Sección 1ª: La reivindicación de un derecho “a la gratuidad”
Sección 2: El tratamiento de la gratuidad por el derecho de autor
A. Los intentos de consagrar sistemas de copia privada gratuita o cuasi- cuasi-gratuita
Table 3.2: UMTS OPERATIONAL ENVIRONMENTS Operational environment T-UMTS S-UMTS
Maritime X
V
Aeronautical X
V
Open-Highway
V V
Suburban
V V
Urban
V
XIndoor
V
Xomission of satellite transmitter in the MT.
3.1.5 Pricing
Pricing of the services will be a key factor in the growth of UMTS. Pricing of basic satellite UMTS services will be higher than that for the terrestrial UMTS services due to the inherent cost involved in the satellite service provision. However, the difference in price would be relatively proportional to that between current terrestrial mobile and fixed line communications [21].
3.2 Propagation environment differences
The terrestrial-mobile channel can be modelled by frequency-selective Rayleigh fading and log-normal short-term power fluctuations whereas the mobile-satellite channel is characterised by Ricean fading (when there is a LOS) and otherwise log-normal shadowing. Due to the power constraints and the large propagation loss associated with the satellite channel, operating under LOS avoids large multipath and shadowing margins.
The propagation environment can be categorised in seven main classes as shown in Table 3.2 [65]
3.2.1 Path loss
Unlike the terrestrial cellular system, the overall range of earth-satellite systems is very large with orbital heights varying from 500 km to 36000 km depending on the type of the orbit used.
This leads to a very large free-space loss component. Therefore, compared to the terrestrial system, a higher power is needed to be transmitted from the satellite, earth station and the MT to overcome this loss. Limitations stem from the MT (health reasons, battery size and lifetime) as well as from the satellite payload (amplifier power and antenna size). Link margins have a major impact on the system cost. A 3 dB excess margin would almost double the user charges [61]. Although the use of larger antennas with higher gain would reduce the transmitted power requirements, the maximum antenna size that a payload can carry is limited by technology of deployable carriers (around 2001).
3.2.2 Blocking and shadowing in the channel
The satellite channel consists of two ground based systems (FES and MT) connected through two radio links (UL and DL). Blocked and shadowing states occur due to the combination of the relative motion of the satellite in terms of elevation angle variation, the terminal speed and the type of environment. As the power margin is limited, the link quality can fluctuate rapidly from high quality to failure during a conversation. The propagation channel fluctuations have a large impact on the link quality and several algorithms must be implemented in the transceivers in order to prevent blocking and shadowing [2]. In mobile satellite systems, the elevation angle from the MT to the satellite is much larger than for terrestrial systems, with the minimum elevation angles in the range of 8° to 25°. Shadowing effects due to clutter, therefore, tend to result mainly from the clutter in the immediate vicinity of the mobile [66]. As the mobile moves along the street, multipath attenuation may change relatively rapidly since the buildings contributing to this process change rapidly. Another consequence of this effect is that there may be rapid and frequent transitions between LOS to non line of sight (NLOS) states in the satellite-mobile case. On the other hand, terrestrial systems involve elevation angles of order of 1° or less, therefore, a large number of buildings along the path are significant. The rate of change of shadowing is, therefore, smaller.
3.2. Propagation environment differences 59
3.2.3 Doppler shift
The Doppler shift and the rate of change of Doppler is significantly larger for satellite systems, due to the fast moving nature of the satellites in LEO/MEO/HEO constellations. The largest Doppler shifts can be expected in LEO systems (i.e.50 kHz at 2000 MHz)[60]. The Doppler shift complicates the signal acquisition and spectrum management procedures. For LEO and MEO orbits, the shift may need to be individually corrected for each mobile using frequency compensation procedures with the knowledge of the satellite’s motion and the MT’s position.
Although Doppler compensation procedures reduce the Doppler shift, the rate of change of Doppler remains unchanged and relatively fast. In IMT-2000, the large frequency separation between UL and DL (i.e. 190 MHz) induces different Doppler shifts in the UL and DL for a particular speed of a moving user.
3.2.4 Multipath delay
As far as multipath is concerned, low elevation angles of the urban environments have been found to be the most hostile environment, whereas, open environments with higher elevation angles have hardly any multipath. Fig 3.2 shows a power delay profile of one such urban envi
ronment at the elevation angle of 45°. It can be observed that even in the urban environment, all the resolvable echoes arrive very close to the LOS signal. As shown in Fig 3.2, there are only 2 major multipath components. Due to such a low delay spread, the propagation channel can be considered as a non-frequency selective channel for a signal bandwidth not exceeding 10 MHz [2] (see section 3.3.6). Therefore, in satellite systems, multipath delays are often short enough to be ignored as they are much smaller than the bit duration due to the comparatively high elevation angle of the radio path (except for aircraft and ships).
3.2.5 Propagation delay
!
One of the major problems with the satellite channel is the large propagation delay. Total i round-trip delay is about 250-280, 110-130 and 20-25 ms for GEO, MEO and LEO systems
I
| respectively. Furthermore, this propagation delay is not the same for all the spot beams due I to the large coverage area in a GEO system. Satellite beams are an order of magnitude larger0 0.2 0.4 0.6 0.8 1.0 1 2 1.4 1.6 1.8 2.0