Interference Cancellation Algorithm for 2 2 MIMO System without Pilot in LTE

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1 Commuicatios ad Networ, 13, 5, Published Olie September 13 ( Iterferece Cacellatio Algorithm for MIMO System without Pilot i LE Otgobayar Bataa 1, Erdeebayar Lamav 1, Uugabayar Purevdor 1, Youg-il Kim, Khishigargal Gochigsumlaa 1 School of Iformatio ad Commuicatio echology (SIC) of Mogolia Uiversity of Sciece ad echology, Ulaaabaatar, Mogolia Electroics ad elecommuicatios Research Istitute (ERI), 138 Gaeeogo, Yuseog-gu, aeeo, Korea otgobayar@sict.edu.m, erdeebayar@sict.edu.m, uugabayar_p@sict.edu.m, im@etri.org, Ghishigargal@yahoo.com Received May, 13 ABSRAC Iterferece cacellatio system (ICS) for 3GPP/LE system is the broadbad cacellatio system, which receives forward sigal through the door atea. We proposed ew algorithm of received sigal with pilot ad o-pilot desig. Although repeater desig eeds our proect, so i this paper we discuss about iterferece cacellatio algorithm for x MIMO systems without pilot i LE. First explai the geeral priciple structure of 3GPP/LE, ext determie our ew desig ad algorithm. Fially, we simulated our mathematic extractio of proposed ew algorithm o MA- LAB. Keywords: MMSE; Looup-table; hreshold; Cost Fuctio; Viterbi Algorithm 1. Itroductio Iterferece cacellatio system (ICS) for the door ad service atea amplifies the received sigals ad seds them to base statio ad mobile to get stroger sigal. urig this process, it cacels the iterferece sigal betwee the door ad receivig atea. here are usig types of cacellatio algorithms, some existig adaptive cacellatio algorithms wor i the frequecy domai, usig the referece toes carried i the OFM sigal. Other algorithms use the time domai methods lie the LMS. I geeral these algorithms are classified by iterferece cacellatio algorithm with ad without pilot sigals. We described iterferece cacellatio algorithm without pilot sigals i this paper. It is feasible to implemet ICS i OFMA systems such as LE. Although IC techiques ca be applied to both dowli ad upli of LE, due to complexity cosideratios, IC is cosidered maily as a techique for the UL ad implemeted i the base statio receiver. ICS techiques ca be used to cacel both itra-cell ad iter-cell iterferece.. Proposed Chael Estimatio Method ad Equalizatio.1. No Pilot Chael Estimatio he MMSE estimator miimizes the MSE of the chael estimates, but the complexity is high compared to the ML or LS estimators. he LS ad MMSE method were compared i ad for OFM systems ad the MMSE was foud to outperform the ML i low SNRs. he calculatio of the MMSE estimate requires a large matrix iversio. he complexity of the MMSE estimator ca also be reduced by cosiderig oly the high eergy chael taps. rasform domai techiques may also be used for obtaiig the chael estimates for the whole badwidth. he iverse FF trasforms the chael frequecy respose ito time domai where the low power taps ca be elimiated ad the oise reduced chael ca be trasformed bac to frequecy domai with the FF. MMSE filterig ca also be used to predict the chael of the curret OFM symbol based o chael estimates from previous symbols, i.e. time ad frequecy domai correlatio of the chael frequecy respose ca be exploited i the chael estimatio. For improved performace i MIMO-OFM systems, the spatial correlatio ca be icluded i the MMSE chael estimatio... Equalizatio echiques I geeral, there are three categories of equalizatio techiques. Our techique is the time frequecy domai equalizatio with chael shorteig. A time-domai equalizer is iserted to reduce the MIMO chaels to the oes with the chael legth shorter tha or equal to the Copyright 13 SciRes.

2 3 O. BAAA E AL. CP legth, ad the, a oe-tap frequecy-domai equalizer is applied to each subcarrier. Whe the MIMO chaels are shorteed by the time-domai equalizer, residual ICI ad ISI are itroduced. hey caot be elimiated by the subsequet frequecy-domai equalizer ad, thus, limit the performace. his has resulted i ew receiver cocepts usig differet equalizatio techiques. hese techiques are briefly explaied below. ime-domai equalizer (EQ) he time domai equalizatio (EQ) is a short FIR filter at the receiver iput that is desiged to shorte the duratio of the chael impulse respose (L). hus it allows a reductio i the guard iterval legth. Usig a filter with up to coefficiets, the effective chael impulse respose of a typical AWGN chael ca easily be reduced by a factor of 1. ifferet cost fuctios such as miimum mea squared error, maximum shorteig sigal-to-oise ratio (SNR), ad miimum iter-symbol iterferece (ISI), ad maximum bit rate have bee proposed to desig the time domai equalizer (EQ)..3. Iterative Algorithm I OFM, chael estimatio ca be performed with a blid or a o-blid techique. he blid chael estimatio method does ot require the use of traiig sequeces or pilot symbols ad eables a more efficiet use of the available badwidth. he chael estimates are obtaied usig the statistical properties of the received data which is collected over a certai time period. ML equalizer, ad ca be used to compute the coefficiets of suboptimal but lower-complexity equalizers such as the miimum mea-squared error (MMSE) liear equalizer (LE). Eve though the MMSE-LE ca be estimated directly, havig the chael estimates allows us to choose which equalizer is more appropriate for the chael. 3. Mathematic Extractio for No Pilot Chael Estimatio We assume a sigle UE receivig desired sigal from the servig BS as well as iter-cell iterferece sigals from eighborig BSs. he BSs from differet cells are assumed to be sychroized i time ad frequecy. he UE has NR receive ateas which are used for performig iter-cell iterferece suppressio. Each BS has oe atea to trasmit oe stream of iformatio. We defie s as the vector that stads for the d pilot symbols, which are multiplexed with s to form a bloc of N=Nd+Np trasmitted symbols s. For simplicity, we X Figure 1. he bloc diagram of a OFM system. Figure. Proposed Iterative model for MIMO receiver. Copyright 13 SciRes.

3 O. BAAA E AL. 33 Figure 3. Proposed system model of ICS Repeater without Pilot sigal for 3GPP/LE. cosider uit-eergy QPSK with the symbol alphabet α, =1,,4, which are used for desired sigal as well as all other iterferece sigals. I this wor, the receiver is assumed to have perfect CSI betwee the servig BS ad the UE. If SIC is applied, the UE also requires owledge about the iterferece li, e.g., the chael betwee the UE ad the iterferig BS ad the modulatio scheme of the iterferece sigals. Notice that the pilot symbols here are oly used to calculate the beam formig weights, ot for performig chael estimatio MMSE Estimator his effectively equalizes the frequecy-selective chael. First, cosider the ifiite legth filter case: he output of the equalizer is xˆ[ ] t[ ] q x[ ] [ ] Is where the equalized chael IR is q f he differece betwee the x.ed data ad the equalizer output is: [ ] x[ ] xˆ [ ] ad the MMSE cost fuctio is: E E x x ˆ Priciple of orthogoality: { [ ] } { [ ] [ ] } Et {[ ] [ ]}, We ca calculate the MMSE equalizer by either miimizig over w: E{ [ ] } E{ x[ ] xˆ [ ] } { [ ] [ ] } L1 [ ] ( l [ ( )] [ ]) } l E x t E x f x l v he mi is, E E E x xˆ { [ ] } { [ ] [ ]} { [ ]( [ ] [ ])} ue to the priciple of orthogoality, E xˆ {[] []} the E x mi 1 {[] []} { [ ] } { [ ] [ ]} E x E t x f 3.. MMSE Calculatio b K xˆ[ ] q x[ ] q x[ ] [ ] K where the WMF output/equalizer iput is q f ad the covolutio of the equalizer ad the equivalet chael IRs is Copyright 13 SciRes.

4 34 O. BAAA E AL. t [ ] f x [ ] [ ] Obviously, the variace of oise is he ISI terms are N c qx[ ] For a fixed sequece of iformatio symbols x {[ x]}, he, the probability of error for this sequece is M 1 PM( ) P{( N) q} M K M 1 ( q ) Q ( ) M N [ ] K Average probability is foud by averagig over all P P { } P{ x } M M x PM { } is domiated by the sequece yieldig highest which occurs whe x[]= ±(M-1) ad the sigs of x[] s match the correspodig {q} Error Model ( M 1) q MMSE equalizer aims at miimizig ˆ E E x x E ex x ˆ x 1 x x xn L Expadig the cost fuctio ˆ H H H E e x x E e x w E e w F x w x e F w w Optimum equalizer coefficiets are: H w R p w w p R e F FF I H 1 H H x x Substitutig bac to the MSE term p R p mi H 1 x x 1 1 H H x xe F xff I xfe H e I F F e 4. Flowchart of Proposed Sigal Processig echique without Pilot Sigal for ICS I result sectio, the simulatios of the algorithms developed two cases. Our proposed desig implemeted both cases, which are pilot ad o-pilot receiver simulated o MALAB. Figure 5 shows o-pilot desig for - x-rx MIMO system. his figure icluded I ad FI based matrix iterpolatio that requires modulatio Figure 4. Proposed combiatio sigal processig techique of ICS for 3GPP/LE. Copyright 13 SciRes.

5 O. BAAA E AL. 35 QPSK, 16QAM, 64QAM. he first iterative step eded to chec maximum threshold is larger tha mi value. If true o more iterative step, otherwise ext iterative step cotiue. Proposed methods based two cases of ICS receiver structure. So we defie oly oe geeral algorithm, which is used to ay system with ICS bloc. If system determied iput sigal with pilot, the sigal go to matched filterig bloc, otherwise select to adaptive filter. Figure 5. he symbol error rate compariso of the proposed o-pilot techique with that of a x Rx at the MMSE receiver. Figure 6. he symbol error rate compariso of the proposed o-pilot ad pilot techique with ay modulatio order at the receiver. able 1. Simulatio parameters. Parameter Modulatio schemes Cyclic prefix Value QPSK,16QAM,64QAM Normal FF/IFF bloc size 14 Number of iteratios 6< hreshold 1. Pacet size i symbols 1^3 Chael tape 6 Chael estimator with pilot MLSE usig Viterbi algorithm Chael estimator without pilot MMSE Chael equalizer with pilot Least Square equalizer Chael equalizer without pilot Liear MMSE equalizer Receiver atea MMSE receiver 5. Coclusios ad Future Wors I fially, liear MMSE time-domai equalizatio techique has bee proposed for geeral MIMO-OFM systems. he added CP at the ed of each MIMO-OFM symbol coverts the liear covolutio i the chael ito circular covolutio. Simulatios have demostrated that the proposed MMSE time domai equalizer techique is effective i suppressig ICI ad ISI ad robust agaist the umber of shifts i excess of the CP legth. Fially, i this proect the MMSE equalizatio used to o-pilot system, LS equalizatio used to pilot system, that are maily cosidered as proved by ay wors. It would be very iterestig to exted the ideas of the polyomial approach ad trasceiver/repeater desigs to ew practical system based chael iterferece cacellatio methods. 6. Acowledgemets his wor was supported by the I R& program of KCC/MKE [1-1356, he evelopmet of IM- Advaced Mobile IPV core techology], Electroics ad elecommuicatios Research Istitute of Korea ad Mobicom corporatio of Mogolia. REFERENCES [1] S. Mahboob, Iterferece Cacellatio System, Simo Frazer Uiversity, 9. []. Ketoe, Equalizatio ad Chael Estimatio Algorithms ad Implemetatios for Cellular MIMO-OFM dowli; Uiversity of Oulu Graduate School; uvees Prit, ampere 1. [3] A. Mehmood ad W. A. Cheema, Chael Estimatio for LE owli, Bleige Istitute of echology, 9. [4]. C. Heao, Y.. Che, K. Fag, M. Grieger, P. Helbig ad R. Legouable, Advaced Receiver Sigal Processig echiques: Evaluatio ad Characterizatio. [5] B. M. ola, Ultra-widebad Narrowbad Iterferece Cacellatio ad Chael Modelig for Commuicatios. [6] Mr. C. Pira Spatial Iterferece Cacellatio ad Chael Estimatio for Multiple-iput Multiple-output Wireless Commuicatio Systems, octor hesis. Copyright 13 SciRes.