34
March 2016
Establishing mobile High
Throughput Satellite services
A challenge to engineers and enterpreneurs
High Throughput Satellite (HTS) is a trade-off between
coverage, service performance, cost and complexity.
The maximum data transmission rate achievable
via any satellite transponder is limited by its bandwidth
and the available carrier-to-noise (C/N)ratio. Carrier
power increases with the gain (aperture) of the satellite
and/or earth station antennas, and/or by increasing
transmit power in each direction of transmission, but it
is ultimately limited by interference thresholds. Noise
comprises thermal and interfering components, but can
be minimised by choice of frequency band, inter-system
coordination, equipment, and the key to HTS — system
design.
Users want smaller terminals with simpler antennas
and consistent levels of performance over designated
service areas. HTS achieves this with higher gain
(narrower beam) satellite antennas that increase the
satellite’s transmitted power spectral density and receive
sensitivity.
HTS mobile uses spot beams, which individually
cover a relatively small area, albeit with high
performance. Exceeding one-spot beamwidth requires
multiple contiguous spots, which collectively cover the
required service area. The challenge for satellite operators
is that some beams may have little demand or need a
wider coverage area, while some may need to satisfy high
demand by using multiple transponders, and steerable
beams may be required for major events.
The provision of large numbers of beams, multiple
transponders and associated control and switching
systems adds major cost and complexity to HTS satellites,
and also to the gateway earth stations that serve them.
Spot beam frequencies (and polarities) must be re-used
several times to minimise interference and achieve high
efficiencies.
Higher throughput requires increasing modulation
efficiency in line with increased C/N (8bps per Hertz
is now achievable). Modulation and coding are also
undergoing continuous development and improvement
(for example, DVB-S2X now supports 4K/UHDTV).
The higher frequency bands are preferred for HTS due
to their higher available bandwidth. Ka-band offers the
most bandwidth and is relatively interference-free and
uncongested compared with Ku-band, but Ka- and Ku-
bands are the worst affected by heavy rainfall. Ku-band is
however, already well established for wide area services.
Rain loss can be compensated by uplink power
control, geographically diverse/redundant gateways
and/or Adaptive Coding & Modulation throttling back
throughput during severe rain.
Smaller mobile antennas may be used at Ka- and
Ku-bands compared to lower frequency bands at similar
throughput. However, mobile antennas must be carefully
selected to achieve defined service levels and pointing
accuracy.
Lower frequency bands offer much lower throughput,
but higher availability. A 30cm diameter antenna at
Ka-band can support 10-20Mbps HTS, whereas the same
sized antenna at L Band may support only 0.5-1Mbps.
The choice of frequency band for any application is
crucial.
Solutions, applications and options
Satellite-on-the-Move (SOTM) land, maritime and
aeronautical services benefit most from mobile HTS, but
require complex, costly, bulky and high maintenance
electro-mechanically tracking Ku- and Ka-band
antennas. Accurate tracking is essential for service
quality and the minimisation of interference to/from
adjacent satellites.
Flat-plate antennas with electronically steered beams
(Kymeta and Phasor) are being deve