Optical Fiber Distributed Antenna Systems - IEEE Xplore

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2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications

HSDPA System Performance of Optical Fiber Distributed Antenna Systems in an Office

Environment

Zekeriya Uykan and Klaus Hugl

NOKIA Research Center Radio Technologies Laboratory P.O. Box 407, FIN-00045 Nokia Group, Helsinki, Finland E-mail: {zekeriya.uykan, klaus.hugl} @nokia.com Abstract- This paper presents the HSDPA system performance of an optical fiber distributed antenna system (ODAS) in an offlce indoor environment using an advanced dynamic system simulator. Our simulation results show that the investigated ODAS with a low remote unit output power supports high data rate communication for interactive HSDPA in indoor environments and meets QoS requirements.

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I. INTRODUCTION Using a distributed antenna system (DAS) is one possibility

to increase indoor coverage of mobile communication systems [1]. Distributed antennas systems provide cost and spectrum efficient radio coverage for micro and picocells in indoors and outdoors like offices/buildings, shopping malls, airports etc. In addition to coaxial feeders also optical fiber distributions networks in DAS have been suggested [2] and are in use in commercial mobile communication systems, e.g. in the Nokia Advanced Indoor Radio (AIR) product [3] for WCDMA. In this paper, we present the HSDPA (high speed downlink packet access) system performance of such an optical fiber distributed antenna system (ODAS) in an indoor office environment using an advanced dynamic WCDMA system simulator [8]. HSDPA [4] is an evolution technology of third generation mobile system WCDMA standardized in 3GPP to provide high-volume and high data rate packet data use. The HSDPA system performance has been extensively studied in literature (e.g. [5] among many others). On the other hand, the coaxial and fiber DAS has been the focus of many papers e.g.

[1][6][7], showing the basic ability of distributed antennas systems to provide basic indoor coverage. In this paper, we investigate the capability of ODAS to provide high-data rate services based on HSDPA in an indoor environment. Thereby, we evaluate the HSDPA throughput of an indoor ODAS cell as well as the maximum achievable user data rates of such a system. The paper is organized as follows: Section II presents a sketch of the ODAS architecture. The used HSDPA ODAS

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indoor simulation setup is described in Section III. The results of our simulations of an indoor ODAS HSDPA system are presented in Section IV, followed by concluding remarks in Section V. OPTICAL FIBER DISTRIBUTED ANTENNA SYSTEM Traditional distributed antenna systems use coaxial feeders to distribute the signals between the base station and the antenna system. Thereby, lossy feeder cables are used in the signal distribution network. In contrast, an optical fiber distributed antenna system (ODAS) uses optical fibers for signal distribution as shown in Figure 1. II.

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In the base station (BS), the signals are converted from radio frequency domain to an optical analog signal and vice versa in order to distribute the radio signals over the fiber to the Remote Units (RU). These RUs contain an optical unit converting the signals from radio frequency (RF) domain to the optical domain and back. Moreover, the RF signals in case of downlink are amplified using low output power amplifiers before being transmitted through the antennas of the RUs.

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The signals of several remote units connected to the same base station are combined/split using an optical combiner and splitter in analogy to the RF combiners/splitters used in the traditional coaxial DAS. Thus, all RUs connected to the same transmitter transmit exactly the same signal in downlink direction. The optical fiber distribution network of the ODAS has some advantages compared to the coaxial feeders of the traditional DAS: The ODAS doesn't need expensive high output power amplifiers in the BS cabinet and requires just low output power amplifiers in the RUs [2]. For example, in case of Nokia AIR product, 14 dBm output power RUs are used to provide high-data rate indoor coverage and there an optical combiner/splitter is built into the BS cabinet instead of the expensive power amplifiers, as shown in Figure 1. In addition to cost advantage, optical fibers are easier and cheaper to install in already shared cables channels and are nowadays in new complexes of buildings already installed during construction. Moreover, in case of uplink reception, lossy RF feeder cables reduce the overall receiver sensitivity. All these advantages are the reason for the application of optical fiber distributed antenna systems to provide high-data rate HSDPA/WCDMA indoor coverage. III. SIMULATION ENVIRONMENT The system level performance of an indoor ODAS HSDPA system is evaluated using an advanced dynamic WCDMA radio network simulator developed at NOKIA Research Center in Helsinki [8]. The dynamic system simulator includes, besides the WCDMA Rel. 99 features, also full HSDPA functionality, e.g. adaptive modulation and coding (AMC), hybrid automatic repeat request (HARQ) as well as packet scheduling. In order to create a realistic usage scenario, we used the floor layout of a Nokia office in Espoo and modeled it in the dynamic system simulator. As indoor propagation model for our single floor setup, the Motely-Keenan [9] or COST 231 multi-wall-model (MWM) [10] has been applied N, L [dB] = Lo + 10 log,o(d) + EL.aii(j) (1) j=l

In the pathloss formula in (1), Lo denotes the minimum path loss (chosen to be 30.6dB), d is the distance between RUs and the User Equipment (UE) and ,¢-3.2 stands for the path-loss exponent. Lwall (j) is the attenuation in dB through wallj. The modeled floor plan with one, two and four RUs, indicated by red circles, together with the corresponding pathloss maps are shown in Figure 2 to Figure 4. Note that these pathloss maps show the total attenuation (in dB) between the UE and the closest RU using the formula (1), but do not include macrodiversity by reception and transmission from several RUs as done in an ODAS system. Remote unit macro diversity is taken into account in the system simulator by considering all available RUs - not just the closest one. Further simulation parameters are given in Table 1. We applied an indoor mobility model based on the characteristics given in UMTS 30.03 [11]. During the simulation, each UE

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randomly selects one target room and moves to that room with the speed of 3 km/h. Once the UE has reached the room, it stays there 30 seconds. Then it randomly chooses another target room, and moves there, and so on. During each interactive HSDPA data session, a certain number of documents are downloaded to the terminal. After each successful document download, the user evaluates the received infonnation, modeled by the reading time, before downloading the next document. In order to guarantee the full buffer assumption and a high number of ended calls in our simulations, the document length is selected to be relatively long (1.5 MB) and the number of documents downloaded in one user data session is restricted to 2. Quality of Service (QoS) in HSDPA is presented in e.g. [16]. The output power of the RUs has been chosen to be 14 dBm, according to the Nokia Advanced Indoor Radio (AIR) product [3]. 80% of the RU output power is allocated for the HS-DSCH, the HSDPA data channel. The rest of the available power is used for the primary common pilot channel (PCPICH, 10%) and other channels, e.g. dedicated channels (DCH) and other common channels like SCH, PICH, AICH or CCPCH. The considered HSDPA terminals/UEs support 15 multicodes according to UE category 10. In the presented simulation campaign, we considered the basic Round Robin (RR) scheduler [14] as well as the more advanced Proportional Fair (PF) [12] scheduler. The PF scheduler maximizes the average throughput by exploiting the channel information before making scheduling among the UEs while the RR sequentially schedules the UEs [15]. As system performance criteria, the single cell throughput/offer traffic as well as the average received bitrate per UE have been monitored. Parameter Environment Mobility model

Channel model Concrete wall attenuation Thin wall attenuation Service type Ratio of number of UEs in rooms to that in corridors:

Duration of staying in room: Number of HS-DSCH codes RU maximum transmit power Pilot power Max HSDPA power allocation Number of UEs Number of RUs

Scheduler Chip Rate DL system Noise level Isotropic antenna gain:

Setting Indoor, office environment

Indoor, UMTS 30.03

Average UE speed 3 km/h

Pedestrian A Independent fading for each RU

10 dB 2 dB Interactive HSDPA Document length: 1.5 MB

80% in rooms; 20% in corridors 30 seconds 15 25 mW (14 dBm) 2.5 mW (10% of max RU power) 20 mW

_1, 2, 3,5_ 7 10, { _1, 2_ 4_ {Proportional Fair, Round Robin} 3.84 Mchi_ s -100.1 dBm 3 dB

Table 1. Simulation parameters.


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IV. HSDPA ODAS SIMULATION RESULTS In this section, we illustrate the results of our simulation E for an HSDPA ODAS system in an indoor 1campaign environment. In subsection A we present the cell throughput and achievable UE data rates in an isolated cell scenario for a variable number of UEs and RUs, followed by a more detailed investigation of the achieved multi-user gains by using advanced packet scheduling in subsection B. The effect of intercell interference on the ODAS HSDPA system is finally shown in subsection C.

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neo 39n A. Maximum throughputs and data rates I| The HSDPA UE average bitrates and the cell throughput ~140 1SQ AN 1+ 1 - -9 wfor an ODAS HSDPA system in an isolated cell scenario with a variable number of RUs and users is given in Figure 5. Figure 2. Floor plan and pathloss map of the one RU reference case (depicted as RU1). UE Average Bitrate kblseletl 0 NO

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Figure 4. Floor plan and pathloss map of the four RU case (depicted as RU4).

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~~~~~rateand system throughput improvement in this isolated cell 0case. Without intercell interference and jUSt considenng baSiC noise limitation, a single low output power RU is sufficient to provide coverage in this about 60xl7m large office space. Increasing the number of RUs does not just improve the received signal strength but also increases the produced intracell interference. The situation is different when also

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considering intercell interference as will be shown in subsection C. Naturally, increasing the number of users in the system reduces the achievable bitrates of each of the UEs. However, with a larger number of UEs, the PF scheduler [12] is able to introduce multi-user diversity and thereby increases the cell capacity as seen in Figure 5.b. The cell throughput in our isolated cell case is boosted from 3.5Mbps to almost 9Mbps with an increase of up to maximum 10 simultaneous HSDPA users. This is in contrast to basic WLAN, where the throughput decreases with increasing number of users due to collisions e.g. [13]. The reason for the increase in system throughput with increasing number of UEs is the higher probability of using high-rate modulation and coding schemes (MCS). The modulations distributions for 1, 5 and 10 UEs are plotted in Figure 6. The results indicate that the PF scheduler increases the probability of using higher modulation and coding schemes and thereby increases the sector throughput/data rate by providing multi-user diversity. The higher the number of UEs in the system, the better the PF scheduler performance because it has more UEs to choose from (depending on their channel conditions). A more detailed analysis of the effect of multiuser diversity on the single user data rates as well as system throughput will be presented in the next subsection. 100 90 80 70 c 60 i 50

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Figure 6. Distribution of QPS and 16QAM modulations for RUI case with the PF scheduler for 1, 5 and 10 UEs.

Comparing Figure 5.a to Figure 5.b, it is seen that the system throughput is lower than the single UE data rate multiplied with the maximum number of simultaneous UEs. This is due to the fact that actual number of active UEs at a given time during the simulation is equal to or lower than the maximum number of UEs. B. Multi-user diversity gain by the PF scheduler Figure 7 presents average UE received bitrate and offered traffic in the isolated cell scenario for the RUl and RU2 cases (Figure 2 and Figure 3) with 10 UEs and for the PF and RR schedulers. Figure 7 shows that the multi-user diversity provided by the PF scheduler results in a throughput improvement of 70% to 80% compared to the RR scheduler.

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so far have been generated for an isolated office environment with downlink system noise floor of -100.1 dBm. In this subsection we examine the effects of interference/noise level on the HSDPA performance. To be independent of a special interfering base station setup and building surroundings, we assume here the total downlink intercell interference to be evenly distributed over the cell/building. Therefore, we model different intercell interference conditions as an increase in the total background noise (i.e., noise + total interference) at the UE. We denote this increase as Noise + Interference Factor (NIF) in this paper. The (total) downlink system background noise is then equal to -1 00. l dBm+NIF in dB scale. By varying the system noise floor in the advanced dynamic simulator, different amount of DL intercell interference is emulated. Figure 8 shows the offered traffic and average UE received bitrate for different NIF cases. The results indicate that the RU setup giving the highest throughputs depends on the downlink interference level in the system: For the NIF=0 dB (isolated cell case, i.e., no intercell interference), only one RU is sufficient to provide high data rates HSDPA services. However, as the NIF increases, e.g. for the NIFs of 30 dB and 40 dB (relatively large intercell interference cases), a higher number of used RUs results in higher average bitrates due to the macro diversity provided by the ODAS. Thus, increasing the number of RUs improves the UE average bitrates and the cell throughput in high interference conditions.


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ACKNOWLEDGMENT The authors would like to thank Mr. Mika Kolehmainen and Mr. Tero Henttonen from NOKIA Research Center for their helps during the HSDPA simulations campaign.

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The RU setup giving the best HSDPA performance (highest throughputs) depends on the downlink interference level in the system. Increasing the number of RUs, macro diversity increases the system throughput in high intercell interference cases. For isolated hotspots, it is basically sufficient to guarantee basic coverage with a minimum number of RUs.

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Figure 8. (a) Offered traffic and (b) Average UE Received Bitrate for different NIF cases. V. CONCLUDING REMARKS In this paper, we present the HSDPA system performance of an optical fiber distributed antenna system in an office indoor environment using an advanced dynamic system simulator. The system simulation results for the office environment in Figure 2 to Figure 4 indicate that * ODAS HSDPA can support interactive HSDPA service and meet QoS requirements. * A low RU output power (in our case 14 dBm) is sufficient to provide high data rate indoor services. * With 10 HSDPA UEs in the system, bitrates in the region of 1 Mbps can be provided, corresponding to a system throughput of about 9 Mbps. * With a single HSDPA UE in the ODAS system, average bitrate about 4.8 Mbps can be provided. * The multi-user diversity provided by the PF scheduler resulted in a throughput improvement of 70% to 80% compared to the RR scheduler in the considered isolated cell

[I] A.A.M. Saleh, A.J. Rustako and R.S. Roman, "Distributed Antennas for Indoor Radio Communications", IEEE Transactions on Communications, Vol. COM-35, No. 12, pp. 1245-1251, Dec. 1987. [2] T.S. Chu and M.J. Gans, "Fiber Optical Microcellular Radio", IEEE Trans. on Vehicular Technology, Vol. 40, No. 3, pp. 339-344, Aug. 1991. [3] Nokia Advanced Indoor Radio (AIR), information available at vww.nok,a.cron. [4] 3GPP Technical Report, TR 25.858 (Release 5), "High Speed Downlink Packet Access: Physical Layer Aspects", March 2002. [5] T.E. Kolding, F. Frederiksen, and P.E. Mogensen, "Performance aspects of WCDMA systems with high speed downlink packet access (HSDPA)", Proc. IEEE VTC 2002-Fall, vol. 1, pp.477481, Sep. 2002 [6] D. Wake, and K. Beacham, "A novel switched radio over fiber architecture for distributed antenna systems", Lasers and Electro-Optics Society,. LEOS, 2004., vol: 1, Nov. 8-9, pp. 55-56. [71 V. Nikolopoulos, M. Fiacco, S. Stavrou, S.; S.R. Saunders, "Narrowband fading analysis of indoor distributed antenna systems", IEEE Antennas and Wireless Propagation Letters , vol. 2,pp. 89-92, 2003. [8] S. H m I inen, H. Holma and K. Sipil , "Advanced WCDMA radio network simulator", in Proceedings of PIMRC '99, pp. 509-604, Aalborg (Denmark), October 1999. [9] A.J. Motley and J.M. Keenan, "Personal Communications radio Coverage in Buildings at 900 MHz and 1700 MHz", Electrnic Letters, vol. 44, no:12, 1998, pp.763-764. [10] "Digital Mobile Radio Towards Future Generation Systems", COST 231 Final Report, European Communities 1999, ISBN 92-828-5416-7, pp.167-174. [II] UMTS 30.03 version 3.2.0, Selection procedures for the choice of radio transmission technologies for UMTS, TR 101 112, v.3.2.0, "UE Radio Transmissison and Reception", 1998-04. [12] P. Bender et. Al., "CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users", IEEE Communications Magazine, Vol. 38, No. 7, pp. 70-77, July 2000. [13] G. Bianchi, "Performance Analysis of the IEEE 802.11 Distributed Coordination Function", IEEE JSAC, Vol. 18, No. 3, pp. 535-547, March 2000. [141 L. Kleinrock, Queing Systems, volume 1: Theory, Wiley-Interscience Publication, New York, 1975. [15] J. Ramiro-Monero, K.I. Pedersen, P.E. Mogensen, "Network performance of transmit and receive antenna diversity in HSDPA under different packet scheduling strategies", Proc. of IEEE VTC 2003-Spring, April 2003, vol.2, pp. 1454-1458. [16] P.Jose and A. Gutierrez, Packet Scheduling and Quality of Service in HSDPA, PhD thesis, Dep. of Comm. Tech., Aalborg University, 2003.

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