## 3 System and Link Level Simulators  
In order to simulate the behavior of a large-scale wireless cellular network, different instances of all variables involved should be considered. In this context, the concept of \"drop\" is defined as one simulation runs over a certain time period, during which large-scale parameters of channel remain constant, while the small-scale parameters change as a function of time. To start a new drop, users' positions are initialized again, with the channel updated at the same time [44]. Hence, the simulator can run over a large number of drops, in order to obtain statistically solid results. During each drop, a large number of packets per active user is transmitted and received, while channel parameters change according to the coherence time of the wireless orientation. Typical performance metrics include the number of active users in the network, Signal to Interference plus Noise Ratio (SINR) per user and subcarrier (in case of Orthogonal Frequency Division Multiple Access - OFDMA), BER, and outage probability. Additional parameters of particular importance, such as handover ratio, can be examined as well. After a sufficient number of MC simulations (i.e., different number of drops), the cumulative distribution function (CDF) or probability density function (pdf) of the aforementioned parameters can be extracted and evaluated for various operating scenarios. The overall procedure is depicted in Figure 4, where for simplicity only the SINR calculation is considered. The requested service per MS can be a simple voice call or a bandwidth demanding operation, such as the streaming of a high-definition video. Once this service has been defined, the simulator examines the appropriate modulation scheme along with channel assignment, based on predefined criteria, such as the minimization of total transmitted power, the minimum number of channels assigned, etc. In cases where full or partial Channel State Information (CSI) is available at the transmitter via an error free feedback channel, the precoding matrix assignment takes place, based on appropriate codebooks calculated a priori. In general, the aforementioned framework can be divided into two separate procedures [45]: The first concept concerns system level (SL) simulations, which are more related to networking issues, such as resource allocation and scheduling, multi-user handling, mobility management, admission control, interference management, and network planning optimization. On the other hand, link level (LL) simulations make feasible the investigation of channel estimation, tracking, and prediction algorithms, as well as synchronization algorithms, MIMO gains, Adaptive Modulation and Coding (AMC), and feedback techniques. In a typical approach, the performance evaluation of large-scale wireless cellular networks is performed in two steps [46; 47]: During the first step, limited LL simulations take place, where associated parameters are calculated for wireless orientations with a limited number of nodes. In addition, LL simulations do not define any network geometry such as cell size or user position, and therefore do not require path loss or shadow fading models. On the other hand, in SL simulations, a user's signal to noise ratio (SNR) is an input parameter to the simulation, rather than a simulation result and is directly controlled via the transmit power, path loss, and noise power level. Figure 4: Cellular network simulation per channel drop. In [48], an SL simulator is presented for 3G networks, where system orientation can include an increased number of cells and beams per cell. In order to reduce execution times and make feasible the simulation of scenarios with increased number of tiers and users per cell, a grid-enabled problem solving environment (PSE) has been developed (Figure 5). The framework exploits the parallelizability of MC simulations and the well-known task farming parallelization pattern. The independent MC simulations may be computed at the same time, thus reducing the simulation execution time.