ViaSat ArcLight, User Manual, Page: 50 / 368

ArcLight Hub User’s Guide
General System Description
1-14 1081006 Rev. 001
1.2.3.1 Return Channel Features
The features of the ArcLight return-channel are:
•
Random-access packet system that avoids the overhead,
delays, and complexity associated with reservation schemes
•
Spread-spectrum multiple access provides efficient
bandwidth use for all traffic types
•
CRMA allows a single PN code to be re-used many times thus
reducing the Hub correlator’s complexity
•
Use of GMSK modulation, a true constant envelope
modulation, allows the use of inexpensive, non-linear up
conversion techniques and non-linear VMT amplifiers
•
Use of a short block length Turbo code to reduce the
sensitivity to multiple-access interference, increasing the
return-channel capacity
• Use of an underlying near-antipodal modulation for high
phase noise immunity and maximum robustness to
frequency estimation errors
•
Use of flexible chip rates and data rates to accommodate a
variety of network sizes and applications
•
Use of uplink power control to combat link fade conditions
1.2.4 Return channel access
Configurations for the ArcLight return channel are called access-types.
Each access-type, as a minimum, is comprised of a burst rate, a chip
rate and a number of cells per burst. These parameters are selected via
screens in the NMS and then communicated to the RtNMS and the
MCD, via the hub network. The mapping for access-type to VMT is
configured by the NMS and sent to the RtNMS whenever a change
occurs.
The login process allows a VMT that is just powered up to enter an
ArcLight network and provide communications services. To login, a
VMT must know what access-type to use to communicate with the
RtNMS at the hub. The RtNMS sends a broadcast Bulletin Board
message with a login access-type. The VMT uses the log-in access-type
to establish communication with the RTNMS. If the VMT has been
provisioned in the network, the RTNMS has received this data from the
NRTS and the RTNMS will confirm back to the VMT that it has received
the VMTs login request. Once the VMT is logged in using the log-in
access-type, the RTNMS will send a Parameter Change Message (PCM)
instructing the VMT to change to its assigned access-type and begin
using it for all communication. Multiple access-types may be used in a
single network.
The communications between the RtNMS, MCD and VMT is shown in
Figure 1-9.