GAUSS: Galileo and UMTS synergetic system

• Forschungsstelle: EU FRP
• Projektnummer: 00.0015
• Projekttitel Englisch: GAUSS: Galileo and UMTS synergetic system

Schlüsselwörter (Englisch)

CDMA; S-UMTS; satellite return link; design; implementation; trials

Alternative Projektnummern (Englisch)

EU project number: IST-1999-20532

Forschungsprogramme (Englisch)

EU-programme: 5. Frame Research Programme - 1.2.1 Systems and services for the citizen

Kurzbeschreibung (Englisch)

See abstract

Partner und Internationale Organisationen (Englisch)

Coordinator: Telespazio Spa, Roma (I)

Abstract (Englisch)

This abstract gives an overview of the design and implementation of the GAUSS demonstrator return link and trial phase during the second project year. The design of the return link was specifically tailored to the needs of the GAUSS location based services, i.e. the return link had to support highly bursty, low rate data services. Therefore delay was a minor issue in the return link. Nevertheless all components in the link were designed to be close to real-time operation. Since for simplicity reasons the components from different partners of the project were connected with serial interfaces and non-real-time operating systems were used, a careful design of the communication via these serial interfaces and of the host software on the physical layer hardware was mandatory.
In the GAUSS demonstrator two transport channels were provided: a random access channel (RACH), which is specified in the S-UMTS standardisation documents and an additional GAUSS packet channel (GPCH) providing a non-continuous packet transmission scheme. The time scale in the return link is divided in consecutive super frames each consisting of one RACH burst followed by one GPCH burst. The total burst length is 4 and 18 frames for the RACH and the GPCH, respectively. The frame duration for the GAUSS demonstrator was chosen to be 80ms. During RACH periods the user terminals are allowed to issue requests for transmission capacity based on a slotted ALOHA scheme. The scheduler in the gateway station then allocates GPCH slots according to the request and transmits this information via the forward link to the user terminal. Now the user terminal can transmit its data on a contention-free basis. Since all mobile terminals have timing and positioning information from the GAUSS navigation subsystem, it is possible to synchronise the return link signals in such a way that they are (quasi) synchronous at the satellite receiver. Since the burst duration in comparison with the round trip delay is very low, closed loop power control inherently is not meaningful for the GAUSS system.
A lot of effort was spent to design and implement the access scheme for the return link described above in order to provide a realistic real-time behaviour of the physical layer for the GAUSS services. Therefore maintaining an absolute time reference in the physical layer and the synchronisation of the MAC layer to the physical layer was mandatory. This was a quite difficult task regarding the serial interface between the physical and MAC layer and the non-real-time operating systems involved. The primitives for interface between the physical and MAC layer considered in the GAUSS demonstrator is compliant to the S/T-UMTS standard.
The mapping of transport blocks to radio frames and the air interface was designed to be compliant to the S/T-UMTS standards. An additional physical channel was introduced on which the gateway station receiver searches for transmitted signal in a wide frequency range. This synchronisation emulation channel (SECH) is invisible to the upper layers and emulates the tight coupling of the forward and return link in real systems where the user terminal is roughly synchronised to the gateway station.
The GAUSS demonstrator aimed at very low rate data services and operates on a chip clock, which was chosen to be eight times lower than in S/T-UMTS. Therefore the symbol duration in GAUSS is very long and frequency offsets had to be compensated very carefully. This required very accurate frequency estimation and tracking algorithms in the gateway station receiver. In order to emulate additional users in the return link, a traffic generator was included at the gateway station, which was synchronised to the access scheme in the return link. A comprehensive set of configuration and management commands are handled by the user terminal modulator and gateway station demodulator, which can be issued by the configuration and management subsystem and are sent via a dedicated serial interface from the configuration and management subsystem to the modems.
Shortly before the start of the trials, ITALSAT F2 was withdrawn from the service because of a failure and INMARSAT 3F5 capacity was allocated. Due to the careful design of the return link no modification were necessary despite a modified frequency plan and the lack of frequency stabilisation in the new satellite set-up. GAUSS was successfully closed in budget and on time.

Datenbankreferenzen (Englisch)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 00.0015