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Comparison of scm, scme, and winner

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  • 1. Comparison of SCM, SCME, and WINNER Channel Models Milan Narandžić, Christian Schneider, Reiner Thomä Tommi Jämsä, Pekka Kyösti, Xiongwen Zhao Technische Universität Ilmenau, PSF 100 565, D-98694 Ilmenau, Germany milan.narandzic@tu-ilmenau.de Elektrobit, Tutkijantie 7, FIN-90570 Oulu, Finland tommi.jamsa@elektrobit.com Abstract—This paper is summarizing and comparing properties of channel models used for Beyond-3G (B3G) MIMO simulations: 3GPP Spatial Channel Model (SCM), its extension (SCME), and models developed by WINNER. Compared models are offering complete channel model description in a sense of large-scale as well as small-scale effects in MIMO radio-channel. WINNER targeted model was supposed to provide reliable tool for estimation of system performance, covering frequencies up to 5 GHz and bandwidths of 100 MHz in different types of environment. Since SCM was originally proposed for 2 GHz range and 5 MHz bandwidth, certain extensions (SCME) were necessary. However, SCME performance was restricted since it has been design as backward compatible with SCM. That was the motivation to start using the new WINNER generic channel model, where model parameters are extracted from channelsounding measurements covering targeted frequency range and bandwidth. This paper describes all important differences and compares features and performances of the models. Index Terms—Generic multipath MIMO channel, spatialchannel-modelling, measurement-based parameterization, system-level correlation. W I. INTRODUCTION (SHORT HISTORIC OVERVIEW) INNER project [1] has begun in 2004, and 3GPP SCM (Spatial-Channel-Model) was adopted for its initial channel model. SCM was developed in 3GPP/3GPP2 ad hoc group for spatial channel models and released in September 2003 [2]. In the beginning of 2005 the first extension of the original model was proposed by WINNER under the name SCME (SCM Extension [3]). SCME was later modified for 3GPP LTE purposes in [4] – [7]. In the end of 2005 further improvements and extensions are resulted in the model with the new name: WINNER channel Model – Phase I (WIM1). WIM1 model is described in the deliverable D5.4 [8] and published in [9]. SCM, SCME, and WIM1 models were briefly compared in [10]. The comparison tables of this paper also include WINNER – Phase II (WIM2) interim model that was recently published in the deliverable D1.1.1 [11]. The final WIM2 model will be available in autumn 2007 (D1.1.2). In this paper, WIM refers to WINNER model in general, WIM1 specifically to Phase I, and WIM2 specifically to Phase II. In the course of the WINNER project all encountered models are implemented in MATLAB/C and made available through the official web site [1]. The paper is organized as follows: In Section II the modeling methodology applied in SCM/SCME/WIM is briefly summarized. In Section III different features of the compared models are analyzed. In Section IV the performance figures are given, and finally Section V concludes the paper. II. MODELING METHODOLOGY The basic modelling philosophy behind all SCM/SCME/WIM is the same: the sum of specular components is used to describe the changes in the channel impulse response (CIR) between each transmitting and receiving antenna element (so called sum-of-sinusoids (SOS) method). Due to the different spatial position of elements inside Tx/Rx antenna array, different channel characteristics are obtained and MIMO concept is supported. All compared models can be classified as stochastically controlled spatial channel models what means that parameters of each specular component (sinusoid) are related to a spatial propagation of the single multipath-component (MPC). To fully describe a MPC following (low-level) parameters are used: departure (from Tx) and arriving (to Rx) angles, propagation delay and power. However, an evolution of these parameters is not based on the ray-tracing since positions of scattering clusters are not known. Instead, MPC parameters are chosen randomly from the appropriate probability distributions. Since the modeled radio-channel shows non-stationary behaviour, distributions of low-level parameters are changing in time. In order to support this phenomenon probability distributions are parameterized and their changes are modeled through the change of the control parameters. Since these parameters are controlling probability distributions of other (low-level) parameters in WINNER terminology they are called Large-Scale-Parameters (LSPs). The large-scale control parameters themselves also represent random variables that are governed by the appropriate probability distributions. These distributions are related to and measured in certain radiopropagation environments, called scenarios (Section III.A.1).
  • 2. III. FEATURES A. Applicability Range 1) Environment dependence: scenarios To characterize different environments (scenarios) WINNER models are using (temporal and spatial) parameters obtained from measured CIRs. For each scenario measured data is analyzed and processed to obtain scenario-specific parameters. After this point, same generic channel is used to model all scenarios, just by using different values of control parameters. SCM was originally dedicated to outdoor propagation, and defines three environments: Suburban macro, Urban macro, Urban micro. In SCME number of scenarios is not extended. WINNER models are supporting considerably larger number of scenarios then SCM/SCME. WIM1 defines the following scenarios: A1–Indoor (small office/residential), B1–Typical urban micro-cell, B3–Indoor hotspot, B5–Stationary feeder with sub-scenarios a, b, and d having different deployment assumptions, C1– Suburban, C2–Typical urban macro-cell and D1–Rural macro-cell. The covered number of scenarios is further extended for the WINNER Phase II model: A2–Indoorto-outdoor, B4– Outdoor-to-indoor, B5–Stationary feeder subscenarios c, and f, D2a–Moving networks, and Bad Urban extensions for micro-cell (B2) and macro-cell (C3) scenarios. Usually, even for the same scenario, existence of LOS component substantially influences values of model parameters. Regarding to this property, each WINNER scenario is differentiating between LOS and NLOS conditions. Originally this was not the case with SCM where LOS condition was analyzed only in the context of Urban Micro scenario, but SCME provided the extensions for the other SCM scenarios. 2) System dependence Carrier frequency: Dependence on carrier frequency in SCM/SCME/WIM is found in path-loss models. In SCM, COST 231 Urban Hata (macro-cell) and COST 231 WalfishIkegami (micro-cell) path-loss models are adopted and adjusted for frequencies of 1.9 GHz. Applicability of proposed models in different frequency ranges is not analyzed. In SCME frequency range for SCM scenarios is extended based on correction of the frequency dependant free space loss: ⎛f ⎞ ΔPL( f c ) = 20 log⎜ c ⎟ , ⎝2⎠ where (1) f c is carrier frequency in [GHz]. Additionally, in SCME COST 231 Walfish-Ikegami path-loss model was applied also for macro-cell environments in 5GHz range since usage of higher frequencies decreases coverage area. Following SCME approach, all scenarios defined by WIM support frequency dependant path-loss models valid for the ranges of 2 – 6 GHz. WIM path-loss models are based on measurements that are mainly conducted in 5.2 GHz frequency range. From WINNER measurement results and literature survey it was found that model parameters delay-spread (DS), angular spread (AS) and Rician K-factor do not show significant frequency dependence [3]. From that reason these parameters show only dependence on environment (scenario) in SCM/SCME/WIM. For modeling systems with time-division-duplex (TDD) all models (SCM/SCME/WIM) are using same (large-scale as small-scale) parameters for both uplink (UL) and downlink (DL). If system is using different carriers for duplexing (FDD), than (additionally to path loss) random phases between UL and DL are independent. Bandwidth: In 3GPP-SCM document there is a note that usage of 6 paths (clusters) may not be suitable for bandwidths higher than 5MHz. This reflects that influence of system bandwidth to the complexity of model structure (number of clusters, number of MPCs per cluster) is not completely investigated. For the WINNER purposes it is required that channel model supports bandwidths up to 100 MHz. Following SCM note and approach described in [12] (for indoor propagation modeling) SCME used intra-cluster delay spread as a mean to support bandwidth extension. However, since SCM forced backwardcompatibility (to attain comparability with SCM) number of clusters and total number of MPCs are not increased! Instead, SCM cluster with 20 MPCs is subdivided into 3-4 zero-delay sub-clusters (“mid-paths”), keeping total number of MPCs constant. Introduced delay spread per cluster (10ns) in not based on targeted 100 MHz bandwidth, but on PDP matching since the equal power of all MPCs belonging to the same cluster is enforced. From the above discussion it is not apparent that intra-cluster spread concept (unchanged number of MPCs) better reflects bandwidth dependence than introduction of the new clusters. However, the latter also increases complexity since total number of MPCs would be increased. In WIM different philosophy is applied: since measurement systems were supporting 100 MHz bandwidth, during WIM parameterization number of clusters was traced in delay and angular domains from measured CIR. In this way number of clusters reflects both system bandwidth and scenario dependence. B. System Level Description In order to calculate path-loss, information about distance between transmitting (Tx) and receiving (Rx) station is necessary for all compared models. This means that system layout information about positions of all stations is necessary. Note that system layout usually does not contain any information about environment characterization. Only exception is related to the positions of far scatterers in the SCM and WIM2. Correlation at system-layout level: In all models all LSPs
  • 3. are fully correlated for links between MS and different sectors of the same BS. In this way, influences of selective antenna gain pattern or different LOS conditions between certain sectors to the level of LSP correlation are not regarded. Additionally, WIM uses positions of MS to introduce scenario-specific correlation of link LSPs for MSs being connected to the same BS. In SCM/SCME correlation of LSPs between different MS is not supported. In SCM, standard deviation of shadowing fading for links from one MS to different BSs (“site-to-site”) has constant correlation coefficient equal to 0.5. Introduced correlation does not depend on distances between BSs or their relative angular positions as seen from MS and therefore it is not layout dependant. Currently WIM1 does not support this type of correlation and same applies to SCM/SCME implementations supported by WINNER. Antenna arrays: SCM/SCME/WIM1 introduce additional support for cross-polarized antenna arrays because used representation for antenna arrays was not general enough (the reference polarizations of antennas and environments are not modeled separately). The full polarimetric antenna description will be developed in WIM2 [13]. C. Complexity Issues WINNER channel models are intended for system level evaluation. In order to keep complexity reasonably low, SCM/SCME/WIM models are relaying on some quasi-realistic assumptions and concepts: Channel segment/drop: Channel segment (drop) represents period of quasi-stationarity in which probability distributions of low-level parameters are not changed. During this period all large-scale control parameters, as well as velocity and direction-of-travel for mobile station (MS), are held constant in SCM/WIM models. To the opposite, SCME has allowed simultaneous drifting of arriving angles and delays for every MPC at each simulation step inside a drop. In respect to this property SCME can be classified as model with continuous evolution (in discrete steps, being smaller than drop). Consequence was substantial increase of complexity and simulation time length in comparison to SCM/WIM1. The new approach to the model evolution will be included in WIM2. Zero-Delay-Spread-Cluster (ZDSC): MPCs belonging to the same cluster (path in SCM terminology) have “close” values of both delay and angular parameters. If all MPCs belonging to the same cluster have exactly the same delay, it is possible to define extremely simple relation between SCM/WIM and tapped-delay-line (TDL) model. Due to mentioned similarity, the latter is also called cluster-delay-line (CDL) model in WINNER terminology. This type of reduced-complexity model is offered in SCM/SCME/WIM for link-level simulations and calibration purposes. The support of intracluster delay spread concept in SCME increases number of “taps” in TDL/CDL model 3 or 4 times, since cluster is represented by 3 or 4 zero-delay sub-clusters. Predefined angular offsets from the cluster center: This property is used by SCM/SCME/WIM to avoid random generation of MPC angles in each drop. Difference is that WIM scales initial offsets with standard deviation of angles per cluster, while SCM uses predefined values for intra-cluster (sub-path) departure angle spreads of 2 (macro) and 5 (micro) degrees and 35 degrees for spread of arrival angles. If separate drifting (continuous evolution) of MPCs inside cluster in used by SCME, angular offsets are not constant any more. D. Comparison Tables TABLE I FEATURE COMPARISON Feature SCM SCME WINNER I WINNER II Yes Yes Bandwidth > 100 MHz No Yes Yes Indoor scenarios No No Yes Outdoor-to-indoor and indoor-to-outdoor No No No scenarios AoA/AoD elevation No No Yes Intra-cluster delay No Yes No spread TDL model based on No Yes Yes* the generic model Cross-correlation No No Yes between LSPs Time evolution of No Yes** No model parameters *TDL model is based on the same measurements as generic model, but analyzed separately. **Continuous time evolution. Yes Yes Yes Yes Yes Yes TABLE II NUMERICAL COMPARISON Parameter Unit SCM Max. bandwidth MHz 5 Frequency range GHz 2 No. of scenarios 3 No. of clusters 6 No. of mid-paths 1 per cluster No. of sub-paths 20 per cluster No. of taps 6 BS angle spread º 5 – 19 MS angle spread º 68 Delay spread ns 170 – 650 Shadow fading standard dB 4 – 10 deviation * artificial extension from 5 MHz bandwidth ** based on 100 MHz measurements 100* 2–6 3 6 WINNER I 100** 2–6 7 4-24 WINNER II 100** 2–6 12 4-20 3–4 1 1–3 SCME 20 10 20 18 – 24 4.7 – 18.2 62.2 – 67.8 231 – 841 4-24 3.0 – 38.0 9.5 – 53.0 1.6 – 313.0 4-24 2.5 – 53.7 11.7 – 52.5 16 – 630 4 – 10 1.4 – 8.0 2–8 DS values in WIM2 scenarios are generally lower then for WIM1 due to the new estimation procedure that regards only upper 20 dB of the observed signal range in CIR. The higher DS range in Table II corresponds to the bad urban scenarios that are modelled with an additional far-cluster option. Additionally, other parameters of WIM1-supported scenarios are also slightly tuned for WIM2 according to the newly performed measurements.
  • 4. IV. COMPARISON: PERFORMANCE FIGURES A. Fading Distribution and Autocorrelation The amplitude fading, autocorrelation, level crossing rate and Doppler spread are important measures of the channel model performance. However, statistical analysis of these parameters have shown very similar results in all compared models. Figure 1 illustrates as example the temporal autocorrelation. Auto-correlation (d/λ) 1 0 0 1 2 3 4 5 Distance/Wavelength, d/λ 6 7 8 Figure 1. Temporal auto-correlation functions of the equivalent narrowband channels. B. Frequency Correlation Under the wide sense stationary uncorrelated scattering (WSSUS) assumption frequency correlation function (FCF) is related to the average power delay profile (PDP) through a Fourier transform. Since the compared models are using only specular components, FCF is estimated by the sum equation: R (Δf ) = ∑ Pi exp(− j 2πτ i Δf ) i ∑P , (2) i i where Δf is the frequency difference, τi is the delay and Pi is the power of the ith path. 1 SCM SCME WIM1 0.9 C. Outage Capacity Both spatial and polarization characteristics of the models are investigated by the means of a channel capacity. Based on the assumption that the channel state information is unknown to the transmitter, the narrowband capacity is calculated by P ⎛ ⎞ (3) C = log 2 det⎜ I + 2 HH H ⎟ , ⎝ σ ⎠ 2 where I is the identity matrix, P σ is the average SNR, H SCME urban macro SCME urban micro SCM urban macro WIM C2 Classical Doppler 0.5 -0.5 SCME by removing the intra-cluster delay spread. This figure shows that the correlation of SCM model for bandwidths exceeding 10 MHz is considerably higher than correlation of SCME and WINNER models. 0.8 0.7 is the narrowband channel matrix and (.)H denotes the Hermitian transpose operation. Simulated capacity curves for all (three) scenarios supported by SCM/SCME and for signal to noise power ratio of 14 dB are shown in Figures 4 to 6. The complementary cumulative distribution function of capacity is plotted for a several independent channel snapshots and compared to Gaussian i.i.d. channel matrix. The reference antenna configuration is 4x4 MIMO with two sub-groups of ±45° slanted single polarized elements whose spacing is 4 wavelength on BS and ½ wavelength on MS, see Figure 3. Figure 3. MIMO antenna configuration for capacity calculation. For the all three scenarios we can observe, that the median outage capacity is about 7 bits/s/Hz lower than Gaussian i.i.d. reference curve. As expected, SCME gives equal outage capacities to SCM because its spatial and polarization characteristics are not changed in the respect to the basic SCM. The outage capacity of the WINNER model (WIM1) deviates slightly from SCM/SCME in Urban Micro scenario, while in Urban Macro scenario WINNER model shows higher capacity than SCM/SCME. FCF(Δf) 0.6 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 60 70 Frequency difference, Δf [MHz] 80 90 100 Figure 2. Frequency correlation function of SCM, SCME, and WIM1 for Suburban scenario. Figure 2 depicts frequency correlation of SCM, SCME, and WIM1 models in 100 MHz bandwidth [4], [14]. For this comparison, we selected suburban NLOS scenarios (TDL model) from each of them. The TDL of SCM was taken from Figure 4. Outage capacity of SCM, SCME, and WINNER Suburban Macro scenario.
  • 5. by WINNER models. Special WIM quality also comes from the fact that parameterization is based upon the real channelsounding measurements. The WINNER models reproduce correlations of LSPs at link and system level (intra-site). This property is observed in measured datasets and has not been modeled in SCM/SCME. Additionally, WIM2 is proposing a reduced complexity time evolution of the low-level model parameters in comparison to SCME. In this paper SCM/SCME/WIM models are briefly compared against each others. The future validation work should show how well these models are representing targeted system level aspects (when compared to measured channels). Figure 5. Outage capacity of SCM, SCME, and WINNER Urban Macro scenario. ACKNOWLEDGMENT This work has been performed in the framework of the IST project IST-4-027756 WINNER II, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues. REFERENCES [1] [2] [3] [4] Figure 6. Outage capacity of SCM, SCME, and WINNER Urban Micro scenario. [5] V. CONCLUSION [7] At the beginning of the WINNER project, before having a full insight into the complete measurement analysis results, the mainly theoretical extension of SCM (SCME) is supported. In SCME possibility of using intra-cluster delay spread to model extended bandwidths is investigated. It was found that the main limitations of the proposed approach come from the decision to attain backward compatibility with SCM. Also the complexity increase, as a consequence quasi-deterministic drifting of small-scale parameters inside drop, was evaluated. As could be seen from previous sections, modelling methodology and certain SCM concepts are still part of WIM. The similarity is observed in the narrowband capacity measures that reflect the spatial and polarization properties of the models. However, without extensions provided in WINNER it would not be possible to apply SCM-based methodology to the frequency ranges and extended system bandwidths. The main strength of WIM is based in scenariocustomized parameters that are making possible to apply generic spatial-channel-model to the numerous of different environments. Up to now 12 different scenarios are supported [6] [8] [9] [10] [11] [12] [13] [14] https://www.ist-winner.org 3rd Generation Partnership Project; technical specification group radio access network; spatial channel model for MIMO simulations, 3GPP TR 25.996 V6.1.0, 2003. Available: http://www.3gpp.org D. S. Baum, J. Salo, G. Del Galdo, M. Milojevic, P. Kyösti, and J. Hansen, An Interim Channel Model for Beyond-3G Systems, in Proc. IEEE VTC 2005 Spring, Stockholm, May 2005. Elektrobit, Nokia, Siemens, Philips, Alcatel, Telefonica, Lucent, Ericsson, Spatial Radio Channel Models for Systems Beyond 3G, contribution to 3GPP RAN4, R4-050854, London, September 2005. 3GPP, E-UTRA MIMO channel model – text proposal, R1-061002 (agreed) 3GPP, System level channel models for E-UTRA evaluations, R1-061594 (agreed) 3GPP, Update of polarization description in link level channel model, R1-061371 (noted) IST-2003-507581 WINNER D5.4 Final report on Link Level and System Level Channel Models v1.4, https://www.ist-winner.org. H. El-Sallabi, D. S. Baum, P. Zetterberg, P. Kyösti, T. Rautiainen, and C. Schneider, Wideband Spatial Channel Model for MIMO Systems at 5 GHz in Indoor and Outdoor Environments, in Proc. IEEE Veh. Technol. Conf. VTC’06 Spring, Melbourne, Australia, May 7-10, 2006. Elektrobit, Telefonica, Siemens, Comparison of SCM, SCME, and WINNER models, contribution to 3GPP RAN4, R4-060521, Shanghai, China, May 2006. IST-4-027756 WINNER II D1.1.1 WINNER II interim channel models, v1.0, December 2006. A. Saleh, and R. A. Valenzuela, A statistical model for indoor multipath propagation, IEEE J. Select. Areas Commun., vol. SAC-5, no. 2, Feb. 1987, pp. 128–137. M. Narandžić, M. Käske, C. Schneider, M. Milojević, M. Landmann, G. Sommerkorn, R. S. Thoma, 3D-Antenna Array Model for IST-WINNER Channel Simulations, IEEE VTC2007-Spring, April 23 - 25, Dublin, IR, April 2007. Elektrobit, Siemens, Ericsson, Lucent, Telefonica, Alcatel, France Telecom, Spatial Radio Channel Models for Systems Beyond 3G, contribution to 3GPP RAN4 meeting #38, Denver, USA, 13 – 17 February 2006.