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LTE-Advanced ─ Evolution of LTE ─ Radio Transmission Experiments LTE-Advanced Multi-antenna Technology Experiment NTT DOCOMO Technical Journal LTE-Advanced—Evolution of LTE— Radio Transmission Experiments The expanded use of smartphones and tablet computers in recent years has brought about a dramatic increase in data traffic in the radio access network, which is expected to grow to even higher levels in the years to come. To meet this demand for radio access, the standardization of LTEAdvan
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  22 ©2012 NTT DOCOMO, INC.Copies of articles may be reproduced only for per-sonal, noncommercial use, provided that the nameNTT DOCOMO Technical Journal, the name(s) of the author(s), the title and date of the article appearin the copies. *1 OFDMA : A multiple access scheme that usesOrthogonal Frequency Division Multiplexing(OFDM). OFDM uses multiple low data ratemulti-carrier signals for the parallel transmis-sion of wideband data with a high data rate,thereby implementing high-quality transmis-sion that is highly robust to multipath interfer-ence (interference from delayed paths). LTE-Advanced  Evolution of LTE  Radio Transmission Experiments LTE-Advanced  Multi-antenna Technology  Experiment 1.Introduction Launched in December 2010,NTT DOCOMO’s “Xi” (Crossy) is amobile communications service con-forming to the LTE standard [1]. Itachieves higher data rates, highercapacities and lower latencies thanFOMA. LTE (whose initial release cor-responds to Release 8 specifications)adopts a variety of radio access tech-nologies such as intra-cell orthogonalmultiple access schemes (downlink:Orthogonal Frequency Division Multi-ple Access (OFDMA) *1 ; uplink: SingleCarrier-Frequency Division MultipleAccess (SC-FDMA) *2 ), frequencydomain scheduling *3 and Multiple-InputMultiple-Output (MIMO) *4 transmis-sion in the downlink [2].Against this background, the spreadof data-intensive services such as videodelivery is expected to increase datatraffic even further in the years tocome. To meet this growing demandfor radio access in a timely manner,NTT DOCOMO looks to make furtherimprovements in system performancein the radio access network and is pro-moting the standardization of LTE-Advanced, an evolution of LTE. TheLTE standard has been continuouslyupdated since Release 8, and LTE-Advanced corresponds to LTE Release10 and beyond [3]. Standardization of Release 10 specifications for the LTE-Advanced radio interface has alreadybeen completed and standardizationactivities for Release 11 are now under-way. LTE-Advanced must maintainbackward compatibility with LTE(Release 8/9) and must satisfy therequirements of International MobileTelecommunication (IMT)-Advanced[4]. LTE-Advanced has therefore LTE-Advanced—Evolution of LTE—Radio Transmission Experiments NTT DOCOMO Technical Journal Vol. 14 No. 2 Teruo Kawamura Yoshihisa Kishiyama Yuichi Kakishima Shinpei Yasukawa Keisuke Saito Hidekazu Taoka The expanded use of smartphones and tablet computers inrecent years has brought about a dramatic increase in datatraffic in the radio access network, which is expected to growto even higher levels in the years to come. To meet thisdemand for radio access, the standardization of LTE- Advanced—an evolution of LTE—is now in progress. Thisarticle describes experimental equipment constructed for testing the radio access technology adopted by LTE- Advanced and presents the results of field and indoor experi-ments on these LTE-Advanced radio access technologiesusing an LTE-Advanced transceiver. Radio Access Network Development Department DOCOMO Communication Laboratories Europe GmbH  23 NTT DOCOMO Technical Journal Vol. 14 No. 2 adopted Carrier Aggregation (CA) *5 toachieve wider transmission bandwidthsup to 100 MHz [5]. It has also adoptedadvanced multi-antenna technologiesincluding MIMO transmission in theuplink, extensions of MIMO transmis-sion in the downlink (such as Multi-User (MU)-MIMO *6 ) and CoordinatedMulti-Point (CoMP) transmission/recep-tion *7 [6][7].The authors have constructed LTE-Advanced real-time radio-transmissionexperimental equipment based onRelease 10 specifications and have sofar performed experimental evaluationsof radio access technologies in LTE-Advanced [8]-[15]. In this article, wedescribe the experimental equipmentthat we have constructed and presentthe results of field and indoor experi-ments on these LTE-Advanced radioaccess technologies using an LTE-Advanced transceiver. 2.Implementation onExperimentalEquipment The experimental equipment thatwe constructed for evaluating LTE-Advanced radio access technology sup-ports asymmetric CA (downlink: 100MHz; uplink 40 MHz), which meanswider transmission bandwidth achievedby using contiguous bands and differenttransmission bandwidths in the down-link and uplink in the Frequency Divi-sion Duplex (FDD) *8 scheme. Thisequipment also supports advancedmulti-antenna technologies such as 2  2 Single User (SU)-MIMO (spatialmultiplexing and closed-loop transmitdiversity *9 with precoding) in theuplink, 4  2 MU-MIMO in the down-link, and CoMP transmission in thedownlink using Remote Radio Equip-ment (RRE) with an optical-fiber con-nection.  2.1CA  The basic concept of CA in LTE-Advanced is shown in Figure 1 . CA isa technology that achieves wider trans-mission bandwidths while maintainingcompatibility with LTE (Release 8/9).It treats the frequency block (or Com-ponent Carrier (CC)) having a maxi-mum bandwidth of 20 MHz as a basicunit of connection with an LTE termi-nal and supports expanded bandwidthsup to 100 MHz using 5 CCs. LTE-Advanced supports not only CA usingcontiguous CCs but also CA using non-contiguous CCs as well as asymmetricCA that allocates a different number of CCs to the downlink and uplink in theFDD scheme [16][17]. These schemesmake for flexible spectrum allocation.Asymmetric CA can be used, for exam-ple, in packet-type services in whichtraffic is quite heavy in the downlink (as when downloading Web content)compared to that in the uplink. Theexperimental equipment introducedhere implements asymmetric CA usingcontiguous CCs.LTE-Advanced performs AdaptiveModulation and Coding (AMC) *10 andHybrid Automatic Repeat reQuest(HARQ) *11 for each transmission streamof each CC as well as in LTE. To pro-vide flexible support for AMC andHARQ in the downlink in asymmetricCA, the MS transmits Uplink ControlInformation (UCI) such as ChannelQuality Indicator (CQI) *12 andAcknowledgement (ACK)/Negative *2 SC-FDMA : A multiple access scheme thatemploys single carrier transmission for an indi-vidual user and achieves multiple access byallocating the signals for different users to dif-ferent frequencies. *3 Frequency domain scheduling : A tech-nology for scheduling radio resources allocatedto each user and obtaining a diversity effectbetween users by using fluctuation on the prop-agation channel in the frequency domain. *4 MIMO : A signal transmission technology thatimproves communications quality and spectralefficiency by using multiple transmitter andreceiver antennas for transmitting signals at thesame time and same frequency. *5 CA : A technology that achieves high-speed com-munications through bandwidth expansion whilemaintaining backward compatibility with existingLTE by performing simultaneous transmissionand reception using multiple component carriers. *6 MU-MIMO : A technology that improves spec-tral efficiency by applying MIMO multiplexedtransmission to the signals for multiple users. *7 CoMP transmission/reception : A technol-ogy that transmits or receives signals frommultiple sectors or cells to and from a givenUE. By coordinating transmission/receptionamong multiple cells, interference from othercells can be reduced and the power of thedesired signal can be increased. FrequencyCC#1CC#2CC#3CC#4CC#5(Example) 20 MHz(Example) 100 MHzLTE-AdvancedLTECA using non-contiguous CCs also possible Figure 1 Concept of CA in LTE-Advanced  ACK (NACK) from one predeterminedCC. In this way, UCI can be transmittedwithout increasing the Peak-to-AveragePower Ratio (PAPR) *13 in an uplink based on SC-FDMA. In this experi-mental equipment, UCI for all down-link transmission streams and all CCs isencoded all together and mapped to thePhysical Uplink Shared Channel(PUSCH) for transmission.  2.2Uplink SU-MIMO LTE-Advanced aims for a highpeak spectral efficiency of 15 bit/s/Hzin the uplink, and to achieve this, it sup-ports MIMO spatial multiplexing hav-ing a maximum of four transmissionstreams. It also supports closed-looptransmit diversity using multiple trans-mitter antennas. In transmit diversity, ascheme with precoding based on acodebook  *14 is adopted, and to maintaina low PAPR, wideband precoding usinga common precoding weight over con-tiguously allocated Resource Blocks(RB) *15 is also adopted. The concept of uplink SU-MIMO is shown in Figure2 . Here, a Mobile Station (MS) periodi-cally transmits a Sounding ReferenceSignal (SRS) to each transmitter anten-na. The Base Station (BS) estimates theMIMO channel response using thereceived SRS and performs adaptiveradio link control (i.e., determines rank number, precoding weight, and theModulation and Coding Scheme(MCS) *16 ). The MS then generates andtransmits a data signal based on theparameters determined at the BS. In theevaluation, switching of rank number isperformed semi-statically which doesn’taim to track instantaneous fading varia-tions. Furthermore, rank 1 correspondsto the closed-loop transmit diversitywith precoding and rank 2 to theMIMO spatial multiplexing in whicheach antenna transmits an independent-ly encoded stream using a 2  2antenna configuration.The sub-frame configuration in theuplink for this experimental equipmentis shown in Figure 3 . To measurereception quality across the entire band,the SRS transmission bandwidth hasbeen set to 96 RB (17.28 MHz) per CCregardless of the transmission band-width for the data signal. A sub-frame,which is 1 msec in length, consists of 14 SC-FDMA symbols *17 . Here, SRS ismultiplexed on the last symbol (the 24 NTT DOCOMO Technical Journal Vol. 14 No. 2 *8 FDD : A scheme for transmitting signals usingdifferent carrier frequencies and bands in theuplink and downlink. *9 Closed-loop transmit diversity : A tech-nology that obtains diversity gain andimproves communications quality by using thedifference in the fluctuation of propagationchannels between transmitter antennas. Amethod within this technology that uses feed-back from the receiver side. *10 AMC : A method for adaptively controllingdata rate by selecting the most appropriateMCS (see * 16) according to the quality of received signals based, for example, on SINR. *11 HARQ : A technology that combines Automat-ic Repeat request (ARQ) and error correctingcodes to improve error-correcting ability on aretransmission and reduce the number of retransmissions. A packet retransmissionmethod that improves reception quality andachieves efficient transmission by combiningresent data with previously received data. *12 CQI : An index of reception quality measuredat the mobile station expressing propagationconditions on the downlink. LTE-Advanced  Evolution of LTE  Radio Transmission Experiments MIMO spatial multiplexingBS  (  1 ) S R S  (  2 ) R a n k   M C S   p r e c o d i n g  w e i g h t (  3 ) D a t a  s t r e a m (  s )    (  R a n k  1 ) MS MS (1) (2)(3)Transmit diversity (  R  a n k   2   )  Figure 2 Concept of SU-MIMO transmission in the uplink SRS    R   B   (   1   8   0   k   H  z   )   P   U   S   C   H  r  e  g   i  o  n   (   1   7 .   2   8   M   H  z   )   C   C   (   1   8   M   H  z   )   F  r  e  q  u  e  n  c  y Sub-frame (1msec)TimeSC-FDMA symbol    S  u   b  c  a  r  r   i  e  r DM-RS (streams # 1-2)SRS(transmitter antennas #1-2)(Example: 5 msec) T   SRS  Figure 3Uplink sub-frame configuration in experimental equipment  25 NTT DOCOMO Technical Journal Vol. 14 No. 2 14th symbol) and transmitted periodi-cally at intervals corresponding to a T  SRS  sub-frame. Code Division Multiplexing(CDM) *18 is applied to the multiplexingof SRS from each transmitter antennathrough a cyclic shift of the same Ref-erence Signal (RS) sequence. In addi-tion, Demodulation RS (DM-RS),which is used in channel estimation *19 for data demodulation purposes, usesthe same band as the data signal multi-plexing at the 4th and 11th symbols of each sub-frame. When applying MIMOspatial multiplexing, CDM by cyclicshift is applied to the multiplexing of DM-RS for two streams as well as forSRS.  2.3Downlink MU-MIMO Downlink MU-MIMO has beenspecified as a required technology inLTE-Advanced to satisfy the spectral-efficiency requirements of IMT-Advanced [4]. This experimental equip-ment applies four BS transmitter anten-nas and two MS receiver antennas(maximum two transmission streamsper MS)—the same as the evaluationrequirements of IMT-Advanced—andevaluates MU-MIMO performancewith 2 MSs. The target here is toachieve a total peak throughput of approximately 1 Gbit/s by transmittinga total of four streams simultaneously tothese 2 MSs. The concept of MU-MIMO transmission in the downlink for this experimental equipment isshown in Figure 4 . Here, the BS peri-odically transmits Channel State Infor-mation (CSI) *20 -RS used for estimatingCSI for each transmitter antenna at eachMS, and each MS returns feedback tothe BS on CSI estimated by using thereceived CSI-RS. Using this CSI feed-back, the BS generates precodingweights to suppress mutual interferencebetween the 2 MSs using beam form-ing. Then, for each data stream, the BSmultiplexes the generated precodingweights on both the data signal andDM-RS, and finally transmits the sig-nals.When having a BS spatially multi-plex and transmit more data streamsthan the number of MS receiver anten-nas as in the MU-MIMO configurationof this experimental equipment, highlyaccurate CSI feedback is needed to gen-erate precoding weights for transmis-sion (beam forming) to suppress mutualinterference between the transmissionstreams of the 2 MSs. For this reason,we have implemented high-accuracyCSI feedback in the unit of sub-bandfor the experimental equipment asdescribed below. This form of CSIfeedback has not yet been specified inLTE-Advanced.The sub-frame configuration in thedownlink for this experimental equip-ment is shown in Figure 5 . Each CCconsists of multiple sub-bands, where asub-band is the frequency unit for pro-viding CSI feedback and applying com-mon precoding weights. Sub-band size( F  CSI  ) is defined as a multiple number of RBs, where each RB consists of 12 sub-carriers *21 (180 kHz). In addition, a sub-frame, which is 1 msec in length, con-sists of 14 OFDM symbols. Here, CSI- *13 PAPR : An index expressing the peak magni-tude of the transmitted waveform defined asthe ratio of maximum power to average power.If this value is large, the power amplifier back-off has to be made large to avoid signal distor-tion, which is particularly problematic formobile terminals. *14 Codebook : A set of predetermined precod-ing-weight matrix candidates. *15 RB : The smallest unit of time or frequencyallocation when scheduling radio resources. *16 MCS : A predetermined combination of datamodulation and channel coding rate when per-forming AMC. *17 Symbol : A unit of data for transmission. InOFDM, it comprises multiple subcarriers (see * 21). A CP (see * 36) is inserted at the head of each symbol. *18 CDM : When transmitting multiple signalsequences on the same radio system band, mul-tiplexing them using spreading sequences. *19 Channel estimation : The process of esti-mating the amount of fluctuation in receivedattenuation and phase rotation in the receivedsignal when a signal is transmitted over a radiochannel. *20 CSI : Information describing the state of theradio channel traversed by the received signal. BS MS#2 ( 1) C SI-RS ( 2) C SI f eedback MS#1 (1) (2) ( 3  )  D a t a  s t r e a m s  Figure 4 Concept of MU-MIMO transmission in the downlink
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