A Lean Carrier for LTE

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

    INTRODUCTIONFourth-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 worlds 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

    ABSTRACTThe next major step in the evolution of LTE

    targets 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].

    HOYMANN LAYOUT_Layout 1 1/28/13 3:37 PM Page 74

  • 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

    1 ms subframe

    Frequency

    All subframes containingcell-specific reference signals (CRS)

    ...

    Time

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

    Resources used for cell-specificreference signals (CRS)Resource used forUE-specific reference signals

    Densification of a

    cellular network by

    using complemen-