Chapter 8 Looking Towards 4G: LTE-Advanced

Chapter 8

567LTE and the Evolution to 4G Wireless: Design and Measurement Challenges, Second Edition, Edited by Moray Rumney.Copyright Agilent Technologies, Inc. 2013. Published by John Wiley & Sons, Ltd.

Looking Towards 4G: LTE-Advanced

8.1 Summary of Release 8The baseline LTE radio access network (RAN) and evolved packet core (EPC) network were defined in 3GPP Release 8, which was functionally frozen in December 2008. This provided the world with a comprehensive and highly capable new cellular communications standard that, according to a November 2012 Global Suppliers Association report, has been successfully launched in 113 commercial networks in 51 countries. The main attributes that differentiate this new standard from previous standards are the following:

• Single-channelpeakdataratesofupto300Mbpsinthedownlinkand75Mbpsintheuplink• Improvedspectralefficiencyoverlegacysystems,particularlyfortheuplink• FullintegrationofFDDandTDDaccessmodes• Packet-basedEPCnetworktoeliminatecostandcomplexityassociatedwithlegacycircuit-switchednetworks.

Some of the key technologies introduced in Release 8 that enable the new capabilities include:• AdoptionofOFDMAandSC-FDMAforthedownlinkanduplinkairinterfacestoenablenarrowbandscheduling

and efficient support of spatial multiplexing• Supportforsixchannelbandwidthsfrom1.4MHzto20MHztoenablehighdataratesandalsoefficientspectrum

re-farming for narrowband legacy systems• Baselinesupportforspatialmultiplexing(MIMO)ofuptofourlayersonthedownlink• Fasterphysicallayercontrolmechanismsleadingtolowerlatency.

Despite the substantial capabilities of LTE in Release 8, the 3GPP standard has continued to evolve through Releases 9, 10, 11, and now 12. The following sections summarize important additions to the LTE specifications that have been made since the first edition of this book. These include the most significant changes to the 3GPP standard, which occurred in Release 10 for the support of LTE-Advanced, 3GPP’s submission to the ITU-R IMT-Advanced (4G) program. Full information about 3GPP releases can be found at www.3gpp.org/Releases. The most useful summary of the work items for each release can be found at ftp.3gpp.org/Information/WORK_PLAN/Description_Releases. This web page includes a short explanation of each work item. A complete list of all work items from Release 4 through Release 12 (nearly 4,000 items) can be found at ftp.3gpp.org/Specs/html-info/WI-List.htm. The list provides links to the specification documents impacted by each work item plus the specific change requests.

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CHAPTER 8 | Looking Towards 4G: LTE-Advanced

8.2 Release 9Release 9 was considered a “short” release in that it came between the major effort required to finish Release 8 and the definition of Release 10. The work on Release 9 was done with the knowledge that significant changes were due in Release 10 as part of the plans for LTE-Advanced. Some of the items in Release 9 were carryovers from Release 8 that had not yet been completed; others were new items not in the original Release 8 definition. At a formal level, Release 9 included over 80 identifiable features. Since it is not within the scope of this book to go through each Release 9 feature individually, a few of the key items have been selected here for further explanation, with a focus on the radio aspects.

8.2.1 New Frequency BandsEvery release introduces new frequency bands. As discussed in Section 2.1.1 and defined in 36.307 [1], new frequency bands are specified independent of release. This is a pragmatic approach to managing the evolving specifications such that bands defined in later releases can be applied to an earlier release without the need to modify that release. Within Release 9 four FDD bands were added as shown in Table 8.2-1.

Table 8.2-1. Frequency bands added during Release 9

Band number

Uplink Downlink Bandwidth Duplex spacing Gap Duplex

modeLow High Low High18 815 830 860 875 15 45 30 FDD19 830 845 875 890 15 45 30 FDD20 832 862 791 821 30 -41 11 FDD21 1447.9 1462.9 1495.9 1510.9 15 48 33 FDD

Bands 18 and 19 are referred to as the extended LTE 800 bands and were specified for use in Japan. The background can be found in 36.800 [2]. Band 20 was added for the so-called “digital dividend” spectrum within Europe made available through the switchover to digital television. Note that the uplink and downlink frequencies are reversed from the usual arrangement. The background to band 20 can be found in 36.810 [3]. The final new band is the extended LTE 1500 band in Japan. The background can be found in 36.821 [4].

8.2.2 Home Base StationWork on femtocell inclusion in UMTS was ongoing during Release 8, and this work continued in Release 9 for the home base station (home BS, also known as home eNB or femtocell). The femtocell concept is not unique to LTE or LTE-Advanced, but there was an opportunity for LTE to incorporate this technology from the start rather than retrospectively designing it into legacy systems such as UMTS and GSM. Figure 8.2-1 shows the topology of a femtocell deployment.

From a radio deployment perspective the femtocell operates over a small area within a larger cell. The radio channel can be the same as that of the larger cell (known as co-channel deployment) or on a dedicated channel. The femtocell concept is fundamentally different from relaying (covered in 8.3.3.4) since the femtocell connection back into the core network is provided locally by an existing wired internet connection rather than over the air back to the macrocell. Most femtocell deployments will

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be indoors, which helps provide isolation between the femtocell and the macrocell. Also shown in Figure 8.2-1 is a femtocell outside the coverage area. This could be, for example, a way to provide local cellular coverage in rural areas where DSL exists but there is no cellular coverage provided by the operator.

Figure 8.2-1. Femtocell (Home eNB)

Although the name femtocell suggests that the major difference from existing systems is one of coverage area, the defining attributes of femtocells are actually far more significant than coverage area alone. Table 8.2.2 compares the main attributes of traditional macro-, micro- and picocell systems with those of femtocells.

Table 8.2-2. Traditional cellular versus femtocellular technology

Attribute Traditional cellular FemtocellularInfrastructure cost $10,000–100,000 $100–200

Infrastructure finance Operator End userBackhaul Expensive leased E1/T1 lines Existing end-user DSL or cable broadbandPlanning Operator End-user (no central planning)

Deployment Operator truck roll End user one touch provisioningQuality of Service (QoS) Operator controlled Best effort

Control Operator via O&M Operator via InternetMobility Good/excellent Nomadic/best effort

Data throughput Limited Excellent

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The two main deployment scenarios for femtocells are as follows:• Inruralareaswithpoorornocoverage,probablyusingco-channeldeployment• Indenseareastoprovidehighdataratesandcapacity(primarilyindoors).

In both cases it must be decided whether the femtocell will be operated for closed subscriber group (CSG) UE or for open access. This along with other practical considerations such as pricing can be considered commercial issues, although in the co-channel CSG case (see 8.3.4.1), the probability that areas of dense femtocell deployment will block macrocells becomes an issue.

Although the potential gains from femtocells are substantial, many challenges remain:

• Needforcognitivemethodstoreduceinterferencetothemacronetwork• Needforradioresourcemanagementrequirements• Securityconcernsfrommakingbasestationtechnologywidelyavailable—includingbackhaulprotection,device

authentication, and user authentication• Verificationofgeographiclocationandroamingaspects• Businessmodelsofopenversusclosedaccess• Noobvioussolutionyetforcross-networkfemtocells—anyonefemtocellcansupportonlyonenetworkoperator.

Therefore, household members can only share a single femtocell if they all choose the same network operator.• Netneutrality—whoownsthebackhaul?Theanswerwillvarybycountry.• Possiblepublicsafetyconcerns• Needforoptimizedandbalanced interworkingbetweenmacro-andfemtocells tominimizeunnecessary

handovers (ping pong)• Potentialbottleneckoverfixedbroadbandbackhaul(suchasDSLorcable)connection,especiallyontheuplinkfor

services requiring symmetric bandwidths, prioritization, and congestion management• QoScontrolforreal-timeservicesandapplicationsrequiringguaranteedbitrates,suchasvoice,withalltheother

traffic types on the broadband access network• AccesscontrolprovidingCSGlocalandroamingaccess• Self-configuration(plugandplay),self-organization,andself-optimizationincludingfaultmanagementandfailure

recovery (self-healing).

Despite these issues, studies have shown that increases in average data rates and 100 times greater capacity are possible with femtocells over what can be achieved from the macro network. On the other hand, femtocells do not provide the mobility of macrocellular systems, and differences exist in the use models of these systems, as shown in Table 8.2-3.

Table 8.2-3. Comparison of macro- and microcellular with femtocellular use models

Macro-/microcellular Femtocell/hotspotUbiquitous mobile data and voice Opportunistic nomadic dataMobility and continuous coverage Hotspot coverage

Ability to control QoS Limited QoS for lower value dataLimited capacity and data rates Distributed cost (not low cost)

High costs, acceptable for high value traffic Free or chargedOften outdoors and moving Sitting down indoors

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For these reasons, femtocellular and hotspot deployments should be considered as complementary rather than competitive with the macro-/microcellular systems.

The background to the home BS Release 9 work can be found in 36.921 [5] for FDD and 36.922 [6] for TDD. The work had two objectives: first, to complete the RF specifications in 36.104 [7] for the introduction of the home BS class and second, to introduce features in the home BS and network that enable control of the home BS output power, in order to mitigate interference to the macro network or other home BS. A number of relaxations to the RF specifications were introduced, not least in importance the maximum output power, which is limited to 20 dBm and lower in some scenarios. The expected low UE speeds in home BS deployments enabled a five times looser requirement for frequency error and there are various other relaxations for spurious emissions.

However, to enable effective interference mitigation, the home BS must be able to measure the signal strength of other base stations in the neighborhood. Downlink measurement is not an issue for TDD, but for FDD a downlink measurement function is required in the home BS although some measurements may also be gathered from UEs connected to the home BS.

The need for interference mitigation is most important when the home BS is deployed in a co-channel closed subscriber group. In this mode the home BS is deployed on the same frequency as the macro network and access to the home BS is restricted to a closed group of users. In this environment, UEs not part of the closed group would likely experience a loss of coverage when close to the home BS whereas UEs that are part of the closed group would hand over to the home BS. For this reason it is important to limit the potential for the home BS to interfere with the macro network when the home BS is operated in a co-channel CSG mode. The general term applied to this form of interference mitigation is inter-cell interference coordination (ICIC). Interference mitigation work continues in Release 10 with enhanced ICIC or eICIC and in Release 11 further enhanced ICIC (FeICIC) is introduced. These developments are described later.

8.2.3 Multimedia Broadcast Multicast Service (MBMS)The MBMS television service was specified at the physical layer in Release 8 but was not functionally complete until Release 9. The features in Release 9 provide a basic MBMS service carried over an MBMS single frequency network (MBSFN).

In Release 9 only the guaranteed bit rate (GBR) bearers were specified, which means that the maximum bit rate (MBR) is always equal to the GBR. This is not good for variable bit rate services which, by exploiting statistical multiplexing, would otherwise allow the MBR to exceed the GBR.

Another limitation of the Release 9 definition was the lack of a feedback mechanism from the UEs that would inform the network if sufficient UEs were present in the target area to justify turning on the MBSFN locally.

In Release 11 further MBMS enhancements for service continuity were specified including support on multiple frequencies, reception during RRC idle and RRC connected states, and support to take UE positioning into account for further optimization of the received services.

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8.2.4 Positioning Support Positioning support work included specifications for support of the Assisted Global Navigation Satellite System (AGNSS) in 36.171 [8]. The GNSS incorporates the following satellite positioning systems:

• Galileo• GlobalPositioningSystem(GPS)andmodernizedGPS• GLObal’nayaNAvigatsionnayaSputnikovayaSistema(GLONASS)• Quazi-ZenithSatelliteSystem• SpaceBasedAugmentationSystem(SBAS).

The LTE physical layer was augmented to support the observed time difference of arrival (OTDOA) positioning scheme with the introduction of the positioning reference signal (PRS). See Section 3.2.12.3. Network-based positioning for LTE was included in Release 11 with a further study item in Release 12 on positioning based on RF pattern matching.

8.2.5 RF Requirements for Multicarrier and Multi-RAT Base StationThis enhancement is better known as multi-standard radio (MSR). It is introduced in Section 2.1.10 with design and test aspects covered in Section 6.4.7. The work was continued in Release 10 with non-contiguous (inter-band) MSR and in Release 11 with the specification of a medium-range and local area MSR base station classes.

8.2.6 RF Requirements for Local Area Base StationsThe local area BS (picocell), along with the home BS (femtocell), is another important introduction to the LTE specifications. The local area BS enables the deployment of a heterogeneous network comprising macro- (wide area BS), pico-, and femtocells. The RF requirements for local area base stations are based on a reduced UE-to-BS coupling loss of 45 dB compared to the 70 dB used for macrocells. This allows for a lower maximum output power requirement of 24 dBm and other relaxations such as for frequency error and unwanted emissions consistent with small cell deployment.

8.2.7 Enhanced Dual-Layer TransmissionRelease 8 specified seven downlink transmission modes (TMs). Transmission mode 7 (TM7) introduced the concept of UE-specific reference symbols (RS), described in Section 2.4.4.7, which enabled non-codebook precoding of the physical downlink shared channel (PDSCH) for single layer transmission. In Release 9 TM8 was introduced, which adds dual-layer transmission to TM7. See Section 2.4.4.8 for further details.

8.2.8 Self Organizing Networks (SON)Today’s cellular systems are very much centrally planned and the addition of new nodes to the network involves expensive and time-consuming work, site visits for optimization, etc. The background to SON can be found in 32.500 [9].

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This technical report identified a number of use cases in which SON could be applied:• Automationofneighborrelationlists intheE-UTRANandUTRANandbetweendifferent3GPPradioaccess

technologies • Self-establishmentofaneweNBinthenetwork• Self-configurationandself-healingoftheBS• Automatedcoverageandcapacityoptimization• Optimizationofparametersaffectedbytroubleshooting• Continuousoptimizationtoaccommodatedynamicchangesinthenetwork• Automatedhandoveroptimization• OptimizationofQoS-relatedradioparameters.

The use cases are further elaborated in 36.902 [10]. Release 8 introduced a basic version of SON including automatic neighbor relations (ANR) list management and self-establishment of new base stations.

In Release 9 SON was extended to include the following operation and maintenance features:• Loadbalancing• Handoverparameteroptimization.

The SON work was continued in Release 10 with specification of the management aspects for the following:• Interferencecontrol• Capacityandcoverageoptimization• Randomaccesschannel(RACH)optimization.

The concept of self-healing was also developed in Release 10. This feature involves the detection and, analysis of network faults and identification of the corrective action required of the network to respond to disruptive events with minimal manual intervention.

8.3 Release 10 and LTE-AdvancedRelease 10 developed the 3GPP proposal for the International Telecommunications Union Radiocommunication Sector (ITU-R) International Mobile Telecommunications Advanced (IMT-Advanced) program. This program is often referred to as “4G” although that term in not formally defined by the ITU or any other body. Due to the involvement of ITU-R in setting the requirements for Release 10, the specification process was more complicated than for previous or subsequent releases:

• ITU-RdefinedtherequirementsforIMT-AdvancedinITU-RM[IMT-TECH][11].• 3GPPdefinedrequirementsforLTE-Advancedin36.913[12]tomeetorexceedtheITU-Rrequirementsin[11].• 3GPPundertookafeasibilitystudy36.912[13]thatproposedLTE-AdvancedasanIMT-Advancedcandidate

technology.• 3GPPthencreatedworkitemstodevelopthemanydetailedspecificationinRelease10todefineLTE-Advanced.

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To explain the evolution to IMT-Advanced it is helpful to remember what came before. The term “third generation” (3G) has been widely and consistently used to describe the ITU-R’s IMT-2000 cellular communications project. The requirements for IMT-2000, defined in 1997, were quite simple, being expressed in terms of peak user data rates:

• 2048kbpsforindooroffice• 384kbpsforoutdoortoindoorandpedestrian• 144kbpsforvehicular• 9.6kbpsforsatellite.

Early 3G systems, of which there were five, did not immediately meet the high peak data rate targets in practical deployment although they did in theory. However, later improvements to the standards brought deployed systems closer to and even beyond the original 3G targets. From a 3GPP perspective, the addition of high speed downlink packet access (HSDPA) to UMTS ushered in the informally named 3.5G, and the subsequent addition of the enhanced dedicated channel (E-DCH), better known as high speed uplink packet access (HSUPA), completed 3.5G. The combination of HSDPA and HSUPA is now referred to as high speed packet access (HSPA).

At the time the IEEE 802.16e standard (Mobile WiMAX) was being developed, and later 3GPP’s LTE/SAE, the ITU-R framework for IMT-Advanced (at the time informally referred to as 4G) was not in place. For this reason the term 3.9G was widely used to describe the first release of LTE and sometimes 802.16e with the expectation of their evolving towards official “4G” status in due course. However, 802.16e was also described by some as 4G and more recently 4G has been used to describe the evolution of UMTS HSPA. This inconsistent application of the term 4G has effectively devalued its meaning to that of a marketing term used to describe anything new and fast. It is therefore always more accurate to use the term IMT-Advanced when referring to the ITU-R “4G” program.

The formal definition of IMT-Advanced was developed by Working Party 5D of the ITU-R. A timeline of the program and the parallel 3GPP activities for LTE-Advanced is shown in Figure 8.3-1.

Figure 8.3-1. Overall IMT-Advanced and LTE-Advanced timeline

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In naming its IMT-Advanced program, the ITU consciously reused the “IMT” (International Mobile Telecommunications) from the IMT-2000 program. This naming is significant because it was agreed that spectrum currently allocated for exclusive use by IMT-2000 technologies will now be known as just “IMT” spectrum and will be available to any approved IMT-Advanced technology. At the 2007 World Radio Conference (WRC-07), new IMT spectrum was identified in the following bands: 450 MHz, 698–960 MHz, 2.3 GHz, and 3.4–4.2 GHz. Crucially, no plans for exclusive IMT-Advanced spectrum were proposed. This is pragmatic since spectrum is scarce and largely occupied. Also noteworthy was the addition in 2008 of 802.16e to the list of approved IMT-2000 technologies. This addition opened up the entire IMT spectrum to access by 802.16e prior to the possibility of 802.16m gaining the same access via the IMT-Advanced route. 802.16m is a planned enhancement to 802.16e, and it was submitted to the ITU-R as a candidate IMT-Advanced technology known as Wireless Mobile Area Network Advanced (Wireless MAN-Advanced).

After considering the attributes of the candidate technologies, the ITU in January 2012 formally approved both LTE-Advanced and Wireless MAN Advanced as meeting the requirements of the IMT-Advanced program. In the period between formal submission and acceptance, 3GPP developed the specifications for LTE-Advanced in Release 10, which were functionally frozen in March 2011.

It is worth pointing out that both of the approved IMT-Advanced technologies are based heavily on pre-existing standards and the modifications that were required of these technologies to meet IMT-Advanced requirements are not considered major, in particular for LTE Release 8, which already met most of the IMT-Advanced requirements.

8.3.1 IMT-Advanced and LTE-Advanced High Level RequirementsThe high level requirements for IMT-Advanced defined by ITU-R in [11] are the following:

• Ahighdegreeofcommonfunctionalityworldwidewhileretainingtheflexibilitytosupportawiderangeoflocalservices and applications in a cost efficient manner

• CompatibilityofserviceswithinIMTandwithfixednetworks• Capabilityofinterworkingwithotherradioaccesssystems• Highqualitymobileservices• Userequipmentsuitableforworldwideuse• User-friendlyapplications,services,andequipment• Worldwideroamingcapability• Enhanceddownlinkpeakdataratestosupportadvancedservicesandapplications(100Mbpsforhighmobility

and 1 Gbps for low mobility were established as targets for research).

The first seven of the eight requirements are “soft” and are largely being pursued by the industry already. However, the eighth requirement, for 100 Mbps high mobility and 1 Gbps low mobility, is quite a different matter and has fundamental repercussions on system design. The 1 Gbps peak target for IMT-Advanced is akin to the 2 Mbps target for IMT-2000 set some ten years earlier. Like its predecessor, the 1 Gbps peak figure is not without qualification since it applies only for low mobility in excellent radio conditions and may require up to 100 MHz of spectrum. Nevertheless, if publicity focuses on the peak rates without taking account of the caveats, the expectations for IMT-Advanced may outstrip practical reality for what could be a long time.

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The work by 3GPP to define a candidate radio interface technology (RIT) started in Release 9 with a study phase for LTE-Advanced. The requirements for LTE-Advanced have been captured in 36.913 “Requirements for Further Advancements for E-UTRA (LTE-Advanced)” [12]. These requirements are defined based on the ITU-R requirements for IMT-Advanced as well as on 3GPP operators’ own requirements for advancing LTE. Key elements include the following:

• ContinualimprovementtotheLTEradiotechnologyandarchitecture• Scenariosandperformancerequirementsforinterworkingwithlegacyradioaccesstechnologies(RATs)• BackwardcompatibilityofLTE-AdvancedwithLTE;i.e.,anLTEterminalcanworkinanLTE-Advancednetwork,

and an LTE-Advanced terminal can work in an LTE network. Any exceptions will be considered by 3GPP.• AccounttobetakenofrecentWRC-07decisionsfornewIMTspectrumaswellasexistingfrequencybandsto

ensure that LTE-Advanced accommodates geographically available spectrum for channel allocations above 20 MHz. Also, requirements must recognize those parts of the world in which wideband channels will not be available.

8.3.2 IMT-Advanced and LTE-Advanced Detailed RequirementsWhen IMT-2000 was defined, the only requirements were for peak data rates with no targets for latency or for the more important average or cell-edge performance, which defines the experience for the typical user. Fortunately, this requirement gap has been eliminated with IMT-Advanced, which specifies a much broader range of performance. The ITU-R requirements, specified in [11], were used by 3GPP along with operator requirements to develop TR 36.913 [12], which defines detailed requirements for LTE-Advanced in the following areas:

• Peakdatarate:1Gbpsdownlink,500Mbpsuplink• Latency

− Control plane: idle to connected < 50 ms, un-sync to in-sync < 10 ms (see Figure 8.3-2)− User plane: Improvements over Release 8 for with and without scheduling assignment

• Spectralefficiency− Peak spectral efficiency—see Table 8.3.1− Average spectral efficiency—see Table 8.3.1− Cell-edge user data throughput—see Table 8.3.1− VoIP capacity

• Mobility− Support for up to 350 km/h and for some frequency bands 500 km/h − Enhanced performance for 0–10 km/h over Release 8 with no degradation and preferred enhancement for

higher speeds• FurtherenhancementstoMBMS—improvedrequirementsforspectrumefficiencyoverRelease8.

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Figure 8.3-2. Requirements for state transitions (36.913 [12] Figure 7.1)

The link between LTE’s targets and the retrospective performance derived for UMTS Release 6 is given in Chapter 1 of this book in Tables 1.4-3 and 1.4-4. Looking forward, the relationship between a few of the LTE targets and those for LTE-Advanced and IMT-Advanced is given in Table 8.3-1. The cell and cell-edge spectral efficiency figures are for inter-site distance (ISD) of 500 m.

Table 8.3-1. LTE, LTE-Advanced and IMT-Advanced spectral efficiency performance targets

Item Sub-category LTE (Release 8) target [14] LTE-Advanced target [12] IMT-Advanced requirement [11]

Peak spectral efficiency (b/s/Hz)

Downlink 16.3 (4x4 MIMO) 30 (8x8 MIMO or less) 15 (4x4 MIMO)Uplink 4.32 (64QAM SISO) 15 (4x4 MIMO or less) 6.75 (2x4 MIMO)

Downlink cell spectral efficiency b/s/Hz/user Microcellular 3 km/h,

500 m ISD

(2x2 MIMO) 1.69 2.4

2.6(4x2 MIMO) 1.87 2.6

(4x4 MIMO) 2.67 3.7Uplink cell spectral

efficiency b/s/Hz/user Microcellular 3 km/h,

500 m ISD

(1x2 MIMO) 1.21.8

(2x4 MIMO) 2.0

Downlink cell-edge user spectral efficiency (b/s/Hz/user), (5 percentile,

10 users), 500m ISD

(2x2 MIMO) 0.05 0.07

0.075(4x2 MIMO) 0.06 0.09

(4x4 MIMO) 0.08 0.12Uplink cell-edge user

spectral efficiency (b/s/Hz/user), (5 percentile,

10 users), 500m ISD

(1x2 MIMO) 0.040.05

(2x4 MIMO) 0.07

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The first point of note is that the peak efficiency targets for LTE-Advanced are substantially higher than the IMT-Advanced requirements—thus the desire to drive up peak performance is maintained despite the average targets and requirements being very similar. However, 36.913 [12] states: “The target for average spectrum efficiency and the cell edge user throughput efficiency should be given a higher priority than the target for peak spectrum efficiency and VoIP capacity.” Another point of note is that with the exception of uplink spectral efficiency, LTE Release 8 meets the requirements for IMT-Advanced. The next section will discuss the challenge of raising the average and cell edge performance.

Improving the Average and Cell Edge Spectral Efficiency

As discussed in Chapter 1, the LTE targets for average and cell-edge spectrum efficiency are based on 2x to 4x improvements to Release 6 HSPA. The reference HSPA configuration is receive diversity with no equalizer for the downlink and a single transmitter for the uplink (25.913 [15] Subclause 7.1). This reference configuration was analyzed during the LTE study phase to provide an average downlink cell spectral efficiency of around 0.53 b/s/Hz/cell using a 500 m ISD [14].

Increasing peak data rates by using more spectrum or higher-order modulation in a good radio environment is a well-understood process that has been in use for years. However, improving the targets for average and cell-edge performance is a much harder task due to radio propagation and interference issues, which are independent of the air interface technology. Many of the UMTS enhancements in Release 7 and Release 8 as well as Release 8 LTE have addressed the challenge of increasing average efficiency. LTE-Advanced takes this challenge to the next level.

The underlying problem of interference is illustrated in Figure 8.3-3, which shows a cumulative distribution function plot of the geometry factor within a typical urban cell.

Figure 8.3-3. Geometry factor distribution in a typical urban cell with frequency reuse 1

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The geometry factor is the term used in UMTS to indicate the ratio of the wanted signal to the interference plus noise. It is equivalent to the signal to interference plus noise ratio (SINR). From the figure it can be seen that 10% of users experience a better than 15 dB geometry factor but 50% of users experience worse than 5 dB. The exact shape of the curve varies significantly depending primarily on the frequency reuse factor followed by the cell size and cell loading. An isolated cell (e.g., a hotspot) would exhibit a shift to the right, indicating that most users are experiencing very good signal conditions. A cell in an urban area with significant co-channel inter-cell interference would shift to the left. Penetration loss through buildings, as experienced when indoor coverage is provided from an external cell, would also cause a shift to the left. However, on the assumption that the deployment in a particular area has resulted in a certain geometry factor distribution, the challenge then becomes how to deal with the interference to improve cell-average and cell-edge performance.

In 2G systems, performance was obtained by means of interference avoidance through the use of high frequency reuse factors of up to 21. In 3G systems the frequency reuse was optimized at 1 and methods such as scrambling and spreading were used to minimize the impact of interference with resulting gains in average spectral efficiency. Later systems employed receive diversity, equalizers, transmit diversity, and limited spatial multiplexing (MIMO). For LTE-Advanced, the planned performance enhancement techniques will take further steps by using more advanced MIMO and beamsteering, interference cancellation, fractional frequency reuse, and other advanced methods.

It is worth explaining how the cell-edge performance targets used by ITU-R and 3GPP were developed based on simulated geometry-factor distributions for the target deployment environments. Ten users were randomly distributed within each cell and the resulting geometry factor was calculated for each UE. This information was converted into a data throughput rate that was in turn used to plot a distribution of throughput. The process was repeated many times in a multi-drop simulation to create a smooth throughput distribution. The cell-edge performance was then defined as the fifth percentile of the throughput distribution. Because the simulation was carried out using groups of 10 UEs, the units are b/s/Hz/user and therefore appear to be 10 times lower than might be expected. It is not straightforward to take the cell-edge figure per user and multiply by 10 to predict the cell-edge average since the distribution is complicated by the type of scheduler used. The scheduler may have allocated more resources to the cell-edge users in a proportionally fair system. Leaving this complication aside, it can be seen that the cell-edge performance is about one third that of the average of the cell.

8.3.3 Release 10 LTE-Advanced EnhancementsAs first noted in 8.3.1, the LTE-Advanced submission to ITU-R was made in September 2009 in 36.912 “Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)” [13]. This document outlines those features identified for development in Release 10 relevant for the IMT-Advanced requirements. This subset of Release 10 is what is meant by the term LTE-Advanced. The original key features of LTE-Advanced proposals were the following:

• Supportofwiderbandwidths• Uplinktransmissionscheme• Downlinktransmissionscheme• Coordinatedmulti-pointtransmissionandreception(CoMP)• Relaying.

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Not all the above were essential to meet the IMT-Advanced requirements and not all aspects were subsequently developed in Release 10 (e.g., CoMP, which is a work item in Release 11 and will be covered later in this chapter). There were other areas for development also identified in 36.912 for which details were not elaborated. These included mobility enhancements, radio resource management enhancements, MBMS enhancements, and further work on SON. 36.912 concludes with a self-evaluation that reports how LTE-Advanced meets or exceeds the ITU-R IMT-Advanced requirements. The following sections outline the main functional areas that were developed in Release 10 for LTE-Advanced. These sections are followed by other work items in Release 10 that were not part of the ITU-R submission.

8.3.3.1 Support of Wider Bandwidths: Carrier Aggregation

Support of wider bandwidths is primarily aimed at addressing the IMT-Advanced requirements for peak single user data rates up to 1 Gbps, although there are additional system-level benefits in terms of deployment flexibility and associated trunking gains that come from the availability of a wider instantaneous transmission bandwidth. Today’s spectrum allocations (frequency bands) offer almost no opportunity for finding 100 MHz of contiguous spectrum needed for 1 Gbps peak data rates. Some new IMT spectrum was identified at the World Radio Conference in 2007 (WRC-07), but there are still only a few places where continuous blocks of 100 MHz might be found (e.g., at 2.6 GHz or 3.5 GHz). One possible way of increasing available bandwidths would be to encourage network sharing, which reduces fragmentation caused by splitting one band between several operators. However, sharing the spectrum, as opposed to just the sites and towers, is a considerable step up in difficulty. The ITU-R recognizes the challenge that wide-bandwidth channels present and so expects that the required 100 MHz will be created by the aggregation of non-contiguous channels from different bands in a multi-transceiver mobile device.

The beginnings of such aggregation techniques are already showing up in established technologies—first with EDGE Evolution, for which aggregation of two non-adjacent 200 kHz channels was specified in Release 7, potentially doubling the single-user data rates that are possible with standard EDGE. Along similar lines, there were 3GPP specifications introduced in Release 8 for dual-carrier HSDPA that close the bandwidth gap between 5 MHz UMTS and 20 MHz LTE. Contiguous multicarrier cdma2000 (3xRTT) has also been defined, which avoids the need for multiple transceivers.

Carrier aggregation is clearly not a new idea; however, the proposal to extend aggregation up to 100 MHz in multiple bands presents numerous design challenges, particularly for the UE in terms of additional cost and complexity. At each of the layers in the radio protocol, from the physical layer up through radio resource control (RRC), changes are required for carrier aggregation. An overview of these can be found in 36.912 [13] Section 5. Although it is possible to conceive of applications for 1 Gbps data rates to a single mobile device, the commercial viability has yet to be understood. It should also be noted that carrier aggregation does not fundamentally increase spectral efficiency (or network capacity) per se, although wider channels do offer better trunking efficiency. Taking all these factors into account suggests that 100 MHz multi-transceiver carrier aggregation as a means of delivering extreme single-user peak data rates is not likely being pursued at this time. The scenarios that are being actively studied are two-carrier aggregation for intra-band and for inter-band. Section 2.1.11 provides more details about specific band combinations and Section 6.4.8 discusses test aspects.

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8.3.3.2 Uplink Transmission Scheme

Several enhancements were introduced to the uplink for LTE-Advanced:• Spatialmultiplexinguptofourlayers• Transmitdiversity• ClusteredSC-FDMA• SimultaneousPUCCH/PUSCH.

Spatial Multiplexing and Transmit Diversity

The introduction of spatial multiplexing and transmit diversity to the uplink makes a significant departure from the UE architecture of Release 8 since both enhancements require the support of more than one uplink transmitter. This has implications for cost, space, power handling, and many new spurious emission scenarios that need to be studied and will require new designs. The benefits of spatial multiplexing provide the necessary improvements in spectral efficiency over Release 8 for LTE-Advanced to meet the requirements for IMT-Advanced. This subject is covered more fully in Section 2.4.

Clustered SC-FDMA

The uplink multiple access scheme has been enhanced by adopting clustered discrete Fourier transform spread OFDM (DFT-S-OFDM). This scheme is similar to SC-FDMA but has the advantage that it allows non-contiguous (clustered) groups of subcarriers to be allocated for transmission by a single UE, thus increasing the flexibility available for frequency-selective scheduling. Clustered SC-FDMA was chosen in preference to pure OFDM in order to avoid a large increase in peak-to-average power ratio (PAPR).

Simultaneous PUCCH/PUSCH Transmission

In Release 8 the user data carried on the physical uplink shared channel (PUSCH) and the control data carried on the physical uplink control channel (PUCCH) are time-multiplexed as shown earlier in Figure 3.2.13. It is also possible to multiplex control data with user data on the PUSCH. LTE-Advanced introduces a new mechanism for simultaneous transmission of control and data by allowing the PUSCH and the PUCCH to be transmitted simultaneously. This mechanism has some latency and scheduling advantages over time-multiplexed approaches although it does generate a multicarrier signal within one component carrier of the uplink. Simultaneous PUCCH/PUSCH transmission should not be confused with carrier aggregation, which involves more than one component carrier. Simultaneous PUCCH/PUSCH transmission is known to increase PAPR, which makes it more likely that the power amplifier will create unwanted intermodulation products. This effect is similar to the one described for clustered SC-FDMA in Section 2.3.5.

8.3.3.3 Downlink Transmission Scheme

The enhancement to the downlink for LTE-Advanced include the following:

• Extensionofspatialmultiplexingfromfourtoeightlayers• Enhancementstodownlinkreferencesignals.

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Eight Layer Spatial Multiplexing

The increase in the number of spatial multiplexing layers on the downlink from four to eight may appear to be a symbolic extension to the standard, since performance requirements through Release 11 have existed only for two-layer transmission to a single UE, even though four-layer transmission has been defined since Release 8. The main drawback to the implementation of eight-layer single user spatial multiplexing is not so much at the base station end, where eight-antenna systems already exist, but at the UE receiver, which would require implementation of eight receive antennas per carrier. This proposition is not practical today due mainly to space constraints.

The potential for eight spatial layers does open up new possibilities for multi-user spatial multiplexing (MU-MIMO), which offers new combinations for the simultaneous support of more than one user sharing the eight layers. This is discussed further in Section 2.4.4.9 for transmission mode 9. Also, the potential for eight transmitters at the base station opens up the potential for enhanced transmission using beamforming; for example, in an 8x2 configuration. Eight-antenna beamforming in covered Section 6.9.

Downlink Reference Signals

The UE-specific reference signals are extended to support up to eight layers and a new class of channel state information reference signals (CSI-RS) are introduced. The purpose of the CSI-RS is limited to channel state information reporting of the channel quality indicator (CQI), precoding matrix indicator (PMI), and rank indication (RI). Specifically, the CSI-RS are not used in support of PDSCH demodulation, which is the task of the precoded UE-specific RS and the non-precoded cell RS. The CSI-RS are discussed further in Section 3.2.12.4.

8.3.3.4 Relaying

The concept of relaying is not new but the level of sophistication continues to grow. The most basic relay method is the use of a repeater, which receives, amplifies, and then retransmits the downlink and uplink signals to overcome areas of poor coverage. The repeater could be located at the cell edge or in some other area of poor coverage. Repeaters are relatively simple devices operating purely at the RF level. Typically they receive and retransmit an entire frequency band; therefore, care is needed when repeaters are sited. In general repeaters can improve coverage but do not substantially increase capacity.

More advanced relays can in principle decode transmissions before retransmitting them. This gives the ability to selectively forward traffic to and from the UE local to the relay station thus minimizing interference. Depending on the level at which the protocol stack is terminated in the relay node (RN), such types of relay may require the development of relay-specific standards. This can be largely avoided by extending the protocol stack of the RN up to Layer 3 to create a wireless router that operates in the same way as a normal eNB, using standard air interface protocols and performing its own resource allocation and scheduling. The distinguishing feature of such relays compared to normal eNBs is that the backhaul connecting the relays to the other eNBs operates as an in-band LTE radio link to the donor eNB. This link, called the Un interface, can be on the same frequency as the RN-to-UE link (inband) or on a different frequency (outband). The concept of the relay station can also be applied in low density deployments where a lack of suitable backhaul would otherwise preclude use of a cellular network. The use of in-band or in-channel backhaul can be optimized using narrow point-to-point connections to avoid creating unnecessary interference in the rest of the network. Multi-hop relaying is also possible as shown in Figure 8.3-4.

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Figure 8.3-4. In-channel relay

Since the RN cannot simultaneously receive from the donor eNB and transmit to a local UE at the same time and frequency, downlink transmission gaps during which the eNB communicates with the RN can be created by configuring MBSFN subframes at the RN. This principle is shown in Figure 8.3-5.

Figure 8.3-5: Relay-to-UE communication using normal subframes (left) and eNB-to-relay communication using MBSFN subframes (right). (36.912 [13] Figure 9.1)

The essential functionality to enable relaying is specified in Release 10 but the radio requirements for the RN transmitter and receiver performance are specified in Release 11. In Release 12 a study item has started to investigate mobile relaying as a solution for improving performance on high speed trains. Currently, the handover success rate from high speed trains is problematic due to the large number of UE attempting to handover at the same time. By using a mobile relay, possibly equipped with a group handover mechanism, the signaling load on the macro network could be substantially reduced.

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8.3.4 Release 10 Other EnhancementsLTE-Advanced is a subset of Release 10 and the following sections describe further work items in Release 10 that were not originally identified to meet the ITU-R requirements for IMT-Advanced in 36.912.

8.3.4.1 Enhanced Inter-cell Interference Coordination (eICIC)

Basic support for ICIC started in Release 8 and is enhanced in Release 10 with the eICIC work item. Before discussing eICIC it is worthwhile reviewing the attributes of the CDMA and OFDM air interfaces to see how they behave with regard to inter-cell interference and the techniques that can be applied to mitigate it.

In the CDMA systems that dominate 3G, cell-edge interference is now a well-understood phenomenon and techniques for dealing with it continue to advance. This was not always the case and early CDMA systems were dogged with unexpected issues such as “cell breathing” in which the cell boundary moves as a result of power-control problems and excessive soft handover activity. Cell breathing can now be used with care as a tool for inter-cell load balancing. UMTS Release 7 introduced the HSDPA Type 3i receiver, which incorporated diversity reception, an equalizer, and dual-input interference cancellation capability. Due to the use of cell-specific scrambling codes and the presence of patterns within the signal caused by frequency selective fading, a cell-edge interferer in a CDMA system has considerably more structure than additive white Gaussian noise (AWGN). This structure can be used by an interference-cancelling receiver to remove significant portions of the co-channel interference.

The introduction of OFDMA to cellular systems—starting with 802.16e and continuing with LTE—has significantly changed the nature of cell-edge interference. In CDMA systems all the transmissions occupy the entire channel and are summed to create a signal with relatively stable dynamics. In OFDMA the potential for frequency-selective scheduling within the channel opens up new possibilities for optimizing intra-cell performance but also creates dynamic conditions in which inter-cell co-channel interference may occur. Work continues in 3GPP to better understand the effect of this interference on operational performance. In particular it has been noted that the narrowband and statistical (temporal) nature of the interference can influence the behavior of subband CQI and PMI reporting. While the presence of interference in CDMA systems is largely consistent across the channel bandwidth, the presence of interference in OFDMA systems using frequency-selective scheduling can change rapidly from the time of CQI reporting to its impact on the next scheduled transmission.

The interference protection between CDMA cells offered by the use of scrambling codes is not available in narrowband OFDMA transmissions, which leaves the narrowband signals vulnerable to narrowband interference. However, the ability of cells to coordinate their narrowband scheduling offers some potential for interference avoidance. Support for coordination of resource block (RB) allocation between cells in the downlink was introduced in Release 8 with the inclusion of the relative narrowband transmit power (RNTP) indicator. This support feature is a bitmap that can be shared between base stations over the X2 interface. It represents those RB for which the base station intends to limit its output power to a configurable upper limit for some period of agreed-upon time. This allows schedulers to agree on how cell-edge RB will be used so that, for instance, cell-edge users who cause the most interference can be restricted to certain parts of the channel. This coordination could be implemented using a semi-static agreement for partial frequency reuse at the cell edge or might involve more dynamic scheduling based on real-time network loading.

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Two interference coordination mechanisms based on RB bitmaps are available for the uplink. The first is a bitmap called the overload indicator (OI), which can be provided by a base station to neighbor base stations indicating the level of uplink power plus noise as being “low,” “medium,” or “high.” The second is more proactive and is the high interference indicator (HII). This is communicated to neighbor base stations prior the UE being scheduled, giving other base stations the chance to avoid the identified RB rather than allowing interference to occur and then having to deal with the consequences. These basic frequency domain approaches to ICIC are elaborated in Release 10 with the additional ability to coordinate inter-cell scheduling in the time domain.

Heterogeneous Networks

The original cellular deployment scenario in Release 8 was the traditional cellular pattern of adjacent cells sharing the same frequency. By Release 10 a variety of new base station types were introduced including the afore-mentioned local area BS (picocell), home BS (femtocell), and relay node. The inter-cell coexistence techniques that might be employed in a Release 8 network comprising wide area base stations are well understood; however, the introduction of the new base station types creates new coexistence scenarios. The issue is not that a network incorporating only one base station class might be deployed—in which case existing techniques might suffice—but that the network might include a mixture of different base station classes, all occupying the same frequency. This scenario has been termed the heterogeneous network or “het-net” for short. In the het-net environment new co-channel interference scenarios arise that require new inter-cell interference coordination solutions.

There are two forms of co-channel heterogeneous deployment, each requiring a different approach to interference avoidance. The first is the open subscriber group (OSG), a type of deployment that might be used by an operator with a macro network providing broad coverage overlaid with local area base stations in areas where coverage issues exist or where higher capacity is needed—for example, in a shopping mall. In this scenario a user is free to roam between the macro network and any local area BS deployed by the operator on the same frequency. For OSG deployment, the local area BS is located in the center of the area in the network where the increased capacity is required. At the perimeter of this area the strengths of the wide area and local area base stations are similar and performance may be significantly degraded. Closer to the local area BS the interference becomes less problematic. It is also possible to have an OSG scenario with a home BS, provided that the home BS is configured to be open to all users of that operator.

The second form of co-channel deployment is the closed subscriber group (CSG). This type of deployment is essentially limited to a home BS scenario in which access to the home BS is limited to a fixed group of subscribers, most likely the occupants of a dwelling or employees of an enterprise who have installed the home BS. This form of deployment provides good service for the closed subscriber group but creates a much more difficult interference situation for all other users since the problem area is no longer limited to a ring around the local area BS or home BS but extends to the entire coverage area of the home BS. This situation could be acceptable in low density rural areas but is likely to cause severe difficulties for macro network coverage in more densely populated areas. The obvious solution to home BS CSG is to assign different channels to the home BS and the macro network, thus reducing the interference to that which exists between adjacent home BS. This approach, however, is not available to operators with only a single channel. Some form of partial frequency reuse is also possible although this does not solve interference in the control channels, which always occupy the central 1.08 MHz of the channel. Given the difficulty of CSG, the initial work on eICIC in heterogeneous networks has been focused on the OSG case.

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Almost Blank Subframes

The frequency domain ICIC techniques available in Release 8 and Release 9 are effective in managing interference that is caused by data traffic, but these techniques are not suited to minimizing interference between the control channels, which always occupy the same central 1.08 MHz of the channel regardless of channel bandwidth. To deal better with control-channel interference issues, Release 10 introduces the almost blank subframe (ABS) as the primary mechanism for eICIC. In this time-domain approach, the macro network chooses to minimize scheduled transmissions on certain subframes so that they can be used by the local area BS with minimal degradation of performance. These subframes are considered “almost blank” since minimal control traffic on the PDCCH may still be present in order to schedule macro uplink traffic and maintain HARQ ACK/NACK feedback to the macro UE. Backward compatibility to Release 8 and Release 9 UEs must also be maintained, which requires that the base station downlink still be measurable by legacy UEs. To do this, the downlink subframe must contain the cell RS, synchronization signals, and the paging channel. If the downlink subframe is designated as an MBSFN subframe, then fewer signals will be required.

As with the RNTP indicator introduced for frequency-domain ICIC, the use of ABS by the macro BS is indicated by an ABS pattern bitmap, but in this case we are not dealing with frequency domain RBs but with the time-domain subframe. There is also a secondary indicator known as the measurement subset, which indicates to the victim BS those subframes that the UE connected to the victim BS should use to assess the interference from the macro network when ABS is not configured. There is a great deal of flexibility in how ABS can be used and as such the standards specify the mechanisms for use in proprietary implementations but does not mandate specific solutions.

Further enhanced ICIC (FeICIC)

Some of the work on eICIC was not completed in Release 10 and so the FeICIC work item was created for Release 11. This includes specification of system performance requirements for scenarios involving a dominant downlink interferer.

Carrier-based Het-Net ICIC

The ICIC requirements developed through Release 10 are all based on co-channel (intra-frequency) scenarios. It was originally planned to develop ICIC further in Release 11 to take advantage of network-based carrier selection and this work has now been carried over to Release 12.

8.3.4.2 Minimization of Drive Test

It has long been the case that the planning and operational optimization of networks has been facilitated by the use of drive testing (see Section 6.14). Drive testing is a powerful technique; however, it is time-consuming and expensive to carry out. To alleviate some of the cost associated with drive testing a new set of UE measurement capabilities are introduced in Release 10 under the minimization of drive test (MDT) work item. The Release 10 work was focused on coverage and Release 11 added QoS verification. The MDT technical report is in 37.320 [16].

8.3.4.3 Machine-Type Communications (MTC)

For most of the history of cellular communications the goal has been to provide services between people. However, since the advent of data services there has been an increasing desire to support cellular communication between machines. These could

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be vending machines communicating with a corporate server to indicate sales activity and the need for restocking, or perhaps machines providing remote meter reading. The types and frequency of traffic in such scenarios is quite different from that for which LTE was originally developed. Machine-type communications often involve small amounts of data sent infrequently, preferably using very low cost infrastructure. These attributes are well-served by legacy systems such as GSM but are not well-suited to the footprint provided by LTE Release 8, whose lowest UE category mandates support for at least 10 Mbps in the downlink with two receivers and 5 Mbps in the uplink. The purpose of the MTC work item is therefore to develop additional UE categories more suited to the lower requirements of MTC.

The work on MTC started in Release 10 and has continued through Release 11 into Release 12. The scope has been clarified to indicate a target improvement in coverage over legacy systems of some 20 dB for very small data packets on the order of 100 bytes per message in the uplink and 20 bytes per message in the downlink. This may be achieved through drastically reduced latency of up to 10 seconds in the downlink and one hour in the uplink. High overall system efficiency can then be delivered through scheduling during quiet times. The MTC technical report is in 36.888 [17].

8.3.4.4 New Frequency Bands

The new frequency bands added in Release 10 are shown in Table 8.3-2.


Band number Uplink Downlink Bandwidth Duplex spacing Gap Duplex modeLow High Low High

22 3410 3490 3510 3590 80 100 20 FDD23 2000 2020 2180 2200 20 180 160 FDD24 1626.5 1660.5 1525 1559 34 -101.5 67.5 FDD25 1850 1915 1930 1995 65 80 15 FDD

41 2496 2690 2496 2690 194 0 0 TDD42 3400 3600 3400 3600 200 0 0 TDD43 3600 3800 3600 3800 200 0 0 TDD

8.4 Release 11Of the 48 radio-related work items identified for Release 11, some 31 relate to work in new frequency bands and various band combinations for carrier aggregation. These work items are covered in some detail in Section 2.1.11. Several other work items are continuations of work started in earlier releases and are referenced in Sections 8.2 and 8.3. The remaining ten work items introduce new concepts to the LTE specifications:

• Verificationofradiatedmulti-antennareceptionperformanceofUEsinLTE/UMTS• LTERANenhancementsfordiversedataapplications• Signalingandprocedureforinterferenceavoidanceforin-devicecoexistence• Coordinatedmulti-pointoperationforLTE• NetworkenergysavingfortheE-UTRAN• Enhanceddownlinkcontrolchannel(s)forLTE-Advanced

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• PublicSafetyBroadbandHighPowerUEforBand14,Region2• ImprovedminimumperformancerequirementsforE-UTRA:interferencerejection• AdditionalspecialsubframeconfigurationforLTETDD• Carrieraggregationenhancements.

8.4.1 Verification of Radiated Multi-Antenna Reception Performance of UEs in LTE/UMTSThis work item is the culmination of the study item introduced in March 2009 for MIMO over the air (OTA) performance verification. This subject is covered in detail in Section 6.10.

8.4.2 LTE RAN Enhancements for Diverse Data ApplicationsThe diverse range of mobile data applications is now very extensive and includes short message service (SMS), instant messaging, web browsing, social networking, and a variety of push services. Modern devices such as tablets and smartphones will often activate some or all of these services in parallel, putting considerable strain on the radio network—not just due to the volume of data but also the substantial signaling overhead created by the “chatty” nature of many applications. In addition, the user expectation of an always-on mobile broadband experience puts great demands on battery consumption since the device may be prevented from reaching the idle state. Moreover, most modern applications were not developed with the unique characteristics of cellular networks in mind and so the use of network resources is often inefficient. How to balance the demands of user experience with battery consumption and network efficiency will depend on the characteristics of individual applications that may vary over time.

The outcome of the RAN enhancement work item is captured in 36.822 [16]. It has resulted in the specification of a power-preference feature that allows the UE to signal the network its preference for a configuration that reduces power consumption.

8.4.3 Signaling and Procedure for Interference Avoidance for In-device CoexistenceSo that users can access various networks and services wherever they are, an increasing number of UEs are equipped with multiple radio transceivers for LTE, Wi-Fi, Bluetooth, GNSS, etc. As a result, UEs are challenged to avoid coexistence interference between those co-located radio transceivers. The studies done for this work item have shown that existing RRM mechanisms in some cases are not effective enough to handle the coexistence issues, and some enhanced signaling and other procedures are necessary to avoid or mitigate the coexistence interference in the identified usage scenarios.

As a result of this work item a new in-device coexistence (IDC) indication message has been defined. This message enables the UE to alert the network of an interference issue and provide information regarding the direction and nature of the interference, which may be identified in either the time or frequency domain. Upon receipt of the IDC message the network will take appropriate steps to alleviate the problem by reallocating radio resources. The stage 2 specification for this message is in 36.300 [17] Section 23.4 and the details are captured in 36.331 [18].

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8.4.4 Coordinated Multi-Point Transmission (CoMP)Section 2.4.7 briefly introduced the concept of co-operative MIMO, which also goes by the name of network MIMO or coordinated multi-point (CoMP). The goal of CoMP is to improve the coverage of high data rates and cell-edge throughput, and also to increase system throughput. Figure 8.4-1 compares standard MIMO with CoMP.

The primary difference between standard MIMO and CoMP is that for the latter, the transmitters are not physically co-located. In the case of downlink CoMP, however, there is the possibility of linking the transmitters at baseband (shown as the link between the transmitters on the right half of Figure 8.4-1) to enable sharing of payload data for the purposes of coordinated precoding. This sharing is not physically possible for the uplink, which limits the options for uplink CoMP. For the standard network topology in which the eNBs are physically distributed, provision of a high capacity, low latency baseband link is challenging and would probably require augmentation of the X2 inter-eNB interface using fiber. However, a cost-effective solution for inter-eNB connectivity is offered by the move towards a network architecture in which the baseband and RF transceivers are located at a central site with distribution of the RF to the remote radio heads via fiber.

Figure 8.4-1. Standard MIMO versus coordinated multi-point

The physical layer framework for CoMP is described in the Release 11 feasibility study in 36.819 [19].

8.4.4.1 CoMP Deployment Scenarios

Four downlink scenarios were defined for the feasibility study:

Scenario 1 is a homogeneous network (all cells have the same coverage area) with intra-site CoMP. This is the least complex form of CoMP and is limited to eNBs sharing the same site.

Scenario 2 is also a homogeneous network but with high Tx-power RRHs. This is an extension of scenario 1 in which the six sites adjacent to the central site are connected via fiber optic links to enable baseband cooperation across a wider area than is possible with scenario 1.

Scenarios 3 and 4 are heterogeneous networks in which low power RRHs with limited coverage are located within the macrocell coverage area. In scenario 3 the transmission/reception points created by the RRHs have different cell identifications than does the macro cell and for scenario 4 the cell identifications are the same as that of the macro cell.

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8.4.4.2 CoMP Categories

The introduction of CoMP enables several new categories of network operation, which are defined for the downlink as follows.

Joint processing (JP): Data for a UE is available at more than one point in the CoMP cooperating set (see Section 8.4.4.3) for the same time-frequency resource.

• Joint transmission (JT): This is a form of spatial multiplexing that takes advantage of decorrelated transmission from more than one point within the cooperating set. Data to a UE is simultaneously transmitted from multiple points; e.g., to coherently or non-coherently improve the received signal quality or data throughput.

• Dynamic point selection (DPS)/muting: The UE data is available at all points in the cooperating set but is only transmitted from one point based on dynamic selection in time and frequency. The DPS includes dynamic cell selection (DCS). DPS may be combined with JT, in which case multiple points can be selected for data transmission in the time-frequency resource.

Coordinated scheduling and beamforming (CS/CB): Data for a UE is only available at and transmitted from one point in the CoMP cooperating set but user scheduling and beamforming decisions are made across all points in the cooperating set. Semi-static point selection (SSPS) is used to make the transmission decisions. Dynamic or semi-static muting may be applied to both JP and CS/CB.

Hybrid JP and CS/CB: Data for a UE may be available in a subset of points in the CoMP cooperating set for a time-frequency resource but user scheduling and beamforming decisions are made with coordination among points corresponding to the CoMP cooperating set. For example, some points in the cooperating set may transmit data to the target UE according to JP while other points in the cooperating set may perform CS/CB.

New categories in the uplink are the following.

Joint reception (JR): The PUSCH transmitted by the UE is simultaneously (jointly) received at some or all of the points in the cooperating set. This simultaneous reception can be used with inter-point processing to improve the received signal quality.

Coordinated scheduling and beamforming (CS/CB): User scheduling and precoding selection decisions are made with coordination among points corresponding to the cooperating set. Data is intended for one point only.

8.4.4.3 CoMP Sets

Various sets of eNBs are identified for downlink CoMP purposes.

CoMP cooperating set: The set of eNB points within a geographic area that are directly or indirectly participating in data transmission to a UE. The UE may or may not know about this set. The direct participation points are those actually transmitting data and the indirect points are those involved in cooperative decision making for user scheduling and beamforming in the time and frequency domains.

CoMP transmission point(s): The point or set of points transmitting data to a UE. CoMP transmission points are a subset of the CoMP cooperating set.

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• ForJT,CoMPtransmissionpointsmayincludemultiplepointsintheCoMPcooperatingsetateachsubframeforacertain frequency resource.

• ForCS/CB,DPS,andSSPS,asinglepointintheCoMPcooperatingsetistheCoMPtransmissionpointateachsubframe for a given frequency resource.

• ForSSPS,theCoMPtransmissionpointcanchangesemi-staticallywithintheCoMPcooperatingset.

CoMP measurement set: The set of points about which channel state and statistical information related to the UE radio link is measured and reported.

RRM measurement set: The set of cells for which Release 8 radio resource management (RRM) measurements are performed. Additional RRM measurement methods may be developed; e.g., in order to separate different points belonging to the same logical cell entity or in order to select the CoMP measurement set.

For the uplink, the following sets are identified.

CoMP reception point(s): The point or set of points that is a subset of the cooperating set receiving data from a UE.• ForJR,CoMPreceptionpointsmayincludemultiplepointsintheCoMPcooperatingsetateachsubframefora

certain frequency resource.• ForCS/CB,asinglepointintheCoMPcooperatingsetistheCoMPreceptionpointateachsubframeforacertain

frequency resource.

8.4.4.4 Radio Interface Aspects

To enable CoMP operation, changes to the radio interface will likely be needed in the areas of channel state information (CSI) feedback from the UE, preprocessing schemes for coordination of joint transmission, and possibly new reference signal designs and new control signaling mechanisms. Reuse of existing Release 8 CSI measurements extended for CoMP, called explicit feedback, is expected. Channel parameters (per point) include the channel matrix H, the transmit covariance matrix R, and possibly inter-point properties such as the inter-point phase relationship required for JT. Noise and interference parameters are also required. To take full advantage of CoMP, more advanced implicit feedback will be required based on UE hypotheses about different CoMP transmission and reception processing. The potential for CoMP becomes greater for TDD operation since UE transmission of the sounding reference signal (SRS) can be used by the eNB to precisely determine the downlink channel conditions on the assumption of TDD channel reciprocity.

8.4.4.5 Simulation Results

Extensive simulation of CoMP performance has been performed by multiple companies for the four deployment scenarios identified in Section 8.4.4.1 for uplink and downlink FDD and TDD. Both 3GPP and ITU channel models were used, and the impact of cell loading and inter-cell communication latency and bandwidth was also studied. Although the simulation criteria were specified, the results showed variations in performance that may be due to different assumptions being made for the channel estimation error modeling, channel reciprocity modeling, feedback and SRS mechanisms, the scheduler, and the receiver. The impact of CoMP on legacy UEs is not considered.

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The results show that CoMP gains vary widely depending on the specific scenario and whether the focus is on average cell performance, mean user performance, or improving the performance of the worst 5% of users in the cell. Some scenarios provide no gain at all and others, particularly TDD with its channel reciprocity advantage, show gains of up to 80%. Typical gains fall in the range of 10% to 30%. As a result a work item to progress CoMP is defined in Release 11 and will focus on the following aspects:

• Jointtransmission• Dynamicpointselection,includingdynamicpointblanking• Coordinatedschedulingandbeamforming,includingdynamicpointblanking.

8.4.5 Network Energy Saving for E-UTRANWith the growth in network capacity there is an increasing need to consider the energy costs of operating the network. In particular, opportunities exist to dynamically dimension the network based on traffic loading. The stage 2 definition of network energy saving is defined in 36.300 [17] Section 22.4.4. The basic mechanism is that an eNB containing one or more capacity booster cells in addition to basic coverage cells may choose to deactivate the booster cells based on a drop in the network load. The deactivation may also require communication with peer eNBs over the X2 interface to indicate that the booster cell is to be deactivated. Offload of users from the booster cell to the coverage cells may also be necessary through handovers.

8.4.6 Enhanced Downlink Control Channels for LTE-AdvancedThe addition of carrier aggregation in combination with a new carrier type (see Section 8.5), CoMP, and DL MIMO has resulted in the need to enhance the capabilities of the physical downlink control channel (PDCCH). The enhanced PDCCH (EPDCCH) will be restricted to QPSK modulation. The EPDCCH will be compatible with legacy carriers, provide more signaling capacity, support frequency domain ICIC, improve the spatial reuse of the control channel, support beamforming and diversity schemes, and operate in MBSFN subframes. Frequency-selective scheduling for the EPDCCH is also desirable as is mitigation of inter-cell interference.

8.4.7 Public Safety Broadband High Power UE for Band 14, Region 2The US Federal Communications Commission Public Safety and Homeland Security Bureau has selected LTE as the technology for public safety services in the 700 MHz public safety band (3GPP Band 14; see Table 2.1-1). Due to the coverage and uplink performance requirements for public safety broadband (PSBB) systems, the existing 23 dBm UE power class (class 3) is not considered sufficient.

Public safety “first responders” will rely on handheld UEs as well as vehicular mobile applications that have fewer constraints on size, weight, and power consumption than handheld UEs. A vehicular mobile application also has the possibility of incorporating very efficient vehicle-mounted antennas. Unlike commercial cellular systems, which often generally have a 95% population coverage target, PSBB systems target 99% coverage. Although this change may seem insignificant, to reach the additional 4% of the US population requires a 60% increase in the coverage area.

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Providing such coverage using base stations alone would be very expensive, so a higher power UE (HPUE) power class 1 is being specified for a vehicular mobile form factor with vehicular-mounted antennas. The provisional requirements are captured in 36.837 [20].

In order to optimize reuse of the existing LTE UE ecosystem, the requirements will minimize change that might impact the design of the baseband and lower-power RF components of the UE. The bulk of the changes are expected in the RF front end containing the power amplifier (PA), filtering, and signal-combining components. The headline parameter driving the HPUE specification is the proposed 10 dB increase of maximum output power to 33 dBm. Although it is expected that few other transmitter and receiver requirements will be changed from those defined for the existing power class 3 UE (23 dBm), this increased maximum power has considerable design implications for both the transmitter and the receiver. For instance, the dynamic range of the transmitter increases 10 dB and all fixed-level unwanted emissions become 10 dB harder to meet.

For the receiver to maintain the existing RF sensitivity the duplex filter has to provide 10 dB more isolation from the transmitter. The tighter filtering requirements represent probably the biggest design change for the HPUE because existing miniature surface acoustic wave (SAW) filters measuring perhaps 5 mm3 cannot handle the higher output power or provide the necessary filtering performance. Alternative technologies will be required—for example, ceramic or cavity filters, which are substantially larger at around 8000 mm3. Fortunately, the form factor of the vehicular mobile has more relaxed constraints on size and power than does the standard handheld UE.

One of the few performance requirements likely to change for the HPUE is the ACLR requirement. Studies have shown that to maintain the existing co-existence performance of power class 3 UE, the HPUE will need to have an ACLR in the region of 28 dB, some 5 dB tighter than for power class 3.

In summary, the increase in maximum output power along with the potential for vehicular-mounted antennas means that the power class 1 HPUE will offer substantially better performance in areas of poor reception than was possible with the power class 3 UE. It’s expected that the increased cost of the HPUE will be offset by substantial savings in the number of base stations needed to achieve 99% population coverage.

8.4.8 Improved Minimum Performance Requirements for E-UTRA: Interference RejectionExisting LTE UE demodulation requirements are based on an assumption of a linear minimum mean squared error (LMMSE) dual receiver. This is a powerful receiver architecture capable of suppressing both inter- and intra-cell interference. However, existing demodulation requirements are based on additive white Gaussian noise (AWGN), which is decorrelated between the antennas. This simplified method of modeling interference has been widely used for many years and is suitable for measuring the performance of receivers without interference cancellation capabilities. However, to exploit the full potential an LMMSE receiver with interference rejection combining (IRC) capabilities and achieve a performance gain over standard receivers, it is necessary to more accurately model the interference. Studies carried out in Release 10 showed that enhanced receivers capable of RS-based interference covariance estimation to mitigate spatial domain interference could provide significant throughput gains in the high interference conditions of a heavily loaded network.

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The scope of the Release 11 work includes a variety of deployment scenarios that take into account the number of interfering sources, their structure (including transmission rank and precoding), and their power ratio relative to the total interference from other cells. This ratio is known as the dominant interferer proportion (DIP) ratio. Both synchronized and asynchronous cases are considered since they can have a major impact on interference susceptibility. Also within the scope of Release 11 were definitions of cell RS and UE-specific RS in anticipation of future network deployment scenarios.

8.4.9 Additional Special Subframe Configuration for LTE TDDThe operation of TDD networks requires careful coordination between systems deployed on adjacent channels. Co-existence of LTE TDD with legacy UMTS TD-SCDMA systems is required and for this case, special subframe configuration number 5 is chosen for the normal cyclic prefix case and configuration number 4 for the extended cyclic prefix case (see Table 3.2-2). The special subframe lasts for one ms and always comes between the transition from downlink transmission to uplink transmission, although it is not required from the uplink back to the downlink (see Table 3.2-1). The special subframe comprises the downlink pilot timeslot (DwPTS), a gap period (GP), and an uplink pilot timeslot (UpPTS). The ratio between the DwPTS, GP, and UpPTS is configurable, and for TD-SCDMA co-existence, configuration 5 uses a ratio of 3:9:2 and configuration 4 uses 3:7:2. Although these configurations provide the necessary protection when LTE TDD and TD-SCDMA systems are in adjacent channels, the use of a relatively large GP in these configurations is seen as inefficient since no data can be transmitted during the gap period.

To address this shortcoming, two new special subframe configurations have been specified in Release 11. For the extended cyclic prefix case, a new option for special configuration number 7 has been defined for a ratio of 5:5:2, which provides an additional two symbols for data communication per special subframe. For the normal cyclic prefix case a new special subframe configuration number 9 provides a ratio of 6:6:2, which is three extra useful symbols per special subframe.

8.4.10 Carrier Aggregation EnhancementsMany new band combinations specific to carrier aggregation are being developed in Release 11 and are covered in Section 2.1.11 of this book. Also in Release 11 is the introduction of new CA capabilities. For example, some uplink CA scenarios will require the ability to define different timing advances for each carrier. This could occur in an inter-band case that uses repeaters for one band but not the other. To deal with the situation the UE is allowed to adjust the timing advance of the two carriers independently such that the time orthogonality of the uplink in the cell is preserved.

Another new CA feature introduced in Release 11 is the ability for TDD to support different uplink and downlink configurations for each band. This provides more flexibility than was possible in Release 10, which required that the format of each carrier be the same.

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8.4.11 New Frequency BandsThe new frequency bands added in Release 11 are shown in Table 8.4-1.


Band number Uplink Downlink Bandwidth Duplex spacing Gap Duplex modeLow High Low High

26 814 849 859 894 35 45 10 FDD27 807 824 852 869 17 45 28 FDD28 703 748 758 803 45 55 10 FDD

44 703 803 703 803 100 0 0 TDD

Band 29 is also being specified. This is a downlink only band from 717 MHz to 728 MHz intended for use in CA scenarios which are likely to initially include bands 2 and 4.

8.5 Release 12At the time of this writing Release 12 is still under discussion. A workshop to consider proposals was held in June 2012. The broad areas for future radio evolution were identified as follows:

• Energysaving• Costefficiency• Supportfordiverseapplicationandtraffictypes• Backhaulenhancements.

The following proposals from the workshop were identified as most likely to be developed in Release 12:

• Interferencecoordinationandmanagement• DynamicTDD• Enhanceddiscoveryandmobility• Frequencyseparationbetweenmacroandsmallcells,usinghigherfrequencybandsinsmallcells(e.g.,3.5GHz)• Inter-sitecarrieraggregationandmacrocell-assistedsmallcells• Wirelessbackhaulforsmallcells.

Other possible areas for study include the following:• Supportfordiversetraffictypes(controlsignalingreduction,etc.)• InterworkingwithWi-Fi• Continuousenhancementsformachine-typecommunications,SON,MDT,andadvancedreceivers• Proximityservicesanddevice-to-devicecommunications• FurtherenhancementsforHSPAincludinginterworkingwithLTE.

The timeframe for Release 12 is likely to be 18 to 24 months beyond Release 11 with March 2013 being the date proposed for completion of the Stage 1 specifications. This would put the Stage 3 completion sometime in 2014.

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As of RAN plenary #57 (Oct 2012), twenty new work items have been approved for Release 12. Most of these are spectrum related with three for new frequency bands and thirteen for new carrier aggregation scenarios. The remaining four are unrelated.

8.5.1 New Frequency BandsThree new frequency bands will be defined:

• FDDdownlink1670MHz–1675MHz,uplink1646.7MHz–1651.7MHzforITURegion2(US)• FDDdownlink461MHz–468MHz,uplink451–458MHzforBrazil• FDDdownlink2350–2360MHz,uplink2305–2315MHz,USWirelessCommunicationsService(WCS)band.

Assuming that band 29 is defined in Release 11, the introduction of the bands listed above will require a break in the band numbering since the TDD bands start at band 32.

In addition to the work items, a study item in Release 12 exists for the 30 MHz of paired spectrum immediately above band 1; that is, uplink from 1980 MHz–2010 MHz and downlink from 2170 MHz– 2200 MHz. This band is currently widely allocated for satellite communications but its use for terrestrial communications is now being considered, particularly for ITU Region 3. The potential for a combined 110 MHz paired band including band 1 is also being considered.

8.5.2 Carrier Aggregation ScenariosWith 28 FDD and 12 TDD bands defined up to Release 11, the theoretical number of two-carrier CA combinations is enormous. Fortunately, only those scenarios relevant to specific geographic deployments or potential deployments are covered in the standardization process. In Release 11, work items were created for five intra-band scenarios and twenty inter-band scenarios (see Section 2.1.11).

In Release 12 work items exist for five intra-band scenarios:• Band1(contiguous)• Band3(non-contiguous),carriedoverfromRelease11• Band3(contiguous)• Band4(non-contiguous)• Band25(non-contiguous).

An additional eight inter-band scenarios are defined:• Bands3and5withtwouplinkcarriers• Bands2and4• Bands3and26• Bands3and28• Bands3and19• Bands38and39• Bands23and29*• Bands1and8.

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Band 29 is being specified as part of Release11. It is a downlink-only band from 717 to 728 MHz and to be used only for the purposes of carrier aggregation with other bands.

Future evolution of carrier aggregation is likely to include inter-site aggregation and macrocell-assisted small cells. The goal is to enable the UE to remain connected at all times to the macro network on one carrier, which is likely to be at a lower (< 1 GHz) frequency for coverage reasons, while opportunistically connecting to the macro network on a second carrier provided by a small cell (probably not co-located) to provide higher capacity. The advantage of doing this using carrier aggregation rather than handover is that CA should provide much faster adaptation to the network conditions than handover-based approaches.

Also identified in the Release 12 workshop are opportunities to exploit LTE aggregation with other radio systems such as UMTS and Wi-Fi to optimize connectivity.

8.5.3 Carrier-based Het-Net ICIC for LTESee Section 8.3.4.1.

8.5.4 New Carrier Type for LTEBackward compatibility of LTE carriers has been a priority since the first LTE Release up to Release 11. However, it is also desirable to consider introducing a new carrier type (NCT) for LTE that would not be constrained by legacy requirements such as cell reference symbols (CRS) and signaling overhead that impact overall network efficiency at low-to-medium loads. This NCT could be useful in several deployment scenarios such as at the cell edge in a homogenous network, in the cell range expansion zone of heterogeneous networks, and in low power eNB deployments whether they be isolated or part of a wider macro deployment. There is also potential for energy savings by allowing the downlink signal to switch off when low demand occurs.

The use cases for the NCT also include new carrier aggregation scenarios such as the aggregation of a downlink-only carrier with a legacy carrier (see Section 8.5.2, bands 23 and 29).

8.5.5 Further Downlink MIMO Enhancement for LTE-Advanced A study item in Release 11 considered several aspects of downlink MIMO performance including rank reporting, time misalignment and antenna calibration, and numerous aspects related to CSI feedback. The conclusions of the study are captured in 36.871 [21], which concluded that UE rank reporting is problematic in some conditions such as non-co-located antenna deployments. These issues are best addressed by specifying new performance requirements. The study also identified potential CSI performance improvements through the use of four transmit antennas.

The scope of the work item will cover the following topics:

• FourtransmitantennaPMIfeedbackcodebookenhancementstoprovidefinerspatialdomaingranularityandto support different antenna configurations for macro and small cells, especially cross-polarized antennas, both closely and widely spaced, and non-co-located antennas with power imbalance

• NewCSIfeedbackmodeprovidingsubbandCQIandsubbandPMI

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• Finerfrequency-domaingranularity• EnhancedcontrolofthereportedrankandcorrespondingassumptionsforCQI/PMIderivationtoimprovesupport

for MU-MIMO.

8.5.6 Further Enhancements for H(e)NB Mobility-Part 3Following a Release 11 study item, this work item has been introduced to further enhance the mobility between home (e)NBs, and from a home (e)NB to a wide area eNB. Both UMTS and LTE are considered. The aspects relevant to LTE will focus on RAN sharing for the scenario in which the UE reports the PLMN identities of the home eNB that are accessible and can pass a closed subscriber group check. The home eNB verifies the access check, selecting just one identity if more than one are identified, and finally the MME/SGSN verifies the CSG membership check.

8.5.7 Release 12 Study ItemsLooking further ahead, several radio-related study items that introduce new concepts to LTE have been defined for Release 12.

8.5.7.1 RF and EMC Requirements for Active Antenna Array System

The multiple antenna base station techniques that have been deployed to date are largely proprietary in nature and have no formal specifications or performance requirements. With the increasing sophistication of multiple antenna techniques it has become apparent that the largely omnidirectional assumptions about base station RF and EMC performance are becoming less representative of actual system performance. The current reference point for base station requirements is the antenna connector and excludes the antenna behavior and any multi-antenna array affects such as beamforming. This study item will investigate defining a new point in the system, independent of implementation, that will better represent the true spatial performance of the base station. A natural conclusion would be some kind of radiated over-the-air test point similar to what is being developed for the UE (see Sections 8.4.1 and 6.10). The study item is being captured in 37.840 [22].

In order to progress the work the concept of an active antenna system (AAS) has been defined as a base station system that combines an antenna array with an active transceiver unit array. An AAS may also include a radio distribution network, which is a passive network that physically separates the active transceiver unit array form the antenna array. Figure 8.5-1 shows the general AAS architecture.

Figure 8.5-1. General AAS Radio Architecture (TR 37.840 [22] Figure 4.2-1)

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A focal point for the study has been spatial ACLR. It is understood that if the unwanted emissions from the antenna array are not fully correlated, then the ACLR performance will vary in space. The ACLR variation can be used to advantage or it can create interference conditions worse than what would be seen with an omnidirectional transmission scheme. For these reasons it is necessary to study the spatial aspects of ACLR in order that the use of multiple antenna systems benefit the network.

8.5.7.2 Passive Intermodulation Handling for UTRA and LTE Base Stations

During the development of the non-contiguous (inter-band) MSR work (see Section 8.2.4) it became apparent that the passive inter-modulation (PIM) performance of the physical elements in the base station transmitter had the potential to significantly degrade system performance. The mechanisms through which signal degradation occurs are based on the non-linear behavior of conducting elements in the vicinity of the transmitter when it is subjected to very high RF power. These elements can then re-radiate at intermodulation frequencies determined by the order of the non-linearity in the components. Initial work on resolving this issue will focus on identifying the sources of PIM while the longer term goal will be to measure and control PIM products. Given the diverse nature of PIM sources, this will not be an easy task and may require testing of the entire base station structure in an anechoic chamber.

8.5.7.3 Scenarios and Requirements of LTE Small Cell Enhancements

The contribution that small cells offer in providing increased network capacity through frequency reuse is well established. However, the propagation, mobility, interference, and backhaul assumptions for small cells can be very different from those of a network comprising the homogenous macro cells that were used to develop LTE and legacy systems. Therefore various features in support of small cells have been incorporated into the LTE specifications since Release 8, including the definition of the home area base station class and the ongoing work on such topics as ICIC and mobility in heterogeneous networks (see Sections 2.1.4.2, 8.2.1, 8.3.4.1, and 8.5.6). This Release 12 study item will look broadly at the unique challenges presented by small cell deployment to identify future areas where the LTE specifications can be further enhanced.

8.5.7.4 Feasibility Study for Proximity Services (LTE-Direct)

A study item for UE peer-to-peer communication has been carried out in TR 22.803 [25]. There are many potential applications including public safety and location-based services. The underlying goal is to develop a system whereby UEs can discover each other within a local area without the direct intervention of the network. This might be achieved by having the network allocate uplink subframes to be used by the UEs to broadcast their identity to the local area. Periodically each UE could be allocated a unique timeslot so that large numbers of UEs might have the opportunity to identify themselves over a period of several seconds. During its allocated timeslot a UE could broadcast information that may be of use to other UEs in the immediate area. For instance, a shop owner might use a UE to broadcast a web address for further information. For a UE to receive these transmissions, it would have to be augmented with a receiver in its uplink band, similar to the base station. The UE could then choose to listen to or (not to listen to) the local UEs that are broadcasting their identities and decide what, if any, further action to take. The detailed operation of such a system needs to be carefully considered from a radio interference perspective and the security aspects are also a concern. A further extension to the broadcast-only feature would be to enable peer-to-peer communication between UEs without relying on the network. This has obvious benefits for public safety applications although overlaying such a feature on top of an existing cellular network presents many challenges yet to be solved.

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8.6 References [1] 3GPP TS 36.307 V11.2.0 (2012-09) Requirements on User Equipments (UEs) Supporting a release-independent

frequency band[2] 3GPP TR 36.800 v9.0.0 (2009-09) Extended UMTS / LTE 800 Work Item Technical Report [3] 3GPP TR 36.810 V9.0.0 (2010-03) UMTS / LTE in 800 MHz for Europe [4] 3GPP TR 36.821 v9.1.0 {2010-03) Extended UMTS/LTE 1500 work item technical report [5] 3GPP TR 36.921 V11.0.0 (2012-09) Home eNode B (HeNB) Radio Frequency (RF) requirements analysis[6] 3GPP TR 36.922 v11.0.0 (2012-09) TDD Home eNode B (HeNB) Radio Frequency (RF) [7] 3GPP TS 36.104 V11.1.0 (2012-06) Base Station Radio Transmission and Reception[8] 3GPP TS 36.171 v11.0.0 (2012-09) Requirements for Support of Assisted Global Navigation Satellite System (A-GNSS)

requirements analysis[9] 3GPP TR 32.500 V11.1.0 (2011-12) Self-Organizing Networks (SON); Concepts and requirements[10] 3GPP TR 36.902 V9.3.1 (2011-03) Self-configuring and self-optimizing network (SON) use cases and solutions[11] ITU-R M.[IMT-TECH] “Requirements related to technical performance for IMT-Advanced radio interface(s),” August

2008[12] 3GPP TR 36.913 V11.0.0 (2012-09) Requirements for Further Advancements of E-UTRA (LTE-Advanced) [13] 3GPP TR 36.912 V11.0.0 (2012-09) Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)[14] 3GPP TSG RAN Tdoc RP-070466[15] 3GPP TR 25.913 V9.0.0 (2009-12) Requirements for E-UTRA and E-UTRAN[16] 3GPP TR 37.320 V11.1.0 (2012-09) Radio measurement collection for Minimization of Drive Tests (MDT)[17] 3GPP TR 36.888 V2.0.0 (2012-06) Study on provision of low-cost MTC UEs based on LTE[18] 3GPP TR 36.822 V1.0.0 (2012-09)[19] 3GPP TS 36.300 V11.3.0 (2012-09) Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal

Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2[20] 3GPP TS 36.331 V11.1.0 (2012-09) Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control

(RRC); Protocol specification[21] 3GPP 36.819 V11.1.0 (2012-12) Coordinated multi-point operation for LTE physical layer aspects[22] 3GPP TR 36.837 V0.3.0 (2012-08) Band 14 Public safety broadband high power User Equipment (UE) for Region 2[23] 3GPP TR 36.871 V11.1.0 (2011-12) Evolved Universal Terrestrial Radio Access (E-UTRA); Downlink Multiple Input

Multiple Output (MIMO) enhancement for LTE-Advanced[24] 3GPP TR 37.840 V0.3.0 (2012-10) Study of AAS Base Station[25] 3GPP TR 22.803 V1.0.0 (2012-06) Study on provision of low-cost MTC UEs based on LTE

Links to all reference documents can be found at www.agilent.com/find/ltebook.

Chapter 8 Looking Towards 4G: LTE-Advanced

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