Aloha parameters – Comtech EF Data MIDAS Version 4.X NMS Configuration User Manual
Page 55
MIDAS 4.X NMS Configuration, Rev.6
Setup and Operation 2–27
E
E
n
n
a
a
b
b
l
l
e
e
U
U
p
p
l
l
i
i
n
n
k
k
P
P
o
o
w
w
e
e
r
r
C
C
o
o
n
n
t
t
r
r
o
o
l
l
This checkbox allows the operator to enable/disable the Uplink Power Control
option.
Uplink Power Control (UPC), a standard feature of the MIDAS system, is the
method used to establish a network-wide reference for power. Specifically,
UPC is used to ensure that the Outbound Control Channel carrier is
downlinked from the satellite at a constant power level. This reference allows
each of the nodes in the network to determine and report (via regular polling
from the NMS) if, and to what degree, downlink degradation of a node (and its
associated traffic channels) may be occurring due to fade, whether from rain or
other non-atmospheric conditions.
The SNM-1002 LinkSync
modem is used to constantly measure the
reference (Outbound Control Channel) carrier. The NMS compares the
measured E
b
/N
o
with a “Reference E
b
/N
o
” stored in the database, and adjusts
the Outbound Network Control Modem’s (SNM-1001) transmit power level
from the default power level, compensating for any uplink degradation,
thereby maintaining a constant transmit power level (downlink) at the satellite.
Aloha Parameters
M
M
i
i
n
n
B
B
a
a
c
c
k
k
o
o
f
f
f
f
/
/
M
M
a
a
x
x
B
B
a
a
c
c
k
k
o
o
f
f
f
f
If a node does not receive an acknowledgement from the NMS, it will pick a
random number of frames between the selected Minimum and Maximum
Backoff values to wait before re-transmitting the last message sent. This is
done to minimize data collisions on the control channel. System default
settings are 1 slot minimum, and 10 slots maximum.
The access method by which remote nodes are accessing the inbound channel
is modeled after the Slotted Aloha random access scheme. This is due to the
fact that in most cases, the two causes of relative timing errors (namely local
oscillator drifts and node location uncertainty) are insignificant to the inbound
channel slot size. Slot timing drifts caused by local oscillator drifts are
insignificant, since the beginning of the slot is triggered based on the timing
message received (all remote nodes) on the outbound channel from the NMS.
Node location relative errors is typically in the order of few tens of
microseconds, for satellite with small inclination angles (e.g.,
~
±
15
µ
s for
satellites with 0.05
°
inclination angles, and 10
°
relative node latitude
differences).