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In satellite communications, CDMA is being exploited for low rate, medium rate as well as broadband type of communications. Low rate applications include a.o. SMS (Short Messaging Services), E-mail over satellite, remote meter reading, voice and data services (fax), and positioning and geolocation applications.

When used in the VHF and UHF bands, we talk about the so-called ‘Little LEO’ (Low Earth Orbit) applications. CDMA is used for its capability of coping with high interference levels in these bands, and because it allows to multiplex a high amount of users with limited protocol overhead. In order to provide global coverage, they are usually store-and-forward satellite systems with sophisticated OBP (On-Board Processing) capabilities.

Data and fax services using CDMA are found in the L and S bands (in the case of the so-called ‘big LEO’ constellations) and Ku-bands (when using transponders of GEO satellites). Different systems can share the same part of the spectrum. Cost-effective terminals are possible by exploiting a high degree of on-chip integration. Field trials and proof-of-concept demonstrations are realized with the SC2002 ASTRA Development Board.

The Ku-band frequencies are mostly used by geostationary satellites, for applications such as DBS (Direct Broadcasting by Satellite). With the enormous growth of the demand for medium rate data services (mostly for the transport of Internet data), transponders in Ku-band are increasingly used also for implementing these data services (multiples of 64 kbps net user data rate).

ADVANTAGES

CDMA-based solutions have an important cost advantage over the traditional PSK-based VSAT solutions. Ground station development is facilitated using the CDMA DataSat Development System. Other application examples in the Ku-band include the combination of terrestrial low cost networks (based on DECT) with a S-CDMA (Synchronous CDMA) satellite for multiplexing the different telephone channels. In more and more cases, CDMA is being used as an overlay to existing satellite services. This is a very efficient use of spread spectrum in view of the limited spectrum being available. One such example is D-SNG (Digital satellite News Gathering), where the CDMA-based coordination channels are put on top of the transponder QPSK DVB (Digital Video Broadcasting) signals.

Besides the ongoing developments using existing GEO capacity, LEO-based satellite systems are under development to provide true broadband access to individual users, using CDMA technology. Several Mbit/s can be offered to the individual user. In some cases, these spread spectrum high-rate applications share the spectrum with other, non-CDMA services, by realizing links with very low power spectral density.

Probably the best known satellite application using CDMA technology is navigation. Both GPS (Global Positioning System) and GLONASS (GLObal Navigation Satellite System, the Russian counterpart) use Direct Sequence Spread Spectrum waveforms for obtaining accurate pseudo-range measurements, which are the raw data to calculate a position fix. GPS has been complemented with the EGNOS (European Geostationary Navigation Overlay System) and the American WAAS (Wide Area Augmentation System), to increase the performance.

Sirius Communications is working towards the new European proposed GNSS-2 (Global Navigation Satellite System), which will provide higher accuracy and increased data rate, using more sophisticated waveforms. Besides the vast consumer market for standard receivers, there’s also a professional market for RTK (Real-Time Kinematic) receivers. These applications exploit the increased accuracy associated with tracking (or pseudo-tracking) of the P-Code (Precision Code) of GPS or GLONASS satellites. Last but not least, alternative navigation systems are being developed, based on combined pseudo-range and Doppler measurements. Read the section on Navigation Terminals to learn more about Sirius Communications products and activities in this challenging field. 

In wireless terrestrial communications, we can make a distinction between applications in licensed and in unlicensed bands.

UMTS APPLICATIONS

In licensed bands, the best known system under development is the UMTS (Universal Mobile Telecommunications System), based on W-CDMA (Wideband CDMA) technology. Worldwide, this development fits in the IMT-2000 (International Mobile Telephone System) initiative from the ITU, which aims at realizing a true worldwide applicable 3G standard. The benefits of CDMA exploited here are the ability to merge different types of services (voice, data, video) over the same band, using orthogonal PN sequences of different lengths, leading to the best capacity (bits/Hz). This so-called 3G (3^rd Generation) cellular networks will be commercially exploited from 2001 on, as an extension to the worldwide deployed GSM networks.

Sirius Communications is working on 3G solutions in cooperation with leading European telecom companies.

Furthermore, applications such as from CATV modems and powerline modems also benefit CDMA, as once again the unwanted unpredictable interference (e.g. ingress noisecancelled ) is efficiently through the processing gain of the spread spectrum modulation scheme. 

More about: The birth of Spread Spectrum