Cost Analyss and Effcent Rado Bearer Selecton for Multcastng over UMTS Antonos Alexou, Chrstos Bouras, Vasleos Kokknos, Evangelos Rekkas Research Academc Computer Technology Insttute, Greece and Computer Engneerng and Informatcs Dept., Unv. of Patras, Greece alexua@ct.gr, bouras@ct.gr, kokknos@ct.gr, rekkas@ct.gr Contact Person: Chrstos Bouras Research Academc Computer Technology Insttute,. Kazantzak str, GR-26500 Patras, Greece and Computer Engneerng and Informatcs Dept., Unv. of Patras, GR-26500 Patras, Greece Tel:+30-2610-960375 Fax:+30-2610-969016 e-mal: bouras@ct.gr
1 Cost Analyss and Effcent Rado Bearer Selecton for Multcastng over UMTS Abstract Along wth the wdespread deployment of the Thrd Generaton (3G) cellular networks, the fast-mprovng capabltes of the moble devces, content and servce provders are ncreasngly nterested n supportng multcast communcatons over wreless networks and n partcular over Unversal Moble Telecommuncatons System (UMTS). To ths drecton, the Thrd Generaton Partnershp Project (3GPP) s currently standardzng the Multmeda Broadcast/Multcast Servce (MBMS) framework of UMTS. In ths paper, we present an overvew of the MBMS multcast mode of UMTS. We analytcally present the multcast mode of the MBMS and analyze ts performance n terms of packet delvery cost under varous network topologes, cell types and multcast users dstrbutons. Furthermore, for the evaluaton of the scheme, we consder dfferent transport channels for the transmsson of the multcast data over the UMTS Terrestral Rado-Access etwork (UTRA) nterfaces. Fnally, we propose a scheme for the effcent rado bearer selecton that mnmzes total packet delvery cost. Index Terms UMTS, Multcast n UMTS, MBMS, Power Control I. ITRODUCTIO UMTS consttutes the thrd generaton of cellular wreless networks whch ams to provde hgh-speed data access along wth real tme voce calls. Although UMTS networks offer hgh capacty, the expected demand wll certanly overcome the avalable resources. The 3GPP realzed the need for broadcastng and multcastng n UMTS and proposed some enhancements on the UMTS Release 6 archtecture that led to the defnton of the MBMS framework. MBMS s a pont-to-multpont servce whch allows the networks resources to be shared [4]. A detaled cost analyss model for the evaluaton of dfferent one to many packet
2 delvery schemes n UMTS s presented n [1]. The schemes that the authors consder n the evaluaton are the Broadcast scheme, the Multple Uncast scheme and Multcast scheme. However, n ths approach the authors focus ther evaluaton n the Core etwork of the UMTS archtecture. In [2], a more detaled analyss of the above mentoned one to many delvery schemes s presented. In ths work, the authors consder dfferent transport channels for the transmsson of the data over the UTRA nterfaces. Both these works do not take nto account n the evaluaton a number of parameters such as the dfferent cell envronments (mcro, macro), the power profles of the transport channels, and fnally the selecton of the most effcent transport channel for the transmsson of the data over the UTRA nterfaces. In ths paper, we present an overvew of the MBMS multcast mode of UMTS. We analytcally present the multcast mode of the MBMS and analyze ts performance n terms of packet delvery cost under varous network topologes and multcast users dstrbutons both n macro cell and mcro cell envronments. The analyss of total packet delvery cost takes nto account the pagng cost, the processng cost and the transmsson cost at nodes and lnks of the topology. Furthermore, for the evaluaton of the scheme, we consder dfferent transport channels for the transmsson of the multcast data over the Iub and Uu nterfaces. The transport channels, n the downlnk, currently exstng n UMTS whch could be used to an MBMS servce are the Dedcated Channel (DCH), the Forward Access Channel (FACH) and the Hgh Speed Downlnk Shared Channel (HS-DSCH). However, n our analyss we wll focus on the DCH and FACH transport channels. The fundamental factor that determnes the transmsson cost over the ar (Uu nterface) s the amount of ode B s transmsson power that should be allocated when usng each one of these transport channels. DCH and FACH have dfferent characterstcs n terms of
3 power control. Thus, we present an extended analyss of ode B s power consumpton n order to defne the exact telecommuncaton cost ntroduced by the Iub and Uu nterfaces durng the MBMS multcast transmsson. Fnally, we propose a swtchng pont scheme for the effcent rado bearer selecton n order to mnmze total packet delvery cost. Ths scheme actually consttutes a contrbuton to the MBMS Countng Mechansm [4]. MBMS countng mechansm examnes whether t s more economc to transmt the multmeda servces n pont-to-pont (PTP) or pont-to-multpont (PTM) mode. Ths mechansm evaluates whether t s preferable to use dedcated resources (multple DCHs) or common resources (a sngle FACH). The crtera for the decson of ths swtchng pont should be based on the downlnk rado resource effcency. Ths paper s structured as follows. In Secton 2, we provde an overvew of the UMTS n packet swtched doman. Secton 3 presents the MBMS framework of UMTS. In Secton 4, we present a cost analyss method for the evaluaton of the MBMS multcast mode. Followng ths, Secton 5 provdes mportant aspects of power control n MBMS, whle Secton 6 presents some numercal results. Fnally, some concludng remarks and planned next steps are brefly descrbed. II. OVERVIEW OF UMTS AD MBMS ARCHITECTURE UMTS network s splt n two man domans: the User Equpment (UE) doman and the Publc Land Moble etwork (PLM) doman. The UE doman conssts of the equpment employed by the user to access the UMTS servces. The PLM doman conssts of two land-based nfrastructures: the Core etwork (C) and the UTRA (Fgure 1). The C s responsble for swtchng/routng voce and data connectons, whle the UTRA handles all rado-related functonaltes. The C s logcally dvded nto two servce domans: the Crcut-Swtched (CS) servce doman and the
4 Packet-Swtched (PS) servce doman [9], [10]. The PS porton of the C n UMTS conssts of two knds of General Packet Rado Servce (GPRS) Support odes (GSs), namely Gateway GS (GGS) and Servng GS (SGS) (Fgure 1). SGS s the centerpece of the PS doman. It provdes routng functonalty nteracts wth databases (lke Home Locaton Regster (HLR)) and manages many Rado etwork Controllers (RCs). SGS s connected to GGS va the Gn nterface and to RCs va the Iu nterface. GGS provdes the nterconnecton of UMTS network (through the Broadcast Multcast Servce Center) wth other Packet Data etworks (PDs) lke the Internet. [10]. Fgure 1. UMTS and MBMS Archtecture UTRA conssts of two knds of nodes: the frst s the RC and the second s the ode B. ode B consttutes the base staton and provdes rado coverage to one or more cells (Fgure 1). ode B s connected to the User Equpment (UE) va the Uu nterface (based on the W-CDMA technology) and to the RC va the Iub nterface. One RC wth all the connected to t ode Bs s called Rado etwork Subsystem (RS). In the UMTS PS doman, the cells are grouped nto Routng Areas (RAs), whle the cells n a RA are further grouped nto UTRA Regstraton Areas (URAs). The moblty-management actvtes for a UE are characterzed by two fnte state
5 machnes: the Moblty Management (MM) and the Rado Resource Control (RRC). The Packet MM (PMM) state machne for the UMTS PS doman s executed between the SGS and the UE for C-level trackng, whle the RRC state machne s executed between the UTRA and the UE for UTRA-level trackng. After the UE s attached to the PS servce doman, the PMM state machne s n one of the two states: PMM dle and PMM connected. In the RRC state machne, there are three states: RRC dle mode, RRC cell-connected mode, and RRC URA connected mode [8]. 3GPP s currently standardzng the Multmeda Broadcast/Multcast Servce. Actually, the MBMS s an IP datacast type of servce, whch can be offered va exstng GSM and UMTS cellular networks. It conssts of a MBMS bearer servce and a MBMS user servce. The latter represents applcatons, whch offer for example multmeda content to the users, whle the MBMS bearer servce provdes methods for user authorzaton, chargng and Qualty of Servce mprovement to prevent unauthorzed recepton. The major modfcaton n the exstng GPRS platform s the addton of a new entty called Broadcast Multcast - Servce Center (BM-SC). Fgure 1 presents the archtecture of the MBMS. The BM-SC communcates wth the exstng UMTS GSM networks and the external Publc Data etworks [5], [6]. Three new logcal channels are consdered for PTM transmsson of MBMS: MBMS pont-to-multpont Control Channel (MCCH), MBMS pont-to-multpont Schedulng Channel (MSCH) and MBMS pont-to-multpont Traffc Channel (MTCH). These logcal channels are mapped on FACH. In case of PTP transmsson Dedcated Traffc Channel (DTCH) and Dedcated Control Channel (DCCH) are used and are mapped on the dedcated channel, DCH [4]. III. DESCRIPTIO OF THE MBMS MULTICAST MODE In ths secton we present an overvew of the multcast mode of the MBMS
6 framework. Fgure 2 shows a subset of a UMTS network. In ths archtecture, there are two SGSs connected to a GGS, four RCs, and twelve ode Bs. Furthermore, eleven members of a multcast group are located n sx cells. The BM-SC acts as the nterface towards external sources of traffc [6]. In the presented analyss, we assume that a data stream that comes from an external PD through BM-SC, must be delvered to the eleven UEs as llustrated n Fgure 2. The analyss presented n the followng paragraphs, covers the forwardng mechansm of the data packets between the BM-SC and the UEs (Fgure 2). Regardng the transmsson of the packets over the Iub and Uu nterfaces, t may be performed on common (ex. Forward Access Channel - FACH) or dedcated (Dedcated Channel - DCH) channels. As presented n [11], the transport channel that the 3GPP decded to use as the man transport channel for pont-to-multpont MBMS data transmsson s the FACH wth turbo codng and QPSK modulaton at a constant transmsson power. DCH s a pont-to-pont channel and hence, t suffers from the neffcences of requrng multple DCHs to carry the data to a group of users. However, DCH can employ fast closed-loop power control and soft handover mechansms and generally s a hghly relable channel [10], [12]. Fgure 2. Packet delvery n UMTS
7 Wth multcast, the packets are forwarded to those ode Bs that have multcast users. Therefore, n Fgure 2, the ode Bs 2, 3, 5, 7, 8, 9 receve the multcast packets ssued by the BM-SC. We brefly summarze the fve steps occurred for the delvery of the multcast packets. Frstly, the BM-SC receves a multcast packet and forwards t to the GGS that has regstered to receve the multcast traffc. Then, the GGS receves the multcast packet and by queryng ts multcast routng lsts, t determnes whch SGSs have multcast users resdng n ther respectve servce areas. In Fgure 2, the GGS duplcates the multcast packet and forwards t to the SGS1 and the SGS2 [14]. Then, both destnaton SGSs receve the multcast packets and, havng quered ther multcast routng lsts, determne whch RCs are to receve the multcast packets. The destnaton RCs receve the multcast packet and send t to the ode Bs that have establshed the approprate rado bearers for the multcast applcaton. In Fgure 2, these are ode B2, B3, B5, B7, B8, B9. The multcast users receve the multcast packets on the approprate rado bearers, ether by pont-to-pont channels transmtted to ndvdual users separately or by common channels transmtted to all members n the cell [14]. IV. COST AALYSIS OF THE MBMS MULTICAST MODE A. General Assumptons We consder a subset of a UMTS network consstng of a sngle GGS and SGS SGS nodes connected to the GGS. Furthermore, each SGS manages a number of ra RAs. Each RA conssts of a number of rnc RC nodes, whle each RC node manages a number of ura URAs. Fnally, each URA conssts of nodeb cells. The total number of RAs, RCs, URAs and cells are: RA = SGS ra (1) RC = SGS ra rnc (2)
8 URA = SGS ra rnc ura (3) ODEB = SGS ra rnc ura nodeb (4) The total transmsson cost for packet delveres ncludng pagng s consdered as the performance metrc. Furthermore, the cost for pagng s dfferentated from the cost for packet delveres. We make a further dstncton between the processng costs at nodes and the transmsson costs on lnks, both for pagng and packet delveres. As presented n [7] and analyzed n [1], we assume that there s a cost assocated wth each lnk and each node of the network, both for pagng and packet delveres. For the analyss, we apply the followng notatons: D gs D sr D rb D DCH D FACH S sr S rb S a p gm p sm p rm p b a s a r a b Tx cost of packet delvery between GGS and SGS Tx cost of packet delvery between SGS and RC Tx cost of packet delvery between RC and ode B Tx cost of packet delvery over Uu wth DCHs Tx cost of packet delvery over Uu wth FACHs Tx cost of pagng between SGS and RC Tx cost of pagng between RC and ode B Tx cost of pagng over the ar Processng cost of multcast packet delvery at GGS Processng cost of multcast packet delvery at SGS Processng cost of multcast packet delvery at RC Processng cost of packet delvery at ode B Processng cost of pagng at SGS Processng cost of pagng at RC Processng cost of pagng at ode B The total number of the multcast UEs n the network s denoted by UE. For the cost analyss, we defne the total packets per multcast sesson as p. Snce network operators wll typcally deploy an IP backbone network between the GGS, SGS and RC, the lnks between these nodes wll consst of more than one hop. Addtonally, the dstance between the RC and ode B conssts of a sngle hop (l rb = 1). In the presented analyss we assume that the dstance between GGS and SGS s l gs hops, whle the dstance between the SGS and RC s l sr hops. We assume that the probablty that a UE s n PMM detached state s P DET, the probablty that a UE s n PMM dle/rrc dle state s P RA, the probablty that a UE
9 s n PMM connected/rrc URA connected state s P URA, and fnally the probablty that a UE s n PMM connected/rrc cell-connected state s P cell. In the remander of ths secton, we descrbe a method that models the multcast user dstrbuton n the network. In partcular, we present a probablstc method that calculates the number of multcast users n the network ( UE ), the number of SGSs that serve multcast users (n SGS ), the number of RCs that serve multcast users (n RC ) and fnally the number of ode Bs that serve multcast members (n ODEB ). As ntroduced n [3] and analyzed n [1], we classfy the RAs nto L RA categores n order to create an asymmetrc topology. For 1 L RA there are RAs of L RA ( ) = 1 class. Therefore, the total number of RAs wthn the network s = RA. Suppose that the dstrbuton of the multcast users among the classes of RAs follows the Posson dstrbuton wth λ = θ where 1 L RA. In general, the probablty that k exactly multcast users resde n the RAs of class s calculated from the followng equaton: θ ( θ ) e p( k, θ ) = (5) k! Thus, the probablty none of the RAs of class serves multcast users s p( 0, θ ) e = θ k, whch n turn means that the probablty at least one multcast user s served by the RAs of class s ( ) p = 1 p 0, θ = 1 e θ. RA Snce every class conssts of RAs, the total number of the RAs n the class, that serve multcast users s ( ) ( ) RA 1 e θ. Thus, the total number of the RAs of every class that serve multcast users s: LRA = 1 ( ) ( RA n = 1 e θ ) (6) RA
10 θ ( where RA ) ( represents the number of multcast users for the RA ) RAs of class. If there are n RA RAs that are servng multcast users, the probablty that an SGS does not have any such RA s: RA ra RA /, nra RA r a psgs = nra nra (7), othewse 0 Based on eqn (7), the total number of SGSs that are servng multcast users can be calculated as follows: n ( 1 p ) =. SGS SGS SGS The total number of multcast users n the network s: L RA = θ (8) UE = 1 where θ s the number of multcast users n a RA of class. As n [1], we assume that all RCs wthn a servce area of class have the same multcast populaton dstrbuton densty as n the RA case. Based on a unform densty dstrbuton wthn a sngle RA, the multcast populaton of an RC wthn the ( RC) servce area of a class RA s θ = θ rnc. The total number of RCs of class ( s RC ) =. rnc Assumng that the number of RA categores s equal to the number of RC categores (L RC =L RA ), the total number of RCs that serve multcast users s: LRC = 1 ( ) ( RC n = 1 e RC ( RC ) θ ) (9) The same are appled to the cells wthn the servce area of an RC. The average ( number of multcast users for a sngle cell of class s B ) ( RC) = ( ) θ θ b. ura node ( The number of ode Bs belongng to class s B ) ( RC) =. ura nodeb Assumng that the number of the RC categores s equal to the number of the ode
11 B categores (L RC =L ODEB ), the total number of ode Bs that serve multcast users s: ODEB LODEB = 1 ( ) ( ) ( B ) B θ 1 n = e (10) B. Cost Analyss of the Multcast Mode In the multcast scheme, the multcast group management s performed at the BM-SC, GGS, SGS and RC and multcast tunnels are establshed over the Gn and Iu nterfaces. It s obvous that the cost of a sngle packet delvery to a multcast user depends on ts MM and RRC state. If the multcast member s n PMM connected/rrc cell-connected state, then there s no need for any pagng procedure nether from the SGS nor from the servng RC. In ths case, the packet delvery cost s derved from eqn(11). It has to be mentoned that ths quantty does not nclude the cost for the transmsson of the packets over the Iub and Uu nterfaces snce ths cost depends frstly on the number of multcast users and secondly on the transport channel used for data transmsson. Ccell = pgm + Dgs + psm + Dsr + prm (11) If the multcast member s n PMM connected/rrc URA connected state, then the RC must frst page all the cells wthn the URA n whch moble users resde and then proceeds to the data transfer. After the subscrber receves the pagng message from the RC, t returns to the RC ts cell ID. The cost for pagng such a multcast member s: ( ) C = S + a + S + S + a + S + a (12) URA nodeb rb b a a b rb r If the multcast member s n PMM dle/rrc dle state, the SGS only stores the dentty of the RA n whch the user s located. Therefore, all cells n the RA must be paged. The cost for pagng such a multcast member s:
12 ( ) ( ) ( ) C = S + a + S + a + S RA rnc sr r rnc ura nodeb rb b a + S + a + S + a + S + a a b rb r sr s + (13) After the pagng procedure, the RC stores the locaton of any UE at a cell level. In multcast, the SGS and the RC forward a sngle copy of each multcast packet to those RCs or ode Bs respectvely that are servng multcast users. After the correct multcast packet recepton at the ode Bs that serve multcast users, the ode Bs transmt the multcast packets to the multcast users va common or dedcated transport channels. The total cost for the multcast scheme s derved from the followng equaton where n SGS, n RC, n ODEB represent the number of SGSs, RCs, ode Bs respectvely that serve multcast users. ( ) ( ) Ms = p n D p n D p Y + + + + + + P C + P C = D + D gm SGS gs sm RC sr rm p ( ) _ RA RA URA URA UE packet delvery pagng + (14) where ( ) ( ), nodeb Drb + pb + DFACH, f channel = FACH Y = UE Drb + pb + DDCH f channel = DCH ( ) ( ) ( ) D = p n D p n D p + + + + + Y D = P C + P C packet _ delvery gm SGS gs sm RC sr rm p pagng RA RA URA URA UE Parameter Y represents the multcast cost for the transmsson of the multcast data over the Iub and Uu nterfaces. Ths cost depends manly on the dstrbuton of the multcast group wthn the UMTS network and secondly on the transport channel that s used. D DCH, and D FACH represent the cost over the Uu nterface. More specfcally, D FACH represents the cost of usng a FACH channel to serve all the multcast users resdng n a specfc cell whle D DCH represents the cost of usng a sngle DCH to transmt the multcast data to a sngle multcast user of the network. Regardng the cost over the Iub nterface, n case we use the FACH as transport channel, each multcast packet send once over the Iub nterface and then the packet s transmtted to the UEs that served by the correspondng ode B. On the other hand, n
13 case we use DCHs for the transmsson of the multcast packets over the Iub each packet s replcated over the Iub as many tmes as the number of multcast users that the correspondng ode B serves. V. POWER COTROL I MBMS In ths secton, some mportant ssues regardng power control of the downlnk transport channels (DCH and FACH) are analyzed. Ths analyss s performed n order to determne, as wll be presented n the next secton, the exact values of parameters D and, appeared n eqn(14). It s remnded that the man factor DCH D FACH that determnes the MBMS transmsson cost over the Uu nterface s the amount of ode B s transmsson power that should be allocated when usng one of these transport channels. Power control s one of the most mportant aspects n MBMS due to the fact that ode B s transmsson power s a lmted resource and must be shared among all MBMS users n a cell. Power control s essental n order to mnmze the transmtted power, thus avodng unnecessary hgh power levels and elmnatng ntercell nterference. The man requrement s to make an effcent overall usage of the rado resources: ths makes the common channel, FACH, the favorte choce, snce many users can access the same resource at the same tme. However, other crucal factors such as the number of users belongng to the multcast group and ther dstance from the servng ode B, the type of servce provded and the QoS requrements (represented by Eb 0 targets) affect the choce of the most effcent transport channel n terms of power consumpton. On the pont-to-pont downlnk transmssons, where multple DCHs are used, fast power control s used to mantan the qualty of the each lnk and thus to provde a
14 relable connecton for the recever to obtan the data wth acceptable error rates. Transmttng wth just enough power to mantan the requred qualty for the lnk also ensures that there s mnmum nterference affectng the neghborng cells. Transmsson power allocated for all MBMS users n a cell that are served by multple DCHs s varable. It manly depends on the number of UEs, ther locaton n the cell (close to the ode B or at cell edge), the requred bt rate of the MBMS sesson and the experenced sgnal qualty Eb 0 for each user. Eqn(15) calculates the ode B s total transmsson power requred for the transmsson of the data to n users when multple DCHs are used[13]. P T = P P + 1 n = 1 n = 1 ( P + x) L W + p E ( b ) R b, 0 p W + p E ( b ) R b, 0 p, (15) where PT s the total transmsson power for all the DCH users n the cell, P s the L p, R b, power devoted to common control channels, refers to the path loss for user, the bt rate for user, W the bandwdth, P the background nose, p the orthogonalty factor (0:perfect orthogonalty) and ( E b 0) s the sgnal energy per bt dvded by nose spectral densty. Parameter x s the ntercell nterference observed by user gven as a functon of the transmtted power by the neghborng cells PTj, j =1, K and the path loss from ths user to the jth cell L j. More specfcally [13]: x P K = Tj j= 1 L (16) j On the contrary, n pont-to-multpont downlnk transmssons, a sngle FACH s
15 establshed and essentally transmts at a fxed power level snce fast power control s not supported n ths channel. A FACH channel must be receved by all UEs throughout the cell. Consequently, the fxed power should be hgh enough to ensure the requested QoS n the whole coverage area of the cell, rrespectve of the UEs locaton. FACH power effcency depends on maxmzng dversty as power resources are lmted. Dversty can be obtaned by the use of a longer TTI, e.g. 80ms nstead of 20ms, to provde tme dversty aganst fast fadng (fortunately, MBMS servces are not delay senstve) and the use of combnng transmssons from multple cells to obtan macro dversty [18]. Parameter Value Cellular layout Hexagonal grd umber of neghborng cells 18 Sectorzaton 3 sectors/cell Ste to ste dstance 1 Km Cell radus 0,577 Km Maxmum BS Tx power 20 W (43 dbm) Other BS Tx power 5 W (37 dbm) Common channel power 1 W (30 dbm) Propagaton model Okumura Hata Multpath channel Vehcular A (3km/h) Orthogonalty factor (0 : perfect orthogonalty) 0.5 Eb 0 target 5 db 4 W (32Kbps servce) FACH Tx power 7.6 W (64Kbps servce) (no STTD, 95% coverage) 15.8 W (128Kbps servce) Table 1. Macrocell smulaton assumptons Power aspects of MBMS are nvestgated separately for macro and mcro cell envronments. The amount of ntercell nterference s lower n mcrocells where street corners solate the cells more strctly than n macrocells. Moreover, n mcrocells there s less multpath propagaton, and thus a better orthogonalty of the downlnk codes. On the other hand, less multpath propagaton gves less multpath dversty, and therefore a hgher E 0 requrement n the downlnk n mcro than n macro b cells s assumed [10]. The basc smulaton parameters are presented n Table 1 and Table 2 [15], [16], [17] and [19].
16 Parameter Value Cellular layout Manhattan grd umber of cells 72 Block wdth : Road wdth : Buldng to buldng dstance 75m : 15m : 90m Straght lne dstance between transmtters 360m (4 blocks) Maxmum BS Tx power 2 W (33 dbm) Other BS Tx power 0.5W (27 dbm) Common channel power 0.1 W (20 dbm) Propagaton model Walfsh-Ikegam Multpath channel Pedestran A 3Km/h Orthogonalty factor (0 : perfect orthogonalty) 0.1 Eb 0 target 6 db FACH Tx power (no STTD, 95% coverage) 0.36 W (64Kbps servce) Table 2. Mcrocell smulaton assumptons VI. RESULTS A. Smulaton and evaluaton parameters In ths secton we present the evaluaton parameters regardng the MBMS multcast mode. We consder dfferent cell confguratons, dfferent user dstrbutons and fnally, dfferent transport channels for the transmsson of the multcast data over the UTRA nterfaces. Therefore, we assume a general network topology, wth SGS =10, ra =10, rnc =10, ura =5 and nodeb =5. The packet transmsson cost (D xx ) n any segment of the UMTS network depends on two parameters: the number of hops between the edge nodes of ths network segment and the capacty of the lnk of the network segment. Ths means that D gs = l gs /k gs, D sr =l sr /k sr and D rb =l rb /k rb. Parameter k xx represents the profle of the correspondng lnk between two UMTS network nodes. More specfcally, n the hgh capacty lnks at the C, the values of k xx are greater than the correspondng values n the low capacty lnks at UTRA. For the cost analyss and wthout loss of generalty, we assume that the dstance between the GGS and SGS s 8 hops, the dstance between SGS and RC s 4 hops and the dstance between RC and ode B s 1 hop. The above parameters as well as the values of the k xx are presented n detal n
17 Table 3. Regardng the transmsson cost of pagng (S xx ) n the segments of the UMTS network, t s calculated n a smlar way as the packet transmsson cost (D xx ). More specfcally, S xx s a fracton of the calculated transmsson cost (D xx ) and n our case we assume that t s three tmes smaller than D xx. Lnk Lnk Capacty factor (k) umber of hops (l) Transmsson cost (D) GGS-SGS k gs = 0.8 l gs = 8 D gs = 10 SGS-RC k sr = 0.7 l sr = 4 D sr = 4/0.7 RC ode B k rb = 0.5 l rb = 1 D rb = 2 Table 3. Chosen values for the calculaton of transmsson costs n the lnks As we can observe from the equatons of the prevous secton, the costs of the schemes depend on a number of other parameters. Thus, we have to estmate the value of these parameters. The chosen values of the parameters are presented n Table 4. S sr S rb S a p gm p sm p rm p b a s a r a b P RA P URA P cell 4/2.1 2/3 4/3 2 2 2 1 1 1 1 0.6 0.2 0.1 Table 4. Chosen parameters values At ths pont, we have to menton that snce the nodes that are responsble for the forwardng of the multcast packets are the GGS, SGS and the RC, we consder a lower packet processng cost n the ode B than the correspondng costs n the GGS, SGS and RC snce some overhead s needed n the above mentoned three nodes n order to mantan the routng lsts requred for the packet forwardng n the multcast scheme (Table 4). Furthermore, we have chosen approprately the probabltes P RA, P URA and P cell More specfcally, the probablty that a UE s n PMM dle/rrc dle state s P RA =0.6. The probablty that a UE s n PMM connected/rrc URA connected state s P URA =0.2 and the probablty that a UE s n PMM connected/rrc cell-connected state s P cell =0.1. Addtonally, there s a probablty that the UE s not reachable by the network and we consder t to be 0.1. Regardng the transmsson over the Iub and Uu, DCH and FACH channels are examned. Some mportant aspects concernng the power consumpton for these two
18 transport channels were presented analytcally n secton V. It s remnded that the amount of ode B s transmsson power that must be allocated for these two channels s the man parameter that defnes the transmsson cost over the ar (Uu nterface). Parameter D FACH represents the cost, over the Uu nterface, of usng a FACH channel to serve all the multcast users. Smlarly, parameter D DCH represents the cost, over the Uu nterface, of usng a sngle DCH channel to serve one multcast user. In our analyss, we calculate n each cell of the network topology the ode B s power n the case of usng DCHs or FACH. Then, by comparng these power values wth the total avalable ode B s transmsson power, we select the approprate values for the D DCH, and D FACH. Obvously the values D DCH, and D FACH are proportonal to the percentage of the ode B s transmsson power allocated to DCH or FACH n any cell. The D DCH, and D FACH values are then used n eqn(14) that calculates the total telecommuncaton cost of the MBMS multcast mode. Furthermore, we assume that the mnmum value that the total D DCH, per cell and the D FACH. could take s the value of 10 snce ths value s the cost of the data transmsson n the wred lnk between the GGS and the SGS (D gs ), and generally the transmsson cost n a wred lnk s assumed to be lower than the transmsson cost n a wreless lnk. It s true that the performance of the multcast scheme depends manly, on the confguraton of the UMTS network that s under nvestgaton. In our analyss, we assume that we have two classes of RAs. A class =1 RA has multcast user populaton of θ 1 = 1/δ and a class =2 RA has a multcast user populaton of θ 2 = δ. If δ >> 1, the class =1 RA has a small multcast user populaton and the class =2 RA has a large multcast user populaton. Let α be the proporton of the class =1 RAs and (1-α) be the proporton of the class =2 RAs [3]. Thus, the number of class =1 RAs s =α RA and the number of class =2 RAs s =(1-α) RA. Each RA of class 1 2
19 {1,2} s n turn subdvded nto rnc RCs of the same class and smlarly, each RC of class {1,2} s subdvded nto ura nodeb ode Bs of the same class. Takng nto consderaton the above mentoned parameters, eqn(8) can be transformed to eqn(17). It s obvous from eqn(17) that as α decreases and δ ncreases the number of multcast users ncreases rapdly. 2 α UE = θ = 1 θ1+ 2 θ2 = RA + δ αδ (17) = 1 δ B. Telecommuncatons cost of the data transmsson over the Uu nterface In ths secton, analytcal smulaton results, dstnctly for the cases of macro and mcro cell envronments are presented. ode B s transmsson power levels when usng DCH or FACH channels, for dfferent smulaton parameters are depcted n each one of the followng fgures. The am for ths parallel plottng s to determne the most effcent transport channel, n terms of power consumpton, for the transmsson of the MBMS data. In Fgure 3 the effect of UEs locaton (dstance from ode B) s presented. UEs are assumed to be n groups, located at the same dstance from ode B each tme. When multple DCHs are used, t s obvous that the further a UE s from the ode B the more power s requred for successful delvery of the MBMS servce, both for macro cell and mcro cell envronments. On the other hand, when a FACH channel s used, transmsson power s kept constant (rrespectve of UEs locaton) at a power level that s hgh enough to serve the UE wth the greater dstance from ode B. In Fgure 3, FACH Tx power s set to a value that provdes 95% coverage (cell edge). For smaller coverage areas FACH Tx power could be set n lower levels.
20 (a) (b) Fgure 3. Tx power vs. dstance (a) Macrocell, (b) Mcrocell Fgure 4 reflects the mpact of QoS requrements ( Eb 0 ) on ode B transmsson power. As expected, the hgher the Eb 0 parameter s the more power s requred when transmttng multcast data wth multple DCHs. (a) (b) Fgure 4. Tx power vs. Eb/o (a) Macrocell, (b) Mcrocell Fgure 5 depcts the mpact of the MBMS bt rate on ode B transmsson power. When multple DCHs are used, ncreased MBMS bt rates result to hgher ode B transmsson power. Smlarly, n the case of a FACH channel, more power needs to be transmtted when provdng hgher MBMS bt rates. FACH transmsson power levels for varous bt rates are depcted for a macro cell envronment (Fgure 5a), whle for a mcro cell envronment (Fgure 5b) we consder only a 64Kbps servce.
21 (a) (b) Fgure 5. Tx power vs. bt rate (a) Macrocell, (b) Mcrocell Another crucal factor that has to be taken nto account when selectng the most effcent transport channel s the transmsson power of neghbourng cells, expressed by the parameter PTj n eqn(16). Fgure 6 depcts the mpact of ths factor under the smplfyng assumpton that all neghbourng ode Bs transmt at the same power levels. Of course, t s rather mpossble that all neghbourng ode Bs transmt at the same power level, but ths s assumed here for better understandng of ths parameter s sgnfcant mpact. Hgher transmsson power of neghbourng ode Bs ncreases ntercell nterference, leadng n turn the examned ode B to ncrease ts transmsson power n order to meet the MBMS servce demands. (a) (b) Fgure 6. Tx power vs. neghborng cells Tx power (a) Macrocell, (b) Mcrocell From the above fgures, that present the cost of MBMS transmsson over the Uu nterface, useful nformaton about the swtchng pont between pont-to-pont
22 transmsson (multple DCHs) and pont-to-multpont transmsson (a sngle FACH) can be extracted. Actually, a power-based swtchng pont scheme can be employed n order to mnmze ode B s transmsson power, thus mnmzng the cost for the transmsson of the multcast data over the ar. The transport channel that requres less power resources s selected. For nstance, from Fgure 3a n the case of a macrocell, t can be seen that for a 64Kbps MBMS servce, Eb 0 target 5dB, 95% coverage and neghborng ode Bs transmttng at 5W an effcent swtchng pont should be 9 UEs. Furthermore, n the case of a mcrocell, for a 64Kbps MBMS servce, Eb 0 target 6dB, 95% coverage and neghborng ode Bs transmttng at 0.5W the swtchng pont should be 7 UEs (Fgure 3b). Ths means that, e.g. for a macro cell envronment, for 9 UEs and above a FACH should be used, whle for less than 9 UEs the use of multple DCHs s the most effcent choce. C. Total telecommuncaton cost In Fgure 7 the total costs for the multcast mode usng dfferent transport channels and cell envronments n functon of α are presented. From these plots we can see that the costs decrease as α ncreases. Ths occurs because as α ncreases the number of RAs wth no multcast users ncreases and hence the multcast users are located n a small number of RAs. (a) (b)
23 Fgure 7. Total cost n functon of α wth (a) δ=300, δ=3000 More specfcally, n Fgure 7a the cost n case we use multple DCHs s smaller than the cost n case we use a FACH channel both n macro and mcro envronments. Ths occurs because the small value of δ, results to a reduced number of UEs n the network and hence the DCH s more effcent for the data transmsson n terms of total cost. The opposte occurs n Fgure 7b where the value of δ s ncreased, whch means that the number of UEs s also ncreased. Therefore, the use of DCHs s neffcent for the transmsson of the data over the Iub and Uu nterfaces whle the FACH s the most sutable transport channel n terms of total cost. In Fgure 8, the total costs usng dfferent transport channels and cell envronments n functon of δ are presented. We choose a small value for the parameter α because the multcast mode becomes effcent when there s an ncreased densty of UEs n the network [2]. Therefore, a value of α=0.1 s chosen whch means that there are many RAs n the network wth a great number of multcast users n these. From Fgure 8, t s clear that as parameter δ ncreases (whch means that the number of multcast users ncreases), the total cost for all cases ncreases too. However, the ncrease n total cost for DCHs s greater than that of FACH due to the fact that a DCH s a pont-to-pont channel and strongly depends on the number of multcast users.
24 Fgure 8. Total cost n functon of δ, α=0.1 More specfcally, n Fgure 8, we observe that for small values of δ, the total cost usng DCHs s small because there s a small number of UEs n the network, whle for bgger values of δ, whch mples bgger number of UEs, the total cost usng DCHs overcomes the cost of usng FACH. Thus, for small values of δ the use of DCHs s more effcent whle for bgger values of δ, the use of FACH s more approprate. The smulaton parameters for DCH and FACH transport channels used for the plottng of Fgure 7 and Fgure 8 are taken from the prevous secton. More specfcally, an 64Kbps MBMS servce and 95% cell coverage are assumed. An mportant notce regardng the swtchng pont between pont-to-pont and pont-to-multpont transport channels should be mentoned at ths pont. From Fgure 8 the swtchng pont between multple DCHs and a sngle FACH, n terms of total transmsson cost, s 6 UEs (or δ=1500) for a macrocell and 3 UEs (or δ=750) for a mcrocell. However, from the prevous secton, when only the cost over the Uu nterface (ode Bs total transmsson power) was taken nto account, t was shown that for the same smulaton parameters, the swtchng pont should be 9 UEs for a macrocell and 7 UEs for a mcrocell. Consequently, t s obvous that a reducton n the swtchng pont levels s takng place when consderng total transmsson cost of
25 an MBMS sesson. Ths reducton s caused by the addtonal cost ntroduced by the Iub nterface, representng the transmsson cost of packet delvery between RC and ode B. Recall from eqn(14) that computes the total cost of the multcast scheme, that the parameter Y represents the multcast cost for the transmsson of the multcast data over the Iub and Uu nterfaces. When a FACH transport channel s used each multcast packet s sent once over the Iub, whle when multple DCHs are used each packet s replcated over the Iub as many tmes as the number of multcast users. The cost added from Iub s not neglgble and depends on the lnk capacty whch s, however, operator dependent. For the smulatons presented above, the lnk capacty factor was set to k rb = 0.5. For greater values of k rb, the swtchng ponts converge to the values presented n Fgure 3 - Fgure 6. From the above observaton, t s clear that the selecton of the approprate rado bearer for the multcast data transmsson strongly depends on the cost added by the Iub nterface. The ode B s transmsson power should not be the only crteron for the selecton of an effcent transport channel, but the total transmsson cost (ncludng the Iub cost) should always be taken nto account. VII. COCLUSIOS AD FUTURE WORK In ths paper we presented an overvew of the MBMS multcast mode of UMTS. We nvestgated the performance of the multcast mode of the MBMS n terms of packet delvery cost through an analytc theoretcal model and by smulatons based on ths model. The nvestgatons were made assumng varous network topologes, cell envronments and multcast users dstrbutons. In addton, we examned the DCH and FACH transport channels n terms of data transmsson cost over the Iub and Uu nterfaces. Fnally, we presented a scheme for the effcent selecton of a swtchng pont between pont-to-pont (multple DCHs) and pont-to-multpont (a sngle
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