2009 International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS 2009) December 7-9, 2009

TA2-A-1

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Abstract – This paper addresses broadband radio access techniquesfor the Long-Term Evolution (LTE) specified as the Release 8specifications (Rel. 8 LTE) and LTE-Advanced in the 3rd GenerationPartnership Project (3GPP). We first briefly describe the systemrequirements for the Rel. 8 LTE and LTE-Advanced. Then, wedescribe wider transmission bandwidths around 100 MHz to satisfythe peak data rate of 1 Gbps in the downlink and 500 Mbps in theuplink for LTE-Advanced. Based on multiple access schemes andmultiple-input multiple-output (MIMO) channel techniques in theRel. 8 LTE, enhanced multiple access schemes and MIMO channeltechniques for LTE-Advanced are presented to satisfy the improvedsystem requirements while maintaining backward compatibilitywith the Rel. 8 LTE radio interface.

I. INTRODUCTIONThe specification on the Universal Mobile TelecommunicationsSystem (UMTS) Long-Term Evolution (LTE) called the EvolvedUniversal Terrestrial Radio Access (UTRA) and Universal TerrestrialRadio Access Network (UTRAN) was finalized as Release 8 (simplyRel. 8 LTE hereafter) in the 3rd Generation Partnership Project (3GPP)[1]. The Rel. 8 LTE represents efficient packet-based radio accessand radio access networks that provide Internet Protocol (IP)-basedfunctionalities with low latency and low cost. Following thecompletion of the specifications, development of commercialequipment has begun.

The principle behind international standardization of InternationalMobile Telecommunication (IMT)-Advanced and new spectra forIMT were agreed on at the Radiocommunication Assembly-2007(RA-07) and the World Radiocommunication Conference 2007(WRC-07) in the International Telecommunication UnionRadiocommunication sector (ITU-R), respectively [2]. The circularletter (CL) to invite proposals for IMT-Advanced radio interfacetechnologies was issued in March 2008 [3]. With the CL asmotivation, the 3GPP initiated a Study Item (SI), i.e., feasibility study,for LTE-Advanced in March 2008 [4]. The system requirements forLTE-Advanced were agreed in [5]. The technical components toachieve the system requirements have been discussed in the 3GPP,and summarized in technical reports [6]. Based on the Rel. 8 LTE andcurrent agreements on LTE-Advanced, the 3GPP submitted its finalproposal to the ITU-R this October. Following the SI, Work Item(WI), i.e., specification work, for the LTE-Advanced radio interfacewill be initiated and it is expected that the specifications will becompleted according to the IMT-Advanced standardizationschedule.

According to the agreed issues at the Technical SpecificationGroup-Radio Access Network (TSG-RAN) Working Group 1 (WG1)meeting in the 3GPP, this paper addresses radio access techniquesfor Rel. 8 LTE and LTE-Advanced. Among the key radio accesstechnologies for both systems, multiple-input multiple-output(MIMO) channel transmission techniques are essential to improvethe cell throughput, cell-edge user throughput, and received quality.Hence, the paper focuses particularly on MIMO channel techniquesincluding multiuser MIMO (MU-MIMO) and transmit diversity. Therest of the paper is organized as follows. In Section II, we brieflydescribe the system requirements for Rel. 8 LTE and LTE-Advanced.Then, we present the transmission bandwidth and multiple accessschemes for Rel. 8 LTE and LTE-Advanced in Sections III and IV,

Mamoru Sawahashi†, Yoshihisa Kishiyama††, Hidekazu Taoka††, Motohiro Tanno††, and Takehiro Nakamura††

†Department of Information Network Engineering, Tokyo City University1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557 JapanE-mail: sawahasi@tcu.ac.jp Tel/FAX: +81-3-5707-1226

††Radio Access Network Development Department, NTT DOCOMO, INC.3-5 Hikari-no-oka, Yokosuka-shi, Kanagawa-ken 239-8536 Japan

Broadband Radio Access: LTE and LTE-Advanced

respectively. Subsequently, Section V describes MIMO channeltechniques in Rel. 8 LTE and LTE-Advanced, followed by theconclusion in Section VI.

II. SYSTEM REQUIREMENTS FOR LTE AND LTE-ADVANCEDTable I gives the system requirements for the Rel. 8 LTE [7]. The Rel.8 LTE supports scalable multiple transmission bandwidths including1.4, 3, 5, 10, 15, and 20 MHz. One of the most distinctive features isthe support for only the packet-switching (PS) mode. Hence, alltraffic flows including real-time service with a rigid delay requirementsuch as voice services is provided in the PS domain in a unifiedmanner. The target peak data rate is 100 Mbps in the downlink and50 Mbps in the uplink. The target values for the average or cell-edge user throughput and spectrum efficiency are specified as relativeimprovements from those of High-Speed Downlink Packet Access(HSDPA) or High-Speed Uplink Packet Access (HSUPA) in thedownlink and uplink, respectively. Here, the average cell spectralefficiency corresponds to capacity, and the cell-edge userthroughput is defined as the 5% value in the cumulative distributionfunction (CDF) of the user throughput. Both are very importantrequirements from the viewpoint of practical system performance incellular environments. In particular, improvement in the cell-edgeuser throughput is requested to mitigate the unfair achievableperformance between the vicinity of the cell site and cell edge. Afterextensive discussions in the 3GPP meetings, it was verified that therequirements and targets for the Rel. 8 LTE were achieved by thespecified radio interface using the relevant techniques [8].

The requirements for LTE-Advanced are specified in [5]. Thefollowing general requirements for LTE-Advanced were agreed upon.First, LTE-Advanced will be an evolution of Rel. 8 LTE. Hence,distinctive performance gains from Rel. 8 LTE are requested.Moreover, LTE-Advanced will satisfy all the relevant requirementsfor Rel. 8 LTE [7]. Second, full backward compatibility with Rel. 8LTE is requested in LTE-Advanced. Thus, a set of user equipment(UE) for LTE-Advanced must be able to access Rel. 8 LTE networks,and LTE-Advanced networks must be able to support Rel. 8 LTEUEs. Third, LTE-Advanced shall meet or exceed the IMT-Advancedrequirements within the ITU-R time plan. In Table II, the requirementsand target values for LTE-Advanced and IMT-Advanced are listedwith those achieved in the Rel. 8 LTE [8]. We briefly summarizebelow the major issues, which depend on MIMO channeltransmissions. With respect to the peak data rate, the CL refers toRecommendation ITU-R M.1645, which specifies that the targetpeak data rate for IMT-Advanced should be higher than 1 Gbps in

TABLE IMAJOR SYSTEM REQUIREMENTS FOR REL. 8 LTE

Bandwidth Support of scalable bandwidths(1.4, 3, 5, 10, 15, and 20 MHz)

Peak data rate DL 100 MbpsUL 50 Mbps

Spectrum efficiency(vs. Rel. 6 HSDPA/HSUPA)

DL 3 – 4 timesUL 2 – 3 times

User throughput(vs. Rel. 6 HSDPA/HSUPA)

DL 3 – 4 times (average)2 – 3 times (cell edge)

UL 2 – 3 times (average)2 – 3 times (cell edge)

Bandwidth Support of scalable bandwidths(1.4, 3, 5, 10, 15, and 20 MHz)

Peak data rate DL 100 MbpsUL 50 Mbps

Spectrum efficiency(vs. Rel. 6 HSDPA/HSUPA)

DL 3 – 4 timesUL 2 – 3 times

User throughput(vs. Rel. 6 HSDPA/HSUPA)

DL 3 – 4 times (average)2 – 3 times (cell edge)

UL 2 – 3 times (average)2 – 3 times (cell edge)

2009 International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS 2009) December 7-9, 2009

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nomadic environments. Based on the description in the CL, thetarget peak data rate for the downlink was set to 1 Gbps for LTE-Advanced. Meanwhile, the target peak data rate for the uplink wasset to 500 Mbps. The target values for the peak frequency efficiencyare 2 and 4 fold those achieved in the Rel. 8 LTE, i.e., 30 and 15 bps/Hz in the downlink and uplink, respectively. It is noted, however,that this requirement is not mandatory and is to be achieved by acombination of base stations (BSs) and high-class UEs with a largernumber of antennas. In LTE-Advanced, 1.4 to 1.6 fold improvementsfor the capacity and cell-edge user throughput are expected fromRel. 8 LTE for each antenna configuration.III. WIDER TRANSMISSION BANDWIDTH FOR LTE-ADVANCEDIn LTE-Advanced, it is necessary to support a wider bandwidththan that for Rel. 8 LTE, i.e. 20 MHz to satisfy a high target peak datarate requirement such as 1 Gbps. Therefore, it was agreed that LTE-Advanced should support a bandwidth extension up to around 100MHz [6]. To acquire such a wider transmission bandwidth, bothcontiguous and non-contiguous spectrum allocations are supported.Moreover, focusing on contiguous spectrum allocation, carrieraggregation is adopted to support a wider bandwidth whilemaintaining backward compatibility with the Rel. 8 LTE [6]. In carrieraggregation, multiple basic frequency blocks called componentcarriers (CCs) are aggregated as shown in Fig. 1. Each CC has amaximum of 110 resource blocks (RBs), which is supported in theRel. 8 LTE. The radio parameters including subcarrier spacing andthe subframe length for multiple CCs are based on those for the Rel.8 LTE so that the Rel. 8 LTE UEs can access them. By using carrieraggregation, the Rel. 8 LTE UEs can camp at one of the CCs in thetransmission bandwidth, which is wider than 20 MHz for LTE-Advanced.

With respect to the physical channels, all physical channelsspecified in the Rel. 8 LTE are employed in LTE-Advanced as well.LTE-Advanced is designed so that the center frequency of each CCsatisfies the 100-kHz channel raster condition, which was specifiedin the Rel. 8 LTE and for the existing 3G systems [9]. A UE mustacquire synchronization s ignals including the primarysynchronization signal (PSS), secondary synchronization signal(SSS), and physical broadcast channel (PBCH) carrying system-specific and cell-specific information first to establish the downlinkradio link at the initial acquisition or intermittent reception mode inthe Rel. 8 LTE. Hence, these channels are transmitted from the centralpart of each CC as shown in Fig. 1, since the structure is beneficialin achieving efficient carrier search for Rel. 8 LTE UEs or for LTE-Advanced UEs with a lower capability in a wider frequency spectrum.Furthermore, control channels are designed so that the Rel. 8 LTEUEs can recognize them, while additional features are also beingdiscussed to support LTE-Advanced specific functionalities.

TABLE IISYSTEM PERFORMANCE REQUIREMENTS FOR LTE-ADVANCED

COMPARED TO THOSE ACHIEVED IN REL. 8 LTE

IV. MULTIPLE ACCESS SCHEMES

A. Rel. 8 LTE(1) Downlink Multiple AccessIn the Rel. 8 LTE downlink, orthogonal frequency division multipleaccess (OFDMA) was adopted because of its inherent robustnessagainst multipath interference (MPI), and its affinity to differenttransmission bandwidth arrangements [10]. In the downlink usingOFDMA, intra-cell orthogonal multiplexing among physical channelsis achieved in the time and frequency domains by utilizing localizedor distributed transmission as shown in Fig. 2. Data traffic is carriedvia the physical downlink shared channel (PDSCH) by localizedtransmission using frequency domain channel-dependentscheduling based on the reported channel quality indicator (CQI)measurement, which is employed to improve the user throughputand cell throughput. Real-time traffic including VoIP could be carriedby distributed transmission employing semi-persistent scheduling.Moreover, downlink control channels carrying Layer 1 (L1)/Layer 2(L2) control signals are multiplexed over the entire system bandwidthemploying distributed transmission over the duration of one-to-three OFDM symbols at the beginning of each subframe duration.(2) Uplink Multiple AccessIn the Rel. 8 LTE uplink, single-carrier (SC)-frequency divisionmultiples access (FDMA) is adopted. This is because high priorityis given to achieving wider area coverage than on achieving higherperformance by utilizing the robustness against MPI in a multicarrierapproach. Moreover, discrete Fourier transform (DFT)-SpreadOFDM is adopted to generate SC-FDMA signals to provide highcommonality with radio parameters and signal processing ofdownlink OFDMA as shown in Fig. 3. In the Rel. 8 LTE uplink, intra-cell orthogonal multiplexing is achieved in the time and frequencydomains as well. Only localized FDMA transmission is used for theshared channel, which carries user traffic or control data. DistributedFDMA transmission using a comb-shaped spectrum is employedto multiplex sounding reference signals (SRS) for CQI measurement

Fig. 1. Carrier aggregation.100-kHz channel raster

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

CC

LTE-Advanced: Multi-CC transmission/reception

Frequency

Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE

Subf

ram

e

100-kHz channel raster

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

PSS/SSSPBCH

CC

LTE-Advanced: Multi-CC transmission/reception

Frequency

Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE Rel. 8 LTE

Subf

ram

e

DL/UL Antenna configuration

Rel. 8 LTEachievement

LTE-Advanced

Peak data rate DL – 300 Mbps 1 GbpsUL – 75 Mbps 500 Mbps

Peak spectrum efficiency [bps/Hz]

DL – 15 30UL – 3.75 15

Capacity [bps/Hz/cell]

DL 2-by-2 1.69 2.44-by-2 1.87 2.64-by-4 2.67 3.7

UL 1-by-2 0.74 1.22-by-4 – 2.0

Cell-edge user throughput [bps/Hz/cell/user]

DL 2-by-2 0.05 0.074-by-2 0.06 0.094-by-4 0.08 0.12

UL 1-by-2 0.024 0.042-by-4 – 0.07

DL/UL Antenna configuration

Rel. 8 LTEachievement

LTE-Advanced

Peak data rate DL – 300 Mbps 1 GbpsUL – 75 Mbps 500 Mbps

Peak spectrum efficiency [bps/Hz]

DL – 15 30UL – 3.75 15

Capacity [bps/Hz/cell]

DL 2-by-2 1.69 2.44-by-2 1.87 2.64-by-4 2.67 3.7

UL 1-by-2 0.74 1.22-by-4 – 2.0

Cell-edge user throughput [bps/Hz/cell/user]

DL 2-by-2 0.05 0.074-by-2 0.06 0.094-by-4 0.08 0.12

UL 1-by-2 0.024 0.042-by-4 – 0.07

IMT-Advanced

1 Gbps

156.75

–2.2(*)

––

1.4(*)

–0.06(*)

––

0.03(*)

IMT-Advanced

1 Gbps

156.75

–2.2(*)

––

1.4(*)

–0.06(*)

––

0.03(*)

(*)Required values in base coverage urban environment

Fig. 2. Intra-cell orthogonal multiple access based on OFDMA.

Frequency

Localized transmission

Distributed transmission

(Different colors represent different users)

Time

Subframe

RB bandwidth

Fig. 3. Transmission scheme for DFT-Spread OFDM.

Modulated data symbols

Transmitsignal

DFT IFFT

Subc

arrie

r map

ping

Add

ition

of c

yclic

pre

fix

2009 International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS 2009) December 7-9, 2009

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at a BS. Orthogonal CDMA is used for SRSs, the physical randomaccess channel (PRACH), and physical uplink control channel(PUCCH). In this case, cyclic-shifted sequences generated from theoriginal constant amplitude and zero auto-correlation (CAZAC)sequence are used, which provides good orthogonality amongmultiple cyclic-shifted versions of the original sequence. In additionto cyclic-shift based multiplexing, block spread based multiplexingis used to accommodate a large number of PUCCHs within onesubframe duration.B. LTE-Advanced(1) Downlink Multiple AccessIn the LTE-Advanced downlink, OFDMA is also used since it iscontinuously extended to a wider transmission bandwidth up toaround 100 MHz while maintaining the same subcarrier spacing. Asingle fast Fourier transform (FFT) reception/transmission isapplicable to an LTE-Advanced UE when continuous spectrumallocation is employed. In this case, it is decided that one transportblock (TB), which corresponds to the channel coding block andretransmission unit, is mapped within one CC [6] as shown in Fig. 4to give priority to simple signal processing at a BS and UE. Hence,parallel CC transmission is employed to transmit multiple TBs.(2) Uplink Multiple AccessIn the uplink, parallel N-times DFT-Spread OFDM was adopted,where N denotes the number of CCs transmitted within the samesubframe duration from a UE to extend the transmission bandwidthof the physical uplink shared channel (PUSCH) beyond 20 MHz [6].This was a result of giving priority to the cost requirements byreusing the Rel. 8 LTE specifications. Similar to the downlink, oneTB is mapped into one CC transmitted using one DFT-Spread OFDMcomponent. Hence, although the peak-to-average power ratio (PAPR)or cubic metric (CM) increases according to the increase in thetransmission bandwidth, i.e., the data rate is increased N-times whenN CCs are aggregated, it is considered that one-CC transmission isused in most environments due to the restriction on the transmissionpower of a UE.

V. MULTI-ANTENNA TRANSMISSION IN LTE AND LTE-ADVANCED

A. Multi-antenna Techniques in Rel. 8 LTE(1) MIMO Spatial MultiplexingIn the Rel. 8 LTE downlink, MIMO multiplexing is used to increasethe peak data rate of the PDSCH carrying user data. The target peakdata rate of 100 Mbps in the Rel. 8 LTE downlink is achieved using2-by-2 MIMO spatial multiplexing with 16QAM and a 20-MHzbandwidth. The signal detection functionality for 2-by-2 MIMOmultiplexing is adopted as a basic functionality at a UE. Moreover,a peak data rate of greater than 300 Mbps is achieved using 4-by-4MIMO multiplexing. In MIMO multiplexing, precoding, whichperforms directive beam forming multiplied by the stream-specificadaptive antenna weight, is beneficial in improving the receivedsignal-to-interference plus noise power ratio (SINR). Ideal transmitantenna weights for precoding are generated from eigenvectors forthe maximum eigenvalue for covariance matrix HHH (H representsthe channel matrix and H denotes the Hermitian transposition) [11].However, it is not realistic to feed back the estimated channel stateinformation (CSI) or generate a precoding matrix (PM) withoutquantization because the number of available control signaling bitsis limited. Hence, codebook-based precoding is adopted in whichthe optimum PM is selected among the pre-defined 6 (64) candidatesfor the 2-by-2 (4-by-4) MIMO case as shown in Fig. 5 [10]. Incodebook-based precoding, the optimum PM is selected so thatthe total capacity, i.e., throughput of the transmitted streams, whichare precoded, is maximized. In this case two types of precodingschemes are adopted: frequency selective precoding over a two toeight RB bandwidth or wideband precoding over the entiretransmission bandwidth. Adaptive rank control called rankadaptation is used according to the received channel conditionssuch as the SINR and spatial correlation. Hence, a UE feeds backthe selected PM index and rank order in addition to the CQI to a BS.

Then, the BS dynamically selects the PM and rank order for eachUE according to the reported information as shown in Fig. 6. InMIMO multiplexing, two code words are used at maximum both fortwo and four transmission antennas.

In the Rel. 8 LTE uplink, one-stream transmission, i.e., SIMO, isadopted to simplify the transmitter circuitry and reduce the powerconsumption of a UE. It was shown that the peak data rate of 75Mbps, which exceeds the target peak data rate of 50 Mbps, isachieved using 64QAM with 2-antenna diversity reception and a20-MHz bandwidth [8]. However, in the Rel. 8 LTE uplink, theapplication of MU-MIMO or space division multiple access(SDMA) is considered to improve the capacity, although therequirements are satisfied without MU-MIMO. Thus, RS usage isspecified for channel-dependent scheduling in the spatial domainemploying MU-MIMO in addition to the time and frequencydomains [12]. The same multiplexing of SRSs is used as that withoutMU-MIMO for CQI measurement. Moreover, the assignment ofdifferent cyclic shifts created from the identical original CAZACsequence to demodulation RSs (DM-RS) of simultaneous UEs withMU-MIMO is specified and then the orthogonal DM-RS multiplexingwith the same transmission bandwidth is possible for uplink MU-MIMO.(2) Transmit DiversityWe first focus on transmit diversity for the downlink unicast channel.Transmit diversity is employed for the PDSCH as one option for therank 1 mode. Space frequency block code (SFBC) and thecombination of SFBC and frequency switched transmit diversity(FSTD) are used for two- and four-antenna transmission for localizedtransmission employing frequency domain channel-dependentscheduling and distributed transmission with semi-persistentscheduling. This is because SFBC and its combination with FSTDprovide a higher gain in reducing the required received SINR than

Fig. 4. Transport block mapping scheme in LTE-Advanced.

Transportblock

Transportblock

Transportblock

Transportblock

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

CC IFFT

Transportblock

Transportblock

Transportblock

Transportblock

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

Datamodulation

Mapping

Channelcoding

HARQ

CC IFFT

Fig. 5. MIMO multiplexing employing codebook based precoding.

Transmit data

Precoding

UE

PM index

Stream #1

Stream #2BS

Stream #1#2

Antenna #1

#4

Antenna #1

#4 Channel estimation

PMselection

P/S

Recovereddata

SignaldetectionS/

P

Transmit data

Precoding

UE

PM index

Stream #1

Stream #2BS

Stream #1#2

Antenna #1

#4

Antenna #1

#4 Channel estimation

PMselection

P/S

Recovereddata

SignaldetectionS/

P

Fig. 6. Adaptive rank control (rank adaptation).

UE #2UE #1

BS

• Low received SINR• High spatial correlation

Rank = 2

• High received SINR• Low spatial correlation

Rank = 1

Adaptive control of rank order

UE #2UE #1

BS

• Low received SINR• High spatial correlation

Rank = 2

• High received SINR• Low spatial correlation

Rank = 1

Adaptive control of rank order

2009 International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS 2009) December 7-9, 2009

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other schemes. Similarly, SFBC and the combination of SFBC andFSTD are applied to common or shared control channels. Meanwhile,precoding vector switching (PVS) transmit diversity is used for thePSS and SSS. This is because only selection type transmit diversityis applicable to the PSS/SSS, since the PSS/SSS is the first physicalchannel a UE acquires and decodes without knowledge about thetransmitter antennas at a BS.

In the Rel. 8 LTE uplink, open-loop or closed-loop switchingtransmit diversity is employed since only one transmitter is equippedat a UE despite having two antennas. Closed-loop antenna selectiontransmit diversity is employed for the PUSCH as an optional function.Time switched transmit diversity (TSTD) is applicable to the controlchannels including the RACH and PUCCH, although the applicationof TSTD is irrelevant to the radio interface, i.e., it is an implementationmatter.B. Multi-antenna Techniques for LTE-Advanced(1) Single-User MIMO (SU-MIMO)� Downlink SU-MIMOIn the Rel. 8 LTE a cell-specific orthogonal RS (CS-RS) is used forMIMO multiplexing and transmit diversity because a maximum offour antennas is supported. However, to achieve further improve-ment in the target peak spectral efficiency such as 30 bps/Hz, higher-order MIMO multiplexing is necessary for the LTE-Advanced down-link. Hence, it was agreed to extend the maximum number of trans-mission layers for MIMO multiplexing to eight in the downlink [6].To support up to eight antennas, the most important issue regard-ing the radio interface is the RS structure, i.e., orthogonal RSs forCQI measurement to support up to eight antennas is necessary.Meanwhile, from the viewpoint of the backward compatibility re-quirement with Rel. 8 LTE, LTE-Advanced must support Rel. 8 LTEUEs within the same bandwidth. Then, if the number of CS-RS sym-bols is increased to support up to eight antennas, the additional RSsymbols cause strong inter-cell interference to the coded data sym-bols for the Rel. 8 LTE UE. Therefore, in addition to the CS-RSsymbols for up to four antennas, the channel state information RS(CSI-RS) is newly specified for more than four to up to eight anten-nas in the LTE-Advanced downlink. The CSI-RS is transmitted at amuch longer interval than the subframe duration at the sacrifice oftracking the channel variation in the time domain. However, theassumption is valid since the application of greater than four anten-nas is focused on low mobility. Additionally, UE-specific DM-RS tosupport eight-layer MIMO multiplexing is also newly specified. Asa consequence, three types of orthogonal RSs are specified in LTE-Advanced: CS-RS, CSI-RS, and DM-RS. CSI-RS is used for CSI(CQI) measurement for more than four antennas. UE-specific DM-RS is transmitted together with user traffic data after the signal areprecoded with the same precoding weight as the coded data sym-bols.� Uplink SU-MIMOTo achieve an improved target peak spectral efficiency such as 15bps/Hz, SU-MIMO with up to four antennas is specified in the LTE-Advanced uplink. In particular, SU-MIMO functionality with twoantennas is adopted as a baseline for the LTE-Advanced UE. Thisis one of the distinct enhancements of LTE-Advanced from the Rel.8 LTE. Similar to the case in the downlink, precoding is used toimprove the cell-edge user throughput by way of increasing theantenna gain. However, due to the small gain from the frequencyselective precoding considering the increasing control signaloverhead and increased PAPR, wideband precoding, i.e., frequencynon-selective precoding, is adopted in the LTE-Advanced uplink[6]. In the LTE-advanced uplink, two code words are used at maximumfor SU-MIMO similar to the case in the Rel. 8 LTE downlink. Hence,assuming two antennas, per-antenna rate control is performedindependently for two streams. This enables the application of turbosuccessive interference canceller (SIC) based signal detection, whichis advantageous to SC-FDMA using DFT-Spread OFDM [13].

(2) MU-MIMOFurther improvement in the capacity compared to that in the Rel. 8LTE is required in LTE-Advanced. MU-MIMO is a promisingtechnique to improve the capacity. In both links, MU-MIMO iseffective in increasing the capacity when the number of antennasequipped at a BS is equal to or greater than four. In this case, threedimensional, i.e., time, frequency, and spatial domain, channel-dependent scheduling is performed. By extending SU-MIMO toMU-MIMO, only slight modification to the radio interface includingthe RS structure is required especially in the uplink. However, theapplication of block spreading multiplexing of the DM-RS isproposed to accommodate UEs with different transmissionbandwidths using MU-MIMO in the uplink [14]. Hence, furtherstudy on the RS structure for MU-MIMO is necessary consideringthe RS structure in the Rel. 8 LTE, which will be inherited in LTE-Advanced as well.(3) Transmit DiversityIn the LTE-Advanced downlink, the same transmit diversity schemesas those in the Rel. 8 LTE are applied, since each CC comprises Rel.8 LTE physical channels, i.e., SFBC and the combination of SFBCand FSTD with two- or four-antenna transmissions for the PDSCHwith rank one mode and common/shared control channels. On theother hand, in the LTE-Advanced uplink, the applied techniquesare different from those in the Rel. 8 LTE due to the adoption ofmultiple transmitters. For selecting transmit diversity schemes inthe uplink, the increase in the PAPR specified by the CM should besuppressed to a low level. Moreover, it is noted that furtherimprovement in the coverage area is unnecessary for the controlchannel since the LTE-Advanced cell is designed so that the Rel. 8LTE UE can achieve the required quality at the cell edge [13].

Furthermore, coordinated multi-point, i.e., multi-cell, (CoMP)transmission and reception schemes have been actively investigatedin the 3GPP associated with multi-antenna implantations at a BSand UE [6]. CoMP mainly contributes to increasing the cell-edgeuser throughput (which means it can also extend the effectivecoverage) although some advanced schemes can increase thecapacity as well.

VI. CONCLUSIONThis paper addressed broadband radio access techniques for theRel. 8 LTE and LTE-Advanced in the 3GPP. We first briefly describedthe system requirements focusing on the physical layer for the Rel.8 LTE and LTE-Advanced. Then, we described wider transmissionbandwidths around 100 MHz to satisfy the peak data rate of 1 Gbpsin the downlink and 500 Mbps in the uplink for LTE-Advanced. Wedescribed the extended multiple access schemes for LTE-Advancedbased on those for the Rel. 8 LTE, parallel CC based OFDMA in thedownlink and N-times DFT-Spread OFDM based SC-FDMA in theuplink. Finally, we addressed MIMO channel techniques includingSU-MIMO, MU-MIMO, and transmit diversity for the Rel. 8 LTEand LTE-Advanced. It should be noted that the LTE-Advanced radiointerface using multiple access schemes with the explained key techniquessatisfies the system requirements of IMT-Advanced [15].

REFERENCES[1] 3GPP, TS36.201 (V8.3.0), Feb. 2009.[2] Final Acts WRC-07, Geneva, Nov. 2007.[3] ITU-R Document IMT-ADV/1, Mar. 2008.[4] 3GPP, RP-080138, Mar. 2008.[5] 3GPP, TR36.913 (V8.0.1), Mar. 2009.[6] 3GPP, TR36.814 (V1.0.0), Feb. 2009.[7] 3GPP, TR25.913 (V7.3.0), Mar. 2006.[8] 3GPP, TR25.912 (V8.0.0), Dec. 2008.[9] 3GPP, TS36.104 (V9.0.0), May 2009.[10] 3GPP, TS36.211 (V8.6.0), Feb. 2009.[11] E. Biglieri, et al., MIMO Wireless Communications, Cambridge

University Press, 2007.[12] 3GPP, TS36.213 (V8.6.0), Feb. 2009.[13] T. Lunttila, et al., Proceedings of WPMC2009, Sept. 2009.[14] 3GPP, R1-074865, Nokia Siemens Network, Nokia, Nov. 2007.[15] A. Furuskär, et al., Proceedings of WPMC2009, Sept. 2009.