# Worldwide Coverage Mobile Systems for
## 3 Terrestrial Segment Requirements  
In this section, the terrestrial segment requirements regarding Tx and Rx antennas gain, Tx power, and tracking systems are analysed.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Type** & **Features** \\ \hline Global & global coverage: One downlink antenna beam \\  & simple antenna \\  & small data capacity \\  & transparent repeaters \\ \hline Spot beams & separated beams \\  & simple antenna \\  & better data capacity \\  & transparent repeaters \\ \hline Multiple spot beams & separated \& contiguous \\  & complex antennas (Beam Forming Networks) \\  & High data capacity (frequency re-use) \\  & comn payload has regenerative repeaters \\  & on Board Processing \& routing capabilities \\ \hline Dual polarization & overlapped beams \\  & increased data capacity (twice frequency re-use) \\  & higher interference: uplink \& downlink \\ \hline \hline \end{tabular}
\end{table}
Table 7: Satellite space segment antennas for earth coverage strategies.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline  & **OneWeb** & **SpaceX** & **Telesat** \\ \hline Constellation & LEO & LEO & LEO \\ Height (km) & 1200 & 550 & 745–1000 \\ No. Satellites & 720 & 4425 & 117 \\ Elevation angle & 55 & 40 & 20 \\ Full coverage & yes & yes & yes \\ Sat. weight (Kg) & 150 & 227 & 100 \\ Freq. band & Ku\&Ka & Ku\&Ka \\ User terminal antenna & Phase array & Phase array & low profile ESA or traditional \\  & & & mechanically steered \\ Antenna size & 36 \(\times\) 16 cm & 1 m wide & from 65 cm \\ Max. Data rate (Gbps) & 9.97 & 21.36 & 38.68 \\ Latency (ms) & 25 & 40 & 10–15 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Comparison of the main features of OneWeb, SpaceX, and Telesat.  
Massive LEO-constellation, such as OneWeb and Space-X, extend mobile operator network coverage to wider areas around the world, including the oceans and polar regions, using nano-satellites as a relay. This capability is enabled using two different terrestrial segments: the gateway and the user terminals. Gateway stations receive the requested data from users through satellites constellation and send back the proper data-packet.  
Additionally to the basic connectivity functionality, Gateway stations also provide Telemetry, Tracking and Command System (TT&C) for the whole constellation. Usually, the Gateway consists of big fixed antennas operating in the Ka-band, wide bandwidth, and high power. For example, in the OneWeb constellation, the Gateway uses a TDM transmission scheme for a 250 MHz wideband carrier, and obtain two-times frequency re-use, by use of orthogonal circular polarization.  
Once the carrier is received as an RHCP and the data is extracted, users' terminals transmit bursts of data SC-TDMA/FDMA in short periods of time and on a narrow-band carrier (1.25 to 20 MHz) in order to minimize the Tx peak power. It's required because terminals, including the LHCP-TX-antennas, are small-size and light, to make them real portable devices, and the dissipated power should be minimized. Moreover, battery life must be extended as long as possible. This can be accomplished by reducing the required Tx power.  
As it was remarked above, LEO-constellations nano-satellites turn around the world at high speed, and handovers frequency is high given that each satellite is tracked by the terminal during few seconds, and it is needed to guarantee the communication service. Minimizing the problem requires using terrestrial steerable dish-antennas which, by using an appropriate DOA algorithm, can track the satellite optimizing the link performance and battery consumption. However, steerable dish-antennas require complex mechanical mechanisms to point the antenna to the satellite. On the other hand, phased array antennas use beamforming array processing techniques [16], to achieve the appropriate radiation pattern. Figure 1 draws a typical phase array antenna that consists of 8 \(\times\) 8 antennas.