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From LTE-Advanced to the Future

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                                                                                                                                        THE WORLD’S NEWSSTAND®




            LTE-ADVANCED AND 4G WIRELESS
            COMMUNICATIONS



           From LTE-Advanced to the Future
           Matthew Baker, Alcatel-Lucent



                                                 ABSTRACT                             should be avoided, and careful evaluation must
                                                                                      continue to inform the selection of new features.
                                   LTE and LTE-Advanced have undeniably                  In the concluding chapter of [1], several
                               provided a major step forward in mobile com-           aspects are identified as being of particular
                               munication capability, enabling mobile service         importance for the future evolution of LTE-
                               provisioning to approach for the first time that       Advanced. These include increased capacity and
                               available from fixed-line connections. However,        spectral efficiency, improvement of the through-
                               market demands typically do not evolve simply          put actually experienced by the user, fairness of
                               in discrete steps; therefore, the future evolution     throughput provision, reduction of cost per bit,
                               of LTE-Advanced will be a story of continuous          and energy saving.
                               enhancement, on one hand taking advantage of              The following sections explore some key
                               the advancing capabilities of technology, while        aspects of potential enhancement in more detail.
                               on the other aiming to keep pace with the expec-
                               tations and needs of the end users. In this arti-
                               cle, the likely directions of this continuing
                                                                                                SPECTRAL EFFICIENCY
                               enhancement are discussed, and some areas              While the peak data rate (or peak spectral effi-
                               where further technical advancement will be            ciency) continues to be important from a mar-
                               required are identified. In particular, potential      keting perspective, the end-user experience is
                               measures to enhance the efficiency of spectrum         more closely associated with the degree of uni-
                               utilization by joint multicell optimization, dynam-    formity of service provision; this implies that
                               ic adaptation of the network to traffic character-     future enhancements will need to augment cell
                               istics and load levels, and support for new            edge data rates rather than simply addressing
                               applications are highlighted.                          peak rates.
                                                                                          At the level of the individual radio links, pos-
                                              INTRODUCTION                            sibilities are limited for further improvements to
                                                                                      the performance in a given bandwidth. Turbo
                               The factors that have led to Long Term Evolu-          codes have raised link-level spectral efficiency in
                               tion (LTE)-Advanced becoming the leading               fading propagation environments almost to the
                               broadband mobile communications system for             Shannon limit, and the performance of single-
                               the fourth generation (4G) are in large part asso-     user multiple-input multiple-output (SU-MIMO)
                               ciated with the close partnership between net-         is largely constrained by the feasible number of
                               work operators and equipment vendors in its            uncorrelated antennas (the size of the mobile
                               development. LTE has, on one hand, been built          terminal, and factors such as wind loading and
                               to provide support for applications for which          aesthetics at the base station), the characteristics
                               there is active consumer demand, while on the          of the propagation channel, and the carrier fre-
                               other hand incorporating advanced self-optimiza-       quency. It is therefore challenging to identify
                               tion and self-configuration functionality to facili-   further incremental enhancements that can be
                               tate practical deployment for network operators.       introduced at the link level without incurring
                               This has resulted in uniquely balanced system          unreasonable complexity.
                               specifications that are technically advanced but           This means that system-level enhancements
                               practical, cost effective, and, above all, useful.     must generally be targeted considering multicell
                                  The direction of the future evolution of LTE-       operation in order to realize useful improve-
                               Advanced will be guided by the principle that          ments in spectral efficiency and user experience,
                               these balances must be maintained. Not every           both overall and at the cell edge.
                               theoretical claim of potential improvement is
                               cost effective and practical to deploy. Factors                      NETWORK TOPOLOGY
                               such as robust conformance testing and perfor-         Historically, mobile networks have consisted of a
                               mance verification must also be taken into             single homogeneous “layer” of macrocells of
                               account, and care must be taken not to fragment        broadly uniform size. Early deployments of LTE
                               the market unnecessarily, in order to ensure that      have required wide-area coverage to be achieved
                               economies of scale are not lost; such factors as       in a short space of time, often to meet regulato-
                               these imply that the introduction of multiple          ry requirements, and these requirements were
                               alternative options offering similar performance       most appropriately satisfied by the homogeneous


           116                         0163-6804/12/$25.00 © 2012 IEEE                    IEEE Communications Magazine • February 2012



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           macrocellular approach, as long as the number           but there is still a need for further study to
           of LTE terminals was small.                             design effective but practical CoMP schemes             Existing techniques
               However, as the density of LTE-equipped             suitable for application in real networks. Stan-
                                                                                                                            for interference
           terminals increases, smaller cells are needed to        dardization for CoMP in future evolutions of
           deliver sufficient capacity; such capacity increas-     LTE-Advanced will focus on the enablers for                coordination
           es from reducing the cell size are often referred       CoMP operation, including suitable reference            generally focus on
           to as cell-splitting gains. Since the small cells are   signals, measurements, and CSI feedback, while
           thus introduced in a pre-existing network of            the actual transmission techniques (e.g., coher-           interference
           macrocells, the result is a heterogeneous net-          ent or non-coherent joint transmission, coordi-        avoidance. However,
           work of macro- and small cells operating at dif-        nated scheduling, or coordinated beamforming)
                                                                                                                          future developments
           ferent transmission powers and with different           and reception techniques (e.g., minimum mean
           coverage areas. The magnitude of the cell-split-        squared error [MMSE]-SIC) will generally be            will tend to be based
           ting gains is dependent on suitable cell associa-       designed and optimized in proprietary vendor-             on cooperative
           tion/handover decisions to ensure that sufficient       specific ways without explicit standardization.
           users are served by the small cells.                        The CoMP schemes deployed in practical              transmission rather
               In such networks, intercell interference man-       LTE-Advanced networks will be governed by the            than interference
           agement becomes significantly more complex              availability of high-bandwidth low-latency fiber
                                                                                                                           avoidance, leading
           than in a homogeneous network. Some intercell           optic interconnections between transmission
           interference coordination techniques have been          points. Such links are not yet widely available, so       to coordinated
           a part of LTE since its inception, with the possi-      early CoMP schemes should not be overly                multipoint operation.
           bility of coordinating the cell edge data transmis-     dependent on their existence. Nevertheless, cen-
           sions of neighboring cells in the frequency             tralized (or cloud) processing will clearly become
           domain to control the intercell impact. In LTE-         an important enabler for realizing the full poten-
           Advanced, additional possibilities for time-            tial of CoMP, and efficient schemes for the
           domain scheduling coordination are introduced           transfer of scheduling/precoding information, as
           [1], which are particularly suited to heteroge-         well as data, between transmission points will
           neous networks of small cells overlaid on macro-        need to be identified.
           cells; time-division multiplexing (TDM) of the              It is worth noting that centralized processing
           control signaling and data between the different        has the potential to provide some practical bene-
           types of cells can help to ensure that the cell-        fits in addition to those arising from centralized
           splitting gains can be achieved to the maximum          CoMP scheduling/beamforming and joint system
           extent. Further optimization of interference            optimization: centralized processing can also
           coordination is one means by which overall sys-         bring possibilities to reduce the costs of network
           tem capacity and performance is likely to be able       operation by reducing the number of secure sites
           to be enhanced in the future, by either standard-       needed for housing power-hungry, high-value,
           ized or proprietary mechanisms. Such mecha-             and security-critical network equipment, as well
           nisms are likely to increasingly exploit                as facilitating network reconfiguration and
           centralized processing in the network architec-         robustness.
           ture in order to jointly optimize the operation of          For the uplink, the benefits of CoMP recep-
           clusters of cells.                                      tion are more readily achievable [5, 6], since no
                                                                   additional transmissions are needed compared to
               COORDINATED MULTIPOINT OPERATION                    reception at a single point. Each reception point
           Existing techniques for interference coordination       can use interference cancellation techniques to
           generally focus on interference avoidance. How-         isolate the weaker signals from mobile terminals
           ever, future developments will tend to be based         in neighboring cells and convey them to a cen-
           on cooperative transmission rather than interfer-       tral node for combining with the versions of the
           ence avoidance, leading to coordinated multi-           signal received at other points.
           point (CoMP) operation.
               Theoretical research has provided some opti-                      MULTI-USER MIMO
           mistic and impressive indications for the poten-        Within a given cell, MU-MIMO may be
           tial performance of CoMP with schemes known             employed to increase spectral efficiency without
           as Network MIMO (e.g., [2]). In the downlink,           necessarily needing the coordination or central-
           Network MIMO involves extending multi-user              ized processing capabilities required by CoMP
           MIMO (MU-MIMO) at the transmit side such                schemes.
           that the transmissions from multiple points are             Downlink transmissions in the latest releases
           jointly precoded for coherent combining at the          of LTE utilize precoded reference signals, which
           receivers. The gains are dependent on the num-          can facilitate advanced beamforming and space-
           ber of cooperating transmission points, the relia-      division multiplexing techniques. Nevertheless,
           bility of channel state information at the              there remains a significant gap between the the-
           transmitter (CSIT) (in the case of coherent             oretical promises of MU-MIMO based on dirty
           CoMP, full Network MIMO requires not just               paper coding (DPC) techniques [7] and what is
           CSIT of a single link but CSIT for the joint            achieved in practice. DPC itself will not be
           channel matrix for all cooperating transmission         implemented in the foreseeable future, due to its
           points), and the bandwidth and latency of the           complexity, its sensitivity to practical non-ideali-
           internode communication links.                          ties, and the unrealistic level of CSIT it requires;
               Practical CSI feedback considerations have a        it should be seen simply as an interesting refer-
           significant impact on CoMP performance [3].             ence point to indicate the upper bound of theo-
           Much investigative work has already been under-         retical performance.
           taken in the Third Generation Partnership Pro-              A number of reasons may be identified
           ject (3GPP) [4], especially for downlink CoMP,          behind the gap between theoretical MU-MIMO


           IEEE Communications Magazine • February 2012                                                                                        117



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                                     performance and that practically achievable so            Some particular deployment scenarios may
             Energy efficiency is    far in LTE deployments:                                emerge where the use of relays or repeaters may
                                        •Good spatial separation of users is most           be beneficial (e.g., with very-low-power relays,
            an area of increasing    easily achieved with correlated arrays of uni-         outdoor-indoor coverage boosting, or on high-
                 importance. All     formly spaced antenna elements, whereas many           speed trains), but in most general cases the loss
             future evolution of     antenna configurations deployed in practice are        of bandwidth on the backhaul link and the chal-
                                     cross-polarized, with low correlation between the      lenge of interference control seem to make
             LTE-Advanced will       polarizations.                                         relays relatively unattractive for realizing useful
             therefore have to          •CSI at the base station is far from perfect,       capacity gains in LTE-Advanced systems in the
                                     due to the constraints of uplink feedback over-        near future.
             give a high priority    head and latency; channel reciprocity may be
             to such considera-      exploited in time-division duplex (TDD) systems,                     OTHER DRIVERS
                 tions. This will    but the accuracy of the CSIT remains limited by
                                     factors such as non-reciprocal transmit-receive                      ENERGY EFFICIENCY
             involve minimizing      antenna configurations, mobility of terminals,         Although LTE has been designed from the
              the transmission       and the accuracy of uplink channel estimation          beginning to facilitate energy-efficient imple-
                                     achievable with reasonable uplink reference sig-       mentations of both mobile terminals and base
            overheads and pro-       nal overhead.                                          stations, energy efficiency is an area of increas-
            cessing requirements        •The availability of sufficient spatially separa-   ing importance due to the rising cost of energy
            when traffic loading     ble users with non-empty data queues is limited;       and ecological concerns. All future evolution of
                                     nevertheless, as the LTE terminal population           LTE-Advanced will therefore have to give high
                 is light, and in    increases exponentially, this will become less of a    priority to such considerations. This will involve
             general enhancing       difficulty, even when bursty traffic models are        minimizing the transmission overheads and pro-
                                     assumed. It is shown in [8] that, for a given CSI      cessing requirements when traffic loading is
              the ability of the     feedback overhead, better MU-MIMO capacity             light, and in general enhancing the ability of the
             network to adapt        can be achieved with high-accuracy CSI feed-           network to adapt dynamically to the actual load
             dynamically to the      back from a small number of terminals (even            levels. This may include more advanced tech-
                                     randomly selected) than with low-accuracy CSI          niques for load balancing/transfer so that parts
             actual load levels.     feedback from a large number of terminals.             of the system which are not essential may be
                                        Future performance improvements with                switched off whenever possible, while retaining
                                     downlink MU-MIMO will therefore depend on              flexibility to bring them online rapidly to handle
                                     whether it is possible to significantly improve the    high traffic demand when needed.
                                     accuracy of the CSIT, given practical channel
                                     estimation, realistic antenna configurations, and                 BACKWARD COMPATIBILITY
                                     typical terminal mobility, without incurring unre-     One of the design principles of LTE has been
                                     alistic feedback overhead; some study of the           backward compatibility between successive
                                     potential benefits of MU-MIMO enhancements             releases of specifications. This implies that user
                                     relative to LTE-Advanced Release 10 has                equipment of an earlier release can work in a
                                     already been carried out by 3GPP [9], but there        network of a later release, and that user equip-
                                     remains room for further study in this area.           ment of a later release can work in an older net-
                                        In the uplink, MU-MIMO can be implement-            work. This is important since it ensures
                                     ed more easily because accurate CSIT is not            continuity of service for existing subscribers and
                                     required. The complexity of the uplink MU-             protects the investments of network operators.
                                     MIMO operation is managed at the receiver              The requirement for backward compatibility
                                     rather than the transmitters, and sophisticated        should not, however, prevent the introduction of
                                     non-standardized receivers, such as those based        new features, since new functionality can be con-
                                     on MMSE-SIC, can theoretically attain the max-         figured or enabled by the network specifically
                                     imum multiple access channel capacity [10].            for the user equipment that supports it, without
                                                                                            requiring all users to upgrade their equipment
                                                          RELAYS                            immediately.
                                     Relay nodes provide an interesting example of             In the future, for maximum efficiency, the
                                     small-cell-based heterogeneous networks, and           requirement for backward compatibility will not
                                     much has been written about their potential. In        be imposed on every LTE-Advanced carrier;
                                     general, the benefits of relays can be categorized     provided there is always at least one carrier on
                                     into coverage extension (including filling cover-      which legacy terminals can operate, new carriers
                                     age holes) and capacity boosting [11]. The future      may be aggregated for the sole use of newer ter-
                                     evolution of LTE-Advanced will be directed             minals; sometimes referred to as extension carri-
                                     mainly at the latter, where the practical benefits     ers, such non-backward-compatible carriers
                                     of relays appear to be still somewhat equivocal.       could then exploit new features to the full, with-
                                     While significant capacity gains have been pre-        out being constrained to carry all the signals
                                     dicted theoretically for systems using relaying        (e.g., reference signals and control signaling)
                                     under some ideal assumptions, these gains are          expected by legacy terminals; this can improve
                                     typically dependent on advanced “network cod-          spectral efficiency by reducing overhead, as well
                                     ing” schemes that are complex and dependent            as further facilitate energy-saving techniques on
                                     on high channel estimation accuracy. Particular        the new carriers.
                                     attention also needs to be paid to the interfer-
                                     ence generated by relay nodes, which, without                        SELF-OPTIMIZATION
                                     careful control, can undo much of the gain that        The deployment of a mobile communications
                                     results from the improved received signal power        network is a complex task, especially its opti-
                                     delivered by the relay nodes.                          mization in different deployment scenarios and


           118                                                                                  IEEE Communications Magazine • February 2012



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           with variations in traffic patterns. The LTE sys-      mechanisms to reduce cost is economy of scale;
           tem broke new ground with the way in which it          therefore, excessive fragmentation of the LTE           Mobile communica-
           was designed with certain self-optimizing net-         chipset market may turn out to be counterpro-
           work (SON) functions built in from the start [1].      ductive.                                                  tions traffic is no
               These functions include the facilitation of            The delivery of multimedia broadcast/multi-         longer dominated by
           deployment of new cells, with automatic detec-         cast services (MBMS) over the mobile network              traditional voice
           tion of neighbor cells, self-configuration of net-     has attracted considerable interest in the last
           work interfaces, and automatic configuration of        decade, and LTE is in a better position than any         services, and it will
           cell identifiers. SON functions in LTE can also        previous cellular system to offer such services in        be important to
           optimize network operation by automatic load           a spectrally efficient manner by exploiting the
           balancing, and by refinement of handover               orthogonal frequency-division multiplexing               continue to adapt
           parameters by automatic detection of handover          (OFDM) signal properties for multipoint trans-            LTE-Advanced to
           failures and identification of their causes (e.g.,     mission in a single-frequency network. The con-           ensure efficient
           handovers that start too late or too early, or tar-    sumer take-up of such services has thus far been
           get an inappropriate cell).                            very mixed, with large differences in popularity        support for unique
               Further enhanced SON functionality, to aug-        in different regions of the world. It remains to        traffic characteristics
           ment the ability of an LTE-Advanced network to         be seen whether new applications will emerge to
           reconfigure itself and adapt to changing scenar-       inspire more widespread MBMS adoption in                of applications that
           ios, will be an essential part of reducing the cost    regions where it has not yet met with significant       become relevant in
           per bit of operating the networks of the future.       consumer confidence; if this were to occur, it               the future.
           One tool likely to be increasingly developed in        could provide motivation for further enhance-
           this area is automatic logging of measurements         ment of LTE’s MBMS modes of delivery, for
           by user equipment in order to reduce the need          example, to support higher data rates or more
           for manual “drive tests” to identify any network       simultaneous service channels.
           coverage problems.                                         Other areas on which it is worth keeping a
                                                                  keen eye are mobile gaming and active social
                       SPECTRUM AVAILABILITY                      networking: the traffic characteristics of such
           The limited availability of suitable radio spec-       applications are quite different from more con-
           trum will increasingly impact the future evolu-        ventional voice or data services. Gaming applica-
           tion of LTE-Advanced. This is already evident in       tions typically give rise to bursty traffic with a
           the carrier aggregation features provided by           requirement for very-low-latency communica-
           LTE-Advanced, and it is inevitable that the            tion; in some cases, accurate knowledge of the
           range of band combinations that have to be sup-        location of mobile terminals may also be
           ported will continue to increase. Techniques to        required. If consumer demand for such services
           enhance dynamic load management between                takes off, it may be worthwhile to consider
           carriers according to traffic demand will also         whether specific enhancements can support
           become an increasingly valuable tool for ensur-        them in a more optimized way.
           ing full and efficient use of scarce spectrum              The uplink-downlink traffic balance is another
           resources.                                             factor that may impact the development of LTE-
              Such dynamic techniques are likely in due           Advanced in the future. Historically, development
           course to evolve in the direction of cognitive         of the uplink in most mobile communications sys-
           radio solutions, with increasing utilization of        tems has lagged behind the downlink, in terms of
           spectrum sharing and white space detection as          both market demand, and system specification
           spectrum becomes ever more crowded.                    and practical implementation. Mobile networks
                                                                  have typically tended to experience downlink-
                                                                  heavy traffic loads; nevertheless, applications such
                   NEW APPLICATIONS AND                           as file sharing in the fixed Internet have resulted
                                                                  in a more even balance, and this may perhaps be
                     TRAFFIC PATTERNS                             seen as an indicator of future trends in mobile
           Mobile communications traffic is no longer dom-        networks. It is likely, therefore, that in the future
           inated by traditional voice services, and it will be   further attention will need to be paid to enhanc-
           important to continue to adapt LTE-Advanced            ing the uplink performance to more closely match
           to ensure efficient support for unique traffic         that delivered by the downlink. Constraints to be
           characteristics of applications that become rele-      overcome include the lower transmit power of
           vant in the future.                                    mobile terminals, which results in uplink coverage
               Traffic in mobile communications networks is       limitations, and the cost of mobile terminals.
           no longer solely generated by traditional mobile       More advanced multiple-antenna schemes than
           terminals. Increasingly, mobile networks are           the four-transmit-antenna single-user MIMO
           being used to gather data from or transfer data        techniques standardized in LTE-Advanced
           between machine-type devices. Such communica-          Release 10 may provide one route to address
           tions have certain particular characteristics, typi-   these challenges.
           cally including small packet size, tolerance of
           transmission delay, low mobility, low frequency
           of use, a need for long battery life, and poten-
                                                                       IMPLEMENTATION ADVANCES
           tially a very large number of terminals; the ter-      Not all enhancements to LTE-Advanced will
           minals themselves need to be very low in cost.         require changes to the system specifications. In
           Specific features to optimize the support provid-      parallel with the introduction of new features in
           ed for these types of communications may be            the specifications, implementation advances are
           introduced in the future, although it should be        also to be expected, as processing capability
           borne in mind that one of the most effective           increases and new algorithms emerge. For exam-


           IEEE Communications Magazine • February 2012                                                                                         119



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                                     ple, algorithms for channel estimation or inter-      of the LTE and LTE-Advanced standards, as
                 It is clear that    ference cancellation that once seemed infeasible      well as his colleagues Federico Boccardi, Fang-
                                     at reasonable cost will tomorrow be entirely pos-     Chen Cheng, Howard Huang, Sudeep Palat,
             LTE-Advanced will       sible.                                                Philippe Sehier, Antonia Tulino, and Sivarama
            need to continue to          On the UE side, such advances are likely          Venkatesan, and the anonymous reviewers, for
                  pursue bold        from time to time to result in new, tighter per-      their helpful comments in the development of
                                     formance requirements being standardized to           this article.
                 evolutions and      identify superior terminals.
                 embrace new             On the network side, some features have                                 REFERENCES
                                     been highlighted above that can benefit from          [1] S. Sesia, I. Toufik, and M. Baker, LTE — The UMTS Long
            technologies to keep     new network architectures with centralized pro-           Term Evolution: From Theory to Practice, 2nd ed.,
             pace with market        cessing; significant effort will also be directed         Wiley, 2011.
                                                                                           [2] S. Venkatesan, A. Lozano, and R. Valenzuela, “Network
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                                     work optimization in order to manage interfer-        [10] H. Huang, C. B. Papadias, and S. Venkatesan, MIMO
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                                         Network adaptability will be a key focus, with    [11] F. Boccardi et al., “An Industrial Perspective of Relay-
                                                                                               ing for Cellular Systems,” Proc. 44th Asilomar Conf.
                                     the aim of responding dynamically to changes in           Signals, Systems and Computers, Pacific Grove, CA,
                                     traffic patterns and reducing the operating costs         Nov. 2010, pp. 1311–15.
                                     of the network; enhanced SON functions will
                                     play a key role in such dynamic optimization.                               BIOGRAPHY
                                     Adaptability will also extend to flexible exploita-
                                                                                           MATTHEW BAKER (matthew.baker@alcatel-lucent.com) is the
                                                                                                             ___________________
                                     tion of available spectrum as radio frequencies       Chairman of 3GPP TSG Working Group 1. He holds degrees
                                     become increasingly crowded.                          in engineering and electrical and information sciences from
                                         Finally, the characteristics and requirements     the University of Cambridge, United Kingdom. From 1996
                                     of emerging applications and device types must        to 2009 he worked at Philips Research where he conducted
                                                                                           research into a variety of wireless communication systems
                                     be carefully monitored and assessed to ensure         and techniques, including propagation modeling, DECT,
                                     that LTE-Advanced has the necessary capabili-         HiperLAN, and UMTS, as well as leading the Philips 3GPP
                                     ties to meet their challenges.                        RAN standardization team. He has been actively participat-
                                                                                           ing in the standardization of both UMTS WCDMA and LTE
                                                   ACKNOWLEDGMENTS                         in 3GPP since 1999. He has been with Alcatel-Lucent since
                                                                                           2009. He is a Chartered Engineer, a member of the Institu-
                                     The author gratefully acknowledges the work of        tion of Engineering and Technology, and a visiting lecturer
                                     all participants in 3GPP toward the development       at the University of Reading, United Kingdom.




           120                                                                                  IEEE Communications Magazine • February 2012



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Description: LTE and LTE-Advanced have undeniably provided a major step forward in mobile communication capability, enabling mobile service provisioning to approach for the first time that available from fixed-line connections. However, market demands typically do not evolve simply in discrete steps; therefore, the future evolution of LTE-Advanced will be a story of continuous enhancement, on one hand taking advantage of the advancing capabilities of technology, while on the other aiming to keep pace with the expectations and needs of the end users. In this article, the likely directions of this continuing enhancement are discussed, and some areas where further technical advancement will be required are identified