1. Satellite Broadcasting

1. Satellite Broadcasting

In this chapter the fundamentals of Satellite Communications are described, with particular attention to the structures of networks, hardware and all the parameters that permit a quality evaluation of a satellite link.

1.1 – Generalities

A satellite is the only device that can permit the communication between multiple users located in a very wide area, especially between two different continents; image

1.1 shows typical configuration for a system that uses a satellite.

(Image 1.1 – Common structure of a satellite network)

Chapter 1 Satellite Broadcasting

is possible to achieve a totally connected network even if there is a large number of users, provided that resources of bandwidth and energy are sufficient.

Most of satellites are repetitive that is they only receive a signal from an Earth

Station, they do some operations (described in the next paragraphs) and they

rebroadcast without demodulating. Regenerative satellites also do some operations of signal processing or switching. The majority of Eutelsat’s satellites belongs to the first type, so we’ll take into account only them.

Satellites work with microwaves: actually, Super High Frequency (SHF), included between 3 and 30 GHz, and Extremely High Frequency, included between 30 and 300 GHz are used. Moreover, the bandwidth shown is conventionally divided into more sub-bands, as Table 1.1 lists:

Referring to Image 1.1, signals coming from each Terrestrial Network that require to reach another one, are transmitted to the Earth Station, which modulates them with one or more RF carriers and transmits them to the satellite. It acts as a repeater, so it receives carriers, amplifies and retransmits them over other frequencies, in order to avoid interferences. Actually, the part of the transmission included between the transmitting Earth Station and the satellite is called Uplink while the one included between the satellite and the receiving Earth Station is called Downlink.

Because of economical reasons, it’s common using wide bandwidth between satellite communications; and so, lots of commercial devices treat band 20 to 500 MHz wide. Band C (6/4 GHz) is the most used, but nowadays it’s very crowded, so recent applications run on the Ku (14/12 GHz) and on the Ka (14/29 GHz) bands, but in the last ones, the rain attenuation is more significant, especially for mobile terminals.

1.2 – Common Satellite Architecture

The available band to the satellite is usually divided into smaller bands (typically 4 ÷ 24) managed by as many transponders, installed into the satellite itself. The Image 1.2 shows a portion of the disposable downlink band to Hot Bird 8 on the X polarization.

Chapter 1 Satellite Broadcasting

(Image 1.2 – Transponder configurations in schematic (a) and real (b) view)

As we can see, the total bandwidth is 400 MHz: there are 10 transponders, each of them occupies a band 33 MHz wide (be aware that in the same satellite there are other types of TXPs with other features), with a guard band of around 5 MHz between two adjacent ones.

Transponders (TXPs) can be used in different ways. For example, in a satellite used for linking Europe and United States together, we can find TXPs destined to the

connection of two Earth Stations located in the same continent (East-East or

West-West coverages) or in different ones (East-West-West or West-West-East coverages).

Image 1.3 shows a typical diagram of the Radio-Frequency path over a satellite. The

received signal is divided into more, in a way pertinent to the band assigned to the

TXPs. This operation is necessary in order to make the frequency translation possible

and to optimize all the devices afterwards used. For each sub-band we find a pre-amplification obtained by a Low Noise Amplifier (LNA), a fixed frequency-shift and a Power Amplification, which is the most delicate step for a Satellite, because of the limited resources aboard. A Travelling Wave Tube Amplifier (TWTA) is usually used: it is light, it needs a little energy to operate but it is a non-linear component, so, since it is used near the saturation point to assure a maximum efficiency, it can bring problems like spectral spreading, intermodulation and so on.

The output signal is then filtered, in order to reduce the out-of-band components and sent to a combiner, which passes all the signals coming from the TXPs to the transmitting antenna. Clearly, all these operations are made by specially made and space qualified microwave components.

Chapter 1 Satellite Broadcasting

(Image 1.3 – Common Satellite Architecture)

1.3 – Common Earth-Station Architecture

Here the usual structure of an Earth Station which transmits/receives to/from the satellite digital contents is quickly described. Be aware that digital contents are not the only present in usual satellite transmissions, but also analogical ones. We’ll study the first type because it is more common.

First of all the data flows are multiplexed together as a unique binary transmission. Because of the noise intervening in the communications between the two Earth Stations and the Satellite, an opportune channel coding is required, in order to minimize errors can occur during the radio transmissions. To do this, a Coder is introduced; its most relevant feature is the Coding Rate, defined as:

n m

where: R is the Coding Rate,

m is the number of the bit of information being coded into a block, n is the number of coded bit for every block of information.

The advantages of a Coder come at a price, because usually n > m, so to conserve the same bitrate of an uncoded transmission, more bandwidth is necessary.

After the Coder, a Modulator is present: it transmits over an Intermediate Frequency

(IF) Carrier (usually 70 or 140 MHz) the data flows, converting the transmission from

digital to analogical. These frequencies are far from those used by satellite communications, so an Up-Converter that modulates the signal to higher is necessary. After that, the radio signal enter into a High Power Amplifier (HPA): it’s important to note that this component is not so critical like the one on the satellite, because on the Earth there aren’t problems with regard to energy or weight, so usually non-linear effects introduced by it are not so critical.

The communication is now sent to the antenna, connected via a Duplexer, which permits to use it both to transmit and to receive.

During the receiving process, similar operations are made in the reverse order, so we can find subsequently a Down-Converter, a De-modulator a Decoder and, finally, a

Chapter 1 Satellite Broadcasting

(Image 1.4 – Common Earth Station Architecture)

1.4 – Antennas

Antennas with a parabolic reflector are generally used in satellite communications. If

A is the geometric area of the aperture and f is the frequency of the communication,

the Gain G of the antenna is:

2 2 2 2 4 4 4 c A f A A G e ! e " ! " ! # = = = (1.2)

where ! (usually included between 0.5 and 0.6) is the efficiency which considers the

losses into the feed, the radiation diagram of the feed itself, the mechanical imperfections; ! is the wave length defined as:

f c =_& ! (1.3) and _3!108 =

c is the light speed in free space. Finally, A_e =_&"!A is the Equivalent

Area of the antenna.

The device feeding the antenna establishes the polarization of the radiated field. In

Dual Polarization systems two different feeds exist, and, usually, they are included in

the same physical structure. The polarizations generally used in the systems are X/Y

Linear or Left-Hand/Right-Hand Circular couples. This technique permits to double

the available communications resources if the system is well designed, and Cross-Pol isolation is significant (usually it means that the level of the Cross-Pol component is < -35 dB).

A directive antenna with a gain of G, fed by a sinusoidal signal of power P produces along the direction of maximum radiation a Power Superficial Density (average level of the module of the Poynting’s Vector) G times greater than an isotropic antenna with the same efficiency and fed by the same input signal power. It is for this reason we can define an Effective Isotropic Radiated Power as:

G P

EIRP=& ! (1.4)

which permits to compare between them various antennas without knowing all the features of everyone.

Chapter 1 Satellite Broadcasting

1.5 – G/T Ratio

If T is the equivalent temperature of the receiver referred to the LNA input, the G/T

Ratio is a parameter we can use to evaluate performance of a receiver, either on the

satellite or in an Earth Station. It is important because it is the only quantity that appears in a Link Budget (see next paragraph) depending on the receiver. It is a convention considering G reduced of the attenuation L caused by the wave-guide _w

which links the antenna feed and the LNA together.

Clearly, greater is the G/T Ratio, better will be the ratio between the power of the received signal and the spectral power density of the noise the communication arrives to the antenna with. That is, if the G/T Ratio is fixed, smaller will be all the resources need to be employed in the system.

Furthermore, the G/T value is independent from the reference point according to which the equivalent temperature is calculated. But the input of the LNA is the most significant one, because it lets us to calculate immediately the effects of the LNA, which is the most crucial component according to the noise introduced into the receiver.

1.6 – Link Budget

In the following description we’ll treat a communication over a single carrier, but the conclusions could be widespread to a multiple carrier scenario, taking into account necessary corrections to the various parameters.

Ignoring for the moment the rain attenuation, the transmitting antenna of the first

Earth Station can be considered isotropic, with an radiated power equal to:

t

t G

P

EIRP= ! (1.5)

where P is the power of the signal feeding the antenna and _t G is the gain of the _t

transmitting antenna. The satellite receives:

) ( ) ( ) (t s t n t r_u = _u + _u (1.6)

where s_u(t) is the useful signal and n_u(t) is the Additive White Gaussian Noise (AWGN), the Power Spectral Density of which is bound with the Equivalent Temperature of the satellite receiver.

On the other hand, the power density of the useful signal at the input of the receiver is: L d EIRP u2 4! " = (1.7)

where d is the length of the uplink side and L is the attenuation caused by _u

Chapter 1 Satellite Broadcasting If we write A as: _u ! " 4 2 u u u G A = (1.9) then, 2 2 2 4 ) 4 ( !!_" # $$ % & ' ' = ' ' ' = u u u u u u u d L G EIRP L d G EIRP C ( ) ( ) _{. (1.10)}

If we define the Free Space Loss as:

2 4 !! " # $$ % & = u u u d L ' ( _(1.11)

then the collected power of the useful signal is:

u u u L L G EIRP C ! ! = . (1.12)

If k is the Boltzmann’s constant, B is the bandwidth of the transmission and T is the _u

equivalent temperature of the satellite receiver referred to the input of the LNA, the noise power on the uplink side is:

B T k

N_u = ! _u! . (1.13)

Now we can define the C/N Ratio that gives us the idea of how much power of the signal is greater than noise. So, we can write:

B T k L L G EIRP N C N C u u u u u u ! ! ! ! ! = = " # $ % & ' & . (1.14)

Recognizing the G/T Ratio in the previous relation and defining Monolateral Power

Spectrum Density of noise as:

N

N u

the C/N Ratio is: k L L T G EIRP N C u u u ! ! " # $ % & ' ! = "" # $ %% & ' 0 (1.16)

or, in a logarithmic form:

k L L T G EIRP N C u u u ! ! ! " # $ % & ' + = "" # $ %% & ' 0 (1.17)

Indicating with C)_u and N)_u the signal and the noise powers respectively at the input of transmitting antenna of the satellite, and if EIRP is the power radiated by the _s

satellite, it’s easy to show that:

s u s u EIRP N C EIRP C ! " # $ % & ' + = ₍₁ 1 ) ; (1.18)

moreover, if the satellite works well, it doesn’t degrade the C/N Ratio of the input, so we can impose that:

u u u N C N C ! " # $ % & = ) ) (1.19) So, u s u u u N EIRP N C N C ) ) ) = = ! " # $ % & _(1.20) s EIRP N) = (1.21)

Chapter 1 Satellite Broadcasting

where s)(t) is the useful signal and n)(t) is the noise retransmitted by the satellite, while n_d(t) is the noise bound with the downlink section.

So, if EIRP is the power the satellite emits, G is the receiving antenna gain, _s L is the _d

free space loss in the downlink, and 'L takes into account the losses caused by the

atmosphere and a non-perfect tracking between the two antennas, the useful signal power is: ' L L G EIRP C d s ! ! = . (1.23)

Likewise, the power of the noise retransmitted by the satellite and reaching the

Earth-Station antenna is:

' L L G N N d u ! ! = ) ) (1.24)

Remembering the (1.21), we obtain:

' L L G N C EIRP N d u s ! ! " # $ % & ' = ) _{. (1.25)}

If T is the equivalent temperature of the earth receiver and supposing that n)(t) and )

(t

n_d are two AWGN independent processes, defining ) ( ) ( ) (t n t n t n = )_& + _d (1.26) and B T k N_d = ! ! (1.27)

B T k L L G N C EIRP N N N d u s d + ! ! ! ! " # $ % & ' = + = ' ) . (1.28)

So, the total _!

" # $ % & N C is: B T k L L G N C EIRP L L G EIRP N C d u s d s ! ! + ! ! ! " # $ % & ' ! ! ! ! = " # $ % & ' ( ( ( ( ( 1 1 1 1 1 ' ' (1.29)

Finally, defining the _!

" # $ % & N C

Ratio for the downlink section as:

' L L B k T G EIRP N C d s d ! ! ! " # $ % & ' ! = " # $ % & ' & (1.30)

we obtain the overall equation which describes the ! " # $ % & N C

Ratio on a satellite link,

without taking into account possible losses because of rain:

1 1 1 ! ! ! " # $ % & ' + " # $ % & ' = " # $ % & ' d u N C N C N C . (1.31)

It’s important to note that very often, especially in the links on which Eutelsat’s satellites work, d u N C N C ! " # $ % & >> ! " # $ % & _(1.32)

Chapter 1 Satellite Broadcasting

Using the logarithmic form and the (1.17) we have:

k L L T G EIRP N C d s "! ! ! # $ % & ' + = "" # $ %% & ' ' 0 (1.34) 