Self-Organizing Network-based Cell Size Adaption and Traffic Adaptive eicic in LTE-A HetNets *

Transcription

1 JOURNAL OF INFORMATION SCIENCE AND ENGINEERING 32, (2016) Self-Organzng Network-based Cell Sze Adapton and Traffc Adaptve eicic n LTE-A HetNets * Department of Computer Scence Natonal Chao Tung Unversty Hsnchu, 300 Tawan E-mal: {yhhsu; kwang}@cs.nctu.edu.tw The Long Term Evoluton-Advanced (LTE-A) heterogeneous network (HetNet), whch can ncrease the capacty of LTE-A, s composed of hgh power macrocells and low power pcocells. However, unbalanced loadng between macro and pco cells and nter-cell nterference problems are two major performance bottlenecks n LTE-A heterogeneous networks (HetNets). We propose Self-Organzng Network (SON)-based Cell Sze Adapton (SCSA) for load balancng between macro and pco cells and mtgatng the nter-cell nterference problem. We also propose Traffc Adaptve enhanced Inter-Cell Interference Coordnaton (TAeICIC) to further mtgate the nter-cell nterference problem. The proposed SCSA uses dynamc mult-threshold load management to dynamcally set the transmsson power of each pco evolved NodeB (enb) by adjustng the plot power. In addton, the proposed TAeICIC utlzes a schedulng metrc, proportonal-far (PF), whch s the estmated throughput based on the channel qualty ndcaton (CQI) reported by a user equpment (UE) dvded by the estmated long term average throughput acheved by the UE, to dynamcally allocate an approprate number of Almost Blank Subframes (ABSs) n each ABS perod n a macrocell so as to mtgate the nterference from the macrocell to ts adjacent pcocells. Smulaton results show that the proposed SCSA+TAeICIC s better than DI, PF-ABS, and IL-ABS n terms of average throughput of enbs (15.43%, 25.66%, 31.49% mprovement, respectvely), average throughput of UEs (16.25%, 29.39%, 39.38% mprovement, respectvely), average rado nterface delay of UEs (81.01%, 84.97%, 86.52% mprovement, respectvely), and average energy consumpton per pco enb (54.38% mprovement for each related method). Therefore, the proposed SCSA+ TAeICIC can sgnfcantly enhance the Qualty of Experence (QoE) of moble users and reduce the operatng expendture (OPEX) of the operators n LTE-A HetNets. Keywords: enhanced nter-cell nterference coordnaton (eicic), HetNet, LTE-A, load balancng, self-organzng network (SON) 1. INTRODUCTION Wreless data traffc has seen prolfc growth n recent years because the use of smart handheld devces and new emergng servces are wdespread, such as real-tme vdeo streamng and multmeda fle sharng. Therefore, the future wreless cellular network must be desgned to satsfy these bandwdth hungry servces [1-3]. In order to enhance the capablty of the cellular network, an operator needs to pay a large amount of money to upgrade and mantan ts cellular network. In addton, users are not wllng to pay more money to the operator to mprove the cellular network s capablty. Thus, Receved October 8, 2015; revsed February 7, 2016; accepted May 2, Communcated by Jann-Lang Chen. * The support by the Mnstry of Scence and Technology under Grants NSC E MY3 and MOST E s gratefully acknowledged. 1503

2 1504 how to reduce the captal expendture (CAPEX) and operatng expendture (OPEX) has become a crucal ssue for the operator. A self-organzng network (SON), as a promsng soluton for both operators and users, has been proposed to effectvely reduce CAPEX and OPEX by mnmzng human nvolvement n networkng processes and optmzng network capacty, coverage, and servce qualty [4, 5]. The SON conssts of three phases: self-confguraton, self-optmzaton and self-healng [6]. Self-confguraton ncludes pre-operatonal functons such as regsterng wth the network, parameter settng, and software downloadng [6]. Self-optmzaton ncludes load balancng, coverage and capacty optmzaton, and nter-cell nterference coordnaton [3]. Self-healng ncludes cell outage detecton and compensaton [3]. In ths paper, we focus on all the above three aspects of self-optmzaton. Wth the rapd growth n wreless data, we need to make the avalable rado spectrum as spectrally effcent as possble. The Long Term Evoluton-Advanced (LTE-A) heterogeneous network (HetNet) s a promsng soluton to expand capacty and to effcently utlze the rado spectrum of the moble network [2]. The LTE-A HetNet, whch conssts of hgh power macrocells and low power pcocells, as shown n Fg. 1, s deployed n a hgh user equpment (UE) densty area, so called a hot spot area, so as to ncrease the capacty of the LTE-A [7, 8]. Note that the notaton CRE n Fg. 1 represents a cell range extenson regon, whch wll be descrbed later. However, LTE-A HetNets encounter two problems. Frst, snce low power pcocells share the same frequency spectrum wth hgh power macrocells, the performance of low power pcocells may be severely nterfered by hgh power macrocells. Second, a UE by default connects to an evolved NodeB (enb) whch has stronger reference sgnal power (RSP). That s, UEs wll connect to macro enbs n most locatons, because a macro enb has stronger RSP than a pco enb. Ths may result n underutlzed pco enbs and defeat the purpose of deployng pco enbs. In Fg. 1, the UEs not close to pcocells may be nterfered by the macrocell n the downlnk and may connect to the macrocell because of the macrocell s hgh power. In order to solve the above problems, the 3rd Generaton Partnershp Project (3GPP) proposed the enhanced nter-cell nterference coordnaton (eicic) known as the Fg. 1. LTE-A HetNet archtecture.

3 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1505 tme doman ICIC [9] to protect the downlnk behavor of the pcocell. In the eicic, each macro enb remans slent durng the Almost Blank Subframes (ABS) over whch pco enbs can transmt n reduced nterference. Moreover, 3GPP also proposed Cell Selecton Bas (CSB) [10], whch can create a CRE regon for a pcocell, to ncrease the reference sgnal receved power (RSRP) of UEs receved from the pcocell so as to offload some UEs from the macrocell to the pcocell. In addton to the CSB, 3GPP also proposed an SON soluton [11] to adjust the transmsson power of an enb. By ncreasng the transmsson power of a pco enb, some UEs can be offloaded from a macrocell to the adjacent pcocell. In ths paper, we utlze the SON to create a CRE regon for a pcocell snce ncreasng the transmsson power of a pco enb can mtgate the nterference from the correspondng macro enb to those UEs located n the CRE of the pco enb. Therefore, we propose an SON-based Cell Sze Adapton (SCSA) for load balancng and mtgatng the nter-cell nterference problem and propose Traffc Adaptve Enhanced Inter-cell Interference Coordnaton (TAeICIC) scheme to further mtgate the nter-cell nterference problem n LTE-A HetNets. The objectves of ths paper are conquerng the above two problems of LTE-A HetNets by achevng load balancng between a macocell and adjacent pcocells and reducng the nterference from the macrocell to the pcocells so as to ncrease the throughput performance of each enb and each UE and to decrease the rado nterface delay experenced by each UE. The rest of ths paper s organzed as follows. Secton 2 brefs background and related work. Secton 3 descrbes an LTE-A HetNet archtecture. Secton 4 presents the proposed SCSA+TAeICIC. In secton 5, an analytc model s descrbed. Performance evaluaton results are dscussed n secton 6. Secton 7 concludes ths paper. 2. BACKGROUND AND RELATED WORK 2.1 Enhanced Inter-cell Interference Coordnaton (eicic) [9] In the eicic, each macro enb remans slent durng the ABS over whch a pco enb can transmt wth low nterference so as to protect the downlnk transmsson of the pco enb. The macro enb can only send control sgnals, whch only occupy a small fracton of an Orthogonal Frequency Dvson Multple Access (OFDMA) subcarrer, durng the ABS. Fg. 2 shows the frame structure of the eicic [9]. T ABS = 8 subframes Frame structure SF-1 SF-2 SF-3 (ABS) SF-4 SF-5 SF-6 (ABS) SF-7 SF-8 Macro Tx subframes Macro transmsson Macro lmted transmsson (only control sgnals) Pco Tx subframes Pco transmsson wth hgh nterference Pco transmsson wth low nterference Fg. 2. Frame structure of eicic [9].

4 Cell Selecton Bas (CSB) [10] Assume s the CSB of a cell and p s the RSRP (n dbm) from cell, as measured by a UE. Note that the cell can be ether a macocell or a pcocell. Then, the UE assocatees wth cell k accordng to Eq. (1) k arg max ( p ). (1) Thus, by assgnng a larger (smaller) CSB to pcocells compared to macrocells, we can ensure more (fewer) UEs to connect to pcocells. The bas values are broadcast by cells to help UEs to select an approprate cell. 2.3 Related Work Ma et al. [12] proposed an SON load balancng mechansm based on the renforcement learnng (RL) to dynamcally change the coverage areas of relay nodes n a multhop cellular network envronment. Note that Ma et al. [12] focuses on macro enbs mxed wth relay nodes nstead of pco enbs. Erlnghagen et al. [7] used an enb s utlty to determne the CSB of the enb for each UE to select an approprate enb (macro or pco) so as to ncrease the resource utlzaton of a pco enb and to determne the number of ABSs a pco enb can use. In addton, t utlzes the CSB to create the CRE of a pco enb. However, adoptng the CSB to adjust the RSRP has the followng two dsadvantages. Frst, UEs located n the CRE of a pco enb stll have strong nterference from the correspondng macro enb. Second, UEs need to perform addtonal computaton n the CSB scheme, whch may ncrease the power consumpton of UEs. Soret and Pedersen [13] proposed an nstantaneous-load-based algorthm (IL-ABS) whch allocates ABSs accordng to the loads of macro and pco enbs. They also proposed a proportonal-far-based algorthm (PF-ABS) whch ncreases or decreases one ABS allocated n an ABS perod (as shown n Fg. 2) accordng to the average throughput acheved by UEs and ther nstantaneous channel condtons. In [13], t showed that PF-ABS has better performance than IL-ABS. However, n [13], t dd not propose any SON-based or CSB mechansm that can acheve load balancng among macro and pco enbs. The above problems of the related work motvate us to propose the SCSA to adaptvely change the transmsson power of a pco enb so as to mtgate the nter-cell nterference problem from a macro enb to ts adjacent pco enbs and acheve load balancng between macro and pco enbs, and to propose the TAeICIC to allocate ABSs more effcently so as to further mtgate the nter-cell nterference. Table 1 summarzes the dfferences between the proposed SCSA+TAeICIC and the three related works. Table 1. Comparson of the proposed SCSA+TAeICIC wth three related works. Method Feature Ma et al. [12] Desgn focus CRE crteron Small cell Proposed an SON load balancng mechansm based on RL to dynamcally change the coverage areas of relay nodes n a mult-hop cellular network envronment. SON (adjust relay node s transmsson power) Relay node

5 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1507 Table 1. (Cont d) Comparson of the proposed SCSA+TAeICIC wth three related works. Use the enb s utlty to determne the Erlnghagen et al. CSB for each UE to select an approprate CSB (adjust Pco (DI) [7] enb and the number of ABSs a pco enb UE s RSRP) can use. Soret and Pedersen (IL-ABS and PF-ABS) [13] Proposed SCSA+TAeICIC Proposed IL-ABS whch allocates ABSs accordng to the load of macro and pco enbs and proposed PF-ABS whch ncreases or decreases one ABS allocated n an ABS perod accordng to the average throughput acheved by UEs and ther nstantaneous channel condtons. Proposed SCSA whch uses dynamc mult-threshold load management to dynamcally adjust the transmsson power of a pco enb and proposed TAeICIC to allocate an approprate number of ABSs n an ABS perod accordng to the average throughput acheved by UEs and ther nstantaneous channel condton. N/A SON (adjust pco enb s transmsson power) Pco Pco 3. LTE-A HETNET ARCHITECTURE Fg. 3 shows the proposed LTE-A HetNet archtecture whch has three pcocells n a macrocell. The macro/pco enbs communcate wth each other by the X2 nterface. In ths paper, we suppose that a macro enb s coverage area cannot be changed so as to avod the coverage holes problem and the pco enb s coverage can be dynamcally changed to releve the hgh loadng problem of macro enbs and to acheve load balancng among macro and pco cells. Pco- enb Correspondng macro enb of Pco- enb Neghborng macro enb of Pco- enb Fg. 3. Proposed LTE HetNet archtecture.

6 1508 A UE s assocaton strategy s based on the RSRP. The RSRP s measured by the UE over the cell-specfc Reference Sgnals (RSs) wthn the measurement bandwdth over a measurement perod [14]. Note that the RSRP s a type of sgnal strength measurement and s ndcatve of the cell coverage [14]. The RSRP s calculated based on the RSP, as shown n Eq. (2) [12]: RSRP j = RSP - PL j + SF j. (2) where RSP s the RSP transmtted by the th enb, RSRP j s the correspondng RSRP measured by the jth UE, PL j s the path loss between the th enb and the jth UE, and SF j s the shadow fadng between the th enb and the jth UE. A UE always assocates wth the enb whch has the hghest RSRP measurement. Thus, adjustng the RSP, whch can modfy the RSRP measured by a UE and affect the UE assocaton decson, can redstrbute the network load [12]. The loadng of the th pco enb at tme t can be defned as the utlzaton of the th pco enb, as shown n Eq. (3): N pco, t (3) pco, t N pcocapacty where N pco-,t and N pco-capacty represent the number of UEs beng served by the th pco enb at tme t and the total number of UEs can be served by a pco enb, respectvely. Note that the loadng of the th pco enb at tme t, pco-,t, s n the range of [0,1]. We also defne N macro-cor-,t, N macro-ne-,t, and N macro-capacty to represent the number of UEs beng served by a correspondng macro enb of the th pco enb at tme t, the number of UEs beng served by a neghborng macro enb of the th pco enb at tme t, and the total number of UEs that can be served by a macro enb, respectvely. Note that the correspondng macro enb of the th pco enb and the neghborng macro enb of the th pco enb are llustrated n Fg. 3. The proportonal-far (PF) of the th macro enb, PF m- (t), and the PF of the pco enbs whch are n the coverage area of the th macro enb, PF p- (t), are defned n Eqs. (4) and (5), respectvely [13]: where m PF ( t) 1 r () t Nmacrototal ( t) nmarcroprb macro u, k (4) N () t u1 k1 R () t marcototal macrou N macro-total- (t) s the total number of actve UEs that connect to the th macro enb n the tth ABS perod. r macro--u,k (t) s the estmated throughput of UE u, whch connects to the th macro enb, at the kth physcal resource block (PRB) n the tth ABS perod. The estmated throughput s based on the channel qualty ndcaton (CQI) reported by UE u. R macro--u (t) s the estmated long term throughput of UE u, whch connects to the th macro enb, n the tth ABS perod. n macro--prb s the total number of PRBs used by UE u, whch connects to the th macro enb.

7 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1509 p PF ( t) 1 r () t Npcototal () t npcoprb pco u, k (5) N () t u1 k1 R () t pcototal pcou N pco-total- (t) s the total number of actve UEs, whch connect to pco enbs, that are located n the coverage area of the th macro enb n the tth ABS perod. r pco--u,k (t) s the estmated throughput of UE u, whch connects to pco enbs that are located n the coverage area of the th macro enb, at the kth PRB n the tth ABS perod. The estmated throughput s based on the CQI reported by UE u. R pco--u (t) s the estmated long term throughput of UE u, whch connects to pco enbs that are located n the coverage area of the th macro enb, n the tth ABS perod. n pco--prb s the total number of PRBs used by UE u, whch connects to pco enbs that are located n the coverage area of the th macro enb. 4. PROPOSED SON-BASED CELL SIZE ADAPTION (SCSA) AND TRA- FFIC ADAPTIVE ENHANCED INTER-CELL INTERFERENCE CO- ORDINATION (TAEICIC) The proposed SCSA uses dynamc mult-threshold load management to dynamcally set the transmsson power of each pco enb by adjustng the plot power so as to acheve load balancng between macro and pco cells and mtgate the nter-cell nterference problem. In addton, the proposed TAeICIC utlzes a schedulng metrc, PF, whch s the estmated throughput based on the CQI reported by a UE dvded by the estmated long term average throughput acheved by the UE, to dynamcally allocate an approprate number of ABSs n each ABS perod n a macrocell so as to mtgate the nterference from the macrocell to ts adjacent pcocells. In the proposed SCSA, we defne three dynamcally changed loadng thresholds (upper threshold T U, mddle threshold T M, and lower threshold T L ) for each pco enb. The adjustment of loadng thresholds s to reflect the loadngs of the correspondng macro enb, the neghborng macro enb, and the pco enb for load balancng among macro and pco enbs. 4.1 Proposed SON-based Cell Sze Adapton (SCSA) The proposed SCSA algorthm s composed of a transmsson power adapton algorthm (Algorthm 1) and a mult-threshold updatng algorthm (Algorthm 2). Algorthm 1 s trggered when σ pco-,t > T U or σ pco-,t < T L. In Algorthm 1, f σ pco-,t > T U, the th pco enb needs to reduce ts transmsson power so as to offload some UEs to ts correspondng macro enb and the neghborng macro enb, and f σ pco-,t < T L, the th pco enb needs to strength ts transmsson power so as to reduce the loadngs of ts correspondng macro enb and the neghborng macro enb. Algorthm 1 ams to adjust the loadng of the th pco enb to T M so as to avod that Algorthm 1 s trggered too often owng to overloadng or underloadng. Therefore, the transmsson power of the th pco enb s set to the maxmum plot power of the th pco enb T M. Note that a pco enb strengthens ts transmsson power to ncrease ts loadng by servng more UEs and decreases ts transmsson power to reduce ts loadng by servng fewer UEs.

8 1510 Algorthm 1: Transmsson power adjusted for the th pco enb Input: N pco-,t, N pco-capacty Output: Transmsson power of the th pco enb 1. f pco-,t > T U σ pco-,t < T L 2. pco-,t = T M // The th pco enb adjusts ts transmsson power by settng ts plot power to the maxmum plot power of the th pco enb T M 3. end f In Algorthm 2, we base on N macro-cod-,t, N macro-ne-,t, and N pco-,t to dynamcally adjust the T U, T M, and T L of the th pco enb. The dea of determnng T U s that f the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are hgher (lower), the hgher (lower) T U of the th pco enb s set. The reasons why we set T U n ths way are descrbed as follows. If the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are heavy, the th pco enb wll ncrease ts T U so as to avod offloadng ts loadng to the two heavy loaded macro enbs. On the contrary, f the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are lght, the th pco enb wll decrease ts T U so as to offload ts load to the two lght loaded macro enbs. The dea of determnng T L s that f the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are hgher (lower), the hgher (lower) T L of the th pco enb s set. The reasons why we set T L n ths way are descrbed as follows. If the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are heavy, the th pco enb wll ncrease ts T L so as to share the two heavy loaded macro enbs loads. On the contrary, f the loadngs of the correspondng macro enb and the neghborng macro enb of the th pco enb are lght, the th pco enb needs to decrease ts T L so as to avod unnecessary handovers from UEs served by the two lght loaded macro enbs. In ths way, each pco enb can effectvely adjust ts transmsson power to reflect ts correspondng and neghborng macro enbs loadngs. In addton, T M s the average of T U and T L. Note that the parameters used n Algorthm 2 wll be updated n the begnnng of each ABS perod through the X2 nterface. Remnd that snce the th pco enb and ts correspondng macro enb are n the same cell, n Algorthm 2 s a weghtng factor that s used to pay more attenton to the loadng of the correspondng macro enb than that of the neghborng macro enb. Algorthm 2: Mult-threshold updatng for the th pco enb Input: N macro-cor-,t, N macro-ne-,t, N pco-,t, N macro-capacty, N pco-capacty Output: T U, T M,, T L 1. f the th pco enb receved updated N macro-cor-,t or N macro-ne-,t (1 ) pco, t macrocor, t macrone, t 2. L avg 3 N (1 ) N macrocor, t macrone, t 3. N N N pco, t macrocor, t macrone, t 4. T U = L avg + L avg

9 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC T L = L avg L avg (1 ) 6. T T U L T M 2 7. end f 4.2 Proposed Traffc Adaptve Enhanced Inter-cell Interference Coordnaton (TAeICIC) In the proposed TAeICIC, we use the rato of the loadng of a pco enb whch has the hghest loadng among the pco enbs located wthn the same correspondng (the th) macro enb over the total loadng of all the pco enbs located wthn the same correspondng (the th) macro enb plus the loadng of the correspondng macro enb as the correspondng (the th) macro enb s mutng rato μ n the frst ABS perod (T ABS ), as shown n Eq. (6): Max( N, N, N ) pco1, t pco2, t pco3, t 3. N N macro, t 1 pco, t (6) In other words, mutng rato μ s the rato of ABSs allocated n an ABS perod for the th macro enb. In ths way, each macro enb can allocate ABSs for ts pco enbs accordng to the loadng of the pco enb wth the hghest load. Then, the th macro enb uses Algorthm 3 to adjust mutng rato μ at the begnnng of each ABS perod. Therefore, Algorthm 3 can be used to obtan the number of ABSs allocated n each T ABS. In Algorthm 3, a hgher mutng rato wll be obtaned f the th pco enb s performance decreases (PF p- ncreases) and at the same tme ther correspondng macro enb s (the th macro enb) performance ncreases (PF m- decreases). Otherwse, f the th macro enb s performance decreases (PF m- ncreases), a lower mutng rato wll be obtaned. Note that n Algorthm 3, the number of ABSs allocated n each ABS perod depends on the growth rates of PF m- and PF p-, as detaled n Algorthm 3. In contrast, the PF-ABS proposed n [13] only ncreases or decreases one ABS n each ABS perod whle the proposed TAeICIC ncreases or decreases a dfferent number of ABSs accordng to the growth rates of PF m- and PF p-. In ths way, the proposed TAeICIC can allocate a more approprate number of ABSs than the PF-ABS. Therefore, the proposed TAeICIC has better performance than the PF-ABS. Algorthm 3: ABS allocaton for the th macro enb Input: PF m- (t), PF m- (t 1), PF p- (t), PF p- (t 1) Output: 1. f (PF p- (t) > PF p- (t 1)) and (PF m- (t) < PF m- (t 1)) p PF () t If ( 1 ) p PF ( t 1) T T ABS ABS p PF ( t) 1 3. p PF ( t 1) TABS 4. else

10 f ( 1 ) T ABS 1 6. T ABS 7. end f 8. end f 9. else 10. f (PF m- (t) > PF m- (t 1)) m PF () t f ( ) m PF ( t 1) T T ABS ABS m PF () t m PF ( t 1) TABS 13. else f ( ) T ABS end f 17. end f 18. end f 19. end f 1 T ABS 5. ANALYTIC MODEL The cellular concept does not apply broadcastng over large areas. Smaller areas, called cells, nstead are handled by less powerful enbs that use less power for transmsson. The avalable spectrum can then be re-used from one cell to another to ncrease the capacty of the system [15]. To support more UEs n urban areas, the heterogeneous cellular archtecture has been proposed. The LTE-A heterogeneous network (HetNet), whch s a heterogeneous cellular archtecture that can ncrease the capacty of LTE-A, s composed of hgh power macrocells and low power pcocells. In ths secton, we analyze the performance of the proposed SCSA+TAeICIC, whch focuses on achevng load balancng among macrocells and pcocells and mtgatng the nterference from macrocells to pcocells so as to reduce the rado nterface delay experenced by each UE. Two performance metrcs related to the rado nterface delay, the packet error rate (PER) and the expectaton of the packet retransmsson count (PRC), for UEs are analyzed. In order to analyze the proposed SCSA+TAeICIC, we model the packet transmsson over the wreless channel between UE and enb by the M/mxE k /1/1 queung model [16] and assume the packet transmsson of a UE s ether n success state (SS) or falure state (FS). The SS state represents that the packet transmsson from a UE to an enb s successful, and the FS state represents that the packet transmsson from a UE to an enb s faled. Fg. 4 shows the state transton dagram of the Markov process for packet transmsson. We assume that the arrvals of faled packet transmssons are a Posson process wth arrval rate λ, and the departure rate of the FS state s θ. The nter-arrval tme of two consecutve faled packet transmssons s exponentally dstrbuted wth the probablty densty functon (PDF), as shown n Eq. (7) [17-19].

11 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1513 f(t) = e -t t 0 (7) Let δ denote a random varable that represents the duraton of a faled packet transmsson. Assume that δ has the mxed Erlang dstrbuton wth the PDF I r 1 ( t) f () t c e 1 ( r 1)! where c 1. I 1 t (8) Note that n Eq. (8), I, c, r and determne the shape and scale of the dstrbuton [18]. We selected the mxed Erlang dstrbuton because t has been proven that t can well approxmate many other dstrbutons [17-19]. Thus, the expectaton of δ s expressed as t 0 E f ( t) tdt I 1 r c ( r 1)! r t t e t 0 dt. (9) By Laplace transform, we get E I r c. (10) 1 Fg. 4. State transton dagram of the Markov process for packet transmsson. Then, the departure rate, θ, can be expressed as θ = 1/E(). Furthermore, let the state probabltes of SS and FS be 0 and 1, respectvely. From the Brth-Death process of queung theory, we get and 0 = (1 + /) -1 (11) 1 = (/)(1 + /) -1. (12) The packet error rate (PER) s expressed as PER t p 0 t0 1 t t p 0 t0 1 0 f () t dt ( e ) 1 e t p (13)

12 1514 where t p s the average rado nterface delay experenced by a UE. Let denote the packet retransmsson count for a UE successfully transmttng a packet to an enb. Therefore, the expectaton of σ s expressed as follows: t n1 p tp 0 0 E n 1 e e 1. (14) n1 λt n1 p λt p Note that n the frst part of Eq. (14), n1 π0e π 0e n1, t represents the expectaton of 1 (the frst packet transmsson) plus the packet retransmsson count. Thus, Eq. (14) represents the expectaton of the packet retransmsson count for a UE successfully transmttng a packet to an enb. Table 2. Parameters settng for performance evaluaton [12, 20, 13]. Parameter Values Macro enb macro enb dstance km Macro enb pco enb dstance 0.7 km Path loss log(d[km]) Shadowng std 4 db Macro enb power (Plot power) 20 db (Tx) 46 dbm Pco enb power (Plot power) 0~20 db (Tx) 37 dbm Uplnk data rate 500 Mbps Downlnk data rate 1 Gbps β weghtng factor n Algorthm 2 2/3 ABS perod (T ABS ) 8 subframes [13] N macro-capacty 300 UEs N pco-capacty 150 UEs Cells layout See Fg. 3 Packet sze 512 bytes s a mxed Eralng-2 random varable c 1 = c 2 = = 2, 2 = 3 (/ms) 6. PERFORMANCE EVALUATION In ths secton, we frst descrbe smulaton and analytc setup, and evaluaton metrcs. Then, we compare the proposed SCSA+TAeICIC wth DI [7], PF-ABS [13], and IL-ABS [13] n terms of average throughput of enbs, average throughput of UEs, average rado nterface delay of UEs, and average energy consumpton per pco enb. In addton, based on the analytc model derved n the last secton, we also compare analytc results and smulaton results of the proposed SCSA+TAeICIC wth DI [7], PF-ABS [13], and IL- ABS [13] n terms of PER and PRC. A smulaton model usng NS-2 was developed to valdate the analytc model. The smulaton model s based on the analytc model de-

13 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1515 scrbed n Secton 5. The same parameter settngs shown n Table 2 were used for smulaton as well as for analyss. Smulaton results were obtaned by the average of 10,000 runs. 6.1 Smulaton and Analytc Setup, and Evaluaton Metrcs We smulated an LTE-A heterogeneous network, whch has 7 macro enbs and 3 pco enb wthn each macro enb, as shown n Fg. 3. There are 750 UEs n each macrocell, and all UEs are dstrbuted unformly and randomly. The percentage of actve UEs s between 0.7 and 1 [12]. Table 2 shows the smulaton setup. Frst, we defne the average throughput of UEs as the throughput averaged over all actve UEs, the average throughput of enbs as the throughput averaged over all macro and pco cells, the average rado nterface delay of UEs as the rado nterface delay averaged over all actve UEs, and the average energy consumpton per pco enb as the energy consumpton averaged over all pco enbs. In addton, for the analytc setup, wthout loss of generalty, we assume δ s a mxed Eralng-2 random varable and the coeffcents for δ are set to c 1 = c 2 = 0.5 [18, 19]. 6.2 Smulaton Results In ths secton, we compare the proposed SCSA+TAeICIC wth DI [7], PF-ABS [13], and IL-ABS [13]. Fgs. 5 and 6 show the average throughput of enbs and the average throughput of UEs, respectvely. We observed that the throughput degrades as the number of UEs ncreases. Ths s because collsons may occur as the number of UEs ncreases and the number of UEs granted to uplnk/downlnk access decreases. Fg. 5 demonstrates that the proposed SCSA+TAeICIC s average throughput of enbs s 15.43% hgher than DI s, 25.66% hgher than PF-ABS s, and 31.49% hgher than IL-ABS s. Fg. 5 also shows that the proposed TAeICIC s average throughput of enbs s hgher than PF-ABS s as well as IL-ABS s. Fg. 6 shows that the proposed SCSA+TAeICIC s average throughput of UEs s 16.25% hgher than DI s, 29.39% hgher than PF-ABS s, and 39.38% hgher than IL-ABS s. Fg. 6 also demonstrates that the proposed TAeICIC s average throughput of enbs s hgher than PF-ABS s as well as IL-ABS s. Fg. 7 shows the average rado nterface delay of UEs. The rado nterface delay ncreases as the number of UEs ncreases. The reason s that when the number of UEs ncreases, collsons may ncrease. Ths results n the ncrease of rado nterface delay. Fg. 7 demonstrates that the proposed SCSA+ TAeICIC s average rado nterface delay of UEs s 81.01% lower than DI s, 84.97% lower than PF-ABS s, and 86.52% lower than IL-ABS s. Fg. 7 also shows that the proposed TAeICIC s average rado nterface delay of UEs s lower than PF-ABS s as well as IL-ABS s. Fg. 8 shows that the proposed SCSA s average energy consumpton per pco enb s 54.83% lower than each of the three related methods. Ths s because the proposed SCSA can adaptvely adjust the transmsson power of each pco enb accordng to ts utlzaton whle the related methods use fxed transmsson power. In addton, the load balance metrc (LBM) [21], rangng from 1 to the number of enb, of the proposed SCSA s 1.1, whch s very close to 1. Note that small values of the LBM ndcate better load balancng performance than large values [21]. Ths proves that the proposed SCSA ndeed acheves load balancng.

14 1516 Fg. 5. Average throughput of enbs. Fg. 6. Average throughput of UEs. Fg. 7. Average rado nterface delay of UEs.

15 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1517 Fg. 8. Average energy consumpton per pco enb. Fg. 9. PER comparson between the proposed SCSA+TAeICIC and DI. 6.3 Analytc Results By Eq. (13), Fgs demostrate that the PER of the proposed SCSA+TAeICIC s 15.61% lower than that of the DI, and 23.51% lower than that of the PF-ABS, and 25.38% lower than that of the IL-ABS, respectvely. Fgs. 12 and 13 demostrate that the PER of the proposed TAeICIC s lower than that of the PF-ABS as well as that of the IL-ABS. As shown n Fgs. 9-13, analytc and smulaton results are very close, whch confrms the valdty of our analytc model. By Eq. (14), Fgs demostrate that the PRC of the proposed SCSA+TAeICIC s 13.79% lower than that of the DI, and 23.13% lower than that of the PF-ABS, and 32.63% lower than that of the IL-ABS, respectvely. Fg. 17 and 18 demostrate that the PRC of the proposed TAeICIC s lower than that of the PF-ABS as well as that of the IL-ABS. As shown n Fgs , analytc and smulaton results are very close, whch confrms the valdty of our analytc model.

16 1518 Fg. 10. PER comparson between the proposed SCSA+TAeICIC and PF-ABS. Fg. 11. PER comparson between the proposed SCSA+TAeICIC and IL-ABS. Fg. 12. PER comparson between the proposed TAeICIC and PF-ABS.

17 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1519 Fg. 13. PER comparson between the proposed TAeICIC and IL-ABS. Fg. 14. PRC comparson between the proposed SCSA+TAeICIC and DI. Fg. 15. PRC comparson between the proposed SCSA+TAeICIC and PF-ABS.

18 1520 Fg. 16. PRC comparson between the proposed SCSA+TAeICIC and IL-ABS. Fg. 17. PRC comparson between the proposed TAeICIC and PF-ABS. Fg. 18. PRC comparson between the proposed TAeICIC and IL-ABS.

19 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1521 In summary, the mprovements of the proposed SCSA+TAeICIC n terms of PER and PRC explan why the proposed SCSA+TAeICIC s better than DI, PF-ABS, and IL-ABS n terms of average throughputs of enbs and UEs, and average rado nterface delay of UEs. 6.4 Implementaton Issues of the SON n LTE-A HetNets In ths secton, we wll descrbe how the proposed SON mechansms, SCSA+TAeI- CIC, can be mplemented n LTE-A HetNets. In 3GPP specfcatons [22, 23], the X2 nterface can be used to exchange overload and traffc load nformaton, such as utlzaton of an enb for the proposed SCSA and proportonal far of an enb for the proposed TAeICIC, between enbs n LTE-A HetNets. In summary, the enbs can control the traffc load approprately. In addton, n a 3GPP SON related specfcaton [11], the SON load balancng algorthm can be mplemented n an enb to optmze the coverage of each enb. Therefore, the proposed SCSA can be mplemented n an enb of LTE-A SON HetNets. 7. CONCLUSION In ths paper, we have presented the SON-based Cell Sze Adapton (SCSA) and Traffc Adaptve enhanced Inter-cell Interference Coordnaton (TAeICIC) schemes to enhance spectrum effcency n LTE-A HetNets. The proposed SCSA uses dynamc mult-threshold load management to dynamcally adjust the transmsson power of each pco enb. In addton, the proposed TAeICIC utlzes the proportonal-far (PF) to dynamcally allocate an approprate number of ABSs n an ABS perod n a macrocell so as to mtgate the nterference from the macrocell to ts adjacent pcocells. Smulaton results have shown that the proposed SCSA+TAeICIC s average throughput of enbs s 15.43% hgher than DI s, 25.66% hgher than PF-ABS s, and 31.49% hgher than IL- ABS s. Its average throughput of UEs s 16.25% hgher than DI s, 29.39% hgher than PF-ABS s, and 39.38% hgher than IL-ABS s. Its average rado nterface delay of UEs s 81.01% lower than DI s, 84.97% lower than PF-ABS s, and 86.52% lower than IL- ABS s. The proposed SCSA s average energy consumpton per pco enb s 54.38% lower than all the above three related methods. Therefore, the sgnfcance of the proposed SCSA+TAeICIC s that t can enhance the Qualty of Experence (QoE) of moble users and reduce the OPEX of the operators n LTE-A HetNets. REFERENCES 1. T. Maksymyuk, L. Han, X. Ge, H.-H. Chen, and M. Jo, Quas-quadrature modulaton method for power-effcent vdeo transmsson over LTE networks, IEEE Transactons on Vehcular Technology, Vol. 63, 2014, pp S. Deb, P. Monogouds, J. Mernk, and J. P. Seymour, Algorthms for enhanced nter-cell nterference coordnaton (eicic) n LTE HetNets, IEEE/ACM Transactons on Networkng, Vol. 22, 2014, pp O. G. Alu, A. Imran, M. A. Imran, and B. Evans, A survey of self-organzaton n

20 1522 future cellular networks, IEEE Communcatons Surveys and Tutorals, Vol. 15, 2013, pp M. Peng, D. Lang, Y. We, J. L, and H. Chen, Self-confguraton and self-optmzaton n LTE-advanced heterogeneous networks, IEEE Communcatons Magazne, Vol. 51, 2013, pp V. Montero, K. M. S. Huq, S. Mumtaz, C. Polts, and J. Rodrguez, Energy effcent load balancng for future self-organzed shared networks, Telecommuncaton Systems, Vol. 59, 2015, pp P. Semasnghe and E. Hossan, Downlnk power control n self-organzng dense small cells underlayng macrocells: A mean feld game, IEEE Transactons on Moble Computng, Vol. 15, 2016, pp K. Erlnghagen, B. Dusza, C. Wetfeld, and J. Huschke, Dynamc cell sze adaptaton and ntercell nterference coordnaton n LTE HetNets, n Proceedngs of IEEE Vehcular Technology Conference, 2013, pp I. Somna and D. Yuan, Load balancng n heterogeneous LTE: Range optmzaton va cell offset and load-couplng characterzaton, n Proceedngs of IEEE Internatonal Conference on Communcatons, 2012, pp GPP TS V12.4.0, 3rd generaton partnershp project techncal specfcaton group rado access network evolved unversal terrestral rado access (E-UTRA) and evolved unversal terrestral rado access network (E-UTRAN) overall descrpton stage 2 (Release 12), December, Huawe, R , Cell assocaton analyss n outdoor hotzone of heterogeneous networks, 3GPP TSG RAN WG1 #60, San Francsco, USA, February, GPP TS V11.2.0, 3rd generaton partnershp project techncal specfcaton group servces and system aspects telecommuncaton management self-organzng networks (SON) polcy network resource model (NRM) nformaton servce (IS) (Release 11), June C. Ma, R. Yn, G. Yu, and J. Zhang, Reference sgnal power control for load balancng n downlnk LTE-A self-organzng networks, n Proceedngs of IEEE Personal Indoor and Moble Rado Communcatons, 2012, pp B. Soret and K. I. Pedersen, Centralzed and dstrbuted solutons for fast mutng adaptaton n LTE-advanced HetNets, IEEE Transactons on Vehcular Technology, Vol. 64, 2015, pp S. Sesa, M. Baker, and I. Toufk, LTE-the UMTS Long Term Evoluton: from Theory to Practce, John Wley & Sons Inc., M. Zobu and V. Saglam, Control of traffc ntensty n hyperexponental and mxed Erlang queueng systems wth a method based on SPRT, Mathematcal Problems n Engneerng, Vol. 2013, 2013, 9 pages. 17. S. Asmussen, Appled Probablty and Queues, Wley, NY, A.-C. Pang, Y.-B. Ln, H.-M. Tsa, and P. Agrawal, Servng rado network controller relocaton for UMTS all-ip networks, IEEE Journal on Selected Areas n Communcatons, Vol. 22, 2004, pp Y. Fang and I. Chlamtac, Teletraffc analyss and moblty modelng of PCS networks, IEEE Transactons on Communcatons, Vol. 47, 1999, pp I. Luketc, D. Smunc, and T. Blajc, Optmzaton of coverage and capacty of

21 SON-BASED CELL SIZE ADAPTION AND TRAFFIC ADAPTIVE EICIC 1523 self-organzng network n LTE, n Proceedngs of IEEE the 34th Internatonal Conventon on Informaton and Communcaton Technology, Electroncs and Mcroelectroncs, 2011, pp R. B. Bunt, D. L. Eager, G. M. Oster, and C. L. Wllamson, Achevng load balance and effectve cachng n clustered web servers, n Proceedngs of the 4th Internatonal Web Cachng Workshop, 1999, pp GPP TS V8.0.0, 3rd generaton partnershp project techncal specfcaton group rado access network evolved unversal terrestral rado access network (EU- TRAN) X2 applcaton protocol (X2AP) (Release 8), December, GPP TS V8.0.0, 3rd generaton partnershp project techncal specfcaton group rado access network evolved unversal terrestral rado access network (E- UTRAN) X2 general aspects and prncples (Release 8), December, Y-Hua Hsu () receved the B.S. degree n Computer and Informaton Scence from the Natonal Tachung Unversty, Tawan, n 2008, and the M.S. and Ph.D. degrees n Computer Scence from the Natonal Chao Tung Unversty, Tawan, n 2010 and 2015, respectvely. Hs research nterests nclude wreless (ad hoc/sensor/vanet) networks, cloud computng, and LTE-A. Kuochen Wang () receved the B.S. degree n Control Engneerng from the Natonal Chao Tung Unversty, Tawan, n 1978, and the M.S. and Ph.D. degrees n Electrcal Engneerng from the Unversty of Arzona n 1986 and 1991, respectvely. He s currently a Professor and the Char of the Department of Computer Scence, Natonal Chao Tung Unversty. He was a Drector n the Insttute of Computer Scence and Engneerng/ Insttute of Network Engneerng, Natonal Chao Tung Unversty from August 2009 to July He was an Actng/Deputy Drector of the Computer and Network Center at ths unversty from June 2007 to July He was a Vstng Scholar n the Department of Electrcal Engneerng, Unversty of Washngton from July 2001 to February From 1980 to 1984, he was a Senor Engneer at the Drectorate General of Telecommuncatons n Tawan. He served n the army as a second leutenant communcaton platoon leader from 1978 to Hs research nterests nclude SDN/ad hoc/cellular/sensor/vanet networks, moble/cloud computng, and power management for multmeda portable devces.