ADAPTVE CALL ADMSSON CONTROL N TDD-CDMA CELLULAR WRELESS NETWORKS Dhananjay Kumar 1, Chellappan C 2 1 Department of nformaton Technology, Anna Unversty, Chenna 1 dhananjay@annaunv.edu 2 Department of Computer Scence & Engneerng, Anna Unversty, Chenna 2 drcc@annaunv.edu ABSTRACT The Code Dvson Multple Access system wth Tme Dvson Duplex mode (TDD-CDMA), adoptng unbalanced slot allocaton between uplnk and downlnk, can meet the asymmetrc traffc requrement of multmeda servces. Here a call admsson control polcy s proposed to support dfferent multmeda applcatons. The scheme operates at the connecton-level where the CDMA code of ongong call can be dynamcally changed to provde an acceptable trade off level between connecton blockng and droppng probabltes for dfferent traffc class. Although Orthogonal Varable Spreadng Factor (OVSF) code s used n smulaton here, the paramount nterest s on algorthm that allows optmum use of the TDD-CDMA resources.e. code and tme slots. Smulaton results shows that on the expense of resource allocated to non real-tme servces, the call droppng and blockng rate for hgh prorty (real-tme) servces can be mnmzed. Keywords: TDD-CDMA; nterference; Multmeda; Call Blockng/Droppng. 1 NTRODUCTON n future wreless networkng envronments, the data traffc for nternet, real-tme voce, and multmeda traffc wll coexst. For the multmeda applcaton such as streamng audo/vdeo or web servces, the downlnk traffc wll be the bottleneck of the system. On the other hand, the uplnk traffc may be bursty and rregular when moble users use the applcaton lke fle uploadng servces. The dynamc change of the traffc asymmetry between uplnk and downlnk makes the resource allocaton of the future wreless system dffcult. The code dvson multple access system wth tme dvson duplex mode (TDD- CDMA) s a promsng soluton to cope wth the traffc asymmetry problem [1, 2]. The TDD-CDMA mode of 3GPP, named UTRA- TDD (Also called UMTS-TDD), s based on TD- CDMA technology, whch s a mxture of TDMA and CDMA [3]. n Frequency Dvson Duplex (FDD) mode, a par of frequency band s chosen for uplnk and downlnk communcaton, but n TDD same frequency s used for both drectons (Fg.1). Although FDD s domnant canddate n W-CDMA (Orgnally developed by NTT DoCoMo, Japan), whch uses a par of 5 MHz-wde rado channels, the TDD mode s more popular n the case of mcro/pco Downstream Upstream Frequency Band Frequency Dvson Duplexng (FDD) Same Upstream & Downstream Frequency band Tme Dvson Duplexng (TDD) Fg.1 FDD and TDD mode of communcaton cell envronment because of ts capacty and flexblty to support asymmetrc traffc [4,5]. Even though TDD-CDMA s not drectly compatble wth UMTS (Whch s based on W- CDMA), t s closely related to W-CDMA, and UbCC Journal, Volume 4, Number 3, August 2009 834
provdes the same types of channels where possble. n ths paper, we nvestgate and analyze an adaptve algorthm for resource allocaton n TDD-CDMA system whch s one of the canddates for multple access technque for the fourth generaton (4G) systems [2, 6]. The resource of TDD-CDMA s dvded n both tme and code doman. n a cell of a TDD-CDMA system, the base staton and all mobles occupy a sngle band employng a drect sequence spreadng waveform. The bdrectonal communcaton between base and mobles s accomplshed by a TDD scheme [7]. Each slot n TDD carres traffc for dfferent moble staton separated by CDMA code. Fgure2 shows an example of the TDD frame structure. A TDD frame conssts of a fxed number of tme slots. At least one slot s dedcated to uplnk (from moble to base) and at least another one slot s dedcated to downlnk (from base to moble). The number of uplnk and downlnk slot can be adaptvely controlled by the base staton. DL DL DL UL DL DL DL UL TDD Frame Fg.2 Uplnk and downlnk n a TDD frame n ths paper, we characterze servces nto two types: adaptve, and non-adaptve. n the case of a non-adaptve servce, the bandwdth of a call s fxed throughout ts lfetme and needs strct bandwdth guarantees, else the call wll be dropped. But n the case of an adaptve servce, the call wll not be dropped, but wll suffer bandwdth degradaton. Many real-tme multmeda servces are adaptve n nature and can operate over a wde range of bandwdth. Adaptve Code Allocaton (ACA) algorthm s called to support the bandwdth requrement of a new hgh prorty calls n the system. The ACA algorthm decreases assgned bandwdth of ongong connectons n the cell dependng on the network load stuaton, by allocatng dynamc OVSF code. N. Nasser and H. Hassanen [8,9], has proposed an adaptve framework to support multmeda applcatons, but not wth respect to any exstng systems. Further t does not consder the multple access technques whch nherently govern the resource allocaton & hence call control mechansm. N. Nasser agan n a smlar paper [10] talks about adaptablty enhancement framework, but once agan wthout consderng under layng technques. Zhhua Zheng [11] has proposed an effcent dynamc channel assgnment wth channel locked for TDD- CDMA communcaton consderng three sectored cell. oanns Spyropoulos et al.[1] has proposed a UL DL decentralzed scheme for TDD-CDMA systems, whch combnes an nterference-aware dynamc channel allocaton algorthm wth space tme lnear mnmum-mean-square-error (LMMSE) jont detecton at the base and moble statons. Ther analyss ncludes outage and average throughput va analytcal approxmatons. The proposed algorthm here, ams to support multmeda calls whle optmzng the resource allocaton n TDD-CDMA system, thereby reducng both the blockng and droppng rates of real-tme multmeda calls. The proposed Call Admsson Control (CAC) algorthm s capable of offerng servces even when there s nsuffcent number of codes avalable by ntellgently swappng the OVSF codes of ongong calls. Ths results n reduced volume of the blockng and droppng rates; thus strkng a proper balance between Qualty of Servce (QoS) fulflment and code/bandwdth utlzaton. Ths paper s organzed as follows. n Secton2 nterference pattern for TDD-CDMA system s presented. An ntroducton to OVSF and CAC algorthm s explaned n Secton3. The smulaton envronment for the proposed adaptve algorthm s presented n Secton4. Smulated result s dscussed n Secton5. We conclude wth an overvew of the smulaton carred out n Secton6. 2 NTERFERENCE PATTERN N TDD-CDMA n TDD-CDMA each slot wll carry many user data on dfferent channel separated and dentfed by pseudo nose (PN) code. Consderng ntra-cell and nter-cell nterference separately n a mult-cell envronment, the bt energy to nose rato can be modelled as Pr. SF Eb / N = 0 nt + ext + N 0. W (1) Where P r s the receved power, SF s the spreadng factor, nt s the nternal nose wthn the cell, ext s external nose comng from other cells, s the nose power spectral densty, and W s the total transmsson bandwdth. 2.1 nterference n Uplnk channels Suppose m k be the number of MS served by a channel, where k = 1,2,3,..K, represent the k th channel to support K type of servces n a cell. Let P k denote the transmt power of th MS to mantan certan qualty of servce (QoS) for a k th servce, and G k the gan between th MS and t s BS. The nternal nterference nt n uplnk for k th channel carryng data of th MS may be gven by nt = K m k k = 1 = 1 k G k P k (2) UbCC Journal, Volume 4, Number 3, August 2009 835
A mult-user detecton (MUD) factor (1-β), where β s MUD effcency, s multpled wth the external nterference nt to acheve 3GPP requrements for CDMA-TDD [12]. n other words, β s an nterference reducton factor. For example, MUD n uplnk, β = l s a case of deal MUD, whle β = 0 represents absence of nterference cancellaton technque and hence employng a rake flter. To compute external nterference ext, let m l be the number of MS served on l th channel, where l = 1,2,..L represents L channels n the neghbour cell j supportng L type of multmeda applcaton. w ext may be expressed as ext = J L m l j = 1 l = 1 = 1 G P (3) Where J s the number of nterferng neghbour cells, G the lnk gan between th MS n neghbour cell and the tagged BS, and P the transmt power of th MS to support ts QoS requrement n ts cell. 2.2 nterference n Downlnk Channels Data n downlnk channels (for example n W- CDMA) are transmtted wth orthogonal codes; n other word, they are coded such that mutual nterference s mnmal. Assumng perfect tme synchronzaton between MS and BS, and f the type of channel s flat fadng.e. f the orthogonalty s preserved durng downlnk slot, then the nternal nose nt s absent. But the multpath propagaton destroys some of ths orthogonalty n downlnk. An orthogonalty factor (α) whch s the percentage of downlnk orthogonalty remanng at the moble recever, s ntroduce to compute nt. w, the nternal nterference nt arsng due to nonorthogonalty of the receved sgnals s gven by L = l = 1 G P nt α l lt (4) Where P lt s the total base staton power allocated to sgnals usng the same scramblng code for l th channel, G l thelnk gan between th MS and tagged BS for the same l th channel. To compute the external nterference ext, we take the advantages of smlarty wth (3) and can be represented as ext = J L m l j = 1 l = 1 = 1 G P (5) Where G s the lnk gan between th tagged MS and a MS n neghbor J th cell, m l the number of MS n a j th cell, and P the transmt power of a MS n cell J. 3 OVSF CODE AND CAC ALGORTHM The use of OVSF codes (Fg.3) to support wde varety of multmeda calls has been wdely advocated [13,14]. Dfferent spreadng factor (SF) means dfferent code length. The requrement s to combne dfferent messages wth dfferent spreadng factors and keep the orthogonalty between them. We therefore need codes of dfferent length that are stll orthogonal. t s assumed that voce traffc wll requre constant bandwdth but other components of multmeda lke mages, audo/vdeo streamng wll demand hgher varable data rate support. So, the resource pool mantans OVSF code representng dfferent data rate R, 2R, 3R. correspondng to SF = 1, 2, 3,. 3.1 CAC Algorthm When a new or a handoff call of a class arrves n a cell, the base staton (BS) calls the CAC algorthm (Fg.4). CAC selects a code form resource correspondng to the types servce requested. After allocatng a code, the E b / s computed for the current slot. f SNR falls below the threshold (γ m ), next code wth hgher spreadng factor s selected. f no hgher SF codes are avalable n resource pool, and f the call has hgher prorty, then ACA s called. The ACA fnds whether an exstng low prorty non-real tme call can be bandwdth degraded, and hence the exstng code s swapped wth a low SF code. On falure of that, the subsequent slot s declare the same status, and the new call s accommodated n the newly declared slot. f the subsequent slot cannot be declared same (UL or DL) because of the exstng traffc pattern, then the new call s rejected. A new call may be a handoff calls whch have hgher prorty than the new calls orgnatng from moble user. 4 SMULATON ENVRONMENT Smulaton envronment to represent TDD-CDMA was mplemented usng Network Smulator-2 (NS- 2). A Unversal Moble Telecommuncatons System (UMTS) patch was ncorporated n exstng NS-2 to create the cellular envronment. A sngle cell envronment wth a BS and rado network controller (RNC) n UMTS s created, but nterference pattern were mplemented correspondng to the mult-cell scenaro. As per UMTS specfcaton n a frame length of 10ms, 15 tme slots were consdered. n the smulaton, 150 user equpment (UE) were created, allowng them to move randomly across the cell and make random request for calls accordng to Posson rate. To communcate wth the BS, UE uses dedcated channels assgned to them. UbCC Journal, Volume 4, Number 3, August 2009 836
SF = 1 SF = 2 SF = 4 SF = 8 C 2,1 = (1, 1, 1, 1 ) C 2,1 = (1, 1) C 2,1 = (1, 1, 1, 1 ) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1,1 = (1) C 2,1 = (1, 1, 1, 1 ) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 2,1 = (1, 1) C 2,1 = (1, 1, 1, 1 ) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) C 1 = (1, 1, 1, 1, 1, 1, 1, 1) Fg.3 Orthogonal varable spreadng factor code Followng are the major functons mplemented n NS-2. 4.1 Request for Connecton by UE New call arrval rate s assumed to follow Posson Process wth rates λ..e. λ calls per second. n the smulaton, λ = 1 call/sec to 16 calls/sec were consdered and each runs for about 10 seconds. A UE sends request for call connecton to BS by sendng a request packet. The request packet contans the class type, call type and lnk requred by the call. 4.2 Processng Request by BS The BS receves random call request from the users accordng to Posson dstrbuton. Ths call request may be for a new call or a handoff call. The BS on recevng request packet for a new call, try to allocate avalable code (bandwdth) effcently among users, whch s done by callng the CAC algorthm. Also, the BS mantans detals of currently ongong calls for each class n lnked lsts. 4.3 Termnaton of Connecton by UE We have also consdered call holdng tme, wth an upper bound, whch s the watng tme between the perod, when the call gets connected and the resource gets allocated. Once the tmer correspondng to call holdng expres, a termnate packet s sent to BS. On recevng ths packet BS calls the termnate procedure to free the resources allocated for that call. 4.4 Recordng by BS For each λ value for call arrvng, the smulaton s run for 10 seconds and the BS mantans the number of calls blocked and dropped n the process. These wll be recorded after the entre smulaton and wll be used for plottng graphs. 4.5 Segregaton of Calls We consder two types of calls, namely class1 and class2. Tme nsenstve calls are consdered n class1, e.g. Web Browsng, Data upload/download, emal servces etc. Tme senstve calls are consdered n class2, e.g. Audo/Vdeo streamng, Telephonc etc. Handoff calls consdered here falls under class2.e. t has hgher prorty than class1 servce. UbCC Journal, Volume 4, Number 3, August 2009 837
Start Select a code correspondng to the BW requrement Estmate SNR n the current slot s SNR > γ m Choose a code wth next hgher SF Allocate the code for th call Update code resource pool s SF > A m Pck up next unallocated call s t class2 call Call ACA procedure All call accommodate Fnd f t can be accommodated n next slot Code avalable End Next slot Declare next slot n same drecton UL/DL Reject th call Fg.4 Flow chart of CAC algorthm 5 RESULTS The record created by BS after the smulaton s used to plot connecton droppng and blockng rate. 5.1 Call Droppng Fg.5 shows call droppng rate vs. call arrval rate (Posson Rate). As expected, as the call arrval rate ncreases the call droppng rate ncreases. Also the call droppng rate for class1 s greater than class2. Ths s because class2 has hgher prorty than class1. We also note that, for a call arrval rate λ = 6, for both class1 and class2, the call droppng rate s less than 0.5, n partcular for class2, t s less than 0.2. UbCC Journal, Volume 4, Number 3, August 2009 838
5.2 Call Blockng The plot n Fgure6 corresponds to call blockng rate vs. call arrval rate (Posson Rate). Call blockng rate gradually ncreases as arrval rate ncreases, and for Class2 that ncludes handoff calls has hgher prorty, blockng rate s low. Although for a call arrval rate λ = 6, call blockng rate s around 0.55 for class 1, t s less than 0.3 for class 2. Fg.5 Call droppng rate vs. arrval rate 6 CONCLUSON Fg.6 Call blockng vs. arrval rate A call admsson control algorthm for multmeda call s mplemented n TDD-CDMA system. The algorthm consders three parameters namely SNR, avalable OVSF code, and tme slot. The framework acheves a better connecton droppng and blockng rate for hgher prorty calls on the expense of nonreal tme lower prorty calls. Class2 calls nclude handoff calls whch are of hgher prorty n any systems. Smulaton result shows that connecton droppng rate for class2 s less than 0.2 for an arrval rate λ = 6. For the same parameter (λ = 6) although connecton blockng rate for class 1 s more than 0.5, for class2, t s below 0.3 Although we have categorzed only two classes of calls, there could be more than two, and exhaustve smulaton needs to be carred out to study the connecton droppng and blockng rate separately. Durng system operaton, and because of the statstcal nature of the arrval and departure processes, the occuped codes wll be randomly scattered across the code tree, so countermeasures need to be taken. 7 REFERENCES [1] oanns Spyropoulos, and James R. Zedler: Supportng Asymmetrc Traffc n a TDD/CDMA Cellular Network va nterference-aware Dynamc Channel Allocaton and Space Tme LMMSE Jont Detecton, EEE Transacton on Vehcular Technology Vol.58,.2, February (2009). [2] Mng Yang, and Peter Han Joo Chong: Uplnk Capacty Analyss for Multhop TDD-CDMA Cellular System, EEE Transacton on Communcaton Vol.57,.2, February (2009). [3] 3GPP Techncal Specfcaton for CDMA- TDD:TS25.221, TS25.222, TS25.223, and TS25.224. http://www.3gpp.org. [4] Overvew of The Unversal Moble Telecommuncaton System, http://www.umtsworld.com/technology/overve w.htm. [5] Maryam Arabshah and Peter Han Joo Chong: Hgh-Speed Multmeda Servces for TDD- CDMA Multhop Cellular Networks, nternatonal Conference on Wreless Communcatons and Moble Computng, 2008, EEE xplore (2008). [6] R. Esmalzadeh, M. Nakagawa, and Alan Jones: TDD-CDMA for the 4th Generaton of wreless communcatons, EEE Wreless Communcaton Magazne, pp.8-15, August (2003). [7] K-Dong Lee; Y, B.K.; Kun-Nyeong Chang; Leung, V: nformaton Exchange Based Low- Complexty Slot Allocaton n TDD-CDMA Cellular Systems, nternatonal Conference on Wreless Communcatons and Moble Computng, 2008, EEE xplore (2008). [8] N. Nasser and H. Hassanen: Connecton-level Performance Analyss for Adaptve Bandwdth Allocaton n Multmeda Wreless Cellular Networks, Proceedngs of the EEE nternatonal Performance Computng and Communcatons Conference (PCCC)-2004, Phoenx, Arzona, pp. 61-68 (2004). [9] N. Nasser and H. Hassanen: Prortzed Multclass Adaptve Framework for Multmeda Wreless Networks, Proceedngs of the EEE nternatonal Conference on Communcatons (CC)-2004, Pars, France, Vol. 7, pp. 4295-4300 (2004). UbCC Journal, Volume 4, Number 3, August 2009 839
[10] Ndal Nasser: Adaptablty Enhanced Framework for Provsonng Connecton-level QoS n Multmeda Wreless Networks, EEE and FP nternatonal Conference on Wreless and Optcal Communcatons Networks (WOCN)-2005, Duba, UAE, pp. 275-279 (2005). [11] Zhhua Zheng: An Effcent Dynamc Channel Assgnment wth Channel Locked for TDD- CDMA Systems, 4th EEE nternatonal Conference on Crcuts and Systems for Communcatons, May 2008. CCSC (2008). [12] T. Ojanpera, R. Prasad: Wdeband CDMA for Thrd Generaton Moble Communcaton, Artech House Publcaton (1998). [13] Sun-Ho Lee and Dong-Ho Cho: OVSF Code Assgnment Method Consderng Traffc charactersrcs n W-CDMA Systems, http://whtepapers.zdnet.co.uk/0,1000000651,2 60235555p,00.htm. [14] Angelos N. Rouskas and Dmtros N. Skoutas: OVSF Codes Assgnemnt and Reassgnement at the Forward Lnk of W-CDMA 3G Systems, EEE PMRC (2002). UbCC Journal, Volume 4, Number 3, August 2009 840