Phương pháp tổ hợp tối ưu trên mạng không dây thế hệ mới

CNTT trong TDH<br /> <br /> <br /> <br /> <br /> PHLiONG PHAP TO HOP TOI UU<br /> TREN MANG KHONG D A Y THE HE M 6 |<br /> • •<br /> <br /> <br /> <br /> <br /> Cac mang khong day the he mdi yeu cau chat lu-omg am thanh cao dat cat tieu chuan di dong hien<br /> tai va phai cung cap dirge cac djch vu truyen dir lieu len tdi 2 Mbit/giay. Vdi yeu cau ve cac thiet bj di<br /> dong ngay cang phai gon nho hon, hoat dong t o t trong cac moi trudng khac nhau (thanh thj, nong<br /> thon, ngoai thanh, trong nha, ngoai trdi...) thi hai phLTong phap dirge de xuat la phan tap anten phat<br /> va phirong phap phan tap anten thu deu nham cai thien chat iirgng cua he thong khong day hien dai.<br /> <br /> <br /> KHANH LINH<br /> <br /> Tdng quan _ khien cdng suat phat, NghTa la neu kenh truyen ben<br /> Cac he thdng vien thdng the he sau ngay cang may thu dupe xac dinh tai may phat thi ben may<br /> phai cd chat lupng cao hon, tan dung dai thdng tdt phat cd the lap dat mdt bd doan trude tin hieu de<br /> hon va hoat ddng tdt trong eac mdi trudng khac loai tru cae hieu ung kenh truyen. Tuy nhien phuang<br /> nghiet khae nhau, Cac cong nghe hien tai deu phai phap nay lai cd 2 nhupe diem ehinh, Nhupe diem<br /> gang sue dap ung cac nhu cau thi trudng ngay cang dau tien (cung la han ehe Idn nha't) dd la can mpt dai<br /> md rpng, Tuy nhien yeu td kinh te la khdng the bd ddng Idn tai may thu. Cu the dd la de may phat loai<br /> qua khi ap dung cdng nghe mdi vao thtic tien vi eau tru duac phading thi phai khueeh dai tin hieu len<br /> true cae tram se cang phuc tap hpn nua de dap ung eiing mdt mue, dieu nay ra't khd thue hien bdi nhirng<br /> dupe eac yeu cau eiia mang the he sau. gidi han ve cdng suat phat va eau triic bp khueeh<br /> Mdt hien tupng ludn gay nhung anh hudng xau dai se rat phuc tap. Han che thu hai cua phuang<br /> tdi truyen thdng khdng day khdng thd bo qua dd la phap nay dd la may phat khd ma cd the biet du'pc<br /> nhilu da dudng theo thdi gian [1]. Hien tupng nay dae trung ciia kenh truyen tdi may thu (tru trudng<br /> chinh la han che eua truyen thdng khdng day so ydi hop dudng len va dudng xud'ng su dung chung mot<br /> spi quang, cap ddng trtjc, sdng vi ba truyen thang tan sd sdng mang). Vi the, thdng tin kenh truyen se<br /> (LOS) va ve tinh. _ phai dupe phan hdi trd lai may phat. Dieu nay lai<br /> Tang chat luang vagiam ti le ldi tren cae kenh lam giam thdng lupng cua he thd'ng va lam tang do<br /> phadinh da dudng la rat khd. Vdi trudng hop nhidu phuc tap tai ca may thu va may phat. Hpn nua, trong<br /> trang cdng tinh cd su dung cac phupng phap dieu rat nhieu ung dung thi khdng cd phan hdi tir may thu<br /> che phd bien va eac phuang phap ma hoa thi de ed trd ve may phat dupe,<br /> the giam ty le ldi bit (SNR) tu 10-2 xudng 10-3 thi ty Mdt phuang phap khae dd la phan tap theo tan<br /> Id tin hieu tren nhieu (SNR) chi' can tang 1 - 2 dB, sd. Xao trdn tin hieu theo thdi gian ciing vdi ma st/a<br /> Tuy nhien vdi trudng hap nhieu phadinh da dudng sai ed the cai tien viee phan tap. Ky thuat trai phd<br /> thi SNR phai tang them 10 dB. Giai phap tang SNR cung dem lai hieu qua td't. Tuy nhien, xao tron tin<br /> bang each tang eong suat tin hieu hay dai thdng rd hieu se gay trd Idn khi kenh truyen bie'n ddng cham,<br /> rang khdng phai la giai phap tdt vi nd di ngupc lai Tuang tu, cac ky thuat trai phd cung se khdng hieu<br /> lieu chi cua he thong the he sau (NGN), Vi vay, can qua khi dai thdng lien ke't cua kenh truyen Idn hon<br /> cd eac giai phap loai phadinh tai ea cac may di dpng dai tan trai phd.<br /> va cac tram co so ma khdng phai tang cdng sua't Trong hau het cae mdi trudng phan tan thi phan<br /> hay hi smh dai thdng. tap diing anten la thue tidn nha't va vi the phuang<br /> Ve mat ly thuyet phuang phap tot nhat la dieu phap nay dupc ap dung rdng rai trong loai tru nhilu<br /> <br /> <br /> Tftl A j t o m a t o ^ J o d a y 55 5(8,)2oo,<br />  CNTT trong T D H<br /> <br /> da dudng phading, Cae phuang phap truyen thd'ng la ngau nhien tu nhien ciia truyen sdng vd tuyen bdi<br /> su dung eac anten tai may thu de ket hop, lua chpn each tim ra phuang phap tao va riit ra eac dudng tin<br /> hay chuyen dudng, Khd khan ehinh cua phan tap tai hieu ddc lap. Mdt khai niem gan vdi phan tap dpn<br /> may thu lai la gia tien, kich thudc va cdng sua't eiia gian la; neu mot dudng tin hieu bj suy hao manh tai<br /> may thu, Viee su dung nhieu anten va cae mach xu ly mpt thdi diem xac dinh thi tai mdt dudng dpc lap<br /> tan sd vd tuyen lam cho may thu trd nen cdng kenh khac ed the tin hieu se manh len. Khi ed nhieu<br /> va dat tien. Vay la cae ky thuat phan tap nen ung dudng tin hieu de chon lua thi ca SNR tue thdi va<br /> dung vao cae tram ca sd. Cac tram ea sd thudng SNR trung binh se dupe cai thien dang ke d anten<br /> phuc vu cho vai tram tdi vai ngan thue bao di ddng vi thu.<br /> the nen se rat tiet kiem. Vi vay, phan tap d may phat Oho dii ky thuat da dudng nao dupc su dung,<br /> da danh dupe sir quan tam rpng rai nha't. Thdng anten thu cGng phai xu ly cae tin hieu da dudng<br /> thudng thi mdt anten nua se dupc dat them tai tram nham dat dupe hieu suat eong sua't Idn nhat ciia<br /> ca sd de tang chat lupng cho tat ea eac thue bao di ca he thong, Cd nhung phuang phap da dudng cd<br /> ddng ma tram co sd aiy dang phuc vu. the su dung d anten thu. Cac ky thuat thong<br /> thudng nha't dd la: Da dudng It/a chpn (SC), Ke't<br /> Phading trdng kenh thdng tin hop he sd can bang (EGO) va Td hpp tdi uu<br /> Phading la mdt thuat ngu dung de md ta su (MRO). Nhung so sanh giua cac phuang phap SC,<br /> thang giang bien dp rat nhanh d anten thu tin hieu EGO va MRO dupc minh hoa tren hinh 2. 0 dd eho<br /> vd tuyen trong mdt ehu ky thdi gian ngdn, Phading ta thay quan he giua ti le tin hieu tren tap (SNR)<br /> la mdt hien tupng thdng thudng xay ra d trong kenh vdi ty le ldi bit (BER) trong mdt he thdng BPSK.<br /> truyen thdng di ddng, d dd cd nhiiu giua 2 hoac Phuang phap EGO cho ket qua tdt hon phuang<br /> nhieu phien ban tin hieu phat di, dupe truyen tdi phap SO va phuang phap MRO eho ket qua tdt<br /> anten thu, thu dupe tai cae thdi diem khae nhau. Tin hon phuang phap EGO.<br /> hieu thu dupe rat ed the bi meo ve bien do va pha,<br /> phu thudc vao su thay ddi eac he sd nhu cudng dp,<br /> thdi gian truyen sdng, dp rdng bang thdng cua tin - phuong phap SC<br /> <br /> hieu truyen - phuong phap EGG<br /> " phuong phap M R C<br /> Hinh 1 md ta quan he giua ty sd tin hieu tren tap<br /> am (SNR) vdi ty le ldi bit (BER) ung vdi trudng hap<br /> kenh ed nhieu trang cdng tinh (AWGN) va trudng<br /> hop kenh chiu tae ddng cua phading (Rayleigh) +<br /> AWGN, Chung ta cd the tha'y tac ddng tram trong<br /> ciia nhieu phading nhu the nao.<br /> <br /> ::::::::::::c:::::::::::c::::::::<br /> Rayleigh + AWON ::<br /> ;; 1;<br /> '•• - - - - - k e n h AWGN<br /> ^^>*^^^,^^-.-•..•...•..••.[• •.•-.-....<br /> <br /> P ^ : : : TTr???'^^^:::: c::::::::::: i:::::::::::: i:::::::::::: j<br /> "••i ^^^((^<br /> <br /> V 1 ^^'^^^'^-^J Hinh 2. So sanh giua SC, EGC va MRC<br /> h!!f:!!!!h^f:!f:::::::f :::::: =TET*|4I^^^!<br /> ' » ^^H^^<br /> Theo phuang phap MRO: tin hieu tu tat ca cae<br /> = nniNN = hE^MHi = iNMMfMNlMMENEM = h m t S ^<br /> hudng dupe ddng bd pha va dupe nhan trpng sd de<br /> ;;;;;;::;:;:::::%;;:::;::::::;::::;::::::;:::;;::::::::::::<br /> \ • ..; eo dupe SNR tdi uu d ldi ra (xem hinh 3). Ta tha'y<br /> [ .1. . : . - • i - SNR ldi ra Idn nhat khi tin hieu d mdi anten dupc<br /> nhan vdi cac he sd. Trudng hop he thdng da dudng<br /> : :";":::::K::;:::::::::::::;;:;::::::;;::;::::::<br /> ; %.; • ; eo 2 anten, phuPng trinh ke't hop eho bdi;<br /> i yi L ....i<br /> ) 5 10 15 20 25 Zk=r I k ^ i k ''2kZ2k<br /> E^/N„(dl)<br /> <br /> Hinh 1 Ty le loi bit trong mPi truong eo nhieu d dd r^k va rj^ dien ta bien dp tuc thdi eua cae<br /> tin hieu thu dupe tai cac nhanh cua he thdng da<br /> dudng, Ti sd SNR tren mdi bit tai ldi ra ciia bp td hop<br /> Phan tap tdi uu /h cd the viet dudi dang:<br /> Mdt trong nhung each cd hieu qua nhat va ky E '<br /> thuat dan gian de giam hieu ung phading la su dung<br />