TD-SCDMA HSUPA Technology

HU JIinling

（Datang Mobile Communications Equipm ent Co.，Ltd， Beijing 100083， China）

Abstra c t:The High Speed Up link Packet Access（HSUPA）technology for Time Division-Synchronous Code Division Multip le Access（TD-SCDMA）is used to imp rove the overall up link performance:to inc rease the peak data rate and throughput and to reduce the transm ission de lay of up link.The considered enhancements inc lude Adap tive Modulation and Cod ing（AMC）,Hyb rid Automatic Repeat Request（HARQ）,Node B fast scheduling and sharing of up link channel resources between User Equipm ents（UE）.New Med ia Access Control p rotocol entities（MAC-e and MAC-es）are adop ted to enhance and op tim ize p rotocols.Simulation results of these technologies show that the system performance（inc lud ing the peak rate,throughput,and delay）can be im p roved significantly.

With the rapid developmentof mobile communications and the Internet,more andmore data services,such as video c lipp ing,stream med ia and download ing,emerge.These services require inc reasing data rate and throughputand reducing delay,putting forth higher requirements for themobile communication system.Tomeet the increasing demand forpacketdata services,3rd Generation Partnership Projec t（3GPP）,has p roposed and standard ized the High Speed Downlink PacketAccess（HSDPA）p rotocol,which is themain feature of3GPPRelease 5（both Frequency Division Dup lex（FDD）and Time Division Dup lex（TDD）are inc luded）.The research and standardization ofHSDPA enab le the downlink performance of3G systems to be g reatly imp roved.This raises a question:can the technologies for HSDPA be used in the up link packet services to op tim ize the up link performance,for instance,coverage,throughputand delay？To answer this question,3GPP first focused its research on High Speed Up link PacketAccess（HSUPA）technologies,and setup the work item for feasibility study on up link enhancement forWideband Code Division Multip le Access（WCDMA）FDD.Then the TDD equipmentvendors p roposed the work item s for TDD up link enhancement to evaluate such technologies as Node B fastscheduling,Adap tive Modulation and Cod ing（AMC）and Hybrid Automatic RepeatRequest（HARQ）[1-2].As an im portantpartof the 3GPP,Time Division-Synchronous Code Division Multip le Access（TD-SCDMA）working g roup has also donemuch research and evaluation on HSUPA.Follow ing technicalevaluation,the working g roup has began the work item for TD-SCDMAHSUPA,mainly tomake the specific standards based on the feasibility study,inc luding up link channel structure definition,signaling and physical layer p rocessing.Atp resent,this work item is stillin p rog ress.In this artic le,the key technologies used in TD-SCDMAHSUPA,as wellas the issues to be taken into accountin standard ization,willbe covered.

1 Key TD-SCDMA HSUPA Technologies

The enhancementofup link for TD-SCDMA is used to increase the peak data rate and throughput,to reduce traffic delay,and to decrease the Frame ErrorRate（FER）.Like HSDPA,HSUPA w illadop tsuch technologies as AMC,HARQ and Node B fastscheduling,and paymuch attention to theway User Equipment（UE）shares up link channel resources.In term s ofup link resources sharing,there is a d ifference between FDD and TDD.For FDD,the High Speed Downlink Shared Channel（HS-DSCH）of HSDPA is shared by users while in HSUPA,each userhas its own data link connec ted to Node B,that is,each user has its own channelresources.The reason for this difference is thata TDD system uses a cell-specific scramb ling sequence to identify each cell,which results in lim ited up link code resources.For TD-SCDMA，the up link enhancement is based on resource sharing.

The p rincip les of AMC,HARQ and Node B fastscheduling in TD-SCDMA HSUPA are sim ilar to those in FDD HSUPA,and they w illnotbe d iscussed here.The focus of this artic le w illbe on the analysis ofusing these technologies tofulfillTD-SCDMA up link enhancement.

1.1 Node B Fast Scheduling

Node B fastscheduling can bring reduced traffic delay and improved throughput.The reason is that the transmission process at the Iub interface is shortened,and that retransmission/UE buffer volume measurement are responded quickly compared to RNC scheduling.These benefits of Node B fast scheduling can be verified with simulations.

▲Figure 1. DG delay CDF， RNC （3GPP Release 5） scheduling（a modified gaming model and round robin scheduler （8 antennae）are used）.

▲Figure 2. DG delay CDF， Node B （Enhanced-Uplink） scheduling（a modified gaming model and round robin scheduler（8 antennas） are used）.

Figures1 and 2 are simulation results of delays using Radio Network Controller（RNC）scheduling and Node B scheduling,respectively.Suppose 99%of Datagrams（DG）should experience a delay of less than 250 ms,in R5（i.e.,RNCscheduling）,five UEs are supported,but in HSUPA（i.e.,Node B scheduling）,up to nine UEs can be supported,meaning that there is an increase of 80%in the number of UE supported.

▲Figure 3. Packet call throughput CDFs at low and high loading（a modified gaming model is used， and 500 ms is the packet call duration （8 antennae））.

Figure 3 is the simulation result of throughputs using Node B scheduling and RNC scheduling.Compared to RNC scheduling,Node B scheduling increases the throughput of packet services by approximately 50%.Further analysis shows the throughput gain is more significant for those packet services with short packet call duration,as short delay has a greater effect on them.

In addition to traffic delay and throughput,TD-SCDMAsystem can benefit from Node B scheduling in resources allocation and interference control.As the TDD uplink code resources are limited,the sharing and fast scheduling of physical resources can mitigate the limitation situation and quickly adapt the system to the changing radio conditions.Besides,by fast control over UE's transmission rate,the base station（Node B）can better control the interference at the air interface.

1.2 AMC

As a link adaptive technology,AMC can increase the system capacity by means of adopting higher order modulation method for good radio condition.Here,simulation analyses regarding the effect of the modulation schemes on the system performance and UEPower Amplifier（PA）are given.

In terms of the effect on system performance,the simulations are performed for three modulation cases:

·Case 1:Quadrature Phase-Shift Keying（QPSK）only

·Case 2:QPSK and 8 Phase-Shift Keying（8PSK）

·Case 3:QPSK,8PSKand 16-order Quadrature Amplitude Modulation（16QAM）

Figure 4 shows the relationship between sector throughput and noise rise.It can be seen here that the throughput in Case 3 is increased by 14%to 18%and 54%to 56%compared to Cases 1 and 2,respectively.

On the uplink,the Peak to Average Power Ratio（PAR）is,among others,an important issue that has to be taken into account.Therefore,the effect of higher ordermodulation on UEPA backoff is analyzed here.Tab le 1 is a summary of UEPA backoffs in the threemodulation modes ford ifferentOrthogonalVariab le Sp reading Factors（OVSF）.

?Figure 4.The relationship between sector throughput and noise rise in three modulation modes（4 uplink timeslots，8 antennas）.

▼Table 1. UE PA backoffs in three modulation modes for different OVSFs

The result ind icates that8PSK can delivera slightly lower PAR than QPSK for the same numberofOVSF codes；for 16QAM,the PAR is 2.1dB higher than thatofQPSK.

1.3 HARQ

Like HSDPA,HSUPA uses HARQ to rap id ly retransm it the erroneously received data and reduce the Rad io Link Control（RLC）retransm issions,thus im p roving the quality ofservices experienced by end users.The twomain aspects involved in adop ting this technology are traffic delay and system throughput.As HARQ has im pacts on the physicaland Med ium Access Control（MAC）layers,other fac tors,such as memory requirementofNode B and UE,signaling load,comp lexity,and UE power lim itation,should also be taken into account.

Figure 5 shows the average number of transm issions in Pedestrian AChannel at3km/h（PA3）with and w ithoutChase combining.Itcan be seen thatata low Carrier-to-Interference Ratio（C/I）,the use ofChase combining allows the numberof transm issions tofall significantly.

The analyses orevaluations of the above-mentioned technologies were made during the feasibility study of TD-SCDMAHSUPA,and in the standardization of TD-SCDMAHSUPA that followed；itis suggested that these techniques be app lied in TD-SCDMA HSUPA.

2 TD-SCDMA HSUPA Standard

2.1 E-DCH Channel

To support the HSUPA characteristics,the Enhanced Dedicated Channel（E-DCH）is introduced in the TD-SCDMA system to carry the high speed up link data.The Transm ission Time Interval（TTI）of this channelis 5ms,and itsupports higherordermodulation and Layer1（L1）HARQ p rocess.The resources the channelused,inc lud ing power,time slots and code resources,are allallocated by Node B.Meanwhile,two other controlchannels,E-DCH Up link ControlChannel（E-UCCH）and E-DCH Random Access Up link Control Channel（E-RUCCH）,are introduced to send the signaling messages related to up link enhancement.E-UCCH is often multip lexed w ith E-DCH to transm it the message related to E-DCHHARQ；E-RUCCH ismapped onto the physical random access resources to send the access requestofup link enhancementservices while E-DCH ismapped onto the E-DCH PhysicalUp link Channel（E-PUCH）.The E-PUCH channel resources fallinto two categories:scheduled and non-scheduled.The non-scheduled E-PUCH resources are allocated by the RNC,but the scheduled ones are allocated and scheduled by the Node BMAC-e entity.

Figure 5.?Average number of transm issions in a PA3with power control.

▲Figure 6. HARQ timing relationship.

On the downlink,two channels are introduced to supportNode B scheduling:E-DCH Absolute Grant Channel（E-AGCH）,which is used to send the base station scheduling information,and E-DCH HARQ Acknow ledgement Ind icatorChannel（E-HICH）,which is used to convey transm ission acknow ledgement messages（such as ACK and NACK）for HARQ p rocesses.

2.2 HARQ Scheme

N-channelstop-and-waitHARQ is adop ted in the TD-SCDMA system,and Chase combining and incremental redundancy can be supported.

The HARQ p rocess is as follows:E-DCH channel resources are first allocated by Node B via E-AGCH,and then the ACK/NACKmessages are returned via E-HICH.The E-DCH/E-AGCH/E-HICH association and tim ing is shown in Figure 6.

In Figure 6,nE-AGCHis the interval between the startof the E-AGCH and the startof the firstactive slotof the subsequentE-DCH transm ission.The value of this parameterdepends on the p rocessing capacity of the UE,and is currently set to be 6 slots（specialtime slots,Downlink Pilot Time Slot（DwPTS）and Up link Pilot Time Slot（UpPTS）,are not taken into account）.nE-HICHis the intervalbetween the lastac tive slotof the E-DCH TTIand the startof the transm ission of the ACK/NACK on E-HICH.This parameter is configured by higher layers w ithin the range of4 to 15 time slots（specialtime slots,Dw PTS and UpPTS,are not taken into account）.The HARQ-related up link/downlink signaling information carried by E-UCCH inc ludes HARQ p rocess ID（3 bits）and Retransm ission Sequence Number,or RSN（2 bits）.Other information such as the numberofHARQ p rocesses and nE-HICH-related messages are configured by higher layers.

2.3 Node B Scheduling Process

（1）A briefNode B scheduling p rocess in HSUPA goes as follows:a.The UEmakes a scheduling request to Node B via E-RUCCH,which contains the scheduling information and UE ID.The UE ID is the E-DCH Radio Network Tem porary Identifier（E-RNTI）.The scheduling information inc ludes suchmessages as the Serving and NeighborCellPath Loss（SNPL）,UE PowerHead room（UPH）,and buffer status.

b.Upon receiving the request,Node B schedulerw illrespond w ith an access grantmessage via E-AGCH if it allows the UE to send up link enhancementdata.In the access grant message,powerg rant,physical resource grant,E-RNTIand E-HICH Indicator（EI）should be inc luded.As E-AGCH is a shared channel,E-RNTIis used to indicate which UE the access g rantis given to.EIis the indicatorof E-HICH,and is used to ind icate which E-HICH w illbe used to convey the acknow ledgementmessage.

c.Then the UEw illdemodulate the access grantmessage on E-AGCH from Node B and find outif themessage is for itself.If itis,the UEwilldecide a rate to begin data transm ission on E-DCH based on allocated resources and power.The UEw ith access grantcan carry scheduling information again at the MAC-e end.

d.Once Node B receives and demodulates the data on E-DCH,itw ill return ACK or NACK to the E-HICH the UE ismonitoring,depending on whether the data is correc tornot.Finally,the UE w illdecide if the retransm ission is necessary according to the received ACK orNACKmessage.

（2）The information carried on E-AGCH inc ludes:

a.Powergrant:specifying the maximum power thatcan be allocated to the UE.

b.Physicalresources grant:denoted bymeans ofa code and a timeslot component.Forsim p lification,all the allocated timeslots use the same code.

c.E-RNTI:used to identify the UE the access g rantis given to.

d.Resource Duration Ind icator（RDI）:ind icating the effective scheduling time for the purpose of reducing the scheduling g rant frequency.

e.EI:ind icating the UE thatwhich E-HICHw illbe used to convey the acknow ledgementmessage.

f.E-UCCH Number Ind icator（ENI）:indicating the numberof E-UCCHs multip lexing w ith E-PUCH.

g.E-AGCH Cyc lic Sequence Number（ECSN）:used forouter loop power controlof E-AGCH.

（3）The contents of the scheduling information are:

a.SNPL:ind icating the path losses of the serving celland neighbor cells.

b.UPH:ind icating the power availab le to the UE.

c.TotalE-DCHBuffer Status（TEBS）:indicating the buffer status of the UE.

d.HighestPriority LogicalChannel Buffer Status（HLBS）:ind icating the usage status of the highest logicalp riority channelbuffer,and the percentage of totalbufferbeing used.

e.HighestPriority LogicalChannel Identification（HLID）:used to identify the highestp riority logicalchannel.

3 Progress of TD-SCDMA HSUPA Standardization

To p romote the standard ization of TD-SCDMAHSUPA,in 3GPPRAN#31 meeting,held in March 2006,Datang,togetherwith other companies,p roposed to setup TD-SCDMAHSUPA.The p roposalwas app roved during the meeting.

Later,several3GPPRAN working groups worked out related technical specifications and started the standard research.Among the working g roups,RAN1 and RAN2 have been working on physicallayerp rotocols and MAC layer p rotocols at the air interface,respec tively；they have been studying themod ification of the p rotocols and its effect.As the entity,MAC-e,is added to Node B,which has certain effects on the network structure,and the new feature and performance parameters have to be analyzed,RAN3 and RAN4 were setup to do related researches.

Since the work item was launched,three RANmeetings were held.Up to now,70%of the work item has been done,inc lud ing basic physical layer structure,HARQ tim ing and signaling,Node B scheduling,modulation scheme,random access p rocess,E-RUCCH and E-AGCH structures and coding,up link signaling,UE capability,overallp rotocol frame,features of E-DCH,QoS control,mobilitymanagement,and Iub interface.However,the stud ies ofsome hot issues,such as E-HICH struc ture,scheduled/non-scheduled transm ission multip lexing and whether20ms TTIis supported ornot,are stillin p rog ress,and they are expected to be com p leted in the RAN p lenarymeeting in 2007.The standard ization of TD-SCDMAHSUPA w illbe documented in R7,which is surely a greatp romotion to the developmentof TD-SCDMA.