PHUONG PHAP DANH GIA LUONG TAI NGUYEN DU TRlT CHO CAC CUOC Goi CHUYEN GIAO
TRONG MANG TE BAO CDMA
ThS. NCS. VO TRUONG SON PGS. TS. LE HUNG LAN TS. NGUYEN THANH HAI
Khoa Dien - Dien tu
Tru&ng Dgi hpc Giao thdng Van tdi
Tdm tdt: Bdi bdo ndy gidi thieu mgt phuang phdp ddnh gid lugng tdi nguyen du tru cho sir chuyen giao cua cdc MS trong mgng te bdo CDMA. Thdng tin cd dugc tir ddu ra ciia md hinh ndy dugc sir dung di phuc vu cho thudt todn dieu khien thdm nhdp cugc ggi trong mgng ti bdo CDMA ndy.
Summary: This paper proposes a method of resei-yed resources estimation for MS's handoff in CDMA Cellular Network. The information from the output of the scheme is used jor CAC schemes in the CDMA Cellular Nenvork.
1. GIOI THIEU
Dieu khien tham nhap cudc ggi (CAC) la mdt trong nhirng chirc nang quan ly tai nguyen quan trgng trong cac mang te bao CDMA. CAC trong cac he thdng CDMA kinh dien deu dua vao viec danh gia lugng tai nguyen chiem dung bdi yeu cau cudc gpi mdi va so sanh vdi kha nang dap img ciia mang, tire la lugng tai nguyen san dimg ciia he thdng. Lugng tai nguyen san diing ciia he thdng la lugng tai nguyen cdn lai ciia he thdng sau khi da trir di lugng tai nguyen bi chiem dung bdi cac cudc ggi dang tdn tai va ca lugng tai nguyen du tru' cho cac cudc ggi chuyen giao [1, 8, 9, 18]. Do vay danh gia lugng tai nguyen dir tru- cho cudc gpi chuyen giao la mot cdng viec quan trpng ddi vdi qua trinh CAC. Lugng tai nguyen nay phu thupc vao kha nang thue bao di ddng (Mobile Station, MS) se chuyen giao sang mpt te bao ke can trong tuong lai.
Tuy nhien, danh gia kha nang nay lai la mdt cdng viec khdng don gian do tinh chat biSn ddi thSt thudng ciia mdi tardng vd tuyen ciing nhu su di chuyen khd du doan trudc ciia cac MS.
Trong [8, 9], viec danh gia lupng tai nguyen du trir dugc thuc hien bang each sir dung y tudng xdc sudt ngdt cuoc goi tuang dirong cua te bdo ldn can. Tuy nhien day lai la md hinh khdng phii hgp vdi sir di chuyen va chuyen giao cua MS khi tat ca cac te baa lan can deu dugc tham gia vao viec tinh toan nay, trong khi chuyen giao chi cd the xay ra vdi mpt hay hai te bao lan can cd mirc cdng suat tin hieu Pilot thu dupe tai MS Idn nhat. Mdt giai phap khac dugc dua ra la tinh xac suat chuyen giao sang tat ca cac te bao k^ can tren ca sd mirc cdng suat tin hieu Pilot thu dugc tai MS, ma cac gia trj cdng suat nay vupt qua mot gia trj nguong dat trudc. Sau
70 Tap chi KHOA HQC GIAO THONG VAN TAI So 29 - 03/2010
dd tai nguyen se du tru cho cudc gpi chuyen giao tai te bao nao cd xac suat chuyen giao ciia MS sang dd la ldn nhat va mirc tai nguyen du trir se ty le vdi xac suat nay. Md hinh nay la kha phii hgp vdi dac tinh di chuyen va chuyen giao ciia MS. Tuy nhien viec tinh toan gia trj xac suat chuyen giao lai khdng de dang. Cach tinh don gian nhat la dua vao ty sd giira cdng suat tin hieu Pilot thu dupe tai MS tir BS thupc te bao lan can vdi cdng suat tin hieu Pilot thu dugc tai MS tir train gdc (Base Station, BS) thude te bao phuc vu. Ve mat ly thuyet, ty sd nay se phan anh tdt mdi quan he ve khoang each gitja MS vdi BS phuc vu vdi BS lan can. Neu ty sd nay nhd, MS gan vdi BS phuc vu va each xa BS lan can, do dd xac suat chuyen giao la thap va ngugc lai.
Tren thuc te, cdng suat tin hieu Pilot thu dirge tai MS tir cac BS chju anh hudng Idn tii- cac hien tugng fading va che khuat. Vi vay each tinh ndi tren la thieu chinh xac.
De giai quyet van de nay, logic md cung cap mdt phuong phap thuc hien xap xi nhung hieu qua cho md hinh danh gia xac suat chuyen giao tai thdi diem hien tai, theo dd neu sii' dung cdng cu toan hgc thi rat phirc tap va khdng de thuc hien. Vdi kha nang cd the giai quyet cac van de mang tinb mo- hd va thieu cbinh xac, logic md dugc hy vgng se cung cap mdt giai phap tdt de thuc hien viec danh gia nay [16]. Tuy nhien, mdt van de nQ-a dat ra la xac suat dugc danh d tren chi thuc hien cho thdi diem hien tai. Van de la phai du bao dugc xac suat nay trong tuong lai, khi cudc ggi cd the da boat ddng trong mang se thuc hien chuyen giao. Mdt md hinh du bao la can thiet cho tinh hudng nay. Vdi loai du' lieu mang tinh thdi gian thuc nhu cac cudc ggi trong mang CDMA, md hinh du bao phii hgp la phuong phap binh phuong nhd nhat de quy (RLS) 119|. Ngoai ra, trong qua trinh danh gia lugng tai nguyen dir tru chuyen giao, [1, 18] dat ra cac tham sd gidi ban tren (va dudi) cho tdng lugng tai nguyen du trir ciia mdi te bao. Dieu nay la tdt neu xet dudi gdc do dam bao ty le tham nhap cudc gpi mdi vi lugng tai nguyen du tru- se khdng chiem qua nhieu. Tuy nhien tren thuc te, viec dam bao cudc ggi chuyen giao khdng bj ngat la quan trpng ho-ii viec cho tham nhap cudc gpi mdi. Neu dat gidi ban tren cho lirpng tai nguyen du trfr va tinh hudng cd nhieu cudc gpi yeu can chuyen giao xay ra, lirpng tai nguyen du trir d cac te bao dich se khdng du dc dap irng cho cac yeu cau nay. Ket qua la, se cd nhieu hon sd cudc gpi chuyen giao bj ngat. Van de nay se dugc giai quyet neu lugng tai nguyen du trfr chuyen giao trong timg te bao khdng cdn bj khdng che d gidi ban tren, tire la du tru tai nguyen cd the dat tdi da.
Bai bao nay gidi thieu md hinh ket hgp he suy dien md va phuong phap binh phuong nhd nhat de quy de danh gia lugng tai nguyen du trfr chuyen giao cho cac MS trong mang te bao CDMA | 4 | . Phuong an dir tru tai nguyen tdi da phuc vii chuyen giao cung dugc dua vao nghien ciru nay. Trong md hinh nay, cdng suat tin hieu Pilot tir BS phuc vu va cac BS lan can thu dugc tai MS dugc he suy dien md sir dung de danh gia xac suat chuyen giao ciia MS sang te bao lan can tai thdi diem hien tai. Sau dd, bd RLS can cir vao xac suat cd dugc nay de du bao xac suat chuyiSn giao cho cac thai diem tiep theo. Thdng tin di chuyen ciia MS cd dugc tii- dau ra ciia md hinh nay dugc su dimg de tinh toan lugng tai nguyen du trir chuyen giao, phuc vu cho thuat toan ilicu khien tham nhap cudc ggi. Phan cdn lai ciia bai bao nay bao gdm cac muc sau: phan 2 gidi thiiiu khai quat ve md hinh he thdng CDMA te bao va cac khai niem ca ban. Phan 3 trinh bay tdm tat ca sd ly thuyet cua phuong phap RLS. Phan 4 gidi thieu md hinh danh gia thdng tin di chuyeii bang \ iec kUl hgp he suy dien md vdi thuat toan RLS. Ap dung thdng tin cd dugc tir md
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hinh cho bai toan CAC va ket qua md phdng dugc gidi thieu d phan 5. Cuoi cimg, ket luan dugc cho d phan 6.
H. CAC KHAI NIEM CO BAN
Gia thiet mpt he thdng te bao CDMA bang rpng vdi cac te bao luc giac cd kich thude bang nhau. Mdi te bao cd mgt BS tai tam ciia nd vdi mdt anten dang hudng (omni). Phd tan sd vd tuyen nhu nhau dugc tai sir dung d tat ca cac ti bao. Cac MS lien lac vdi BS phuc vu nd qua giao tiep vd tuyen, mpt vai BS dugc ket ndi tdi trung tam chuyen mach cac djch vu di ddng (MSC), va MSC nay dugc ket ndi tdi mdt mang true.
Trong tuyen xudng, mdi mdt BS phat ini3t tin bieu Pilot duy nhat quang ba xudng cac MS.
Cac tin hieu Pilot nay tir cac BS khac nhau dugc phan biet bdi cac ma khac nhau. MS cd the phat hien dugc tin hieu Pilot nay tir bat ky BS nao khi cudng do tin hieu Pilot vugt qua mdt gia trj ngudng nao dd. Trudc khi truyen dan, mi5t MS kiem tra mirc cdng suat tin hieu Pilot thu dugc tir cac BS gan dd va chgn BS phuc vu nd theo cdng suat tin hieu Pilot thu dugc circ dai.
Gia thiet rang cac MS, BS va MSC dugc thiet ke thich hgp tdi mirc trong khi bam theo tin hieu Pilot ciia te bao hien tai, mpt MS se tim kiem tat ca cac tin hieu Pilot cd the va duy tri mpt danh sach tat ca cac tin bieu Pilot cd mirc cdng suat ldn hon mdt ngudng djnh trirdc. Danh sach nay dirge phat ve MSC mdt each djnh ky qua BS phuc vu. MSC sir dung nhung thdng tin nay de quyet djnh khi nao chuyen giao nen bat dau va danh gia thdng tin di chuyen. Trong bai bao nay, thdng tin di chuyen la kha nang cd the xay ra tinh hudng MS se boat ddng trong cac te bao khac tai mdt thdi diem trong tirong lai. Khi mdt yeu cau cudc gpi xuat hien, MSC sir dung thdng tin di chuyen ciia tat ca cac MS trong te bao dd va cac te bao lan can cimg vdi dac tinh luu lugng ciia chimg de danh gia xem cd dii hay khdng ngudn tai nguyen de cung cap cho cudc ggi mdi ma van khdng lam suy giam QoS ciia cac cudc ggi dang dugc phuc vu.
Hinh 1. Cdc li bdo luc gidc
Hinh 1 bieu dien cac te bao luc giac, d dd, MS xem xet dat tai diem M. D^ don gian trong trinh bay, chimg ta se chi tap trung tdi thdng tin di chuyen ciia MS cd lien quan tdi BS phuc vu (ky hieu la BSo) va sau BS lan can vdi nd thugc ldp dau tien (ky hieu la BS,, BS., .... BS,,). Gpi d:(t), 1 = 0, 1 6 la ky hieu khoang each giiia MS va BS, tai thdi diem t. Gia thiet may thu
72 Tap chi KHOA HQC GIAO THONG V^N TAI S6 29 - 03/2010
phat dupe thit't ke tich hpp, tap am ciia kenh chii yeu la do hien tupng che khuat va suy hao dudng truyen. Gia trj trung binh ciia cdng suat tin hieu Pilot tir BS| thu dugc tai MS cd the dugc md ta nhu [4]:
a (t) = y d ( t ) / D 10' + v ( t ) (1) Trong dd yi la hang sd ty le vdi cdng suat tin hieu dugc phat di, r la sd mii cua suy hao dudng truyen. Do la la khoang each tham khao va ^](t) theo dB tai bat ky thdi diem t nao la mdt bien ngau nhien Gaussian (vdi gia trj trung binh bang 0 va dp lech chuan bang as) dac trung cho hien tugng bdng md. Vdi 1 f k, ^i(t) va ^k(t) la cac qua trinh ngau nhien ddc lap. Neu cac mirc cdng suat phat ciia cac tin hieu Pilot la gidng nhau thi Yi = y vdi 1 = 0, 1, ..., 6. V|(t) bieu dien cdng suat tap am nen va nhieu da tham nhap (MAI) tii- cac tin hieu mang tin ciia tuyen xudng tdi tat ca MS. Khi cd mdt sd lugng ldn cac user trong he thdng, MAI cd the dugc md hinh boa xap xi bang mdt qua trinh ngau nhien Gaussian.
Trong he thdng te bao CDMA bang rdng, chuyen giao mem dugc sir dung dan den viec viing phii sdng ciia te bao dugc md rdng va lain tang dung lugng tuyen len [1]. Ty sd giira nang lugng bit ciia kenh Pilot vdi mat do tap am va nhieu (Ei,/Io) dugc sir dung lam thdng sd do phuc VII chuyen giao. De do-n gian, ta gia thiet rang trong chuyen giao mem, mdt MS dugc ket ndi tdi hai tram BS gan nhat vdi cac tin bieu Pilot manh nhat trong khi nd dugc dieu khien cdng suat bdi tram BS, ma tram nay yeu cau nd phat tai mirc cdng suat yeu hon.
HI. PHUONG PHAP HINH PHUONG NHO NHAT DE QUY
Cho mdt IK; thdng rdi rac dugc md ta bang phuong trinh sai phan sau:
y(ki + a,y(k-l) + a:y(k-2) = b„u(k-l) + b,u(k-2) (2) Trong dd y(k) la tin hieu ra va u(k) la tin hieu vao; ai, a., bo, b| la cac tham sd he thdng.
Pliirong trinh (2) cd the dugc viet dudi dang vecto- nhu sau:
y(k) = 4, ^e (3) Trong do (j) va 0 lan lugt la \-ec to' hdi quy (vecto- du lieu do dugc) va vecta tham sd, dugc cho nhu sau:
(t) = [-y(k-l) -y(k-2) u(k-l) u ( k - 2 ) ] \ va 6 = [a, a, bob,]^
Phirong trinh (3) la phuong trinh chinb xac md ta he thdng khi khdng cd sai sd. Tuy nhien da>' chi la InrcTiig hgp ly tudng. Do vay phuo-ng trinh (2) dugc viet thanh dang sau:
y(k) = (t)^e + e(k) (4) Trong dd e(k) la sai sd. Tir phuo-ng trinh (4) chung ta cd:
e(k) = y(k) - (t)'e (5)
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Y tudng cua phuang phap RLS la cue tieu hda ham gia trj sau:
J = i e 2 ( i )
k i=l (6)
Tir phuang trinh (6) ta thay gia trj ciia ham gia trj nay tiep tuc dugc tfch liiy cac gia trj ciia sai sd, nham lam giam sir tich luy va anh hudng tii' dQ- lieu "cu" ngudi ta ap dung he sd quen nhu sau:
k-i 2 ,
J =ix^''s^{i) k i=l
(7)
Trong dd X la "he sd quen" cd gia trj nhd ban don vj va thudng dugc chgn trong khoang 0.995-0.998 [19],
Cue tieu hda ham gia trj va thirc hien mdt sd budc bien ddi chimg ta thu dugc thuat toan sai sd binh phuong nhd nhat de quy nhu sau:
Tai thdi diem t = k+1, tinh cac budc sau:
Budc 1: Cap nhat vecta dii-lieu : ct)(k+l) = [-y(k-l) -y(k-2) u(k-l) u(k-2)]''^
Budc 2: Tinh sai so md hinh hda: e(k+l) = y(k+l) - (j)\k+l)0(k) Budc 3: Cap nhat ma tran ddng phuong sai:
P(k + l)=P(k) (p(k + l)(p^(k + l)P(k) M >. + (p'^(k)P(k)(p(k + l) I
Budc 4: Cap nhat vecta tham sd: (t)(k+l) = e(k) + P(k+1) (t)(k+1) e(k+l) Budc 5: Quay trd lai budc 1.
Chii y ning ma tran dong phuang sai P(k) cd gia trj ban dau la P(0) = alw trong dd a la mdt gia trj tu chpn, thudng cd cac gia trj a = 1000 va M la sd cac tham sd chua biet ciia md hinh trong phirong trinh (4). Gia trj ciia a anh hudng tdi tdc dp hdi tu ciia cac tham sd dugc udc
liro-ii".
Tir thuat toan nay chiing ta thay rang vec ta tham sd dugc cap nhat lien tiic tai cac budc.
Neu ma>- tinh dugc noi vdi be ddng va dir lieu dugc cap nhat lien tuc thi cac gia trj udc lugng ciia vecta tham sd ciing dirge cap nhat lien ti^ic. Day la dac diem ciia phuong phap RLS va phuong phap nay duac ap dung trong cac he thdng dieu khien tu chinh cung nlur cac he thdng dieu khi^n theo thdi gian thirc. Tir phuong trinh cap nhat vec ta tham sd ni;u chimg ta thSy so hang thir hai cd chira sai sd d ve phai P(k+l)(()(k+l)e(k+l) va neu sai sd nay cd gui trj rat nhd hoac bang khdng thi chimg ta se cd (j)(k+l) = 9(k) bieu dien tinh hdi tu ctia cac tham sd he thdng. Sir hpi tu ciia cac tham sd dugc udc lugng la dac diem quan trpng ciia phuang phap nay [19].
74
Tap chi KHOA HQC GIAO THONG VAN TAI S6 29 - 03/2010IV. MO HINH DANH GIA T H O N G TIN DI CHUYEN
Md hinh danh gia thdng tin di chuyen ciia MS gdm cd hai khdi chinh, do la (i) he suy diln md diing de danh gia xac suat chuyen giao ciia MS tai thdi diem hien tai va (ii) bd du bao xac suat chuyen giao ciia MS tai thdi diem trong tuang lai gin sir dung thuat toan RLS. Cau triic ciia md hinh nay, theo [4], dupe bieu dien trong hinh 2. He thdng suy dien md danh gia xac suat de mdt MS se boat ddng trong mdt te bao tai thdi diem tn tren ca sd do cudng dp cdng suat tin hieu Pilot tai thdi diem dd. Trudc khi cd sd lieu do tai thdi diem tp+i, bd dQ- bao RLS tien hanh du bao xac suat de MS boat ddng trong te bao dd tai thdi diem t„+N, vdi N = 1, 2, ... tren co- sd danh gia cua he suy dien md tdi thdi diem tn. Thdng tin xac suat dugc du bao cd the dugc sii- dung de danh gia su di chuyen cua MS trong cac budc tiep theo, phuc vu cho thuat toan dieu khien tham nhap cudc ggi.
1 1 1 1
Cong . ^ suat 1 tin hieu |
Pilot i do du'p'c 1
1 1 1 1 r
Md hda
Ca cau suy dien md
1 L
Co- sd luat ind
• Giai md
H( l suy dien md
1 • Du bao RLS
•
Dir hao xac suat chuyen giao
Hinh 2. Cdu trite md hinh ddnh gid ihdng tin di chuyin 4.1. He SUV (lien md
Theo (1) ta thay rang, cdng suat tin hieu Pilot tii- cac tram BS thu dugc tai MS la sir phan anh tdt khoang each giua cac BS dd vdi MS, theo dd khi khoang each tang len tQ-c la cdng suat tin hieu Pilot thu dugc tai MS se yeu di va ngugc lai. Do dd, neu cdng suat tin hieu Pilot tir BS phuc VII (BSo) thu dugc tai MS ldn ddng thdi cdng suat tin hieu Pilot thu dugc tu BS thude te bao lan ciin 1 (BS|) nhd thi kha nang MS chuyen se chuyen giao sang te bao 1 tu- te bao phuc vu la thap. Ngugc lai, neu cdng suat tin hieu Pilot tir BSo thu dugc tai MS nhd ddng thdi cdng suat lin Ilicu Pilot thu dugc tu BS; ldn thi kha nang MS chuyen se chuyen giao sang te bao I tir te bao pluic VII la cao. Vi vay, can cir vao mirc do cdng suat tin hieu Pilot tir cac BSQ va cac BS| thu dugc tai MS, cd the tinh dugc xac suat chuyen giao sang cac te bao lan can cua MS tai thdi diem hien tai. Tuy nhien, do anh hudng cua cac hien. tugng dugc gay ra bdi mdi trudng truyln dan nhu fading, che khuat nen cdng suat tin hieu Pilot thu dugc phan anh khdng hoan toan chinh xac khoang each giQa BS va MS. Do vay, ket qua xac suat chuyen giao tinh dugc cung khdng chinh xac. De giai quyet van de nay, mipt ca cau suy dien md dugc sQ- dung de danh gia xac suat chuyen giao nay.
Xet tai thdi diem hien tai (t = tn), bien ngdn ngu dau vao cua he suy dien md la cac muc
. 9P chi KHOA HQC GIAO THONG VAN TA^ So 29 - 03/2010 75
cdng suat tin hieu Pilot tu' BSo (a„.o) va cac BSi (ani), 1 = 1, ..., 6 thu dugc dugc tai MS dang hoat ddng trong te bao i. Ham thugc dugc lua chgn cho cac bien dau vao cd dang Gaussian vdi cac gia trj ngdn ngu- cua cac bien dau vao cd dang: U(an.i) = {ES, VS, S, SM, M, ML, L, VL, EL}, 1 = 0, 1, ...., 6. Bien ngdn ngu- dau ra cua he suy dien md la xac suat chuyen giao sang te bao 1 cua MS dang boat ddng trong te bao i tai thdi diem hien tai, (pni), 1 = , 1, ..., 6 . Ham thupc dugc lira chpn cho cac bien dau ra cung cd dang Gaussian vdi cac gia trj ngdn ngQ- cua bien dau ra cd dang: U(P„,,) = {ES, VS, S, SM, M, ML, L, VL, EL}, 1 = 1 6. D I don gian trong thuc hien, ta gia thiet rang, xac suat chuyen giao sang te bao 1 lan can cua mdt MS dang boat di3ng trong te bao i chi phu thupc vao khoang each giQ-a MS vdi BSo va vdi BS| dd ma khdng phu thupc vao cac BSj khac (] = 1, ..., 6, j 9^ 1) tire la chi phu thu(?c vao miic cdng suat tin hieu Pilot tii- BSo (aii.o) va miic cdng suat ciia mdt tin hieu Pilot tir BS| (a„.i), thu dugc tai MS thupc te bao i. Khi do, vdi mdi tap cdng suat tin hieu Pilot thu dugc tai MS thupc te bao i bao gdm bay phan tu- a„,i ,(1 = 0, 1,..., 6) ta se cd sau cap tin hieu Pilot thu dugc (ano, Sni), (1 = 1,..., 6). Mdi cap tin hieu nay dugc dua vao he suy dien md mdt each ddc lap. He suy dien md trong trudng hgp nay se bao gdm hai bien ngdn ngQ- dau vao la (a„,o, an i). Co- sd luat md cua he dirge xay dirng dua tren y kien cua chuyen gia va kmh nghiem tich luy, ket hgp vdi cac yeu td anh hudng cua mdi trudng truyen dan. Luat thir k se cd dang: Neu an.o la Ao.i, va a„j la AL,I< thi Pn.i la Pi.^, k = 1,..., K, vdi K la sd lugng luat.
4.2. Dy bao xac suat chuyen giao RLS
Viec di chuyen cua cac MS la khd du doan trudc, do dd du bao xac suat chuyen giao cua MS trong tuong lai la cdng viec khdng de dang. Ly thuyet cua phuang phap binh phuang nhd nhat de quy dupe ap dung trong phan nay de dir bao xac suat ma MS se chuyen giao sang te bao ke can tai cac thdi diem sap tdi. Dau vao cua bd du bao la xac suat chuyen giao cua MS tai thdi diem hien tai, cd dugc tir he suy dien md. Dau ra cua nd la xac xuat ma MS se chuyen giao sang te bao ke can tai cac thdi diem sap tdi. Trong du bao nay, sd budc dir bao la L = 3, tire la khi cd xac suat tai thdi diem t,,, xac suat tai thdi diem tn-L, L = 3, se dugc du bao.
V. AP DUNG C H O MO HINH CAC VA KET QUA MO P H O N G
Trong cac md hinh CAC ap dung cho mang te bao CDMA, mdi khi cd yeu cau cudc ggi mdi hay cudc gpi chuyen giao, yeu cau ve tai nguyen cua cudc gpi se dirpc xac djnh, dong thdi tai nguyen san diing cua he thdng cQng dupe danh gia. Neu tai nguyen san dimg cua he thong cdn du de dap irng cho yeu cau nay, cu(3c gpi se dugc chap nhan, neu khdng nd se bi tir choi.
Trong dd lugng tai nguyen san dimg ciia mang la lugng tai nguyen cua mang trir di lugng tai nguyen bi chiem dung bdi cac cudc ggi dang ket ndi trong te bao va d cac te bao lan can. Ngoai ra lupng tai nguyen san diing cua mang cdn cd the bj lay bdt bdi viec dir trQ- cho cac cudc gpi chuyen giao [1, 8, 9, 18].
76 Tap chi KHOA HQC GIAO THONG VAN TAI So 29 - 03/2010
Hhth 3. Du tru tdi nguyen giao Hinh 4. Cdc ti bdo trong hi thdng md phong Trong cac md hinh CAC, viec danh gia tai nguyen san dung cua he thdng phai tinh tdi yeu td dir tru ke tren. Khi dd md hinh trong bai bao nay la phuong an hieu qua cho viec xac djnh lugng tai nguyen can du trQ- cho cac cudc ggi chuyen giao. Khi xac suat, ma theo dd mdt MS cd kha nang chuyen giao sang mdt te bao lan can, ldn hon mdt gia trj ngudng djnh trudc thi te bao nay phai danh mpt lugng tai nguyen de du trQ- cho cudc ggi chuyen giao ndi tren. Mirc tai nguyen ma mdt BS du trQ- cho mdt cudc gpi chuyen giao ty le vdi xac suat ma MS chuyen giao sang BS dd. Tire la BW, = m*P, trong dd BW, la lupng tai nguyen du tru cho cudc gpi chuyen giao, P la xac suat chuyen giao tuong irng va m la he sd ty le [1], Lugng tai nguyen du trQ- ciia mdt te bao, Br, danh cho cac cudc gpi tir cac te bao ke can cd the chuyen giao vao nd, bang tdng lugng tai nguyen dir trQ- ciia tat ca cac cuipc ggi dang dugc phuc vu trong cac te bao ke can, xet tren gdc do chuyen giao vao nd (xem hinh 3). Gia trj Br nay phai ldn ho-n mdt gia trj circ tieu B,„n„ dat trudc. De danh gia anh hudng ciia viec dir tru tai nguyen cho cudc ggi chuyen giao, viec md phdng thuat toan CAC dugc thuc hien cho ba tinh hudng: khdng dir trQ- nhu d [7] (UR- FCAC), du trQ- vdi gia trj Br bj khdng die chan tren B„„a,^ nhu d [ 1, 18] (L-FCAC) va du tru vdi gia tri Br khdng bj khdng che chan tren (UL-FCAC).
Cac thdng sd md phdng: IK; thdng CDMA cd bang thdng tuyen len la W = 3.75 MHz, va K
= 25 cells, nhu trong hinh 4. dugc md phdng. Mdi cell cd ban kinh la 1.500m. Hinh mau di chuyen ciia MS dugc md phdng nlur sau: (1) vj tri ban dau cua cac cudc gpi mdi la phan phdi di;u trong 25 cell; (2) tdc dp di chuyen ciia cac MS la phan phdi deu giiia 5in/s va 25m/s va khdng thay ddi theo thdi gian; (3) cac MS di chuyeii tdi bat ky mdt cell nao ke can vdi nd vdi xac stuit nlur nhau va hudng di chuyen cua nd van duy tri nbu cu trong sudt thdi gian ciia ket ndi; (4) chi quan tam tdi cac MS d trong 25 cell dugc dua ra.
Ciic tham sd trong (1) la: y, = 1. D = 100m, r = 4, a^ = 2 dB. Cac tham sd ciia dir tru tai nguyen la P„,.,x.i = 0.3, B, „,.,^ = 200 KHz, B, „„, = 50 KHz. Thdng tin di chuyen dugc cap nhat mdi Is \a dugc di^r bao vdi L = 1, 2, 3. Cac thdng sd duac sir dung de danh gia boat ddng cua md liinh la xiic siuit ngat cudc goi chu\en giao, xac suat chan cudc gpi mdi va xac suat gian doan Ihdng lin. trong dd Ihdng sd thir nhat la quan trpng nhat.
.>p chi KHOA HQC GIAO THONG Vy^N TAI So 29 - 03/2010 77
Voice call arriving rale
a)
Hinh 5. Xdc sudt chgn cugc ggi mdi Voice vd Data
Hinh 5 (a, b, c) bieu dien xac suat chan cudc gpi mdi tu-Q-ng ung vdi ba djch vu Voice, Data va video. Ta thay rang, phuang phap khdng du trQ- tai nguyen chuyen giao cd gia trj nay la be nhat, tiep den la du trQ- ban che va cudi cimg la dir tru' khdng ban che. Dilu nay la do trong cac phuang phap du trQ-, mdt phan tai nguyen da danh cho cac cudc gpi chuyen giao, lupng tai nguyen cho cac cudc gpi mdi giam xudng do dd xac suat chan cudc gpi tang len.
0 2 - 0 3 035 -Video call a'nnno lale
c)
TTinh 5c. Xdc sudt chdn cudc ggi Video
a)_
Hinh 6a. Xdc sudt chgn cugc ggi chuyin giao Voice
Hinh 6 (a, b, c) bieu dien xac suat ngat cudc gpi chuyen giao cung tuang irng vdi ba djch vu ndi tren. Rd rang rang, phuong phap khdng dir tru tai nguyen chuyen giao cd mirc dp ngat cudc gpi chuyen giao la ldn nhat, tiep den la dir trQ- ban che va cudi cimg la du trQ- khdng ban che cd nnic dp ngat la be nhat. Day chinh la mong mudn cua nghien ciru nay. Ket qua dd la do trong cac phuang phap du tni, mdt phan tai nguyen da danh cho cac cudc ggi chuyin giao, dir trQ- cang it bj khdng che thi xac xuat ngat cudc ggi chuyen giao cang thap. Ket qua nay cung khdng anh hudng tdi hieu suat sir dung tai nguyen bdi vi khi cd cudc ggi can chuyin giao thi tai nguyen mdi dugc dir trQ-. neu khdng cd nhu cau chuyen giao thi tai nguyen van dugc danh cho cudc ggi mdi
i Bl
Dsia cai imng liie
b)
Hinh 6. X 'dc sudt chgn cudc ggi chuyin giao Data vd Video
78
Tsip.chiKH<\
Xac xuat gian doan thdng tin ciia cac phuong phap ung vdi cac loai djch vu dugc bieu dien d hinh 7 (a, b, c). Cac kit qua cQng cho thay rang, d cac phuong phap cd du trQ- tai nguyen va du IrQ- khdng ban che, gia trj nay cQng nhd ban, cd nghia la he thdng boat ddng tdt hon vdi trirdng hgp khdng dii- tru tai nguyen hoac du tru bj ban che.
Voice call Sfmrig fale
a) • b )
Hinh 7. Xdc sudt gidn doan thdng lin Voice vd data
01 015 02 0,25 0,3 035 V * o call anwnj lale
I : 05
Hinh 7c. Xdc sudt gidn doan ihdng tin Video
VI. KET LUAN
Trong bai bao nay, viec kit hgp he suy dien md va phuong phap du bao RLS dugc ket hgp dl danh gia thdng tin di chuyin cua cac MS va sau dd la du tru tai nguyen chuyen giao, trong mang t l bao CDMA dupe thuc hien. Y tudng du trQ- tai nguyen khdng ban che cung dugc dua vao nghien ciru nay. Thdng tin di chuyin cd dugc tir md hinh nay la dQ- lieu hQ'u ich phuc vu cho qua bai toan dilu khiln tham nhap cudc ggi ciia mang te bao CDMA. Ket qua md phdng cho thay rang, he thong su dung phuong phap du tni- tai nguyen cho cudc ggi chuyen giao se cd xac suat ngat cupc gpi chuyin giao thap hon khi khdng sir dung va neu lugng tai nguyen dir trQ- khdng bj chan tren thi xac suAt nay cdn thap hon nQa, tire la cac cudc ggi chuyen giao dugc bao ve tdt ho-n.
l i i i lic'ii tham kh;io
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.9P chi KHOA HQC GIAO THONG VAN TAI So 29-03/2010 79
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