ViaSat ArcLight, User Manual, Page: 37 / 368

ArcLight Hub User’s Guide
General System Description
Rev. 001 1081006 1-1
1. General ArcLight
TM
System Description
1.1 Introduction
The ArcLight
TM
Broadband Interactive System consists of an ArcLight
Hub and many ViaSat Mobile Terminals (VMTs) containing ViaSat
Mobile Broadband Routers (VMBRs). The ArcLight Hub couples
revolutionary ViaSat technology with high-volume, industry-standard
transmission equipment to deliver broadband access, two-way via
satellite, in a star network topology. ArcLight is an IP-based network
used to relay internet and intranet content to the customer equipment
(typically laptops, etc.). The ArcLight Forward Link uses DSS QPSK
modulation in spread or unspread mode with Parallel Concatenated
Convolutional Code (PCCC) Turbo Code. User IP traffic is encapsulated
in simplified HDLC frame for the data link layer. It occupies 1.5 MHz to
36MHz BW and bit rates from 500Kbps up to 5Mbps for spread and
10Mbps for unspread.
The ArcLight system features two ViaSat-proprietary technologies: Code
Reuse Multiple Access (CRMA) and Asymmetric Paired Carrier Multiple
Access (A-PCMA). CRMA is a subset of Code Division Multiple Access
(CDMA) and is used for return channel access to a central control
station (Hub) by VMTs.
ArcLight systems can function in two operational configurations:
standard and cross-strapped. A-PCMA (paired carrier multiple-access)
is used at the Hub (through the PCMAHC) when an ArcLight system is
in its standard configuration (i.e., when a single satellite transponder is
used for both forward and return link traffic). In the standard
configuration the return channel signal is overlaid on the forward
channel signal and the PCMAHC is used to cancel out the forward link
signal at the Hub and retain the return channel signal. This allows both
the forward and return link signals to be carried on a single satellite
transponder, which is a significant cost savings.
When separate transponders are used for forward and return link
traffic the PCMAHC is not used. In this configuration (cross-strapped)
the return signal goes to the MCD directly and the Utility Card (UC)
uses the low-pass bandwidth filter circuitry to filter the incoming
signal. This filtered signal is then forwarded to the MCD CDC cards for
demodulating. In the standard configuration where the PCMAHC
cancels the forward link signal, the UC filtering is not needed since
filtering is provided by the PCMAHC. For this configuration the MCD
chassis is rewired so that the return channel signal from the PCMAHC
bypasses the UC.