LA IDENTIDAD CAMPESINA

4.3.2.1 Multi Port Amplifiers

The most important aspect of the OBDPA system is to efficiently share the radiated power, reducing inter-beam interference and allowing high flexibility in the power allocation among antenna ports. The adaptive beamforming network is composed of N elements, one per each antenna feeds, clustered by a set of MPA devices. The MPA is a multi-input and multi-output system that is capable to amplify multiple

input signals at the same time by using shared amplifiers. The multiple input signals and outputs are amplified separately via different output ports to reduce any mutual interference.

The MPA scheme is a well known and consolidated technique to carry out power reconfiguration among spot beams. The technique has been implemented in the Japanese satellite WINDS for an experiment strictly related to power reconfiguration aimed to the compensation of Ka-band rain attenuation.

The general scheme of the antenna is shown in Fig. 4.3, where each beam is radiated by a single feed via distributed amplification and final composition of the different amplified contributions. The amplified contributions are collected by a High- Power BFN (HP-BFN) which has the goal to focus the whole power on the single feed in charge for radiating the current beam.

The classical layout of the HP-BFN is based on a set of MPA, composed by a number of hybrid couplers properly connected to each other. The basic assembly, named Butler-Like Matrix (BLM) because similar to Butler Matrix layout with the exception that phase shifters between successive layers of hybrid couplers are not present. Apart from the trivial BLM with 2 inputs and 2 outputs, i.e. a single hybrid coupler, the lowest level of BLM is the one with 4 inputs and 4 outputs, also called 4_× 4; it is shown in Fig. 4.2, where a number of 4 Hybrid couplers 3dB/90o is recognized.

ESA/ESTEC 20887/07/NL/HE

Final Report

Page 9-18

Space Engineering S.p.A. Via dei Berio, 91 - 00155 ROMA

Figure 9-12 a) Ground stations and service area b) Frequency Reuse Scheme for the beamforming network

The numerical values that have been defined to devise the system data are reported in the following

table:

Table 9-1 Reconfigurable System Parameters

Number of feeds

72

Number of pixels

1988

MPA order

4 x 4

Transmission Line Loss [dB]

0.5

LNA Temperature [K]

150

Symbol Rate [MBaud]

45

Rx Station Gain [dB]

42

Radiated Power [W]

2000

The reconfigurable system makes use of meteorological data as the inputs of the optimization

process which are provided by the ERA40 Database. This database collects files which give the

Figure 4.1: Coverage area over Europe: reference grid points and multibeam antenna scheme [2]

Chapter 4: Fade Mitigation Techniques 102

Figure 2.3.

Spots distribution and coverage area for satellite longitude 16

East.

frequencies. The matter of a Reconfigurable Antenna is to efficiently share the radi-

ated power reducing inter-beam interference and allowing high flexibility in the power

allocation among antenna ports. The adaptive beamforming network is composed of M

elements clustered by a set of MPA devices. The MPA is a multi-input and multi-output

system that is capable to amplify multiple input signals at the same time by using shared

amplifiers. The multiple input signals and outputs are amplified separately via different

output ports to reduce any mutual interference. An example is reported in fig. 2.4, where

an MPA elements of order 4x4 is shown [17]. The device is composed of an input and an

output hybrid matrix (made of 3dB, 90◦

hybrid couplers) and a set of 4 shared power

amplifiers. Commonly the order of a MPA is 4x4 or 8x8.

In such a system, the available on board power is firstly equally divided among the

MPAs and afterwards a set of coefficients assign the power to the 4 (or 8) ports that

belong to the same MPA. In order to best exploit the power allocation between the

ports of an MPA, the weather condition detected inside the spots covered by that feeds

should be not correlated. So, in case there is high rain intensity over two spots of the

MPA, the needed power could be subtracted to the spot that are not interested by rain

events. Usually, we considered a frequency reuse scheme reported in fig. 2.5, where each

color identifies the area covered by elements clustered in a single MPA. This scheme,

by assuming the maximum distance between elements with the same frequency, tries to

ensure the higher dissimilar weather condition among these feeds.

Figure 2.4.

Functional block diagram of a

4-ports MPA [17]

Figure 2.5.

Map of reuse frequency color

Figure 4.2: MPA general scheme

As a result, the available on board power is firstly equally divided among the MPAs and afterwards a set of coefficients assign the power to the 4 (or 8) ports that belong to the same MPA. Reconfiguration is actuated in MPA scheme by means of a variation of the carrier level among the different beams at the MPA input.

In order to best exploit the power allocation between the ports of an MPA, the weather condition detected inside the spots covered by the MPA feeds should be not correlated. Consequently, in case there is high rain intensity over one spot linked to an MPA, the needed power could be subtracted to the same MPA remaining spots. As a consequence, the spot assignment to each MPA assumes the maximum geographical distance between elements of the same MPA, trying to ensure the highest dissimilar weather conditions among the feeds. In this way, the co-frequency interference among the same MPA beams is also minimized.

4.3.2.2 Antenna Front-End

The front-end architecture for the TLC antenna under analysis is based on fixed amplifiers it is divided into 2 main sections:

• the reconfiguration control section is composed by a set of beam amplifiers and it is responsible for supplying the amplification section with optimum amplitude distribution among beams according to the selected criterion;

• the amplification section is composed of MPAs of 4_×4order.

From a reconfiguration point of view, the optimum beam amplitude distribution is obtained by a single amplitude-only optimization including all the beams [74].