User Relay assisted Traffic Shifting in LTE-Advanced Systems

Transcription

1 Use Relay assisted Taffic Shifting in LTE-Advanced Systems Lexi X 1, Ye hen 1, KoK Keong hai 1, Dantong Li 1, Shaoshi Yang, John Schomans 1 1 School of Electonic Engineeing and ompte Science, Qeen May Univesity of London, London, United Kingdom School of Electonics and ompte Science, Univesity of Sothampton, Sothampton, United Kingdom lexi.x@eecs.qml.ac.k Abstact In ode to deal with neven load distibtion, mobility load balancing adjsts the handove egion to shift edge ses fom a hot-spot cell to the less-loaded neighboing cells. Howeve, shifted ses eceive the edced signal powe fom neighboing cells, which may eslt in link qality degadation. This pape employs a se elaying model and poposes a se elay assisted taffic shifting (URTS scheme to addess this poblem. In URTS scheme, a shifted se selects a sitable nonactive se as elay se to fowad signal, ths enhancing the link qality of the shifted se. Since the se elaying model consmes elay se s enegy, a tility fnction is designed in elay selection to each a tade-off between the shifted se s link qality impovement and the elay se s enegy consmption. Simlation eslts show that the URTS scheme can impove and capacity of shifted ses. Also, URTS scheme keeps the cost of elay se s enegy consmption at an acceptable level. Keywods: mobility load balancing; link qality; elay selection I. INTRODUTION De to sevice development and se mobility, LTE/LTE- Advanced systems have the andom, time-vaying and often neven taffic distibtion [ []. Mobility load balancing (MLB is an impotant esoce management fnctionality that aims at balancing the taffic demand between the hot-spot cell and lightly loaded cells to avoid possible congestion and to incease the spectm efficiency [3]. Geneally, MLB schemes follow two stages: initially, a hotspot cell chooses some less-loaded neighboing cells as patnes; then the hot-spot cell calclates the eqied offloading taffic and adjsts cell-specific handove offsets (HO off to shift edge ses to the selected patnes by handove. These two stages ae designed in o pevios wok in [4] [5]. MLB can addess neven load distibtion. Howeve, shifted ses may eceive low efeence signal eceived powe (RSRP and sffe link qality degadation. As shown in Fig.1, ell 1 is the hot-spot and ties to offload taffic to the lightly loaded ell. BS 1 inceases HO off towads BS, in ode to tigge handove of ell 1 edge se. Afte MLB, the shifted se eceives a edced RSRP, compaed with RSRP 1 befoe MLB. Fthemoe, the edced RSRP may eslt in low. In this pape, the phenomenon of the shifted ses edced RSRP is called link qality degadation. This poblem impacts netwok pefomance. The shifted se may expeience handove faile de to poo link qality. Fthemoe, afte sccessfl handove, BS needs to assign moe sbcaies to meet the shifted se s data ate eqiement, which will edce the spectm tilisation. Fig.1. RSRP compaison of shifted se To deal with link qality degadation, this pape employs a se elaying model: a non-active se is teated as a elay to fowad signal to a shifted se. The spatially independent tansmission paths (elay link, BS diect link can achieve spatial divesity, enhancing the shifted se s link qality. The se elaying model enhances the shifted se s link qality at the expense of elay se s enegy consmption. Hence, this pape fthe poposes a se elay assisted taffic shifting (URTS scheme. In this scheme, a tility fnction consideing above two factos is designed, which selects an appopiate elay se, enhancing the shifted se s link qality nde low cost of elay se s enegy consmption. This pape is oganized as follows: Section II pesents the se elaying model. Section III analyses this model. Section IV descibes the URTS scheme. Simlation eslts and conclsions ae pesented in Section V and VI, espectively. II. USER RELAYING MODEL This wok follows o pevios eseach on MLB [4] [5]. Afte MLB implementation, the hot-spot cell offloads its edge ses to lightly loaded neighboing cells. These shifted ses may sffe the link qality degadation (see Fig.1. Meanwhile, thee ae many non-active ses in each neighboing cell (a se in the idle mode [ is called the nonactive se in this pape. In the downlink of each non-active se, the taffic channel is idle. Hence, the idle taffic channel can be tilised to fowad signal to the shifted se /13/$ IEEE

2 Hence, this pape employs a se elaying model. As shown in Fig., the shifted se selects a non-active se located in the lightly loaded cell, which is the handove taget cell of the shifted se, as the elay se. When BS tansmits data to the shifted se, the elay se eceives these data in the fist time slot and then fowads to the shifted se in the second time slot. : ommon vaiance of the Gassian white noise. B: Bandwidth in the se elaying model. III. ANALYSIS OF USER RELAYING MODEL Based on the se elaying model, Section III analyses the capacity of shifted Use, as well as the impact of enegy consmption of Relay. Fom the se elaying model, the eceived signals at Use and Relay in TS n ae given by (1 and (, espectively. y [ n] a x [ n] + Z [ n] + I [ n] (1 b b b y [ n] a x [ n] + Z [ n] + I [ n] ( b b b Fig.. Use elaying model In ode to simplify the desciption, this pape descibes the se elaying model as: a destination shifted se, defined as Use ; seveal non-active ses, defined as Relay 1 ; and a lightly loaded BS, defined as BS b. Theefoe, fo a specific se elaying model, it consists of one Relay, one shifted se and one soce BS b. The downlink tansmission mode is shown in Fig., inclding two consective time slots [] [6]. In time slot (TS n, both Use and Relay listen to the signal of BS b; in TS n+1, both BS b and Relay tansmit to Use simltaneosly [6]. Note that we assme BS b tansmits the identical se signal to the shifted se at two consective time slots [] [6]. In this pape, Relay is opeating in the amplify-andfowad (AF mode [7]. In the AF mode, the elay se amplifies all eceived signals, inclding intefeence, noise and se signal. Then it fowads these signals to the shifted se. The AF mode sits the se device, as the AF mode eqies lowe signal pocessing capability than the decodeand-fowad (DF mode does. The paametes that will be sed in sbseqent sections ae listed in Table I. Table I Paametes of se elaying model : Relay s eceived signal fom BS b. : Use s eceived signal, in BS b to Use link. : Use s eceived signal, in Relay to Use link. : hannel gain fom BS b to Relay. : hannel gain fom BS b to Use. : hannel gain fom Relay to Use. : Amplification facto of Relay. : Tansmit powe of BS b. : Tansmit powe of Relay. : Inte-cell intefeence at Relay in TS n. : Inte-cell intefeence at Use in TS n. : apacity of Use with Relay assistance. : apacity of Use withot elay assistance. : Relay s capacity, with the same nmbe of sbcaies being allocated to Relay. eflects Relay s capacity loss. : Use signal fom BS b. whee and ae the noise at Use and Relay, espectively; and ae the inte-cell intefeence at Use and Relay, espectively; is the channel gain fom BS b to the Use ; is the channel gain fom BS b to Relay. A. apacity of Use in Use Relaying Model In AF mode [7], Relay amplifies all eceived signals and fowads to the shifted Use in TS n+1. Fom (, the amplification facto of Relay is denoted as, sing (3: λ ( a P + σ + I [ n ] P b b P λ (3 a P + σ + I [ n ] b b whee and ae the tansmit powe of BS b and Relay, espectively; is the common vaiance of the Gassian white noise; denotes the magnitde of the symbol. Fo example, is the intefeence powe at Relay. In TS n+1, Use eceived signals fom Relay and BS b ae discssed in i and ii. i Use eceived signal fom Relay (Link L in Fig. in TS n+1 is denoted as 1, sing (4: y [ n+ λ y [ n] a + Z [ n+ + I [ n+ (4 b ( λ a a x [ n] + ( λ a Z [ n] + Z [ n+ + λ a I [ n] + I [ n+ b b whee efes to (; is the channel gain fom Relay to Use. Accoding to (4, of Use in TS n+1 fom L can be expessed as,, sing (5: P a a λ ( L b b n, + 1 a λ ( I [ n] + I [ n+ ii Use eceived signal fom BS b (Link L b in Fig. in TS n+1 is denoted as 1, sing (6. Fom (6, of Use in TS n+1 fom L b is denoted as,, sing (7: y (5 [ n + a x [ n + + Z [ n + + I [ n+ (6 b b b P a ( L b b b n, + 1 I [ n+ (7

3 iii In TS n, Use eceived signal fom BS b (Link L b in Fig. is shown in (1. Hence, of Use in TS n fom L b can be expessed as ( L b Pb ab n, (8 I [ n] Based on (1 (4 (6, and 1 ae the identical se signal fom thee sepaate links. Use combines them to enhance the signal qality. The estimated at Use is 4 4 AF ab Pb ab Pb ab σ + ab I[ n] ab σ + ab I[ n λ a ab Pb a ab + a ab + a ab I[ n] + a ab I[ n λ σ λ σ λ λ + P a b b n, + 1 I [ n+1 ] (13 whee is the tansmit powe of BS b; 1 is the intefeence powe at Relay in TS n+1. is the channel gain fom BS b to Relay. Accoding to (13, can be calclated as B P a log {1 } BS b b + I [ n+1 ] (14 Relay selection impacts the vale of and 1. Eqation (14 indicates that selecting an appopiate elay can keep the capacity loss of the elay se at a low level. ab Pb ab Pb λ a ab Pb + I[ n] + I[ n+ a ( +I[ n] + + I[ n+ + + σ σ λ σ σ Eqation (9 is the estimated, which is sed to select sitable elay. Then, the estimated capacity of Use with Relay assistance is AF B AF log (1 + (1 whee B is the bandwidth; denotes that Use eceives the identical se signal in two consective time slots [] [6] [7]. Relay selection impacts the vale of, and. Fom Eqation (1, selecting a sitable elay se can impove the capacity of the shifted se. B. apacity of Use withot Relay If thee is no elay link, Use only eceives signal fom BS b (Link L b in Fig. in TS n and TS n+1. Fom (7 (8, the capacity of Use withot elay is (9 IV. USER RELAY ASSISTED TRAFFI SHIFTING SHEME Fom the analysis above, the se elaying model povides a complementay link to impove the capacity of the shifted se. Howeve, this model also consmes the battey powe of the elay se and shotens its woking time, which will edce the elay se s total capacity. Both the facto of the shifted se s capacity and the facto of the elay se s capacity loss shold be consideed jointly in elay selection. Theefoe, based on the se elaying model, this pape poposes a se elay assisted taffic shifting (URTS scheme. The key of URTS scheme lies in designing a tility fnction to select an appopiate elay fo the two factos tade-off. A. Weight of Taffic Shifting In ode to select a pope Relay to incease the capacity of the shifted Use, this pape designs the weight of taffic shifting (WTS as,. As shown in (15,, eqals the atio of Use s capacity with Relay assistance (, see (1 to Use s capacity withot elay (, see (11. Hence,, indicates the capacity gain of Use. B P a P a log {1 } NO AF b b b b + + I [ n] I [ n+ (11 ψ WTS, {1,... R} (15 AF NO AF. apacity Loss of Relay Fom the se elaying model, Relay amplifies signal powe and fowads to Use in TS n+1. This consmes the enegy of Relay and shotens Relay battey woking time, which will eslt in the capacity loss of Relay. This pape ses capacity as the single metic, which allows s to compae the benefit to shifted ses and the cost to elays diectly. We define as Relay s capacity, with the same nmbe of sbcaies (the same bandwidth being allocated to Relay. Hence, eflects Relay s capacity loss, and indicates the impact of enegy consmption of Relay. If Relay becomes active, the eceived signal at Relay in TS n+1 is given by (1. oespondingly, the achieved of Relay in TS n+1 is defined as,, sing (13. yb[ n + abxb[ n + + Z[ n + + I[ n + (1 B. Weight of apacity Loss The enegy consmption of Relay will shoten its battey woking time and edce its total capacity. Unde the simila enegy consmption of the non-active Relay, this pape designs the weight of capacity loss (WL to compae the capacity loss of Relay, and the capacity impovement fo Use., is calclated as (16: BS WL, AF NO AF ψ {1,... R} (16 * eflects the capacity impovement of Use, with Relay assistance. * eflects Relay s capacity loss itself (see (14., indicates the impact of enegy consmption. In (16, the highe capacity loss of Relay leads to the highe,.

4 . Utility Fnction based Relay Selection In ode to select a sitable se to each the tade-off between the weight of taffic shifting and the weight of capacity loss, this pape poposes a tility fnction as (17: ψ ψ ( AF AF NO AF WTS, NO AF BS WL, {1,.. R} (17 Accoding to (17, the highe Use s capacity with Relay assistance can lead to highe. Meanwhile, the lowe capacity loss of Relay can also lead to highe. Hence, URTS scheme ties select Relay k to maximize : Relay k agmax {1,... R} AF AF NO AF ( agmax NO AF BS {1,... R} (18 Fom (18, the tility fnction elates to,,. Use has its coespondingly fixed, given by (11. and ae vaying with diffeent Relay. Fom (1, is based on thee vaying paametes:,,. Fom (14, is based on and 1. D. URTS Scheme Pocess Based on the analysis above, Use can calclate the tility fnction to select a sitable elay, when Use knows the vale of,,, 1. To edce the complexity and signalling load, the URTS scheme calclates them accoding to existing/measable paametes in othe esoce management fnctionalities, e.g., cell selection, admission contol. Specifically, they can be estimated as: i (channel powe gain fom BS b to Relay : Since Relay knows its RSRP fom BS b, as well as BS b s tansmit powe (which cold be infomed fom BS b in contol channel [, Relay estimates as (19: b Relay 's RSRP fom BS b BS b's tansmit powe P b a (19 ii (channel powe gain fom Relay to Use : Afte Relay esponding to Use, Use knows its eceived esponse signal powe fom Relay. Besides, Relay epots P to Use, as shown in Fig.3. Use estimates as Use 's eceived esponse powe fom Relay Relay 's tansmit powe P a ( iii, 1 (intefeence powe of Relay in TS n and n+1: In the fll feqency ese LTE-Advanced cellla netwoks [ [4], pecise intefeence estimation is difficlt. It is becase Relay s intefeence, which is imposed by othe cells sing the co-channel sbcaies, is vaying de to the dynamic sbcaies allocation of neighboing cells. To edce the estimation complexity, Relay consides the RSRP fom all neighboing BSs as the intefeence, and then calclates the theoetically heaviest intefeence. The flowchat of URTS scheme is shown in Fig.3, which involves the pocess of shifted Use and Relay. Fig.3. Flowchat of URTS scheme As shown in Fig.3, if a se in the hot-spot BS needs to be shifted to the taget BS b, the shifted Use boadcasts its coopeation eqest and its taget BS ID (denotes BS b. Afte eceiving the boadcast, the non-active se jdges whethe it is in the coveage of BS b and whethe it is available to assist Use (Since a non-active se can only assist a shifted se at a time. If it is, this non-active Relay calclates fom (19. Besides, Relay estimates as the sm of RSRP fom all neighboing BSs. Then Relay esponds and sends,, and P to Use. Afte eceiving the esponses, Use estimates fom (. Fthemoe, Use estimates, and. Based on the estimated, and, Use calclates the tility fnction of all esponding non-active ses. Then Use selects a non-active se with the lagest as elay. Afte elay se selection, the selected non-active se stats to assist Use to fowad the signal. Note that mltiple shifted ses may eqest one non-active se at the same time. Unde this scenaio, the non-active se chooses one shifted se, fom which the non-active se eceives the stongest boadcast powe. V. SIMULATION ANALYSIS A. Simlation Scheme Intodction A downlink system-level LTE-Advanced simlation platfom is designed based on [ [8]. As shown in Fig.4, thee ae thee hot-spot aeas, which cove 7% active ses and non-active ses. Othe impotant paametes ae shown in Table II and [5]. Besides, this pape simlates fo schemes. Table II Simlation Platfom Paametes Paamete Vale Sbcaie and Total bandwidth Sbcaie: 15K Hz; Total: 5M Hz Physical esoce blocks(prb Total 5 (1 sbcaies pe PRB Feqency G Hz

5 Inte-site distance Log-nomal shadow fading Total BS tansmit powe Total se tansmit powe Schedle Taffic model m (3-secto antenna splits site Standad Deviation: 8dB 43 dbm 1 dbm Max /I Active se: Gaanteed 64Kbps Non-active se: no sevice eqiement Fig.4. ells layot and ses distibtion (nit: mete i URTS (called LB with tility fnction se elay scheme in Fig.6-9 Fig.5. Oveall simlation flowchat of URTS scheme The poposed (tility fnction based URTS scheme is simlated. Fig.5 shows its oveall simlation flowchat. A hotspot cell employs o pevios coopeative load balancing (LB scheme, which consists of se-vote assisted patne selection [4] and coopeative taffic shifting [5], to shift ses to patne cells. Afte LB, the shifted se employs the poposed tility fnction based elay selection. Finally, the elay se fowads signal fo the shifted se. ii LB scheme This section also simlates the standalone LB scheme (withot se elay [4] [5]. In the LB scheme, the hot-spot cell adjsts HO off towads patne ell b. Then Use in the hot-spot cell will be shifted to ell b withot elay assistance. iii Typical MLB scheme The typical mobility load balancing (MLB scheme in [3] is simlated fo compaison. In [3], the hot-spot cell selects all lightly loaded neighboing cells as patnes. Then the hotspot cell estimates its shifted ses eqied sbcaies in each patne cell. The hot-spot cell gadally adjsts HO off towads each patne to offload ses ntil two cells each a simila load Active se Non-active se Thee hot-spot aeas 34 iv WTS se elay scheme (called LB with WTS se elay scheme in Fig.6-9 In ode to evalate the pefomance by adopting the poposed tility fnction, the efeence LB with WTS se elay scheme is simlated. Its simlation flowchat is simila to Fig.5. The diffeence is that in LB with WTS se elay scheme, a shifted se only consides the poposed WTS (weight of taffic shifting ding the elay selection. As discssed in Section IV A, LB with WTS se elay scheme aims at selecting the elay which can best impove the capacity of the shifted se, while it does not conside the capacity loss of elay ses. In addition, this scheme also eflects the widely sed elay selection citeia, which aims at obtaining the maximm capacity fo each se in fixed elay cellla netwoks, e.g., shotest distance based elay selection, minimm path-loss based elay selection, and maximm eceiving powe based elay selection. B. Simlation Reslts Load balancing (LB handove faile ate eflects the link qality of shifted ses [, becase the bette the link povided by a patne cell, the moe shifted ses can be handed ove sccessflly. Fig.6 shows that the LB scheme has lowe LB handove ate than the typical MLB scheme. This is becase the LB scheme can addess the heavily loaded pblic patne, as discssed in [5]. ompaed with the LB scheme, the poposed LB with tility fnction se elay scheme can fthe edce the LB handove faile ate. This is becase the elay link can enhance the link qality of the shifted se. As a eslt, the impoved link qality deceases the LB handove faile ate. LB Handove Faile Rate [%] LB with tility fnction se elay LB with WTS se elay LB Typical MLB Total Nmbe of Active Uses Fig.6. LB handove faile ate compaison In ode to evalate the poposed LB with tility fnction se elay scheme fo helping shifted ses of diffeent link qalities, fo categoies of shifted ses ae consideed accoding to thei (withot se elay assistance: lowe than 1; between 1 and ; between and 6; between 6 and 1. Among fo categoies, the poo link qality shifted ses (<1, 1<<, expeience lage impovement via the poposed LB with tility fnction se elay scheme. Fo example, the poposed scheme can incease nealy 7% fo shifted ses in <1 and 1<< categoies. The poposed scheme also effectively impoves fo the medim link qality shifted ses. Fo example, 3%

6 impovement fo shifted ses in <<6 categoy. The poposed scheme also inceases the of good link qality shifted ses, e.g., 6<<1. Bt the enhancements ae not as otstanding as poo/medim link qality ses. Fo example, shifted ses in 6<<1 categoy expeience nealy % incease. Fom the analysis above, the poposed scheme is moe sefl fo the shifted ses who sffe poo link qality. De to the impoved and the edced handove faile ate, Fig.8 shows that the LB with tility fnction se elay scheme can impove the oveall capacity of all shifted ses, compaed with the LB scheme. Aveage of Shifted Uses in Each ategoy Total Nmbe of Active Uses Total apacity of All Shifted Uses [Kbps] Fig.7 compaison of shifted ses in diffeent categoies LB with tility fnction se elay LB with WTS se elay LB Total Nmbe of Active Uses Fig.8. Oveall capacity of all shifted ses LB with tility fnction se elay (1>>6 LB (1>>6 LB with tility fnction se elay (6>> LB (6>> LB with tility fnction se elay (>>1 LB (>>1 LB with tility fnction se elay (<1 LB (< Fig.6, Fig.8, and Fig.9 fthe evalate the tility fnction in the tade-off between shifted ses pefomance and elay ses pefomance. Fig.6 demonstates that LB with tility fnction se elay scheme has a simila LB handove faile ate, compaed with the LB with WTS se elay scheme. In Fig.8, both the LB with tility fnction se elay scheme and LB with WTS se elay scheme can impove the oveall capacity of shifted ses. Fo example, compaed with the LB scheme, the LB with tility fnction se elay scheme can incease the capacity by 31%, and the LB with WTS se elay scheme can incease the capacity by 35%, nde 7 ses scenaio. The eason of the slight diffeence is that the WTS based elay selection only consides the capacity impovement of the shifted se, while the tility fnction based elay selection also consides the capacity loss of the elay se. Fig.9 depicts the oveall capacity loss of elay ses. The capacity loss in the LB with tility fnction se elay scheme is nealy 3%~3% less than that in the LB with WTS se elay scheme. Total apacity Loss of Relay Uses [Kbps] LB with tility fnction se elay LB with WTS se elay Efficiently save Relay se's capacity loss Total Nmbe of Active Uses Fig.9 Oveall capacity loss of all elay ses Fom the analysis above, both LB with tility fnction se elay scheme and LB with WTS se elay scheme bing simila pefomance fo shifted ses. Meanwhile, LB with tility fnction se elay scheme can effectively edce the capacity loss of elay ses. Theefoe, the poposed tility fnction can each a tade-off between shifted ses pefomance and elay ses pefomance. VI. ONLUSION This pape employs a se elaying model to enhance the link qality of shifted ses in mobility load balancing. Fthemoe, based on this model, a se elay assisted taffic shifting (URTS scheme is poposed. URTS scheme can effectively incease the link qality of shifted ses nde accepted cost of elay ses enegy consmption. REFERENES [ 3GPP TS 36.3 V9.5., "E-UTRAN Oveall desciption," Sept. 1 [] Liping Wang, Ysheng Ji, Fqiang Li, "Resoce allocation fo OFDMA elay-enhanced system with coopeative selection divesity," in Poc. IEEE WN, Ap. 9, Bdapest, Hngy, pp.1-6 [3] Raymond Kwan, Rob Anott, Robet Pateson, Piccado Tivisonno, Mitshio Kbota, "On mobility load balancing fo LTE systems," in Poc. IEEE VT-fall, Sept. 1,Ottawa, anada, pp.1-5 [4] Lexi X, Ye hen, John Schomans, Laie thbet, Tianki Zhang, "Use-vote assisted self-oganizing load balancing fo OFDMA cellla systems," in Poc. IEEE PIMR, Sept.11, Toonto, anada, pp.17-1 [5] Lexi X, Ye hen, Kok Keong hai, Tianki Zhang, John Schomans, Laie thbet, "oopeative load balancing fo OFDMA cellla netwoks," in Poc. Eopean Wieless, Ap. 1, Poznan, Poland, pp.1-7 [6] Basak an, Halim Yanikomeogl, Fzan Atay Onat, Elisabeth De avalho, Hioyki Yomo, Efficient coopeative divesity schemes and adio esoce allocation fo IEEE 8.16j," in Poc. IEEE WN, Ap. 8, Las Vags, USA, pp [7] Jn ai, Xemin Shen, J. W. Mak, A. S. Alfa, "Semi-distibted se elaying algoithm fo amplify-and-fowad wieless elay netwoks," IEEE Tans. Wieless ommn., vol.7, no. 4, Ap. 8, pp [8] Zheny Wang, E. K. Tameh, A. R. Nix, "Statistical pee-to-pee channel models fo otdoo ban envionments at GHz and 5GHz," in Poc. IEEE VT-fall, Sept. 4, Los Angeles, USA, pp