Tdma

TDMA vs. SCPC
SatLink System Technical Notes
1. Background
TDM/TDMA and SCPC are the main alternative technologies for satellite networking in the world today. The modem and management technologies underlying both approaches have been advancing rapidly in recent years, causing some confusion as to which technology is better for a given set of networking requirements. This technical note will explain the important trends and trade-offs. These two alternative technologies are illustrated in Figure 1 and Figure 2 for a simple star topology network, highlighting their key elements and configuration differences.
(Time Division Multiplexing with Time Division Multiple Access)

Figure 1: TDM/TDMA Network

Figure 2: SCPC Network
(Single Channel Per Carrier)

1

TDMA vs. SCPC Technical Note
SCPC uses a separate dedicated carrier to each remote terminal ("VSAT") to receive information from the central site, and another dedicated carrier for each VSAT to transmit information back to the central site. Both carriers are modulated in "continuous mode". Usually some non-standard coding techniques are used since SCPC technology was never standardized. In contrast, TDM/TDMA technology uses a single high-speed TDM carrier transmitted from the central site or "Hub", from which many VSATs can receive information. For this TDM forward link, the DVB-S2 standard is most commonly used. It is also the most flexible for multiplexing many concurrent streams of traffic to different sites, and the most efficient with its support of Adaptive Coding and Modulation (ACM). ACM dynamically adjusts the modulation and coding on the "virtual link" to each VSAT individually, as local conditions (e.g., weather, interference) at the VSAT change. To transmit back to the central site efficiently, the VSATs in a TDM/TDMA network are synchronized, and they transmit information in "burst mode" within a series of short, scheduled timeslots. Timeslots may be assigned across multiple TDMA carriers