Reed, Mark

Shi, Zhenning

Wang, He (Jackson)

Zhao, Ming

The Australian National University

On the Uplink Coverage and Capacity of UMTSFemtocells in Enterprise EnvironmentZhenning Shi‡, Ming Zhao∗†, Mark C. Reed∗†, and He Wang†∗∗NICTA, Canberra Research Laboratory†The Australian National University‡Alcatel Lucent-Shanghai BellEmail: {Mark.Reed, Ming.Zhao}@nicta.com.auAbstract— This paper investigates the uplink capacity andcoverage of femtocells for enterprise UMTS applications to betterunderstand the trade-offs and the expected performance of such adeployment. A radio system model is used where a large numberof randomly distributed users are in an open office layout, whereHome NodeBs are deployed. The interference effect is studiedand the performance with interference mitigation is determined.The results and methodology could be used to plan a systemdeployment. Furthermore, results show that advanced receivertechnology implemented in the Home NodeB will significantlyenhance the uplink capacity and coverage, and minimize theoverall cost of a femtocell enterprise solution.I. INTRODUCTIONFemtocells are wireless access points (or Home NodeB),which were first proposed as a solution for residential use [1],[2], where users in households with poor coverage could getimproved service by connecting a femtocell to their broadbandrouter [3]. The introduction of femtocells gives rise to anumber of technical challenges, including the IP interface tothe backhaul network, closed or open access, synchronizationand interference [4]. In the residential use-case co-channelinterference of the femtocell underlay system has been studiedby a number of authors [5]–[7].What hasn’t been very well addressed is the utilization offemtocells in the enterprise or office environment [8]. Here theutilization of femtocells has significant advantages in that usertraffic is offloaded from the outdoor macro wireless system tosave cost for the operators and enterprise.In this paper, we consider the uplink interfering scenarios ina 3GPP UMTS network for the early deployment of femtocellsin enterprise envrionment, where a closed-loop power controlalgorithm to accommodate a large number of femtocell con-nected User Equipments (UEs) is proposed. The approach canbe extended to plan high speed access solutions. We focus onthe femto-to-femto interaction in a shared carrier for multiplefemtocells. Furthermore, we consider interference mitigationtechniques to enable the network operation even in extremecases. The overall capacity and coverage of the enterprisefemtocells can be enhanced dramatically by employing suchadvance receiver technology.II. SYSTEM MODELIn this study, a static network simulator is proposed toevaluate the performance of the enterprise femtocell system.Through modeling channel, femtocell connected UEs andNodeBs, the results over a large number of simulations areused to generate the average stastics for the system.A. Channel ModelThe network simulations are conducted at a carrier fre-quency of 2GHz. The signal transmission power is subjectto a propagation characterized by the ITU-R.P 1238 model[9], which is suitable for the indoor office scenario. Thepropagation pathloss is given by:PL = 20× log10(fc) + 30× log10(d)− 17, (1)where fc is the carrier frequency, d is the distance betweenthe transmitter and the receiver. The Additive White GaussianNoise (AWGN) floor at the NodeB receiver is −95dBm overan 5MHz spectrum. There are no internal walls/windowsassumed to attenuate the transmitted signals.B. Femtocell Connected UE ModelingThe enterprise femtocell UEs (FUE) in the simulationscan transmit at 12.2kbps (voice call), 64kbps, 144kbps and384kbps, with the corresponding spreading factor being 64,16, 8 and 4 respectively. The requirement on the receivedSignal-to-Interference Ratio (SIR) conforms to the minimumrequirement on 3GPP UMTS base stations [10], [11]. The UEtraffic portfolio can be adjusted to reflect a specific trafficmodel in the realistic scenario. The FUE transmit power iscapped at 20dBm. The UEs are modeled to camp on theNodeB associated with the minimum pathloss.C. Home NodeB ModelingThe Omni-directional antenna is employed at the enterpriseHome NodeB. Round-Robin scheduling is employed at thebase stations to provide equal service to all active UEs. TheRise over Thermal (RoT) threshold at the NodeB scheduler isset to be 12dB.III. CLOSED-LOOP POWER CONTROL ALGORITHM FORENTERPRISE FEMTOCELLSA. Link Budget and InterferencesGiven the pathloss PL(k, ks)1 between the UE and itsserving NodeB ks, the NodeB AWGN floor N0(ks), and the1The antenna gain and loss are absorbed in the pathloss term, for the sakeof simplicity.UE’s SIR requirement SIRk, i.e. the target SIR, the initialtransmit power Pt(k) of the kth UE can be computed as:Pt(k) = PL(k, ks)×N0(ks)× SIRk, (2)where k ∈ Ψ(ks) and Ψ(ks) is the FUEs’ set on the servingNodeB ks. The initial transmit power Pt(k) is the requiredtransmit power for the kth UE in the single-user channel,which is the lower bound of user power in multi-user systems.The intracell as well as the intercell interference for thekth user’s transmission link can be computed using the initialtransmitting power as follows:Iintra(k) =∑j∈Ψ(ks),j 6=kPt(j)PL(j, ks), (3)Iinter(k) =∑l 6=Ψ(ks)Pt(l)PL(l, ks). (4)B. Femtocell UE Link Adjustment ProcedureIn the closed-loop power control algorithm proposed in thispapser, the instantiate SIR at nth iteration for the kth UE canbe computed as:SIRnk =Pnt (k)(N0(ks) + Iintra(k) + Iinter(k))× PL(k, ks).(5)Compare to the target SIR, denoted as SIRk, we canmeasure the SIR difference as:∆nSIRk =SIRnkSIRk. (6)If this SIR difference at nth iteration is greater than thepower control step, i.e. |∆nSIRk | > ∆, we need to increase ordecrease the UE transmitting power and update the intracelland intercell interference to mth UE (m 6= k) as follows:Pn+1t (k) =Pnt (k)∆nSIRk, (7)In+1intra(m) = Inintra(m) + (1/∆nSIRk− 1) Pnt (k)PL(k, ks)(8)In+1inter(m) = Ininter(m) + (1/∆nSIRk− 1) Pnt (k)PL(k, ks). (9)It is worth mentioning that the we skip the incremental powercontrol steps in above link adjustment for faster power controlconvergence.On the other hand, if the SIR difference at nth iteration issmaller than the power control step, i.e. |∆nSIRk | ≤ ∆, wewill claim that the power control is converged for the kth at(n + 1)th iteration.We start the above closed-loop power control from thefirst femtocell UE, i.e. k = 1, until the power of all UEsare converged to satisfy the target SIR. In order to managethe convergence of the power control algorithm, we limit themaximum number of power control iterations to be 30. In thecase of non-convergence happens, we will claim the UEs arein outage if they cannot meet the target SIR.Fig. 1. Uplink capacity for enterprise femtocellsFurthermore, in the above mentioned algorithm, single-userdetector (SUD) receiver is used as an illustrative example indetermining the co-channel interference as well as receivedSIR. For more advanced receiver techniques, the procedures tocompute the effective co-channel interference and SIR updatesneed to be adjusted accordingly. For example, if an iterativeParallel Interference Cancelation (PIC) based multiuser detec-tor (MUD) is employed, the interference term should be theeffective interference remaining after a number of iterations.IV. SIMULATION RESULTSA. Simulation SetupIn this section, we present the system simulation resultsfor an enterprise femtocell serving high speed UEs with adata rate of 384 kbps. Two types of enterprise Home NodeBsare considered in the simulation. “NodeB SUD” employsconventional SUD receiver, and ”NodeB IFX” represents theadvanced NodeB with intra-cell interference cancelation capa-bility. The office environment under consideration is confinedto a rectangular region (9600m2, 80m× 120m). There are upto four NodeBs in the office, and the NodeBs are deployed inthe middle of the service area so that the whole office areacan be covered evenly. The FUEs are deployed randomly withuniform distribution in the office area. The cell capacity isevaluated at an outage probability of 5%.B. Numerical Results1) Capacity for Uplink Enterprise Femtocells: Fig. 1 showsthe capacity of the enterprise femtocell by varying the numberof NodeBs and comparing the performance of conventionalNodeBs to that of the advanced NodeBs. It can be observedthat the femtocell with conventional NodeBs can only supportup to 1.5Mbps, 3Mbps and 4.5Mbps high speed UEs withone NodeB, two NodeBs, and four NodeBs respectively. Onthe other hand, the femtocell with one advanced NodeB cansupport up to 4.6Mbps before substantial outage shows up.This indicates a 200% increase in capacity can be obtainedby deploying a single advanced NodeB, which is roughlythe same as the improvement by deploying four conventionalNodeBs.Fig. 2. Noise rise at uplink enterprise femtocells2) Noise Rise at Uplink Enterprise Femtocells: Fig. 2shows the noise rise at NodeBs in the enterprise femtocells. Asthe femtocell with advanced NodeB can support three timesof the high speed UEs as the NodeB SUD, the noise risesaturates around 10dB, which is close to the RoT. However, thenoise rise at conventional NodeB only saturates at 6dB for oneNodeB, and 9dB for two and four NodeBs. This observationshows that the advanced NodeB can work in heavily loadedscenarios to maximize the system capacity, while for theconventional NodeBs the noise rise may experience a drasticincrease by admitting one more user into the system withmoderate loading. Therefore, cells with NodeB SUD areunable to handle high-speed traffic.3) Uplink Capacity vs. Coverage in Enterprise Femtocells:Fig. 3 shows the capacity versus coverage if the area of theoffice is varying between 104m2 to 106m2. It can be seenthat generally one or two conventional NodeBs can provide acoverage up to 105m2 , while four conventional NodeBs canprovide a coverage up to 3×105m2 . For an office with area of104m2, an advanced NodeB can triple the servicing 384kbpsUEs if one conventional NodeB is deployed and double theservicing UEs if two conventional NodeBs are deployed. Itcan even support two more UEs compared to deploying fourconventional NodeBs. Furthermore, deploying one advancedNodeB has better coverage up to 7× 104m2 compared to thedeployment plan of four conventional NodeBs if four UEsare supported in the femtocell. Therefore, deploying femtocellwith advanced NodeB will improve the system coveragesignificantly.V. CONCLUSIONIn this paper, we studied the uplink capacity and coverageof a UMTS femtocell networks for the enterprise scenario. Weanalyzed the femto-to-femto interaction with the interferenceamong high speed (384kbps) FUEs. We simulated the systemcapacity and coverage by adopting the conventional single userdetection and advanced interference mitigation techniques inthe Home NodeBs. The results show that there are great ben-Fig. 3. Uplink capacity vs. coverage for enterprise femtocellsefits by deploying femtocell with advanced NodeB Receivers.The system capacity can be tripled while the noise rise at theNodeB is close to the designed RoT. The UEs served by theadvanced NodeB generally transmit less power than served bythe conventional NodeBs so that longer connecting time canbe experienced by the UEs. Meanwhile, the coverage can alsobe enhanced, which means the cost of the enterprise femtocellsolution is minimized for the offices with extensive area.ACKNOWLEDGMENTM. Zhao, M. C. Reed and H. Wang are with NICTA andaffiliated with the Australian National University. NICTA isfunded by the Australian Government as represented by theDepartment of Broadband, Communications and the DigitalEconomy and the Australian Research Council through theICT Centre of Excellence program. Z. Shi was with NICTAwhen the work was done and is currently with Alcatel Lucent-Shanghai Bell.REFERENCES[1] V. Chandrasekhar and J. G. Andrews, “Femtocell networks: A survey,”IEEE Commun. Mag., vol. 46, pp. 59–67, Sep 2008.[2] D. Das and V. Ramaswamy, “On the reverse link capacity of a CDMAnetwork of femtocells,” in Proc. IEEE Sarnoff Symposium, Princeton,NJ, Apr 2008, pp. 1–5.[3] S. Saunders et al., Femtos: Opportunities and Challenges for Businessand Technology. Wiley, 2009.[4] H. Claussen, L. Ho, and L. Samuel, “Self-optimization of coverage forfemtocell deployments,” in Proc. Wireless Telecommunications Sympo-sium, Pomana, Califonia, Apr 2008, pp. 278–285.[5] V. Chandrasekhar and J. G. Andrews, “Uplink capacity and interferenceavoidance for two-tier femtocell networks,” in Proc. IEEE GLOBECOM,Nov 2007, pp. 3322–3326.[6] “Interference management in UMTS femtocells,” Femto Forum WhitePaper, Dec 2008.[7] R4-081345, “HNB and macro uplink performance with adaptive atten-uation at HNB,” Qualcomm Europe.[8] R4-080154, “Simulation results for Home NodeB to Macro NodeBuplink interference within the block of flats scenario,” Ercisson.[9] Propagation data and prediction methods for the planning of indoorradiocommunication systems and radio local area networks in thefrequency range 900 MHz to 100 GHz, ITU-T Std. P.1238-6, 2009.[10] Base Station (BS) radio transmission and reception (FDD), 3GPP Std.TS25.104 Rel-7, 2008.[11] Base Station (BS) conformance testing (FDD), 3GPP Std. TS25.104Rel-7, 2008.

On the Uplink Coverage and Capacity of UMTS Femtocells in Enterprise Environment

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