Measurements for Distributed Antennas in WCDMA Indoor Network

Transcription

1 Measurements for Distributed Antennas in WCDMA Indoor Network Tero Isotalo, Jarno Niemelä and Jukka Lempiäinen Institute of Communications Engineering, Tampere University of Technology P.O.Box 553 FI-33 TAMPERE FINLAND Tel , Fax tero.isotalo@tut.fi, Abstract The target of the paper is to provide guidelines for indoor antenna selection for passive distributed antenna system or radiating cable implementation. Distributed antenna systems are commonly used for indoor installations, and radiating cables are often used in tunnels, airplanes, or similar constricted areas. However, guidelines for optimizing WCDMA indoor network antenna configuration are lacking in the literature. In this paper, the effect of different distributed antenna configurations on signal quality and system performance is studied, taking also radiating cable in comparison. A special attention is paid to the future requirements of HSDPA. The measurement results show that planning of distributed antenna system is not very sensitive to the amount of antennas, as long as good coverage can be ensured. Also radiating cable can be used, but providing good coverage requires very dense installation of cables. Key words: DAS, field measurements, indoor, radiating cable, WCDMA. Introduction The need for macrocellular 3G (3rd generation) networks is fast increasing in densely built-up areas. In the near future, a significant number of high data rate users are located indoors. Therefore, providing good indoor coverage and capacity also for indoor users will be an important topic for network operators. Interference and coverage limitations prevent efficient usage of outdoor networks for serving indoor users. Thus, dedicated indoor systems should be used []. Available solutions for building indoor coverage with dedicated systems are distributed antenna system (DAS), radiating cables () [], or pico base stations [3]. Optimizing the behavior of basic UMTS (universal mobile telecommunication system) indoor systems is an up-to-date topic, but the future needs of implementing an HSDPA (high speed downlink packet access) functionality should be taken into account when planning new indoor systems. To take a maximal advantage of the HS- DPA enabled network, good radio conditions are needed for high bit rate applications that in indoor emphasizes the need of dedicated indoor systems [4]. In GSM indoor system planning, the main target was to ensure good coverage, whereas optimizing WCDMA (wideband code division multiple access) indoor system might not be that straightforward task. Indoor environment may cause some challenges for WCDMA system due to, e.g., lack of multipath diversity [5]. Thus, excluding few simulation related references, e.g., [6], [7], guidelines for a planning dedicated WCDMA indoor systems can not be found from the literature.. WCDMA Indoor Planning.. Indoor Environment A system is considered to be a wideband, if the signals transmission bandwidth is much wider than the coherence bandwidth of the radio channel. The coherence bandwidth of macrocellular environment varies typically between.53 MHz and MHz, which is clearly less than the bandwidth of WCDMA system (3.84 MHz). Therefore, WCDMA system is rather robust for frequency selective fast fading in typical outdoor environments due to additional multipath diversity. However, in indoor environment the coherence bandwidth can be more than 6 MHz. [5] This leads to WCDMA signal being flat fading in most of typical indoor environments, which might cause some unintended behavior and lowered the system performance in indoor locations... Configuration Planning Due to interference limited nature of WCDMA networks [8], the requirement of high throughput for a single user in the cell edge or in an indoor location can consume great amount of the downlink transmit power of the whole cell that naturally affect the available capacity of the whole cell. High bit rate users are rather often in positions with high propagation loss, such as buildings or cars. If outdoor network is not planned to provide coverage and capacity for indoor users, or some indoor location is introducing high load to some cells, a dedicated indoor solution inside the building is worth considering for providing good indoor coverage [6], [9]. Different antenna configurations have been studied in various cellular technologies, i.e., [], []. Moreover, simulations have been performed to analyze the capacity of a dedicated WCDMA indoor system [6], [7]. However, measurement-based comparison of different antenna configurations and radiating cable providing background information and guidelines for planning an indoor network can not be found in the literature. There are some possible solutions for building indoor coverage. Macro/microcellular networks can be planned

2 in such a manner that in-building coverage is taken into consideration, which reduces the need for deploying dedicated indoor systems. However, the high in-building penetration loss makes it an inefficient solution. Most typical solution is implementation of independent indoor base station, with either DAS, [], or even distributed base station system [4]. The indoor environment constitutes a challenge for radio network planning due to the difficulty to perform accurate simulations in indoor environment. In macrocellular radio network planning, estimating the propagation of the signal can be based on propagation models, such as Okumura-Hata. In indoor, simple models can be used, for instance 3GPP path loss model for indoor environment [3], but very high accuracies should not be expected. On the other hand, ray-tracing or similar accurate models can be exploited, but this requires very detailed information on the building, which may lead to higher planning cost. Practical knowledge on earlier successful installations has been used in GSM indoor planning, but a part of the phenomena caused by WCDMA system remain currently undiscovered..3. Indoor Antenna Systems Distributed antenna system is the most common approach for providing in-building coverage. Antennas used in DAS are small discrete antenna elements designed specially for indoor use. Typically they are either omnidirectional or low gain directional antennas. In a passive DAS implementation, signal is transferred from base station by network of feeder cables, connected by splitters and tappers. Advantages of DAS are easy planning and good coverage, while drawbacks include high installation costs compared to, e.g., indoor pico cells or outdoor-to-indoor repeaters [4]. Radiating cable (), often also called as leaky feeder, is simply a feeder cable with small holes or groove in the outer conductor of coaxial cable, and the signal leaks in a controlled way from the cable. The end of a radiating cable needs to be either terminated, or one can install a discrete antenna there. Radiating cables are typically used in, e.g., tunnel installations. Due to small (effective isotropic radiated power) of, it is also a good choice for interference sensitive environments, such as hospitals and airplanes. Another possible approach for indoor building an indoor systems is usage of pico cells; a single pico cell or a dense enough network of small base stations with, e.g., in-built omnidirectional antenna in desired area for indoor coverage. Note that the fundamental difference between a passive and an active DAS is the losses in the feeder lines. However, from antenna configuration point of view, this separation cannot be made if coverage targets can be achieved. Hence, the results of this paper can be applied to both distributed antenna systems. 3. Measured Quality Indicators Coverage of a cell is estimated by P- RSCP (received signal code power for the primary common pilot channel). Standard deviation (STD) of RSCP caused by free space attenuation together with slow and fast fading indicates the smoothness of RSCP in the network. Large variations in RSCP leads to greater slow fading margins. Quality of the coverage is indicated by P- E c /N, the received energy per chip on P- divided by the power density in the band. The P- E c /N is defined as ratio between RSCP and RSSI (received signal strength indicator): E c = RSCP N RSSI () In addition, interference level is indicated with signal to interference ratio (SIR) for the P-: P P CP ICH SIR = SF 56 ( α)p T OT + (I inter + p n )L where SF 56 is the processing gain for P-, P P CP ICH is the transmit power of P-, α is the downlink orthogonality factor, P T OT is the total transmit power of the base station, I inter is the received inter-cell interference, p n is the received noise power, and L is the path loss. The instantaneous root mean square (RMS) delay spread σ RMS is evaluated in this paper from scanner measurement according to following definition: σ RMS = τ τ (3) () where τ k = a k τ k a (4) k is the mean excess delay and τ k = a k τ k a (5) k is the mean delay with a k as the amplitude of kth multipath component and its a relative delay of τ k. 4. Measurement Setup Measurements were conducted in an university building, in two different environments; open area and office corridor. Open area environment consisted of an open m long large corridor, having an open hall in the other end and being floors in height. The antennas were located in the nd floor level, whereas the measurements were carried out in the st floor level. Antennas did not have LOS (line-of-sight) between each others, where as connection between antenna and UE consisted of a mixture of LOS and NLOS (non-lineof-sight) connections. The office corridor measurements

3 a) -Antenna -.db 5 m -4.dB dbi 7.66dBm < b) -Antenna -.db 5 m -4.dB dbi 4.66dBm < (a) -antenna / (b) -antenna c) 3-Antenna -.db -5 db 5 m -4.dB dbi.66dbm d) 4-Antenna -.db 5 m -4.dB dbi.66dbm (c) 3-antenna (d) 4-antenna e) Radiating Cable -.db m.66db (e) Measurement routes, st floor (f) Measurement routes, nd floor Figure. System block diagram and antenna configuration scenarios for open area measurements. were carried out in a narrow corridor opening to other corridors, and having various small rooms along. Antennas were located in the nd floor, and measurements were conducted in the st as well as in the nd floor. Antennas had LOS between each others. Measurement route in the nd floor had almost continuous LOS for the antennas, whereas measurement route in the st floor had only NLOS connections. The measurements were carried out in an experimental WCDMA network, consisting of an /Iub (radio network controller) simulator running on a PC, and a commercial WCDMA base station, connected to the an antenna system. The antenna system consisted of a feeder cable connected to varying antenna configuration. The antenna configuration for open area measurements is shown in Fig., and the configuration for office corridor measurements is otherwise similar, except the attenuation in feeder cable is.9 db smaller. Also a 4- antenna scenario was excluded from measurements in office corridor. Depending on the antenna configuration, the signal was transmitted either directly to an antenna or a, or split into -4 equal parts (-3 for office corridor). Antenna locations are shown in Fig.. (a)-(d). Antenna locations for the office corridor are shown in blue/upper markings, and for the open area with red/lower markings. Antennas used for the measurements were omnidirectional indoor antennas with dbi gain. Radiating cable and feeder cable were /" coaxial cables, and the length of the radiating cable was m. Measurement routes are shown in Fig..(e) for the st floor, and Fig..(f) for the nd floor. The shorter measurement routes in Fig..(f) are measured underneath Figure. Antenna positions for -4 -antenna and radiating cable scenarios (a)-(d), and measurement routes (e-f). (a)-(d) are located on the nd floor. Upper/blue markings are for the office corridor and lower/red for the open area. the radiating cable. Measurement equipment consisted of a WCDMA UE and a WCDMA scanner, connected to a radio interface measurement software. The cell scenario was isolated, i.e., no inter-cell interference was present and the measurements were conducted in connected mode having a. kbps speech connection. The measurement routes were repeated several times in order to increase the reliability of results. 5. Measurement Results All measured values are are shown in Table. For the open area measurements, E c /N values remain at the level of 4 db for all configurations, and moreover the values are at the same level for all the antenna configuration. This is natural when the load of the network is low, and no coverage limitations occur, i.e., RSCP remains at good level above the thermal noise floor at the mobile. The RSCP level is improved when increasing the amount of antennas. However, the coverage improvement achieved by adding more than two antennas is rather small partly due to lower ( db improvement of having four antennas instead of two). However, with radiating cable the average RSCP is clearly lower (3..7 db difference compared to discrete antennas). A decrease in STD of the level of RSCP was expected after increasing the number of antennas, but it remains almost unchanged in open area environment in all discrete antenna scenarios. Radiating cable provides clearly the smallest deviation in RSCP. The average delay spread in discrete antenna scenarios is between µs and 3 µs, whereas with radiating cable

4 Table. Idle mode measurement results. Open area -antenna -antenna 3-antenna 4-antenna E c /N [db] RSCP [dbm] RSCP STD [db] Delay spread [µs] SIR [db] Office corridor, nd floor -antenna -antenna 3-antenna E c /N [db] RSCP [dbm] RSCP STD [db] Delay spread [µs] 4 SIR [db] Office corridor, st floor -antenna -antenna 3-antenna E c /N [db] RSCP [dbm] RSCP STD [db] Delay spread [µs] 8 SIR [db] antenna antenna 4 antenna antenna antenna F(X) F(X) Figure 3. of downlink SIR measurements in open area for all antenna configurations. Figure 4. of downlink SIR measurements in office area for all antenna configurations (nd floor). slightly higher, 9 µs. The results indicate that certain amount of multipath diversity would be available in the particular measurement scenario. Also SIR values remain at the level of db. The (cumulative distribution function) of SIR in open area is shown in Fig. 3. Increasing the amount of antennas is not affecting SIR, since the variations between different antenna configurations are inside one decibel. In the office corridor in the nd floor, the measurements were conducted close to antennas with LOS conditions. Again, E c /N remains close to 4 db due to same reasons mentioned before and the level of RSCP is again increasing when adding more antennas. There is an improvement of.4 db when adding a second antenna, and another.3 db when adding a third antenna. Radiating cable has again clearly the worst average RSCP. In the office corridor, STD of RSCP is behaving expectedly, decreasing while increasing the number of antennas and again, radiating cable provides the smallest signal level variations. Also the delay spread values follow the ones seen in the open area measurements. When being very close to the antennas, increasing the number of antennas seems to have negative effect on SIR. Moreover, radiating cable provides high values for SIR. Measurements in office corridor in the st floor repeat the measurements with same configurations than in the nd floor, differing only by added one floor attenuation. Worsened coverage is affecting E c /N ; slight decrease with discrete antennas, and clear decrease with radiating cable due to coverage constraints. Being still relatively close to antennas, radiating cable is already almost unusable with one floor attenuation, discrete antennas still having adequate RSCP values. Direction is again the same; increasing the number of antennas improves RSCP values slightly, while also decreasing signal level variations (RSCP STD). It is good to notice that with floor attenuation, signal variations with are now at the same level with -antenna scenario, whereas close to cable radiating cable provided the smoothest cover-

5 F(X).. antenna antenna Figure 5. of downlink SIR measurements in office area for all antenna configurations (st floor). age. Average delay spread remains at the same level regardless of floor attenuation. SIR values are clearly increased when adding more antennas, and radiating cable is Showing lowest SIR values due to reduced coverage. The of SIR in the office corridors is shown in Fig. 4 for measurements under the antennas (the nd floor) and in Fig. 5 for measurements with one floor attenuation (the st floor). 6. Conclusions and Discussion This paper covers the measurements results for indoor UMTS network with different antenna configurations. These antenna configurations consisted of a passive distributed antenna configuration with one to four omnidirectional antenna or of a radiating cable. The results indicate that the average level RSCP increases if the amount of antennas is increased in open area indoor and also in more closed corridor -type of indoor environment. The increase of RSCP was observed even though the signal was divided into increasing parts having lower s for different antennas. The standard deviation of RSCP decreases while the amount of antennas is increased, which means more uniform signal conditions over the measured area. A radiating cable provides in general more uniform signal level. However, in order to achieve a uniform signal level and adequate coverage, an extremely dense cabling is required. The delay spread values for all configuration were almost the same indicating the balance between the amount of multipath diversity and loss of downlink code orthogonality. Without any floor attenuation it was observed that increasing the number of antennas actually decreases the SIR. However, with a floor attenuation, the situation is opposite. Based on the measurement results, it can be seen that increasing the amount of antennas in DAS improves the coverage and smooths the average signal level in the network, but does not provide significant gain in signal quality. Hence, the measurement results shows that the capacity of the network does not change at all if the number of antennas are increased. Radiating cable can provide decent quality as long as the coverage is not limiting. Therefore it can be discussed, whether the focus of planning WCDMA networks should be more in coverage than capacity. Future studies will include comparison of DAS and pico cells in indoors, as well as measurements with indoor HSDPA implementation. Acknowledgment Authors would like to thank Elisa Oyj, European Communications Engineering (ECE) Ltd, Nemo Technologies, and Nokia Networks for helpful comments concerning the measurements, and for enabling the measurement setup. This work was partly funded by National Technology Agency of Finland. REFERENCES [] K. Hiltunen, B. Olin, and M. Lundevall. Using dedicated in-building systems to improve HSDPA indoor coverage and capacity. In IEEE 6st Vehicular Technology Conference, VTC 5-Spring, pages , 5. [] C. Seltzer. Indoor coverage requirements and solutions. In IEE Colloquium on Antennas and Propagation for Future Mobile Communications, pages 3/ 3/4, Feb 998. [3] H. Andersson, R. S. Karlsson, P. Larsson, and P. Wikström. Improving system performance in a WCDMA FDD network using indoor pico base stations. In IEEE 56th Vehicular Technology Conference, VTC Fall, pages ,. [4] H. Beijner. The importance of in-building solutions in thirdgeneration networks. Ericsson Review, ():9 97, 4. [5] J. Lempiäinen and M. Manninen, editors. UMTS Radio Network Planning, Optimization and QoS Management. Kluwer Academic Publishers, 3. [6] D. Hong, S. Choi, and J. Cho. Coverage and capacity analysis for the multi-layer CDMA macro/indoor-pico cells. In IEEE International Conference on Communications, ICC 99, pages , 999. [7] R. E. Schuh and M. Sommer. W-CDMA coverage and capacity analysis for active and passive distributed antenna systems. In IEEE 55th Vehicular Technology Conference, VTC Spring, pages ,. [8] J. Laiho, A. Wacker, and T. Novosad. Radio Network Planning and Optimisation for UMTS. John Wiley & Song Ltd,. [9] T. Chen, P. Hautakangas, Q. Cai, and X. Che. Fill-in solution for the coverage hole in the indoor environment. In IEEE 6th Circuits and Systems Symposium on Emerging Technologies: Frontiers of Mobile and Wireless Communication, 4., pages 69 7, 4. [] I. Stamopoulos, A. Aragh, and S. R. Saunders. Performance comparison of distributed antenna and radiating cable systems for cellular indoor environments in the DCS band. In Twelfth International Conference on Antennas and Propagation, ICAP 3, pages , 3. [] K. J. Grandell. Indoor antennas for WCDMA systems. In Eleventh International Conference on Antennas and Propagation, pages 8,. [] M. Liénard, Ph. Mariage, J. Vandamme, and P. Degauque. Radiowave retransmission in confined areas using radiating cable: Theoretical and experimental study. In IEEE 44th Vehicular Technology Conference, pages , 994. [3] 3GPP TR 5.95 V6.., Release 6. Universal Mobile Telecommunications System (UMTS);Base Station (BS) classification (FDD). [4] J. Borkowski, J. Niemelä, T. Isotalo, P. Lähdekorpi, and J. Lempiäinen. Utilization of an indoor DAS for repeater deployment in WCDMA. IEEE 63th Vehicular Technology Conference, VTC 6 Spring, to be published.