06917417

IEEE Communications Magazine October 2014 1530163-6804/14/$25.00 2014 IEEE

Yifei Yuan, Zhisong Zuo,Yanfeng Guan, XianmingChen, and Wei Luo arewith ZTE Corporation.

Qi Bi, Peng Chen, andXiaoming She are withChina Telecom.

INTRODUCTIONWith the fast expansion of smart phone usage,cellular networks need to accommodate theexplosive growth of wireless traffic. LTE/LTE-Advanced, which features OFDM and variousadvanced technologies, is helping to bridge thegap between the traffic demand and the achiev-able system capacities. With OFDM and MIMOtechniques, LTE-Advanced systems [1] shouldoffer peak spectral efficiency of 30 b/s/Hz on thedownlink (DL) with 88 MIMO, and 15 b/s/Hzon the uplink (UL) with 44 MIMO. In macrocell deployment, via advanced MIMO technolo-gies, the average cell spectral efficiency of down-link 44 MIMO and uplink 24 MIMO canreach 3.7 b/s/Hz and 2 b/s/Hz, respectively. Fur-thermore, with carrier aggregation, the peakdata rate can be directly scaled up. Carrieraggregation also improves the spectrum flexibili-ty with the bandwidth up to 100 MHz. Heteroge-neous deployment opens a new space for systemcapacity enhancements, where various types oflow power nodes (LPN) such as pico node,Home eNB, and relay node are added to themacro network to exploit the cell splitting gains.Meanwhile, enhanced inter-cell interferencecoordination (eICIC) can be employed to dealwith the increased interference in the overlaiddeployments.

Many of the LTE/LTE-A techniques aremotivated by improving the high peak rate andhigh spectral efficiency, rather than enhancingthe cell coverage. Compared to the dramaticadvancement of system throughput, littleimprovement is seen in the cell coverage from

the technologies standardized so far for LTE/LTE-Advanced. In fact, some key features suchas orthogonal frequency modulation and shorttransmission time interval (TTI) are designed forincreasing the spectral efficiency at the expenseof coverage performance. Some techniques suchas coordinated multi-point transmission (CoMP),MIMO, and enhanced control channel touchsome aspects of coverage, but they do notaddress the overall balance between the cell cov-erage of control and data channels, and betweenthe downlink and the uplink. Consequently, thecoverage of LTE is generally worse than that of3G systems, if operated in the same frequency.For example, assuming the UL VoIP with AMR12.2 kb/s, the maximum coupling loss for 3G isseveral dBs worse than LTE. The coverage issueis further acerbated by particular spectrumarrangements, for example, when higher fre-quency bands are allocated for LTE, renderingthe harsher propagation environment. In orderto match the coverage of 3G networks, morebase stations are needed to compensate the linkbudget shortfall. Such a situation incurs signifi-cant financial burden on operators as they planto smoothly migrate their 3G systems to 4G, ontop of the existing 3G site grid.

Several technologies in previous releases ofLTE/LTE-A can mitigate the coverage issue.The fractional frequency reuse (FFR) in LTERelease 8 facilitates the orthogonal frequencyallocations among adjacent base stations (eNBs),thus reducing the inter-cell interference andimproving the cell coverage. Adding low powernodes such as Release 10 relay nodes can com-bat the excessive pathloss/shadowing and extendthe coverage. While the cost of LPNs is lowerthan macro eNBs, careful network planning isneeded to handle the complicated interferencescenarios. The TTI bundling in Release 8 [2] dis-tributes a VoIP packet transmission over longertime duration. Hence, the instantaneous trans-mit energy can be accumulated and the link bud-get is improved.

Note that there is an ongoing work item in3GPP on improving the coverage of low-costdevices for machine type communications(MTC) [3]. That scenario is quite specific, tar-geting very low data rate applications and lowmobility. Hence, the techniques discussed in [3]may not be applicable to general deploymentwhere cellular users tend to be moving andengaged in fast communications.

Coverage of LTE/LTE-A systems is a generic

ABSTRACTVarious technologies in LTE/LTE-Advanced

have significantly improved the data throughputof 4G systems. However, the coverage of LTEnetworks, a very important performance metricto operators, receives relatively less attention. Inthis article we describe the motivations of LTEcoverage enhancements from several aspects.Through link budget analysis, the limiting linkand channels are identified. Then potential solu-tions to LTE coverage enhancements are dis-cussed in a comprehensive manner, with thefocus on schemes of specification impact. Thisarticle provides insights on how to design practi-cal schemes to improve the coverage of LTE/LTE-Advanced systems.

ACCEPTED FROM OPEN CALL

Yifei Yuan, Zhisong Zuo, Yanfeng Guan, Xianming Chen, Wei Luo, Qi Bi, Peng Chen, and Xiaoming She

LTE-Advanced Coverage Enhancements

YUAN_LAYOUT_Layout 9/25/14 12:56 PM Page 153

IEEE Communications Magazine October 2014154

issue and solutions can be of an evolutionarynature for backward compatibility consideration,or they can be rather revolutionary. In this arti-cle we focus on the former. The article is orga-nized as follows. In the following section, theLTE coverage issue is described. Next we illus-trate the process of limiting link identification.Then we discuss several candidate solutions toLTE coverage enhancements. The conclusionsare provided in the final section.

LTE COVERAGE ISSUESFUNDAMENTAL LIMITATIONS

OFDM is a key feature of mainstream 4G wire-less standards including LTE/LTE-A. The coher-ence nature of multiple carriers in OFDM leadsto a higher peak to average power ratio (PAPR)compared to the modulation schemes in 2G and3G. Because of this, the linearity requirementfor power amplifiers is more stringent, whichresults in poor efficiency of power amplifiers.This poses serious issues for coverage, especiallyin the uplink since the transmission power ofmobile terminals (UEs) is quite limited. In LTEuplink, single-carrier frequency division multipleaccess (SC-FDMA) is adopted, with the purposeto reduce the PAPR. SC-FDMA requires that aUE be allocated with a continuous but mutuallynon-orthogonal block of resources that forms asingle carrier.

Even with SC-FDMA, LTE uplink is still anorthogonal frequency division multiple access(OFDMA) system. It does not have soft han-dover as code division multiple access (CDMA)in 3G which can improve the SINR at cell edges.The inter-cell interference statistics of OFDMAsystems cannot be accurately modeled as Gaus-sian as in the case of CDMA systems, making itharder to maintain robust links at cell edges.

To reduce the air-interface latency, a shortsubframe structure is adopted for LTE/LTE-Asystems. The 1 ms duration of TTI, or one sub-frame, is shorter than 2ms-TTI in HSPA and20ms-TTI in Release 99 UMTS. Since they allcarry the similar payload, transmit energy is moreconcentrated in time for LTE/LTE-A, whichinherently burdens the link budget. While theshorter TTI would allow more accurate link adap-tation and reduce the latency, which can improvethe transmission efficiency to some extent, in gen-eral higher instantaneous transmission power isexpected for LTE than HSPA or CDMA.

As seen in Fig. 1, 4G system design empha-sizes spectral efficiency improvement where theoperating SNR is noticeably higher than that of3G systems. The green squares in Fig. 1 corre-spond to practically achievable spectral efficien-cies of various physical channels in 3G and 4G.The solid green curve is the fitting of thosesquares. Higher operating SNR will inevitablycause higher power consumption. The UE bat-tery lifetime for 4G will become more criticalcompared to that for 3G. In contrast, power effi-ciency was a major design consideration in 3G,which generally benefits the coverage. So evenoperating in the same frequency, the receiversensitivity of EV-DO or UMTS would be severaldBs better than that of LTE, for both VoIP andother low data rate services.

DEPLOYMENT SPECIFICAs a truly global standard, LTE/LTE-A supportsdiverse operations that cover different spectrumbands. However, in many cases LTE/LTE-A islikely to be deployed at higher frequencies thanthose in existing 2G and 3G systems, eventhough in the United States of America LTEwas firstly deployed at near 700MHz, which wasoriginally for TV broadcasting. In higher bands,the channel would experience severe path lossand building penetration loss. Less scatters areseen at high frequencies, indicating the lack ofdiversity, which can otherwise be used to miti-gate the severe path loss. With the higher bandsallocation for LTE/LTE-A compared to 3G,operators face challenges in ensuring propercoverage in practical deployments.

Wireless operators may pursue different pathsfor migration from 3G to LTE, depending ontechnologies used in 3G, spectrum allocation, andother factors. Smooth migration is preferred bymany operators, for example, to reuse the cellsites of 3G systems and to avoid excessive initialexpenditure in adding the new sites. For opera-tors who had chosen CDMA 1x and EV-DO fortheir 3G networks, the coverage issues may bemore challenging and there may be more cellplanning work to be done as the systems evolve toLTE. Table 1 shows the numbers of base stations(BSs in Ev-DO or eNBs in LTE) needed in ChinaTelecoms network in the Shanghai metropolitanarea. Link budget analysis is used to predict thecell size. It is observed in the case of EV-DO thatthe actual cell radius is smaller than estimated.This may be explained by the non-hexagonal cellshape and less uniform site-to-site distance in realdeployment. A similar situation is expected inLTE. The difference in the estimated cell radiusbetween EV-DO and LTE is due to:

Figure 1. 3G and 4G system design targets.

Eb/N0 (dB)

00.01

Bits

/s/H

z

0.1

1

10

2 4 6 8

3G system design focuson power efficiency

4G system design optimizedfor high spectral efficiency

Shannons limit

Practically achievable

10 12 14-2


IEEE Communications Magazine October 2014 155

Lower SNR required for EV-DO receiversin order to achieve the similar rate.

About 16 dB path loss shortfall of an LTEsystem that operates at 2.535 GHz, ratherthan 835 MHz for EVDO.

The much increased path loss for the higherband would make the cell size much reducedand the number of eNBs significantly increased.

LIMITING LINK IDENTIFICATIONUplink LTE/LTE-A physical channels have dif-ferent formats and thus require different SNRsfor correct decoding. Coverage is usually limitedby one or two channels. So the first step of thecoverage enhancements study is to identify limit-ing links.

EVALUATION METHODOLOGYFor practical systems, coverage can be measuredin various ways. In standards development bod-ies, three methods are commonly used to quanti-fy the coverage.

Maximum Coupling Loss (MCL) The cou-pling loss is basically the total long-term channelloss over the link between the transmit antennaports and the receiving antenna ports. MCL isthe upper limit of the coupling loss, whichdefines the coverage of a physical channel.MCL depends only on the transmit power andthe receiver sensitivity. The receiver sensitivityis determined by the required SINR, thermalnoise density, receiver noise figure, and theoccupied bandwidth.

Link Budget Link budget can be seen asa refined version of maximum coupling loss.The exact formula for link budget calculationmay be channel/service dependent. Neverthe-less, a typical link budget table would includea few extra entries, for example: hybrid auto-matic repeat request (HARQ) gain, whichcaptures the effective rate due to early termi-nation; system loading, which reflects theinter-cell interference level; and edge cover-age rel iabil i ty , which measures the confi-dence of coverage. These addi t ionalparameters help to more accurately capturethe real deployment. Similar to MCL, linkbudget analys i s re l ies pr imari ly on therequired SINR to calculate the coverage, andthe required SINR is based on the link levelsimulations.

System Level Simulations The cell edgethroughput, usually counting the 5 percentileUEs in the systems, is highly related to the cov-erage. Edge throughput is typically used for traf-fic channels, although control channels such asphysical downlink control channel (PDCCH)and physical uplink control channel (PUCCH)may also be studied using this metric. Theobtained cell edge rate can be affected by manyfactors, much more than those for MCL and linkbudget calculations. Among them are the sched-uler implementations, inter-cell interferencecoordination, which can reduce the interferenceseen by cell edge UEs, multiuser diversity, andso on. In a 3GPP LTE enhancements study [4],

MCL is chosen as the evaluation methodologyfor its simplicity.

As the uplink is usually more coverage limitedthan the downlink, in this article we focus onuplink channels. Interested readers can refer to [4]for more comprehensive simulations for an LTEcoverage enhancement study. Table 2 summa-rizes the simulation parameters for the bottle-neck channel identification Major uplink physicalchannels are simulated. Note that for physicaluplink shared channel (PUSCH, essentially theuplink traffic channel) simulations, the numberof UL physical resource blocks (PRB), the mod-ulation and coding scheme (MCS), and the max-imum number of HARQ transmissions are notspecified in Table 2. It is up to each company tochoose appropriate settings to get the best per-formance for each service/rate. The initial blockerror rate (BLER) is counted after the firstHARQ transmission, and the residual BLER iscounted after the maximum number of HARQtransmissions is reached. For PUSCH Message 3,Message 4, and UL VoIP, the inter-subframefrequency hopping is enabled. For VoIP, TTIbundling of Release 8 is used. PUCCH format 2is assumed to carry four bits.

COVERAGE COMPARISONSThe MCL of LTE uplink channels are comparedin Table 3, which is originally from [4]. It isobserved that the coverage of PUSCH mediumdata rate 384 kb/s is much worse than otherchannels. VoIP traffic channel is also a limitinglink, even with Release 8 TTI bundling. Randomaccess channel (RACH) and its subsequentPUSCH message 3 with transport block size(TBS) of 144 bits are the potential bottleneck.Note that for RACH, a relaxed performance tar-get, for example 10 percent miss-detection prob-ability (Pmiss), can be used if the latencyrequirement for the random access procedure isnot critical. The relaxed target can improve theMCL of RACH by 4~6 dB.

Based on the MCL evaluations, the PUSCHmedium data rate of 384 kb/s and UL VoIP areidentified as the limiting links that need cover-age enhancements.

POTENTIAL SCHEMESBRIEF DESCRIPTION OF SOLUTIONS

From the above discussions, it is seen that thecoverage issue of LTE/LTE-A stems from thedifferent design principles of 3G and 4G, as well

Table 1. Number of eNBs predicted and actually deployed in Shanghaimetropolitan area.

EV-DO LTE

Predict r (km)

Actual r (km)

Predict# BSs

Actual # BSs

Predict r (km)

Predict # eNBs

Dense urban 0.46 0.38 287 425 0.23 1096

Urban 0.83 0.51 194 503 0.40 846

Suburb 2.25 1.21 144 495 0.94 818



as the deployment-specific factors. For such ageneric problem, the ultimate solution requiresan overhaul of the entire LTE specifications,which is certainly not desirable. Instead, morerealistic goals can be set, for example, 1 dBimprovement, so that we can focus on a smallnumber of enhancement features that have lessspecification impacts.

Coverage enhancement solutions can be cate-gorized as: Air interface specification impacting. Implementation based without specification

changes, for example, no work needed in3GPP RAN1 working group. In the former category, the following schemes

have been proposed.

TTI Bundling Enhancements More energycan be accumulated by increasing the transmis-sion time of a packet. In Release 8 LTE, TTIbundling is used for uplink. It can support up to4-TTI bundling and limited to 3 PRBs. For cov-erage enhancements, TTI bundling with morethan three PRBs per subframe can be consid-ered for the PUSCH medium data rate to makethe channel coding more efficient. In addition,for uplink VoIP, more flexible HARQ timingand bundling size can be studied.

Using Spreading Codes Spreading code isanother way to achieve the lower rate and thusimprove the coverage. Compared to the straight-forward repetition, spreading can strengthen the

link robustness against interference, similar toCDMA systems. For LTE uplink, spreading isalready supported in PUCCH format 3, and itforms a good base for CDMA channelization forUL VoIP for coverage enhancement. Note thatthis scheme requires resource coordinationamong neighboring cells.

Frequency Hopping Enhancements Fre-quency hopping can provide frequency diversity,which is crucial to coverage. On the other hand,frequency hopping makes it difficult to jointlyestimate the channel over multiple subframes.Such joint estimation can improve the accuracyif the channel varies slowly. LTE Release 8offers very limited choices of frequency hopping.There may still be room for further enhance-ments to provide a better trade-off between thefrequency diversity and channel estimation accu-racy [5]. Fig. 2 shows two examples, correspond-ing to inter-subframe hopping only and bothinter/intra subframe hopping. With these newpatterns, both the gains of joint decoding andtime-frequency diversity can be obtained.

Implementation-based solutions are listedbelow. Although none of them requires stan-dardization in RAN1, 3GPP needs to define theminimum performance requirements for someschemes.

1.8 Rx Antennas at eNB Extending thenumber of receive antennas to eight is an effi-cient way to improve LTE UL coverage. It isequivalent to increasing the aperture of thereceiver, thus improving the sensitivity. LTEalready supports eight Tx antennas on the down-link, and therefore it is straightforward to sup-port eight Rx antennas on the uplink One typicalconfiguration is the cross-polarization antennaswhere in each polarization direction, four close-ly-spaced antennas form a uniform linear array(ULA). In 3GPP, a work item [6] was created tospecify the minimum performance requirementfor an eNB receiver of eight Rx antennas.

Advanced Receiver Compared to the base-line receiver, advanced receivers such as aninterference rejection/cancellation (IRC) receiv-

Table 2. LTE uplink simulation parameters for limiting link identification.

Parameters Values

Services and bit rates Service 1: VoIP (UL 12.2 kb/s) Service 2: Web browsing (UL 384 kb/s)

System bandwidth 10 MHz

UE Tx power 23 dBm

Num. of eNB Rx antennas 2

Antenna configuration UE 1 Tx

eNB receiver noise figure 5 dB

Radio channel Enhanced Pedestrian A, 3 km/h

Thermal noise PSD 174 dBm/Hz

Max number of HARQtransmissions for PUSCH 4

Performance target

1 percent Pmiss and 0.1 percent Pfa for RACH format 2

1 percent Pmiss and 1 percent Pfa for PUCCH format 1/1a

1 percent BLER for PUCCH format 2 10 percent residual BLER for PUSCH Message 3

and 4 2 percent residual BLER for VoIP 10 percent initial BLER for PUSCH media data

rate 384 kb/s

Table 3. MCLs of LTE uplink channels [4].

Channel MCL (dB)

RACH format 2 141.8

PUCCH format 1 146.5

PUCCH format 1a 147.2

PUCCH format 2 146

Message 3 TBS 56 146.7

Message 3 TBS 144 143.3

VoIP AMR 12.2 kb/s 141.7

Medium data rate 384 kb/s 132.4



er or a non-linear interference cancellationreceiver could mitigate both intra-cell and theinter-cell interference. In a RAN4 study, theIRC receiver has shown better performance thanthe baseline receiver at cell edge. The non-linearinterference cancellation receiver could providesignificant gain over linear receivers by suppress-ing strong interference from common referencesignal (CRS), primary synchronization signal(PSS), secondary synchronization signal (SSS),and primary broadcast channel (PBCH).

Power Boosting of Demodulation Refer-ence Signal As the operating SNR is pushedlower to improve the coverage, the receivingquality of the reference signal becomes a seriousissue. Boosting the power of demodulation refer-ence signal (DMRS) would improve the robust-ness of the channel estimation in low SNRregions. Since QPSK is most likely to be used incoverage-limiting scenarios, increasing the powerof DMRS does not require specification change.On the uplink it is more or less a UE implemen-tation issue. However, this solution may bringsome challenges to UE vendors. For example, ina subframe of LTE uplink, the DMRS is timemultiplexed with the data symbols (PUSCH).Certain transition time is needed to ramp upand ramp down the transmit power. The round-off waveform of the DMRS would reduce theoriginal gain of DMRS power boosting. Also,the un-even power level within a subframe tendsto lower the efficiency of the amplifiers.

In the following sections we will focus onspecification-impacting solutions, in particularthe TTI bundling enhancements that quicklybecame the popular choice among interestedcompanies. For performance evaluations of othercandidate schemes, readers are referred to [4]for more details.

ENHANCED SUBFRAME BUNDLING SCHEMESTTI bundling in LTE Release 8 is mainly forVoIP, and the total transmit energy of a VoIPpacket is evenly distributed over multiple sub-frames. TTI bundling can be extended toPUSCH medium data rate, with the followingmotivations: To reduce the overhead of upper layer sig-

naling. To improve the efficiency of the channel

coding. To have more time diversity as the trans-

mission of an aggregated packet can spanover a longer time duration.Unlike in VoIP, a packet size of PUSCH 384 kb/s

can easily go beyond 400 bits. Hence the restric-tion of three physical resource blocks (PRBs) inRelease 8 TTI bundling can be lifted so that theTTI bundling for medium data rate PUSCHwould be done more efficiently. Fig. 3 is anexample of 384 kb/s transmission where foursmaller packets are bundled into a larger packetof 1504 bits. Those four subframes that carry theaggregate packet may not be adjacent to eachother in time, as seen in Fig. 3.

In the case of no TTI bundling, at the radiolink control (RLC) layer, the transmission rateof 384 kb/s corresponds to one packet conver-gence protocol (PDCP) packet data unit (PDU)

of 368 bits plus a 2-byte RLC header at eachsubframe. After adding a 3-byte medium accesscontrol (MAC) header and a 24-bit cyclic redun-dancy check (CRC), each physical layer (PHY)PDU has 432 bits. Hence, four such PHY PDUscontain 1728 bits in total.

With TTI bundling, four PDCP PDUs areconcatenated into a 1472-bit PDU, to be sent infour subframes. With a 5-byte RLC/MAC head-er and a 24-bit CRC, the size of the single PHYPDU becomes 1536 bits. In this case, the savingof upper layer signaling overhead is 192 bits,roughly 0.5 dB.

The gain from larger Turbo interleaver size,for example, 432 to 1536 bits, is about 0.3~0.6 dB(Table 4) for additive white Gaussian noise(AWGN) channel and enhanced pedestrian A(ePA) channel. So combining the gain of over-head reduction and the gain of longer inter-leaver, the TTI bundling for medium data rateof 384 kb/s is roughly 1 dB.

Figure 2. Examples of frequency hopping enhancements. a) Inter-subframehopping; enhancements; b) Inter- and intra subframe hopping enhance-ments.

Slot

Hopping period

(a)

Hop

ping

band

wid

th

Slot

Hopping period

(b)

Hop

ping

band

wid

th

Figure 3. An example of bundled packets for 384 kb/s transmission at RLClayer.

CRCRLC

headerMAC

header

1472 bits from PDCP

1472 bits from PDCP16 24

# 7# 6

24

Subframe Packet for4 TTIs

RLC/MAC

# 5# 4# 3# 2# 1# 0



During a talk spurt, a VoIP packet arrives atthe physical layer every 20 ms. In theory, a maxi-mum of 20 subframes can be used for the trans-mission of each VoIP packet. However, if thedelay bound is imposed, the actual number ofsubframes used for a VoIP packet can be lessthan 20. For Release 8 TTI bundling, maximumusage of 12 or 16 subframes can be achieved for50 ms or 52 ms delay bound, respectively. RLCsegmentation is an alternative scheme toimprove the subframe utilization. However, dueto the very high overhead, the overall perfor-

mance of RLC segmentation is worse than TTIbundling. Thus RLC segmentation was not fur-ther studied in [4].

Figure 3 shows several schemes of TTIbundling enhancements for UL VoIP. Color codeis used to differentiate transmissions of differentVoIP packets, and the same color blocks are thetransmission or HARQ retransmission for a VoIPpacket. LTE uplink HARQ is synchronous, forexample, the time gap between HARQ (re)trans-missions is fixed, exhibiting a period pattern. Fig.4a illustrates a straightforward extension of 4-TTIbundling in Release 8 to 8-TTI bundling. Theround trip time (RTT) is still kept 16 ms. In thisscheme, up to two HARQ transmissions can besupported, that is, the maximum number of trans-missions (MaxTx) = 2. Otherwise, the thirdHARQ transmission would collide with the trans-missions of other VoIP packets (although notexplicitly shown in this figure). The number ofsubframes for a UL VoIP packet would be up to16 in this scheme. In Figs. 4b and 4c, the TTIbundling size is 4 TTI, similar to that of Rel-8.The difference is that the round trip time isreduced, which requires faster signal processing atthe eNB receiver to decode the VoIP packet intime. Both schemes can fully utilize the subframeresource, for example, 20 subframes. The schemein Fig. 4d can reuse the frame structure of circuitswitch voice, that is, 20-TTI aggregation, withoutHARQ retransmissions. Note that the subframescan be interleaved, as shown in the lower plot ofFig. 4d. This can improve the time diversity. Thescheme in Fig. 4e bundles 10 TTIs, and allowstwo HARQ transmissions. The scheme in Fig. 4fextends Release 8 TTI bundling by increasing oneof the bundles to eight TTIs, for example, trans-mitting eight TTIs in the first HARQ transmis-sion. Bundling size of five TTIs is shown in Fig.4g. Simulations show that the gain from the aboveTTI bundling ranges from 0.6 dB to 2 dB.

Among them, schemes (c)/(f)/(g) perform thebest. In light of the low complexity and compati-bility with Release 8 TTI bundling, scheme (c)was adopted for VoIP coverage enhancement inRelease 12.

CONCLUSIONSIn this article the LTE coverage issue was

described, not only from the aspect of funda-mental technologies of 3G and 4G, but also inthe context of deployment scenarios such as car-rier frequencies. Evaluation methodology wasthen discussed. From the maximum coupling lossanalysis, PUSCH medium data rate 384 kb/s anduplink VoIP were identified as the bottlenecklinks for LTE. Both specification-impactingschemes and implementation-based solutionswere presented. Among the specification-impact-ing solutions, TTI bundling enhancements showpromise in improving the coverage of PUSCHmedium data rate and UL VoIP.

REFERENCES[1] 3GPP TR 36.814, Evolved Universal Terrestrial Radio

Access (E-UTRA); Further Advancements for E-UTRAPhysical Layer Aspects.

[2] 3GPP, R1-080339, On VoIP Uplink Coverage for LTE,Ericsson, Jan. 2008.

Table 4. MCL gain from TTI bundling enhancement for PUSCH 384 kb/s.

Overhead reduction Turbo interleaver gain Total gain in MCL

~0.5 0.3~0.6 dB 0.8~1.1 dB

Figure 4. TTI bundling enhancement schemes for UL VoIP. a) 8-TTIbundling, RTT = 16 ms, MaxTx = 2; b) 4-TTI bundling, RTT = 8 ms,MaxTx = 5; c) 4-TTI bundling, RTT = 12 ms, MaxTx = 5; d) 20-TTIbundling, MaxTx = 1; e) 10-TTI bundling, RTT = 30 ms, MaxTx = 2; f)Flexible bundling sizes, RTT = 16 ms, MaxTx = 4; g) 5-TTI bundling,RTT = 15 ms, MaxTx = 4.

(a)

60 ms40 ms20 ms0 ms

60 ms40 ms20 ms0 ms

60 ms40 ms20 ms0 ms

(b)

60 ms40 ms20 ms0 ms

(d)

(e)

60 ms40 ms20 ms0 ms

60 ms40 ms20 ms0 ms

(f)

(g)

(c)

60 ms40 ms20 ms0 ms



[3] 3GPP TR 36.888, Technical Specification Group RadioAccess Network; Study on Provision of Low-Cost MTCUEs Based on LTE.

[4] 3GPP TR 36.824, Evolved Universal Terrestrial RadioAccess (E-UTRA); LTE Coverage Enhancements.

[5] 3GPP, R1-122130, TTI Bundling Enhancements Detailsfor PUSCH Medium Data Rate, ZTE, May 2012.

[6] 3GPP, RP-121709, Proposed WI on PerformanceRequirements of 8 Rx Antennas for LTE UL, China Tele-com, Barcelona, Dec. 2012.

BIOGRAPHIESYIFEI YUAN ([email protected]) received Bachelor andMaster degrees from Tsinghua University of China, and aPh.D. from Carnegie Mellon University, USA. He was withAlcatel-Lucent from 2000 to 2008, working on 3G/4G keytechnologies. Since 2008 he has been with ZTE, responsiblefor standards research on LTE-Advanced physical layer, andrecently on 5G technologies. His research interests includeMIMO, iterative codes, and resource scheduling. He wasadmitted to the Thousand Talent Plan Program of China in2010. He has extensive publications, including two bookson LTE-A.

ZHISONG ZUO ([email protected]) received his degreeof Master in computer science from Portland State Univer-sity, Oregon. He joined ZTE Corporation in 2003 and cur-rently serves as the chief delegate in 3GPP RAN1. He hasbeen involved in the research and standardization of theLTE physical layer protocol since 2005. His research inter-ests include OFDM system, small cells, control channel, andchannel coding.

YANFENG GUAN received a B.S. degree from North ChinaUniversity of Water Conservancy & Electric Power in 2000,an M.S. degree from HeHai University in 2003, and a Ph.D.from Southeast University in 2007. From 2007 to 2013 hewas a senior engineer at the Wireless Research Institute atZTE, and worked in wireless communication standards andresearch. His research interests are in the broad area ofcommunication, particularly resource management and sig-nal processing for wireless communication.

XIANMING CHEN ([email protected]) received theMaster degree in communication and information systemfrom Harbin Engineering University of China in 2009. Hehas been at ZTE as a senior engineer since 2009, engagedin research on wireless communication technologies andstandards. His main areas of research include LTE coverageenhancement technologies and 5G key technologies, forexample, massive MIMO.

WEI LUO ([email protected]) received the Masterdegree in communication engineering from Huazhong Uni-versity of Science and Technology of China in 2008. Shehas been in ZTE since 2008, where she works on the Wire-less Advance Research team. Her main areas of researchinclude wireless system design and LTE radio access net-work technologies, for example, CoMP interference coordi-nation and small cell enhancement.

QI BI ([email protected]) is the President of the Technolo-gy Innovation Center and the CTO of Beijing ResearchInstitute of China Telecom. He received his M.S. fromShanghai Jiao Tong University and Ph.D. from Pennsylva-nia State University. Prior to joining China Telecom in2010 he worked at Bell Labs for 20 years and became aBell Labs Fellow in 2002. He is an IEEE fellow. He has pub-lished extensively and holds 42 US patents and 66 Euro-pean patents.

PENG CHEN ([email protected]) received his Ph.D.degree in electronic engineering in 2006 from Beijing Uni-versity of Posts and Telecommunications. He has been withChina Telecom Innovation Center since 2010, working onLTE-A standardization in 3GPP RAN and cutting-edge tech-nology for IMT-2020. He is the rapporteur for the 3GPP LTERel-11 coverage enhancement study item.

XIAOMING SHE ([email protected]) received his Ph.D.degree in electronic engineering in 2004 from TsinghuaUniversity. He has been with China Telecom InnovationCenter since 2012, working on LTE-A standardization in3GPP RAN and cutting-edge technology for IMT-2020. Heis currently the rapporteur for the 3GPP LTE Rel-12 cover-age enhancement work item.


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