The protocol processor broadcasts timeplan messages on the downstream carrier that allocate upstream traffic slots to the remotes in the network and acquisition slots to remotes that are not in the network. When a remote is ready to join an iDirect network, it first locks to the downstream carrier and waits for a timeplan message from the hub that invites the remote to send an acquisition burst to the hub. The timeplan message identifies the upstream carrier, acquisition slot, and frequency offset that the remote should use when transmitting the acquisition burst. The remote may be assigned acquisition slots on any upstream carrier in the inroute group.
Based on the timeplan message and configuration parameters, the remote calculates the correct Frame Start Delay (FSD) and frequency for the acquisition burst. The remote calculates the transmit power of the acquisition burst from the TDMA initial power and reference carrier parameters configured in iBuilder.
The initial power must be set such that the hub receives acquisition bursts from the remote at an acceptable C/N. Beginning in iDX Release 3.2, the characteristics (MODCOD, symbol rate, etc.) of the upstream carriers in an inroute group may differ from carrier to carrier. If the remote were to transmit at the same initial power on all inroutes, the C/N of the acquisition bursts received at the hub would vary depending on the characteristics of the inroute on which the remote was acquiring. This variation could cause interference on the upstream carrier or missed acquisition bursts from the remote.
To prevent this variation in C/N, reference carrier parameters are configured in iBuilder along with the TDMA initial power. If the remote is assigned an acquisition slot on a carrier that differs from the configured reference carrier, the remote adjusts its initial transmit power to compensate for the differences between the reference carrier and the assigned carrier. This allows the C/N of the acquisition bursts to remain within the acceptable range regardless of the inroute on which the remote acquires. For a description of the reference carrier
parameters, see Reference Carrier Parameters on page 41. The Installation and
Commissioning Guide for iDirect Satellite Routers contains Instructions for setting the TDMA initial power and reference carrier parameters for a remote.
Acquisition Process
If the hub fails to detect the acquisition burst from the remote in the assigned acquisition slot, it allocates another upstream acquisition slot to the remote. The hub changes the remote’s frequency offset for the new burst if the acquisition step size for the carrier is smaller than the total sweep range. The sweep range is mainly determined by the stability of the hub downconverter.
This process continues until the hub detects an acquisition burst from the remote. Once the hub detects an acquisition burst, the hub sends the frequency offset correction to the remote and the Upstream Control Process takes over to keep the remote in the network at the correct power, frequency and symbol timing. (For more information, see Uplink Control Process on
page 79.)
The performance of the acquisition process is determined by the speed with which remotes join the network and the number of acquisition bursts the remote must transmit before a burst is successfully demodulated. If a remote can acquire the network more quickly by trying fewer frequency offsets, the number of opportunities that other remotes have to acquire is increased and the number of remotes that are out of the network at any one time is reduced. Therefore optimization of the acquisition process involves reducing the number of acquisition bursts that remotes must transmit to acquire the network.
iDX Release 3.3 supports two types of remote acquisition: Traditional Acquisition and
Superburst Acquisition. The type of acquisition is configured per upstream carrier in iBuilder. Superburst Acquisition greatly improves the time and bandwidth required for remotes to join the network and should be used whenever possible. However, in this release Superburst Acquisition can be used only on upstream carriers being received by multichannel line cards or by receive-only eM1D1 line cards.
When receiving a traditional acquisition burst, the TDMA demodulator at the hub has a narrow tolerance for frequency offset (approximately 1.5% of the upstream carrier symbol rate for all modulation types). Because of this, the hub may fail to demodulate an acquisition burst at the assigned frequency offset. Therefore, the hub varies the frequency offset in the timeplan messages causing the remote to burst at different frequencies within a defined frequency range (or “sweep range”) until the demodulator at the hub detects the upstream burst. When receiving a Superburst, the hub demodulator’s tolerance for frequency offset improves to approximately 7.5% of the symbol rate. A Superburst is also a much more robust waveform that is independent of the carrier MODCOD. These advantages allow the hub to detect a Superburst over a much wider frequency range and at a much lower C/N when compared to a traditional acquisition burst. Therefore, in most cases, a remote must only transmit a single Superburst to acquire the network. When using Superburst, frequency sweeping is typically not required since the sweep step size is generally larger than the instability of the hub downconverter. In the rare cases when sweeping is required, the remote sweeps the frequency range using the same “fast acquisition” method as a remote acquiring with traditional acquisition bursts.
The frequency sweeping algorithm is described in the next section. For more on Superburst, see Superburst Acquisition on page 130.
In iDX Release 3.3, Traditional Acquisition is still required in the following cases: • Acquisition over Spread Spectrum upstream carriers
• Acquisition in TRANSEC networks
• Acquisition on upstream carriers received by single channel line cards except receive-only eM1D1 line cards configured for Single Channel TDMA (Adaptive) Receive Mode.