Research and Development of New Broadband Wireless Access System

Zhang Xiaodong Bu Zhiyong You Xiaohu

（1. Shanghai Research Center for W ireless Communications， Shanghai 200050， China；

2. Jushri Technologies， Inc.， Shanghai 200050，China；

3. State Key Laboratory of Mobile Communication， Southeast University， Nanjing 210096， China）

Abstra c t:Broadband Wireless Access（BWA）has become one of the m ajor evolution trends in the fields of b roadband access and mobile communications.However,som e d ifficulties em erged during the app lication and industria lization of the BWA system,which ca lls for typ ical service models,key techno logy innovations and backbone support from b roadband Internet.This artic le introduces a Broadband Wireless Mu ltim ed ia（BWM）access system.It aim s to build a new-type“trip le p lay”BWA network by integ rating terrestrial d igital TV system and the BWA system,and by taking b roadband Internet as its core network p latform.A p rim ary research shows that the new BWA system is an effective solution to the existing d ifficulties of BWA system.Moreover,it is quite com petitive in service p rovision,service coverage,frequency p lanning,term inal power consump tion and cost for network construc tion.

I EEE 802.16 task g roup has

released two Broadband Wireless Access（BWA）standards:IEEE 802.16,aim ing at fixed access modes,and IEEE 802.16e,aim ing at mobile accessmodes.Itstarted to d raft IEEE 802.16j[1-3]at the beginning of2006,which is a new BWA standard based on relay topology.

China started an influentialmobile communication research p lan called FuTURE,in 2003.Since then,China has selec tively listed key BWA technologies to be tackled.One afteranother importantachievementhas been accom p lished,and quite a lotof intellectualp roperties have been accumulated.Meanwhile,China's standardization organizations started to map out industrialstandards associated to IEEE 802.16.Atp resent,relevant nationalstandards are under p reparation.

Here are some typicalstandard ization organizations from China:the BWA working g roup,jointly estab lished by NationalDevelopmentand Reform Comm ission（NDRC）,Ministry of Information Industry（MII）and Ministry of Science and Technology（MST）；China Communications Standards Association（CCSA）TC5WG3；and Standardization Adm inistration ofChina（SAC）Broadband Wireless Multimedia（BWM）standard working group.

The SAC BWM standard is obviously d ifferent from other typ icalBWA system standards in terms ofpositioning.The BWM imp lements inner convergence of homegrown BWA system and terrestrial d igitalTV system.Its fundamental technologymainly comes from FuTURE research p lan,China NextGeneration Internet（CNGI）research p lan and China homeg rownmobile TV standard.Furthermore,italso absorbs advantages of IEEE 802.16 standards to some extent.The BWM standard recommends Ultra High Frequency（UHF）,although itcan also supportother frequency bands（such as 3.5GHz）.

As foroperationmodes,b roadcast TV system may be operated separately from the BWA system.Jointoperation of these two system s is also supported.Therefore,it is an effec tive w ireless“trip le p lay”solution.

As for core network,the intelligent node overlay network,which is based on Point-to-Point（P2P）Session Initiation Protocol（SIP）,is recommended.The overlay network is a QoS-guaranteed virtualnetwork,overlaid on pub lic Internet.Besides,the NGN/IMS interface is supported.The air interface adop ts Generalized Multi-Carrier（GMC）technology,which targets for low power consump tion op tim ization,in the up link direction.

1 Key Technologies for BWM Access Network

1.1 W ireless Network Architecture

The BWM system has to solve the p rob lem of resource coord ination and unified transm ission ofb roadcastTVand BWA system s.Therefore,the BWM system has tomake fulluse of the existing resources of terrestriald igitalTV network and w ireless Metropolitan Area Network（MAN）,realizing convergence of the network architectures.The BWM network p rovides three topologies to supportbroadcastTV and BWA services.

◄Figure 1.Large-cell coverage mode.

◄Figure 2.Small-cell coverage mode.

1.1.1 Large-CellCoverage Mode

The large-cellcoveragemode is popular in existing terrestriald igitalTV system,which p rovides thew ireless network for b roadcastTV services.The BWM system inherits this technology.As required,it can also supportmulti-cellSignal Frequency Network（SFN）to p rovide regionalb roadcastTV services.The corresponding network topology is shown in Figure 1.

Accord ing to the technical features of the BWM system,the large-cell coveragemodemay be deemed as eitheran independentoperationmode or a p re-dep loymentmode inwhich itis converged w ith the BWA network.In the second operation scenario,once the BWA network,which has the small-cell coveragemode,is dep loyed and becomes themain body of the whole system,the large-cellcoveragemode of the BWM system may be transited to the large-and small-cellm ixed coverage mode to p rovide broadcastTV and BWA services.

1.1.2 Small-CellCoverage Mode The small-cellcoveragemode is the p rem ise for the BWM system to p rovide two-way services.Accord ing to p rac tical operation demands,the small-cell coveragemodemay be further divided into hotspotcoverage and cellular coverage.The cellular coveragemode is sim ilar to the one used in traditional mobile communication cellularnetworks.Based on small-cellcoveragemode,the BWM system can also supportSFN topology w ithin a certain range to p rovide b roadcast TV services.Therefore,the small-cellcoveragemode w illbe one of themain networking modes when BWM system is putinto commercialoperation.The correspond ing network topology is shown in Figure 2.

Based on the small-cellcoverage mode,the BWM system may p rovide integ rated services,inc lud ing b roadcast TV and BWA services,to some extent.This w ireless network p lanning mode is com patib le to the traditionalcellular mobile communication architectures.Hence,it's possib le that the BWM system w illuse the same sites used by 2G and 3G.

1.1.3 Large-and Small-CellMixed Coverage Mode

The large-and small-cellm ixed coveragemode is the w ireless networking mode recommended for the BWM system.The correspond ing network topology is shown in Figure 3.In thismode,b roadcast TV services are p rovided by the terrestriald igitalTV system,which adop ts the large-cell coveragemode,while BWA services are p rovided by the small-cellcoverage mode.Having a region-oriented coverage,b roadcast TV system does not care aboutnetwork capacitymuch.Therefore,macro-cellbase stations adop ted in thismode can notonly avoid the overhead forbase station synchronization and cellhandover,but also greatly reduce the cost forboth system networking and equipment.

The large-and small-cellm ixed coveragemode is a new technology inthe BWM system.Therefore,its feasibility needs to be verified through a trial-and-errorp rocess.

Figure 3. ►Large- and small-cell mixed coverage mode.

◄Figure 4.Structure of wireless frame shared by broadcast TV and BWA.

▲Figure 5. GMC transmitter principle diagram.

1.2 Key Technologies for Physical Layer

At the physicallayer,the BWM system mainly usesmultip le access,broadcast and otherenhanced technologies（such as Multip le InputMultip le Output（MIMO）,Adap tive Modulation and Coding（AMC）and Hyb rid Automatic RepeatRequest（HARQ））.This artic le willintroduce multi-add ress and broadcast technologies.

1.2.1Wireless Frame Structure Atair interface of the BWM system,a w ireless frame consists ofbroadcastTV data and BWA data.Both share the frame resources by the Time Division Multip lexing（TDM）mode,as shown in Figure 4.Moreover,BWA data is d ivided into up link data（UL）and downlink data（DL）to imp lement Time Division Dup lex（TDD）for two-way communications.

1.2.2Up link Multi-Add ress Solution UL capability is traditionally weak than DL capability due to asymmetric ULand DLbandw id th.As theWeb2.0 era is com ing,the UL/DL asymmetric phenomena w illdisappear.An enhanced ULmulti-add ress solution is urgently desired to imp rove transm ission capability.The BWM system adop ts GMCmulti-add ress solution forup link accessing,which is an achievementof the FuTURE p lan.The solution has equivalentoverallperformance to internationalmainstream technologies.Moreover,itdec lares robustand stab le transm ission performance.Figure 5 illustrates GMC transm itterp rincip le used in the BWM system.

Compared to IEEE 802.16e OrthogonalFrequency Division Multip le Access（OFDMA）solution,GMC solution is better in smallcalculation amount,and has less hardware comp lexity.

（1）Com pared to OFDMA,GMC transm ittergenerates higher Peak-to-Average Ratio（PAR）,reaching 2-4 dB.

（2）GMC sets frequency domain p rotection sub-band and time domain cyc le p refix for transm itting signals so that there is lower requirement for accuracy of transm ittersynchronization than in the OFDMA solution.

（3）GMC can effec tively supp ress multi-add ress interference due to near-fareffect,poorsynchronization or poorpower control.In otherwords,GMC ensures robustand stab le transm ission performance.

1.2.3 Downlink Multi-Add ress Solution Currently,OFDMAmulti-add ress technology is w idely recognized as the bestdownlink solution forb roadband w ireless communications due to its easy im p lementation and integ ration ofother enhanced technologies such as MIMO.Afterextensive d iscussions,this technology is still recommended by the BWM standard working group.Add itionally,Unified Multip le Input Multip le Output（U-MIMO）,one of the FuTURE p lan achievements,is hopefully absorbed as a BWM solution for multi-antenna transm ission.This aim s to effectively im p rove throughputofsystem and term inals,and moderately restrain fad ing effectofmobile channels.Figures 6 and 7 show the typicalOFDMA Tx p rincip le and MIMO transm ission p rincip le,respectively.

Com pared to otherMIMO transm ission technologies,U-MIMO owns the follow ing technicaladvantages:

（1）Strong EnvironmentAdap tability Based on environmentdetection mechanism,itis easy tomeet communication requirements in a situation where spatialcorrelation is various.

（2）Sim p le Tx and Detec tion Algorithm

Pre-cod ing mechanism is used to generate Tx data in unified form.2-D detec tion ofspace-time signals is based on MMSE/SIC.Therefore,the algorithm is notvery comp licated.

（3）Separate ChannelCod ing from Space-time Transm itting/DetectingAlgorithm.

Figure 6.►OFDMA Tx principle.

With GMD orthogonaltransformation,spatialmulti-stream shares the same rate ofchannelcoding stream,which obviously reduces the p ressure on c losed-loop power controland feedback signaling channel.

1.2.4 Downlink BroadcastSolution

The w ireless frame structure shows that both b roadcastdata and BWA data adop t the TDM mode to im p lement multip lexing.Accord ing to the d raft assessment,thismultip lexing structure makes the separate operation of broadcastand BWA services easy.According ly,itmostly needs to reduce coup ling and mutualdependency between downlink b roadcastand BWA solutions.Ifso,the pub lished innovative achievements can be largely reused in the area ofmobile TV and BWA.Finally,the technology and industry w in-w in goalis surely fulfilled.

STiMi,recently issued by the State Adm inistration ofRad io Film and Television（SARFT）ofChina,is a homegrownmobile TV standard.Itis based on an OFDMA industrialstandard,named BroadcastChannelFrame Structure,ChannelCod ing and Demodulation.The STiMiis converged w ith the BWA system at the levelof parameterdesign（such as bandw id th setting）and partial technology.In p rincip le,the BWM system may be converged w ith other terrestriald igitalTV system s,which is com p letely determ ined by operation requirements.Therefore,it is flexib le.However,details w illnotbe described here.

1.3 Key Technologies for MAC Layer

As a center forw ireless resource management,Media Access Control（MAC）p rotocoland relevant technologies determ ine the efficiency and robustness of the entire access network.Aim ing at the packetdomain,IEEE 802.16e specifies signaling structure and QoS architecture for the MAC layer.Wireless resource scheduling policies are taken tofulfill full-service scheduling and controlling functions.This effec tively releases the p ressure to increase the costofhardware supporting system while im p lementing functions in the circuitdomain or circuitand packet m ixed domain.To sum up,itshould be recognized as am ilestone during the developmentofnew generation b roadband w ireless communication networks.

Figure 7.►MIMO transmissionprinciple.

In the BWM system,IEEE 802.16e is mostly taken as reference to design signaling structure and QoS architec ture ofMAC stack.Sincemobile TV services have special requirements,MAC stack is redesigned to integ rate both b roadcast TV and BWA services,called Convergence Med ia Access Control（CoMAC）stack.Figure 8 shows the basic structure ofCoMAC.

In general,CoMAC stack has the follow ing technicalfeatures:

（1）Three typicalsub-layer c lassification:Network Adap tation Sub-layer（NAS）,Media Controland ManagementSub-layer（MCMS）and Security Sub-layer（SES）.

（2）In p rincip le,QoS architec ture supports four service c lasses:Unsolicited GrantService（UGS）,Real-Time Polling Service（RTPS）,Non Real-Time Polling Service（NRTPS）and BestEffort（BE）service.Mobile TV service belongs to UGS,butit is initiated at the network side.

（3）The whole signaling system is connection oriented.Dynam ic stream is adop ted tomanage allconnections.

（4）Allpacketservices aremanaged and scheduled by MCMS.MCMS is also responsib le formobile handoverand w ireless resourcemanagement.

2 Research Progress

China started generalresearch on the BWM system in 2004.Some key technologies were verified on FuTURE field testin the second halfof2006.After that,aim ing formore achievements,a BWM field demonstration in Chongm ing Island was p lanned.Thiswas also used as a verification field by SAC to d raft BWM standards.This p lan was supported by three Shanghaimunicipal departments,namely the Science and Technology,Developmentand Reform,and Information Comm issions,w ith kind efforts.Themain participants were ShanghaiResearch Center forWireless Communications,Shanghai Institute of Microsystem and Information Technology,and JushriTechnologies.There were also other institutes involved:other institutes in Chinese Academ y of Sciences,SAC,SoutheastUniversity,Tsinghua University,and ShanghaiJiao Tong University.At the same time,the correspond ing equipmentand software system for the BWM system were also researched and developed.This inc ludes core chips,which would exert tremendous ac tions in future industry.

Up to now,the firstphase of the BWM field demonstration in Chongm ing Island is being constructed.In this phase,the BWA network coversmain scenic spots of the island.However,broadcastTV and b roadband access services cannotbe organically integ rated yet.That is to say,the firstphase network is being constructed based only on the BWA technology.Figure 9 basically describes the firstphase network coverage.According to the p lan,a“trip le p lay”BWM system is expected to be demonstrated in Chongm ing Island by the end of2007.

◄Figure 8.CoMAC stack structure.

▲Figure 9. Phase 1 of BWM field demonstration in Chongm ing Island.

3 Conclusions

This artic le introduces the latest achievements accom p lished by the SAC BWM standard working g roup.Aw ireless“trip le p lay”solution is p roposed,through which,broadcast TV services can be organically integrated w ith BWA services.Some key technologies are briefly introduced from the aspec ts of network,transm ission and p rotocol.At p resent,the R&D on BWM system is

under the stage ofstandard formulation and verification.The industry alliance has notbeen estab lished.Therefore,there is stilla long way tofulfilling the industrialization ofBWM system.