Improved Wideband Precoding with Arbitrary Subcarrier Grouping in MIMO-OFDM Systems

Transcription

1 Improved Widebad Precodig with Arbitrary Subcarrier Groupig i MIMO-OFDM Systems ag Log, Kyeog Ji Kim, Wei Xiag, Shasha She, Ka Zheg, ad Webo Wag Precodig i the multiple-iput multiple-output orthogoal requecy divisio multiplexig system is ivestigated I covetioal widebad precodig (), oly oe precoder, obtaied rom the decompositio o the subcarrier idepedet chael matrix, is used or all subcarriers With a ivestigatio o the relatioship betwee the subcarrier idepedet chael matrix ad the temporal/requecy chaels, a improved scheme is proposed or practical scearios i which a part o subcarriers are allocated to a user he improved scheme is a geeralized scheme o which arrow-bad precodig ad covetioal schemes are special modes Simulatio results demostrate that the improved scheme almost achieves the optimum perormace o a sigle precoder ad outperorms the covetioal scheme i terms o the bit error ratio ad ergodic capacity with slight complexity icrease he largest advatage o the improved scheme o sigal-tooise ratio i simulatio results is over db Keywords: MIMO, OFDM, widebad precodig Mauscript received Ja 8, 0; revised May, 0; accepted Jue, 0 his research was supported by the Natioal Basic esearch Program o Chia (97 Program) uder Grat 0CB6005, the Natioal Sciece Foudatio or Post-doctoral Scietists o Chia uder Grat , ad the esearch Fud or the Doctoral Program o igher Educatio ag Log (phoe: , hlog@bupteteduc), Shasha She (shachegeiya@gmailcom), Ka Zheg (zka@bupteduc), ad Webo Wag (wbwag@bupteduc) are with the Wireless Sigal Processig ad Network Lab, Key Laboratory o Uiversal Wireless Commuicatio, Miistry o Educatio, Beijig Uiversity o Posts & elecommuicatios, Beijig, Chia Kyeog Ji Kim (kyeogjkim@hotmailcom) is with the UWB Wireless Commuicatios esearch Ceter, Iha Uiversity, Icheo, ep o Korea Wei Xiag (WeiXiag@usqeduau) is with the Faculty o Egieerig ad Surveyig, Uiversity o Souther Queeslad, oowoomba, QLD, Australia I Itroductio ecetly, theoretical work has show that i suicietly rich scatterig eviromets, multiple-iput multiple-output (MIMO) systems hold the potetial or tremedous spectral eiciecy improvemets [] O the other had, the orthogoal requecy divisio multiplexig (OFDM) techique is widely used i high-data-rate wireless commuicatio systems due to its high spectrum eiciecy ad tolerace o itersymbolitererece []-[4] he MIMO-OFDM system has attracted sigiicat worldwide research eorts Uder the assumptio that chael state iormatio (CSI) is available at the trasmitter, the perormace o MIMO systems ca be sigiicatly improved with precodig techiques [5]- [8] he well-kow sigular value decompositio (SVD)- based precodig techique [5], [6] ca be used or chael diagoalizatio, whereas the Q decompositio (QD)/ geometric mea decompositio (GMD)-based precodig schemes [7], [8] are used or chael trigoalizatio Covetioal precodig schemes [5]-[8] are mostly proposed or arrow-bad MIMO systems with lat-adig hese schemes are ot suitable or MIMO-OFDM systems with requecy selective adig It is either possible or ecessary to compute oe precoder or each subcarrier he use o a sigle precoder or multiple cotiuous subcarriers is preerable Precodig with limited eedback has attracted sigiicat attetio i the past decade, or example, the vector quatizatio codebook-based scheme [9], the Grassmaia codebook-based scheme [0], [], the iterpolatio based scheme [], ad the householder rotatio-based scheme [] hese schemes require sophisticated quatizatio to miimize iormatio loss [4] he complexity ad eedback overhead EI Joural, Volume 4, Number, February 0 0 ag Log et al 9

2 o precodig i MIMO-OFDM systems ca be urther reduced A widebad precodig () scheme is preseted i [4], [5], where oly oe precoder is used or all subcarriers i the MIMO-OFDM system he precoder is obtaied rom the subcarrier idepedet chael matrix which is costructed through the temporal chael vectors he CSI eedback overhead ca be sigiicatly reduced i the umber o subcarriers is large I this paper, we poit out that the scheme i [4], [5] is suitable oly whe a sigle user is allocated with all subcarriers i the system I practical OFDM systems, the umber o available subcarriers is usually smaller tha the discrete Fourier trasorm (DF) size [], [4] Additioally, i the orthogoal requecy divisio multiple access (OFDMA) based cellular systems [], all available subcarriers are allocated to multiple users accordig to their past ad curret chael gais [6] he covetioal scheme i [4], [5] is ot suitable or these scearios, especially whe the subcarriers allocated to a particular user are ot cotiuous hus, improved schemes or arbitrary subcarrier groupig are highly desirable he relatioship betwee the subcarrier idepedet chael matrix ad the temporal/requecy chael matrices is studied i this paper he, we propose a improved scheme with arbitrary subcarrier groupig where a user is allocated with oly part o all the subcarriers he proposed scheme is show to outperorm the covetioal scheme, ad perorms similarly as the exhaustive optimum precodig scheme Additioally, the improved scheme icludes the covetioal oe ad arrow-bad precodig schemes as its degraded modes Simulatio results validate the advatage o the proposed scheme he advatage o the improved scheme ca be more sigiicat i the group size is small or the subcarriers are ot cotiuous he remaider o this paper is orgaized as ollows he system model is described i sectio II he covetioal scheme is reviewed i sectio III ad the improved scheme is preseted i sectio IV Simulatio results are give i sectio V Fially, sectio VI cocludes this paper Notatio A, A, ad A deote the traspose, cojugatetraspose, ad pseudoiverse o matrix A, respectively I ad 0 m, deote the idetity matrix ad the m all-zero matrix, respectively Mathematical expectatio is deoted by E(), ad deotes the Kroecker product o two matrices he m vector space o all m complex matrices is deoted by he largest itegral smaller tha x is deoted by x II System Model We cosider the MIMO-OFDM system with a DF size o C&M C&M C&M d,0 d, d,l MCS choosig x Precodig matrix F x Feedback MIMO detector CSI Fig Illustratio o MIMO system over -th subcarrier y x dˆ,0 dˆ, d ˆ, L N ad L iput spatial streams N ad N ateas are equipped at the trasmitter ad receiver, respectively N L, N L, ad the maximum path delay is assumed to be shorter tha the legth o the cyclic preix (CP) he equivalet MIMO system over the -th subcarrier ca be writte as y = x + z, {0,,, N }, () where N x ad N y are the trasmitted ad received sigal vectors, respectively; z is the additive white Gaussia oise at the receiver, z ~ CN ( 0N,, σ IN ); ad N N is the requecy MIMO chael matrix over the -th subcarrier he equivalet MIMO system is also illustrated i Fig, where d,i is the modulated symbol o the i-th spatial data stream over the -th subcarrier, i {0,,, L }, that is, d = [ d,, d,,, d, L] is the data vector beore precodig It is assumed that, ', I L = E( dd ' ) = L () 0LL,, ' F is the precoder over the -th subcarrier, ad x =F d As described i [4], [5], i () ca also be deoted pq, by the temporal chaels hl is the temporal chael o the l-th tap betwee the p-th trasmit atea ad the q-th receive atea, l {0,,, N }, p {,,, N}, ad q {,,, N} N deotes the chael order determied by the ollowig equatio τ Δt max N = + where τ max is the maximum path delay, ad Δt is the temporal samplig iterval he temporal chael vector betwee the p-th trasmit atea ad the q-th receive atea ca be writte as, () 0 ag Log et al EI Joural, Volume 4, Number, February 0

3 h = [ h, h,, h ] (4) pq, pq, pq, pq, 0 N Usig (4), ca be writte i terms o h p,q as i [4], [5] where, N, wh wh =,, N N, N wh wh j π / N j π / N j π ( N )/ N (5) w = [,e,e,,e ] (6) III Widebad Precodig I the covetioal arrow-bad precodig scheme, i CSI is kow at the trasmitter, the optimum precoder over the -th subcarrier ca be achieved with the SVD o [0], [], [4] = U ΣV, (7) F = V [ I, 0 ] (8) L L, N L A scheme is preseted i [4], [5], where oly oe precoder is used or all subcarriers I this way, the CSI eedback overhead ca be sigiicatly reduced Equatio (5) ca be rewritte as where ( N ) = I w, (9), N, h h =, N N, N h h (0) is the subcarrier idepedet chael matrix he SVD o is calculated at the receiver as [4], [5] = UΣV () he irst L-th sigular vectors are used as the precoder, that is, F = F = V IL 0L N L, N } () [,, ] {0,,, I (), V ca also be obtaied with the eigevalue decompositio o he liear zero-orcig (ZF) equalizer is assumed to be employed i this study or simplicity We assume that all subcarriers are employed ( {0,,, N } ) As metioed beore, this assumptio is ot valid or practical OFDMA systems o overcome these diiculties, we propose a improved scheme with arbitrary subcarrier groupig i the ollowig sectio IV Widebad Precodig with Arbitrary Subcarrier Groupig A alterative explaatio o the scheme i [4], [5] is give i this sectio Firstly, we ivestigate the relatioship betwee the subcarrier idepedet chael matrix ad the temporal/requecy MIMO chael matrices he temporal pq, MIMO chael matrix o the i-th tap is costructed by { h l } as ollows: (t) i, N, h i hi =, i {0,,, N }, N N, N hi hi () he relatioship betwee ad the temporal MIMO (t) chael matrices { } ca be summarized as i N (t) (t) = i i i= 0 [ ] (4) hat is, the covariace matrix o the subcarrier idepedet chael is the sum o the covariace matrices o the temporal MIMO chael matrices he proo o (4) is straightorward, ad thus we omit its details or brevity Propositio he relatioship betwee ad the requecy MIMO chael matrices { } is give by N = (5) N = 0 hat is, the covariace matrix o the subcarrier idepedet chael is the average o the covariace matrices o the requecy MIMO chael matrices Proo: Accordig to (9), we have ( ) ( ) * N ( ) = I w I w N N = I w w (6) * he elemets i matrix ww have the same modulus but dieret agles as ad Θ ww = e, (7) * j π Θ / N 0 ( N ) 0 ( N ) = ( N ) ( N ) 0 is the phase oset matrix satisyig to the property o N, N (8) Θ + Θ =0 Due EI Joural, Volume 4, Number, February 0 ag Log et al

4 N jπ k/ N 0, k { N +,,,,,, N }, e = (9) = 0 N, k = 0, we have N * ( ww ) = I N (0) N = 0 Accordig to (6) ad (0), (5) is proved As show i propositio, the precoder i () is obtaied rom the average covariace matrix o the requecy MIMO chaels over all the N subcarriers ece, it ca be cosidered as a average choice or precodig through N subcarriers owever, the umber o available subcarriers is usually smaller tha the DF size N i practical systems For example, i the third geeratio partership project (GPP) log term evolutio (LE) system [], [] with a badwidth o 0 Mz, 600 subcarriers are used while the DF size is,04 Besides, i OFDMA systems, all available subcarriers are allocated to multiple users Covetioal schemes ca be urther improved i a user is allocated with a set o subcarriers For special scearios where oly oe subcarrier is allocated to the user, or example, the -th subcarrier, is so dieret rom that the covetioal scheme is much worse tha the optimum precoder give i (8) Propositio motivates us to propose a improved scheme, where the precoder is calculated by the averaged covariace matrix o the chael matrices over the subcarriers that are allocated to the user It is assumed that the user is allocated with a subcarrier group: S {0,,, N } We deie the average covariace matrix as * S = IN ( ), ww () S S where S is the umber o subcarriers i S he eigevalue decompositio o S is carried out at the receiver as S = EΠE, () where E is costructed with the eigevectors o S, ad Π is a diagoal matrix with the descedigly sorted eigevalues o S he precoder or S is costructed o the irst L eigevectors o S :, L L, N L F = E I 0 () Similar to the covetioal scheme i (0) to (), oly oe precoder is required or the improved scheme hus, the CSI eedback overhead o the improved scheme is the same as the covetioal oe i [4], [5] Note that i oly oe subcarrier is allocated to the user, or example, S = {}, the precoder i () degrades to the covetioal arrow-bad precoder i (8) ece, the improved scheme icludes the covetioal arrow-bad precodig scheme ad the covetioal scheme i [4], [5] ( S = {0,,, N } ) as special degraded modes Similarly, QD/GMD ca also be utilized i the improved scheme I a small umber o subcarriers are allocated to the user, the covetioal scheme caot utilize the subcarrier group iormatio, whereas the improved oe is able to make use o the iormatio or the precodig desig as show i () Ituitively, the improved scheme outperorms the covetioal oe i the size o subcarrier group is small ad is quite dieret rom S hese properties will be veriied by the simulatios i sectio V As show i (), subcarrier group iormatio is required both at both the trasmitter ad the receiver hus, additioal sigalig overhead betwee the trasmitter ad receiver is ievitable i OFDMA systems with the use o subcarrier schedulig eve i the improved scheme is ot i use For example, i the GPP LE dowlik system, subcarrier allocatio iormatio belogig to each user is trasmitted through the so-called physical dowlik cotrol chael which occupies the irst three OFDM symbols o each subrame [] owever, the proposed improved scheme does ot require additioal sigalig overhead i OFDMA systems Compared () to () with (0) to (), the proposed scheme slightly icreases the computatioal complexity o the receiver Oce the subcarriers are allocated to the user, the group relative matrix ca be calculated as S * ( ) ww (4) he, it ca be used util the subcarrier allocatio is chaged he decompositios i () ad () require the same operatios he computatio o (4) etails aroud about S N complex additios, ad that o () requires two additioal matrix multiplicatios at the receiver or each subrame, amely, less tha N N ( N N + N ) complex multiplicatios Compared with the complexity o the SVD ad other matrix operatios, the additioal computatioal complexity with the improved scheme is early egligible V Simulatio esults Our simulatio parameters or perormace evaluatio are listed i able he ergodic capacity ad bit error rate (BE) are adopted as the perormace metrics he system ergodic capacity is deied as [7], [8] ag Log et al EI Joural, Volume 4, Number, February 0

5 Samplig requecy (/ Δ t ) able System parameters 960 kz DF size (N) 64 Available subcarriers 64 OFDM symbol duratio Legth o subrame Chael estimatio Sychroizatio Atea coiguratio N N ) ( Data: 6667 µs CP: 456 µs ms (4 OFDM symbols) ideal ideal, 4 4 Number o data streams (L),, Chael model Pedestria B i [9] N 4 MS speed Modulatio MIMO equalizatio km/h Biary phase-shit keyig (BPSK) ZF Capacity (bit/chael use) BE Improved ESP N =N =, L=, S (a) L C = E log ( + γ i, ) S, (5) S i= 0 where γ i, is the post-sn o the i-th data stream over the -th subcarrier ater ZF equalizatio Some reerece schemes are also evaluated or purpose o perormace compariso he precoder with exhaustive search is used as the perormace upper boud A vast umber o uit vectors/uitary matrices are radomly geerated ad the oe with the optimum capacity/be perormace is selected accordig to F = arg max C( F ) (6) ad F F arg mi BE( F ) (7) = F he precoder with exhaustive search is deoted by ESP i the sequel Note that the results o (6) ad (7) are dieret hree subcarrier groups are cosidered i simulatios: i) S = {0,,,}; ii) S = {0,,,5,,,,47}; ad iii) S = {0,,,5} It is ituitive that the covetioal scheme perorms poorly with S sice the umber o allocated subcarriers is small ad the matrix i (4) is ar rom a scaled idetity matrix he improved scheme outperorms its covetioal couterpart or ay subcarrier groupig he simulatio results will validate the aalytical coclusio draw i the precedig sectio 0 N =N =, L=, S Improved ESP Fig (a) Capacity perormace ad (b) BE perormace with N =N =, L=, ad S he capacity ad BE perormace compariso with N =N = ad S is show i Fig he precoder is a vector, which makes exhaustive search possible,000 vectors are radomly geerated at each subrame to id the optimum precoder deied i (6) or (7) With the icrease o N, exhaustive search grows i prohibitive complexity, so that the results o ESP caot be show i Figs to 5 Similar ergodic capacities or the three precodig schemes ca be observed i Fig Sice oly hal o the subcarriers are allocated to the user, the improved scheme perorms closely to the optimum ESP scheme ad outperorms the covetioal scheme by 0 db as show i Fig (a) where the curves with the circle ad upper triagle symbols are almost overlapped he diereces i the BE amog the three schemes are oticeable i Fig (b) he improved scheme outperorms the covetioal oe by about 07 db at BE=0 More ateas at the trasmitter ad receiver are cosidered (b) EI Joural, Volume 4, Number, February 0 ag Log et al

6 Capacity (bit/chael use/stream) Improved N =N =4, S L= L= L= Capacity (bit/chael use) Improved N =N =4, S L= L= L= (a) (a) 0 0 BE N =N =4, S Improved (b) Fig (a) Capacity perormace ad (b) BE perormace with N =N =4, L=,,, ad S i Figs to 5 with the three subcarrier groups he improved scheme achieves a better perormace tha the covetioal oe or ay subcarrier groupig ad umber o data streams Sice oly hal o the subcarriers are allocated to the user with S, ad the requecy idepedet chael matrix i (0) caot represet the requecy chaels o the irst N/ subcarriers I Fig (a), the improvemet betwee the two schemes becomes oticeable whe L is large he observatio o Fig (b) is cotrary sice the BE perormace with large L is worse tha that with a small umber o data streams he improved scheme outperorms the covetioal oe by about 05 db i the average SN at C=80 bits/chael use ad L= his is observed i Fig (a) he largest improvemet i the SN i Fig (b) is about 06 db or BE=0 4 with L= Similar observatios ca be obtaied rom Figs 4 ad 5 With S, the subcarriers are ot cotiuous ad the group relative matrix i (4) is close to a scaled idetity matrix, so that the requecy idepedet chael ca be L= L= L= BE N =N =4, S Improved (b) Fig 4 (a) Capacity perormace ad (b) BE perormace with N =N =4, L=,,, ad S approximately used to compute the precoder he covetioal ad improved schemes perorm similarly as show i Fig 4 he advatage o the improved scheme is more evidet or S as show i Fig 5, sice the size o S is smaller tha those o S ad S he largest distace betwee the two curves show i Fig 5(b) is about db Accordig to the simulatio results, the proposed scheme achieves ear optimum capacity ad BE perormace Additioally, it outperorms the covetioal scheme or arbitrary subcarrier groupig he perormace dierece becomes oticeable whe a small umber o cotiuous subcarriers are allocated to the user VI Coclusio with oe precoder or all subcarriers allocated to users i the MIMO-OFDM system is ivestigated i this paper he covetioal scheme is show to be easible oly whe all subcarriers are allocated to a particular user he relatioship L= L= L= 4 ag Log et al EI Joural, Volume 4, Number, February 0

7 Capacity (bit/chael use) BE Improved N =N =4, S (a) N =N =4, S Improved Fig 5 (a) Capacity perormace ad (b) BE perormace with N =N =4, L=,,, ad S L= L= L= L= L= L= (b) betwee the subcarrier idepedet chael matrix ad the temporal/requecy chael matrices is derived he, a improved scheme with arbitrary subcarrier groupig is proposed, which icludes the covetioal ad arrowbad precodig schemes as special degraded modes Simulatio results veriy that the improved scheme achieves ear optimum capacity ad BE perormace ad outperorms the covetioal scheme, especially whe the allocated subcarriers are located withi a small badwidth eereces [] GJ Foschii ad MJ Gas, O Limits o Wireless Commuicatios i a Fadig Eviromet Whe Usig Multiple Ateas, Wireless Persoal Commu, vol 6, o, Mar 998, pp -5 [] GPP 684 v900, Further Advacemets or E-UA Physical Layer Aspects (elease 9), Mar 00 Available: [] GPP S 6 v880, Physical Chaels ad Modulatio, Sept 009 Available: [4] IEEE P80/D04, Wireless LAN Medium Access Cotrol (MAC) ad Physical Layer (PY) Speciicatios: Ehacemets or igher hroughput, Sept 006 [5] GG aleigh ad JM Cioi, Spatio-emporal Codig or Wireless Commuicatio, IEEE ras Commu, vol 46, o, Mar 998, pp [6] A Scaglioe, S Barbarossa, ad GB Giaakis, Filterbak rasceivers Optimizig Iormatio ate i Block rasmissios over Dispersive Chaels, IEEE ras I heory, vol 45, o, Apr 999, pp 09-0 [7] J Zhag ad KJ Kim, Near-Capacity MIMO Multiuser Precodig with QD-M Algorithm, Proc Asilomar Co Sigals, Syst, Comput, Paciic Grove, CA, USA, 005, pp [8] Y Jiag, J Li, ad W ager, Joit rasceiver Desig or MIMO Commuicatios Usig Geometric Mea Decompositio, IEEE ras Sigal Process, vol 5, o 0, Oct 005, pp [9] JC oh ad BD ao, rasmit Beamormig i Multiple- Atea Systems with Fiite ate Feedback: A VQ-Based Approach, IEEE ras I heory, vol 5, o, Mar 006, pp 0- [0] DJ Love ad W eath Jr, Limited Feedback Precodig or Spatial Multiplexig Systems, Proc IEEE Globecom, vol 4, Sa Fracisco, CA, USA, Dec 00, pp [] DJ Love, W eath Jr, ad Strohmer, Grassmaia Beamormig or Multiple-Iput Multiple-Output Wireless Systems, IEEE ras I heory, vol 49, o 0, Oct 00, pp [] J Choi ad W eath Jr, Iterpolatio Based rasmit Beamormig or MIMO-OFDM with Limited Feedback, IEEE ras Sigal Process, vol 5, o, Nov 005, pp [] Q Li ad X Li, Compact Feedback or MIMO-OFDM Systems over Frequecy Selective Chaels, Proc IEEE VC Sprig, Stockholm, Swede, May 005, pp 87-9 [4] KJ Kim, MO Pu, ad A Iltis, QD-Based Precoded MIMO-OFDM Systems with educed Feedback, IEEE ras Comm, vol 58, o, Feb 00, pp [5] KJ Kim, MO Pu, ad A Iltis, QD-Based Precoded MIMO-OFDM Systems with educed Feedback, Proc IEEE ICC, Beijig, Chia, May 008, pp [6] Kim et al, A Proportioal Fair Schedulig or Multicarrier rasmissio Systems, Proc IEEE VC004-Fall, LA, CA, USA, 004, pp [7] Bolcskei, D Gesbert, ad AJ Paulraj, O the Capacity o OFDM-Based Spatial Multiplexig Systems, IEEE ras Comm, vol 50, o, Feb 00, pp 5-4 [8] S Gault, W achem, ad P Ciblat, Perormace Aalysis o a EI Joural, Volume 4, Number, February 0 ag Log et al 5

8 OFDMA rasmissio System i a Multicell Eviromet, IEEE ras Comm, vol 55, o 4, Apr 007, pp [9] GPP 5996 v60, Spatial Chael Model or Multiple Iput Multiple Output (MIMO) Simulatios (elease 6), Sept 00 Available: ag Log received the BS ad PhD rom Beijig Uiversity o Posts ad elecommuicatios (BUP), Beijig, Chia, i 005 ad 00, respectively Sice 00, he has bee with BUP as a postdoctoral researcher is curret research iterests are i the ield o sigal processig or wireless commuicatios with emphasis o MIMO ad cooperative commuicatios Kyeog Ji Kim received the MS rom the Korea Advaced Istitute o Sciece ad echology (KAIS) i 99, ad the PhD i electrical ad computer egieerig rom the Uiversity o Calioria, Sata Barbara, i 000 Durig 99 to 995, he was a research egieer at the Video esearch Ceter o Daewoo Electroics Ltd, ep o Korea I 997, he joied the Data rasmissio ad Networkig Laboratory at the Uiversity o Calioria, Sata Barbara, USA Ater receivig his degrees, he joied the Nokia esearch Ceter i Dallas as a seior research egieer From 005 to 009, he worked with the Nokia Corporatio i Dallas as a L specialist is research has ocused o the trasceiver desig, resource maagemet, ad schedulig i the wireless commuicatio systems e is curretly workig at Iha Uiversity, ep o Korea theory, particularly codig ad sigal processig or multimedia commuicatios systems Shasha She received the BS rom Beijig Uiversity o Posts ad elecommuicatios (BUP), Beijig, Chia, i 009 Sice 009, she has bee pursuig her MS at BUP er curret research iterests are i the ield o MIMO ad cooperative systems Ka Zheg received the BS, MS, ad PhD rom Beijig Uiversity o Posts ad elecommuicatios (BUP), Beijig, Chia, i 996, 000, ad 005, respectively Sice 005, he has bee with the Wireless Sigal Processig ad Network Lab, BUP Sice 008, he has bee workig at the School o Iormatio ad elecommuicatio Egieerig (SIE), BUP, as a associate proessor is curret research iterests are i the ield o sigal processig or digital commuicatios with emphasis o spacetime codig ad cooperative commuicatios Webo Wag received the BS, MS, ad PhD rom Beijig Uiversity o Posts ad elecommuicatios (BUP), Beijig, Chia, i 986, 989, ad 99, respectively e is curretly a proessor ad the Dea o the Graduate School, BUP is research iterests iclude sigal processig, mobile commuicatios, ad wireless ad hoc etworks Wei Xiag received the BEg ad MEg, both i electroic egieerig, rom the Uiversity o Electroic Sciece ad echology o Chia, Chegdu, Chia, i 997 ad 000, respectively, ad the PhD i telecommuicatios egieerig rom the Uiversity o South Australia, Adelaide, Australia, i 004 Sice 004, he has bee with the Faculty o Egieerig ad Surveyig, Uiversity o Souther Queeslad, oowoomba, Australia, where he was irst a associate lecturer i computer systems egieerig rom 004 to 006, the a lecturer rom 007 to 008, ad curretly holds a aculty post o seior lecturer I 008, he was a visitig scholar at the School o Electrical ad Electroic Egieerig, Nayag echological Uiversity, Sigapore Durig 00 ad 0, he was a visitig scholar at the Uiversity o Mississippi, Oxord, MS, USA e was awarded a prestigious Queeslad Iteratioal Fellowship i 00 is research iterests are i the broad area o commuicatios ad iormatio 6 ag Log et al EI Joural, Volume 4, Number, February 0

9 Copyright o EI Joural is the property o Electroics & elecommuicatios esearch Istitute ad its cotet may ot be copied or ed to multiple sites or posted to a listserv without the copyright holder's express writte permissio owever, users may prit, dowload, or articles or idividual use