Truyen song true xa (LOS) va khong true xa (NLOS) trong cong nghe WiMAX
TS. Thai Van Lan ThS.Vu Long Oanh KS. Le Xuan Trung Tom tat: Hien nax dd cd nhieu gidi phap cdng nghe dp dung de truyen dan khong day bang thong rong trong dieu kien trite xa (LOS). Tuy nhien cdc cdng nghe truyen thdng nay se kho phat huy hieu qua trong dieu kien mdi trudng truyen song bi che chan. Cdng nghe WiMAX chuan IEEE 802.16 Id gidi phap huu hieu de truyen dan bang thdng rong ke cd trong mdi trudng LOS vd NLOS. Pham vi phii song cd the tdi 50km trong trudng hop LOS vd 5km trong trudng hop NLOS.
Bai viet nay gioi thieu torn tat dac diem cong nghe WiMAX bang rong trong moi truong truyen song true xa va khong true xa va kha nang ung dung tren mang Vien thong.
1. Truyen song trong dieu kien NLOS va LOS
Kenh vo tuyen ciia mot he thong thong tin khong day thudng duoc mo ta dudi dang ket noi vo tuyen LOS hoac NLOS. Trong mot lien ket LOS, tin hieu di thang va thong suot tir bp phat den bo thu. Mot lien ket LOS yeu cau bo phat va bo thu nam trong mien Fresnel thir nhat va khong co vat can, (xem hinh 1). neu dieu kien nay khong thoa man, tin hieu se bj suy giam dang ke. Khoang trong Fresnel yeu cau phu thuoc vao tan so hoat dong va khoang each giira vj tri bp phat va bp thu.
Hinh 1: Mien Fresnel trong tnrdng hop truyen LOS
Trong mot lien ket NLOS, tin hieu di tdi bo thu qua sir phan xa, phan tan, sir nhieu xa. Tin hieu den bo thu bao g6m cac thanh phan: di thing, cac dudng phan xa, phan tan va cac dudng lchuc xa. Nhung thanh phan nay cd do trai tre, dp suy giam, dp phan cue va dp on djnh khac vdi thanh phan di thang.
Tram goc
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Wimax CPE
Hinh 2: Truyin NLOS
Hien tuong cd nhieu thanh phan tin hieu den bp thu la nguyen nhan gay ra su phan cue va lam bien ddi tin hieu. Do do, vide sir dung su phan cue cd nghTa la tai dung tan so, gidng nhu van thuc hien trong truyen LOS nhung co the se kho thuc hien trong truyen NLOS.
Tan dung dupe lpi the ciia viec ket hop nhieu thanh phan tin hieu tai bp thu la ldi giai cho bai toan cung cap dich vu trong dieu kien truyen NLOS. Mot thiet bi dupe tao ra de cai thien kha
nang di xuyen qua vat can cua tin hieu khong phai la cdng nghe NLOS bdi vi nd van dua vao cac thanh phan tin hieu di thang khong su dung nang luong cua cac thanh phan tin hieu di vong. Ca dieu kien truyen LOS va NLOS deu bi anh hudng bdi dac tinh lan truyen cua mdi trudng, suy hao dudng truyen va sd lupng kenh lien kdt vd tuyen.
Co mot so lgi the khi trien khai cong nghe truyen dan NLOS. LOS han che boi 2 ly do: Yeu cau chat che ve quy hoach kien true ciia dia ban, gap bat lgi khi dp cao ciia anten thay doi.
Khi trien khai nhieu cell lan can, so lugng tan so tai sir dung thi han che, ha thap anten se khong co lpi trong viec giam kenh giao thoa giira cite cell lien ke. Trudng hgp nay yeu cau tram goc hoat dong d che do NLOS. He thong LOS khong the giam do cao anten bdi vi no yen cau tam nhin thang tir ngudi sir dung den tram goc.
Cong nghe NLOS ciing giam chi phi lap dat thiet bi do co the lap dat thiet bi khach hang tai cite vi tri thuan lpi ma van thu dupe tin hieu. Cong nghe nay cung giam thieu chi phi khao sat trudc khi lap dat thiet bi va chf phi cho viec nang cao dp chinh xac ciia cac cong cu quy hoach NLOS.
Hinh 3: Lap dat CPE trong dieu kien truyen NLOS
Kha nang truyen trong dieu kien NLOS la tinh nang noi trpi ciia WiMAX, no rat huu ich voi nhirng CPE dat trong nha. No co 2 uu diem: Khong bi mat tin hieu ben trong cac toa nha va bao phii dupe trong khoang each trung binh vdi cong suat phat thap, phii hop vdi do cao anten gan vdi CPE trong nha. Truyen NLOS cd the con dugc cai tien hon nua dua vao va kha nang ciia cong nghe WiMAX.
2. Cac giai phap ky thuat NLOS trong cong nghe WiMax Ci ng nghe WiMAX cai tien ky thuat truyen NLOS bang viec sir dung:
• Ky thuat OFDM
• Kenh phu
• Anten dinh huong
• Phan tap thu va phat
• Mo hinh tuong thich
• Ky thuat sua loi
• Dieu khien cong suat 2.1 Ky thuat OFDM
Ky thuat dieu che tan so true giao (OFDM) cung cap kha nang hoat dong hieu qua trong moi trudng lan truyen NLOS. Ky thuat OFDM cua WiMAX dem lai lpi the ve kha nang hoat dong vdi do trai tri ldn ciia moi truong NLOS. Dua vao co chi chen thoi gian bao ve, tin hieu OFDM tranh duoc nhiiu giao thoa va giam do phiic tap ciia phuong trinh bien ddi. Vi du sau so sanh giua tin hieu OFDM va tin hieu song mang don, trong do thong tin dupe gui song song doi vdi OFDM va noi tiep vdi song mang don:
Dana song mang don Dang song OFDM J
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* hieu co 1011 SO rpng, lhai gian
I s Cac ky hieu
| * co tan so hep, I thai gian dai
Dong ky hieu noi tiep dirge dung de
dieu che song mang don bang rong Moi ky hieu duoc dung de dieu che mot song mana rieng Dong du lieu noi tiep dirge chuyen thanh cac ky hieu (Moi ky hieu co the tugng trung cho mot hoac nhieu bit dir lieu)
Hinh 4: Song mang dan va OFDM
Kha nang vuot qua do trai tre. da duong, va xuyen nhieu giua cac ky hieu (ISI) cho phep tin hieu di qua vdi van t6c ldn hon. Mot vi du khac, de dang can bang cac thanh phan tan so song mang OFDM hon so vdi can bang cac thanh phan tan so rong cua tin hieu song mang don.
Dang sons mang dan Dana sons OFDM
Viing chain ma tuong trung cho triim phat Vung dac kin tugng trung cho vao bo thu
Hinh 5: Song mang don va tin hieu duoc dieu che bdi OFDM
Tir nhung ly do tren ma cac tieu chuan qu6c te nhu IEEE 802.16, ETSI BRAN, va ETRI da chgn lua chgn ky thuat OFDM.
2.2. Kenh phu (kenh con)
Kenh phu trong duong uplink la mot phuong an dugc chgn trong ky thuat WiMAX. Khi khong co kenh phu, mot so han che thudng gap la: can cac thiet bi CPE dat tien, quy cong suat tuyen se bi bat doi xung - nguyen nhan gay ra han che dudng uplink ciia he thong. Kenh phu cho phep quy cong suat tuyen trd len can bang do do ma he thong dat dugc su can bang giua duong up link va dudng down link. Kenh phu tap trung nang lugng phat it hon vao song mang OFDM; lam tang kha nang vuon xa ciia he thong va kha nang di vong qua vat can.
hoac giam cong suat tieu thu ciia CPE. Sir dung kenh phu cung co the md rong truy nhap OFDM (OFDM Access) cho phep md rong do linh hoat ciia he thong phuc vu khach hang vang lai va cac he thong thong tin di dong.
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Ph6 phat OFDM hudng xu6ng, tir tram g6c, moi mot khe tugng trung cho mot song mang RF.
L.U.d.i..Ua.,UiLldJJ H i ! 1: i Vi i'"n~n . ' I U TTT1, , , 1 1 Phd phat OFDM huong len, tir CPE, tat ca song mang dugc phat cti nhung a miic lA ciaa miic tai tram goc, do do tin hieu se nho hem
i I i S 1 1 S I I S 1 1 Pho phat OFDM hudng len, tir CPE, chi su dung XA song mang nhung cung miic voi tram goc, do do tin hieu se bang lA dung lugng kenh Hinh 6: Tdc dung cua kenh phu
2.3. Anten djnh huong
Anten dinh hudng tang do du tru fading nho tang he so tang ich. Lam tang kha nang hen ket, dung he so K de so sanh giua anten dinh hudng va anten khong dinh hudng. Giam do trai tre bang each diing anten dinh hudng d ca phfa tram goc va phfa thiet bj CPE. Dan anten khir nhieu cho mgi tin hieu hoat ddng d che do bup song ben (sidelobes) va blip song sau (backlobes). Hieu qua ciia cac phuong phap nay da dugc kiem chiing va chiing minh thanh cong trong thuc te, khi thuc hien cung cap dich vu dudi dieu kien co fading NLOS .
He thong anten thich nghi (AAS) la mot lua chgn trong chuan 802.16. O He thong nay, chum tia co tinh chat: cd the lai theo mot hudng rieng hoac tat ca cac hudng. Dieu nay co nghia rang trong khi truyen tin hieu, tin hieu cd the dugc gidi han theo hudng yeu cau ciia bo thu;
giong nhu su rpi sang. Ngugc lai khi nhan tin hieu, he thong anten AAS cd the dinh hudng theo huong ma tin hieu phat truyen den. He thong nay cung cd the giam nhieu giao thoa tdi tir thanh phan khac. He thdng anten AAS se phat trien trong tuong lai, cai tien dung luong va kha nang tai sir dung chum tia ciia mang WiMAX.
2.4. Phan tap bo thu - phat
So dd phan tap nay dugc sir dung de tan dung lgi the cua tin hieu phan xa va da dudng trong dieu kien truyen NLOS. Tinh phan tap la mot dac tfnh tuy chgn cua ky thuat WiMAX. Thuat
loan phan tap duoc WiMAX cung cap cho ca bo thu va bo phat. lam tang kha nang cua lie thong. Truyen phan tap trong WiMAX su dung ma thoi gian-khong gian tao nen su doc lap ciia nguon phat; dieu nay lam giam yeu cau du tru1 phading va chong lai su giao thoa. Doi voi bo thu. mot so ky thuat phiin tap ket hop duoc su dung de cai tien kha nang hoat dong ciia he thong. Vi du, ly so ket hop cue dai (MRC) co loi the hon la 2 chuoi nhan rieng biet, tranh hien tuong fading va giam do suy hao tuyen. Tinh phan tap da duoc kiem cluing va tro thanh cong cu huu hieu cho cac phien ban tiep theo, tao co' hoi cho lan truyen NLOS.
2.5. Dieu che thich nghi
Dieu che thfch nghi cho phep he thong WiMAX dieu chinh tin hieu dieu che hoat dong dua vao ty so SNR cua kef noi vo tuyen. Khi ket noi vo tuyen dat chat luong cao, dieu che 6' miic cao nhat se duoc sir dung, md rong dung luong ciia he thong. Khi tin hieu bi suy giam, he thong WiMAX co the chuyen sang boat dong dieu che d miic thap hon de duy tri chat luong ket noi va on dinh ciia tuyen. Dac tfnh nay cho phep he thong khac phuc duoc hien tuong lading dinh thoi. Uu the ciia he thong dieu che thfch nghi la no co the thuc hien heat dong dieu che o1 mire cao hon, do do he thong thfch nghi duoc vdi cac dieu kien fading thuc te, Jam bao hoat dong a dieu kien xau nhat.
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SNR = 22 dB Ban kinh cell tuong ung vai cac kieu dieu che
Hinh 7: Ban kinh cell 2.6. Ky thuat sua loi trudc
Ky thuat sira loi duoc tich hop trong cong nghe WiMAX nham giam ty so SNR ciia he thong ihco yeu cau. Ma hoa Strong Reed Solomon FEC. ma xuan, va thuat toan chen ma duoc su
dung de khir va siia loi, nang cao kha nang thong suot ciia he thong. Ky thuat sua loi nay giiip khoi phuc cac khung bj loi ma no co the da bi mat do fading tai 1 tan so nao do hoac do loi ngau nhien. Ngoai ra co che yeu cau lap lai tu dong (AHQ) dugc su dung de siia nhirng ldi ma khong dugc sua bdi FEC, thdng qua cac thdng tin bao ldi. Dieu nay cai tien dang ke ty le ldi bit hoat ddng d cung miic nguong.
2.7. Dieu khien cong suat
Thuat toan dieu khien cdng suat dugc sir dung de cai tien hoat ddng ciia he thdng, thuc hien d cac tram gdc nham gui thdng tin dieu khien cdng suit cho mdi CPE, yeu cau CPE phat d 1 miic cdng suat can thiet sao cho mire cdng suit nhan dugc d cac tram dat tdi miic dinh trudc.
Trong mdi trudng fading ddng, miic cdng suit dinh trudc dugc thoa man khi moi CPE chi phat cdng suat dii theo yeu ciu. Trong didu kien ngugc lai, miic phat cua CPE tuy thuoc vao dieu kien ket ndi xau nhit. Thuc hien dieu khien cdng suit lam giam cdng suit tieu thu ciia CPE va giam toan bd thanh phan cdng suat va kha nang gay nhidu vdi cac tram gdc lan can.
Ddi vdi truyen LOS, cdng suit phat ciia CPE ty Id thuan vdi khoang each tir no tdi tram gdc, con ddi vdi truyen NLOS cdng suat nay con phu thudc nhidu vao khoang trdng va cac vat can.
3. Mo hinh lan truyen NLOS
Trong dieu kien kenh NLOS; tin hieu co the suy giam do tan xa, nhidu xa, thay ddi do phan cue va phan xa. Nhung he sd nay se anh huong tdi dd ldn ciia tin hieu thu. Do suy giam nay thuong khong xuat hien trong trudng hgp bd phat va bg thu truyen tin hieu trong didu kien LOS.
De md phong va tinh.toan cac tham sd dudng truyen trong mdi trudng bj che chin, mot sd md hinh mau da dugc nghien ciiu va dd xuit tuy thuoc vao bang tin, miic suy hao, dd che chan va dac diem dja hinh.
3.1 Cdc md hinh NLOS
Nhung nam qua, cd nhidu md hinh dri duoc phat tridn dd phu hgp vdi dac thu mdi trudng truyen din tin sd vd tuydn (RF) va dinh trudc dd ldn tin hieu RF. Cac md hinh nay, dua tren
kinh nghiem do dac sau do dirge dung de dinh trudc cho vung rong hon doi vdi cac he thong thong tin vo tuyen te bao cell. Nhung mo hinh nay cung cap kha nang danh gia do suy hao dudng truyen giua bo thu va bo phat, cac thong so ve dia hinh, do cao anten thu phat va cac tan so cell.
AT&T Wireless lua chon md rong viing cung cap du lieu tir mot vai vung dgc nude My de danh gia chinh xac moi trudng RF khong day co dinh. Mo hinh AT&T Wireless phat trien tir dii' lieu da dugc phe chuan doi vdi viec trien khai cac he thong khong day co dinh va theo cac ket qua so sanh. Mo hinh nay dua tren mo hinh tieu chuan cong nghiep va dugc sir dung trong cac phan noi dung ciia chuan IEEE 802.16. To chirc IEEE da chap thuan mo hinh AT&T Wireless la tai lieu tham khao ciia IEEE 802.16.3c-01/29r4, "Channel Models for Fixed Wireless Application by Erceg Ethernet al.," va co the tim thay trong website ciia IEEE. Mo hinh dudng suy hao ciia AT&T Wireless bao gom cac thong so ve io cao anten, tan so so song mang va loai dia hinh.
3.2. Mo hinh SUI
Mo hinh Stanford University Interim (SUI) dugc md rong trudc mo hinh ciic. AT&T Wireless va Erceg ..
Co 3 loai dia hinh co ban:
Catelogy A - Hilly/ Mat do cay to vira phai
Catelogy B - Hilly/ Mat do cay coi thap hoac bang phang/ Mat do cay to vira phai Catelogy C - Hilly/ Mat do cay coi thap
Nhung ban do nay cung cap 1 phuong phap don gian de udc luong chinh xac mire suy hao ciia kenh RF trong trudng hgp truyen NLOS. Dugc thong ke trong thuc te, mo hinh nay co the tugng trung cho nhirng kinh nghiem due ket ve mire suy hao trong cac tuyen RF thuc te.
Cac mo hinh kenh SUI da dugc lira chgn de thiet ke, phat trien va kiem tra ddi vdi ky thuat WiMAX trong 6 kich ban khac nhau (SUM den SUI-6). Sir dung nhirng mo hinh kenh nay, co the tien doan chi'nh xac moi trudng lan truyen ma song co the di tdi ben trong cac sector tram goc. Su thiet lap moi trudng lan truyen cd the dugc sir dung de thiet lap them cdc ban do. Vi du no co the sir dung de xac dinh so lugng tram can thiet de cung cap dich vu trong 1 vung dia ly. Nhung mo hinh nay khong thay the dugc nhirng ban do chinh xac nhung co the sir dung de udc lugng trudc khi thiet lap dugc ban do chi'nh thirc. No rat quan trgng trong
viec tim hieu cac thong so ciia moi truong, giao thoa ddng kenh, nhieu thuc te va cac hieu ling dia chat.
3.3. Dinh trudc vung phii song
Trong dieu kien truyen LOS, vung phii song phu thuoc vao dudng vd tuyen dat tdi trong mien Fresnel thu nhat. Trong trudng hop truyen NLOS, mot khai niem ciia vung phii song dugc dua ra la ty sd sac xuat thdng ke sd lugng khach hang tiem nang tren vung phii song tien doan co the thu dugc tin hieu. Vi du, sac xuat ciia vung bao phii la 90% cd nghTa la 90%
khach hang tiem nang dudi vung bao phii tien doan se dugc dam bao vd hat lugng khi lap dat. Tieu chuan hoa dudng vd tuyen WiMAX se cho phep cac nha cung cap thiet bi co ban dd RF de phat trien cac ling dung cho truyen NLOS. Noi each khac, neu cd 100 khac hang
"xanh" trong ban dd vung bao phii tien doan, thi 90 khac hang trong sd dd co the dugc cai dat de sir dung dich vu mac dii vln ton tai vat can giua tram gdc va khach hang CPE. Ban dd RF va vung bao phii tien doan yeu cau phan tich chat che trong ky thuat NLOS cho phep biet trudc chinh xac khach hang nao co the dugc lap dat.
4. Vung bao phii ciia tram gdc WiMAX
Co 2 loai tram gdc va cac kha nang tuong irng nhu sau:
Mot tram gdc chuan bao gom:
- Cac thanh phan he thdng WiMAX co ban tdi thidu (nhung tinh nang bat budc) - Cdng suat RF ra chuan cho tram gdc (dac thu nha cung cap)
Mot tram gdc cai tien (con ggi la dac tinh tang cudng) bao gdm:
- Cdng suat ra d miic cao hon so vdi cdng suat chuan (dac thu nha cung cap)
• Phan tap Tx/Rx vdi ma thoi gian - khong gian va chap nhan MRC Kenh phu
• ARQ.
"a tram goc chuan va tram gdc cai tidn cd the dugc nang cap bdi cdng nghe WiMAX, tuy ihien ket qua thuc thi cd the khac nhau. Bang 1 chi ra sir khac nhau giua cac loai, cho cau imh he thdng tham khao. Nd rat quan trgng dd biet sd luong cac ung dung ciia cdng nghe ViMAX va cac nha cung cap thiet bj cd thi xay dung mang phu hop vdi cac djch vu trien :hai ciia minh. Dudng uplink cue dai dugc chi ra trong bang 1 md ta mot kenh phu dugc sir lung de md rong cell bien cang Ion cang tdt.
Gia dinh
9 n s~
3 = 2 :
Tan s6: 3.5GHz Bang thong: 3.5 MHz Tren sector 60°
Cai tien Chuan Gia dinh
9 n s~
3 = 2 :
Tan s6: 3.5GHz Bang thong: 3.5 MHz
Tren sector 60° Tir Den Tir Den
Gia dinh
9 n s~
3 = 2 :
LOS 30 50 10 16
Gia dinh
9 n s~
3 = 2 : NLOS (Erceg-Flat) 4 9 1 2
CPE tu lap dat indoor 1 2 0.3 0.5
Toe do cue dai tren Sector (Mbps) Toe dp cue dai tren CPE tai cell bien (Mbps)
Downlink 11.3 8 11.3 8 Toe do cue dai tren Sector (Mbps)
Toe dp cue dai tren CPE tai cell bien (Mbps)
Uplink 11.3 8 11.3 8
Toe do cue dai tren Sector (Mbps) Toe dp cue dai tren CPE tai cell bien (Mbps)
Downlink 11.3 2.8 11.3 8 Toe do cue dai tren Sector (Mbps)
Toe dp cue dai tren CPE tai cell bien
(Mbps) Uplink 0.7 0.175 11.3 2.8 So thue bao cue dai Ldn hon Nho Iron Bang 1: Thong so ciia tram goc cai tien va tram goc chuan
Giong nhu da chi ra co the hoat dpng vdi su cai tien cho cac CPE indoor, vung dien tich bao phii tang hon 10 lan so vdi nguong chuan. Hinh 8 dua ra mot bieu do tupng trung ciia truyen LOS va NLOS cho hai loai tram goc khac nhau
Cai tien Chuan Hinh 8: Ban kinh cell ciia tram goc day dii tinh nang vd tram gdc chuan Mot giai phap mang toi uu se la he thong ket hop ca cac tram chuan va tram cai tiSn.
4. Ket luan
WiMAX la ky thuat co the* ho trg ca truyln trong moi trudng NLOS va LOS. Truyen NLOS co nhieu uu diem, nd cho phep phat du heu bang thdng rong tdi vung mat dd khach hang cao.
Ky thuat WiMAX cd rat nhieu uu diem: nd cho phep cung cap giai phap truyen NLOS, vdi ky thuat chinh la ky thuat OFDM, md hinh tuong thich va sua ldi. Hon nua, cdng nghe WiMAX co nhieu tinh nang lua chgn, vi du nhu ARQ, Kenh phu, Da dang va ma chen thoi gian-khdng gian cung cap tinh nang uu viet cho cac nha khai thac nham dem lai chat lugng dich vu canh tranh vdi ky thuat co day. Dau tien, cac nha khai thac con ; nghe khong day bang thdng rong cd the trien khai cac thiet bj da dugc chuan hoa vdi gia ca hgp ly; lua chgn tinh nang phii hop vdi dac thu ve dich vu khach hang va vung phii song.
Tdi lieu tham khdo:
1. Eugene Crozier ; Allan Klein (SR Telecom). WiMAX's technology for LOS and NLOS environments. WiMax Forum.
2. Deepark Pareek. The Business of WiMax. John Wiley & Sons,. LTD. 2006.
3. Loutfi Nuaymi. WiMax Technology for Broadband Wireless Access. John Wiley &
Sons,. LTD. 2007.
4. Jeffvey G. Andrews; Arunabha Ghosh; Rias Muhamed. Fundamental of WiMax.
Prentice Hall. 2007
5. Erceg, et. al, "An Empirical Based Path Loss Model for Wireless Channels in Suburban Environments, " IEEE Selected Areas in Communications, Vol. 17, No. 7 July 1999.
