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Figure 1: Instead of sending data using individual DCHs, HSDPA extends the DSCH, allowing packets destined for

many users to be shared on one higher-bandwidth channel called the HS-DSCH.

By David Maidment

Product Manager

picoChip Designs LtdE-mail: david.maidment@picochip.com

Since its introduction, 3G tech-nology has been heralded forits ability to deliver more voicechannels and higher-bandwidthpipes. But operators havestarted to realize that while 3Gallows for high-quality voice andmedia streaming services, it isa poor fit for high-speed data.

High-speed downlink pa-

cket access (HSDPA) techno-logy promises to bridge the gapbetween 3G and the Internet,providing an overlay for the ex-isting protocol stack that en-ables the delivery of high-speeddata to many users in a cell. In-stead of limiting high-speeddata access to fewer than fiveusers in a cell, HSDPA can de-liver 384Kbps data to up to 30users.

Not simpleBut HSPDA is not a simple soft-ware upgrade to 3G systems. Inmany respects, the change fromRelease 99 to HSDPA is as sig-nificant as that from voice-onlyGSM to EDGEchanging bothmodulation and the way packetsare processed.

There are parts of theHSDPA standard that are rela-tively simple to implement us-ing existing hardware. But,taken as a whole, HSDPA will

simply break many deployedarchitectures and will requirenew hardware. Most base sta-tions (also known as Node Bs) will need significant upgradesto cope with the increased datathroughput and consequencesof moving to a more complexprotocol.

HSDPA increases the down-link data rate within a cellto a theoretical maximum of14Mbps, with 2Mbps on the

uplink. However, it is not aboutdelivering Ethernet bandwidthto one fortunate user. What isimportant is the ability to reli-ably deliver many sessions ofhigh-speed, bursty data to alarge number of users within

that cell. The changes thatHSDPA enables include better

quality and more reliable, morerobust data services. In other words, while realistic datarates may only be a few Mbps,the actual quality and numberof users achieved will improvesignificantly.

Burst problemsIP is a bursty protocol that de-mands changes to the wideband-CDMA (W-CDMA) protocolstack to support IP efficiently.Bursty protocols are a poor

fit with dedicated channels(DCHs) that are used in existing W-CDMA networks. Althoughthe DCH can support many dif-ferent types of traffic, use of thechannel for bursty traffic is quitelow. This is because the processof channel reconfiguration thatcan be used to tune the DCH fora change in traffic mix traffic isslow, taking on the order of500ms.

These issues have been ad-

dressed in Release 5 of the 3GPartnership Project (3GPP)standards, which radicallychanges the network to make it better suited to data traffic.Support for IPv6 has been in-corporated into the core net-work with a key enhancementto provide high-bandwidth

support for bursty IP traffic forthe mobile user.

Instead of sending data us-ing individual DCHs, HSDPAextends the downlink sharedchannel (DSCH), allowingpackets destined for many usersto be shared on one higher-bandwidth channel called thehigh-speed DSCH (HS-DSCH).As with wired networks such asEthernet, this allows for themore efficient use of available bandwidth. On top of that, afaster channel configurationprocess allows the base station

to control the channel more ef-fectively (Figure 1).

Too many optionsThere are many options avail-able to base-station designersand operators when dealingwith HSDPA. This complicatesthe provision of HSDPA as thenetwork is upgraded, but intel-ligent choices over base-stationimplementation can result inhigher throughput for high-

revenue services, improvingoperators margins.The maximum bandwidth

that can be achieved withHSDPA depends largely on cellsize. To limit the power neededto send each bit of information,the maximum achievable bitrate tends to fall away for users

Understanding HSDPAsimplementation challenges

at the edge of the cell. For alarge cell with a diverse range of

users, the peak aggregate datarate will be in the range of 1-1.5Mbps. This can increase tomore than 6Mbps as the cellsize decreases to the microcelllevel and beyond. In principle,a picocell could see data rates of8Mbps or more.

To achieve higher raw datarates, HSDPA uses higher-levelmodulation schemes such as16QAM at the PHY layer, to-gether with an adaptive codingscheme based on turbo codes.

Note that the modulationscheme is adaptive and changedon a per-user basis. The spread-ing factor used for the HS-DSCH remains fixed at 16, butthe coding rate can vary on aper-user basis between 1/4 and3/4. In theory, the protocol al-lows an uncoded link of 4/4, but that is only useful for labtests to achieve the theoreticalmaximum of 14Mbps using16-QAM modulation.

However, many of the re-sults announced so far fall shortof what is expected from a ma-ture, robust system. In smallcells (the 3G equivalent of hotspots), very high data ratesshould be realistically attained.

Under poor reception condi-tions, the modulation can vary

HS-PDSCH

HS-PDSCH

HS-PDSCHHS-PDSCH

HS-SCCH

HS-DPCCHHS-DPCCH

HS-DPCCH

HS-DPCCH

Downlink

Uplink

High-speed physical downlink shared channel"The data bearer"

Up to 15, always associated with a DCHData bearer: Peak data rate 14.4MbpsSF = 16

High-speed shared control channel"The signaling channel"

Carries HARQ information and format parametersUp to 4 logical channels per UESF = 128

High-speed dedicated physical control channel"Packet flow control signaling"

Carries HARQ, channel quality informationSF = 256

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Figure2

: Protocols such as HSDPA work well on parallel-processing architectures.

as well, possibly reverting toQPSK from the higher-ordermodulation of 16-QAM. Linkadaptation ensures the highestpossible data rate is achievedboth for users with good signalquality who are typically closeto the base station and for moredistant users at the cell edge who may receive data with alower coding rate. The link ad-aptation is performed on eachtransmission timing interval(TTI), with the user equipmentsending an estimate of thechannel quality to Node B thatis then used to select the modu-lation and coding rate for thatuser on the next transmission.

Moving MAC controlThe more important changethat HSDPA makes is to movecontrol of the medium access

control (MAC) layer from theradio network controller (RNC)into the base station. Crucially,this move enables the use of fast-scheduling algorithms where,under constructive fading con-ditions, users are served databased on channel-quality esti-mates. This compares to riskinghigh error rates to be experi-enced by users in poor receptionconditions using a conventionaluser-priority or round-robinscheme, where the scheduleruses average channel conditionsto select the modulation andcoding scheme used. As a result,fast scheduling works hand-in-hand with the algorithms usedto select optimum modulationand coding schemes.

This increases the respon-siveness of the base station. The16-QAM coding change in-creases the peak speed in thesame way that a high-poweredengine can boost the perfor-

mance of a car, but it is theMAC change that makesHSDPA deliver a real-worldspeed increase, much like re-placing a learner driver with aFormula One racing driver.

Performance will be notice-ably better even if 16-QAMmodulation cannot be used.This demonstrates how a shiftin the 3G architecture from atraditional dumb pipe with in-telligent center to a moredatacom-like smart edge canyield better results.

Overall, the data rate has in-creased sevenfold, the responsetime was reduced by 80 percentand the algorithms, schedulingand complexity have increased

significantly. These changeswill be difficult to achieve in ahardware design that was notarchitected to support them.

Indeed, some early demon-strations only implemented afew of these features or onlyachieved limited data rates.While 16-QAM modulation isthe most obvious change andeasy to demonstrate at a tradefair, the capabilities of theMAC-hs and adaptive controlloops are more important, butless visible. Developing, testingand hardening these algo-rithms for field deployment arethe major challenges for manu-facturers.

Increasing complexityThe improved performance andprocessor power provided byHSDPA implies an increase in

complexity. Many high-speedfeedback loops are needed toimplement HSDPA efficientlyand provide users with the bestdata rates possible.

For example, the TTI usedfor modulation and coding se-lection for individual frames inHS-DSCH is just 2ms, com-pared with a typical time of10ms (and up to 80ms) for theTTI used for power control inthe existing Release 99 sharedchannel. Furthermore, the al-gorithms needed to make gooduse of all the possibilities pro- vided by fast scheduling will be more complex than thoseimplemented by existing RNCsoftware, but those decisionshave to be made within amillisecond.

When link errors occur, data

packets can be retransmittedquickly at the request of themobile terminal. In existing W-CDMA networks, these re-quests are processed by theRNC. As with fast scheduling, better responsiveness is pro-vided by HSDPA by processingthe request in the base station.

The hybrid automatic repeatrequest (HARQ) protocol de-veloped for HSDPA allows effi-cient retransmission of drop-ped or corrupted packets. Theprotocol was designed to allowthe average delivered band-width of HSDPA to be higherthan would be possible if moreextensive forward error correc-tion were to be used. However,it puts significant demands onthe base station, since supportfor HARQ calls for low latency.The latency demanded for effi-

cient HARQ support calls forretransmissions to be pro-cessed within 2-7ms. But thefeedback loop that allowsHARQ to be implemented is not

one that exists in Release 99 base stations, as that functionsits in the RNC for existingDCH and DSCH transmissions.Hence, not only must thingswork faster, but many functionsare new, adding to the capabili-ties and intelligence of Node B.

In addition to fast retrans-missions, a number of tech-niques are used to provide themobile terminal with a betterchance of receiving the datacorrectly. For users with a highcoding rate, simple chase com-bining may be used, which sim-ply repeats the packet. For us-ers with a low coding rate, in-

cremental redundancy can beused. In this scheme, parity bits are sent to allow the mo- bile terminal to combine theinformation from the firsttransmission with subsequentretransmissions.

The scheduler and retrans-mission manager require large buffers to hold all the packetsthat might need to be re-sent.This function was not presentin earlier functions, so the sup-porting hardware needs to havebeen designed in readiness forit for existing implementationsto support HSDPA at suffi-ciently high data rates.

Factors impacting schedulingIt is simple to devise a schedu-ling algorithm that will workwell for a few users in the labo-ratory with artificially gene-

rated constructive fading condi-tions, but it is much harder todevelop one that works robustlyin the field for many usersallwith different, complicated and

changing situations. Many cir-cumstances will affect real-world systems, not the least ofwhich are evolving capabilitiesof the terminals themselves,whether they are handsets ordata cards inserted into PCs.Latency demands of HSDPAmean that designs will react dif-ferently to changing fading con-ditions and packet deliveryspeeds.

Similar problems were seenin the early days of the Internet,when interactions between thedifferent layers of the protocolstack led to less-efficient band-width use than expected. Tech-

lub

Transport

layer

termination

R4 Tx symbol rate

lub/FP

termination

HS Tx

encode

MAC-hs

SDRAM SRAM

R4 Tx chip rate

HS Tx chip rate

Channel element controller

Tx

sample

rate

Rxsample

rate

R4/HS-DPCCHRx chip rate

PowerPC

Radio network layer

- NBAP (TS5.433);

- L1 resource management and

measurement

- Framing protocol termination for

dedicated and common channels

Transport network layer

- NBAP transport (ATM, AAL5)

- Transport signaling (ALCAP)

- Data stream transport (ATM, AAL2)

- Specific O, A and M

To/from

ADC/DAC

and RF

Device 0 (PC 102)

HS-DPCCH decode

R4 Rx symbol rate

Device 1 (PC 102)

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niques were developed to over-come the problems and in-serted into terminal equipmentand infrastructural systems to bring performance back up totheir expected levels.

If a scheduler is not designedto react to problems, operatorsmay see some users with ter-minals that are able to handlehigh-speed transfers starved of bandwidth, while other users with less-capable systems useup too much of the HS-DSCHbandwidth. Such a situationwill see much lower data usethan expected. A more intelli-gent scheduler that watches forchanges to channel and termi-nal conditionsand schedulespackets for terminals that canreceive at higher data rateswill improve overall revenue.

However, the need to sup-

port different quality-of-service(QoS) contracts with each ter-minal will further complicatethe situation for the scheduler,as it cannot simply deny band-width to a terminal with a highQoS setting just because it hap-pens to be in a poor receptionarea or is unable to reactquickly enough to the data itreceives from the base station.

As well as allowing for evo-lution in scheduler design, inmany cases, it will be desirableto have different schedulingpolicies in action at differenttimes of the day or tuned for cer-tain locations, such as an air-port waiting lounge. Testing

this requires multiple scenariosto be evaluated under differentloading conditions. As a result,architectures that maximizeflexibility will be key to effi-cient HSDPA implementation.

Granularity neededProcessing granularity will bean major consideration for theefficient implementation of anHSDPA-compliant base station.Systems based on a small num-ber of high-performance DSPstend to demand large buffersand work on large groups of dataat any one time to reduce theoverhead of switching betweentasks. This makes thingsclumpy with high latency.However, such a coarse-grainedapproach to task scheduling is apoor fit for algorithms such asscheduling that need low la-

tency to work effectively.Advanced silicon processes

available today make it possibleto implement hundreds of pro-cessors on a single chip withdistributed memory blocks andan interconnect structure thatefficiently delivers data neededto implement the required feed- back paths. Protocols such asHSDPA, as with earlier versionsof W-CDMA, work well on par-allel-processing architectures,as many different processesneed to happen at the sametime (Figure 2).

Fine-grained control will benecessary to implement featuressuch as fast scheduling and per-

user coding and modulation ad-aptation. With a large numberof processing elements, it be-comes possible to dedicate pro-cessing and buffer resources al-most on a per-user or per-func-tion basis. For example, oneprocessor may collate informa-tion for a processor that justruns an advanced scheduling al-

gorithm. This allows the proces-sor to perform scheduling deci-sions all the time. This will yieldmuch lower latencies than a sys-tem where scheduling is shared with other tasks on a general-purpose processor or DSP.

Future-proofingA flexible, software-based de-sign will be vital for future im-provements to the W-CDMA ser-vice offering. HSDPA is an un- balanced system with a maxi-

mum of 14Mbps on the downlinkand 2Mbps on the uplink fromthe terminal to the network.That can be a concern, as TCPcan easily be uplink-choked ifacknowledgments are slow, re-ducing the downlink rate.

Release 6 of the 3GPP speci-fication will change that by in-troducing high-speed uplinkpacket access (HSUPA). This al-lows users to take advantage offaster uplinks with lower latencywhen sending large files or e-mails. That in turn improves theefficiency of the link, increasingeffective throughput even if themodulation has not changed.Indeed, without the improved

efficiency of HSUPA, it is highlylikely that HSDPA will be im-paired in applications thathave more balanced bandwidthneeds.

HSUPA puts even morestrenuous demands on the base-station design and processingelectronics will have to deal witha much more complex decode

environment in the same waythat HSDPA demands muchmore of the terminals in deco-ding. HSUPA means movingcontrol functions from the RNCto the Node B. As is the case forHSDPA, these will likely breakmany installed architectures.Given the speed that thesechanges are arriving, having aflexible or upgradeable plat-form is important.

HSDPA significantly im-proves the quality and perfor-

mance of wireless data for 3G,with a corresponding impact onthe operators profit. Changesto the modulation, architectureand networking control algo-rithm are all required. How-ever, despite some claims, thisupgrade is not simple and manybase stations will require newextensive hardware if they areto deliver on the potential. Im-mediately following HSDPA isits counterpart for the up-stream. This also has great ad- vantages, but likely requiresfurther hardware changes. Car-riers should plan for the majoropportunity, but also minimizethe disruption.