General aspects of mobile satellite systems

General aspects of mobile satellite systems

Differences between satellite and terrestrial systems exist in spite of common objectives for high quality services and excellent spectrum efficiency. Some differences arise because:- user costs are closely related to satellite transmit power the satellite propagation channel is highly predictable satellite paths introduce significant propagation delays and Doppler shifts frequency co-ordination has to be on a global basis frequency re-use options are more limited, hence bandwidth is a tight constraint satellite beam shaping and sizing opportunities are limited.The first two points lead naturally to the emphasis placed on the line-of-sight satellite link budget when establishing the system design. The base link budget is derived from theoretical path losses to which link margins are added to
compensate for inevitable impairments in equipment and propagation characteristics. All impairments, even if not directly calculable in terms of signal loss (e.g. group delay and rate of change of Doppler shift), are converted accurately to dB so that the compensating increase in transmit power can be established. The total margin over the theoretical ideal
path is only a few dB and precision in calculating the contributory impairments is essential. The resulting link budget then allows the availability and quality of service to be estimated over the coverage area.Large link margins have a major impact on system build cost and operating tariffs simply because of the impact of additional power requirements on spacecraft size — a 3dB excess margin would almost double user charges. For this reason, mobile satellite communication systems have lead the way in very power-efficient modulation formats and low bit rate voice codecs (2,4 kbit/s and 4,8 kbit/s) as well as adaptive power control. The drive for efficient use of satellite power is noticeably reflected in terminal equipment design with:

- very low loss antennas coupled with very low loss receive filters;
- very tight transmit/receive filter specifications;
- very low noise amplifiers;
- excellent carrier/signal acquisition in presence of Doppler, noise and interference;
- power-saving and spectrum-efficient forward error correction;
- multi-path discrimination techniques might facilitate low signal-to-noise demodulator operation

The satellite-mobile uplink and downlink are inevitably more fragile than the corresponding feeder links (land earth station-satellite). However the feeder link itself needs a very substantial link margin in order that the aggregate up/down performance may be largely determined by the mobile link. These feeder links operate in higher frequency bands where Doppler and atmospheric/meteorological disturbances can become even more significant. The following clauses of this TR focus on particular characteristics, capabilities and limitations of mobile satellite systems together with typical values for key parameters where possible. However it must be recognised that most parameters are inter-dependent and will also vary with architecture of the ground infrastructure, the satellite orbital arrangement, and the user terminal configuration.