La creación de nuevas excepciones por el Juez

Constellation types

Although, the ESA proposals are independent of the constellation type, there are many param­

eters (e.g.diversity, HO) within these two proposals that depend on the type of constellation.

The INX system (by Iridium LLC), proposes either LEO or MEO constellations whilst the

re-maining systems have designed and optimised their air interfaces to a specific constellation (i.e ICO by ICO global communications-MEO, SAT-CDMA by TTA-LEO, Horizons by Inmarsat- GEO)

Elevation angle distribution

Elevation angle distribution has a great impact on availability and blockage in a satellite system.

The elevation angle also influences the link budget in terms of path losses and link margin dimensioning. Blockage probability of the satellite decreases with increasing elevation angle.

Generally LEO systems have low mean elevation angle and must rely on satellite diversity.

SAT-CDMA system (LEO) has a minimum elevation angle of 17.5° (for service link) and does not depend heavily on satellite diversity. Simulation results [29] concluded that in a dual satellite diversity environment, maximum ratio combining becomes more effective than equal gain combining , as the difference in elevation angles of the two satellites increase. Although GEO systems provide consistent availability at lower latitudes, they are not good for higher latitudes nor for providing satellite diversity. ICO (MEO) provides minimum and average elevation angle 25° and 50° respectively for regions between 20° to 50° of latitude. It also has two or more satellites visible for at least 90% of the time. Iridium has a minimum user elevation angle of 15°.

Diversity

To make use of the Rake receiver architecture in the satellite environment, path delays are deliberately introduced by the BS such that when the signal arrives via two visible satellites (in dual satellite diversity scenario) the Rake receiver at the receiving station can coherently combine the signals. Satellite diversity is an essential part of S-UMTS in order to implement the soft HO (when the signal quality falls below a threshold, connection to another satellite is made before terminating the connection with the current service satellite) and to improve the quality and availability of the system particularly under shadowed conditions.

Satellite diversity is provided by all the proposals (for LEO constellations in ESA and Iridium proposals) except the Horizons system which is a GEO system.

2.5. Brief comparison of IMT 2000 satellite systems proposals 39

Frequency diversity techniques have not been used by any of the proposals to compensate medium impairments. This is due to the short delay spreads associated with the satellite channel not providing much improvement.

Adoption of satellite diversity is very difficult in a TDMA system such as ICO as it needs very precise timing advances. A large amount of signalling would be required if satellite switching is to be used in order to identify how to swap from one satellite to another. Moreover, termi­

nal complexity will also increase unlike the CDMA based mobile terminal (MT) where Rake architecture can be implemented to exploit diversity.

Multiple access technique

The W-C/TDMA proposal is more suitable to the employment of multi user detection to in­

crease the capacity (compared to W/CDMA proposal) since the number of codes to be pro­

cessed simultaneously is less.

Full frequency reuse is allowed in both the ESA proposals simplifying the frequency planning compared to TDMA and FDMA proposals such as ICO (FDMA & TDMA) and Horizons (F/TDMA). In the ICO system, a 4-cell frequency re-use pattern is implemented such that the given frequency is never available simultaneously to two beams with insufficient isolation. The frequency plan is also adaptive to the traffic variation and the evolution of the constellation.

Both ESA proposals use randomisation codes (optional for SW-CDMA) to make adjacent beam and inter-satellite interference appear noise like. Since a common code is used in the synchro­

nisation burst, the DL acquisition becomes simpler. If a minimum mean square error (MMSE) interference mitigation technique is not used, very large m-sequences are used as randomisation codes. Short codes can be used in the presence of MMSE detection.

The Iridium system intends to use both TDMA and CDMA multiple access techniques and both FDD and TDD modes in order to increase the efficiency of spectrum utilisation. The performance of a CDMA system depends heavily on the effectiveness of the power control loop. Long round-trip delay associated with satellite systems introduces limitations to accurate power control. The CDMA air interface is used for applications such as data services in which the environment is more stable and where the power control can be effective. The TDMA

interface is used for mobile voice types of service in which the environment and hence the signal level changes rapidly. However, adoption of two different air interfaces increases the complexity in the MT as well as the payload. The air interface is able to support the required bandwidth for users by aggregation of multi-time slot, multi-codes and/or multi-carrier due to the employment of two different air interfaces.

Duplex method

Among all the proposals, only the INX (Iridium LLC) system considers pure TDD mode of operation. The W-C/TDMA (ESA) proposal considers F/TDD operation which reduces the terminal complexity as it requires less demanding antenna duplexers compared with FDD (but will need complex filters). The rest of the proposals consider only the FDD mode of operation.

This is mainly due to the symmetric frequency allocation for S-UMTS.

2.5.2 Physical layer issues