The demodulator at the hub functions optimally when the received power spectral density is similar for all remotes transmitting on a given upstream carrier. Therefore, the goal of the Uplink Control Process is to adjust the individual transmit powers of the remotes such that all bursts transmitted on a TDMA upstream carrier are received at the same C/N as measured at the hub. For a given remote, the C/N may vary due to fade conditions or as mobile remotes move across the satellite footprint, requiring UCP to adjust the remote’s transmit power to keep the C/N within range.
To normalize the C/N across remotes, the C/N deviations detected by the line cards are averaged by the protocol processor for each remote. If the deviations are within the
acceptable range of the target C/N, no corrections are sent to the remote. If the deviations are outside the acceptable range, corrections are sent to the remote until the C/N is within range.
In earlier releases, all TDMA carriers in any inroute group had the same MODCOD and symbol rate. Therefore, if network conditions (such as a severe fade) did not allow a remote’s power to be increased to a sufficient level for the hub to demodulate the remote’s burst, the remote would drop out of the network and be forced to reacquire when conditions were favorable. Since all upstream carriers were identical, there was no opportunity to move the remote to a more protected carrier during the fade condition.
In iDX Release 3.2 the power control algorithm was redesigned to accommodate the heterogeneous nature of the upstream carriers in adaptive inroute groups. The goal of the power control algorithm is for each remote to be received at the target C/N on the remote’s nominal carrier. Therefore, the new algorithm is responsible not only for adjusting the remote’s power on the current nominal carrier, but also for selecting a new nominal carrier when required.
TDMA Uplink Control
A remote may be assigned slots on an upstream carrier that does not match its current nominal carrier. For example, during upstream bandwidth contention, a remote may be granted slots on a less efficient carrier if there are no available slots on the nominal carrier. In that case, the remote automatically adjusts its transmit power such that the power matches what is required on the assigned carrier.
In earlier releases, the target C/N (or TDMA Nominal C/N) was an operator-entered value determined by adding the C/N threshold for the inroute from the Link Budget Analysis Guide to the additional operating margin determined by the Link Budget Analysis for the network. Beginning in iDX Release 3.2, the target C/N is calculated using the C/N thresholds for the inroutes from the Link Budget Analysis Guide and the following margins configured in iBuilder:
• The Fade Slope Margin (M1) allows for incremental fade that can occur during the reaction time of the power control algorithm as well as the uncertainty in the C/N0 estimations.
• The Hysteresis Margin (M2) is added to the Fade Slope Margin to prevent unnecessarily frequent switching between carriers.
The system adds the sum of the Fade Slope Margin and the Hysteresis Margin to the C/N thresholds from the LBA Guide to determine the target C/Ns for each carrier in the inroute group. iDirect provides a dimensioning tool to assist network designers in determining suitable values for these parameters.
If a remote’s C/N falls below the target C/N, the power control algorithm increases the remote’s transmit power if possible to bring the signal back up to the target level. If the remote’s power cannot be increased on the nominal carrier and a more protected carrier is available, then the remote’s nominal carrier is changed to the more robust carrier. This keeps the remote in the network at the expense of diminished throughput. If the remote is
consistently below the threshold defined by the target C/N minus the Hysteresis Margin on the most protected carrier in the inroute group, the remote is “logged out” of the network. A remote that has been logged out must re-acquire the network before it can continue to transmit user traffic.
If a remote’s C/N is above the target C/N plus the hysteresis margin, the power control algorithm looks for a more efficient carrier on which the remote can maintain the target C/N. If such a carrier is found and if UCP estimates that remote is capable of transmitting on that carrier, the remote’s nominal carrier is changed to the new carrier. If the remote cannot switch to a better carrier, the power control algorithm decreases the power as necessary to return the remote’s signal to the target C/N.
The power control algorithm reacts faster when the hub determines that a remote has entered an upstream fade. When monitoring the remote’s signal, the algorithm samples the C/N periodically. The algorithm determines a “fade slope” for the remote based on analysis of the C/N samples. The algorithm adjusts the measurement interval depending on the fade slope, such that it is shorter when the fade varies rapidly. This tends to keep small and constant the amount of margin required in order to account for the reaction time.
If a remote fade is detected, then the update interval is decreased based on the severity of the fade. Decreasing the update interval may require the system to allocate additional
NOTE: The Acquisition Margin (M3) is used only to select the initial nominal carrier when a remote acquires the network. This margin is discussed on page 80.
TDMA Uplink Control
upstream slots to the remote. For idle remotes, the configured Minimum Information Rate is ignored and more slots allocated if additional burst measurements are required by the Uplink Control Process.
The Measurement Spacing configured in iBuilder on the Uplink Control tab of the inroute group defines the steady-state update interval. This parameter is set to 2 seconds by default but can be modified by the Network Operator. The smaller update intervals used during fades are set by the software and are not configurable.
In pre-iDX 3.2 releases, the power adjustment algorithm applied corrections to the remote’s power using coarse and fine step sizes configured in iBuilder. These parameters are no longer used and have been removed from iBuilder. Beginning with iDX Release 3.2, the power adjustment step size is automatically determined by the power control algorithm.
Figure 11-3 illustrates the application of uplink power control (UPC) to a fading remote in an Adaptive system.
Figure 11-3. Uplink Power Control During Remote Fade
In Figure 11-3:
• C/N0 = C/N + 10log10 (Rs) : The C/N adjusted for the carrier symbol rate (Rs). NOTE: Because iDX Release 3.2 supports Inroute Groups with different sized upstream carriers, C/N0 has replaced C/N as the measurement of signal quality used to monitor and control remote transmissions on the upstream carriers. See
C/N0 and C/N on page 38 for details.
C3 C2 C1 M2 M1 C3 C2 C1 M2 M1 Carrier 1 Carrier 2 ' H H C3 C2 C1 M2 M1 UPC keeps C/N0 close to C3 Out of UPC: Drops below C2;
Switch down Correction aims for C3 of next carrier down
UPC keeps C/N0 close to C3
Sufficient headroom to
switch up Correction aims for C3
of next carrier up
UPC keeps C/N0 close to C3
Carrier 1
Increasing fade Decreasing fade
Hysteresis margin