spectrum: Introduce a MIMO- and performance-oriented channel model based on 38.901

This MR aims to introduce the changes related to the implementation of the simplified channel model into ns-3.

Specifically, the model, which has been developed in the context of my GSoC project, aims to provide a performance-oriented alternative to the 3GPP TR 38.901 framework which is currently implemented in the mainline ns-3.

This model, which follows the general approach of [1], provides the means for computing a 3GPP TR 38.901-like end-to-end channel gain by combining:

• the path loss and shadowing terms provided by the ThreeGppPropagationLossModel class;
• the array and beamforming gain, computed as outlined in [2] using the novel MimoSpectrumPropagationLossModel class. This term supports the presence of multiple antenna elements both at the transmitter and at the receiver and arbitrary antenna radiation patterns; and
• a fast fading term, sampled using the Fluctuating Two Ray (FTR) model distribution [3]. The latter is a fading model which is more general than typical ones, and which has been shown to provide a better fit to fading phenomena at mmWaves. Since the model requires on input a set of parameters, the goal is to calibrate it using the 3GPP channel model and to produce a table which associates the simulation parameters (such as carrier frequency and LOS condition) to the FTR parameters providing the best fit to the corresponding TR 38.901 channel statistics. As a consequence, it will be possible to use this channel model for all the frequencies which are supported by the 38.901 model, i.e., 0.5-100 GHz. Please note that this calibration is still missing (hence the draft tag): it will be taken care of in the next few weeks, after a calibration of the 38.901 model itself will be carried out.

The class has been thoroughly documented and an ad-hoc test (mimo-splm-test) has been implemented, to check the correctness of the related computations. In particular, the test checks the consistency of the FTR empirical mean with the expected value, and compares the overall array gain at boresight with the theoretical value, using different array configurations.

Furthermore, the array radiation pattern versus the relative azimuth of transmitter and receiver has been plotted (see below) and visually compared to that presented in [4], matching the expected behavior.

UPA with 3GPP 38.901 antenna elements:

UPA with isotropic antenna elements:

P.S.: Due to the draft status, I have not documented yet the changes in RELEASE_NOTES.md, CHANGES.html and the manual. As soon as I will get some positive feedback on the general structure of these modifications I will take care of this as well.

[1] Michele Polese and Michele Zorzi. “Impact of channel models on the end-to-end performance of mmwave cellular networks”. In: 2018 IEEE 19th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC). IEEE. 2018, pp. 1–5

[2] Mattia Rebato et al. “Study of realistic antenna patterns in 5G mmWave cellular scenarios”. In: 2018 IEEE International Conference on Communications (ICC). IEEE. 2018, pp. 1–6.

[3] Juan M Romero-Jerez et al. “The fluctuating two-ray fading model: Statistical characterization and performance analysis”. In: IEEE Transactions on Wireless Communications 16.7 (2017), pp. 4420–4432.

[4] Asplund, Henrik, et al. Advanced Antenna Systems for 5G Network Deployments: Bridging the Gap Between Theory and Practice. Academic Press, 2020.