## 3 System and Link Level Simulators
By using a large-scale grid-infrastructure, the parallel MC simulations may be massively executed with relatively low cost, providing solutions to time critical or exceptionally time-consuming simulations. The same platform was also used in [49], where the goal was to examine 3G networks for different radio resource management strategies. In both the aforementioned papers however, only system level analysis was performed. Moving a step forward, a hybrid system link level simulator was introduced in [50], where the performance of several precoding techniques is analyzed for MIMO-WCDMA orientations. A key advantage of the specific simulator is that now system and link level simulations are not decoupled. To tackle with the overall computational complexity, independent MC simulations were executed in parallel, using the developed framework in [48]. The concept of hybrid system-link level simulations is also followed in [51], where a 4G MIMO-OFDMA network has been considered. In [52], the open source framework LTE-Sim is presented. LTE-Sim simulates among others uplink and downlink scheduling strategies in multicell/multiuser environments, taking into account user mobility, radio resource optimization, frequency reuse techniques and the adaptive modulation and coding module. In [53], an LL simulator for the LTE downlink has been described. In this context, various parametric curves have been extracted (BER vs SINR) for several AMC schemes. These curves can be then used as input to an SL simulator. In [54], video streaming applications in LTE networks are simulated, with the help of Vienna LTE SL Simulator. The video server was integrated to the simulator, while other types of traffic were also present. In [55], a computationally efficient dynamic system-level LTE simulator for enterprise femtocell scenarios is presented. A key advantage of the specific simulator is the use of a 3D office scenario with realistic mobility and propagation models. The tool has been designed to reduce computations in real time by precomputing propagation matrices and mobility traces. In the era of 5G, shifting Quality of Experience (QoE) towards high bandwidth and zero latency applications, the design and development of 5G network simulators should be able to incorporate  
Figure 5: Grid flow for independent MC (Monte Carlo) simulations. among others a set of diverse technologies (i.e., mmWave channel modeling, massive MIMO, adaptive Radio Resource Management - RRM, NOMA, etc.) and support interoperability with currently established 4G networks. A list of all advanced features and requirements is provided in Table 1, along with a related literature review. In this context, in [56], the challenges of developing a simulation platform for 5G brought by emerging new technologies and network architectures are presented. In this framework, and in order to meet the strict requirements, a cloud-based two-level framework of an SL simulator is proposed. In [57], the Vienna LTE Simulators [58] extension to 5G networks is presented, which is a MATLAB-based LL simulation tool. In the same context, in [59], a verification of the Vienna 5G LL and SL simulators is provided. The Vienna 5G SL Simulator [60] considers wireless networks on a large scale, offering the possibility to simulate scenarios that include hundreds of nodes. In order to keep the computational complexity at a low level, link abstraction is performed; hence, link quality models (LQMs) and the link performance model (LPM) are employed. The simulation of large-scale networks with several BSs and user types and several thousand active network nodes is now possible, due to efficient implementation. WiSE is an efficient SL simulator that has been released for the use of the industrial and academic communities. Simulation results have been validated by 3 GPP calibration processes. In [61], the structure of the simulator as well as specific guidelines are presented in detail.