difficult to accommodate in wide areas in macrocells either because of space limitations on the eNode B (eNB) side, e.g., in terms of radio frequency (RF) equipment, the antenna size, and coverage limitations, e.g., higher path loss, or because of cost issues due to the need to alter the already established network infrastructure. The proposed way forward is to use lower frequency bands such as existing cellular bands to provide basic coverage and mobility and to use separate frequency bands such as higher frequency bands to provide high-speed data transmission in local areas as shown in Fig. 3. The figure shows that, in local area, we can consider using a wider spectrum bandwidth in the higher frequency bands and local area specific technologies for smaller/denser cell deployments.

Perfo

rman

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Rel. 10Rel. 10/11/11

Rel. 8Rel. 8/9/9Pico/Femto

CA/eICIC/CoMP for HetNet

LTELTE

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LTELTE--AARel. 12 Rel. 12 onwardonward

Common specifications for wide & local areas

General enhancements to improve performance also in wide areas

Local area specific enhancements if sufficient gain is identified

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LTELTE--AARel. 12 Rel. 12 onwardonward

Common specifications for wide & local areas

General enhancements to improve performance also in wide areas

Local area specific enhancements if sufficient gain is identified

Figure 2. eLA as an integrated part of general LTE enhancements.

Existing cellular bands(High power density for coverage)

Higher frequency bands(Wider bandwidth for high data rate)

Frequency

Very wide(Ex. > 3 GHz)

Super wide(Ex. > 10 GHz)

Wide area Local area

Existing cellular bands(High power density for coverage)

Higher frequency bands(Wider bandwidth for high data rate)

Frequency

Very wide(Ex. > 3 GHz)

Super wide(Ex. > 10 GHz)

Wide area Local area

Figure 3. Frequency-separated deployments between wide area and local area.

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IV. CANDIDATE SOLUTIONS A set of radio access technologies is required in order to expand the current cellular capacity and performance, and to be able to meet future requirements and challenges. “The Cube” depicted in Fig. 4 provides a holistic view on the main directions of evolution [4]. Hereafter, we present candidate solutions to address the future requirements along with each dimension of the Cube, i.e., spectrum efficiency, spectrum extension, and network densification. A. Candidate Technologies for Enhanced Spectrum Efficiency 1) General Enhancements LTE radio access based on orthogonal frequency division multiple access (OFDMA) achieves radio link performance that is almost at the Shannon capacity limit for point-to-point communications although further overhead reduction or further enhancement in a higher signal-to-interference and noise ratio (SINR) region could still be considered. Therefore, the main challenges in LTE-A to enhance the spectrum efficiency further have focused on multi-point/antenna to multi-user radio links including interference solutions. Such technical challenges for intra-cell access, i.e., multi-user multiple-input multiple-output (MU-MIMO), and inter-cell access, i.e., coordinated multi-point (CoMP) transmission technologies will continue to be studied in LTE-B. However, in order to enhance the spectrum efficiency further hopefully without significant increase in the amount of channel state information (CSI) feedback [6], we consider that the following approaches should be investigated further. a) Active beamforming with more antenna elements Three dimensional (3D) MIMO/beamforming with an Active Antenna System (AAS) is one of the major technologies that would be studied in LTE-B [7]. The AAS, in which vertical antenna elements are integrated with independent RF units, allows beam control in both horizontal and vertical directions as shown in Fig. 5(a). From a specification perspective, 3D MIMO/beamforming will require the use of more than 8 antenna elements although the current physical layer specifications (Release 10) only support up to 8 antenna ports in the downlink (DL), e.g., for the DL reference signals (RSs) and CSI feedback scheme. Furthermore, extending 3D MIMO/beamforming to MU-MIMO and CoMP operations would need to be investigated.

Massive MIMO, which is to some extent 3D MIMO/beamforming with AAS, is an interesting technology for future higher frequency bands, e.g., beyond 10 GHz [8]. For a higher frequency, antenna elements can be miniaturized and more elements can be implemented in the same space, e.g., more than 100 antenna elements. As shown in Fig. 5(b), very narrow beamforming created by massive number of antenna elements will be essential to support practical coverage areas by compensating for the increased path loss in higher frequency bands. If the ideal beamforming gain is assumed, two dimensional mapping of antenna elements can compensate for the path loss with the frequency factor of 20 dB/decade. However, to achieve such massive-antenna technologies, there are technical issues that must be resolved, e.g., 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 explained later. We anticipate that Phantom cells plus massive MIMO will become a key technical combination to utilize much higher frequency bands, e.g., beyond 10 GHz. This kind of technology that assumes future higher frequency bands might be a long-term research topic, e.g., for LTE-C and so on.

(a) 3D MIMO/beamforming (b) Massive MIMO

Very narrow beamforming

(a) 3D MIMO/beamforming (b) Massive MIMO

Very narrow beamformingVery narrow beamforming

Figure 5. 3D MIMO/beamforming and massive MIMO.

b) Advanced receiver cancellation Receiver cancellation technologies do not require significant CSI feedback and are generally beneficial to both local and wide areas. In LTE Release 11, an advanced receiver was investigated for user equipment (UE) that employs interference rejection combining (IRC) using 2 Rx antennas [9]. Thus, further study on the IRC receiver with more Rx antennas, e.g., 4 or 8, will be a natural enhancement and be beneficial because it efficiently utilizes the increased degrees of freedom at the UE receiver to suppress multiple interference resources as shown in Fig. 6. This kind of further advanced receiver might be a standardization topic in LTE-B. CoMP technologies considering the IRC receiver, e.g., interference alignment [10], may be a potential topic in LTE-B or C although it might be achieved through eNB implementations.

2 Rx IRC receiver 4 Rx IRC receiver

Desired signal

Desired signal

2 Rx IRC receiver 4 Rx IRC receiver

Desired signal

Desired signal

Figure 6. Advanced receiver with more Rx antennas.

In addition to the IRC receiver, non-linear interference cancellation

such as successive interference cancellation (SIC) could be considered as a future advanced receiver technology, e.g., for LTE-C. However, one issue regarding the SIC receiver for the UE terminal side is that the SIC receiver requires a condition in which interference signals to be cancelled are decodable at the UE receiver. Thus, it may be challenging to apply a SIC receiver to cancel the inter-cell interference since those signals are typically difficult to decode. Compared to that, application to intra-cell multiple access schemes might be more feasible. It was reported that intra-cell non-orthogonal multiple access using a SIC receiver, as depicted in Fig. 7, improves spectrum efficiency and user fairness [11].

Non-orthogonal multiple access

with interference cancellation

freq/time

OFDM-basednon-orthogonal

Power control to achieve user fairness

Efficient utilization of difference in path loss

Non-orthogonal multiple access with interference cancellation

freq/time

OFDM-basednon-orthogonal

Power control to achieve user fairness

Efficient utilization of difference in path loss

Figure 7. Intra-cell non-orthogonal multiple access using SIC receiver.

Spectrum extension

Required network capacity

Spectrumefficiency

Current capacity

Traffic offloading

Network density

Spectrum extension

Required network capacity

Spectrumefficiency

Current capacity

Traffic offloading

Network density

Figure 4. Directions of evolution: “the Cube”.


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2) eLA Focused Enhancements The aforementioned approaches are generally applicable to both wide and local areas. However, if we focus on the requirements for local area, the following enhancements can be additionally considered. a) Dynamic time division duplex (TDD) and interference coordination / management for dense small cells In small cell deployments, it is expected that the number of UEs per small cell will not be so large and the traffic tendency in each small cell, e.g., uplink (UL) heavy or DL heavy, will fluctuate widely due to user applications. As a result, spectrum sharing between the UL and DL, the so-called dynamic TDD [12], will provide significant gains in terms of spectrum efficiency compared to semi-static partition of the UL and DL. A study on dynamic TDD is already underway in Release 11. However, when small cells are densely deployed and the traffic load around the small cells is relatively high, interference issues such as UL-to-DL or DL-to-UL interference, as shown in Fig. 8, become more significant. Therefore, interference coordination / management schemes for dense small cells especially for dynamic TDD need to be established in LTE-B.

UL DL UL2DL interference

DL2UL interference

UL DL UL2DL interference

DL2UL interference

Figure 8. Interference issue in dynamic TDD.

b) Enhancements for higher SINR region, e.g., UL OFDMA and 256QAM In small cell deployments, especially in the aforementioned frequency-separated deployments between wide and local areas, it is expected that a high SINR at the eNB or UE receiver will often be observed due to lower inter-cell interference compared to macrocellular deployments. Therefore, link-level performance enhancement in a higher SINR region, e.g., higher than 20 dB, is beneficial in improving the user-experienced throughput in a local area. Higher order modulation such as 256QAM can be effective in improving the DL throughput performance in a high SINR region. For UL enhancement in a higher SINR region, UL OFDMA will need to be considered before introducing higher order modulation. In this case, hybrid access with OFDMA and the current single-carrier frequency division multiple access (SC-FDMA) [13] as shown in Fig. 9 may be considered as UL radio access schemes.

Resource mapping& Power control

DFT (SC)

S/P (MC)Tx data IFFT Transmission

Local area

Wide area Resource mapping& Power control

DFT (SC)


Resource mapping& Power control

DFT (SC)


Local area

Wide area

Figure 9. UL hybrid access with OFDMA and SC-FDMA. B. Spectrum Extension Using Higher Frequency Bands The spectrum below 2.5 GHz is currently being fully utilized for cellular systems. Hence, future capacity expansion for LTE is envisaged at 3.5 GHz and higher. As described in Section III, the proposed concept for efficient use of higher frequency bands is based on frequency-separated deployments between wide and local areas. Thus, spectrum extension and network densification are highly correlated. The concept of frequency-separated deployments provides motivation to introduce macro-assisted small cells, i.e., Phantom cells, which can basically be a common target for LTE-B and future steps. However, when we look into further higher frequency bands, e.g., beyond 10 GHz, the current LTE radio interface, which was optimized for the existing cellular bands, i.e., around 2 GHz, will not be optimum. Therefore, at a certain point in time in the future, it may be necessary to redesign the radio interface assuming the

requirements for future new spectra that will be potentially identified, e.g., in the World radiocommunication conferences (WRC-15).

One more point we would like to note here is that higher frequency bands should be supported by many UEs in the future. To achieve this, the market size for utilizing higher frequency bands needs to be large from a commercial service point of view. Therefore, it is desirable from the operator perspective that higher frequency bands be used in wide 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 although they are used in local area, i.e., small cells. Fig. 10 shows two typical scenarios for outdoor small cell deployments, i.e., a normal high traffic spot such as railway stations in suburban areas in which small cells are sparsely deployed, and a super high traffic area such as the downtown area in a large city in which small cells are densely deployed in continuous or partial coverage.

Normal high traffic spot (Outdoor hotspot cell)

Super high traffic area(Outdoor dense small cells)

Normal high traffic spot (Outdoor hotspot cell)

Super high traffic area(Outdoor dense small cells)

Figure 10. Outdoor deployment scenarios for higher frequency bands.

C. Network Densification with Phantom Cell Concept 1) Macro-assisted Small Cell – Phantom Cell In the current deployments, there are a number of capacity solutions for indoor environments such as WiFi, Femto cells, and in-building cells with distributed antenna systems (DAS). However, there is a lack of capacity solutions for the outdoor environment that support good mobility and connectivity. Consequently, high capacity seamless communications in cellular systems is required for outdoor environments. Thus, we propose a macro-assisted small cell, called the Phantom cell, for higher frequency bands as a capacity solution that is suitable for both indoor and outdoor environments and that offers good mobility support while capitalizing on the existing LTE network.

In the Phantom cell concept, a Control (C)-plane / User (U)-plane split configuration is proposed as shown in Fig. 11. The C-plane is provided by a macrocell in a lower frequency band to maintain good connectivity and mobility. The macrocell also works as a normal cell supporting both C-plane and U-plane signaling. On the other hand, the U-plane is provided by a small cell, i.e., a Phantom cell, using a higher frequency band in order to boost the user data rate. Hence, the Phantom cell is only intended to contain UE-specific signals, and the radio resource control (RRC) connection procedures between the UE and a Phantom cell such as channel establishment and release are managed by the macrocell. Therefore, the Phantom cell does not need to transmit any cell-specific signals/channels to UEs except for discovery signals as explained later. To actualize the Phantom cell, a new carrier type (NCT) must be specified without legacy cell-specific

2 GHz (Example)

3.5 GHz (Example)

Macro cell

Phantom cell

C-pl

ane

(RRC

)

U-plane

2 GHz (Example)

3.5 GHz (Example)

Macro cell

Phantom cell

C-pl

ane

(RRC

)

U-plane

Figure 11. C-plane/U-plane split and Phantom cell.


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signals/channels such as primary/secondary synchronization signals (PSS/SSS), cell-specific RS (CRS), and master information block/ system information blocks (MIB/SIB). By applying the NCT, the Phantom is able to provide further benefits such as efficient energy saving, lower interference thus higher spectrum efficiency, and a reduction in cell-planning efforts. 2) Technical Issues Related to Phantom Cell a) Network architecture To establish a network architecture that actualizes a C/U-plane split, interworking between the macrocell and Phantom cell is required. Therefore, 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 intra-eNB carrier aggregation (CA) using RRHs [14]. However, such a tight CA-based architecture has some drawbacks such as that it requires tight optical fiber connections between the macro and Phantom cells. Therefore, more flexible network architectures should be investigated in LTE-B to allow for relaxed backhaul requirements between macro and Phantom cells and support separated network nodes for each. This kind of approach may be referred to as inter-eNB CA. b) Macro-assisted discovery In frequency-separated deployments, efficient Phantom cell discovery in higher frequency bands is an important technical issue. To achieve efficient discovery, e.g., for UE battery savings, this macro-assisted property should be fully utilized. In our proposal, newly defined discovery signals, which are transmitted in a time-synchronized manner with macro DL signals, are introduced in the Phantom cells as shown in Fig. 12. Assisted by control information from the macrocell, the UE tries to detect the discovery signals only in a short time interval when all Phantom cells simultaneously transmit the discovery signals. As a result, the UE can detect the surrounding Phantom cells with a minimum wake-up or inter-frequency measurement time to conserve the UE battery. The application of discovery signals is also effective in achieving efficient network energy savings since when the Phantom cell has no data traffic to send, it does only need to transmit the discovery signal with a relatively long time periodicity, e.g., in the order of several seconds. Furthermore, it is preferable that the discovery signals are designed to be orthogonal to each other so that the so-called pilot pollution problem can be avoided especially in densely deployed scenarios.

Macro cellPhantom cell

Discovery signal transmissions

Macro cell assists local areafor battery-efficient discovery

Macro cellPhantom cell

Discovery signal transmissions

Macro cell assists local areafor battery-efficient discovery

Figure 12. Macro-assisted Phantom cell discovery.

c) Flexible duplex support In frequency-separated deployments, different duplex schemes, i.e., frequency division duplex (FDD) and TDD, may be used for lower and higher frequency bands, respectively, as shown in Fig. 13. In particular, FDD for macrocell coverage and TDD for the Phantom cell with wider bandwidth transmission may be a promising combination in future deployments. Therefore, it is desirable to support the Phantom cell (or inter-eNB CA) mechanism irrespective of the combination of duplex schemes in lower and higher frequency bands.

Uplink Downlink

FDD carrier(Macro cell, legacy carrier type)

Downlink Uplink

TDD carrier(Phantom cell, new carrier type)

Frequency

Uplink Downlink

FDD carrier(Macro cell, legacy carrier type)

Downlink Uplink

TDD carrier(Phantom cell, new carrier type)

Frequency Figure 13. Flexible duplex support for Phantom cell.

V. CONCLUSION In this paper we presented our views on the requirements, the evolution concept, candidate technologies for the future steps of LTE-A. The establishment of the evolution concept is of particular importance in setting the evolution framework so that 3GPP standardization can focus the specification work on usage cases and deployment scenarios that really matter not only in the short term but also on the long term. In the proposed evolution concept we emphasize the importance of integrating eLA into wide area enhancements. Furthermore, we identified frequency-separated deployment between wide area and local area as an important scenario towards efficient utilization of higher/wider frequency bands in local area while still maintaining basic mobility and connectivity in wide area over existing lower frequency bands. For this scenario, the macro-assisted small cell, called the Phantom cell, with a split C/U plane between the macrocell and small (Phantom) cells is proposed. Also a set of radio access technologies that we see as most promising for improving spectrum efficiency in wide area and local area was introduced. Taking into consideration technology maturity and future spectrum assignments, a tentative technology roadmap was also drawn up for proposal to 3GPP [3]. We hope the views shared here can also provide some guidance to the research community.

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