MODCODModulation and Coding
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The ACM scheme in DVB-S2 is used for enhancing the connectivity maximization to use variable information bits of payload according to a certain MODCOD. Fig.
If the wireless link is in good condition with clear sky, a modem selects a high MODCOD then TCP's sending rate is increased with a suitable Congestion window (cwnd).
Buffer overflow can be experienced when bandwidth is changed from a high bandwidth with a high MODCOD to a low bandwidth with a low MODCOD.
The data are then classified according to the MODCOD level, and converted into traffic data formats for ACM transmission.
Classified packets are processed by the IP layer QoS scheduler, and transferred to the MODCOD queues.
In this paper, we consider the maximum capacity of the DVB-S2 forward link and the available capacity of the current MODCODs. The packets processed by the second step scheduler are classified by each MODCOD.
where [c.sub.i] is the transmission capability of the modem according to a certain MODCOD.
To provide fairness to the RCSTs and use the ACM scheme, we share the timeslots with a region with poor weather conditions having low MODCOD.
Because the weight value in each MODCOD is perfectly equal, the round-robin scheduling does not offer a guarantee of QoS, as shown in Fig.
More timeslots are allocated in regions with poor conditions and low MODCOD than in regions with clear skies and high MODCOD, as shown in Fig.
Absolutely, satellite communication system with ACM scheme has higher transmission efficiency, but it does not offer earth station fairness because the data are transmitted with high MODCOD in regions with clear skies, and are transmitted with low MODCOD in regions experiencing rain event.
To achieve our goals which throughput fairness and Qos support, we propose fairness improvement-weighted round robin (FI-WRR) scheduler which the allocation timeslot is calculated by the transmission rate of each MODCOD queue.