A Lean Carrier for LTE

IEEE Communications Magazine • February 201374 0163-6804/13/$25.00 © 2013 IEEE

INTRODUCTION

Fourth-generation (4G) mobile broadband basedon the Third Generation Partnership Project(3GPP) Long Term Evolution (LTE) radioaccess technology [1] is the fastest developingsystem in the history of mobile communication.In mid-2012, LTE covered 455 million peopleglobally, and by 2017 it is expected to coveraround 50 percent of the world’s population [2].Current deployments are based on Release 8,the first release of LTE, but LTE is continuouslyevolving. Release 9 was an intermediate releaseintroducing multicast and broadcast functionali-ty. The first major step in the evolution was LTERelease 10, where carrier aggregation and relay-ing were added. Since it meets the InternationalTelecommunication Union (ITU) requirementsfor IMT-Advanced systems, it is commonlyreferred to as LTE-Advanced. Release 11 canagain be seen as an intermediate release, wherecoordinated transmission and reception of basestations was specified.

Currently, 3GPP is specifying the next majorrelease, LTE Release 12 [3], which targets theincreasing demand for mobile broadband ser-vices and traffic volumes.1 The main focus is onsmall cell enhancements, where low-power nodesprovide high capacity and enhanced user datarates locally (e.g. in indoor and outdoor hotspotpositions), while the macro layer provides wide-area coverage. One of the key components is thenew carrier type [4], henceforth referred to in thisarticle as the Lean Carrier. By minimizing con-trol channel and reference signal overhead, theLean Carrier increases resource utilization andreduces interference, thereby increasing spectral

efficiency. It can increase spectrum flexibilityand reduce energy consumption.

This article provides an overview of the moti-vations and main use cases of the Lean Carrier.Technical challenges such as mobility support, aswell as transmission of data and control arehighlighted, and design options are discussed.Finally, a performance evaluation is presentedwith some key results showing that the LeanCarrier can provide substantial cell edge userthroughput gain in heterogeneous deployments,and macro node energy consumption can be sig-nificantly reduced.

LTE BACKGROUNDLTE supports frequency-division duplex (FDD),where uplink and downlink transmission are sep-arated in frequency, as well as time-divisionduplex (TDD), where uplink and downlink areseparated in time. LTE can be deployed in sixdifferent, specified system bandwidths of 1.4, 3,5, 10, 15, and 20 MHz. With the introduction ofcarrier aggregation in LTE Release 10, up tofive downlink carriers can be aggregated to uti-lize a maximum of 100 MHz.

LTE uses orthogonal frequency-division multi-plexing (OFDM), which divides the available sys-tem bandwidth into multiple orthogonalsubcarriers in the frequency domain and into mul-tiple OFDM symbols in the time domain. In orderto limit the signaling complexity of addressingeach time-frequency resource individually, multi-ple subcarriers and OFDM symbols have beengrouped to form an addressable unit, a physicalresource block (PRB) pair, which is highlighted inFig. 1. Depending on the system bandwidth,between 6 and 110 PRB pairs compose a 1 mssubframe, and 10 subframes form a radio frame.Figure 1 illustrates the frame structure for theFDD downlink. For TDD, the frame structure issimilar; the main difference is that certain sub-frames are used for uplink instead of downlink [1].

Within a subframe, the first few OFDM sym-bols (one to four symbols) are reserved and usedfor control channels, which mostly contain uplinkand downlink scheduling assignments. Controlchannels are distributed across the entire systembandwidth. Data can be transmitted on a per-PRB basis on the remaining OFDM symbols ofa PRB. An example data channel is shown inFig. 1. Cell-specific reference signals (CRSs) aretransmitted on certain resources in every PRBand every subframe. Cell-specific reference sig-nals are used for various purposes, such as

ABSTRACT

The next major step in the evolution of LTEtargets the rapidly increasing demand for mobilebroadband services and traffic volumes. One ofthe key technologies is a new carrier type,referred to in this article as a Lean Carrier, anLTE carrier with minimized control channeloverhead and cell-specific reference signals. TheLean Carrier can enhance spectral efficiency,increase spectrum flexibility, and reduce energyconsumption. This article provides an overviewof the motivations and main use cases of theLean Carrier. Technical challenges are highlight-ed, and design options are discussed; finally, aperformance evaluation quantifies the benefitsof the Lean Carrier.

LTE TECHNOLOGY UPDATE: PART 2

Christian Hoymann, Daniel Larsson, Havish Koorapaty, and Jung-Fu (Thomas) Cheng, Ericsson

A Lean Carrier for LTE

1 Mobile data traffic isexpected to grow approxi-mately 12 times between2012 and 2018 [2].

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IEEE Communications Magazine • February 2013 75

mobility measurements, synchronization, andchannel estimation for demodulation. As analternative to cell-specific reference signals, thedata channel can also be based on UE-specificor demodulation reference signals (DMRS),where UE refers to User Equipment, i.e., amobile terminal. Unlike cell-specific referencesignals, UE-specific reference signals are trans-mitted on certain resources only within PRBsused for the data channel.

In Release 11, an enhanced control channelwas introduced, which, in contrast to the legacycontrol channel, occupies only resources inselected PRBs of a subframe (but not the fullbandwidth). Furthermore, it only uses OFDMsymbols of the data region (i.e., excluding thefirst few OFDM symbols used for legacy control)and is based on UE-specific reference signals(but not cell-specific reference signals). Anexample enhanced control channel is shown inblue in Fig. 1.

MOTIVATIONSAs discussed above, cell-specific reference sig-nals are transmitted in all subframes indepen-dent of the actual system load. The result is thatthe transmission circuitry at base stations has tostay active for a significant fraction of time. Thisleads to the motivation of removing the cell-spe-cific reference signals in order to increase thepotential for lower network energy consumptionby allowing base stations to turn off transmissioncircuitry when there is no data to transmit.

Due to their “always-on” nature, cell-specificreference signals cause interference to neighborcells even when no data is transmitted. Further-more, for data channels based on UE-specificreference signals, cell-specific reference signalsoccupy resources that could otherwise be usedfor data. Legacy control channels are distributedacross the entire system bandwidth, which causesuncontrollable intercell interference. In addition,resources of the first symbols of a subframereserved for control cannot be used for dataeven if there are no legacy control channel trans-missions. This leads to the motivation for mini-mizing control channel overhead. In combinationwith the removal of the cell-specific reference

signals, the reduced overhead and interferencelevel at low to medium loads enables higher end-user throughput and improved system spectralefficiency.

Finally, some of the operators’ spectrum allo-cation cannot be fully utilized with the currentlyspecified LTE system bandwidths. This leads tothe motivation of easing the adaptation of theLean Carrier to various bandwidths in order toallow operators to leverage their spectrum assetseven more efficiently.

Note that in uplink, there are neither full-bandwidth transmissions nor always-on signals;hence, the uplink of legacy LTE already fulfillsthe main goals of a Lean Carrier. As a conse-quence, the major modifications of the LeanCarrier target the downlink, while the uplink ismuch less affected.

SCENARIOS AND USE CASES

LEAN CARRIER FOR LOW-POWER NODES INRELEASE-12 SMALL CELL SCENARIOS

Densification of a cellular network by usingcomplementary low-power nodes is one of themain scenarios for LTE Release 12 [5]. In such aheterogeneous deployment, low-power nodesprovide high capacity and enhanced user datarates locally (e.g., in indoor and outdoor hotspotpositions), while the macro nodes provide reli-able wide-area coverage.

The Lean Carrier fits nicely into the Release12 concept of dual connectivity, where the UEmaintains its connection to the macro nodewhile a simultaneous connection to a low-powernode can be added. For instance, the legacyLTE connection to the macro can be used forsystem information and basic control signaling,while the Lean Carrier connection to the low-power node can be used for high-capacity datatransmissions [3]. Figure 2 illustrates such asmall cell scenario using dual connectivity wherethe terminal is connected to the macro nodeusing a legacy LTE carrier and to the low-powernode through a Lean Carrier. Since macro nodesprovide all vital control connections, low-powernodes can use the Lean Carrier with minimumoverhead. Due to a potentially large number of

Figure 1. LTE time-frequency grid (FDD) containing example control and data channels with a focus on aPRB pair showing cell-specific and UE-specific reference signals.

3 OFDM symbols reserved forlegacy control channels

Physical resource block (PRB) pair

10 ms radio frame

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Densification of a

cellular network by

using complemen-

tary low-power

nodes is one of the

main scenarios for

LTE Release 12.

In such a

heterogeneous

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low-power nodes

provide high capacity

and enhanced user

data rates locally.


IEEE Communications Magazine • February 201376

low-power nodes and a typically low average buthighly bursty load per node, reducing energyconsumption of and interference from nodeswithout traffic becomes important. With theremoval of the cell-specific reference signals,nodes utilizing the Lean Carrier can be shutdown in a larger fraction of subframes where nouser is to be served. If deployed in dense clus-ters, low-power nodes potentially interfere witheach other even at very low loads, and the LeanCarrier can reduce such interference.

Low-power nodes can be deployed on a fre-quency band that is separate from the one usedby the macro nodes or on the same frequency. Afrequency-separated deployment allows forsmooth integration of the Release 12 Lean Car-rier into a legacy macro network, which providesbasic coverage and serves all legacy terminals,while Release 12 terminals can be offloaded tothe local-area layer on separate frequency bands.

AGGREGATION OF LEGACY LTE AND LEANCARRIER FOR FLEXIBLE BANDWIDTH SUPPORT

System bandwidths of 1.4, 3, 5, 10, 15, and20 MHz have been specified for 3GPP LTE.LTE Release 10 introduced carrier aggregation,which allows utilizing even larger spectrum allo-cations. Furthermore, aggregating contiguouscarriers of those six different bandwidths allowsoperators to efficiently utilize their frequencybands of various sizes. For instance, a 5 MHzLTE carrier and a 3 MHz LTE carrier can beaggregated to fully utilize an 8 MHz frequencyallocation. Nevertheless, certain spectrum alloca-tions may still not be fully utilized. It is withinthe scope of the Release 12 work to investigatesolutions to achieve enhanced spectrum flexibili-ty support, including the possibility to reduce“full-bandwidth” channels and signals.

Contiguous aggregation of a legacy and aLean Carrier also allows for smooth integration:the legacy LTE carrier serves all legacy terminalson a legacy bandwidth, while Release 12 termi-nals can access the entire band including legacyand Lean Carrier frequencies.

LEAN CARRIER FOR MACRO NODESHomogeneous wide-area deployments also bene-fit from the Lean Carrier. Minimizing interfer-ence of cell-specific reference signals is especiallyappealing at the cell edge, where cell-specificreference signals of neighbor cells are a mainsource of interference in networks with low tomedium loads. If low-power nodes complementmacro nodes using the same carrier frequency,

reduced interference from the (lean) macronode is beneficial in the cell range of small cellsin heterogeneous deployments, which leads toincreased capacity. Finally, reduced power con-sumption of macro nodes will reduce operatingexpenses of the wireless service providers.

Macro nodes can first use the Lean Carrieron new frequency bands, which have not yetbeen allocated to legacy LTE. In the future, car-rier aggregation of a legacy LTE and a LeanCarrier can serve as a potential migration pathto the Lean Carrier for networks operating onexisting cellular frequency bands.

DESIGN OPTIONS ANDTECHNICAL CHALLENGES

As discussed earlier, the motivations forenhanced spectral and energy efficiency lead toremoval of the cell-specific reference signals andminimization of control channel overhead on theLean Carrier. Of course, those signals and chan-nels have served certain functionalities that alsohave to be maintained on the Lean Carrier. Thissection discusses the main challenges posed bythe changes to the cell-specific reference signalsand control channels on the Lean Carrier.

SYNCHRONIZATIONOn the Lean Carrier, cell-specific reference sig-nals, which were commonly used by terminals tosynchronize with the base station, are removed.For synchronization purposes, a new referencesignal was added, which in the following isreferred to as an extended synchronization signal(eSS). The eSS is based on the cell-specific ref-erence signals, but it appears only once everyfive subframes. The reduction in overheadachieved by using the eSS is shown conceptuallyin Fig. 3, which shows the legacy synchronizationsignals (primary and secondary synchronizationsignals, also referred to as the PSS/SSS in LTE,used for coarse time and frequency synchroniza-tion as well as cell search) in addition to the eSSwith each of the signals appearing at a maximumfrequency of only once every five subframes.

CELL SEARCHOn a legacy LTE carrier, a terminal will identifya cell by detecting its (legacy) synchronizationsignals (PSS/SSS) first, followed by the detectionof the cell-specific reference signal. Cell searchfor the Lean Carrier is built on the same princi-ple, but the reduced overhead eSS is usedinstead of the cell-specific reference signal.

Having found a cell, the terminal performsreceived signal power measurements to evaluateif the cell is a likely candidate to which to attach.On a legacy cell, the cell-specific reference sig-nals are used for such measurements. On a LeanCarrier, such measurements could be performedon the eSS.2

INITIAL ACCESS AND HANDOVERIf the terminal has found a good candidate,there are two different procedures to access thecell. When the terminal is already attached to acell, it performs a handover from its source cellto the target cell; or, when the terminal is not

Figure 2. Small cell scenario where the terminal is connected to a macro nodeusing legacy LTE and to a low-power node through a lean carrier.

Legacy LTE carrier

Lean carrier

Low-power nodeMacro node

2 Alternatively, measure-ments could be performedon user equipment (UE)-specifically configured ref-erence signals originallyused for dynamic channelmeasurements (channelstate indicator-referencesignal [CSI-RS]).



attached yet, it performs an initial access to thenetwork.

In the case of initial access, the terminal willacquire the basic system information for thatparticular cell such as transmission bandwidth byreading the master system information. On a lega-cy carrier, this system information is transmittedusing a broadcast channel, which is based oncell-specific reference signals. The Lean Carriercan use a similar function, with the main differ-ence that the broadcast channel has to be basedon UE-specific reference signals or the eSS.Having acquired the master system information,the terminal needs to receive the remainingcomponents of system information, which areusually transmitted on a data channel scheduledby common control channels. On the Lean Car-rier, the remaining system information has to bereceived on a data channel scheduled by theenhanced control channel.

In the case of handover, the terminal will getthe master system information from its sourcecell prior to performing the handover.

Alternative to a standalone operation usingthe above procedures, the Lean Carrier can beaggregated with a legacy LTE carrier. In thatcase mobility as well as acquisition of systeminformation is based on the legacy carrier.

DATA TRANSMISSIONIn general, downlink data transmission is sup-ported by cell- or UE-specific reference signals.When using cell-specific reference signals, theterminal performs channel state and interferencemeasurements on those reference signals, andsends the measurement reports to the base sta-tion. The base station then transmits data, whichthe terminal can demodulate using the cell-spe-cific reference signals.

Instead of cell-specific reference signals, UE-specific reference signals can be used, which areonly present in subframes and PRB pairs wheredata or control is transmitted, as seen in Fig. 3.Since UE-specific reference signals are only usedfor demodulation purposes, another type of ref-erence signal is needed specifically for channelmeasurements.4 These signals are used by theUE to measure channel state information andreport the corresponding information to the net-

work. Interference can be measured on specifi-cally configured interference measurementresources, which are not used for data or controltransmissions by the serving cell. On the LeanCarrier, data transmissions can only be per-formed using this approach based on UE-specif-ic reference signals.

Uplink-based transmissions can generallyreuse the legacy carrier’s uplink schemes. This ismainly because legacy uplink transmissionsalready fulfill most of the motivations for theLean Carrier.

CONTROL TRANSMISSIONAs discussed earlier, legacy control channels mayoccupy one to four OFDM symbols out of 14symbols in each subframe. In Release 11, anenhanced control channel has been defined fortransmitting control information that is specificto a terminal. The Lean Carrier solely relies onthis enhanced control channel, as shown in Fig.3.

The enhanced control channel is transmittedin a similar fashion as is unicast data. Therefore,it can be transmitted in only a few PRB pairsinstead of in the entire bandwidth as is the casefor the legacy control channels. The enhancedcontrol channel uses UE-specific reference sig-nals for channel estimation when demodulatingthe control information. Thus, advanced trans-mission techniques such as beamforming canalso be used for control. In addition, frequencydomain interference coordination can be used tomanage interference on control transmissionsfrom neighboring cells by using different PRBsfor control in different cells.

On a legacy carrier, due to the necessity forcommon control, the first few symbols that areused for legacy control channels are not avail-able for transmission of data. On the Lean Car-rier, all control transmissions are sent on theenhanced control channel, and legacy controlchannels are not used. Hence, all symbols can beused for data transmissions (or for the enhancedcontrol channel), as seen in Fig. 3.

On the legacy carrier, information that iscommon to multiple terminals (e.g., systeminformation) has to be scheduled by the legacycontrol channels. For the Lean Carrier, the

Figure 3. Lean carrier with legacy synchronization signals and eSS transmitted only every fifth subframe.3

10 ms radio frame

1 ms subframe

Frequency

Subframes containingextended synchronization signal (eSS)

Time

Example allocationof a data channelExample allocation of anenhanced control channelSynchronization signals(PSS/SSS)Broadcast channel

Resource used forUE-specific reference signals

Physical resource block (PRB) pair

3 Exact details of the eSSare currently being verifiedin 3GPP.

4 CSI-RS.

On the Lean Carrier,

all control

transmissions are

sent on the

enhanced control

channel and legacy

control channels are

not used. Hence,

all symbols can be

used for data

transmissions (or for

the enhanced

control channel).



enhanced control channel has to be improved sothat it can multicast common information aswell.

OTHER DESIGN CHALLENGESFlexibility on the bandwidth capabilities of ter-minals may also serve to increase spectrum flexi-bility for operators in the future, which was oneof the scenarios described earlier. However,addressing lower-bandwidth (legacy) terminalson a higher-bandwidth carrier may pose somechallenges on both base station and terminaldesign. For instance, a lower-bandwidth legacyterminal that normally expects a guard band oneither edge of its carrier may now have differentsignals within the guard band because a LeanCarrier is contiguously aggregated with the lega-cy carrier to extend the usable bandwidth. Whilethis may not be a difficult problem for terminaldesign, it is still a new aspect that needs to betaken into account when designing terminals forthe Lean Carrier. The base station schedulermust keep track of the different bandwidth capa-bilities of different terminals and the spectrumbeing accessed by each one of them. Such chal-lenges will need to be taken into considerationin the development of the Lean Carrier.

PERFORMANCEThe goals of the Lean Carrier, as described ear-lier, are as follows:• Improved spectral efficiency• Improved network energy efficiency• Improved support of heterogeneous deploy-

ments• Improved spectrum flexibility support

Improved spectral efficiency is achieved main-ly in low and medium load scenarios on theLean Carrier compared to the legacy carrier typebecause the signals that are mandatory to trans-mit from the base station have been reduced onthe Lean Carrier. An example is that the cell-specific reference signals defined for the legacycarrier on the Lean Carrier are replaced by theeSS, which needs to be transmitted only in everyfifth subframe. The downlink transmissions onthe Lean Carrier are instead based on the use ofUE-specific reference signals that are only pre-sent within assigned downlink data transmis-sions. The UE-specific reference signal overheadscales with the amount of data being transmitted

as opposed to being a constant overhead as onthe legacy carrier. Another differentiating factoris that the scheduling on the legacy carrier typeutilizes wideband control channels and the band-width-flexible enhanced control channel com-pared to the Lean Carrier, which only uses theenhanced control channel. The drawback withutilizing the legacy control channel is that theoverhead scales with increased scheduling loadwith a large granularity (using an integer numberof OFDM symbols as described earlier). In con-trast, the overhead of the enhanced controlchannel scales more directly with the transmittedscheduling messages.

Increased energy efficiency at the networkside is enabled by the base station applying microsleep operations, wherein if the base station doesnot transmit anything, it is able to reduce thepower consumption within its radio and digitalunits. The typical quiet period can be on theorder of one or several OFDM symbols. On theLean Carrier the quiet period can be up to foursubframes long since this is the maximum periodwithout any mandatory transmissions. In order tomeasure network energy savings, several compa-nies promoted a model based on the frameworkfrom the EU project EARTH [6]. The modelsdefined within EARTH were adjusted to fit thescenarios studied in 3GPP [7]. In addition to theabove micro sleep, during Release 12, 3GPP willstudy energy savings that may be achieved byhaving even longer quiet periods (e.g., up toapproximately 1 s). One potential use case is thesmall cell scenario described earlier, where low-power nodes can be quiet while the macro tem-porarily serves the terminals.

Improvements for heterogeneous deploy-ments, as described earlier, mainly target deploy-ments utilizing a cell selection offset (CSO),where the cell area covered by a low-power nodeis expanded in order to serve a larger percentageof the total traffic from the low-power node. Theapproach to achieve this on the Lean Carrier istwofold. First, the Lean Carrier relies completelyon enhanced control channels, which allows themacro node to protect the control in the low-power node by avoiding high interference on thePRB pairs used for control. This can be done byblanking out these PRB pairs or transmittingwith a lower power. This is in contrast to theapproach of almost blank subframes defined inLTE Release 10, wherein the macro node blanks

Figure 4. Cell edge user throughput of the Lean Carrier for high and low loads compared to the legacy carrier type (LCT) with and with-out CRS interference cancellation.

Heterogeneous deployment CSO 4dB low load

Lean carrier

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out entire subframes. Second, the Lean Carrierminimizes the amount of interference caused byreference signals to low-power nodes. In 3GPPRelease 11, some terminals are able to cancelinterfering cell-specific reference signals. Instead,the Lean Carrier avoids the transmission ofthese cell-specific reference signals, which allowssimpler terminal implementation and a poten-tially larger cell selection offset for the low-power node.

Initial performance evaluation in a heteroge-neous network deployment is presented in Fig.4. The evaluation assumptions are based on theassumptions defined within 3GPP [8]. Briefly,the evaluations are performed with a non-fullbuffer traffic model in a high and low load sce-nario,5 where four low-power nodes are deployedper macro node. Evaluations are performed witha cell selection offset of 4 dB with the low-powernode and the macro nodes being deployed onthe same frequency. The results are presentedfor the 5th percentile user, which corresponds toterminals operating on the cell edge.

In the evaluations, the 5th percentile userthroughput is compared between a networkusing the Lean Carrier and a network using thelegacy carrier. For the legacy carrier, two casesare studied. In the first scenario, it is assumedthat terminals can cancel the cell-specific refer-ence signals from the strongest interfering cell.6In the second scenario, terminals are standardRelease 8, incapable of cancellation.

Figure 4 shows that the Lean Carrier has anapproximately 70 percent throughput gain at thecell edge compared to a legacy carrier with lega-cy Release 8 terminals with low load. At highload there is still a 20 percent gain over a legacycarrier with a standard Release 8 terminal. TheLean Carrier also performs significantly betterthan a legacy carrier serving terminals that arecancelling the cell-specific reference signals fromthe strongest interfering cell. The reason for thisis that after cancelling interference from the sin-gle largest cell, there are many more interferingcells left, for which the cell-specific referencesignals are not cancelled.

Figure 5 shows energy savings achievable in

the same deployment as above. Base stations areassumed to consume power according to themodel defined in [6]. The results are separatedinto macro and low-power nodes. The results arepresented relative to a deployment with the lega-cy carrier.7 Energy savings come from two fac-tors: First and most important, due to fewermandatory transmissions on the Lean Carrier,base stations can go into micro sleep more fre-quently. Second, since the throughput is higheron the Lean Carrier, the terminal buffers emptyquicker, allowing base stations to be quiet overlonger time periods. Furthermore, energy sav-ings are larger for macro nodes than for low-power nodes since the total transmission poweris larger in the macro. Additionally, the fractionof power used for radio transmissions is greaterin a macro node than in a low-power node.

CONCLUSIONSThis article introduces the Lean Carrier, one ofthe key components of LTE Release 12. By min-imizing control channel and reference signaloverhead, the Lean Carrier can increase resourceutilization and reduce interference, therebyincreasing spectral efficiency. It can increasespectrum flexibility and reduce energy consump-tion. One of the primary use cases is a small cellscenario, where complementary low-power nodesprovide very high capacity and enhanced userdata rates locally while macro nodes providewide-area coverage. Technical design aspectssuch as mobility support, synchronization, as wellas transmission of data and control are highlight-ed, and design challenges are discussed. A per-formance evaluation quantifies the benefits: theLean Carrier can provide up to 70 percent celledge user throughput gain in heterogeneousdeployments, and it can reduce macro nodeenergy consumption by approximately 20 percentat low loads.

REFERENCES[1] E. Dahlman, S. Parkvall, and J. Sköld, 4G: LTE/LTE-

Advanced for Mobile Broadband: LTE/LTE-Advanced forMobile Broadband, Academic Press, Apr. 2011.

Figure 5. Lean carrier network energy savings.

System throughput (Mb/s)

Energy savings in macro compared to Rel-10 carrier

Low load0

5

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Energy savings in low power node compared to Rel-10 carrier

0

2

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avin

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perc

ent)

4

6

8

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High loadLow load

5 The low load scenariocorresponds to 50Mb/s/km2 and the highload scenario correspondsto 500 Mb/s/km2.

6 Cell-specific referencesignal interference cancel-lation is a feature intro-duced in LTE Rel-11.

7 It is noted that a legacycarrier can be made moreenergy-efficient byemploying multicastbroadcast single-frequencynetwork (MBSFN) sub-frames, which have lowercell-specific reference sig-nal overhead. However,the Lean Carrier is moreenergy-efficient than alegacy carrier even whenMBSFN subframes areemployed largely due to afurther 40 percent reduc-tion in reference signaloverhead at lower loads.



[2] Ericsson, “Ericsson Mobility Report,” Nov. 2012,http://www.ericsson.com/res/docs/2012/ericsson-mobili-ty-report-november-2012.pdf.

[3] D. Astely et al., “LTE Release 12 and Beyond,” IEEECommun. Mag., to be published

[4] Ericsson: 3GPP LTE Work Item Description “New CarrierType for LTE,”3GPP RP-121415, http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_57/Docs/RP-121415.zip.

[5] 3GPP, TR 36.932, Scenarios and Requirements for SmallCell Enhancement for E-UTRA and E-UTRAN

[6] EARTH project, www.ict-earth.eu.[7] R1-114336, Base Station Power Model, NTT DOCOMO,

Alcatel-Lucent, Alcatel-Lucent Shanghai Bell, Ericsson,Telecom Italia.

[8] R1-112856, Summary of Ad Hoc Session on FeICIC Sim-ulation Assumptions, NTT DOCOMO.

BIOGRAPHIESCHRISTIAN HOYMANN ([email protected])received his Diploma degree in electrical engineering fromRWTH Aachen University in 2002. At RWTH’s Chair of Com-munication Networks he worked toward his doctoraldegree, which he received in 2008. Since 2007 he is work-ing at Ericsson Research, where he focuses on advancing3GPP LTE, especially in the areas of relaying, CoMP, andheterogeneous networks. Currently, he is technical coordi-nator of Ericsson’s RAN1 delegation.

DANIEL LARSSON ([email protected]) received hisM.S.E.E degree from the Royal Institute of Technology in2006. He joined Ericsson in 2007, for which he has attend-ed standardization meetings since 2008 covering a broad

area of topics such as mobility, positioning, and carrieraggregation. Recently, he became the 3GPP Rapporteur forthe work item on the New Carrier Type.

HAVISH KOORAPATY ([email protected])received his B.S., M.S., and Ph.D. degrees from North Car-olina State University in 1991, 1993, and 1996, respective-ly. He has been with Ericsson Research since 1996, wherehe has worked on various topics in the area of cellular andsatellite communications including error control coding,location determination and tracking, phone systems engi-neering, 4G broadband wireless system design, and wire-less backhaul solutions. Recently, he has worked onenhancements to LTE and participated in standardizationefforts in 3GPP.

JUNG-FU (Thomas) Cheng ([email protected])is with Ericsson Research in Ericsson Sil icon Valley,where he drives research and development of advancedwireless communication technologies. His research inter-ests include iterative processing, coding and decodingtechniques, signal processing algorithms for wirelesscommunications, and application of information theoryto wireless system design. He has contributed to theevolution of cellular wireless PHY and MAC layer evolu-tion from 2.5G EDGE to 3G HSPA to 4G LTE. He was theprincipal contributor to the LTE turbo and convolutionalcoding and rate matching specifications. He received hisB.S. and M.S. degrees in electrical engineering fromNational Taiwan University, Taipei. He received his Ph.D.degree in electrical engineering with additional study insocial science from California Institute of Technology,Pasadena.

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A Lean Carrier for LTE