Access Schemes

TDMA

 TDMA means allocating the overall bandwidth of the transponder to every carrier in sequence for a limited amount of time, called a time slot. The sequence may be random, every station transmitting a data packet on a carrier burst with duration equal to a time slot whenever it has data to transmit, without being coordinated with respect to other stations. This is named ‘random TDMA’ and is best represented by the so-called ALOHA type protocols. As a result of the random nature of transmissions, such multiple access schemes do not protect two or more carrier bursts transmitted by separate stations from possibly colliding within the transponder (that is overlapping in time). The interference which results then prevents the receiving stations from retrieving the data packets from the corrupted bursts.

With TDMA, carriers are transmitted in bursts and received in bursts. Every burst consists of a header made of two sequences of bits: one for carrier and bit timing acquisition by the receiving VSAT demodulator, another named ‘unique word’ indicating to the receiver the start of the data field. The header is followed by a data field containing the traffic associated with either one or several one-way connections. If only one, the burst is a Single Channel Per Carrier (SCPC) burst, if several, the burst is a Multiple Channel Per Carrier (MCPC) burst and is divided into sub bursts, each sunbursts corresponding to one one-way connection. Synchronization is necessary between earth stations, and the earth station must be equipped with rapid acquisition demodulators in order to limit burst preambles to a minimum.

FDMA

FDMA means allocating a given sub band of overall transponder bandwidth, to every carrier. The allocated sub band, for a specific carrier must be compatible with the carrier bandwidth which depends on the bit rate it conveys and the type of modulation and. The bit rate on the carriers may correspond to the traffic of one one-way connection: this is a Single Channel Per Carrier (SCPC) mode, or to several one-way connections which are time division multiplexed (TDM), and then this is a Multiple Channels Per Carrier (MCPC) mode.

CDMA

CDMA is a multiple access technique which does not consider any frequency–time partition: carriers are allowed to be transmitted continuously while occupying the full transponder bandwidth, B. Therefore interference is inevitable, but is resolved by using spread spectrum transmission techniques based on the generation of high-rate chip sequences (or ‘code’), one for every transmitted carrier. These sequences should be orthogonal so as to limit interference. Such techniques allow the receiver to reject the received interference and retrieve the wanted message.

MF-TDMA allows a group of terminals to communicate with the HUB using a set of carrier frequencies each of which is divided into time slots. Thus a terminal has access to all available carriers. Using MF-TDMA capacity is fully utilized, avoiding fragmentation and unused bandwidth.

D‑TDMA (Deterministic Time Division Multiple Access ) is a technology that facilitates capacity sharing by dividing it into fixed time slices, allows a single allotment of bandwidth to be efficiently and effectively shared among multiple remotes.  D‑TDMA has been proven to be the most efficient in terms of throughput and satellite resource requirements.   Bandwidth is available when required; organizations do not have to pay for a channel that they are not using.  This type of bandwidth sharing maximizes the use of available capacity.