Channel Characteristics

4

Channel Characteristics

4.1 Introduction

This chapter considers the propagation environment in which a mobile-satellite system oper￾ates. The space between the transmitter and receiver is termed the channel. In a mobile￾satellite network, there are two types of channel to be considered: the mobile channel,

between the mobile terminal and the satellite; and the fixed channel, between the fixed

Earth station or gateway and the satellite. These two channels have very different character￾istics, which need to be taken into account during the system design phase. The more critical

of the two links is the mobile channel, since transmitter power, receiver gain and satellite

visibility are restricted in comparison to the fixed-link. The basic transmission chain is shown

in Figure 4.1.

By definition, the mobile terminal operates in a dynamic, often hostile environment in

which propagation conditions are constantly changing. In a mobile’s case, the local opera￾tional environment has a significant impact on the achievable quality of service (QoS). The

different categories of mobile terminal, be it land, aeronautical or maritime, also each have

their own distinctive channel characteristics that need to be considered. On the contrary, the

fixed Earth station or gateway can be optimally located to guarantee visibility to the satellite

at all times, reducing the effect of the local environment to a minimum. In this case, for

frequencies above 10 GHz, natural phenomena, in particular rain, govern propagation impair￾ments. Here, it is the local climatic variations that need to be taken into account. These very

different environments translate into how the respective target link availabilities are specified

for each channel. In the mobile-link, a service availability of 80–99% is usually targeted,

whereas for the fixed-link, availabilities of 99.9–99.99% for the worst-month case can be

specified.

The following reviews the current status of channel modelling from a mobile and a fixed

perspective.

4.2 Land Mobile Channel Characteristics

4.2.1 Local Environment

Spurred on by the needs of the mobile-satellite industry, the 10 years spanning the mid-1980s

Mobile Satellite Communication Networks. Ray E. Sheriff and Y. Fun Hu

Copyright q 2001 John Wiley & Sons Ltd

ISBNs: 0-471-72047-X (Hardback); 0-470-845562 (Electronic)

to the mid-1990s witnessed significant effort around the world in characterising the land

mobile-satellite channel. The vast majority of these measurement campaigns were focused

on the UHF and L-/S-bands, however, by the mid-1990s, with a number of mobile-satellite

systems in operation, focus had switched to characterising the next phase in mobile-satellite

development, that of broadband technology at the Ka-band and above.

The received land mobile-satellite signal consists of the combination of three components:

the direct line-of-sight (LOS) wave, the diffuse wave and the specular ground reflection. The

direct LOS wave arrives at the receiver without reflection from the surrounding environment.

The only L-/S-band propagation impairments that significantly affect the direct component

are free space loss (FSL) and shadowing. FSL is related to operating frequency and transmis￾sion distance. This will be discussed further in the following chapter. Tropospheric effects

can be considered negligible at frequencies below 10 GHz, while impairments introduced by

the ionosphere, in particular, Faraday rotation can be effectively counteracted by the selec￾tive use of transmission polarisation. Systems operating at above 10 GHz need to take into

account tropospheric impairments and these will be considered further when discussing the

fixed-link channel characteristics.

Shadowing occurs when an obstacle, such as a tree or a building, impedes visibility to the

satellite. This results in the attenuation of the received signal to such an extent that transmis￾sions meeting a certain QoS may not be possible.

The diffuse component comprises multipath reflected signals from the surrounding envir￾onment, such as buildings, trees and telegraph poles. Unlike terrestrial mobile networks,

116 Mobile Satellite Communication Networks

Figure 4.1 Mobile network propagation environment.

which rely on multipath propagation, multipath has only a minor effect on mobile-satellite

links in most practical operating environments [VUC-92].

The specular ground component is a result of the reception of the reflected signal from the

ground near to the mobile. Antennas of low gain, wide beamwidth operating via satellites

with low elevation angle are particularly susceptible to this form of impairment. Such a

scenario could include hand-held cellular like terminals operating via a non-geostationary

satellite, for example.

The first step towards modelling the mobile-satellite channel is to identify and categorise

typical transmission environments [VUC-92]. This is usually achieved by dividing the envir￾onment into three broad categories:

† Urban areas, characterised by almost complete obstruction of the direct wave.

† Open and rural areas, with no obstruction of the direct wave.

† Suburban and tree shadowed environments, where intermittent partial obstruction of the

direct wave occurs.

As far as land mobile-satellite systems are concerned, it is the last two of the above

environments that are of particular interest. In urban areas, visibility to the satellite is difficult

to guarantee, resulting in the multipath component dominating reception. Thus, at the mobile,

a signal of random amplitude and phase is received. This would be the case unless multi￾satellite constellations are used with a high guaranteed minimum elevation angle. Here,

satellite diversity techniques allowing optimum reception of one or more satellite signals

could be used to counteract the effect of shadowing.

The fade margin specifies the additional transmit power that is needed in order to compen￾sate for the effects of fading, such that the receiver is able to operate above the threshold or the

minimum signal level that is required to satisfy the performance criteria of the link. The

threshold value is determined from the link budget, which is discussed in the following

chapter. The urban propagation environment places severe constraints on the mobile-satellite

network. For example, in order to achieve a fade margin in the region of 6–10 dB in urban and

rural environments, a continuous guaranteed minimum user-to-satellite elevation angle of at

least 508 is required [JAH-00]. The compensation for such a fade margin should not be

beyond the technical capabilities of a system and could be incorporated into the link design.

However, to achieve such a high minimum elevation angle using a low Earth orbit constella￾tion would require a constellation of upwards of 100 satellites. On the other hand, for a

guaranteed minimum elevation angle of 208, a fade margin in the region of 25–35 dB,

would be required for the same grade of service, which is clearly unpractical. While these

figures demonstrate the impracticalities of providing coverage in urban areas, in reality, for an

integrated space/terrestrial environment, in an urban environment, terrestrial cellular cover￾age would take priority and this is indeed how systems like GLOBALSTAR operate.

In open and rural areas, where direct LOS to the satellite can be achieved with a fairly high

degree of certainty, the multipath phenomenon is the most dominant link impairment. The

multipath component can either add constructively (resulting in signal enhancement) or

destructively (causing a fade) to the direct wave component. This results in the received

mobile-satellite transmissions being subject to significant fluctuations in signal power.

In tree shadowed environments, in addition to the multipath effect, the presence of trees

will result in the random attenuation of the strength of the direct path signal. The depth of the

fade is dependent on a number of parameters including tree type, height, as well as season due

Channel Characteristics 117