Home' RTCA Documents for Review : DO-262D Contents Appendix D
© RTCA, 2018
Certain control and traffic applications implement error correction coding to improve
the link bit error rate, with characteristics tailored for certain traffic and signaling
message applications. The vocoder algorithm provides its own interleaving and
forward error correction. Most of the administrative transmissions used in granting
access to and exerting control of the link implement their own internal error correction
The link protocol does not provide forward error correction to user generated data
transmitted in the payload. Such data is protected from transmission errors by a 24-bit
Frame Check Sequence (FCS) transmitted in every traffic burst containing a data
payload (as opposed to a voice payload). If the FCS does not validate that the
payload data was correctly received, the L-Band Protocol implements error by
retransmission of the Iridium frame. Erroneous information, i.e., payload data that
does not satisfy the Frame Check Sequence, is not passed to the end user. Therefore,
a decrease in channel quality which causes any increase in channel bit-error-rate
results in an increase in the number of retransmissions and a corresponding decrease
in the number of user-generated bits provided to the end user. Iridium circuit-switched
data service has been designed to provide a minimum throughput of 2400 bps user
Traffic channels operate with adaptive power control, discussed below, which acts to
limit power transmissions beyond what is required for appropriate voice and data
The L-Band link has been designed for a threshold channel bit error of 0.02, which is
sufficient to support voice services and data services. Note this does not include any
Forward Error Correction (FEC) or other methods that are employed in the network.
This level is achieved at an Eb/(No+Io) of 6.1 dB in clear line of sight conditions.
When used with a standard Iridium handset, the basic Iridium Satellite Network will
operate with a link margin of 15.5 dB above this level with respect to an average
received signal level. This margin is required to mitigate fading due to the Rayleigh
multipath and shadowing typical of handheld phone operation in urban environments.
This margin is available on a statistical basis based on the underlying Rayleigh
statistics. Under good channel conditions, this level is reduced by adaptive power
control. Even under adaptive power control, the link margin is maintained to mitigate
fades that are too short in duration to be compensated by the power control loop.
In the aeronautical environment, the fading statistics are Rician and the spread of
possible received signal levels is less. Iridium has agreed to make some of the
Rayleigh statistical margin available to meet the needs of aeronautical installations for
the AES type of terminal in use. See Iridium Reference F Iridium Aircraft Multi-Modem.
In effect, this increases the average demand on the satellite transmitter power
necessary to support aeronautical users by a factor of two with respect to the
aggregate average power demand for handheld users. The remainder of the handheld
margin is required to meet the statistical variations of Rician aeronautical environment.
Adaptive power control uses a closed loop algorithm in which the space vehicle and
AES receivers measure the received energy per bit per noise power spectral density
(Eb/No) and command the transmitters to adjust their transmitted power to the
minimum value necessary to maintain high link quality. When the entire available link
margin is not required to mitigate channel conditions, adaptive power control has the
effect of reducing system power consumption. There are slight differences in the
power control algorithms used for voice and data operations. For data operations, the
algorithm is biased toward higher power levels and does not use adaptive power
control, hence ensuring low channel bit error rates and high user throughput.
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