DIPRET

Space Segment - Payload

Status

Ongoing
Status date

2024-09-30
Activity Code

5C.458

Objectives

The objective of the proposed activity is to develop and test efficient broadband digital pre-distortion algorithms and architectures for payloads utilising digital processors.

The developed concept shall be implemented in a combined RF and digital processor testbed.

Tests in a laboratory environment will be carried out to fully evaluate the performance improvements. The basic idea is to consider at least 16 independent transmitting beams generated by a multiple-beam smart antenna, each one capable of using a transmission technology of something as 12 subcarriers organised in an OFDM architecture.

The amplifiers connected with the active antenna array should be linearised to target the following improvements:

at least 10% improvement of payload capacity/throughput
30% higher signal C/I
up to 5% DC-to-RF efficiency improvement

Challenges

The PA is a key element required in every communications system to strengthen the transmitted signal to compensate for the wireless radio channel losses.

The principles of predistortion linearisation are straightforward and precede the PA with a subsystem that counteracts the nonlinear characteristic of the PA.

The main challenge is to create a DPD capable of significantly reducing the output level of intermodulation.

In this case it would become possible to reduce the transmission back off (by increasing the output power level)), improving the efficiency of the transmission channel and at the same time improving the link budget.

Other important challenges are:

make effective the DPD inside in a large bandwidth, in fact the DPD performances are typically better in a narrow band context;
make effective the DPD by observing the return RF feedback by a reduced number of RF paths, so reducing the complexity of the hardware and of the digital algorithms.

Other challenges are to create a test bed capable of making the benefits of DPD fully evident.

For this reason, it is necessary to pay close attention to the performances of the RF and digital boards, in terms of:

digital resources of the Digital module to generate modulated tones with beamforming and multi-beam capabilities;
adequate spurious suppression related to the RF up-conversion process in the RF paths;
Phase and amplitude matching of all the RF paths, in order to make them as similar as possible in performance.

Benefits

Satellite communication systems with digital signal processing and active beamforming capabilities could benefit by employing digital pre-distortion techniques as part of the onboard signal processing.

Benefit will be drawn from ground transceivers and digital systems to develop efficient broadband digital pre-distortion algorithms for communication payloads utilising digital signal processors.

The design activity of RF/Digital Test-Bed with DPD capabilities, should demonstrate as a moderate increase in processor power requirement could be traded off against either increased PA efficiency, leading to a reduction of the mass of the thermal management system, or reduced beamforming and intermodulation interference at the same PA efficiency.

Laboratory tests on the Test-Bed, should demonstrate as DPD reduces transmission back-off allowing for greater output power while meeting transmission spectral requirements.

Far-field radiated measurements should also demonstrate a consistent reduction in the amplitude of intermodulation patterns.
This allows to consider PAs with lower specifications to be used, thus saving design and manufacturing costs.

Features

In this project the Digital Pre-Distortion (DPD) is used to reduce intermodulation effects on RF paths and to increase the communication performances.

This includes:

development of a combined RF and digital signal processor architecture;
generation of multiple modulated tones;
beamforming function with multi-beam generation;
digital pre-distortion of the signals with polynomial models, as the Generalized Memory Polynomial process;
performance tests carried out in a laboratory environment to qualify the performance improvement.

System Architecture

The developed concept will be implemented in a combined RF and digital processor test-bed.

The test-bed can simulate the behaviour of the system, it is composed of the digital and the RF parts (RF part also contains the radiating elements) and by the necessary laboratory instrumentation.

The Digital module contains the following main functions:

generation of multiple modulated tones;
beamforming along four RF paths;
digital pre-distortion of the conducted signal through the four RF paths.
AD and DA converters.

The RF part is formed by four paths, everyone contains:

the Up-conversion stage for the forward signal routed to the radiating element;
the Down-conversion stage for the feedback signal routed to the Digital Module.

Test measurements will be made inside an automatic test procedure, involving the test-bed together the instrumentation.

The external computer will be connected to the test-bed through standard ethernet/USB interfaces and to the instrumentation through standard ethernet/IE488.

Plan

The program foresees a detailed design of a validated Digital Pre-Distortion (DPD) techniques and combined RF and processor testbed with accompanying algorithms in source code, the realization of the prototype and the verification of the characteristics.

The program duration is 24 months with the following timeline: