Progress of 3GPP LTE Standardization

Xu Jing Xu Honglin Zhou Ting

（Shanghai Research Center for W ireless Communications， Shanghai 200050， China）

Abstra ct:The 3rd Generation Partnership Pro ject（3GPP）initiated its Long Term Evo lution（LTE）standard ization work to retain its technology and standard advantages in the fie ld of mobile comm unications.A series of advanced technolog ies and novel concep ts related to the physical layer（Layer 1）,air interface p rotocol structure layer（Layer 2）and network architecture are adop ted.Solutions to Voice over IP（Vo IP）and Multim ed ia Broadcast/Multicast Service（MBMS）are in the op tim ization p rocess.3GPP LTE has achieved the design goals of high data rate,low latency and an all-packet basis.

I n order tomeet the challenge of broadband access technology,and to satisfy the requirements fornew services,the international standardization organization,the 3rd Generation Partnership Project（3GPP）,initiated its Long Term Evolution（LTE）standard ization work at the end of2004.The LTE has the follow ing goals:

（1）Retaining 3GPP's strengths in mobile communication technology and standard ization

（2）Filling the huge gap between 3G and 4Gmobile communication technologies

（3）Maintaining the benefits of spectrum resources by utilizing the spectrum allocated to 3Gmobile communication systems（4）Correc ting the over-concentration of the patent rights for3Gmobile communication systems There are two phases in the schedule of3GPP LTE:the study phase from December2004 to June 2006；and the phase foraccomp lishing the LTE standard ization from June 2006 to June 2007.However,the study phase was extended untilSep tember2006 because ofsome p rob lem s leftunsolved.The standard ization p rocess of3GPPLTE,which is initially aimed atevolving the 3rd generationmobile communication system,is indeed a technological revolution p rocess due to service demands,competition from other technologies,and p ressure from operators.Com pared w ith the 3rd generationmobile communication system,3GPP LTE has revolutionized the physical layer（Layer1）transm ission technologies[1],the air interface p rotocol structure layer（Layer2）,and network architecture[2].

1 Evolution Purposes of 3GPP LTE

3GPP LTE is fulfilling an all-packet-based mobile communication system with high data rate and short latency.Its basic purposes[3]are as follows:

（1）Flexib le Spectrum Bandw id th Configuration

The LTE system shallsupport the bandw id th configuration of1.25MHz,1.6MHz,2.5MHz,5MHz,10MHz,15MHz and 20MHz.In this way,the system can technologically use the spectrum of the 3rd generationmobile communication system.

（2）Imp roved Transm ission Rate at CellEdge

Both transm ission rate and user experience at the celledge shallbe imp roved to enhance the coverage performance of3GPP LTE system.This purpose can be fulfilled by Frequency Division Multip le Access（FDMA）and inter-cellinterference supp ression techniques.

（3）Date Rate and Spectrum Utilization In terms ofdata rate and spec trum utilization,the system enab les a peak downlink rate of100Mb/s and a peak up link rate of50Mb/s.Compared to the High Speed PacketAccess（HSPA）system,ithas 2 to 4-fold inc rease in both spec trum utilization and average user throughput.Multi-antenna technology,adap tivemodulation and cod ing,and frequency selec tive scheduling based on channelquality are adop ted to ensure the streng ths of3GPP LTE system in the field ofspectrum utilization.

（4）Latency

The LTE system enab les less than 5ms one-way transm it time in the user p lane.Italso supports less than 50m s transition time for the controlp lane from a dormantstate to an active state,and less than 100ms transition time from a cam ped state to an active state.

Therefore,ithas a strong support for real-time services.

（5）Various Multimedia Broadcastand MulticastServices Support

Itenhances the supportofmultimed ia b roadcastand multicastservices tomeet the demand of their integration.The frame structure of the physical layer,channelstruc ture of Layer2 and upper-layer rad io resourcemanagement help achieve this goal.

（6）AllPacketSw itching

The LTE system uses allpacket sw itching to im p rove system spectrum utilization.However,its support for Voice over IP（Vo IP）services and smalldelay make itd ifficult for scheduling and signaling design between Layers 1 and 2.

（7）Co-existence

The system can co-existw ith the third generation and othermobile communication system s.

2 Transm ission Technologies for Physical Layer of 3GPP LTE System

2.1 Uplink/Downlink Transm ission Schemes for Physical Layer

The OrthogonalFrequency Division Multip le Access（OFDMA）technology is used for downlink transm issionwhile the Sing le-Carrier Frequency Division Multip le Access（SC-FDMA）technology is used forup link transm ission.The sub-carrier intervalis 15 kHz.SC-FDMA is a sing le-carrier transm ission technology based on OrthogonalFrequency Division Multip lexing（OFDM）.It features a low peak-to-average power ratio.Both OFDMA and SC-FDMA are ab le to support frequency selec tive scheduling well.

2.2 Frame Structure

Each frame on both up link and downlink is 10ms,and has 20 slots and 10 sub-frames,w ith am inim ized physicalresource b lock of180 kHz.

In order to supportb roadcastand unicastservices,both long frame Cyc lic Prefix（CP）and ShortCP are designed fordownlink transm ission.Each shortCP sub-frame comp rises seven OFDM symbols and mainly supports unicast services.Each long CP sub-frame has six OFDM symbols and 16.67m s duration.Itsupportsmulticastservices,helps im p lementsing le-frequency networking,and achieves the combined gain ofmulti-celltransport.

Each up link sub-frame consists of eightOFDM symbols:two shortOFDM symbols and six long OFDM symbols.The shortOFDM symbols are responsib le for transm itting p ilotsignals while the long ones are used for data transm ission.

Additionally,in order to co-existw ith the Time Division Dup lex（TDD）system,the LTE system has special frame structures for Low-Chip-Rate Time Division Dup lex（LCR-TDD）and High-Chip-Rate Time Division Dup lex（HCR-TDD）.

2.3 Inter-cell Interference Control Technology

Inter-cellinterference control technology is used to increase the information transm ission rate at the celledge.Major multi-cellinterference compensation technologies inc lude interference random ization,interference cancellation and multi-cellinterference coordination.Early research work on 3GPP LTE standard ization focused on themulti-cell interference coord ination technology for downlink Frequency Division Dup lex（FDD）system.This technology is used to app ly restric tions to the frequency resources and transm itpower.The users at the cellcenter can utilize allresource b locks,but they are required to use low powerwhen they enjoy certain resource b locks.On the otherhand,the users at the celledge can utilize certain resource b locks in fullpower,accord ing ly im p roving the cell-edge Signal-to-Interference-p lus-Noise Ratio（SINR）and cellcoverage.The up linkmulti-cellinterference controlin the FDD system is im p lemented by power controltechnology,w ith the frequency reuse fac torof1.

2.4 Multi-antenna Technology

Currently,the sing le-userdownlink multi-antenna p refers no d isturbance among layers.This is im p lemented by unitary-matrix-based p re-coding technology,w ith term inal feedback being the unitarymatrix index.Other multi-antenna technologies are for further research.

The downlinkmulti-user multi-antenna p refers unitary-matrix-based term inal feedback,butnotallp re-codingmatrices of the base stations are unitarymatrices.The evaluation and selection ofup link transm itdiversitymainly rely on the poweram p lification requirements.

▲Figure 1. Architecture and functions of 3GPP LTE network.

2.5 Cell Search Technology

The design of cellsearch focuses on synchronized channels and cell sequence.Considering the com p lexity of cellsearch,LTE inc lines to use themajor synchronized channel for cell synchronization,and the secondary synchronized channels for cell ID detection.Common pilotsequence is adop ted in themajorsynchronized channelwhile differentpilotsequences are adop ted in them inorsynchronized channels ofdifferentcells.As for the design ofcellp ilotsequences,requirements on both performance and comp lexity should be taken into consideration.Atp resent,referenced code types inc lude PN,Zadoff-Chu and Frank.

2.6 Random Access Technology

Random access is c lassified into non-synchronized random access and synchronized random access.

As fornon-synchronized random access,the usermessage bits,such as useraccess destination and user ChannelQuality Ind icator（CQI）,are concealed behind the user signature sequence to im p rove the base station's controlefficiency ofuseraccess.The usersignature sequence adop ts the Zadoff-Chu Zero Correlation Zone（ZC-ZCZ）code.There is a contention on whethera longer code ora sim p le short code repetition should be used for the coverage ofa large cell.LTE p refers the latter in consideration of the com p lexity of,and resistibility to,frequency offset.

There are few d iscussions on synchronized random access.In term s of functionality,many com panies have recommended to cancelthe channelfor synchronized random access.As resource requests from users can be delivered to the base station through a vastof transportmodes,the need to design a synchronized random access channelfor the synchronized users to send resource requests is elim inated.

3 3GPP LTE Network Architecture

The 3G network consists of fournetwork nodes:Node B（NB）,Rad io Network

Controller（RNC）,Serving GPRSSupport Node（SGSN）and Gateway GPRS SupportNode（GGSN）.

Themain func tions ofRNC are rad io resourcemanagement,network-related functions,maintenance and operation of Rad io Resource Control（RRC）,and supp ly of the interface w ith Network Management（NM）system.However,sincemany functions related to air interface are located in the RNC,resource allocation and services are not ab le to adap t to channels.Thismakes the p rotocolstruc ture very com p licated,and it results in d ifficultsystem op tim ization.

A decision wasmade at the p lenary meeting of3GPP in March 2006 that the LTE network uses a flatnetwork structure,and consists ofevolved Node B（eNB）and Access Gateway（AGW）.Themain functions ofeNB are as follows:

·selecting an AGW in attachment state

·send ing paging and b roadcast messages

·dynam ic allocation of rad io resources,inc luding multi-cellrad io resourcemanagement

·setting and offering eNB measurement

·rad io bearer control

·rad io adm ission control

·connec tionmobility controlunder ac tive state

Figure 1 shows the network architecture and functions of3GPPLTE network,in which,the interface between eNB and AGW is S1,and the interface between eNBs is X2.Allfunctions related to air interface are centralized in eNB.The Radio Link Control（RLC）,and the Media Access Control（MAC）are im p lemented by the same network node.This facilitates combined op tim ization and design.

4 Layer 2 Standardization

4.1 Layer 2 Technology Overview

In order toformulate a unified standard,the Layer2 standard does notconsider the d ifference between FDD and TDD.Thatis to say,FDD and TDD are expected to be converged and integ rated in Layer2.The all-packet-based p rotocol results in a greatly simp lified 3GPPLTE p rotocolstructure.As both RLC and MAC are located ateNB,the scheduler can divide Service Data Units（SDU）accord ing to the channelquality.This helps reduce padding,and itmaxim izes the channel transm ission capability.It also enab les combined op tim ization for Automatic RepeatRequest（ARQ）at the RLC layerand Hybrid Automatic Repeat Request（HARQ）at the MAC layer.

4.2 Layer 2 Protocol Structure

Figure 2 shows the downlink p rotocol structure of Layer2,which is very sim p le due to the elim ination ofcircuitsw itching.

Com pared to 3G p rotocols,the LTE p rotocolhas a struc ture w ith HARQ and ARQ entities located at the eNB.The leng th ofRLC SDU is changeab le.Besides,the LTE p rotocolsupports re-segmentation ofRLCProtocolData Units（PDU）,enab ling the service payload to adap twell to channelquality.

On the otherhand,as the rigorous delay requirementasks forsmall filling,3GPP LTE Layer2 p rotocolsupports multip lexing ofseveralrad io bearers belonging to the same term inalat the MAC layer.

Figure 3 shows that the LTE-p referred RLC and MAC structure,which can wellsolve the overhead p rob lem resulting from the rigorous delay requirement.

4.3 Design Ideas of ARQ， and ARQ and HARQ Cooperation

ARQ and HARQ ensure the reliab le

transportofmost im portantdata.In 3GPP LTE system,both ARQ and HARQ are located at the eNB,which facilitates combined design of the system.

The ARQ p rotocolis g reatly sim p lified in comparison w ith ARQ ofR6.The design ideas of ARQ are as follows:

（1）In order to ensure independency among p rotocol layers,the RLC and PDCP sequence numbers are independentofeach other.

（2）In order to reduce RLC header overhead,both RLC sequence numbers and re-transportofARQ are based on the RLC PDU.

（3）ARQ re-transportsupports re-segmentation（adap tab le to channel quality）,w ith re-segmentation times larger than 1.

The cooperation of ARQ and HARQ targets rap id triggering ofARQ re-transport,reducing transportdelay of air interface and avoiding w rong activation of Transm ission Control Protocol（TCP）ow ing to ACK/NACK.The cooperation works in this way:HARQ transm itend can trigger ARQ to conduct re-transportonce HARQ transm itend d iscovers a packet transport failure.However,itneeds to be researched furtherwhether the HARQ receiving end reports a failure if it finds one,ornot.

4.4 Vo IP Service Scheduling and Signaling

Vo IP services feature around 20m s of service arrivalinterval,40 bytes of service payload,and 160m s ofsilence interval.Vo IP servicesw illbe a killer app lication in the long run.Therefore,the 3GPP LTE takes severalefforts to op tim ize Vo IP services.

The design p rincip les ofVo IP service scheduling are as follows:

▲Figure 3. Data Format of RLC and MAC.

▲Figure 2. Downlink protocol structure of Layer 2.

▲Figure 4. Transport modes for multicast services.

（1）Vo IP-related signaling overhead should be small（ornone atall）due to smallVo IP packets and short transport intervals.

（2）The schedulershould support re-allocation of resources in static or sem i-static scheduling.

（3）The scheduler is ab le to dealw ith Vo IP resend ing and burstofservice payload.

However,no consensus has been reached on the design of Vo IPservice scheduling and signaling.

4.5 Multicast Services

Due to service demands and strong requirements from operators,3GPP LTE highlights the support forb roadcastand multicastservices.

If there is the demand onmulticast services only in one cell,the services can be transported via the Downlink Shared Channel（DL-SCH）.Moreover,the multicastservice can use the Multicast Channel（MCH）for transm ission to achievemulti-celltransportcombining d iversity.Figures 4a and 4b show two transportmodes in a hyb rid cell.Based on operators'demand,the 3GPP LTE also specified two transportmodes in MBMS-ded icated cell,as shown in Figures 4c and 4d.

5 Conclusions

In this paper,the p rocess of3GPP LTE standard ization is introduced and analyzed.Many advanced technologies and concep ts are adop ted in 3GPPLTE tofillthe gap between 3G and 4Gmobile communication system s.The 3GPP LTE system offers strong support for real-time services,highly-reliab le services,and b roadcastand multicast services.The standard ization p rocess of 3GPP LTE ismore like a technological revolution instead ofan evolution.Ithas basically fulfilled its purposes ofshort delay,all-packetand high data rate.