## 3 New Radio Link-Level Simulator
### Channel Model Generation  
The key element of this simulation framework is clearly the fading channel block: for an NR-compliant link simulator, the 3 GPP has two defined models for NR link and system evaluation purposes [22; 47]:  
* Tapped delay line (TDL): for this channel model, the correlation between different antennas is defined statically by a correlation matrix. The TDL model is based on the description of the impulse response of the channel;
* Clustered delay line (CDL): with this channel model the direction of the signal in the space is modelled. The model is based on the description of the main departure and arrival directions of the signal in the space and the number of clusters corresponds to the number of channel reflections.  
The CDL channel model is adopted for all the simulations presented in this article due to its importance for practical applications. In particular, we select the CDL-B profile. This CDL profile is specific for non-line-of-sight (NLOS) transmissions and can be used for sub 6 GHz frequency, where line-of-sight (LOS) is not mandatory. A characterising property is that the first path in time is also the strongest one.  
The channel between the user and the BS depends on the environment geometry. In our scenario, the user is placed at a distance of 100 m from the BS, with a height of 1.5 m, while the BS has a height of 10 m. The UE direction could be defined as DOMINANT_PATH (the same direction of the main azimuth of arrival (AoA) direction), RANDOMLY (uniformly generated around \([-180^{\circ},+180^{\circ}]\)) and ARBITRARY (statically selecting a UE direction). The delay spread can be selected among the set ('VERY SHORT', 'SHORT', 'NOMINAL','LONG', 'VERY_LONG'). NOMINAL is fixed for our set of simulations, which corresponds to 100 ns. The coherence time of the channel depends on the Doppler spread and on the user speed, which is fixed at 3 km/h (i.e., a walking pedestrian). The CDL profile channel requires the generation of \(N_{c}\) clusters. Each cluster \(n\) will be assigned a different delay (depending on the delay spread times), a set of angular orientations, including AoA, azimuth of departure (AoD), zenith of departure (ZoD) and zenith of arrival (ZoA), and a power gain \(P_{\pi}\). The azimuth angle spread of departure and arrival (ASD and ASA, respectively) and the zenith angle spread of departure and arrival (ZSD and ZSA, respectively), characterize the entire set of clusters. These values are used for the generation of the \(M\) rays that form each cluster. The direction of a certain ray is given by the angular orientation of its own cluster, the angular spreads and a different coefficient for each ray. The angle for azimuth is \(\phi\), while the angle for zenith is \(\theta\). At both the gNB and UE side, three levels of antenna correlation are possible: LOW, MEDIUM and HIGH. The level of correlation is inversely related to the angle spread (i.e., larger angle spreads correspond to lower correlation and vice versa). The CDL model for cluster \(n\), transmitter antenna element \(u\) and receiver antenna element \(s\) is, thus, a combination of \(M\) NLOS rays and a LOS ray when the UE is in a LOS condition:  
\[\begin{array}{ll}\mathrm{H}_{u,s,u}^{\mathrm{NLOS}}=&\sqrt{\frac{P_{\pi}}{M} }\sum_{m=1}^{M}\left[\begin{array}{c}F_{u,s,u}(\theta_{u,2,u},\phi_{u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u,  
reduction in the computational load. However, the frequency domain mode is applicable only in scenarios characterized by low mobility (i.e., when the channel is nearly constant within the OFDM symbol duration) and when the maximum delay of the channel is lower than the cyclic prefix length. In the simulations presented in the following, the time domain mode is selected and an impulse response is generated after a certain time interval, defined as an integer multiple of the slot length, and then interpolated in the time domain. For the frequency domain, an impulse response would be generated for each OFDM symbol.