The document discusses new challenges in modeling 5G networks, including: 1) The need for more realistic channel, deployment, propagation, traffic and mobility models to accurately reflect real 5G scenarios. 2) Developing new physical layer abstractions to simulate new waveforms like FBMC and UFMC. 3) Modeling massive MIMO systems with tens to hundreds of antennas at both the transmitter and receiver. 4) Developing channel models for higher millimeter wave frequency bands with larger bandwidths. 5) Properly modeling interference between cellular and device-to-device links that reuse physical resources.

1. New challenges in the 5G
modelling

2. Introduction
 This section describes a set of challenging characteristics in 5G modeling that
require the development of new models for new features or a great increase in
memory and/or computational complexity for an accurate evaluation. Some of
these challenges have already been well addressed, like the use of real
scenarios, the simulation of moving networks and the D2D link, while others
are being tackled at this moment.

4. Real scenarios
 In order to obtain simulation results that reflect the system performance as precise as in reality,
models exploited in simulation work should be able to reflect the characteristics of a real world
scenario with a manageable computation complexity.
 Therefore, a simulation is only meaningful if it is established and aligned with its specific considered
real scenario, in terms of environment model, deployment, propagation, traffic and mobility models.
Invalidity of these models highly contributes to lack of accuracy of simulation results, which
motivates further strengthening those models that are not proper for 5G network evaluation.
 For instance, current channel models, such as ITU-R pedestrian and vehicular, lack the incorporation
of elevation aspects with respect to transmit and receive antennas. For evaluating beyond 4G
technologies, where massive and ultra-dense antenna deployments are to be expected, more precise
and realistic channel models that take the elevation dimension and the resulting changes in radio
propagation into account are required. In order to check the capability to support new types of
services, simulation and evaluation of different technology components will be conducted in 5G
scenarios.
 Thus, current simulation assumptions need to be rephrased and aligned with the real scenarios, in
order to provide a high validity of simulation results. Since strict communication reliability is
required in some 5G scenarios, a high accuracy of simulation models is critical to obtain valid and
meaningful simulation results.
 For instance, real scenarios should be captured in simulation as precisely as possible for emergency
and traffic safety scenarios to avoid any misleading conclusion

6. New waveforms
 New waveforms are being proposed for 5G, such as Filter Bank Multi-Carrier
(FBMC) and Universal Filtered Multi-Carrier (UFMC). Therefore, new
physical layer abstraction must be provided to simulate the new characteristics
of those waveforms in system-level simulations.
 In particular, the non-orthogonality and synchronism peculiarities that they
have are quite different to previous physical layers.
 Research is being conducted on this topic and proposals are being published.

7. Massive MIMO
 Massive MIMO implies that the transmitter and/or the receiver has more than ten antennas.
 Typically, from tens to hundreds of antennas. In 4G, it is commonly assumed that only the
base station has this high amount of antennas.
 However, in 5G both transmitter and receiver will be equipped with a high number of
antennas. In early phases of massive MIMO evaluations, the modeling can be simplified. For
example, given a receiver with nr antennas, the whole set of antennas could be replaced in
simulation by a unique effective antenna with 10 log10ðnrÞ dB more gain than the antennas
of the array. A more general approach is to consider that a whole antenna array could be
represented in simulation by M effective antennas with 10 log10ðnr=MÞ dB more gain than
the antennas of the original array.
 Each effective antenna would represent a set of nr=M antennas of the original array. In this
simple approach, the channel to each effective antenna would be equal to the channel of the
central antenna of the set of replaced antennas.
 Another simplification that can be made is to assume that channels are uncorrelated, but in
this case, it is suggested that the number of sets assumed is realistically chosen (based on
literature reported values) in order to avoid overestimating the performance. If a more
accurate analysis is required, channels between all the transmit antennas and all the receive
antennas must be considered

8. Higher frequency bands
 The use of higher frequency bands, such as millimeter waves, opens a set of new
challenges. Channel modeling is a first challenge for simulation at higher frequency
bands. Ray tracing is a valid approach for real scenarios, or for synthetic scenarios with
detailed description. Stochastic models are currently being developed as results from
the measurements campaigns are being made available.
 Frequency bandwidths available at higher frequency bands are considerably larger than
those available at the frequencies used by 4G. If frequency sampling is kept equal in 5G
simulations to the values used in 4G simulations, computational complexity could
increase heavily. It can be noted that the delay spread found at higher frequencies is
usually smaller than at lower frequencies, although the reported differences are not too
big.
 The frequency sampling can be reduced in some evaluations without losing too much
accuracy even if coherence bandwidth is not reduced. For example, consider an
assessment with full buffer traffic model, a frequency-selective scheduler and 1 GHz
bandwidth. It could be valid to divide the bandwidth in 100 portions of 10 MHz instead
of considering portions of 180 kHz, as it is common in LTE. The rationale is that the
consideration of 100 portions could be enough to get a multi-user diversity gain close to
its maximum value.

9. Device-to-device link
 As a key enabler for 5G networks, D2D communication can be exploited, on one side,
to offload traffic of cellular network, and on the other side, to decrease the latency.
 Depending on whether cellular resources are reused for D2D communication or not,
mutual interference exists among cellular and D2D links or among D2D links, where
the same physical resources are reused.
 In order to obtain reliable simulation results, the interference channel should be
modeled properly. Since positions of D2D users are generated within runtime, path loss
values for the above-mentioned interference links are hard to be pre-cashed due to the
extreme long calculation time and large storage capacity.
 Meanwhile, special consideration should also be taken into account regarding traffic
models of D2D links, depending on the concrete environment, i.e. whether the D2D
link is established between vehicles or pedestrians.
 The traffic model applied in simulations has critical impact on the evaluation of smart
scheduling schemes for D2D communication.

10. Moving networks
 In legacy systems vehicles bring a penetration loss for their inside-vehicle users. However, due to a better
computation capability, a larger antenna dimension and more storage space, communication facilities
deployed in vehicles are more advanced than current mobile user devices.
 Therefore, vehicles play an important role for 5G networks to improve overall system performance.
Advanced communication infrastructures enable every vehicle to have the flexibility to act as an access
node for its carried users and users in the proximity. Vehicles can be considered in this case with antennas
equipped on top. When simulation is carried out, the rule of vehicle infrastructure should be properly
reflected.
 For instance, if information required by a user is already cashed by one vehicle located in proximity of this
user, a V2X communication link is established to offload traffic from cellular network. Moreover, the
vehicle acts as a transmitter and forwards the information directly to the user.
 In another case where required information by outdoor users is not cashed locally, both the wireless
backhaul link between base station and vehicle and the link between vehicle and users should be simulated
and aligned with legacy evaluation methodology.
 It should be noted that interference should be properly modeled if in-band wireless backhaul is exploited.
Regarding inside-vehicle passengers, the vehicle penetration loss can be efficiently avoided by exploiting
antennas deployed on top of the vehicle.
 The communication link between one vehicle and its carried passengers can be solved very well by current
technologies and therefore does not need to be simulated