Enhancement and Evolution of TD-SCDMA Systems

Peng Mugen Li Yong Wang Wenbo

（W ireless Signal Processing and Network Lab， Beijing University of Posts and Telecommunications， Beijing 100876， China）

Abstrac t:Tim e Division-Synchronous Code Division Multip le Access（TD-SCDMA）,a home-grown standard for the third generation mobile communications,is already mature and ready for commerc ia l dep loym ent.The enhancement and evolution of the TD-SCDMA system is a hot topic recently.Many strateg ies for TD-SCDMA enhancements and evolutions have been p roposed,but there are no integrated solutions that can jointly consider the deve lopment both in the short term and in the long term.This paper discusses four evolution phases,which are p roposed accord ing to the key technolog ies.Phase 1 is about the current TD-SCDMA system.Phase 2 is the short term evolution of TD-SCDMA,i.e.,the period of HSxPA,inc luding the evolution of High Speed Downlink Packet Access（HSDPA）,High Speed Up link Packet Access（HSUPA）and High Speed Packet Access p lus（HSPA+）.Phase 3 is the Long Term Evo lution（LTE）.Phase 4 is the Beyond 3G（B3G）/Fourth Generation（4G）mobile communication Time Division Dup lex（TDD）system,which is based on TD-SCDMA.Key characters for every evolution phase are d iscussed,as well as the advanced technolog ies adop ted in the evolution p rocess such as Multip le Input Mu ltip le Output（MIMO）and Orthogonal Frequency Division Multip lexing（OFDM）.

This w o rk was funded by the 863 Prog ram of China under grant 2006AA01Z257 and by the National Natural Science Foundation of China under grants 60602058 and 60572120.

Time Division-Synchronous Code Division Multip le Access（TD-SCDMA）is the third generation（3G）mobile communication standard p roposed by China thathas become an im portantpartof3rd Generation Partnership Project（3GPP）standards.In the TD-SCDMA system,many advanced w ireless communication technologies have been adop ted,inc lud ing up link synchronization,smartantenna,joint detection,software rad io,baton handoverand Dynam ic ChannelAllocation（DCA）[1].As the Time Division Dup lex（TDD）mode does not require symmetric spectrum s,the rad io resources can be flexib ly allocated between up link and downlink.Therefore,itis quite suitab le forsuch asymmetric services as packetdata transm ission.Considering that the spectrum resources are becom ing scarcerand scarcer nowadays,the TDDmode has g radually been the focus of researches on Beyond Third Generation（B3G）/Fourth Generation（4G）systems.

In China,the Frequency Division Dup lex（FDD）system s,inc lud ing WCDMA and CDMA2000,have been assigned a total bandw id th of2×90MHzwhile the TDD systems have been assigned 155MHz.This ind icates the Chinese government's g reatsupport to TD-SCDMA,as wellas the bright future of the TD-SCDMA system in China.

Compared w ithWCDMA and CDMA2000 systems,the TD-SCDMA system have advantages in technologies,buthave less experience in commercialdep loyment.

In the evolution of TD-SCDMA,the good compatibility between TD-SCDMA and WCDMA systems w illp rovide a solid basis forhandoverand roam ing between the two systems.In particular,both system s use the same Core Network（CN）of 3GPP,allow ingmutualsupportand com p lementin their development.Butin terms of investment,the CN only accounts for30%of the entire system.Therefore,the research on the com patibility of the two systems in other fields is stillan important topic in the evolution p rocess.

As for TD-SCDMA and CDMA2000 systems,their architectures are quite different,but the com patibility between them has to be given focus in order tofulfillg lobal communication service and to p rovide betterservice experience formobile users.

The peak rate of the currentTD-SCDMA system can reach 2Mb/s in a hotspot,384 kb/s in amoderate speed mobility environmentand 144 kb/s in high speed mobility environment.However,as themobile communication services g row rapid ly,these transm ission bit rates in 3G system s can notsatisfy the service requirements.Accord ing to the estimatationmade by the InternationalTelecommunication Union（ITU）,the peak rates in the B3G cellular system can reach 100Mb/s to 1Gb/s by the year2015.

Sim ilar to the CDMA2000 system,TD-SCDMAwillbe configured w ith sing le carrierormultip le carriers.The multip le-carrier configuration can delivera higher transm ission rate,butw illincrease the comp lexity of the system.Moreover,the integration ofmobile communication technologies w ith Broadband Wireless Access（BWA）technologies,such as Wireless LocalArea Network（WLAN）and Worldwide Interoperability forMicrowave Access（WiMAX）,has been in full swing.WithWLAN integ rated,the TD-SCDMA system can p rovide the peak rate of54Mb/s in indoorenvironments and hotspots,and effectively extend its coverage in hotspots.Meanwhile,the integ ration between TD-SCDMA and Wireless Metropolitan Area Network（WMAN）technology（generally called WiMAX）,which is based on the IEEE 802.16 series standards,has also been puton the schedule,and has become a hot top ic in the current technicalresearch field.The peak transm ission rate can reach 75Mb/s w ithin the 20MHz spectrum allocation in theWiMAX system,which is a useful technology for TD-SCDMA enhancement.

In November2004,3GPP started its Long Term Evolution（LTE）p roject.The goals of the p rojec tinc lude expand ing the coverage and capacity,increasing its throughput,reducing costs and service delays,and imp roving the Quality ofService（QoS）so as to offerbetteruserexperience.This p rojectw illbe based on the current3G spectrum s and adop tseveralnew but mature B3G technologies to ensure a smooth transition from current3G system s to B3G/4G system s and tofulfillsustainab le developmentof the communication system s.

The Chinese governmenthas invested the evolution of TD-SCDMA to the future TDD system,as wellas researched the criticaltechnologies for the B3G/4G TDD system,inc lud ing air interface and new network architecture,with aims to achieve full coverage of the entire 3G network and delivera peak rate of 100Mb/s to 1 Gb/s.By using Multip le InputMultip le Output（MIMO）and OrthogonalFrequency Division Multip lexing（OFDM）,the first release of the future TDD system can achieve the downlink peak rate of100Mb/s and an up link peak rate of 50Mb/sw ithin the 20MHz spec trum bandw id th,whichmeans a spec trum efficiency of2-5 bps/Hz.Due to the advantages of the TDD system,the evolution from the currentTD-SCDMA system to the LTE/B3G TDD system is urgently demanded[2].

This paper d iscusses the TDD system evolution p rocess:from the currentTD-SCDMA system to the future B3G/4G TDD system.Since the B3G/4G TDD system w illnotbe commercially dep loyed until2015,and the data rate has a sharp inc rease from the currentTD-SCDMA,there should be some transition phases between them.In terms ofnew technologies and peak data rates,the evolution p rocess can be d ivided intofour phases,each ofwhich can be furtherd ivided accord ing to the key technologies:currentTD-SCDMA system（sing le carrier and multip le carrier）,HSxPA system（sing le carrier,multip le carrier,and integ rated system w ith BWA technologies）,LTE TDD system（sing le carrierand multip le carrier）,and B3G/4G TDD system.

1 Evolution from Current TD-SCDMA to B3G/4G TDD

The goals of the evolution are to offerservicesw ith high rates,short time delays and low costs,aswellas to expand the system's coverage and capacity.To achieve these goals,severaladvanced technologies have to be adop ted,inc lud ing Adap tive Modulation and Cod ing（AMC）,Hyb rid Automatic RepeatRequest（HARQ）,OFDM,MIMO and multip le carriers.Among these,some w illp lay important roles in the evolution and be the basic d rive for peak rate increase.

With the continuous g row th of the users'demands,some cells（especially in bustling areas）ofa signal-carriersystem may be confronted w ith heavy traffic that they cannotbear.Therefore,new solutions,such as sp litting a celland add ing carriers,have to be emp loyed to imp rove the system's capacity.Considering the two solutions,the latterhas less im pacton network p lanning ordesign w ith lower costs.The multip le carrier technology can effectively solve the p rob lem of insufficientsystem capacity.The HSxPA technologies（i.e.,High Speed Downlink PacketAccess（HSDPA）,High Speed Up link PacketAccess（HSUPA）,and High Speed PacketAccess p lus（HSPA+））,ofwhich AMC and HARQ are themostnotab le ones,can further im p rove the downlink and up link throughputs.

As the remarkab le signs in the LTETDD phase,MIMO and OFDM are themost revolutionary technologies in B3G/4G systems.OFDM can effectively im p rove the spectrum efficiency.With the developmentofcom puter science and the advancementofmodern signalp rocessing techniques,the Fast Fourier Transform（FFT）and Inverse FastFourier Transform（IFFT）algorithm s are deduced,which considerab ly sim p lifies the realization ofOFDM in the TDD system.On the otherhand,the MIMO technology introduces the concep t“space domain”into the air link.The space domain,togetherw ith time domain,frequency domain and code domain,ob tains diversity and multip lexing gains,thusmultip lying the capacity of the communication system,and speeding up the transm ission rate.However,the MIMO technology is often suitab le for flattened channels,while in broadband w ireless communication,most channels are frequency selec tive.The combination ofOFDM and MIMO can perfectly solve this p rob lem:the Cyc lic Prefix（CP）technique in OFDM is used to overcome the effectof multipath and change the frequency selec tive channelinto the flattened one；and then,the MIMO technology can be app lied to imp rove the transm ission[3].

▲Figure 1. Evolution of TD-SCDMA systems.

Figure 1 shows the fourevolution phases of the TD-SCDMA system,and each phase can be furtherd ivided into d ifferent levels.They are sing le-carrier/multip le-carrier TD-SCDMA system,sing le-carrier/multip le-carrierHSxPA system,LTETDD system and B3G/4G TDD system.

1.1 Phase 1： TD-SCDMA System

The TD-SCDMA system is d ivided into sing le-carrierand multip le-carrier.The critical technologies adop ted in this phase inc lude CDMA,up link synchronization,smartantenna,joint detection and DCA.With the core network based on 3GPPR4,the peak rate can reach 2Mb/sw ith sing le carrier,and N×2Mb/s w ith N carriers theoretically.

1.1.1 Sing le-Carrier TD-SCDMA System

The originalTD-SCDMA system is focused on the sing le carrier,adop ting some advanced technologies such as smartantenna,jointdetec tion,DCA,up link synchronization,baton handover,low chip rate,multip le timeslots,variab le sp read ing,and adap tive power control,as wellas the higher layerp rotocols p roposed by 3GPP.Its core network is based on 3GPPR4.The TD-SCDMA system is notonly wellcom patib le w ith the WCDMA system,butitalsomeets the requirements specified by ITU and 3GPP for the 3G system to suitvarious 3G environments.As both the up link and downlink work at the same frequency in TDDmode,theirspatial physicalp roperties are sim ilarat the same time.With some adap tive rad io signal p rocessing technologies being used,flexib le sw itching between the up link and downlink can be realized.The advantage of TDDmode is that the timeslotallocation for the up link and downlink can be flexib ly changed by a conversion point tomeet differentservice demands.By changing the sw itch points,all3G symmetric and asymmetric services can be delivered.The introduc tion of jointdetection technology can considerab ly inc rease the transm ission capacity,which can be further im p roved by the smartantenna technology.With its d irec tionaland smartp roperties,the smart antenna technology can reduce the intercell and intracellinterferences,thus allowing the system to offerbetterquality ofservices,increase its capacity and extend the coverage.

1.1.2Multip le-carrier TD-SCDMA System In amultip le-carrier TD-SCDMA system,one cellis configured w ith severalcarriers.Ifeach carrier is regarded as a logic cell,the cellis then a combination of the logic cells thatshare the same common channels.Thus,the system's service bearing capacity can be d ramatically increased.The purpose of introducing the multip le-carrier technology is to increase the network capacity,which is help fulfor imp roving the system.Theoretically,the capacity ofan N-carrier TD-SCDMA system should be N times ofa sing le-carrier system.But the ac tualcapacitymay notbe so large because there are inter-carrier interferences from the ad jacentcell.

1.2 Phase 2： HSxPA System

In this phase,the HSDPA and HSUPA technologies w illbe introduced.Meanwhile,to supporthigh speed data transm ission,the integration ofother BWA systems into the system w illbe taken into consideration（e.g.,the integ ration of TD-SCDMA and WiMAX in term s ofcore network）.The critical technologies adop ted in this phase inc lude AMC,HARQ and FastPacketScheduling（FPS）,and the core network is an IP Multimedia Subsystem（IMS）.The theoreticalpeak rate ofa sing le-carrier HSxPA system is 2.8Mb/s,and it is 8.4Mb/sw ith three-carriers.When TD-SCDMA is integ rated w ith WiMAX,its theoreticalpeak rate can reach 75Mb/s（w ithin a 20MHz spectrum allocation）.

1.2.1 Sing le-carrier HSxPA System

The current TD-SCDMA system can be transformed into HSxPA system so long as necessarymod ifications aremade to the w ireless access network to raise the downlink transm ission rate to 2.8Mb/s per carrierwhile keeping the network structure and the core network unchanged.The HSxPA technologies p rovide a w ider developmentspace for the operator,and lay the foundation forob taining the target thatdata services exceed voice services.Being new UMTS TerrestrialRad io Access Network（UTRAN）transm ission technologies,as wellas enhancementofcommon transm ission technologies,the HSxPA technologies adop thigh-ordermodulation and fast retransm issionmechanism to inc rease the system's throughput,decrease the transm ission delay and imp rove the peak transm ission rate.

Themain d ifference between common transm ission technologies and HSxPA technologies lies in the compensation made for the channelquality.In common transm ission technologies,a constantdata rate is kep tby way of fastpower control,so they are suitab le for transm ission of real-time data,such as voice services.In HSxPA technologies,the total downlink transm itpowerofany user ismaintained ata constant value.As we know,if the powers ofallusers are the same,the usernear the base station w illhave less loss and less interference；thus,its channelcapacity is relatively large.On the contrary,the user,faraway from the base station,w illhave a lower channelcapacity.By adop ting AMC and HARQ mechanism,the HSxPA technologies enab le the data rate to change w ith the channelcapacity.Obviously,these technologies are suitab le fordata services rather than real-time services.In the case ofgood channelquality,the theoretical peak rate of the HSxPA system can reach 2.8Mb/s fora carrier.If N carriers are configured,the peak rate can reach N x 2.8Mb/s,theoretically.In this phase,which is based on 3GPPR5,the all IP-based core network w illbe construc ted and putinto use,and the IMSw illbe introduced to p rovide IP-based QoS.

1.2.2Multi-carrier HSxPASystem

In the TD-SCDMA system,each carrier is allocated a spec trum of1.6MHz.As a result,even if5 downlink timeslots are emp loyed,the HSxPA system can only achieve a downlink peak rate of2.8Mb/s.But the theoreticalpeak rate ofan FDD system is 14.4Mb/s and thatofaWLAN system is 54Mb/s.Therefore,the TD-SCDMA system is far less than the other two kinds of systems in peak data rate,although ithas betterspectrum efficiency.

Ifmultip le carriers are configured for the HSxPA system,the data can then be sentor received on these carriers simultaneously.Forexamp le,in the case ofa three-carrier configuration,the data w illbe transm itted through the three carriers at the same time；thus,the peak rate can reach 8.4Mb/s.Themore carriers,the higher the peak ratew illbe.The advantages ofmultip le-carrier configuration are high peak rate and flexib le configuration.When there is a smalldemand for data services in an area,it is enough to configure only some of the carriers forHSxPA services.Butas the demand g rows,the networkmay have to be re-p lanned and more carriersmay have to be configured to p rovide HSxPA services.

In a WCDMA system,ifHSDPA services are to be p rovided in a spectrum of10MHz,the respec tive 5MHz forup link and downlinkmustbe consecutivewhile in a TD-SCDMA system,6 independent1.6MHz carriers can be used.Taking the availability ofcarrier resources into account,this is no doub ta g reatcom petitive advantage.

1.2.3MIMO Technology

Withmultip le antennas,a communication system can realize space diversity,spatialmultip lexing,and beam form ing in the space domain；thus,the system performance and transm ission capacity are g reatly im p roved,especially during transm ission and recep tion.As the fad ings ofdifferentantennas are relatively independent,the channelcapacity w illincrease linearlyw ith the numberofantennas.In 3GPPR5,many MIMO schemes,like the antenna-based rate control,have been p roposed to enhance system performance,and in 3GPPR6,the MIMO technology is introduced into the TD-SCDMA system,allow ing a peak rate of 14.4Mb/s fora three-carrier configuration.The evolution and enhancementofHSxPA is stillunderstandardization.However,it is suggested to use MIMO and high-ordermodulation and coding scheme to geta higher transm ission rate.

1.2.4 Distributed Antenna System

Once smartantenna and jointdetection technologies are used in the TD-SCDMA system,the system capacity ismainly lim ited by the intercellinterference rather than intracellinterference.As the b reathing effectofa TD-SCDMA cellis notg reat,the decreasing of intercellinterference can increase system capacity.The use ofd istributed antennas can expand system coverage and reduce the impac tof fast fad ing.In a d istributed antenna system,severalad jacentantennas use the same p rocessing unit toform a logic antenna array,whichw illserve the user simultaneously:in the downlink,the distributed antennasw ill transm it the same signals to the userat the same time；while in the up link,theywillreceive the same signals and send them to the p rocessing center.The antenna end should be sim p lified as faras possib le to lower the cost.As a result,only the basic componentswillbe configured there,and signal p rocessing w illbe centralized at the p rocessing center.

In addition to coverage expansion,the d istributed antenna system has otheradvantages.First,itallows an even coverage over the service areas；thus,the system performance,especially the handoverperformance,can be im p roved.When a user moves w ithin a service area,no handover is required even though severalantennas are serving it.Second,as the signals are received via the antennas atdifferent locations,macro d iversity can be realized to resist the effec tofshadow fading.Third,the resourcemanagementof the d istributed antenna system w illbemore flexib le:its p rocessing center dynam ically allocates the resources in a uniform way,so the resource utility can be op tim ized and the spec trum efficiency is g reatly increased.Finally,the software configured-and managed-cell structure can better cope w ith service changes in d ifferent times and d ifferent locations.

1.2.5MBMSTechnology

Multimed ia BroadcastMulticastService（MBMS）is an enhancementof the currentWCDMA network.Seam lessly integrated into currentmobile network,this technology enab les the operator to delivermobile television services.MBMS is p roposed for TDD systems in 3GPPR6.Unlike otherd igital television b roadcasting technologies,MBMS offers a setof b roadcast/multicastchannels thatare comp letely operated and controlled by themobile operator.

Taking advantage of the two-way channelof the cellularnetwork,MBMS services can interactw ith the users.In addition to b roadcast service,MBMS can offerother services:multicast,highly reliab le download ing w ith the point-to-point fixmechanism,flexib le charging via interactive channels,mobile television services,and various kinds of PUSH services,many ofwhich have been offered by currentnetworks.Besides,MBMS can bring the operator p rofits bymeans of increasing the transm ission capacity.

In view ofsomany services offered by MBMS,how to multip lex MBMSw ith cellularnetwork data has become a g reat concern of the researchers.The new p roposals basically fallinto twomodes:Time Division Multip lexing（TDM）and Frequency Division Multip lexing（FDM）.In the TDM mode,long cyc lic p refixes can be used to combatmultipath interference.But this mode does notsupportvariab le spectrum and can only work in the spectrum of10MHz.In the FDM mode,only the cyc lic p refixes of the cellularsystem（which are relatively short）can be used ow ing to the fact thatMBMS and the cellularnetwork data aremultip lexed into an OFDM symbol.But thismode supports variab le spectrum and can work in different,selected spectrum s.

The MBMS technology is no doub tan effective supp lement to the currentcellularsystem.With it,more services can be delivered to the users by way ofadd ing or imp roving some func tionalentities in the currentnetwork.

1.2.6 Integration w ith BWA Systems

When the TD-SCDMA system and BWA networks are compared,the formerhasw ider coverage area,but the latter has a higher peak rate and can offerhigh rate services in a low mobile environment.（Forexamp le,the IEEE 802.11a-based WLAN can achieve a peak rate of54Mb/s.）A system integ rating WLAN and TD-SCDMA canmake use of the advantages ofboth:deliverhigh rate transm ission in hotspots using WLAN,and p rovide w ide network coverage w ith TD-SCDMA.

Meanwhile,the integration of TD-SCDMAwithWiMAXhas been p laced into the agenda and has become a focus in technicaldiscussions.WiMAX can p rovide a peak rate of 75Mb/s in the spectrum of20MHz,which is a powerfuland effective supp lement to the large coverage of the TD-SCDMA system in hotspots.Moreover,the p roposalof802.16e（expansion release ofWiMAX）b rings a g reatenhancementin mobile speed of the integ rated system.To integ rate TD-SCDMA system s w ith BWA,the TD-SCDMA term inals are required to supportboth kinds ofaccesses,and some special functionalentities should be added to support the p rotocols for the integ rated system[4].

1.3 Phase 3： LTE TDD System

The third phase in the evolution of TD-SCDMA system s is LTE TDD,which is a transitionalperiod to 4G TDD system s.In this phase,the technologies for4G system s w illbe used on the 3G p latform so as to lay a solid foundation for smooth transition from the 3G system to the 4G system.Currently,most researches on LTETDD focus on the physical layer.The transm ission performance and communication parameters of this phase are sim ilar to those of the nextphase.Many of the technologies adop ted in this phase are for the purpose ofenhancing the system performance,inc lud ing MIMO,OFDM,flexib le spectrum selection（1.25MHz,1.6MHz,5MHz,10MHz,15MHz and 20MHz）and d istributed w ireless access networks.With MIMO and OFDM technologies,the peak rate of the system can reach 100Mb/s in the downlinkw ithin the spectrum of20MHz,and 50Mb/s in the up link.Allof its services w illbe delivered on the shared and common channels,and its core networkw illbe based on IPv6.

As the Peak-to-Average Power Ratio（PAPR）is quite high in the OFDM scheme for the up link,the OFDM technology w illbe used only in the downlink.In the up link,the sing le carrier technologies w illbe used,for instance,Interleaved Frequency Division Multip le Access（IFDMA）and Disc rete Fourier Transform-Sp read OrthogonalFrequency Division Multip lexing（DFT-SOFDM）；while in the downlink,the OrthogonalFrequency Division Multip le Access（OFDMA）w illbe used.The IFDMA technology is designed to be w ithoutanymultip le access interference.Each user in the system has its own setof sub-carriers,which w illinterleave w ith the sub-carriers of d ifferentusers.In IFDMA,the sub-carriers ofeach userw illbe evenly distributed on the transm ission band to get themaximum frequency d iversity gain.In add ition,as the IFDMA signalis in time domain,a low PAPR is achieved.

In the LTETDD phase,the integration of TD-SCDMA system s w ith other BWA networks is furtherstrengthened,from the IP core network to the w ireless access network.The core network of this phase is based on the IMSof All IPv6,and its services inc lude notonly trad itionalPeer-to-Peer（P2P）multimed ia data services,butalso the MBMS services and more flexib le P2P services.With the emp loymentofadvanced p rocessing mechanism for the physical layer,the spectrum efficiency w illrange between 2 bps/Hz and 5 bps/Hz.

▲Figure 2. Topological structure of traditional point-to-multipoint network.

1.3.1 SC-FDMA

Two schemes have been p roposed for the up link of3GPP LTE:OFDMA formultip le carriers and SC-FDMA for sing le carrier.And the later is finally chosen due to the high PAPR p rob lem in the OFDMA scheme.The SC-FDMA system,which can be further divided into IFDMA and SOFDM systems,hasmany advantages:low PAPR,m inimalintercellinterference due to orthogonalp rocessing of frequency domain between users,less comp licated frequency domain balance,and variab le chip rates.The IFDMA system and DFT-SOFDM system are time domain p rocessed and frequency domain p rocessed,respectively,and the com parison of the two system s has become the focus of the industry.The PAPR of IFDMA is lower than thatofDFT-SOFDM,and so is its spectrum efficiency.The DFT-SOFDM system ismuch comp licated to realize,but is more com patib le w ith the OFDMA system.Therefore,the DFT-SOFDM system is a better choice for the up link.

1.3.2OFDMA

In the downlink of3GPP LTE,the OFDMA system is adop ted.In either3GPP LTE orWiMAX,the OFDMA scheme has,beyond question,been chosen for the downlink because of its better spectrum efficiency and less realization com p lexity.As to the high PAPR p rob lem in themultip le-carrier system,itcan be solved by comp licated p rocessing at the base station.In the finalscheme chosen for the LTETDD system,itis necessary to take into account the balance between the extra comp lexity arising from reducing the PAPR and the im p rovementin link performance.The combination ofMIMOmultip le antenna technology and the OFDMA can b ring the system more gain and betterperformance.

1.3.3MIMO-OFDMA Technology

The MIMO-OFDMA technology,themost revolutionary technology in the nextgeneration communication system,is the basis for3GPP LTE system s to enhance the peak rate and QoS.The MIMOmultip le antenna technology perform s quite wellin imp roving the spectrum efficiency.But thew ider the spectrum it uses,the g reater themultipath interference is.In traditional sing le-carrier systems,the receiver is designed considerab ly comp lex in order to perform MIMO detection.The introduction of OFDM technology solves this p rob lem.OFDM can effectively m itigate the effectof frequency selective fading and decrease the inter-symbolinterference,making itquite suitab le for the high rate data transm ission in w ireless broadband channel.With FFT/IFFT,OFDM can be easily realized.Moreover,OFDM app lies the AMC technology in each sub-carrier,which allows the spectrum to be effectively utilized.These charac teristics of OFDM make itcom petitive in the LTE and B3G systems.By integ rating MIMO and OFDM,the frequency selective MIMO channelcan be divided intomany flattened sub-channels,and the MIMO detection system is sim p lified.

1.3.4 Dynam ic Frequency Selec tion Mechanism Nowadays,the spectrum resources are extremely lim ited.To make effec tive use of the cellularspectrum resources,the LTE TDD system w illadop tvariab le spectrum to enab le a good-quality and high-rate transm ission ateach spectrum.This is called dynam ic frequency selec tion technology.The TD-SCDMA system w illdefinitely be dep loyed before the LTE TDD system.Thismeans 1.6MHz is likely to be used in future systems.On the otherhand,the spectrums 1.25MHz,5MHz,10MHz and 20MHz w illalso be taken into consideration in the LTETDD system in China.Thus,a flexib le spectrum selection can be realized,enab ling the communication system to cope w ith the changes in time and area,and effectively use d ifferent spectrum s.

1.3.5Wireless Mesh

The traditionalcellularnetwork is a point-to-multipoint structure,as shown in Figure 2.This structure is a centralized mechanism where each Base Station（BS）takes charge of the communication ofallusers w ithin a cell.

In the LTETDD system,in order to expand the system coverage and capacity,the“multi-hop”concep tis introduced.Multi-hop means to use the user term inalin the topology as a relay toforward the signals to a farthernode.In this way,the coverage is extended.Meanwhile,the system capacity increase is achieved due to the relay gain.In traditional point-to-multipointnetwork struc ture,the communication ofany link has to go through the base station.Nomatterhow near two term inals are,they have to send the signals to the base station they belong tofirst,and then the base station transm its the signals to the target term inal.If the overhead spenton the signaling interaction is also taken into account,this linkw ill wastemuch of the resource.To avoid this waste,the multipoint-to-multipointstructure is introduced.Thatis to say,any two points in the network can communicate with each other.In this case,the data can be transm itted in a quicker,easierand more econom icalway.

In the trad itionalnetwork structure,supposing the transm it power is certain,the higher the data transm ission rate is,the smaller the coverage is.Once themaximum allowab le transm it power is exceeded,the transm itterhas to lower its transm ission rate so as to extend the coverage d istance.Generally,the transm itpower is lim ited by the standard specifications and the userequipmentbattery.Therefore,the adap tive technologies are necessary in the cellularsystem in order to offer the users of ad jacentbase stations w ith a higher rate.However,the data rate willd rop sharp ly as the space between base stations inc reases.

▲Figure 3. Topological structure of Mesh network.

The Mesh struc ture is not the case.Itis integ rated w ith multi-hop and multipoint-to-multipoint technologies.As shown in Figure 3,the Mesh structure can notonly p rovide a long-distance end-to-end communication w ith a series of intermed iate nodes,butalso offera data transm ission rate which is high enough.Compared w ith the d istance between the transm itterand the receiver,the d istance between nodes（i.e.,d istance ofa hop）ismuch shorter,so the transm ission rate of each hop w illbemuch higher than thatof the transm itter-receiver communication.In thisway,a high transm ission rate is obtained in the long-d istance communication system.The Mesh networkingmode effectively solves the contradic tion between high transm ission rate and w ide coverage,which have long been a contrad ictory pair.In add ition,as each node in a Mesh network is required to transm it the signals fora very shortdistance,its transm itpower can be very low；thus,the interference within the system decreases d ramatically,and the frequency can bemultip lexed in a denser way.Moreover,because the data can be transm itted via the intermediate nodes,a robust routing can be estab lished by avoiding the obstac les or b locks.

The Mesh structure can be d ivided into two kinds:centralized and distributed.The centralized Mesh structure,based on the trad itionalpoint-to-multipointstruc ture,expands the coverage and system capacity by using the user term inals as relays.The d istributed struc ture ismore flexib ly configured to reduce the system delay,avoid network bottlenecks and node faults,im p rove QoSand deliver com p rehensive services.In the distributed structure,the user information and the controlsignaling messages are separated,so service delays are reduced and the costs incurred in system integration and dep loymentare lowered.As a result,the investmentcan be directly turned into the g row th ofnetwork scale,saving the expenses needed fornetwork dep loymentin the initial period.Themultip le choices ofstruc tures enab le a flexib le and efficient transm ission,and the entire network can thus be op tim ized.Therefore,the B3G and 4G w ireless access networks can be introduced into the LTETDD system w ith smallmod ifications at the base stations and the term inals[5].

1.3.6 P2PCommunication Technology

The P2P technology has been w idely app lied in com puternetworks,such as network phone and BitTorrent（BT）downloading.And this technology w illeventually be introduced to the w ireless communication network.3GPP has inc luded the P2P services into its agenda,whichmeans the combination ofP2P technology w ith Mesh topology w illbe enhanced in the LTE and nextgeneration network.In 802.11,the distributed Ad Hoc network adop tsmultipoint-to-multipoint structure,which w illbe surely connected w ith the P2P technology.In Figure 4,the leftis a trad itional point-to-multipointstructure,where the communication between term inals is im possib le and has to go through the base station.Due to the base station's spec trum availability,many user requests cannotbe g ranted.As a result,its download efficiency is quite low on the whole.The rightof Figure 4 is a P2P communication structure,where allthe nodes are peers rather than in a server/c lientarchitecture,so any two points can freely communicate w ith each other.The essence of the P2P system is“node cooperation”.Any system where there is no adm inistrators and tasks are comp leted by sw itching and cooperation between nodes can be called a P2P system.

1.4 Phase 4： B3G/4G TDD System

▲Figure 4. Traditional point-to-multipoint structure vs. P2P structure.

IMT-Advanced has p roposed peak transm ission rates of 100Mb/s and 1 Gb/s in the high and low mobile environments respec tively.Themain purpose ofusing the TDDmode in the B3G/4G system is to achieve its goal.In this phase,the system w illadop tsuch criticaltechnologies as cognitive rad io and network cognition,and support the integration w ith various w ireless communicationmechanisms in theWireless Ubiquitous Service（WUS）environment.In the WUS architecture,several advanced technologies inc lud ing Ultra-Wide Band（UWB）,Ultra-Narrow Band（UNB）cognitive rad io and network cognition,w illbe used to deliverhigh speed data transm ission and develop an op timalnetwork access and dep loyment scheme.A new sub jectas itis,the network information theory w illp lay a very im portant role in the B3G/4G system and become a guide fornetworking and dep loymentof the WUS system.

In the B3G/4G TDD phase,the variety and coexistence ofair interface networks allow diversified services to be delivered to the users,buton the otherhand,they b ring many inconveniences in handoverand roam ing.In particular,one term inalhas to satisfy allkinds ofaccess networks,and the realization p rocess should neitherbe too comp licated,nor cost toomuch.With software rad io technology,the term inalcan download access networkmodes and p rocesses ofdifferent standards to be compatib le w ith differentnetworks.In the future,themodularized,open and distributed architecture w illbe the mainstream mode forservice generation and offering.This mode,trying to shield the details of lower-layernetwork im p lementation,p rovides a framework for the services to be generated and dep loyed in an easierway.In ubiquitous networks,the service typesw illbe very rich and the service offering willbe user-oriented,and the QoSwillbe guaranteed.Otheradvantages of the ubiquitous architec ture inc lude:each network can p rovide the user-centered multi-med ium services；support the seam less handoverand roam ing between networks；balance the d ifferentnetworks'load；and determ ine the op timum network to ultimatelymeet the users'demands.

In this phase,tomeet the high transm ission rate and the ubiquitous coverage requirements,rad io relay and multi-hop transm ission technologieswillbe adop ted.The basic concep tof rad io relay is to use relays to p rocess and send the base station signals.This p rocessmay be simp le,where the relay just receives and amp lifies the signals,or itmay be comp lex,where the p rocessing above MAC layer is required.The introduction of radio relays,either fixed ormobile,can save the transm itpower of the term inal,thus p rolonging the battery life.The app lication ofmulti-hop can notonly expand the coverage ofa cell to the extent that the b lind ang les,such as build ing shadows and underg round,w illbe elim inated,butalso balance the load by transferring the hotspotservices to otherareas.On the other hand,to sim p lify rad io spectrum management,the cognitive radio technology is used to cognize the rad io environment.The essence of the environmentcognition is to extend the network to every corner,use the cognitive ability of the new w ireless communication network node to cognize the status of current network environmentand the user scenario and surround ings,especially the spectrum information,and finallymake p lans,decisions and reactions based on the status and the scenario bymeans ofseveraladvanced physical layer technologies.The spectrum information ofone node w illbe shared among the ad jacentnodes so as tomake fulluse of the spectrum resources.In this way,the coverage w illbe expanded and the utilities ofspectrums and powersw illbe enhanced.

2 Conclusions

The goalof TD-SCDMA system evolutions is to offer the system s w ith high data rate,short time delay,low cost,op tim ized coverage and capacity,as wellas highmobility.The rapid grow th ofmobile services in recentyears has p romoted the evolution ofmobile communication systems to have some advanced features,such as b roadband and w ireless.In the p rocess ofsuch an evolution,the increasing demands forhigh data rate and QoS are inevitab le.The evolution p rocess are d ivided intofour phases:sing le carrier/multip le carrier TD-SCDMA system,sing le carrier/multip le carrier HSxPA system integ rated w ith BWA technologies,LTE TDD system（sing le-carrier configuration in the up link and multip le-carrier configuration in the downlink）,and B3G/4G TDD system.The entire p rocess w illbe quite smooth,allow ing technology update,network structure op tim ization and maximum usersatisfac tion to be achieved at the lowestcost.