Evolution of Key Technologies for WiMAX Physical Layer

Liu Qiao yan Yu Qiu Xing

Abstra c t:Desp ite the rap id development in Worldwide Interoperability for M ic rowave Access（WiMAX）technologies,key technologies for the Physica l Layer（PHY）still need to be further im p roved so as to achieve highly efficient and reliab le communication performance,as well as to support a m obile environment with a higher transm isison rate.As an am endm ent to IEEE 802.16d（for fixed b roadband w ireless access systems）,IEEE 802.16e（for m obile b roadband wireless access systems）introduces the Orthogona l Frequency Division Multip lexing（OFDM）and Multip le Input Multip le Output（M IMO）technolog ies into the PHY,doub ling the transm ission rate while supporting a certain deg ree ofm obility.In the future,m ore advanced Air Interface（AI）technology is to be app lied in the IEEE 802.16m standard.

Wireless b roadband technologies based on IEEE 802.16e have become themainstream for Worldw ide Interoperability for Microwave Access（WiMAX）technologies.Access to w ireless networks has become a reality formany peop le.To satisfy peop le's needs for higher transm ission rates and mobility,the Institute ofElec tricaland Electronics Engineers（IEEE）ismaking the next generation advanced Air Interface（AI）standard,namely 802.16m,on the bases of its p revious ed itions of802.16a,802.16d and 802.16e.The formulation of IEEE 802.16m,in whichmany renowned telecommunication com panies have joined efforts,has started in December 2006.

The technological features of WiMAX's PhysicalLayer（PHY）are as follows[1]:

（1）OrthogonalFrequency Division Multip lexing（OFDM）technology is app lied to achieve efficient frequency utilization.

（2）Time Division Dup lex（TDD）,Frequency Division Dup lex（FDD）and Half FDD（HFDD）are supported.FDD needs coup led frequencies,butTDD does not.TDD can realize flexib le and dynam ic allocation ofup link/downlink bandw id th.HFDD lowers the requirements on end transceiver.

（3）Bothmobile and fixed situations are supported,w ith the highestspeed supported reaching 120 km/h.

（4）TheWiMAX system's bandw id th span is from 1.25MHz to 20MHz.WiMAX has defined two series of bandw id th:1.25MHz and itsmultip les,and 1.75MHz and itsmultip les.The formerseries inc ludes 1.25MHz,2.5MHz,5MHz,10MHz and 20MHz.The latter inc ludes 1.75MHz,3.5MHz,7MHz and 14MHz.

（5）Advancedmulti-antenna technology is used to imp rove system capacity and scope ofcoverage.

（6）Hyb rid Automatic RepeatRequest（HARQ）technology is adop ted,which integrates the Forward ErrorCorrection（FEC）func tion,so thatevery packet sending operation can contribute to the finalcorrectdecoding.HARQ can be d ivided into two types:Chase Combining and Inc rementalRedundancy.

（7）Adap tive Modulation and Cod ing（AMC）technology is adop ted.The technology ad justsmodulation,coding mode and cod ing rate ofpackets at times according to quality ofsignals received,so as to achieve the highest possib le data transm ission rate of the system whilemaintaining adequate reliability.

（8）Power control technology is used tomaxim ize spectralefficiency when othersystem indicators are allmet.

（9）Advanced channelcod ing technology is adop ted to enhance communication performance and expand scope of coverage.

Regard ing the technological demands for InternationalMobile Telecommunications（IMT）-Advanced and NextGeneration Mobile Networks（NGMN）,the future transm ission rate for mobile communications should reach hund red Megabits persecond,and even Gigabits persecond.However,the highest transm ission rate on the PHY acquired in the IEEE 802.16e standard is only 75Mb/s.Therefore,the effec tive evolution ofkey technologies for the PHY mustbe achieved in the future standards in order to achieve both high data transm ission rate and high communications quality.

▲Figure 1. OTDM system.

1 OFDM and OFDMA

OFDM and OrthogonalFrequency Division Multip lexing Access（OFDMA）technologies are adop ted by both IEEE 802.16d and IEEE 802.16e.In the future technologicalevolution on the PHY,OFDM and OFDMA technologies w ill remain themostim portantones.Another technology thatattracts peop le's attention recently is OrthogonalTime Division Multip lexing（OTDM）,which m ightevolve into one of themajor multip lexing technologies on the PHY in the future.

1.1 OFDM

Themain idea ofOFDM[2]is to d ivide a channelinto a numberoforthogonal sub-channels,and transform high-speed data signals to parallel low-speed sub-data flows which are thenmodulated and transm itted on each sub-channel.Orthogonalsignals are d ivided in receivers by related technologies,which can reduce Inter-Channel Interference（ICI）in the sub-channels by a certain deg ree.As signalbandw id th is smaller than channel bandw id th in every sub-channel,each sub-channelcan be regarded as a flat fad ing channel；therefore,Inter-Symbol Interference（ISI）is avoided.Since the bandw id th ofeach sub-channel accounts foronly a smallportion of the originalchannel,channelbalancing becomes relatively easier.

OFDM technology is becom ing more and more eye-catching because of the following unique advantages ithas:

·High spectralefficiency

·Strong anti-multi-path interference and anti-frequency-selective fading ability

·The system can reach the highest possib le bit rate w ith the Dynam ic Subcarrier Allocation Algorithm

·Strong anti-fad ing performances w ith united coding ofsubcarriers

·Discrete Fourier Transform（DFT）-based OFDM has a fastalgorithm.Both the Fast Fourier Transform（FFT）and Inverse FastFourier Transform（IFFT）that the OFDM app lies for modulation and demodulation can be easily realized by DigitalSignal Processor（DSP）

OFDM,however,also have the follow ing d isadvantages:

·Sensitivity tofrequency offsetand phase noise

·Peak-to-Average Power Ratio（PAPR）is high,which leads to low power and efficiency of themagnifierof the sender

·Auto-adap ted modulation technologymakes the system more com p licated

As amodulation solution securing spectralefficiency,OFDM has been adop ted by some standards and systems.Itw illbecome one of the best ways ofmodulation for the downlink of the nextgeneration wireless telecommunications systems,and w ill integrate w ith the trad itional multi-access technology to evolve into a possib le solution for the nextgeneration w ireless telecommunications system.

1.2 OFDMA

In an OFDMA system,users onlymake use ofsome of the sub-carriers.As long as the user's tim ing offsetand frequency offset in one frame are smallenough,there w illnotbe any intra-cell interference in the OFDMA system,which gives OFDMA an edge overCode Division Multip le Access（CDMA）.Since the frequency hopp ing technology and the OFDM technology are combined in the OFDMA system,amore flexib le multip le access solution can be developed on thatbasis.Besides,OFDMA can flexib ly adap t to bandw id th requirementso itcan work collaboratively w ith Dynam ic ChannelAllocation（DCA）technology to supporthigh speed data transm ission.

In the future developmentof the PHY technologies,OFDMAw illcontinue to p lay a very im portant role.

1.3 Single-Carrier Frequency Domain Equalization （SC-FDE ） Technology

Decreasing PAPR in the OFDM system is stillan urgentp rob lem thathas to be solved.To avoid PAPR interferences,many evolution solutions have introduced SC-FDE technology[3-4],orwhatwe call OTDM,in theirdevelopment.Figure 1 shows theworking theory ofOTDM System.

SC-FDE becomesmore and more popularbecause ithas the follow ing advantages:

·Strong anti-multi-path interference ability

·High spectralefficiency（sim ilar to OFDM oreven higher）

·No PAPR

·Smallout-of-band rad iation

·Sim p le im p lementation

·Adap tive technology adop ted

Besides,SC-FDE is easy to be integrated into the follow ing technologies:·Cyc lic Prefix（CP）-CDMA·CP-DirectSequence（DS）-CDMA·OTDM p lus SmartAntenna

（Transm itter）

·OTDM p lus SpatialDiversity Receiving（Receiver）

The new generation w ireless telecommunications system has raised stric t requirements on the system's performance,cost,size,powerand energy consump tion.SC-FDE system has a relatively strong capacity in resisting frequency-selec tive fad ing；thus,ithas overcome the defectofOFDM system and made the imp lementation of receivereasier.SC-FDE can also co-existw ith OFDM in a bid irectional transm ission system so thatadvantages ofboth technologies can be achieved flexib ly and efficiently.Besides,SC-FDE can integ rate w ith Multip le InputMultip le Output（MIMO）technology to raise spectralefficiency and imp rove system performance.Therefore,SC-FDE is a very com petitive solution for future high speed w ireless telecommunications,and itenjoys w ide app lication p rospects in the field ofb roadband w ireless telecommunications.

▲Figure 2. Single input multiple output.

2 Frame Structure

IEEE 802.16e has defined TDD,FDD and HFDD for the PHY.These three dup lex use the burstdata transfer format,which supports the adap tive burstservice data,and can dynam ically ad just transm ission parameters likemodulationmode,cod ing mode and transm itting power w ith the assistance of the MAC Layer.

In the TDDmode,the leng th ofeach physical frame is fixed,and the shifting pointofup link and downlink can be automatically adap ted w ith downlink being the p riority.In this way,com petition from the up link can be avoided.Meanwhile,the transceiving time slotcan be laid between the up link sub frames and the downlink sub frames to leave necessary p rotective space.Resourcesmanagementand d istribution can be controlled at the Base Station（BS）so thatchannels can be flexib ly used in the up link ordownlink,accord ing to realneed.Besides,multip le permutations are defined for different scenarios in the frame structure to raise spectralefficiency,supportvariab le mobility and resistmulti-path fading.IEEE 802.16e also adop ts OFDMA of 128/512/1 024/2048 changeab le sub-carriers,which enab les the channel bandw id th toflexib lymove from 1.25 MHz to 20MHz.In this way,the equipmentcan have stronger channel balancing capability and anti-fast fad ing ability,which assures the normal utilization ofWiMAX end equipment in a mobile environment.

The future frame structure has to make the scheduling ofd ifferent multi-antenna app lications sim p le and effective,and ithas to support the evolution ofkey technologies for the PHY.

▲Figure 3. Multiple input single output.

3 MIMO

Frequency resource is lim ited.Channel capacity w illnotexceed Shannon Lim it whether in time domain,frequency domain or code domain.Utilization of multip le antennasmakes itpossib le for d ifferentusers'signals to be exp ressed by differentspace features,and the space domain resourcemay according ly be used.Space domain utilization can multip ly channelcapacitywhile maintaining bandw id th at the same level,and itcan also imp rove communication quality and transm ission reliability.

3.1 Application Modes of Multiple Antennas

The app lication of themultip le antenna technology in the futuremustbe flexib le and changeab le,and majorapp lication modes are as follows:

（1）SpatialDiversity Receiving（in case of Sing le InputMultip le Output（SIMO））Affec ted by factors like size,powerand cost,some end equipment only have one antenna in theirsend ing part；however,BSusemultip le antennas to im p lement recep tion d iversity.In ideal circum stances,10log N r（dB）increment can be gained.N r stands fornumberof BS receiving antennas.Therefore,the value increases as the logarithm of N r inc reases.The case is shown in Figure 2.

（2）SpatialDiversity Transm itting（in case ofMultip le InputSing le Output（MISO））In idealcircumstances,one receiving antenna at the term inaland multip le transm itting antenna at the BS can ob tain the gain of10log N t（dB）.N tstands for numberof BS transm itting antennas.Therefore,the value increases as the logarithm of N t increases.The case is shown in Figure 3.

（3）Beam form ing（in case ofMISO）

One antenna at the term inaland multip le transm itting antenna at the BS can im p lementbeam form ing Since the transm itting end has got H matrix,the Signal-to-Noise Ratio（SNR）ofwave packetis 3 dBmore than thatof transm it d iversity.So,wave packet form ing can be carried outonly after channel information is collected either through up linkmeasurementorup link feedback.

（4）Space Time Cod ing（in case of MIMO）

In the future communications system s,end equipmentw illbe d iversified and some of them could have many antennas so that the up link and downlink in the communication p rocess can both realize MIMO,like what is shown in Figure 4.Space Time Cod ing is one of themain app lication forms of MIMO.The orthogonalspace time packet codes can have fulld iversity gain.Space time trellis code can acquire both partial diversity gain and code gain.

（5）Space Multip lexing（in case of MIMO）

▲Figure 4. Multiple input multiple output.

Anothermain app licationmode of MIMO is Space Multip lexing.Space Multip lexing technologymakes increasing the channelcapacity by severaltimes possib le.Prem ises for app lying Space Multip lexing technology inc lude:N r≥N t；using zero-forcing and interference cancellation to execute signalby signal test；the channel information and channelcorrec tion is unnecessary for the transm itting side.When channelcapacity d rops,the multip lexing coefficientcanmake auto-adap tive changes according ly.

（6）SmartAntenna（an advanced multi-antenna system）

One of themajor tasks for smart antenna is to im p rove the system's performance by acquiring and utilizing signal's space information and im p roving signal's quality through array signalp rocessing.Weighing ofantenna array is p rocessed and com p leted by digitalsignals atbaseband.As an importantpartof the advanced multi-antenna system,adap tive array technology is the key and future trend of the smartantenna research.

3.2 Space Domain Adaptive Multi-Antenna Technology

The futuremulti-antenna technology w ill surely fulfill the space domain adap tive link.Accord ing to channelchanges,it can realize the adap tive link w ith the highestdata transm ission rate and largestaverage channelcapacity.

Design p rincip le for the adap tive link aim ing at the highestdata transm ission rate:

（1）Undermobile circum stances,MIMO channelis changing,and so is channelcapacity.

（2）As faras low rank channelis concerned,more transm itting antennas unnecessarily bring the channel capacity,and the system capacity can be inc reased by reasonab ly selec ting the right transm itting antenna.

Design p rincip le for the adap tive link aim ing at the largestaverage channel capacity:

（1）When the fad ing coefficients of the transm itting antenna and the receiving antenna are notcompatib le,buthave the same distribution pattern,the MIMO system ob tains a considerab le channel capacity.However,lim ited by antenna array elementspacing and real communication environment,the coefficients of transm itting antenna and receiving antenna are often correlated.It is unveiled by research thatchannel capacity w ill falld ramatically when the coefficients are c losely correlated.

（2）In related fading channels,itis necessary to reasonab ly design antenna array space and array pattern,so as to reduce the related coefficients thataffect channelresponse between antennas.

4 HARQ

HARQ is a new PHY technology combining Automatic RepeatRequest（ARQ）with Forward ErrorCorrection（FEC）.Itcan be d ivided into three types.HARQ Type Ionlymakes a sim p le combination of FEC and ARQ.Though it solves the innate defec ts of FEC and ARQ ata certain deg ree,itcauses low data transm ission efficiency in general because itdoes notmake fulluse of the help fulinformation in the w rong data packets butsimp ly abandons those data packets and requests the data packets again.Basing on Type I,HARQ Type II introduces the idea thatcode combining generates decoding gain,and itmakes fulluse of the help fulinformation in every data packetsent.However,the resent data packetin Type IIinc ludes new redundantinformation（a new data packetgenerated from the integration of help fulinformation）,and not the original data information,so itdoes notpossess a self-decoding capability.If the original data packetis seriously damaged or is m issing,resend ing it,nomatterhow many times,w illnot lead to correct decod ing.This is a big d isadvantage of HARQ Type II.To overcome defects in I and II,HARQ Type IIIinc ludes the originaldata information in both the originaldata packetand the resentone,and can restore originaldata information simp ly through decod ing the resentdata packet.The system performance can be wellim p roved by flexib ly adop ting the HARQ Type IIIand Chase Combining.

5 AMC

The basic idea ofAMC is to allocate transm ission powerand cod ing rate accord ing to channelconditions,so as to upg rade the transm ission rate or system throughout.There are two steps for im p lementing AMC:

（1）Measuring transm ission channel parameters

（2）Selecting one ormore transm ission parameters on the basis of op tim izing p resetcost function

Only the selected channel parameters can be compatib lew ith the p racticalscenarios if the channelvaries less fast.Therefore,the adap tive technology can only app ly to the case where there is no significantexpansion of Dopp ler.The technology has a p rom inent streng th when app lied in indoor circumstances where transm ission delay is short,and the relative speed ofboth the transm itterand receiver is slow.Given this case,the adap tive technology can be used frame by frame.

Themajoradap tive technologies inc lude:

·Adap tively ad justing power rating

·Adap tively ad justing the size of constellation chart

·Adap tively ad justing cod ing rate·Adap tively ad justing both power rating and the size ofconstellation chart

·Adap tively ad justing both the size of constellation chartand symbolrate

·Adap tively ad justing both power and transm ission rate

·Adap tively ad justing cod ing rate,symbolrate and the size of constellation chartsimultaneously.

6 Channel Coding

Channelcod ing technology is ind ispensab le forw ireless communications.Forhigh speed commuications,the error is a key technology for fulfilling error control.802.11d/e standards adop tsuch error correc tion cod ing technologies as RS packetcode,convolution cod ing,convolution Turbo code,packetTurbo code and Low Density Parity Check Code（LDPC）.Moreover,the Woven convolution cod ing has been w ritten into the Guideline ofNew Air Interface Technology inWireless Communications by China Communications Standard ization Association（CCSA）.

There are detailed descrip tions about RS packetcoding,convolution coding,convolution Turbo code and packet Turbo code in Reference[5].As a new app roach forerror correction coding,LDPC is a linear packetcod ing thatcan be defined by low density odd-even p roof-testmatrix；ithas become a key technology forNext-generation Satellite DigitalVideo BroadcastStandard（DVB-S2）.If the LDPC is used inWiMAX,w ith its good anti-fading performance and high coding gain,the receiver can stillacquire a low rate ofcoding error in sp ite of low SNR,expanding the scope of coverage.Although it is sillop tionalin the currentEnhanced Wireless Consortium（EWC）d raft,it is reasonab le to believe that LDPC would p lay an im portant role in the IEEE 802.16 series standards in the future.In 1997,Host,Johannesson and otherpersons put forward Woven Convolution Cod ing[6],which c leverly weavesmember convolution codes together.Therefore,itcan notonly carry onmanymerits ofconvolution coding along w ith considerab le free space,butit can also be com patib le w ith traditional technologies like packetcod ing,convolution cod ing and various Turbo coding.Woven cod ing is the expansion ofSerialConcatenated Convolutional Code（SCCC）thatuses convolution code as a componentcode.We are convinced thatWoven convolution code w illbe w idely app lied in the p rocess of technologicalstandard developmentin the future.

7 Conclusions

As a b roadband network solution,WiMAXhas beenw idely accep ted by many network operators.The 802.16d/e standards have grown outof theWiMAX fam ily,and theywillsoon be joined by 802.16m when time is ripe.802.16m w ill integ rate quite a lotofstate-of-the-art technologies in the fields ofw ireless communications,and w illgenerate p roducts w ith ultra-high data transm ission rate and considerab ly large scope ofcoverage.Itw illalso be easy to use in otherw ireless communication networks.WiMAX standard w illbe a g reatpush to the w ireless broadband marketin a d ramatic way.With increasing app lication and development ofmultimedia technologies in Internet,it is certain that lots ofw ireless network equipmentw ith higher transm ission rate w illemerge,and we are very op tim istic about the p rom ising p rospects of investments in w ireless b roadband equipmentand services.Driven by the w ireless broadband users and many internationalnetwork operators,high speed WiMAXw illbecome one of the mostim portantnetwork technologies in the com ing severalyears.