Radio Link Design

5

Radio Link Design

5.1 Introduction

Unlike terrestrial cellular networks, in a mobile-satellite network, transmissions are

constrained by available power. As illustrated in the previous chapter, the mobile-satellite

channel provides a challenging environment in which to operate. Consequently, efficient

coding and modulation techniques need to be employed in order to achieve a system margin

above the minimum needed to guarantee a particular Quality of Service (QoS).

The transmission chain for a satellite communication system is shown in Figure 5.1.

In Figure 5.1, the transmit (Tx)/receiver (Rx) hardware includes the application of the

multiple access scheme. Of course, not all of the above need be applied to a particular system,

although there is an obvious need for certain components, such as the modulator/demodu￾lator, for example. The selection of particular elements of the chain is driven by the needs of

the system design. This chapter initially considers the approach to developing a link budget

analysis. Here, the influence of the satellite payload characteristics, as well as other opera￾tional characteristics such as frequency, transmit power, and so on, on the overall link design

Figure 5.1 Simplified transmission chain.

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)

are considered. This is followed by a description of the modulation schemes and coding

techniques that are employed on the link. This chapter concludes with a presentation on the

multiple access schemes that are applicable to a mobile-satellite system, followed by an

assessment of the current status of the standardisation of the multiple access scheme for S￾UMTS/IMT-2000.

5.2 Link Budget Analysis

5.2.1 Purpose

A link budget analysis forms the cornerstone of the system design. Link budgets are

performed in order to analyse the critical factors in the transmission chain and to optimise

the performance characteristics, such as transmission power, bit rate and so on, in order to

ensure that a given target quality of service can be achieved.

5.2.2 Transmission and Reception

The strength of the received signal power is a function of the transmitted power, the distance

between transmitter and receiver, the transmission frequency, and the gain characteristics of

the transmitter and receiver antennas.

An ideal isotropic antenna radiates power of uniform strength in all directions from a point

source. The power flux density (PFD) on the surface of a sphere of radius R, which has at its

centre an isotropic antenna radiating in free space a power Pt (Watts), is given by:

PFD ¼ Pt

4pR2 Wm22 ð5:1Þ

In practice, antennas with directional gain are used to focus the transmitted power towards

a particular, wanted direction. Here, an antenna’s gain in direction (u, f), that is G(u, f), is

defined as the ratio of the power radiated per unit solid angle in the direction (u, f) to the

same total power, PT, radiated per unit solid angle from an isotropic source:

Gðu;fÞ ¼

P u;f

PT

4p

ð5:2Þ

Antenna radiation patterns are three-dimensional in nature, however, it is usual to represent

an antenna radiation pattern from the point of view of a single-axis plot. Such a plot is shown

in Figure 5.2.

An antenna’s gain is normally calculated with reference to the boresight, the direction at

which the maximum antenna gain occurs. In this case u, f ¼ 08. Gain is usually expressed in

dBi, where i refers to the fact that gain is relative to the isotropic gain. An important para￾meter that is used in an antenna’s specification is the 3-dB beamwidth, which represents the

angular separation at which the power reduces to 3-dB, or half-power, below that of bore￾sight. For a parabolic antenna, the simplified relationship between the antenna diameter and

3-dB beamwidth, u 3db, as shown in Figure 5.2, is given by:

148 Mobile Satellite Communication Networks

u3dB < 65l

D degrees ð5:3Þ

where l is the transmission wavelength (m); D is the antenna diameter (m).

Here, it can be seen that the half-power beamwidth is inversely proportional to the operat￾ing frequency and the diameter of the antenna. For example, a 1 m receiver antenna operating

in the C-band (4 GHz) has a 3-dB beamwidth of roughly 4.98. The same antenna operating in

the Ku-band (11 GHz) has a 3-dB beamwidth of approximately 1.88.

The level of the antenna pattern’s sidelobes is also important, as this tends to represent gain

in unwanted directions. For a transmitting gain this leads to the transmission of unwanted

power, resulting in interference to other systems, or in the case of a receiving antenna, the

reception of unwanted signals or noise. The ITU-R recommend several reference radiation

patterns, with respect to the antenna’s sidelobe characteristics [ITU-93, ITU-94a], depending

on the application and the antenna characteristics. For example, for a reference earth station:

G ¼ 32 2 25logf dBi; for wmin # w # 488

¼ 210 dBi for 488 # w # 1808

where w min is the greater of 18 or 100l/D.

Figure 5.3 is the recommended radiation pattern for a vehicular-mounted near-omni-direc￾tional antenna operating within the 1–3 GHz band. Here, the gain of the antenna is restricted

to less than or equal to 5 dBi for elevation angles in the range 220 to 908.

Radio Link Design 149

Figure 5.2 Antenna gain characteristics.

As was discussed in Chapter 4, antennas have co- and cross-polar gains, where the recep￾tion of unwanted, orthogonally polarised cross-polar signals will add as interference to the co￾polar signal. As was noted in Chapter 4, the ability of an antenna to discriminate between a

wanted polarised waveform and its unwanted orthogonal component is termed its cross-polar

discrimination (XPD). When dual polarisation is employed, an antenna’s ability to differ￾entiate between the wanted polarised waveform and the unwanted signal of the same polar￾isation, introduced by the orthogonally polarised wave, is termed the cross-polar isolation

(XPI). Typically, an antenna would have an XPI . 30 dB.

If an antenna of gain Gt is transmitting power in the direction of a receiver located on the

boresight of the antenna, then the power flux density at the receiver at a distance R from the

receiver, is given by:

PFD ¼ PtGt

4pR2 Wm22 ð5:4Þ

The product PtGt is termed the effective isotropic radiated power (EIRP).

For an ideal receiver antenna of aperture area A, the total received power at the receiver is

given by:

Pr ¼ PtGtA

4pR2 W ð5:5Þ

In reality, not all of the transmitted power will be delivered, due to antenna reflections,

shadowing due to the feed, manufacturer imperfections, etc. Antenna efficiency is taken into

account by the term effective collecting area, Ae, which is given by:

Ae ¼ hA ð5:6Þ

where h, the antenna efficiency factor, is generally assumed to be in the region of 50–70%.

Therefore, the actual received power is given by:

Pr ¼ PtGtAe

4pR2 W ð5:7Þ

An antenna of maximum gain Gr is related to its effective area by the following equation:

150 Mobile Satellite Communication Networks

Figure 5.3 Reference radiation pattern for vehicle mounted antennas operating in the 1–3 GHz band.

Gr ¼ h

4pA

l2 ð5:8Þ

where l is the wavelength of the received signal.

For a parabolic antenna of diameter D, this equation can be re-written as:

Gr ¼ hp2

D2

l2 ð5:9Þ

Using equation (5.9), the variation in antenna gain for a range of transmission frequencies

that are employed in satellite communications is shown in Figure 5.4, assuming an efficiency

of 60%.

Rearranging equation (5.8) and substituting in (5.7) gives:

Pr ¼ PtGtGrl2

ð Þ 4pR 2 W ð5:10Þ

The term (l/4pR)

2 is known as the free space loss (FSL). The variation in free space loss

against frequency for LEO, MEO and GEO satellites is illustrated in Figure 5.5.

Usually, it is more convenient to express the parameters of the link in terms of dB ratio. For

power ratios, parameters are expressed in terms of dBW or dBm. Here, the term dBW refers

to the ratio, expressed in dB, of the parameter power to 1 W. Similarly, dBm refers to the ratio

of parameter power to 1 mW. So, for example, 20 W is equal to 13 dBW or 43 dBm.

Expressing the above equation in terms of dB results in:

Pr ¼ EIRP 1 FSL 1 Gr 1 Ap dBW ð5:11Þ

In the above expression, an additional parameter, Ap, has been added to the equation to take

Radio Link Design 151

Figure 5.4 Variation in antenna gain with frequency.