Hughes has enhanced and optimized our satellite broadband products and services to directly address the key requirements found in many mobile applications. These features have been fully incorporated and integrated into the HX satellite broadband systems. Some of the key features include:
The use of Adaptive Coding and Modulation (ACM) of the outbound channel enables the remote terminal to continually monitor the received signal level of the outbound channel and to dynamically request changes to the combination of coding and modulation. This allows the downstream channel to be continually optimized as the mobile terminal travels through the various contours of the satellite footprint.
With the AIS (Adaptive Inroute Selection) feature, the TDMA channel (uplink from the mobile device) is continually monitored by the hub and the remote terminal is thus continuously advised of its optimal TDMA transmission coding and power levels. Similar to the DVB-S2/ACM for the outbound channel, this feature means that the TDMA channel is also continually optimized as the remote terminal travels through the various contours of the satellite footprint.
TDMA Channel Spreading
To overcome issues with off-axis emissions, the HX System supports spreading of the TDMA channel by two times and four times the nominal channel bandwidth. A 256 ksps TDMA channel with a nominal channel spacing of 320 Khz can be spread to either 640 khz (2x spreading) or 1280 Khz (4x spreading). This feature enables the use of very small antennas as it mitigates adjacent satellite interference.
The HX System is being used to support aeronautical broadband for commercial airliners. As these vehicles move at a very high rate of speed, Hughes has made enhancements for a Doppler update.
Outbound Flywheel and Fast Reacquisition
As land mobile units will frequently encounter obstructions (trees, bridges, etc.) which prevent receipt of the outbound channel, Hughes has implemented a “flywheel” for the timing synchronization of the outbound channel. The flywheel can “spin” for as long as 30 seconds, whereby if the outbound signal is seen within the 30 seconds, the reacquisition of the outbound channel occurs immediately upon receipt of the first superframe marker.
External 10 Mhz Reference
To ensure fast TDMA transmit capability, the HX remote unit is capable of accepting an external 10 Mhz reference. This eliminates the need for frequency stability to be derived off the outbound carrier (which is not accurate during the flywheel period) and enables the remote terminal to transmit the TDMA carrier immediately upon reacquisition of the outbound channel.
IP Steady State
The HX System maintains the IP session during periods of link outages so that even if the signal is lost for longer than 30 seconds, (i.e., a train in a tunnel) once the link is restored, the users do not have to reestablish IP connectivity.
The HX remote terminals are configured with a ruggedized chassis permitting mounting in a 19 inch rack. Hughes’ experience is that mobile terminals are often installed in environments with high heat, humidity, dust, and vibration.