Device-to-Device (D2D) Communication in Cellular Network - Performance Analysis of Optimum and Practical Communication Mode Selection

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1 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. Devce-to-Devce (D2D) Communcaton n Cellular Network - Performance Analyss of Optmum and Practcal Communcaton Mode Selecton Sam Hakola, Tao Chen, Janne Lehtomäk and Tmo Koskela Noka, P.O. Box 407, Noka Group, Fnland sam.hakola@noka.com tao.t.chen@noka.com Centre for Wreless Communcatons, Unversty of Oulu, anne.lehtomak@ee.oulu.f tmo.k.koskela@noka.com Abstract In a cellular network system one way to ncrease ts capacty s to allow drect communcaton between closely located user devces when they are communcatng wth each other nstead of conveyng data from one devce to the other va the rado and core network. The problem s then when the network shall assgn drect communcaton mode and when not. In prevous works the decson has been done ndvdually per communcatng devce par not takng nto account other devces and the current state of the network. We derve means for gettng optmal communcaton mode for all devces n the system n terms of system equatons. The system equatons capture nformaton of the network such as lnk gans, nose levels, sgnalto-nterference-and-nose-ratos, etc., as well as communcaton mode selecton for the devces. Usng the derved equatons performance bounds for the cellular system where D2D communcaton s an addtonal communcaton mode are llustrated va smulatons. Further, practcal communcaton mode selecton algorthms are used to evaluate ther system performance aganst the achevable bounds. Analyss show the usablty of the system equatons and the potental of havng D2D operaton ntegrated nto a cellular system when there s enough local communcaton occurrng. I. INTRODUCTION In a cellular network user data s transmtted va base staton (BS) or other central network element when one user devce s communcatng wth another user devce. When communcatng devces are relatvely close to each other t could be sensble to have a drect communcaton lnk nstead of conveyng data va base staton n order to save transmsson power of both user devces and base statons, rado access as well as core network resources as dscussed n [1]. The drect communcaton mode requres half of the resources compared to cellular communcaton mode 1 thus offerng double spectral effcency per connecton typcally. Also f devces n drect communcaton mode are close to each other transmsson power levels could be lower than n cellular mode whch can be then turned nto battery savngs at the devce and reduced nterference levels n the system. Further, reduced 1 Cellular communcaton mode means here a conventonal rado access where a user devce s havng a communcaton lnks towards and from a BS nterference levels n the system lead to hgher system capacty and spectrum effcency. On the other hand, there are lots of possble practcal lmtatons for D2D such as: 1) probablty of local communcaton mght be low, 2) propagaton channel between devces s worse than propagaton channel between a BS and devces (manly due to dfference n antenna heghts and antenna gans) and 3) nterference to and from n-band cellular mode users. Probablty of local communcaton of all communcaton n the system should be relatvely hgh to have a real reason for drect communcaton. Thus servces and use cases that would provde possblty for D2D communcaton have hgh mportance. Fnally, n a system where there are both cellular communcaton mode and D2D mode users accessng the same spectrum, the nterference stuatons are dfferent compared to a conventonal cellular system. Hseh et al. show that usng the peer-to-peer network model n cellular wreless data networks s a promsng approach when the network model s complemented wth approprate mechansms [4]. They propose a hybrd network model n whch the am s to combne the throughput and power consumpton advantages of the peer-to-peer model wth the farness, reslence and moblty advantages of the cellular model. [5] Haas et al. provded system analyss results of frequencydvson duplexng (FDD) wdeband code dvson multple access (WCDMA) cellular network where D2D communcaton was enabled on uplnk spectrum [2]. They concluded that permttng D2D communcaton n tme-dvson duplexng (TDD) mode at the cell edge results n a 1.5 tmes capacty mprovement over the scenaro where only FDD cellular mode operaton s permtted. However the decson whether the drect or cellular communcaton mode s selected for the user s based on comparng the path loss between devce and BS to a pre-defned path loss. If the pre-defned path loss s smaller, the drect mode s selected. Thus mode selecton wasn t optmal n ther studes because communcaton mode selecton was made ndvdually for one par at the tme wthout takng nto account other users sharng the same /10/$ IEEE

2 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. spectrum. Performance analyss of spatal reuse of rado resources n a pont-to-pont (P2P) enabled TDD CDMA network has been provded n [3]. Authors dstrbute mobles n the cell randomly and unformly wth a hgh enough densty to guarantee that only the termnals wth most favorable locatons are accessed. In other words, they am to reuse the same rado resource n a sngle cell to the greatest extent by optmzng user locaton. However, that approach may not provde general results because closest devces may not have any traffc between each other. II. FRAMEWORK A general concept comprses a cellular network n whch user devces can communcate wth each other, ether va means of conventonal cellular mode or va means of drect D2D communcaton. In other words, a D2D feature s ntegrated nto the cellular network operaton so that the rado resources of the cellular network are used for both communcaton modes and are controlled by the BSs or other network elements. It s assumed that the cellular network s operatng n FDD duplex mode and D2D connectons are utlzng TDD duplex mode ether usng cellular network uplnk, downlnk (DL) or uplnk and downlnk resources controlled by the base staton(s). The general concept coverng FDD or TDD cellular network where drect communcaton connecton utlze ether FDD or TDD can be found from [7] and another reference of havng P2P communcaton establshed n a cellular network (especally n TDD-CDMA/TD-SCDMA) can be found from [8]. In addton authors n [9] propose to use FDD uplnk for D2D operaton n UMTS rado access network (UTRAN) to release resources on downlnk for other purpose. In ths study, D2D communcaton uses uplnk resources of a cellular network. TDD duplex mode for D2D communcaton s selected because of ts n-bult effcency for asymmetrc data transmsson between peers. And snce the cellular network s n FDD mode t s possble to utlze dynamc UL/DL swtchng patterns n D2D connectons f the nterference can be controlled. Also f when FDD UL resources are utlzed for D2D communcaton only UL recever functonalty has to be added to the moble devce nstead of both DL transmtter and UL recever functonaltes n the case f D2D communcaton was also n FDD mode. Recently the smlar settng of usng uplnk band for drect communcaton n cellular networks has been used n [10] and [11]. As a reference system n ths study we use WCDMA system where channels may not be orthogonal 2 and nterference experenced by recevers n a cell comprse ntra- and nter-cell nterference. III. JOINT COMMUNICATION MODE AND POWER ALLOCATION In [6] Yates et al. consder the combned problem of regulatng transmtter powers and assgnng users to base statons n conventonal cellular communcaton mode. As a result, they formulate the mnmum transmtted power problem n whch total transmtted uplnk power s mnmzed subect to mantanng an ndvdual carrer-to-nterference (CIR) for each moble. The mnmzaton occurs over the set of power vectors and base staton assgnments. In ths secton we extend those system equatons to nclude possblty for D2D mode communcaton n TDD duplexty on the same rado resources as cellular mode operaton on uplnk usng FDD duplexty to fnd optmum communcaton mode and transmsson power for each user devce ontly. A. Background In the analyss we use WCDMA system where all the users are sharng the same frequency resource and are separated n cellular mode (both n UL and DL) and D2D mode by nonorthogonal scramblng codes. In DL frequency band there are only users n cellular mode reservng resources f not control sgnallng from BS to D2D users s not taken nto account. In UL frequency band there are both cellular and D2D mode users sharng the same frequency resource. B. Generaton of System Equatons 2 In WCDMA downlnk based on UMTS standard physcal channels are orthogonal n nature but channel dsperson makes channels non-orthogonal. In uplnk user devces are separated by non-orthogonal scramblng codes. Fg. 1. Lnk gans between each devce and BS, and between devces.

3 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. Let s assume that we have a cellular system where there are both cellular communcaton mode and D2D mode devces sharng the rado resources of the UL band. Fg. 1 depcts lnk gans 3 between devces where devces 1 and 4 are both havng a cellular communcaton lnk and devce 2 and 3 are havng a D2D communcaton lnk wth each other. In addton, the fgure llustrates the occason when devce 3 s transmttng to devce 2 n D2D mode. Let s assume that we have altogether L devces n the system. Further, we defne a local communcaton probablty as p local (D2D canddates). Then there are floor (p local L) locally communcatng devces and cel (floor(p local L)/2) canddate D2D pars. For the dervaton of the new system equatons the followng symbols are used:, User devce ndces wth values 1, L m A communcaton mode selecton vector, of sze L 1, for devces (m =1f devce s n cellular mode and m =0f devce s n D2D mode) a (t) A TDD actvty vector (used only when m =0), where for the odd tme slot t (D2D usng TDD) a (t) = ( ) T and ( for the even ) tme slot t a (t) = T β c β d g c g d b α,b γ, n b n u A target sgnal-to-nterference-and-noser rato (SINR) for cellular communcaton mode physcal rado lnk A target SINR for D2D communcaton mode physcal rado lnk A processng gan for cellular communcaton mode transmsson A processng gan for D2D communcaton mode transmssson A BS assgnment vector,.e. b ndcates the BS to whch the user devce s connected A lnk gan between a user devce and a BS b A lnk gan between user devces and Nose power over the bandwdth at the recever of the BS ncludng both thermal noser power and the recever s nose fgure Nose power over the bandwdth at the recever of the user devce ncludng both thermal noser power and the recever s nose fgure A vector of transmsson powers of user devces where tem gves a transmsson power of a devce n a tme slot t Accordng to Fg. 1 at the odd tme slot t SINR for Devce 1 s targeted to be greater than or equal to β c as follows g c α 1,b1 m 1 1 α 2,b1 m α 4,b1 (1 m 4 )a (t) c (1) 4 p(t) 4 + n b 3 A lnk gan comprses a shadowed path loss attenuaton, and transmtter and recever antenna gans. when transmsson powers and communcaton modes are selected properly. Smlarly SINR for Devce 2 s g c α 2,b2 m 2 2 α 1,b2 m α 4,b2 (1 m 4 )a (t) c (2) 4 p(t) 4 + n b and SINR for Devce 4 (n D2D connecton mode) s targeted to be greater than or equal to β d as follows g d γ 4,3 (1 m 4 )a (t) 4 p(t) 4 γ 1,3 m γ 2,3 m 2 d (3) 2 + n u At the even tme slot t SINR requrement for Devce 1 s g c α 1,b1 m 1 1 α 2,b1 m α 3,b1 (1 m 3 )a (t) c (4) 3 p(t) 3 + n b SINR requrement for Devce 2 s g c α 2,b2 m 2 2 α 1,b2 m α 3,b2 (1 m 3 )a (t) c (5) 3 p(t) 3 + n b and SINR requrement for Devce 3 (n D2D connecton mode) g d γ 3,4 (1 m 3 )a (t) 3 p(t) 3 γ 1,4 m γ 2,4 m 2 d (6) 2 + n u By gnorng greater than condton n SINR requrements and solvng n (1)-(6) and combnng those equatons together we can derve the followng formulae for to have opton for each user devce as follows: = m =1 = + L =1 = + β c n b +(1 m ) a (t) =1 = + L =1 = + β d n u g d γ,+1 β c α,b m (1 m ) a (t) β c α,b β d γ,+1 m G d γ,+1 (1 m ) a (t) β d γ,+1 g d γ,+1 (7)

4 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. for odd t and = m =1 = + L =1 = + β c n b +(1 m ) a (t) =1 = + L =1 = + β d n u g d γ, 1 β c α,b m (1 m ) a (t) β c α,b β d γ, 1 m G d γ, 1 (1 m ) a (t) β d γ, 1 g d γ, 1 for even t. From (7) and (8) we can make a matrx form as follows: M P (t) = (ΦM +ΦA (t) ΦMA (t)) +A (t) (I M) (ΘM P(t) +ΘA (t) ΘMA (t)) (9) +MCN b + A (t) (I M) DN u where dagonal matrx C, of sze L L, has dagonal elements C = β c /( gc α,b() ), D, ofszel L, has dagonal elements D =[β d /(g d γ,+1 ) β d /(g d γ, 1 )... β d /(g d γ L 1,L ) β d /(g d γ L,L 1 )], (8) (10) dagonal matrx A (t) has dagonal elements formed by a (t), dagonal matrx M formed from mode selecton vector of devces m, matrxφ, ofszel L, defnes gans aganst nterference from other devces when devce n queston s n cellular communcaton mode and t has been formulated as follows: Φ = 0 = β c α,b (11) matrx Θ, ofszel L, defnes gans aganst nterference from other devces when devce n queston s n D2D communcaton mode and t has been formulated as follows: 0 = β d γ,+1, s odd Θ = g d γ,+1 β d γ, 1 g d γ, 1, s even, (12) vector N b, of sze L 1, s defned as follows: N b = [ nb n b n b ] T, vector Nu,ofszeL 1, s defned as follows: N u = [ n u n u n u ] T, and A (t) s a dagonal matrx formed from a vector a (t). For P (t) we have to have a non-negatve vector where the element can be zero only f m =0and a =0. Otherwse the element n power vector has to be greater than zero. Equaton (9) can also be formulated lke n [12] or where the nterference matrx s P (t) = I (t) P (t) + ℵ (t) (13) (I I (t) )P (t) = ℵ (t) (14) I (t) = M(ΦM + ΦA (t) ΦMA (t) ) +A (t) (I M)(ΘM + ΘA (t) ΘMA (t) ) and the nose vector (15) ℵ = MCN b + A (t) (I M)N u (16) The set of feasble power vectors under mode selecton m s { } P (t) = P (t) 0 P (t) IP (t) + ℵ (17) The set P (t) descrbes a cone of feasble powers n that f p P (t) then αp P (t) for all α 1. Each cone P (t) s specfed by an nterference matrx I and nose vector ℵ. The non-negatve nose vector dsplaces these cones from the orgn. [6] On the other hand, one of the devces of D2D par has power equal to zero due TDD duplexty dependng on the on-gong tme slot. Snce elements of M,Φ,Θ and A are n the range [0, 1], s non-negatve. Thus Perron-Frobenus theory guarantees the exstence of a domnant, postve egenvalue r [13]. Equaton (18) has a postve soluton f and only f r<1. Ths s referred later as egenvalue crtera. If r<1 then there s a unque soluton P (t) gven by ( P (t) = I I (t)) 1 ℵ (t) (18) C. Utlzaton of System Equatons Usng (15) we can fnd the optmal mode selecton vector m for example va mnmzng the frst norm of power vector combned from two consecutve tme slots arg mn P (t 1) + P (t) 1 (19) M subect to P (t) = ( I I (t)) 1 ℵ (t) =0:m =0, a (t) =0 > 0:m =1ORm =0, a (t) =1 Further one can fnd a probablty for havng a domnant postve egenvalue of the nterference matrx (14) that s less than unty. Then we can analyse system capactes as a functon of system outage when D2D s allowed usng an optmal mode selecton and compare those results to the pure cellular mode system and practcal mode selecton algorthms.

5 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. Parameter Cellular layout System area TABLE I SIMULATION PARAMETERS Value Isolated cell, 1-sector User devces are dstrbuted wthn a range of 500 m from the BS Path loss model for cellular lnk log10(d[km]) [14] Path loss model for D2D lnk log10(d[km]) [14] Shadow fadng standard devaton Nose spectral densty System bandwdth Nose fgure Mnmum couplng loss Antenna gans and patterns (transmtter and recever) 10 db for cellular mode lnks and 12 db for D2D mode lnks [14] -174 dbm/hz 5 MHz 5 db at BS / 9 db at devce 70 db BS-Devce / 40 db Devce- Devce BS: 14 db Devce: Omndrectonal 0 db Processng gan Cellular 30 / D2D 15 Probablty of local communcaton 0.2, 0.4 per devce Maxmum dstance between locally 5, 50 [m] communcatng devces IV. PERFORMANCE ANALYSIS In ths secton system level smulatons are carred out usng prevously generated system equatons to fnd out optmum and practcal capactes of the system when D2D s allowed. The results are compared to pure cellular communcaton mode case. A. Smulaton Model, User Dstrbuton and Smulaton Parameters The network conssts of one BS wth one sector. Probablty for locally communcatng devces s gven as a smulaton parameter per devce. Devces that don t have a local communcatng peer are dstrbuted unformly over the system area. Two consecutve devces that are havng a local communcaton n terms of devces ndces form a par. One devce of the par s dstrbuted unformly over the system area whereas the other devce of the par s dstrbuted unformly upon a dsk centered by the frst devce and radus of the dsk s gven as a smulaton parameter. Thus the radus gves the maxmum dstance between locally communcatng devces. Used smulaton parameters are provded n Table I. B. Smulatons (Numercal Analyss) Here we analyse system capactes aganst system outage n four dfferent cases: CELLULAR All devces are n cellular mode (no D2D communcaton mode avalable). OPT D2D Mode selecton vector for all devces s searched n order to mnmze the frst norm of the devce powers combned over two tme slots (18) and to fulfll egenvalue crtera. PL D2D D2D mode s selected f path loss between communcatng devces s less than mnmum of path losses between each devce and BS. FORCE D2D D2D mode s selected always for locally communcatng devces. To study capacty n a cellular network we are manly nterested n to fnd the capacty when the system outage s around 5 %. The system outage here s referred to a probablty that egenvalue crtera of the nterference matrx (14) cannot be fulflled. We smulate the system realzaton va means of multple ndependent drops and by averagng over the drops we can derve the probablty for the egenvalue crtera wth certan parameter settngs. The obtaned probablty s consdered as the system outage. In Fg. 2 one can obtan the system capacty n terms of the number of users n the system for the outage probablty beng 5 % when maxmum dstance between locally communcatng devces s 50 metres and local communcaton probablty s ether 0.2 or 0.4. It can be seen that when the maxmum D2D dstance s qute large there s not much potental mprovement avalable from D2D capablty n the system. The avalable performance mprovement ncreases as a functon of ncreasng local communcaton probablty. On the other we can see how dangerous t would be to use path loss based or forcng all locally communcatng devces nto D2D communcaton mode n ths case. Those methods really deterorate the system performance and actually as local communcaton probablty ncreases the stuaton goes even worse. Ths s due to fact that these methods don t consder the nterference stuaton at all. System capacty at 5th percentle System capacty at 5th percentle, 50 m CELLULAR OPT D2D PL D2D FORCE D2D p_local 0.2 p_local 0.4 Fg. 2. System capacty at 5th percentle of the system outage when maxmum D2D dstance s 50 m and local communcaton probabltes are 0.2 and 0.4 for dfferent mode selecton algorthms compared to pure cellular communcaton mode system. Impact of decreasng the maxmum dstance between locally communcatng devces can be seen n Fg. 3 where the maxmum dstance s 5 metres. Now we can see that even path loss based and forcng D2D methods behave qute well, almost

6 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subect matter experts for publcaton n the WCNC 2010 proceedngs. reachng the achevable upper bound. In addton to gans wth optmal mode selecton method, gans wth two other mode selecton methods ncrease as the local communcaton probablty ncreases. From above observatons one could thnk a practcal mode selecton algorthm to be such that t could frst check f the path loss or lnk gan between locally communcatng devces were below a certan threshold. The algorthm could then further use above descrbed path loss based mode selecton crtera (PL D2D) to decde the used communcaton mode for the D2D par n queston. On the other hand, n the system where devces share the same rado resource, the practcal mode selecton algorthm could be based on nterferenceawareness n whch nterference measurements from devces could be used for mode selecton especally n the case of the large maxmum dstance between communcatng devces. System capacty at 5th percentle System capacty at 5th percentle, 5 m CELLULAR OPT D2D PL D2D FORCE D2D p_local 0.2 p_local 0.4 Fg. 3. System capacty at 5th percentle of the system outage when maxmum D2D dstance s 5 m and local communcaton probabltes are 0.2 and 0.4 for dfferent mode selecton algorthms compared to pure cellular communcaton mode system. V. CONCLUSIONS In ths paper we derved the system equatons that can be used to analyse a system where both cellular communcaton mode and D2D communcaton mode devces can share the same resources and the communcaton mode selecton as well as power allocaton for the communcatng devces can be assgned ontly and optmally. Further we showed how the derved system equatons can be used to evaluate the system where D2D communcaton mode s ntegrated nto a conventonal cellular network system. Va numercal analyss t was shown that communcaton mode selecton algorthms need to be desgned carefully to prevent deteroratng the system performance. Accordng to the results man affectng factors for the performance gan from D2D are local communcaton probablty and maxmum dstance between communcatng devces, as well as mode selecton algorthm. In the future, nterference-aware mode selecton algorthm could be evaluated aganst above mentoned mode selecton algorthms to fnd a practcal algorthm to acheve close to optmum gans from D2D also when maxmum dstance between communcatng devces ncreases. Also, one could use dfferent deployment scenaros to analyse D2D performance, for example n ndoor envronments. REFERENCES [1] J. Lehtomäk, I. Sulman, J. Vartanen, M. Benns, A. Taparugssanagorn, and K. Umebayash, Drect communcaton between termnals n nfrastructure based networks, n Proceedngs of the ICT Moble and Wreless Communcatons Summt, Stockholm, Sweden, [2] P. E. Omy and H. Haas, Maxmsng spectral effcency n 3G wth hybrd ad-hoc UTRA TDD/UTRA FDD cellular moble communcatons, n Proceedngs of the IEEE Internatonal Symposum on Spread Spectrum Technques and Applcatons, Sydney, Australa, Sep. 2004, pp [3] Y. Zhang, L. Du, D. Shang, and Y. Yang, Performance analyss of a p2p-enabled tdd-cdma network wth ntra-cell spatal resource reuse, Wrel. Commun. Mob. Comput., vol. 9, no. 8, pp , [4] H.-Y. Hseh and R. Svakumar, On usng peer-to-peer communcaton n cellular wreless data networks, IEEE Transactons on Moble Computng, vol. 3, no. 1, pp , [5], A hybrd network model for cellular wreless packet data networks, [6] R. D. Yates and C. yao Huang, Integrated power control and base staton assgnment, IEEE Transactons on Vehcular Technology, vol. 44, pp , [7] S. McLaughln and H. Haas, Cellular communcatons system, Patent Applcaton WO060182, [8] Y. Du, L. Sun, and N. Ma, Method and system for peer-to-peer communcaton management n wreless communcaton networks, Patent Applcaton US , [9] L. Popova, T. Herpel, and W. Koch, Enhanced downlnk capacty n UMTS supported by drect moble-to-moble data transfer, n Proceedngs of NETWORKING 2007, 2007, pp [10] Z. Bharucha and H. Haas, Applcaton of the tdd underlay concept to home nodeb scenaro, n Vehcular Technology Conference, VTC Sprng IEEE, May 2008, pp [11] P. Marques, J. Bastos, and A. Gamero, Opportunstc use of 3g uplnk lcensed bands. n ICC. IEEE, 2008, pp [12] R. D. Yates, A framework for uplnk power control n cellular rado systems, IEEE Journal on Selected Areas n Communcatons, vol. 13, pp , [13] S. V. Hanly, An algorthm for combned cell-ste selecton and power control to maxmze cellular spread spectrum capacty (nvted paper), IEEE Journal on Selected Areas n Communcatons, vol. 13, no. 7, pp , [14] Selecton procedures for the choce of rado transmsson technologes of the UMTS, 3GPP TR 30.03U, verson 3.2.0, 1998.