27NTT DOCOMO Technical Journal Vol. 15 No. 2

2013 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) ofthe author(s), the title and date of the article appearin the copies.

Currently, Radio Access Network EngineeringDepartment

*1 Cell: The smallest unit of area in which trans-mission and reception of wireless signals isdone between a cellular mobile communica-tions network and mobile terminals.

*2 HetNet: A network configuration that over-lays nodes of different power. It typicallyincludes picocell, femtocell, Wi-Fi and otherbase stations of lower power than conventionalbase stations, mixing, linking and integratingmultiple technologies.

Further Development of LTE/LTE-Advanced LTE Release 10/11 Standardization Trends

3GPP Release 11 Interference-rejection Technology Mobile-device Performance

1. IntroductionTo deal with the sharp increase in

traffic due to the recent spread of smart-phones and other factors, cell*1 densityis being increased, and as a result, inter-ference from adjacent cells is alsoincreasing. Also, when operating Het-erogeneous Networks (HetNet)*2 withsmall cells*3 inside macrocells*4 using asingle frequency, interference between

the macro-cell and small-cells is evengreater. In this sort of area, the power ofthe interference signal reaching themobile terminal from adjacent cells islarge compared to that of the noise sig-nal, and this interference degradesthroughput*5 performance.

In this article, we describe exten-sions in the Release 11 LTE specifica-tions (hereinafter referred to as Rel. 11LTE) using technologies studied at the

3GPP to improve the performance ofmobile terminals.

2. Overview of Mobile-device InterferenceRejection/SuppressionTechnology

Rel. 8 LTE mobile terminal perfor-mance stipulations were set assumingreceivers using Minimum MeanSquared Error (MMSE)*6 and the stan-

Improved Interference Rejection and SuppressionTechnology in LTE Release 11 Specifications

Yuta Sagae0

Yusuke Ohwatari0

Yousuke Sano0

To handle recent large increases in traffic, cell density isincreasing. The accompanying increase in interference fromadjacent cells is becoming a issue, so there is a need toreduce this interference.In this article, we give an overview of technologies newlyintroduced into the Release 11 LTE specifications at the3GPP, which increase the performance of mobile devices byrejecting and suppressing interference. Specifically, wedescribe interference rejection combining receivers that usemultiple receiver antennas on the mobile terminal to sup-press interference arriving from adjacent cells. Thisimproves throughput performance, mainly near cell bound-aries.

Radio Access Network Development Department

Research Laboratories

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To handle recent large increases in traffic, cell density is increasing. The accompanying increase in interference from adjacent cells is becoming a issue, so there is a need to reduce this interference. In this article, we give an overview of technologies newly introduced into the Release 11 LTE specifications at the 3GPP, which increase the performance of mobile devices by rejecting and suppressing interference. Specifically, we describe interference rejection combining receivers that use multiple receiver antennas on the mobile terminal to suppress interference arriving from adjacent cells. This improves throughput performance, mainly near cell boundaries.

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*3 Small cell: A general term for cells that trans-mit with power that is low compared to that ofa macrocell transmitting at higher power.

*4 Macrocell: An area in which communicationis possible, covered by a single base station,and with a radius from several hundred metersto several tens of kilometers.

*5 Throughput: The amount of data transmittedwithout error per unit time, i.e., the effectivedata transfer rate. In this article, throughout is

defined as the (data rate on the transmissionside) x (number of packets received withouterror per unit time) / (number of packets trans-mitted per unit time).

*6 MMSE: A method for suppressing interfer-ence from other signals by multiplying thereceived signal with calculated weights.

*7 User throughput: The amount of data thatone user can transmit without error per unittime.

*8 MIMO transmission technology: A signaltransmission technology that improves com-munications quality and spectral efficiency byusing multiple transmitter and receiver anten-nas for transmitting signals at the same timeand same frequency.

*9 Antenna gain: The power emitted by anantenna relative to an ideal antenna.

*10 Null: A direction in the beam pattern forwhich the antenna gain is very small.

Improved Interference Rejection and Suppression Technology in LTE Release 11 Specifications

dard Rel. 8 LTE MMSE receiversprocess reception assuming that theeffects of interference from adjacentcells will be comparable to that of noisereceived by the mobile terminal. Thus,in environments where the power of theinterference signal is larger than that ofthe noise, interference from adjacentcells restricts throughput.

Rel. 11 LTE has introducedMMSE-Interference Rejection Combin-ing (MMSE-IRC) receivers [2] as amobile terminal interference rejectionand suppression technology to mitigatethe effects of these interference signalsand increase user throughput*7 even inareas that are recently experiencinghigh interference. Rel. 8 LTE receiverssupport MIMO transmission technolo-gy*8, so receivers were equipped with atleast two antennas since it was firstintroduced. The MMSE-IRC receiversin Rel. 11 LTE, are able to use the mul-tiple receiver antennas to create points,in the arrival direction of the interfer-ence signal, where the antenna gain*9

drops (nulls*10) and use them to sup-press the interference signal (Figure1). The terminal orients a null towardthe main interference signal, which isthe signal that particularly affects thedegradation of throughput, therebyimproving the Signal-to-Interference-plus-Noise power Ratio (SINR)*11 andimproving throughput performance.However, with the original MIMO mul-

tiplexed transmission, which realizedhigh throughput using multiple transmitand receiver antennas, the receiverantennas are used to separate the signalsbetween layers, so interference fromadjacent cells cannot be suppressed andthroughput cannot be improved, partic-ularly for mobile terminals with tworeceiver antennas.

On the other hand, the 3GPP hasalready included interference rejectionand suppression technology in perfor-mance specifications for mobile termi-nals equipped with W-CDMA/High-Speed Downlink Packet Access(HSDPA)*12 in Rel. 7 of the UniversalMobile Telecommunications System(UMTS). With W-CDMA, receiversnormally use one receiver antenna andperform Rake reception*13, but theeffects of multipath*14 interferencedegrading reception performance wasan issue. Thus, the following threereceiver extensions were studied andintroduced. Type 1/1i extends the Rake receiver

to use two antennas.

Type 2/2i extends the Rake receiverto an MMSE receiver that suppress-es multipath and adjacent-cell inter-ference.

Type 3/3i extends the MMSE inter-ference-suppressing receiverdefined in Type 2/2i to use tworeceiver antennas.

The functional extensions inreceivers in Rel.7 UMTS and Rel. 11LTE are summarized in Table 1. TheMMSE-IRC receivers in Rel. 11 LTEincorporate receiver algorithms that aregenerally equivalent to those in theType 3/3i receivers introduced in W-CDMA/HSDPA. However, in the W-CDMA/HSDPA receivers they alsooperate to suppress inter-coding inter-ference within a cell. There is no inter-ference within a cell in LTE systems, soin the MMSE-IRC receivers introducedin Rel. 11 LTE, they operate to sup-press interference arriving from adja-cent cells.

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Serving cellMMSEreceiver

Interfering cell

MMSE-IRC receiver

Figure 1 Network structure handled with mobile-device extensions

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3. Extension to Rel. 11MMSE-IRC Receivers

As described above, the MMSE-IRC receivers introduced in Rel. 11LTE use multiple receiver antennas todirect a null toward the interference sig-nal from an adjacent cell, suppressingthat interference signal.

We now describe the MMSEreceivers that are standard in Rel. 8LTE. MMSE receivers handle interfer-ence included in the signals theyreceive as noise, and generally combinethe signals received at the receiveantennas as expressed by the followingequation.

(1)

Here, WRx, 1 is a receiver weightmatrix*15 used to combine the signalsfrom the two receiver antennas, and canbe expressed as follows.

(2)

(3)

Here, 1 is the signal after recep-tion processing, r is the received signalvector, H1 is the channel matrix

*16

between the serving cell and the termi-nal, P1 is the transmit power of the serv-ing cell, and I is the identity matrix*17.When computing the channel covari-ance matrix*18, R, the interference sig-nal component is included in the noisepower,2. Since interference signalsare handled as equivalent to noise inthis way, the arrival direction of thenoise is ignored, and the quality is max-imized with respect to the interferenceand noise power.

In contrast, with MMSE-IRCreceivers, interference signals are con-sidered independently of noise compo-nents instead of handling them asequivalent to noise. The covariancematrix, R, in Equation (2) handles thenoise component independently, asshown in the following equation.

(4)

Here, Hi (i>1), is the channel matrixbetween the ith interfering cell and themobile terminal, and Pi is the transmitpower from the interfering cell. Hi Hi

H

expresses the degree of correlation inthe interference signal from the ith cell,and by using the inverse correlation(inverse matrix) of the interference sig-nal as a receiver weight matrix, thecoefficients corresponding to the maininterference signal can be reduced, sup-pressing it. The interference signal issuppressed by orienting a null in thedirection of its arrival. Then, the recep-tion quality of the interference andnoise components after suppression ismaximized. Due to suppression of theinterference signal, the SINR is higherthan for an MMSE receiver, so higherthroughput can be achieved.

Note that Rel. 11 LTE MMSE-IRCreceivers can be used on Rel. 8 LTEbased systems, so interference suppres-sion effects can be obtained on Rel. 8LTE networks that have already beguncommercial services.

The results of throughput improve-ments due to MMSE-IRC receivers areshown in Figure 2. The graphs havethroughput on the vertical axis, andaverage received SINR on the horizon-tal axis, and show the results for mobileterminals moving at 3 km/h and 30km/h, assuming motion at speeds of

R=P1 H1 H1 + Pi Hi Hi +2

IH H i

d

R=P1 H1 H1 +2

IH

WRx, 1=P1 H1 R1H

d1=WRx, 1 r

*11 SINR: The ratio of desired-signal power to thesum of all other interference-signal power andnoise power.

*12 HSDPA: A high-speed downlink packet trans-mission system based on W-CDMA. Maxi-mum downlink transmission speed under the3GPP standard is approximately 14 Mbit/s.Optimizes the modulation method and codingrate according to the radio reception conditionof the mobile terminal.

*13 Rake reception: A technique for improvingreception quality by collecting and receivingsignals that have different propagation delaysand superimposing those signals.

*14 Multipath: A phenomenon that results in aradio signal transmitted by a transmitter reach-ing the receiver by multiple paths due to propa-gation phenomenon such as reflection, diffrac-tion, etc.

*15 Receiver weight matrix: A matrix reflect-

ing fluctuations in amplitude and phase, andused to combine or separate signals received atmultiple receive antennas. Signals can beobtained by multiplying the signal vector of thereceived signals by the matrix to combine orseparate them.

*16 Channel matrix: A matrix composed of thechanges in amplitude and phase on the chan-nels between each transmit and receive antennapair.

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Basic configuration

Mobile deviceextensions

Rel.7 UMTS Rel.11 LTE

Rake receiver (one receiver antenna) MMSE Receiver (two receiver antennas)

Type 1/1i: Rake reception with two receiver antennasType 2/2i: MMSE receiver suppressing multi-path and adjacent-cell interference One receiver antennaType 3/3i: MMSE receiver suppressing multi-path and adjacent-cell interference Two receiver antennas

MMSE-IRC receiver

(two receiver antennas)

Table 1 Receiver function extensions

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*17 Identity matrix: A square matrix with diago-nal elements of one (1) and all other elementszero (0).

*18 Covariance matrix: A matrix whose diago-nal components express the variance of eachvariable in a set of variables and whose otherelements each express the degree of correlationbetween two variables with respect to theirdirection of change (positive/negative).

*19 Radio access network: The network con-sisting of radio base stations and radio-circuitcontrol equipment situated between the corenetwork and mobile terminals.

Improved Interference Rejection and Suppression Technology in LTE Release 11 Specifications

walking and riding in a vehicle respec-tively. These simulations assume high-traffic areas with two main interferingcells and with sufficiently large Inter-ference-to-Noise power Ratios (INR)for each of the interfering signals. Theresults show an improvement in thethroughput performance of approxi-mately 30% compared to Rel. 8 LTEMMSE receivers at cell boundarieswhere SINR is low.

4. ConclusionIn this article, we have described

mobile terminal interference rejection

and suppression technology being stan-dardized in Rel. 11 LTE at the 3GPP. Itis a promising technology for improv-ing user throughput in high-interferenceenvironments such as where cells aredensely installed to handle recent high-traffic. This functionality can also beused with ordinary LTE, such as Rel. 8LTE, so it can support a variety ofintroduction and cell-expansion scenar-ios.

In the future, we will continue topromote standardization towardimproving mobile terminal receptionperformance, to further improve the

performance, functionality and econo-my of radio access networks*19.

References[1] Nakamura et al.: Overview of LTE-

Advanced and Standardization Trends,

NTT DOCOMO Technical Journal, Vol.12,

No.2, pp.4-9, Sep. 2010.

[2] 3GPP TR36.829 V11.1.0: Enhanced per-

formance requirement for LTE User

Equipment (UE), 2013.

[3] 3GPP TS36.101 V11.4.0: Evolved Uni-

versal Terrestrial Radio Access (EUTRA);

User Equipment (UE) radio transmission

and reception, 2013.

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Figure 2 Throughput improvements for MMSE-IRC receiver

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