JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Satellite Systems, VSAT


Spread Spectrum VSAT Systems

VSAT networks consist of a number of terminals and a satellite through which the terminals communicate. Ultra-small fully portable satellite groundstations (picoterminals) with an antenna diameter of approximately 10-20 cm are possible in principle using 20 / 30 GHz bands. Direct sequence Code Division Multiplexing/Multiple Access CDMA appears the most advantageous transmission scheme.

CDMA Multiple Access

Terrestrial communication systems with many transmitters normally use the frequency-hopping technique. This is due to the so-called "near-far" problem. The "near-far" problem arises normally if the signal of a distant transmitter is much weaker than that of a nearby transmitter. In a direct-sequence system this gives rise to an enormous interference of the nearby transmitter when receiving the distant transmitter. Communication systems employing the frequency-hopping technique do not suffer from this problem, because a distant and a nearby transmitter transmit on a different frequency most of the time. This is in contrast to a direct-sequence system.

Satellite communication systems normally use the direct-sequence technique, because a DS system has the best noise and anti-jam performance and does not need expensive frequency synthesizers as a frequency hopping system does. Because the "near-far" problem is not encountered here the frequency-hopping technique is not necessary.

The CDMA scheme can be synchronous or asynchronous. Because synchronous CDMA needs some form of network control to keep the spreading codes aligned at the satellite, in this case asynchronous CDMA is chosen. Moreover, this means that the terminals will be simpler and hence cheaper for asynchronous CDMA compared to synchronous CDMA. As modulation form binary phase shift keying (BPSK) is chosen. This is together with quaternary phase shift keying (QPSK) the most common form in digital satellite communications systems.

(Co-operative Olympus Data Experiment) CODE system

The proposed CODE picoterminal network consists of a hub station and several picoterminals. The hub station uses one 64 kbit/s channel to transmit data towards the picoterminals. The picoterminals use the other 64 kbit/s channel to transmit their messages to the hub station. For this code division multiple access (CDMA) is used.

A picoterminal transmits a spread-spectrum signal with a chip rate of 64 kchip/s. These spread-spectrum signals occupy the same bandwidth as the original microterminal signal as the chip rate is equal to the bit rate in the original CODE channel. The spreading code length has been chosen equal to 511. This means that the bit rate of a picoterminal is 64000/511=125.2 bit/s. Further, in this network, the bit error probability may not exceed 10^-5 for 99% of the time. This requires a signal-to-noise ratio of 9.6 dB.



JPL's Wireless Communication Reference Website © Jean-Paul M.G. Linnartz and Frank Kamperman, 1993, 1995.