Spatial Channel Modeling - SCM / SCME
It has been widely understood that radio propagation has a significant impact on the performance of wireless communication systems. Because of the major influence on the system performance and complexity, radio channel models and simulations have to be more versatile and accurate than in earlier systems. The more we know about the channel in different dimensions (space, time, frequency and polarization) provide us means for successful design and comparison between different wireless communication systems. In this context it is clear that realistic and reliable multidimensional Spatial Channel Modeling (SCM) / Spatial Channel Modeling Extended (SCME) channel models for simulation and testing of advanced wireless systems are necessary.
Why Spatial Channel Models?
Traditional tapped delay line (TDL) models have typically been designed for narrowband single input single output (SISO) systems. Additionally, they are applicable for link level testing only as a fixed parameter models. They do not cover environment variability and thus do not test system adaptivity nor system level performance. Therefore, TDL models are not adequate for simulation and testing of broadband multi-antenna systems. Geometry-based stochastic channel models (GSCM) cover wide range of environments with random number parameters, supports different multi-antenna technologies such as beam forming and spatial multiplexing. As an antenna independent model, GSCM is not restricted to pre-defined array characteristics which makes difference to correlation matrix based models.
Geometry-based Stochastic Channel Modeling
Spatial channel modeling tool follows a geometry-based stochastic channel modeling (GSCM) approach, which allows creating of an arbitrary double directional radio channel model. The channel models are antenna independent, i.e., arbitrary antenna configurations and beam patterns can be inserted. The channel parameters are determined stochastically, based on statistical distributions extracted from channel measurement. The distributions are defined for, e.g., delay spread, delay values, angle spread, and shadow fading. For each channel snapshot the channel parameters are calculated from the distributions. Channel realizations are generated by summing contributions of rays with specific channel parameters like delay, power, angle-of-arrival and angle of departure. Different scenarios are modeled by using the same approach, but different parameters. 3GPP SCM and SCM-Extension (SCME) models are based on the GSCM.
SCM Channel Model
The scope of the 3GPP-3GPP2 Spatial Channel Model (SCM) Ad-Hoc Group was to develop and specify parameters and methods associated with the spatial channel modeling that are common to the needs of the 3GPP and 3GPP2 organizations. The model includes simple tapped-delay line (TDL) models for calibration purposes and GSCM for simulation purposes. SCM model is based on geometry, but a subset of the parameters is stochastic. It is designed for the bandwidth of 5 MHz and the centre frequency of 2 GHz. System-level simulations often include multiple base stations and multiple mobile terminals. A drop-based model is assumed for the generation of the clusters of scatterers. During a drop, the channel undergoes fast fading according to the mobile movement. However, delays and AoA/AoD are kept constant. Two consecutive drops are independent and include randomly located clusters, which make the channel model discontinuous. The channel model includes several parameters, such as number of paths, number of sub-paths, mean angular spread (AS) at BS and MS, AS per path at BS and MS, AoA/AoD distributions, variable delay spread, path loss, shadowing.
SCME Channel Model
The Spatial Channel Modeling (SCM) has a bandwidth of 5 MHz. However, the required bandwidth in the B3G systems is up to 100 MHz. For that reason an extension to Spatial Channel Modeling Extended (SCME) was developed in European WINNER project. The bandwidth extension is carried out by introducing so-called mid-paths which define the intra-cluster delay spread. The mid-paths have fixed delay and power offsets in order to keep the SCME model backwards-compatible with SCM. As a result of the bandwidth extension the number of delay taps increases from six in SCM to 18 or 24, depending on the scenario.