Extending the 3GPP Spatial Channel Model (SCM)
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An Interim Channel Model for Beyond-3G Systems Extending the 3GPP Spatial Channel Model (SCM) Daniel S. Baum and Jan Hansen Jari Salo ETH Zürich, Zürich, Switzerland Helsinki University of Technology, Espoo, Finland {dsbaum,hansen}@nari.ee.ethz.ch [email protected] Giovanni Del Galdo and Marko Milojevic Pekka Kyösti Ilmenau University of Technology, Ilmenau, Germany Elektrobit Ltd., Oulu, Finland {giovanni.delgaldo,marko.milojevic}@tu-ilmenau.de [email protected] Abstract— This paper reports on the interim beyond-3G (B3G) While the WINNER project only started recently, there is channel model developed by and used within the European an immediate demand for models suitable for initial usage. WINNER project. The model is a comprehensive spatial channel This document presents the result of our studies in form of a model for 2 and 5 GHz frequency bands and supports band- model that is used for initial evaluation of B3G technologies in widths up to 100 MHz in three different outdoor environments. It outdoor scenarios within the WINNER project. further features time-evolution of system-level parameters for challenging advanced communication algorithms, as well as a Contributions. Our specific contributions are as follows: reduced-variability tapped delay-line model for improved usabili- • ty in calibration and comparison simulations. We analyze shortcomings of a selected spatial channel model standard with respect to the identified require- Keywords- channel model, beyond-3G, MIMO, SCM, 3GPP ments from other WINNER Work Packages. • We evaluated results found from literature search and I. INTRODUCTION derived from our own measurement data to devise In recent years Multiple-Input Multiple-Output (MIMO) missing parameters. wireless communication techniques have attracted strong • We propose a set of backward compatible extension to attention in research and development due to their potential the 3GPP Spatial Channel Model (SCM). benefits in spectral efficiency, throughput and quality of service. Only recently, however, has this technology been This paper summarizes the results reported in [4]. considered to be included in wireless communication system standards, such as IEEE 802.11n for wireless LANs (WLAN), II. 3GPP SCM IEEE 802.16 for broadband fixed wireless access (FWA), and 3GPP high-speed downlink packet access (HSDPA) for cellu- We have identified two publications ([1], [2]) defining lar mobile communications. spatial / MIMO radio channel models that are commonly accepted and used. Other publications focus mainly on aspects Any wireless communication system needs to specify a and certain effects of the radio channel. As the 802.11n model propagation channel model that can act as a basis for perfor- is targeted towards indoor applications, we have selected the mance evaluation and comparison. With advancing com- 3GPP SCM as a basis for outdoor channel model extensions. munication technologies, these models need to be refined as further characteristics of the channel can be exploited and thus A. Properties need to be modeled. To enable MIMO, the standardization groups 802.11 and 3GPP thus first defined spatial channel The SCM is a so-called geometric or ray-based model models suitable for their applications [1], [2]. based on stochastic modeling of scatterers. It defines three environments (Suburban Macro, Urban Macro, and Urban Upcoming communication systems will be based on a new Micro) where Urban Micro is differentiated in line-of-sight set of system parameters (e.g. extended bandwidth and new (LOS) and non-LOS (NLOS) propagation. There is a fixed frequency bands), a broader range of and additional scenarios number of 6 “paths” in every scenario, each representing a (e.g. mobile to mobile, mobile hotspot), and new communica- Dirac function in delay domain, but made up of 20 spatially tion techniques (e.g., tracking algorithms). This triggers new separated “sub-paths” according to the sum-of-sinusoids requirements on the underlying channel models. method [5]. Path powers, path delays, and angular properties for both sides of the link are modeled as random variables The European WINNER project [3], which is part of the defined through probability density functions (PDFs) and Framework 6 effort, is currently researching the outline of a cross-correlations. All parameters, except for fast-fading, are system design of such a B3G system. In WINNER, it is the drawn independently in time, in what is termed “drops”. goal of Work Package 5 to come up with channel models that suit the needs in the project. This work has been performed in the framework of the IST project IST- 2003-507581 WINNER, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues. 0-7803-8887-9/05/$20.00 (c)2005 IEEE TABLE 1. MIDPATH POWER-DELAY PARAMETERS values, and avoids that single sub-paths become delay- resolvable. Furthermore, lumping together a number of sub- Scenario Suburban Macro, Urban Micro Urban Macro paths keeps the fading distribution of that tap close to Rayleigh and thus aids a potential implementation with a classic No. mid-paths per path 3 4 Gaussian-distributed number generator. We found that 4 is the Mid-path power and 1 10/20 0 ns 6/20 0 ns absolute minimum number of sinusoids to yield a reasonable delay relative to 2 6/20 7 ns 6/20 5.8 ns paths Rayleigh distribution. 3 4/20 26.5 ns 4/20 13.5 ns The number of mid-paths, and the power and delay 4 - - 4/20 27.6 ns parameters chosen for each mid-path are tabulated in Table 1. The mid-path powers, i.e. number of sub-paths, were chosen by B. Shortcomings considering the decreasing power with delay while staying The SCM was defined for a 5 MHz bandwidth CDMA above the minimum number of sub-paths. The delays for the system in the 2 GHz band, whereas the currently defined mid-paths were then derived by employing the method from WINNER system parameters are 100 MHz bandwidth in both 2 [9] with the DS set to the predetermined value of 10 ns and the and 5 GHz frequency range [6]. Other issues are the drop based predetermined set of powers given for the mid-paths. concept, i.e., no short-term system-level time-variability in the In SCM, each sub-path has an angle relative to the path model, the lack of Ricean K-factor models (LOS support) for mean angle assigned to it. By perturbing the set of sub-paths macro scenarios, and the lack of a wider range of scenarios. assigned to a mid-path, the AS of that mid-path can be varied. It has been reported, e.g. [10], that the intra-cluster AS III. INTERIM BEYOND-3G CHANNEL MODEL conditioned on the intra-cluster delay is approximately Our main goal for the extension was to keep it simple, independent of the delay. Hence, the mid-path ASs (ASi, where backward-compatible, and within the conceptual approach of i is the mid-path index) were optimized such that the deviation the SCM. This approach provides consistency and compara- from the path AS (ASn, where n is the path index), i.e. the AS bility. In the following we discuss the underlying concepts and of all mid-paths combined, is minimized. The result is the reasoning behind the proposed extensions. tabulated in Table 2. A. Bandwidth B. Frequency Range To extend the model in a way such that its characteristics 1) Path-Loss Model remain unchanged if compared at the original 5 MHz resolu- The SCM path-loss model is based on the COST-Hata- tion bandwidth, we add intra-path delay-spread (DS), which is Model [11] for Suburban and Urban Macro and the COST- zero in the SCM. A possible power-delay profile (PDP) is a Walfish-Ikegami-Model (COST-WI) [11] for Urban Micro. one-sided exponential function. This approach of so-called Some relevant references on path-loss were found ([12]-[18]), intra-cluster DS was originally proposed by Saleh and however only few of them allow direct comparison between Valenzuela for indoor propagation modeling [7]. The intra- equivalent measurements at 2 and 5 GHz. These few however cluster DS model has also been adopted for outdoor scenarios indicate that the most significant difference can be attributed to in the COST 259 [8] model. Following the SCM philosophy, different gains in free-space path-loss, which is 8 dB higher at which is partly based on COST 259, we use this as our guide- 5 GHz compared to 2 GHz. Thus, for comparability reasons, line. The path DS was chosen under the following considera- we propose a 5 GHz path-loss model that has an offset of 8 dB tions 0 • 3 mid-paths In SCM, all paths within a scenario have the same path -5 4 mid-paths azimuth-spread (AS). Equivalently, we set the path DS exponential PDP to be constant. The path AS and DS then define the -10 minimum observable total (over all paths) AS and DS. -15 • Both from measurements and intuition it follows that this minimum total spread lies somewhere between -20 zero and a fraction of the mean total spread. -25 • The error in power between an exponential PDP and the SCM definition (no DS) is illustrated in Figure 1. -30 For a path DS of 10 ns, this error is slightly below -20 dB and can be considered reasonably small. We set it -35 equivalent to this value for all paths. power belowerror original signal in dB -40 We split the 20 sub-paths into subsets, denoted “mid- -45 paths”, which we then move to different delays relative to the 0 1 original path. Even though a mid-path consists of multiple sub- 10 10 paths, it remains a single tap (delay-resolvable component).