Hughes HX System, Обзор системы

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Chapter 8 • Transmission features
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The CLT feature provides timing adjustment feedback to enable
the satellite router to transmit as close as possible to the middle of
the burst aperture.
Inroutes and inroute
groups This section describes the technical aspects of the Hughes HX
System implementation of inroutes and introduces the concepts
of inroute groups. Inroute groups are used in the HX System to
bundle inroutes and collectively assign them a bandwidth and
other properties.
The inroute portion of the space segment transmits data from the
satellite routers to the HX gateway.
The inroute transmission has the following parameters:
•
Frequency-time division multiple access (F-TDMA). Each
inroute is at an assigned frequency on the satellite (FDM),
and each satellite router accesses the inroute at its assigned
time slot (TDM).
•
Offset quadrature phase shift key (OQPSK) modulation
•
Convolutional or turbo coding
•
Viterbi forward error correction (FEC)
The inroute is divided into units called superframes. Each
superframe is divided into frames, and each frame is divided into
slots. The slot is the basic unit of inroute capacity. The slot size
and number of slots are determined by the inroute data rate.
The satellite router transmits a burst at the assigned time slots.
This burst enables the burst channel demodulator to lock onto the
incoming signal.
Inroute types and burst
types
Inroutes use one of the following access methods to transmit user
data to the HX gateway:
•
Aloha is immediate, contention-based user data transmission,
and is not allocated. The satellite router randomly transmits
the bursts on an aloha channel. If another satellite router is
transmitting at the same time, a collision occurs. The HX
System uses diversity ALOHA, which involves transmitting
two copies of the packet, each in a different ALOHA slot.
The HX gateway will only use one of the packets received on
the ALOHA channel. The satellite router may be assigned to
a stream depending on inroute configuration and network
congestion.
•
Stream allocation is a defined allocation at a defined time.
The satellite router is assigned and allocated the stream after
the HX gateway detects the ALOHA burst. Stream allocation