## 4 Discussion  
Throughout the paper, several results provide insight into the behavior of the indoor radio channel at different environments in two clearly different millimeter-wave frequency bands: The licensed one near 40 GHz and the unlicensed around 60 GHz. Their characteristics allow the definition of advantages and disadvantages of both for deploying high rate wireless networks, which is the aim of this work.  
From the measurement data and the processing of this gathered information, the main observed differences are:  
1. The amount of multipath contributions is clearly reduced in 60 GHz compared to 40 GHz. This is due to the larger propagation attenuation and lower reflection coefficients at 60 GHz. At 60 GHz the energy will be scattered in a more diffuse way as the wavelength is smaller compared to the surface irregularities.
2. This leads to smaller delay spreads at 60 GHz than at 40 GHz (i.e., 2.73 ns versus 8.64 ns at the auditorium).
3. The radio channel is more frequency selective at 40 GHz, with coherence bandwidths for a 0.9 correlation level of 24 MHz at 40 GHz in the auditorium, while a value of 78 MHz was measured in the same environment at 60 GHz.
4. Regarding the different scenarios in LOS conditions, smaller environment dimensions yield to a smaller RMS delay spread at 60 GHz. A value of 0.1 ns is obtained in the corridor while 2.73 ns is obtained in the auditorium. At 40 GHz the difference is not so clear (8.4 ns versus 8.64 ns).
5. On the other hand, smaller scenarios correspond to larger coherence bandwidth values, particularly at 60 GHz, with coherence bandwidth values for a 0.9 correlation level increasing from 6 MHz in the corridor to 78 MHz in the auditorium.
6. OLOS conditions result in larger delay spreads (5.1 ns) compared to LOS conditions (0.1 ns) at 60 GHz. The difference at 40 GHz is no so clear (8.7 ns versus 8.4 ns).  
Figure 15: Theoretical Fleury’s limit and measured results, for different correlation levels \(\alpha\) = 0.5, 0.7, or 0.9 at the corridor in the 40 GHz band: (**a**) LOS conditions, (**b**) OLOS conditions.  
The inverse relation between the delay spread and coherence bandwidth has been confirmed by plotting both parameters against [PERSON]'s limit.
* Due to the reduced multipath propagation conditions at 60 GHz, channel parameters seem to be more sensitive to changes in the size of the environment and on the visibility conditions (LOS/OLOS) than at 40 GHz.
* Regarding ray-tracing simulations, a simple ray-tracing model with just single reflections would be enough to represent all relevant multipath contributions at 60 GHz. Higher order reflections should be considered for modelling all contributions at 40 GHz.  
At 38 GHz, results for the corridor agree with those in the literature [16] where measurements were performed in a wagon with similar dimensions and geometry. The delay spread values reported are 7.63 ns and 9.77 ns, very similar to the 8.4 ns we measured in LOS condition.  
At 60 GHz, similar delay spread values can be found in [17] where a mean delay spread value of 6 ns with 2 ns standard deviation are reported for a conference room and in [18] where delay spreads below 7 ns are measured for 99% of the positions, also at a similar conference room. Finally, in [19] a value of 3.13 ns is found, but for a smaller environment.