# A Novel Link-to-System Mapping Technique Based on Machine Learning for 5G/IoT Wireless Networks
## 2 Background on 5G NR  
3 GPP Rel-15 was approved for NR standards in December 2017, referred to as 5G NR. Different from 4G LTE-A systems with fixed sub-carrier intervals, 5G NR systems have newly flexible frame structures to provide various services. Various transmission frame settings are available by introducing sub-carrier interval parameter \(\mu\) called numerology. 5G NR systems can set \(\mu\) from 0 to 5, and the value of \(\mu\) determines the sub-carrier interval. As the numerology value \(\mu\) increases by one, the sub-carrier interval is doubled. Compared to 5G NR systems, 4G LTE/LTE-A systems only use a fixed sub-carrier interval of 15 kHz (\(\mu=0\)), therefore, it can accommodate various services.  
The most innovative change of 5G NR system is channel coding. Turbo codes [15], low-density parity-check (LDPC) code [16], and polar code [17] were designated as candidate channel codings. In particular, LDPC code and the polar code have been actively studied, and the performance of LDPC code has been shown to be close to the Shannon limit. In a recent study, LDPC code enables fast encoding/decoding with high error correction capability, and achieves high speed, low delay, low cost, and high reliability for 5G communication services. At the first-phase of Rel-15, LDPC code replaces the Turbo code used in 4G LTE data channels, while Polar code replaces the Tail Biting Convolutional Codes used in 4G LTE control channels.  
3 GPP standards technical reports of TR 38.900 [18] and TR 38.901 [19] define clustered delay line (CDL) and tapped delay line (TDL) as 5G channel models for LLS. Each channel model consists of  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Abbreviation: Full Name** & **Abbreviation: Full Name** \\ \hline AWGN: additive white Gaussian noise & BLER: block error rate \\ CQI: channel quality indication & CDL: clustered delay line \\ DNN: deep neural network & EESM: exponential effective SNR mapping \\ eNB: evolved Node-B & LDPC: Low-density parity-check \\ LTE-A: long term evolution-advanced & LLS: link-level simulator \\ L2S: link-to-system & MIMO: multiple input multiple output \\ ML: machine learning & MMSE: minimum mean square error \\ MSE: mean squared error & NS: network-level simulator \\ NR: new radio & RB: resource block \\ RE: resource element & SINR: signal-to-interference plus noise ratio \\ SISO: single input single output & SLS: system-level simulator \\ SNR: signal-to-noise ratio & TB: transport block \\ TDL: tapped delay line & UE: user equipment \\ \hline \hline \end{tabular}
\end{table}
Table 1: Abbreviations.  
three non-line-of-sight channels of A, B, and C types and two line-of-sight channels of D and E types. These channel models support bandwidth of up to 2 GHz in the 500 MHz to 100 GHz operating frequency.  
The CDL model [18; 19] is designed as a signal arriving at the receiver antenna dispersed into 20 signals as it passes through each cluster in the channel. At this time, the delay, power, and four kinds of angles (azimuth angles of arrival and departure, and zenith angles of arrival and departure) are defined. In the case of the TDL model, instead of defining each cluster parameter in the channel as in the CDL model, only the power delay profile of each taps is defined for the entire channel. Therefore, it is a simplified form in which the four angular values do not appear, and the process of distributing signals passing through each cluster to 20 signals is not modeled. It is also possible to generate TDL models by assuming non-isotropic antennas such as directive horn antennas or array antennas. The TDLs and the spatial-filtered TDLs can be used with the correlation matrices for MIMO link-level simulations.  
In contrast, 4G LTE-A system models the wideband characteristics of the channel as a TDL. Each tap independently experiences a fading characteristic by an azimuth direction of departure and direction of arrival angular spectrum. Since the mean direction and angular spread are fixed, TDL represents stationary channel conditions in 4G LTE-A system.