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-33101 TAMPERE FINLAND Tel. +358 3 3115 5128, Fax +358 3 3115 3808
[email protected], http://www.cs.tut.fi/tlt/RNG/
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
1. 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 [1]. Available solutions for building indoor coverage with dedicated systems are distributed antenna system (DAS), radiating cables (RC) [2], 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 HSDPA 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. 2. WCDMA Indoor Planning 2.1. 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 0.053 MHz and 0.16 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 16 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. 2.2. 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., [10], [11]. 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
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, RC [12], 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 [13], 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. 2.3. 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 implementation1 , 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 [14]. Radiating cable (RC), 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 EIRP (effective isotropic radiated power) of RC, 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. 1 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-CPICH 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 PCPICH Ec /N0 , the received energy per chip on PCPICH divided by the power density in the band. The P-CPICH Ec /N0 is defined as ratio between RSCP and RSSI (received signal strength indicator): RSCP Ec = N0 RSSI
(1)
In addition, interference level is indicated with signal to interference ratio (SIR) for the P-CPICH: SIR = SF256
PP −CP ICH (1 − α)PT OT + (Iinter + pn )L
(2)
where SF256 is the processing gain for P-CPICH, PP −CP ICH is the transmit power of P-CPICH, α is the downlink orthogonality factor, PT OT is the total transmit power of the base station, Iinter is the received inter-cell interference, pn is the received noise power, and L is the path loss. The instantaneous root mean square (RMS) delay spread σRM S is evaluated in this paper from scanner measurement according to following definition: p σRM S = τ 2 − τ 2 (3) where P 2
τ =
a2k τk2 a2k
k
is the mean excess delay and P 2 a τk τ 2 = k 2k ak
(4)
(5)
is the mean delay with ak as the amplitude of kth multipath component and its’ a relative delay of τk . 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 100 m long large corridor, having an open hall in the other end and being 2 floors in height. The antennas were located in the 2nd floor level, whereas the measurements were carried out in the 1st 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
a) 1-Antenna RNC
b) 2-Antenna RNC
c) 3-Antenna RNC
d) 4-Antenna
CPICH 30dBm 60 m -10.12dB
25 m -4.22dB
2 dBi EIRP 17.66dBm
25 m -4.22dB
2 dBi EIRP 14.66dBm
< RC 1
2
2
BTS RC
CPICH 30dBm 60 m -10.12dB
-3 dB
60 m -10.12dB
-5 dB
