VSAT Basics

A VSAT terminal can receive and transmit data, voice and video information and is typically connected to an end user’s equipment via LAN port of the satellite router. The router processes an incoming traffic, packetize it and convert into continuous SCPC (Single Channel Per Carrier) or burstable TDMA (Time Division Multiple Access) modulated carrier. The router transmits this modulated carrier to the transceiver (or BUC) via coaxial cable and with the use of Intermediate Frequency (IF) band (typically, L-band). The transceiver converts a band of frequencies from an IF frequency to a higher frequency (C-, X-, Ku- or Ka-band) and transmits the converted and amplified signal to the satellite. A satellite dish antenna performs an additional amplification of the signal and forms the focused beam directed to the satellite.

VSAT networks are typically based on geostationary satellites. A geostationary satellite is a satellite located on the orbit 35’786 km above the Earth’s equator with the same orbital period as the Earth’s rotation period. Therefore, the satellite antennas which communicate with them do not have to rotate to track these satellites. Communication satellites retransmit signals received from the satellite dishes back to the Earth. The satellite amplifies such signals and transmits it using different frequencies. Satellites may support different frequency bands and have several beams with various shapes (coverage zones).

The signals retransmitted by the satellite can be received by unlimited number of satellite antennas whining a coverage zone of the satellite. A satellite dish and an LNB amplify the received frequency band, convert it to the IF band and deliver this signal via the coaxial cable to a satellite router’s demodulator.

The modulator on a transmit side and the demodulator on a receive side use similar modulation and coding (MODCOD) schemes and a symbol rate. Satellite routers apply error correcting code used to control errors in data transmission that happen in microwave communication channels. The demodulated and decoded data will be de-packetized to its original form and dispatched to the destination via the LAN port of satellite router on a receive side. Two-way VSAT terminals simultaneously receive and transmit signals that way.

SCPC (Single Channel Per Carrier) is purposed for point-to-point links and using a single dedicated signal at a given frequency (carrier) which is available only for a single transmitting and single receiving VSAT. Where multiple access is concerned, SCPC is used in DAMA (Dynamically Assigned Multiple Access) mode, where SCPC carriers established on demand using common frequency bandwidth.

TDM (Time Division Multiplexing) is a multiplexing technique where multiple bit streams are transmitted simultaneously as sub-channels within a single carrier. Data transmission in such a channel is separated into fixed time intervals (timeslots) for each sub-channel.

TDMA (Time Division Multiple Access) technology allows several VSAT stations to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one by one, each of them uses its own time slot within the constant and synchronized TDMA-frame. TDMA-frame is a sequence of TDMA-slots of a certain length, assigned by Hub or Master station. The bandwidth is allocated in accordance with current traffic load and defined priorities/QoS.