Home' RTCA Documents for Review : DO-262D Contents Appendix D
© RTCA, 2018
L-Band Link - The LL control provides the functionality to control and monitor the air
channels, determine access privileges, update system programmable data, and
establish and release connections. LL is responsible for the Call Processing-related
signaling associated with Mobile origination or termination and provides for the
signaling procedures associated with the Access portion of the Iridium Network.
Additionally, LL controls the real-time aspects of radio resource management on the L-
band link, such as the allocation and maintenance of L-band resources and handoff
L-Band Physical - LBP represents the control interface that exists between the satellite
and the SDU. The primary distinguishing characteristic of LBP is that unlike ACCHL,
the delivery of messages is not guaranteed. Examples of messages carried in this
manner are ring alerts, directed messaging, Broadcast Channel messages, handoff
candidates, handoff candidate lists, and Doppler/timing/power control corrections.
Associated Control Channel, L-Band - The ACCHL transmission protocol is used by all
entities that need to (reliably) send data via the L-Band traffic channel burst between
the satellite and the SDU. The ACCHL protocol permits sharing the traffic channel
burst with other protocols. The ACCHL Logical Channel is bi-directional and uses
portions of the uplink and downlink Traffic Channel, Link Control Word and the
Payload Field between the satellite and the SDU. The Traffic Channel is described in
the next section. The ACCHL protocol will transport variable size messages on the
ACCHL Logical Channel and is used to guarantee the delivery of messages between
the satellite and the SDU. It relies on LBP only in that LBP arbitrates the access to the
physical layer when there is contention for the Physical Layer resources.
The Iridium satellites, in low earth polar orbit, have highly directional antennas
providing Iridium system access to SDUs. These antennas are configured to project
multiple beams onto the surface of the earth. The beams move rapidly with respect to
SDUs and with respect to other satellites. Handoff, the process of automatically
transferring a call in progress from one beam to another (or sometimes within a beam)
to avoid adverse effects of either user or satellite movement in this highly mobile
environment, is required in three situations. First, an SDU must be handed off
between satellites as they move relative to the SDU (Inter-satellite).
Second, an SDU must be handed off between beams on a satellite as beam patterns
move relative to the SDU (Intra-satellite). Last, an SDU must be handed off to another
channel within a beam for frequency management and to reduce interference (Intra-
beam). Although the Iridium system may force a handoff, handoff processing is
primarily SDU initiated.
As a satellite moves away (for example, moves over the horizon) and a new satellite
approaches (for example, comes into view over the horizon), an SDU must transfer
from the current satellite (the losing satellite) to the new satellite (the gaining satellite).
This Inter-satellite handoff, on the average, occurs approximately every five minutes
during a telephone call. It may be initiated as frequently as five seconds or as long as
10 minutes, depending on link geometry.
As satellites move from the equator to a pole, the actual distance between adjacent
satellites decreases to a few kilometers and then increases to several thousand
kilometers as the satellites again approach the equator. To avoid radio interference,
beams near the edges of a satellite’s coverage field are turned off as the satellite
approaches a pole and then turned on again as it approaches the equator.
Additionally, the same radio channels are never available in adjacent beams on a
satellite or between nearby satellites. Thus, as the satellite and its beams pass by, an
SDU must frequently transition to a new beam. This Intra-satellite handoff occurs
approximately every 50 seconds during a call.
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