An Adaptive Cross-layer Bandwidth Scheduling Strategy for the Speed-Sensitive Strategy in Hierarchical Cellular Networks

Transcription

1 An Adaptve Cross-layer Bandwdth Schedulng Strategy for the Speed-Senstve Strategy n erarchcal Cellular Networks Jong-Shn Chen #1, Me-Wen #2 Department of Informaton and Communcaton Engneerng ChaoYang Unversty of Technology, Tachung, Tawan 1 schen26@cyut.edu.tw 2 s @cyut.edu.tw Abstract Effcent bandwdth schedulng of wreless bandwdth s crtcal to cellular system performance. Bandwdth schedulng methods can be dvded nto cell-layer channel allocaton and call-layer channel assgnment. Prevous studes are focused on the latter type of method. Some problems, such as traffc-load varatons among base statons (BSs) and BSs faled to provde wreless communcaton servce, are the cell-layer problems and are dffcult to handle usng the call-layer methods. Therefore, ths study presents an adaptve cross-layer bandwdth schedulng strategy for herarchcal cellular networks. Ths strategy s mplemented to solve the traffc-adaptaton and the fault tolerance problem accordng to the speed-senstve call-layer method. Keywords Cross-layer, speed sensng, traffc adaptaton, and fault tolerance. 1. INTRODUCTION erarchcal mcrocell/ cell archtectures have been proposed [[3]-[4], [6], [9], [12]-[13], [18]-[19]] to ncrease the traffc-carryng capacty and crcut qualty. A maor drawback s that a large number of handoff procedures usually take place when the calls crosse the cell boundary. The large number of handoffs ncrease wll ncrease the system overheads to do channel swtch, data swtch, and even network swtch. Accordngly, the speed senstve allocaton strategy, termed as the SS strategy, that can decrease the handoff probablty was appled [[7], [12]-[13], [15], [18]]. owever, some system problems such as the mbalance of the traffc loads wth the occuped bandwdth or base statons (BSs) faled to provde wreless communcaton servce wll lessen the traffc-carryng capacty of the SS strategy. Therefore, provdng a traffc-adaptve and fault-tolerant management strategy for the SS strategy s vtally meanngful. A geographcal area s overlapped by both of mcrocells and cells, where each cell s covered by a number of mcrocells. Each cell has a BS to support wreless communcatons of a number of moble hosts (Ms). The avalable system bandwdth for provdng wreless communcaton servce s dvded nto two ndependent subbands, where one s allocated to the mcrocell network and the other s allocated to the cell network. For each band of a network (the mcrocell network or the cell network), the band s parttoned nto a set of non-nterferng channels usng varous technques such as frequency dvson, tme dvson, code dvson, the combnaton of the technques. Accordngly, each cell can acqure a number of channels. A communcaton sesson (or a call) can be establshed f enough channel(s) can be allocated for supportng the communcaton between the M and the BS. Notably, two neghborng cells can not acqure the same channels. Otherwse, the calls usng the same channels at dfferent cells wll nterfere wth other. Ths stuaton s referred to channel nterference. Snce the channels of two neghborng cells are dfferent, when a call carred by a M moves from a cell to ts neghborng cell, a handoff procedure wll take place. In general, the handoff procedure ncludng data tramsson, channel swtchng, and even network swtchng takes tens or hundreds ms. Snce the cell densty of a herarchcal network s large, a M has hgh probablty to cross the cell boundary and needs to do handoffs. To lght the handoff problem, the speed-senstve assgnment strateges are appled,.e., hgh-moblty Ms are prortzed to acqure the channels of cells and low-moblty Ms are prortzed to acqure the channels of mcrocells to establsh the communcaton sessons.

2 Each cell has a set of prmary channels [[5], [10]]. When a call arrves at an area, ths call s handled by the BS a cell. If no prmary channels are avalable n ths cell to serve ths call, the call wll be blocked. Accordngly, the number of channels allocated to a cell wll affect the communcaton qualty n ths cell and the allocatons of system channels among cells wll affect the traffc-carryng capacty of a cellular system. A reasonable allocaton should provde more channels to each cell wth heavy traffc than wth lght traffc. Otherwse, t wll experence that the heavy traffc cells do not have suffcent channels to carry ther traffc loads but the lght traffc cells have many avalable channels. Thus, the traffc-carryng capacty of a cellular system s reduced and the call blockng probablty arses. To consder real-lfe networks, the traffc dstrbutons among cells should be changeable accordng to varous condtons. In order to acheve hgher channel utlzaton, when there are varatons n traffc, the channel allocatons among cells should be effectvely reallocated accordng to current traffc profle. Effcent bandwdth schedulng of wreless bandwdth s crtcal to cellular system performance. Bandwdth schedulng methods can be dvded nto cell-layer channel allocaton and call-layer channel assgnment. The former type of method s responsble for allocatng system channels to cells. Accordngly, when calls arrve at cells, the latter type of method s actvated n each cell to assgn channels to calls to establsh communcaton sessons. Prevous studes are focused on the latter type of method [[2], [5], [11]-[12], [14], [16]-[17]]. Some problems, such as traffc-load varatons among base statons (BSs) and BSs faled to provde wreless communcaton servce, are the cell-layer problems and are dffcult to handle usng the call-layer methods. In lght of above dscussons, ths study presents an adaptve cross-layer bandwdth schedulng strategy for the SS strategy. Ths strategy s mplemented to solve the traffc-adaptaton and the fault tolerance problem accordng to the speed-senstve call-layer method. 2. SYSTEM MODE AND SS STRATEGY Ths secton frst presents the system model and, then, usng the defned system model to present the SS strategy. A mcrocell/ cell cellular system s a herarchcal cellular system, where a cell overlays a set of mcrocells. Each cell has a BS n ts center to handle the wreless communcatons of a number of moble hosts (Ms) n ts covered area. The moblty of a M s valuated as Def. 1. Defnton 1: Gven a moble host mh, the moblty of mh, denoted as M(mh), s the number of mcrocells that mh traverse for a specfed duraton. Moble host mh s termed as a hgh moblty host f M(mh)>s h, where s h s a speed threshold. The avalable system bandwdth s dvded nto two dsont sub-bands: B mcro and B, where B mcro s used for the mcrocells and B s used for the cells. For a mcrocell (or cell) system, the gven sub-band B s dvded nto a number of dsont unts (termed as channels). Each cell C s gven a subset P(C) of B, termed as the prmary channels of C. When a call arrves at C, P(C) s used to serve the call. Two cells cannot concurrently assgn the same channel to calls f ther geographcal dstance s less than D mn ; otherwse, ther communcaton sessons nterfere wth each other. Ths stuaton s referred to channel nterference [[5], [10]]. Defnton 2: Gven a cell C, the set of nterferng neghbors of cell C, denoted by IN(C), s: IN(C) = {cell C' the BSs of cells C and C operate at the same band and Dst(C, C ) < D mn }, where Dst(C, C ) s the geographcal dstance between cells C and C. Accordng to Def. 2, a cell C wth ts nterferng neghbors IN(C) can not contan the same channel(s) as ther prmary channel(s). Therefore, Def. 3 s the condton of prmary channel allocatons. Besdes the prmary channel set P(C) of cell C, we also use S(C), termed as the secondary channel set, to denote other non-prmary channels of cell C, where S(C) = B C P(C). Defnton 3: (The condton of prmary channel allocaton): Gven two dstnct cells C and C', where C' IN(C), the condton of prmary channel allocaton between cells C and C' s P(C) P(C')=. For cell C, P(C) s used to serve the calls arrved at C. For channels P(C), a channel ch s avalable to C f ch currently s not been assgned to any call n C. Defnton 4 accordngly defnes the avalable channels of C. Defnton 4: (Avalable prmary channel): Gven a cell C wth prmary channels P(C), the avalable channels of C, denoted as A(P(C)), s A(P(C))={ch ch P(C) and ch s avalable to C}.

3 Fg. 1 erarchcal nfrastructure network In general, each cell C overlays wth k mcrocells, C 0, C 1,, C k-1. For convenence, C C s used to denote that mcrocell C s overlad by cell C. Moreover, D mcro and D to denote the mnmum reuse dstances of mcrocells and cells, respectvely. Fgure 1 llustrates a cell confguraton. Each cell C overlays 4 mcrocells: C 0, C 1,, and C 4. D mc and D mac are 21 r mcro and 21 r, where r mcro and r are the raduses of a mcrocell and a cell, respectvely. The nterferng neghbors IN(C 32 ) of cell C 32 nclude cells {C 18 -C 20, C 24 -C 27, C 30 -C 34, C 37 -C 40, C 44 -C 46 }. If channel ch s the prmary channel of C 32, other cells n IN(C 33 ) cannot keep ths channel as ther prmary channel. The prmary channel allocatons for mcrocells are also smlar. For nstance, these mcrocells n IN(C 8,0 )={C 1,0 -C 1,1, C 2,0, C 0,3, C 1,2 -C 1,3, C 2,2, C 7,0 -C 7,1, C 8,1, C 9,0, C 7,2 -C 7,3, C 8,2 -C 8,3, C 14,0 -C 14,1, C 15,0 } cannot keep the prmary channels of C 8,0 as ther prmary channels. For any locaton of a cellular network, there are a mcrocell C and a cell C whch can use ther prmary channels P(C ) and P(C ) to handle the arrval calls. For the SS strategy, the procedure to handle the arrval call can be formed as Fg. 2. When a call requestng a number c of channels wth the targeted M mh arrves at the area of C, there are three cases that mh can acqure the suffcent channels to establsh ts communcaton sessons. Otherwse, the call s blocked. In the frst case, mh can acqure the prmary channels of cell C that f mh s a hgh moblty host,.e., M(mh) >s h, and the avalable channels of the cell C are suffcent.e., A(C ) > c. The second s mh can acqure the prmary channels of mcrocell C that f that mh s a low moblty host,.e., M(mh) <s h, and the avalable channels of the mcrocell C are suffcent.e., A(C ) > c. The other s that the avalable channels of the cell C are suffcent.e., A(C ) > c and mh can acqure the prmary channels of cell C. The SS strategy prortes a hgh-moblty M to acqure the channels P(C ) of a cell C and a low-moblty M to acqure the channels P(C ) of a mcrocell C. The SS can be formalzed as Fg. 2. When a call wth the targeted M mh requestng c capacty arrves at the area of mcrocell C, there are 3 cases that the communcaton sesson of ths mh can be establshed. Otherwse, the call s blocked. Case 1: mh s a hgh-moblty M and the avalable channels of C are suffcent,.e., M(mh) > th and A(P(C )) > c. Case 2: mh s a lgh-moblty M and the avalable channels of C are suffcent,.e., M(mh) > th and A(P(C )) > c. Case 3: (Not n Cases 2 and 3): The avalable channels of C are suffcent,.e., A(P(C )) > c.

4 Fg. 2 The SS strategy Therefore, the allocatons of P(C ) and P(C ) wll affect the performance of the performance of the SS strategy. For the conventonal SS strategy, the prmary channel allocaton s fxed and s dffcult to handle the varatons n system profles, such as traffc varatons n cells, base staton faled to provde wreless servces, or wred lnk falures among cells. In ths study, dstnct cells can dynamcally change ther prmary channel allocatons to satsfy the dfferent varatons. et the prmary cells of a channel ch, denoted by PC(ch), be all of the cells, whch contan a prmary channel ch n the system. Accordng to Def. 2, f cell C acqures a new prmary channel ch, the orgnal owners of ths prmary channel ch n IN(C) wll be forbdden to keep channel ch as ther prmary channel. Defnton 5 presents the nterferng prmary cells IP(C, r) wth a channel ch, whch are the cells that cannot keep channel ch as ther prmary channel, when cell C acqures a new prmary channel. Defnton 5: The nterferng prmary cells of channel ch relatve to cell C are denoted by IP(C, r), where IP(C, ch) = PC(ch) IN(C). For nstance n Fg. 1, suppose that cells C 2, C 11, C 20, C 22, C 31, C 40, and C 42 have a prmary channel ch,.e., PC(ch) = {C 2, C 11, C 20, C 22, C 31, C 40, C 42 }. Snce IP(C 32, ch) =IN(C 32 ) PC(ch) = {C 31, C 40 }, f C 32 acqures ch as ts new prmary channel, C 31 and C 40 can not keep ch as ther channel. Moreover, for C 41, C 47 or C 48, snce the number of nterferng prmary cells of ch s 0, one of C 41, C 47 and C 48 can acqure ch as ts new prmary channel. 3. SUBJECT STRATEGY Fgure 3 presents the layout of our cell-layer bandwdth schedulng strategy for the call-layer bandwdth schedulng strategy. Our strategy s responsble for allocatng system channels to each cell. Accordngly, when calls arrve at ths cell, the call-layer strategy s actvated to assgn the allocated channels to calls to establsh communcaton sessons. When there are varatons such as traffc servce profle among cells, the cell-layer strategy wll be actvated, accordngly to

5 the varatons, to allocated system channels to each cell. The new allocaton s submtted to the call-layer strategy. Then, the call-layer strategy uses the new allocaton to carry the traffc. Fg. 3 ayout of the subect strategy Ths strategy uses two allocaton condton functons R mcro and R to valuate the capacty effects of a mcrocell and a cell to allocate a new channel. Accordngly, a dstrbuted channel allocaton strategy can be actvated to allocate the system channels to meet the varatons n traffc profle, servce profle, and lnk-profle. 3.1 Allocaton condton The dscusson of the condton that a cell can acqure a new channel s presented heren. The condton connects wth the traffc of the servce area. To evaluate the traffc, we dvde the physcal servce area of a cellular system nto a number of unts. Each unt contans the covered area of a mcrocell C. The traffc of n the area C s dvded nto hgh-moblty traffc λ (C ) and low-moblty traffc λ (C ). The SS strategy prortzes assgnng the channels of a cell to a hgh-moblty M and the channels of a cell to a low-moblty M. Accordngly, the traffc loads of an area of a mcrocell C s dvded nto hgh-moblty traffc λ (C ) and low-moblty traffc λ (C ), where λ (C ) s handled by usng P(C ) of the overlapped cell C and λ (C ) s handled by usng P(C ) of the overlapped mcrocell C Mcrocell Wth ow-moblty Traffc The evaluaton of usng P(C ) to carryng the traffc λ (C ) can be represented usng Erlang B formula, as shown n (1), n whch P(C ) s the number of channels n P(C ). Based on (1), the valuaton of cell C ncreasng and decreasng a channel ch can be represented as (2) and (3), respectvely. The ncr mcro (C, ch) s used by C to evaluate ts capacty effect f t acqures a new channel ch. The ncr mcro (C, ch) compares the channel utlzaton wth the low-moblty traffc λ (C ) before and after acqurng a channel ch, for cell C. The decr mcro (C, ch) s used by the nterferng neghbor C of C,.e., C IN(C ), to evaluate the capacty effect f C decreases a channel ch. Therefore, C must take the states of C nto consderaton. If C currently cannot provde wreless communcaton servce (Servce Falure),.e., C cannot use any channels to carry the traffc, the capacty reducton of decreasng a channel ch s 0. For the No-Falure case, snce the current operatons of C s normal (No falure), decr(c ) s set as to compare the channel utlzaton wth and wthout havng a channel ch. Accordng to Def. 5, f C takes a channel ch as ts new channel, the orgnal owners IP(C, ch) of ch must gve up ch. The channel transformaton among cells can be represented as (4). Accordng to (2)-(4), the capacty effect of a mcrocell C acqurng a new channel ch and the orgnal owners gvng up ch can be represented as (5). In order to enhance the system capacty, the necessary condton to reallocate a channel ch to C s R mcro (C, ch)> Macrocell Wth gh-moblty Traffc For a cell C, ts allocated channels are prortzed to handle the hgh-traffc loads λ (C ) presented as (6). The evaluaton of usng P(C ) to carryng the traffc λ (C ) can be represented usng Erlang B formula, as shown n (7), where P(C ) s the number of channels n P(C ). Based on (7), the valuaton of cell C ncreasng and decreasng a channel ch can be represented as (8) and (9), respectvely. Accordng to (8) and (9), the capacty effect of a mcrocell C acqurng a new channel ch and the orgnal owners gvng up ch can be represented as (10). In order to enhance the system capacty, the necessary condton to reallocate a channel ch to C s R (C, ch)>0.

6 n ( ) n k C ( C ) EB ( λ ( C ), P( C )) = λ λ, where n = P( C ). n! k= 0 k! 1 ncr decr mcro mcro ( EB( C ) ) ) ( EB( C ) { ch} ) ) λ ( C ) ( C, ch) = ( C,, ch) = ( EB( C ) { ch} ) ) ( EB( C ) ) ) P(C) P(C) {ch} P(C') P(C') {ch}, where C' IP(C, ch). 0 λ ( C ), No, Servce Falure Falure (1) (2) (3) R mcro ( C, r) = ncr( C, ch) λ ( C ) decr( Cf, r) λ( Cf ). Cf IP ( C,, ch) (4) λ ( C ) = λ ( C ), where λ ( C ) s the hgh - traffc load n the covered area of C C, C. (5) n n k ( C ) ( C ) EB ( λ ( C ), P( C )) = λ λ, where n = P( C ). n! k= 0 k! ncr 1 (( EB( C ) ) ) ( EB( C ) { ch} ) )) λ ( C ) ( C, ch) = (6) (7) decr R ( C, ch) = (( EB( C ) { ch} ) ) ( EB( C ) ) )) ( C, r) = ncr ( C ch) λ ( C ) decr ( Cf, r) λ( Cf ). Cf IP ( C, ch) 0 λ ( C ), No falure, Servce falure (8) (9) (10).

7 Perodcally, C sets S(C) as B P(C) and, then, performs the follows. For each C n IN(C), C collects P(C n ) and λ(c n ). Moreover, f C n s servce faled cell, C sets P(C n )=Ø and λ(c n )=0 S(C)=Ø yes Stop no (1) For each ch S(C), C evaluates the value of R(C, ch) and f R(C, ch)<0, C deletes ch from S(C). (2) Fnd a ch max S(C), whose R(C, ch max ) s maxmal for all ch S(C) R(C, ch max ) > 0 no yes C perform: P(C) P(C) {ch max } and S(C) S(C) {ch max } For each C p IP(C, ch max ), C p performs: P(C p ) P(C p ) {ch max } and S(C p ) S(C p ) {ch max } Fg. 4 Allocaton strategy 3.2 Allocaton Strategy The strategy s performed perodcally by each cell. The opportunty that a cell performs the strategy can be formally descrbed as follows. Partton the set of all cells n a system nto G 0, G 1,,and G cs-1 dsont subsets, such that any two cells n the same subset are apart by at least a dstance of D mn. Accordngly, partton the tme nto T 0, T 1,, and T cs-1 dsont tme perods (cs s also termed as the cluster sze). The cells n G are assgned to perform the channel allocaton at tme perod T. For nstance n Fg. 1 wth D = 21 r, cells {C 0, C 1,, C 48 } can be dvded nto G 0, G 1,, G 6, where G 0 ={C 0, C 9, C 18, C 27, C 29, C 38, C 47 }, G 1 ={C 1, C 10, C 19, C 21, C 30, C 39, C 48 }, G 2 ={C 2, C 11, C 20, C 22, C 31, C 40, C 42 }, G 3 ={C 3, C 12, C 14, C 23, C 32, C 41, C 43 }, G 4 ={C 4, C 13, C 15, C 24, C 33, C 35, C 44 }, G 5 ={C 5, C 7, C 16, C 25, C 34, C 36, C 45 }, and G 6 ={C 6, C 8, C 17, C 26, C 28, C 37, C 46 }. Cells n the same group can perform the strategy at same tme perod. A cell C that performs our

8 strategy s to evaluate the current varatons such as traffc loads and the wreless communcaton provsons of ts nterferng neghborng cells IN(C). Accordngly, C reschedules the system channels for IN(C) to meet the varatons The proposed strategy s performed by each cell C, whch accords to the current traffc loads and the prmary channels of t tself and the each nterferng neghbor C n to determne the new prmary channels. Therefore, the nputs of the strategy ncludng B, λ(c), and P(C), where f C s a mcrocell (or cell), B represents the bandwdth B mcro (or B ) and λ(c) represents the low-moblty traffc λ (C) (or the hgh-moblty traffc λ (C)). The output of the strategy s channel sets, ncludng C and ts nterferng neghbor. The formal descrpton of the strategy s shown as Fg. 4. As follows, we gve some examples to descrbe the methods to handle the varatons n traffc profle, lnk-profle, and servce-profle. Suppose, n Fg. 1, the avalable bandwdth of the cell system has 70 channels, denoted as B ={ch 0, ch 1,, ch 69 }. The orgnal prmary channel sets of C 24 s P(C 24 )={ch 0, ch 1,, ch 9 }, of C 29 s P(C 29 )={ch 10, ch 11,, ch 19 }, and of C 18 s P(C 18 )={ch 20, ch 21,, ch 29 }. We contnually suppose, C 29 ncurs servce falure, whch cannot provde wreless communcatons. When C 24 performs the strategy, C 24 sets S(C 24 ) as B P(C 24 ) ={ch 10, ch 11,, ch 69 } and then collects the hgh-moblty traffc loads λ (-) and the current channel allocatons P(-) of IN(C 24 )= {C 10 -C 12, C 16 -C 19, C 22 -C 23, C 25 -C 26, C 29 -C 32, C 36 -C 38 }. owever, snce C 29 ncurs servce falure, C 24 sets P(C 29 ) as and sets λ (C 29 ) as 0. Then, C 24, round by round, evaluates R (C 24, ch) for each ch n S(C 24 ). In each round, C 24 pcks up a channel ch max havng the largest postve value and saves ch max nto P(C 24 ) and deletes ch max from each P(C p ), where C p s the orgnal owner of ch max,.e., C p IP(C 24, ch max ). In each round, C 24 also deletes ch max and other channels wth non-postve R (-) values from S(C 24 ). Untl S(C 24 ) s empty or any channel n S(C 24 ) has non-postve R (-) value, the evaluaton s stopped. After the evaluaton, P(C) denotes the new prmary channel allocaton of cell C, where C belongs to C 24 and IN(C 24 ). In the evaluaton, channels P(C 29 )={ch 10, ch 11,, ch 19 }, belongng to a servce-falure cell C 29, C 29 cannot use ts channels {ch 10, ch 11,, ch 19 } to carry traffc. The decr (C 29, ch), for ch n {ch 10, ch 11,, ch 19 } wll be set as 0 (as shown n (9)),.e., no capacty effect f C 29 gves up the channels. For other nstance n Fg. 1, suppose IP(C 9, ch 39 )={C 8, C 17 },.e., n the nterferng area IN(C 9 ) of C 9, C 8 and C 17 have a channel ch 39. Snce C 8 has channel ch 39, no cells n IN(C 8 )={C 0 -C 3, C 7, C 9 -C 10, C 14 -C 16, C 22 } have ch 39. When a large number of Ms transfers from the neghbors of C 9, such as C 8 and C 17 nto C 9, C 9 must carry more traffc. The condton wll cause the carryng traffc loads wth the occuped channels among C 9 and ts neghbors unbalanced. Therefore, C 9 has opportunty to acqure more channels from ts neghbors. Suppose C 9 performs the strategy and fnd R (C 9, ch 39 )>0, C 9 can acqure ch 39. The orgnal owners IP(C 9, ch 39 ) wll gve up ch 39 and transfer to C 9 to balance the traffc transformaton. Moreover, for C 0 before transferrng ch 39 to C 9, C 8 s the unque cell havng ch 39. After C 8 transfers ch 39 to C 9, no cells n IN(C 0 ) have ch 39, C 0 wll acqure ch SIMUATION RESUTS The smulaton envronment has 49 cells, arranged as 7-parallelogram structure, where the radus of a mcrocell s 400 meters and each cell overlaps 4 mcrocells. The reuse dstances of the mcrocell system and the cell system are 21r and mcro 21 r, where r mcro and r are 400m and 800m, respectvely. The frequency bands for mcrocells and cells are 2.24Mb/s and 4.48Mb/s, respectvely. The average number of moble hosts, whch locate at the area of a mcrocell, s 100. It ncludes 30% hgh-moblty moble hosts and 70% low-moblty moble hosts, generated accordng the random process. The call arrval rate of a non-callng moble host s generated accordng to the random process from 2.20 to 4.95 calls/hour. The envronment ncludes 5 hot mcrocells. A M, located at a mcrocell C, has 2 cases to determne the next locaton. Case 1: No hot-cells are closed to C. The probablty of movng to each neghborng cell s the same. Case 2: If a hot cell s closed to a hot cell, the probablty of movng to ths hot cell s 50% and the probablty of movng to other non-hot cells s 50%. The smulaton results are demonstrated as Fg. 5- Fg. 8, whch nclude no falure, and 1%, 2%, and 8% of cells faled to provde communcaton servces. In whch, SS represents the SS strategy wthout our strategy and Ours represents the SS strategy wth our strategy.

9 Moreover, -a represents the overall call blockng probablty, -h represents the handoff call blockng probablty, and -n represents the new call blockng probablty. The results reveal that our strategy s avalable for the SS strategy and can greatly mprove the traffc-carryng capacty. The reason s descrbed as follows. The orgnal cannot handle the cell-layer varatons such as the change of the traffc loads among cells or the base statons faled to provde servce. Ours strategy can adaptvely tune the bandwdth allocatons accordng the varaton of traffc loads among cells. Moreover, when a cell faled to provde servce, the neghborng non-faled cells can acqure more prmary channels released from the faled cells to lght the effect to the system capacty. Call blockng rate SS-a Ours-a SS-h Ours-h SS-n Ours-n Call arrval rate Fg. 7 2% servce falure SS-a SS-h SS-n Ours-a Ours-h Ours-n Call blockng rate SS-a SS-h SS-n Ours-a Ours-h Ours-n Fg. 5 No falure Call arrval rate Fg. 8 8% servce falure Call blockng rate SS-a Ours-a SS-h Ours-h SS-n Ours-n Fg. 6 1% servce falure 5. CONCUSIONS Bandwdth schedulng methods can be dvded nto cell-layer channel allocaton and call-layer channel assgnment. Prevous studes are focused on the latter type of method. Some problems, such as traffc-load varatons among base statons (BSs) and BSs faled to provde wreless communcaton servce, are the cell-layer problems and are dffcult to handle usng the call-layer methods. Therefore, ths study presented an adaptve cross-layer bandwdth schedulng strategy for herarchcal cellular networks. Ths strategy s mplemented to solve the traffc-adaptaton and the fault tolerance problem accordng to the speed-senstve call-layer method. In mcrocell/ cell

10 cellular systems, when some cells fal to provde communcaton servces, channels n these faled cells can be reallocated to other homogeneous cells and the overlapped heterogeneous cells of faled cells can be assgned more channels. Accordngly, the subscrbers stll acqure the apprecated communcaton servces even though some cells fal to provde communcaton servces. The mechansm used n the proposed strategy s from the cell-layer to schedule system bandwdth to handle the varaton s system profles. It can acheve varous demands for the moble communcatons area by modfyng the allocaton condtons wth more related factors [1-2]. For nstance, the charges of provdng communcaton servces are one of the consderatons for telecommuncaton markets. For nstance, the charges for provdng communcaton servces are an mportant consderaton n marketng telecommuncaton. The charges of calls can be ncluded n the reward functon, accordng to the requrement to maxmze the charge to allocate channels to calls. Ergonomc and economc factors wll be consdered to satsfy new trends n the telecommuncaton ndustry n the future. REFERENCES [1] S. Al A. Fakooran,. Taher and M. Fath "Channel management based on maxmzng customers satsfacton and servce provders revenue," Internatonal Symposum on Wreless Pervasve Computng, pp , May [2] A. Boukerche, K. Abrougu and T. uang, "QoS and fault-tolerant based dstrbuted dynamc channel allocaton protocol for cellular networks," IEEE Conference on ocal Computer Networks, pp.59-67, November [3] A. Byungchan, Y. yunsoo, and C. Jung Wan, "A desgn of -mcro CDMA cellular overlays n the exstng bg urban areas," IEEE Journal on Selected Areas n Communcatons, vol. 19, Issue: 10, pp , October [4]. Bo, W. ChengKe, and A. Fukuda, "Performance analyss of flexble herarchcal cellular systems wth a bandwdth-effcent handoff scheme," IEEE Transactons on Vehcular Technology, vol. 50, Issue: 4, pp , July [5] G. Cao, "Integratng dstrbuted channel allocaton and adaptve handoff management for QoS-Senstve cellular networks," ACM/Kluwer Wreless Networks (WINET), Vol. 9, pp , March 2003 [6] S. P. Chung and J. C. ee, "Performance analyss and overflowed traffc characterzaton n multservce herarchcal wreless networks," IEEE transactons on wreless Communcatons, Vol. 4, No. 3, May [7] C. Dazhong, X. Jng, and W. Peng, "Research on mprovement n the handoff performance for hgh-speed moble servce," IEEE Canadan Conference on Electrcal and Computer Engneerng, pp , May [8]. uang, S. Kumar, and C.-C.J Kuo, "Adaptve resource allocaton for multmeda QoS management n wreless networks," IEEE Transactons on Vehcular Technology., Vol.53, pp , Mar 2004 [9] A. Iera, A. Molnaro, and S. Marano, "andoff management wth moblty estmaton n herarchcal systems," IEEE Transactons on Vehcular Technology, vol. 51, PP , September [10] I. Katzela and M. Naghshneh, "Channel assgnment schemes for cellular moble telecommuncaton systems: a comprehensve survey," IEEE Personal Communcatons, vol. 33, pp , June [11] S. Km and P. K. Varshney, "Adaptve fault tolerant bandwdth management framework for multmeda cellular networks," IEE Proceedngs-Communcatons, Vol. 152, No. 6, pp , December [12] B. C. K. Wu, and A. Fukuda, Performance analyss of flexble herarchal cellular systems wth a bandwdth-effectve handoff scheme, IEEE Transactons on Vehcular Technology, vol. 50, no. 4, pp , July [13] W. Shan, P. Fan, S. Member, and Y. Pan, "Performance evaluaton of a herarchcal cellular system wth moble velocty-based bdrectonal call-overflow scheme," IEEE transactons on parallel and dstrbuted systems, vol. 14, NO. 1, January 2003 [14] S. Tokekar and N. Puroht, "Performablty Analyss of a Fault Tolerant Sectorzed Cellular Network," IEEE Transactons on Annual Inda Conference, pp.1-5, September 2006.

11 [15] A. E. Xhafa, and O. K. Tonguz, "andover Performance of Prorty Schemes n Cellular Networks," IEEE Transactons on Vehcular Technology, pp , January [16] J. Yang, Q. Jang, D. Manvannan and M. Snghal, "A Fault tolerant dstrbuted channel allocaton scheme for cellular networks," IEEE Transactons on Computer, Vol 54, No 5,May [17] J. Yang, Q. Jang and D. Manvannan, "A Fault-Tolerant Channel-Allocaton Algorthm for Cellular Networks Wth Moble Base Statons," IEEE transactons on Vehcular Technology, Vol. 56, No. 1, pp , January [18] Y. Zhang and B.-. Soong, "Take-back schemes n herarchcal cellular system," IEEE Globecom, pp , [19] F. M. Zhao,. G. Jang and. Y. Bng, "Performance analyss of channel borrowng on herarchcal wreless networks," IEEE Conf. on IWVDVT2005, pp , May 2005.