ViaSat ArcLight, User Manual, Page: 57 / 368

ArcLight Hub User’s Guide
General System Description
Rev. 001 1081006 1-21
1.4.3 Return Link Requirements
1.4.3.1 CRMA
The Return Channel waveform utilizes CRMA, a direct sequence spread
spectrum transmission in which all users employ the same spreading
code.
1.4.3.2 Return Channel Physical Layer
1.4.3.2.1 Return Channel Data Rates
The following Return Channel data (information) rates are supported:
32 kbps, 64 kbps, 128, 256, 512 kbps.
1.4.3.2.2 Return Channel Modulation
The Return Channel employs a constant envelope, phase shift keyed
modulation.
The modulation is selected so that the power spectral density of the
modulated signal meets the following requirements when transmitted
by the VMT (non-linear) SSPA, and amplified by the specified VMT
reflector(s):
• PSD sidelobes suppressed 28 dB or more from peak response
outside of the allocated channel bandwidth
• Radiated power less than 4 dBW in any 4 kHz bandwidth
outside of the allocated channel bandwidth.
The details of the selected Return Channel modulation are as defined in
the SkyLink Air Interface specification.
1.4.3.2.3 Forward Error Correction (FEC) Coding
The Return Channel employs a rate 1/3 Parallel Concatenated
Convolutional Code (PCCC).
The FEC block size is dependent on packet size, and shall take on
values of 424, 848, or 1696 information bits.
1.4.3.2.4 Burst Preamble
Each CRMA transmission is pre-pended with a burst preamble to
facilitate signal acquisition at the HUB demodulator.
A unique preamble is defined for each permutation of data rate and
packet size defined in the system. There shall therefore be a total of 15
unique preambles (5 data rates x 3 packet sizes) defined in the system
air interface.