## 4 Propagation and Challenges of the Wireless Communication
In addition, with the capsule traveling at 333 m/s (1200 km/h), the cell radius of the communications system must be as long as possible (>330 m) to allow a maximum cell reselection time of 1 s.  
Figure 5: Simulation with ray tracing technique of 1000 m straight (**a**) and 3000 m curved steel tube (**b**) at 24 GHz. **Doppler shift**: is another of the key elements of the system. For example, Doppler shifts at 24 GHz at top speed (1200 km/h) is \(\pm\)26.6 kHz and can have an important influence on the communications. Maximum Doppler shift is obtained when the capsule passes behind the antennas so that it's important to configure the communications system to operate with Doppler requirements. Thus, in the multi-carrier systems, such as Orthogonal Frequency Division Multiplexing (OFDM) used on 802.11 xx or 5G NR, this parameter is configurable [30] and the requirements are:  
* 2.5 GHz frequency: Maximum Doppler shift \(\pm\) 2.77 kHz at (1200 km/h) intercarrier guard. * 24 GHz frequency : Maximum Doppler shift \(\pm\) 26.6 kHz. In addition, as Doppler shift is deterministic in tubes, Doppler cancellation [31] can be included on the MAC Layer of the communications systems to reduce the impact. This is applicable for 802.11 xx systems that can be adapted to operate in this environment. **Delay spread**: a secondary effect of the speed is the multipath inside the tube due to reflections in an element like intermediate doors, track, antennas, and any discontinuity of the surface of the tube. This multipath will produce time-varying Doppler spreads and non-stationary fading [32] that will produce the overlapping of the sub-carriers and therefore will introduce Inter-Carrier Interference (ICI). ICI will seriously degrade the synchronizer performance and increase the system bit error rate. These problems can be overcome using an expanded intercarrier spacing that will reduce throughput but will increase robustness against ICI. 5G NR can be configured with a subcarrier spacing up to 240 kHz while 802.11 xx can have up to 312.5 kHz. With this configuration, it will be possible to minimize ICI; nevertheless, it is necessary for accurate modeling and testing of propagation inside the tube using channel sounders [33] and propagation measurements to optimize propagation, especially in sections like emergency exits and at the tops of the capsule and to model the influence of the linear synchronous motor track. **Penetration loss**: as is mentioned above the Hyperloop capsule, is operating in the hermetically sealed vacuum tube. To ensure the airtightness, the tube walls are made of steel and so those penetration losses of the tube can be in the range of 40-60 dB if the system is carefully designed so that we can consider that it is a confined space without external interferences and does not require frequency sharing compared with a conventional system. This is especially important for the use of Wi-Fi 2.5 GHz and 5 GHz ISM bands. The capsule is made of carbon fiber without windows so penetration losses are very high (40-60 dB) and must be equipped with external antennas and repeaters to provide coverage inside. **MIMO capabilities**: the use of MIMO in tunnel environments is very limited due to the propagation characteristics of the channel and the difficulty of positioning the antennas with the optimal separation and orientation [34]. The linear propagation in tunnels makes the different propagation channels highly correlated [35]; therefore, the capacity improvement achieved using MIMO is small. To reduce the spatial correlation, it is necessary to separate the antennas considerably, which makes deployment difficult, and is not always possible, especially on the capsule side. Therefore, the use of MIMO in tunnels is usually limited to 2 \(\times\) 2 configurations with a large separation of antennas at the base that allows for achieving a performance improvement.