ViaSat ArcLight, User Manual, Page: 48 / 368

ArcLight Hub User’s Guide
General System Description
1-12 1081006 Rev. 001
The ArcLight system is an IP networking system, carrying user IP traffic
and system monitor and control messages as IP traffic, simplifying the
design and limiting the use of special purpose hardware. The hub
hardware is all COTS with the exception of the MCD, FLM and
PCMAHC.
1.2.3 Satellite Air Interface
The air interface is segmented into forward-link (from the Hub) and
return-link (from the VMT) satellite air interfaces. Both directions are
part of Paired Carrier Multiple-access (PCMA), which permits satellite
transponder to be used for both forward and return link traffic
simultaneously, as explained in the system overview.
Forward-link
– Square root raised cosine pulse shaped QPSK modulation
is used for the forward-link.
The FEC coding supported includes:
Convolutional coding rate -1/3
Data rates are selectable in 100 kbps increments from 500 Kbps to 10
Mbps. The breakdown is:
o
100 Kpbs increments up to 1 Mbps
o
250 Kbps increments up to 3.5 Mbps
o
500 Mbps increments up to 5 Mbps
o
1 Mbps increments up to 10 Mbps
The lowest bandwidth occupied is 1.5 Mbps (500 Kbps * 3 symbols/bit
for unspread).
The VMBR receiver automatically detects the data rate and FEC
encoding rates, and processes the received data selecting only those
packets addressed to the local VMT or broadcast to all VMTs.
Return-link
– The return-link uses a derivative of CDMA, a direct
sequence spread spectrum modulation called Code Re-use Multiple
Access (CRMA) where all users employ the same spreading code. Each
CRMA burst transmission is pre-pended with a 64 bit (192 symbols)
burst preamble to facilitate signal acquisition at the Hub Demodulator.
The return link data is packetized into standard 53 octet Asynchronous
Transfer Mode (ATM) cells. The IP data is encapsulated by the ST-IDU
using ATM Adaptation Layer 5 (AAL5) protocol. The return link data
rate is selectable from 32, 64, 128, 256, or 512 kbps and the Return
Channel burst size can be 1, 2, or 4 ATM cells. The burst size is a
configurable parameter and can be set either locally or remotely via the
NMS.
A Rate-1/3 Turbo Code is used on the forward and return channel. The
packet size may be 424, 848, or 1696 bits. The Packet format is shown
in Figure 1-8.