Lean carrier for lte (for more insights go to: http://trends-in-telecoms.blogspot.com)


Published on

A Lean Carrier for LTE

  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Lean carrier for lte (for more insights go to: http://trends-in-telecoms.blogspot.com)

  1. 1. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® LTE TECHNOLOGY UPDATE: PART 2 A Lean Carrier for LTE Christian Hoymann, Daniel Larsson, Havish Koorapaty, and Jung-Fu ( (Thomas) Cheng, Ericsson ) ABSTRACT efficiency. It can increase spectrum flexibility y and reduce energy consumption. The next major step in the evolution of LTE This article provides an overview of the moti- targets the rapidly increasing demand for mobile vations and main use cases of the Lean Carrier. broadband services and traffic volumes. One of f Technical challenges such as mobility support, as the key technologies is a new carrier type, well as transmission of data and control are referred to in this article as a Lean Carrier, an highlighted, and design options are discussed. LTE carrier with minimized control channel Finally, a performance evaluation is presented overhead and cell-specific reference signals. The with some key results showing that the Lean Lean Carrier can enhance spectral efficiency, Carrier can provide substantial cell edge user increase spectrum flexibility, and reduce energyy throughput gain in heterogeneous deployments, consumption. This article provides an overview and macro node energy consumption can be sig- of the motivations and main use cases of the nificantly reduced. Lean Carrier. Technical challenges are highlight- ed, and design options are discussed; finally, a performance evaluation quantifies the benefits LTE BACKGROUND of the Lean Carrier. LTE supports frequency-division duplex (FDD), where uplink and downlink transmission are sep- INTRODUCTION arated in frequency, as well as time-division duplex (TDD), where uplink and downlink are Fourth-generation (4G) mobile broadband based separated in time. LTE can be deployed in six x on the Third Generation Partnership Project different, specified system bandwidths of 1.4, 3, (3GPP) Long Term Evolution (LTE) radio 5, 10, 15, and 20 MHz. With the introduction of f access technology [1] is the fastest developing carrier aggregation in LTE Release 10, up to system in the history of mobile communication. five downlink carriers can be aggregated to uti- In mid-2012, LTE covered 455 million people lize a maximum of 100 MHz. globally, and by 2017 it is expected to cover LTE uses orthogonal frequency-division multi- around 50 percent of the world’s population [2]. plexing (OFDM), which divides the available sys- Current deployments are based on Release 8, tem bandwidth into multiple orthogonal the first release of LTE, but LTE is continuously subcarriers in the frequency domain and into mul- evolving. Release 9 was an intermediate release tiple OFDM symbols in the time domain. In order introducing multicast and broadcast functionali- to limit the signaling complexity of addressing ty. The first major step in the evolution was LTE each time-frequency resource individually, multi- Release 10, where carrier aggregation and relay- ple subcarriers and OFDM symbols have been ing were added. Since it meets the International grouped to form an addressable unit, a physical Telecommunication Union (ITU) requirements resource block (PRB) pair, which is highlighted in for IMT-Advanced systems, it is commonly Fig. 1. Depending on the system bandwidth, referred to as LTE-Advanced. Release 11 can between 6 and 110 PRB pairs compose a 1 ms again be seen as an intermediate release, where subframe, and 10 subframes form a radio frame. coordinated transmission and reception of base Figure 1 illustrates the frame structure for the stations was specified. FDD downlink. For TDD, the frame structure is Currently, 3GPP is specifying the next major similar; the main difference is that certain sub- release, LTE Release 12 [3], which targets the frames are used for uplink instead of downlink [1]. increasing demand for mobile broadband ser- Within a subframe, the first few OFDM sym- vices and traffic volumes.1 The main focus is on bols (one to four symbols) are reserved and used small cell enhancements, where low-power nodes for control channels, which mostly contain uplink provide high capacity and enhanced user data and downlink scheduling assignments. Control rates locally (e.g. in indoor and outdoor hotspot channels are distributed across the entire system positions), while the macro layer provides wide- bandwidth. Data can be transmitted on a per- area coverage. One of the key components is the PRB basis on the remaining OFDM symbols of f new carrier type [4], henceforth referred to in this a PRB. An example data channel is shown in 1Mobile data traffic is article as the Lean Carrier. By minimizing con- Fig. 1. Cell-specific reference signals (CRSs) are expected to grow approxi- trol channel and reference signal overhead, the transmitted on certain resources in every PRB mately 12 times between Lean Carrier increases resource utilization and and every subframe. Cell-specific reference sig- 2012 and 2018 [2]. reduces interference, thereby increasing spectral nals are used for various purposes, such as 74 0163-6804/13/$25.00 © 2013 IEEE $ IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  2. 2. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Physical resource block (PRB) pair Densification of a cellular network by using complemen- 3 OFDM symbols reserved for All subframes containing legacy control channels tary low-power cell-specific reference signals (CRS) Example allocation of a data channel nodes is one of the Frequency ... Example allocation of an enhanced control channel main scenarios for Synchronization signals (PSS/SSS) LTE Release 12. Broadcast channel In such a Resources used for cell-specific reference signals (CRS) heterogeneous Time Resource used for UE-specific reference signals deployment, 1 ms subframe 10 ms radio frame low-power nodes provide high capacity Figure 1. LTE time-frequency grid (FDD) containing example control and data channels with a focus on a PRB pair showing cell-specific and UE-specific reference signals. and enhanced user data rates locally. mobility measurements, synchronization, and signals, the reduced overhead and interference channel estimation for demodulation. As an level at low to medium loads enables higher end- alternative to cell-specific reference signals, the user throughput and improved system spectral data channel can also be based on UE-specific efficiency. or demodulation reference signals (DMRS), Finally, some of the operators’ spectrum allo- w here UE refers to User Equipment, i.e., a cation cannot be fully utilized with the currently mobile terminal. Unlike cell-specific reference specified LTE system bandwidths. This leads to signals, UE-specific reference signals are trans- the motivation of easing the adaptation of the mitted on certain resources only within PRBs Lean Carrier to various bandwidths in order to used for the data channel. allow operators to leverage their spectrum assets In Release 11, an enhanced control channel even more efficiently. was introduced, which, in contrast to the legacy Note that in uplink, there are neither full- control channel, occupies only resources in bandwidth transmissions nor always-on signals; selected PRBs of a subframe (but not the full hence, the uplink of legacy LTE already fulfills bandwidth). Furthermore, it only uses OFDM the main goals of a Lean Carrier. As a conse- symbols of the data region (i.e., excluding the quence, the major modifications of the Lean first few OFDM symbols used for legacy control) Carrier target the downlink, while the uplink is and is based on UE-specific reference signals much less affected. (but not cell-specific reference signals). An example enhanced control channel is shown in SCENARIOS AND USE CASES blue in Fig. 1. LEAN CARRIER FOR LOW-POWER NODES IN MOTIVATIONS RELEASE-12 SMALL CELL SCENARIOS A s discussed above, cell-specific reference sig- Densification of a cellular network by using nals are transmitted in all subframes indepen- complementary low-power nodes is one of the dent of the actual system load. The result is that main scenarios for LTE Release 12 [5]. In such a the transmission circuitry at base stations has to heterogeneous deployment, low-power nodes stay active for a significant fraction of time. This provide high capacity and enhanced user data leads to the motivation of removing the cell-spe- rates locally (e.g., in indoor and outdoor hotspot cific reference signals in order to increase the positions), while the macro nodes provide reli- potential for lower network energy consumption able wide-area coverage. by allowing base stations to turn off transmission The Lean Carrier fits nicely into the Release circuitry when there is no data to transmit. 12 concept of dual connectivity, where the UE Due to their “always-on” nature, cell-specific maintains its connection to the macro node reference signals cause interference to neighbor while a simultaneous connection to a low-power cells even when no data is transmitted. Further- node can be added. For instance, the legacy y more, for data channels based on UE-specific LTE connection to the macro can be used for reference signals, cell-specific reference signals system information and basic control signaling, occupy resources that could otherwise be used while the Lean Carrier connection to the low- for data. Legacy control channels are distributed power node can be used for high-capacity data across the entire system bandwidth, which causes transmissions [3]. Figure 2 illustrates such a uncontrollable intercell interference. In addition, small cell scenario using dual connectivity where resources of the first symbols of a subframe the terminal is connected to the macro node reserved for control cannot be used for data using a legacy LTE carrier and to the low-power even if there are no legacy control channel trans- node through a Lean Carrier. Since macro nodes missions. This leads to the motivation for mini- provide all vital control connections, low-power mizing control channel overhead. In combination nodes can use the Lean Carrier with minimum w ith the removal of the cell-specific reference overhead. Due to a potentially large number of f IEEE Communications Magazine • February 2013 75C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  3. 3. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® reduced interference from the (lean) macro Legacy LTE carrier node is beneficial in the cell range of small cells in heterogeneous deployments, which leads to Lean carrier increased capacity. Finally, reduced power con- sumption of macro nodes will reduce operating expenses of the wireless service providers. Macro node Low-power node Macro nodes can first use the Lean Carrier on new frequency bands, which have not yet been allocated to legacy LTE. In the future, car- rier aggregation of a legacy LTE and a Lean Figure 2. Small cell scenario where the terminal is connected to a macro node i Carrier can serve as a potential migration path using legacy LTE and to a low-power node through a lean carrier. to the Lean Carrier for networks operating on existing cellular frequency bands. low-power nodes and a typically low average but highly bursty load per node, reducing energy DESIGN OPTIONS AND consumption of and interference from nodes w ithout traffic becomes important. With the TECHNICAL CHALLENGES removal of the cell-specific reference signals, As discussed earlier, the motivations for nodes utilizing the Lean Carrier can be shut enhanced spectral and energy efficiency lead to down in a larger fraction of subframes where no removal of the cell-specific reference signals and user is to be served. If deployed in dense clus- minimization of control channel overhead on the ters, low-power nodes potentially interfere with Lean Carrier. Of course, those signals and chan- each other even at very low loads, and the Lean nels have served certain functionalities that also Carrier can reduce such interference. have to be maintained on the Lean Carrier. This Low-power nodes can be deployed on a fre- section discusses the main challenges posed by y quency band that is separate from the one used the changes to the cell-specific reference signals by the macro nodes or on the same frequency. A and control channels on the Lean Carrier. frequency-separated deployment allows for smooth integration of the Release 12 Lean Car- SYNCHRONIZATION rier into a legacy macro network, which provides On the Lean Carrier, cell-specific reference sig- basic coverage and serves all legacy terminals, nals, which were commonly used by terminals to while Release 12 terminals can be offloaded to synchronize with the base station, are removed. the local-area layer on separate frequency bands. For synchronization purposes, a new reference signal was added, which in the following is AGGREGATION OF LEGACY LTE AND LEAN referred to as an extended synchronization signal CARRIER FOR FLEXIBLE BANDWIDTH SUPPORT (eSS). The eSS is based on the cell-specific ref- erence signals, but it appears only once every System bandwidths of 1.4, 3, 5, 10, 15, and five subframes. The reduction in overhead 20 MHz have been specified for 3GPP LTE. achieved by using the eSS is shown conceptually y LTE Release 10 introduced carrier aggregation, in Fig. 3, which shows the legacy synchronization which allows utilizing even larger spectrum allo- signals (primary and secondary synchronization cations. Furthermore, aggregating contiguous signals, also referred to as the PSS/SSS in LTE, carriers of those six different bandwidths allows used for coarse time and frequency synchroniza- operators to efficiently utilize their frequency tion as well as cell search) in addition to the eSS bands of various sizes. For instance, a 5 MHz with each of the signals appearing at a maximum LTE carrier and a 3 MHz LTE carrier can be frequency of only once every five subframes. aggregated to fully utilize an 8 MHz frequency allocation. Nevertheless, certain spectrum alloca- CELL SEARCH tions may still not be fully utilized. It is within On a legacy LTE carrier, a terminal will identify the scope of the Release 12 work to investigate a cell by detecting its (legacy) synchronization solutions to achieve enhanced spectrum flexibili- signals (PSS/SSS) first, followed by the detection ty support, including the possibility to reduce of the cell-specific reference signal. Cell search “full-bandwidth” channels and signals. for the Lean Carrier is built on the same princi- Contiguous aggregation of a legacy and a ple, but the reduced overhead eSS is used Lean Carrier also allows for smooth integration: instead of the cell-specific reference signal. the legacy LTE carrier serves all legacy terminals Having found a cell, the terminal performs on a legacy bandwidth, while Release 12 termi- received signal power measurements to evaluate nals can access the entire band including legacy if the cell is a likely candidate to which to attach. and Lean Carrier frequencies. On a legacy cell, the cell-specific reference sig- nals are used for such measurements. On a Lean 2 Alternatively, measure- LEAN CARRIER FOR MACRO NODES Carrier, such measurements could be performed ments could be performed Homogeneous wide-area deployments also bene- on the eSS.2 on user equipment (UE)- fit from the Lean Carrier. Minimizing interfer- specifically configured ref- ence of cell-specific reference signals is especially INITIAL ACCESS AND HANDOVER erence signals originally appealing at the cell edge, where cell-specific If the terminal has found a good candidate, used for dynamic channel reference signals of neighbor cells are a main there are two different procedures to access the measurements (channel source of interference in networks with low to cell. When the terminal is already attached to a state indicator-reference medium loads. If low-power nodes complement cell, it performs a handover from its source cell signal [CSI-RS]). macro nodes using the same carrier frequency, to the target cell; or, when the terminal is not 76 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  4. 4. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Physical resource block (PRB) pair On the Lean Carrier, all control transmissions are Subframes containing sent on the extended synchronization signal (eSS) Example allocation of a data channel enhanced control Frequency Example allocation of an enhanced control channel channel and legacy Synchronization signals (PSS/SSS) control channels are Broadcast channel not used. Hence, Resource used for UE-specific reference signals all symbols can be Time 1 ms subframe used for data 10 ms radio frame transmissions (or for the enhanced Figure 3. Lean carrier with legacy synchronization signals and eSS transmitted only every fifth subframe.3 control channel). attached yet, it performs an initial access to the work. Interference can be measured on specifi- network. cally configured interference measurement In the case of initial access, the terminal will resources, which are not used for data or control acquire the basic system information for that transmissions by the serving cell. On the Lean particular cell such as transmission bandwidth by Carrier, data transmissions can only be per- reading the master system information. On a lega- formed using this approach based on UE-specif- cy carrier, this system information is transmitted ic reference signals. using a broadcast channel, which is based on Uplink-based transmissions can generally y cell-specific reference signals. The Lean Carrier reuse the legacy carrier’s uplink schemes. This is can use a similar function, with the main differ- mainly because legacy uplink transmissions ence that the broadcast channel has to be based already fulfill most of the motivations for the on UE-specific reference signals or the eSS. Lean Carrier. Having acquired the master system information, the terminal needs to receive the remaining CONTROL TRANSMISSION components of system information, which are As discussed earlier, legacy control channels may y usually transmitted on a data channel scheduled occupy one to four OFDM symbols out of 14 by common control channels. On the Lean Car- symbols in each subframe. In Release 11, an rier, the remaining system information has to be enhanced control channel has been defined for received on a data channel scheduled by the transmitting control information that is specific enhanced control channel. to a terminal. The Lean Carrier solely relies on In the case of handover, the terminal will get this enhanced control channel, as shown in Fig. the master system information from its source 3. cell prior to performing the handover. The enhanced control channel is transmitted Alternative to a standalone operation using in a similar fashion as is unicast data. Therefore, the above procedures, the Lean Carrier can be it can be transmitted in only a few PRB pairs aggregated with a legacy LTE carrier. In that instead of in the entire bandwidth as is the case case mobility as well as acquisition of system for the legacy control channels. The enhanced information is based on the legacy carrier. control channel uses UE-specific reference sig- nals for channel estimation when demodulating DATA TRANSMISSION the control information. Thus, advanced trans- In general, downlink data transmission is sup- mission techniques such as beamforming can ported by cell- or UE-specific reference signals. also be used for control. In addition, frequency y When using cell-specific reference signals, the domain interference coordination can be used to terminal performs channel state and interference manage interference on control transmissions measurements on those reference signals, and from neighboring cells by using different PRBs sends the measurement reports to the base sta- for control in different cells. tion. The base station then transmits data, which On a legacy carrier, due to the necessity for the terminal can demodulate using the cell-spe- common control, the first few symbols that are cific reference signals. used for legacy control channels are not avail- Instead of cell-specific reference signals, UE- able for transmission of data. On the Lean Car- specific reference signals can be used, which are rier, all control transmissions are sent on the only present in subframes and PRB pairs where enhanced control channel, and legacy control data or control is transmitted, as seen in Fig. 3. channels are not used. Hence, all symbols can be Since UE-specific reference signals are only used used for data transmissions (or for the enhanced for demodulation purposes, another type of ref- control channel), as seen in Fig. 3. 3 Exact details of the eSS erence signal is needed specifically for channel On the legacy carrier, information that is are currently being verified measurements. 4 These signals are used by the common to multiple terminals (e.g., system in 3GPP. UE to measure channel state information and information) has to be scheduled by the legacy y report the corresponding information to the net- control channels. For the Lean Carrier, the 4 CSI-RS. IEEE Communications Magazine • February 2013 77C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  5. 5. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Relative fifth percentile throughput Relative fifth percentile throughput Heterogeneous deployment CSO 4dB low load Heterogeneous deployment CSO 4dB high load 2 1.4 1.8 1.2 1.6 1.4 1 1.2 0.8 1 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 Lean carrier LCT: non ideal CRSIC LCT: no CRSIC Lean carrier LCT: non ideal CRSIC LCT: no CRSIC 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- o out CRS interference cancellation. c enhanced control channel has to be improved so as opposed to being a constant overhead as on that it can multicast common information as the legacy carrier. Another differentiating factor well. is that the scheduling on the legacy carrier type utilizes wideband control channels and the band- OTHER DESIGN CHALLENGES width-flexible enhanced control channel com- Flexibility on the bandwidth capabilities of ter- pared to the Lean Carrier, which only uses the minals may also serve to increase spectrum flexi- enhanced control channel. The drawback with bility for operators in the future, which was one utilizing the legacy control channel is that the of the scenarios described earlier. However, overhead scales with increased scheduling load addressing lower-bandwidth (legacy) terminals with a large granularity (using an integer number on a higher-bandwidth carrier may pose some of OFDM symbols as described earlier). In con- challenges on both base station and terminal trast, the overhead of the enhanced control design. For instance, a lower-bandwidth legacy channel scales more directly with the transmitted terminal that normally expects a guard band on scheduling messages. either edge of its carrier may now have different Increased energy efficiency at the network signals within the guard band because a Lean side is enabled by the base station applying micro Carrier is contiguously aggregated with the lega- sleep operations, wherein if the base station does cy carrier to extend the usable bandwidth. While not transmit anything, it is able to reduce the this may not be a difficult problem for terminal power consumption within its radio and digital design, it is still a new aspect that needs to be units. The typical quiet period can be on the taken into account when designing terminals for order of one or several OFDM symbols. On the the Lean Carrier. The base station scheduler Lean Carrier the quiet period can be up to four must keep track of the different bandwidth capa- subframes long since this is the maximum period bilities of different terminals and the spectrum without any mandatory transmissions. In order to being accessed by each one of them. Such chal- measure network energy savings, several compa- lenges will need to be taken into consideration nies promoted a model based on the framework k in the development of the Lean Carrier. from the EU project EARTH [6]. The models defined within EARTH were adjusted to fit the scenarios studied in 3GPP [7]. In addition to the PERFORMANCE above micro sleep, during Release 12, 3GPP will The goals of the Lean Carrier, as described ear- study energy savings that may be achieved by lier, are as follows: having even longer quiet periods (e.g., up to • Improved spectral efficiency approximately 1 s). One potential use case is the • Improved network energy efficiency small cell scenario described earlier, where low- • Improved support of heterogeneous deploy- power nodes can be quiet while the macro tem- ments porarily serves the terminals. • Improved spectrum flexibility support Improvements for heterogeneous deploy- Improved spectral efficiency is achieved main- ments, as described earlier, mainly target deploy- ly in low and medium load scenarios on the ments utilizing a cell selection offset (CSO), Lean Carrier compared to the legacy carrier type where the cell area covered by a low-power node because the signals that are mandatory to trans- is expanded in order to serve a larger percentage mit from the base station have been reduced on of the total traffic from the low-power node. The the Lean Carrier. An example is that the cell- approach to achieve this on the Lean Carrier is specific reference signals defined for the legacy twofold. First, the Lean Carrier relies completely y carrier on the Lean Carrier are replaced by the on enhanced control channels, which allows the eSS, which needs to be transmitted only in every macro node to protect the control in the low- fifth subframe. The downlink transmissions on power node by avoiding high interference on the the Lean Carrier are instead based on the use of PRB pairs used for control. This can be done by y UE-specific reference signals that are only pre- blanking out these PRB pairs or transmitting sent within assigned downlink data transmis- with a lower power. This is in contrast to the sions. The UE-specific reference signal overhead approach of almost blank subframes defined in scales with the amount of data being transmitted LTE Release 10, wherein the macro node blanks 78 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  6. 6. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Energy savings in macro compared to Rel-10 carrier Energy savings in low power node compared to Rel-10 carrier 25 12 10 20 Energy savings (percent) Energy savings (percent) 8 15 6 10 4 5 2 0 0 0 Low load High load 0 Low load High load System throughput (Mb/s) System throughput (Mb/s) Figure 5. Lean carrier network energy savings. out entire subframes. Second, the Lean Carrier the same deployment as above. Base stations are minimizes the amount of interference caused by assumed to consume power according to the reference signals to low-power nodes. In 3GPP model defined in [6]. The results are separated Release 11, some terminals are able to cancel into macro and low-power nodes. The results are interfering cell-specific reference signals. Instead, presented relative to a deployment with the lega- the Lean Carrier avoids the transmission of cy carrier. 7 Energy savings come from two fac- these cell-specific reference signals, which allows tors: First and most important, due to fewer simpler terminal implementation and a poten- mandatory transmissions on the Lean Carrier, tially larger cell selection offset for the low- base stations can go into micro sleep more fre- power node. quently. Second, since the throughput is higher Initial performance evaluation in a heteroge- on the Lean Carrier, the terminal buffers empty y neous network deployment is presented in Fig. quicker, allowing base stations to be quiet over 4. The evaluation assumptions are based on the longer time periods. Furthermore, energy sav- assumptions defined within 3GPP [8]. Briefly, ings are larger for macro nodes than for low- the evaluations are performed with a non-full power nodes since the total transmission power buffer traffic model in a high and low load sce- is larger in the macro. Additionally, the fraction nario,5 where four low-power nodes are deployed of power used for radio transmissions is greater per macro node. Evaluations are performed with in a macro node than in a low-power node. a cell selection offset of 4 dB with the low-power node and the macro nodes being deployed on 5 The low load scenario the same frequency. The results are presented CONCLUSIONS corresponds to 50 for the 5th percentile user, which corresponds to This article introduces the Lean Carrier, one of f Mb/s/km2 and the high terminals operating on the cell edge. the key components of LTE Release 12. By min- load scenario corresponds In the evaluations, the 5th percentile user imizing control channel and reference signal to 500 Mb/s/km2. throughput is compared between a network overhead, the Lean Carrier can increase resource using the Lean Carrier and a network using the utilization and reduce interference, thereby y 6 Cell-specific reference legacy carrier. For the legacy carrier, two cases increasing spectral efficiency. It can increase signal interference cancel- are studied. In the first scenario, it is assumed spectrum flexibility and reduce energy consump- lation is a feature intro- that terminals can cancel the cell-specific refer- tion. One of the primary use cases is a small cell duced in LTE Rel-11. ence signals from the strongest interfering cell.6 scenario, where complementary low-power nodes In the second scenario, terminals are standard provide very high capacity and enhanced user 7 It is noted that a legacy Release 8, incapable of cancellation. data rates locally while macro nodes provide carrier can be made more Figure 4 shows that the Lean Carrier has an wide-area coverage. Technical design aspects energy-efficient by approximately 70 percent throughput gain at the such as mobility support, synchronization, as well employing multicast cell edge compared to a legacy carrier with lega- as transmission of data and control are highlight- broadcast single-frequency cy Release 8 terminals with low load. At high ed, and design challenges are discussed. A per- network (MBSFN) sub- load there is still a 20 percent gain over a legacy formance evaluation quantifies the benefits: the frames, which have lower carrier with a standard Release 8 terminal. The Lean Carrier can provide up to 70 percent cell cell-specific reference sig- Lean Carrier also performs significantly better edge user throughput gain in heterogeneous nal overhead. However, than a legacy carrier serving terminals that are deployments, and it can reduce macro node the Lean Carrier is more cancelling the cell-specific reference signals from energy consumption by approximately 20 percent energy-efficient than a the strongest interfering cell. The reason for this at low loads. legacy carrier even when is that after cancelling interference from the sin- MBSFN subframes are gle largest cell, there are many more interfering REFERENCES employed largely due to a cells left, for which the cell-specific reference [1] E. Dahlman, S. Parkvall, and J. Sköld, 4G: LTE/LTE- further 40 percent reduc- signals are not cancelled. Advanced for Mobile Broadband: LTE/LTE-Advanced for r tion in reference signal Figure 5 shows energy savings achievable in Mobile Broadband, Academic Press, Apr. 2011. overhead at lower loads. IEEE Communications Magazine • February 2013 79C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  7. 7. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® [2] [ ] Ericsson, “Ericsson Mobility Report,” Nov. 2012, area of topics such as mobility, positioning, and carrier f http://www.ericsson.com/res/docs/2012/ericsson-mobili- ______________________________ aggregation. Recently, he became the 3GPP Rapporteur for ty-report-november-2012.pdf. ________________ the work item on the New Carrier Type. [3] D. Astely et al., “LTE Release 12 and Beyond,” IEEE Commun. Mag., to be published H A V I S H K O O R A P A T Y (Havish.Koorapaty@ericsson.com) ____________________ [4] Ericsson: 3GPP LTE Work Item Description “New Carrier received his B.S., M.S., and Ph.D. degrees from North Car- Type for LTE,”3GPP RP-121415, http://www.3gpp.org/ olina State University in 1991, 1993, and 1996, respective- ftp/tsg_ran/TSG_RAN/TSGR_57/Docs/RP-121415.zip. ___________________________ ly. He has been with Ericsson Research since 1996, where [5] 3GPP, TR 36.932, Scenarios and Requirements for Small he has worked on various topics in the area of cellular and Cell Enhancement for E-UTRA and E-UTRAN satellite communications including error control coding, [6] EARTH project, www.ict-earth.eu. location determination and tracking, phone systems engi- [7] R1-114336, Base Station Power Model, NTT DOCOMO, neering, 4G broadband wireless system design, and wire- Alcatel-Lucent, Alcatel-Lucent Shanghai Bell, Ericsson, less backhaul solutions. Recently, he has worked on Telecom Italia. enhancements to LTE and participated in standardization [8] R1-112856, Summary of Ad Hoc Session on FeICIC Sim- efforts in 3GPP. ulation Assumptions, NTT DOCOMO. J UNG -F U (Thomas) Cheng (thomas.cheng@ericsson.com) _________________ BIOGRAPHIES is with Ericsson Research in Ericsson Silicon Valley, where he drives research and development of advanced C HRISTIAN H OYMANN (christian.hoymann@ericsson.com) ____________________ wireless communication technologies. His research inter- received his Diploma degree in electrical engineering from ests include iterative processing, coding and decoding RWTH Aachen University in 2002. At RWTH’s Chair of Com- techniques, signal processing algorithms for wireless munication Networks he worked toward his doctoral communications, and application of information theory degree, which he received in 2008. Since 2007 he is work- to wireless system design. He has contributed to the ing at Ericsson Research, where he focuses on advancing evolution of cellular wireless PHY and MAC layer evolu- 3GPP LTE, especially in the areas of relaying, CoMP, and tion from 2.5G EDGE to 3G HSPA to 4G LTE. He was the heterogeneous networks. Currently, he is technical coordi- principal contributor to the LTE turbo and convolutional nator of Ericsson’s RAN1 delegation. coding and rate matching specifications. He received his B.S. and M.S. degrees in electrical engineering from DANIEL LARSSON (daniel.n.larsson@ericsson.com) received his ________________ National Taiwan University, Taipei. He received his Ph.D. M.S.E.E degree from the Royal Institute of Technology in degree in electrical engineering with additional study in 2006. He joined Ericsson in 2007, for which he has attend- social science from California Institute of Technology, ed standardization meetings since 2008 covering a broad Pasadena. 80 IEEE Communications Magazine • February 2013C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®