APEXX: Algorithms for Pre-distortion and Equalization of tX and rX terminals - Multicarrier digital equalization/predistortion

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Satellite communication data traffic rates are growing. In the scope of obtaining higher data rates, the increase of the spectral efficiency and the use of higher bandwidth is mandatory. Future systems will also implement multi-carrier operation.

In the frame of this study, some key technologies for on-ground processing techniques to minimize non-linear effects in multi-carrier operation mode are investigated. Pre-distortion and equalization algorithms have been analysed and evaluated by software simulations.  The end-to-end system simulation have been performed with a DVB-S2 system and representative channel impairments caused by IMUX, OMUX and HPAs.

Within this study, different techniques for on-ground pre-distortion and equalization, including fractionally spaced equalization has been surveyed and the performance gain was analysed. The techniques have been benchmarked using different mission scenarios for single and multi-carrier broadcasting. The scenarios included present state and predicted future demands of the end-to-end system requirements for increased data rates and bandwidth.

The study defined realistic mission scenarios (respecting requirements of the near future), surveyed existing algorithms, compared algorithms for on-ground pre-distortion and equalization and select a set of promising techniques to be evaluated in more details for the defined scenarios (task 1+2). The selected algorithms have be added to a already existing software environment (TOPCOM++) which realises a DVB-S2 system. The software is adapted for end-to-end simulations respecting non-linear effects in the channel. The simulation results have be evaluated by the study team and according conclusions for future development work (task 3+4) have been drawn. During the study, the field of techniques was extended to fractionally spaced equalization techniques (FSE, task 5). 

Figure: Distortion of a 32APSK modulated carrier (exemplary)

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The keywords of the APEXX project are:
  • improvement of the spectral efficiency in satellite broadcasting/broadband communication,
  • non-linear effects characterization of the transmission channel,
  • novel pre-distortion and equalization techniques for satellite multi-carrier systems
  • data level pre-distortion
  • fractionally spaced equalization
  • centroid based demapping
  • multi-gateway scenarios
To increase the data rates of today’s satellite communication, new technologies have to be applied in the near future to fulfil these demands.
Pre-distortion and equalization are state-of-the art techniques to counteract non-linearities in the transmission channel, especially, if multi-carrier systems instead of single-carrier systems will be used.
The study has surveyed latest research results with respect to on-ground pre-distortion and equalization techniques and evaluated their performance within future mission scenarios and requirements of future broadcasting systems.
It could have been shown that - in comparison to standard techniques -  a combination of digital data pre-distortion (DPD) on TX and fractionally spaced equalization (FSE) on RX side can gain up to 1dB in total degradation for multi carrier scenarios.  
The conclusions of this study will support decisions for a development of an operational system (hardware) in the near future.  
The project does not develop hardware. The pre-distortion and equalization techniques are evaluated in numerical simulations. The project used the Topcom++ Software tool from ESA, which was extended by adequate models of IMUX, TWTA, OMUX and filters for this investigation. The algorithms for pre-distortion and equalization have been coded and  implemented as C++ subroutines and classes. 

Figure: Simulation chain with models (exemplary)

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In the first 2-3 month of the project, the future mission scenarios to which the techniques shall apply have been defined (task 2). A survey of the state-of-the art algorithms for on-ground pre-distortion and equalization techniques have been performed and an analysis of the candidate algorithms has been performed with respect to the mission requirements (task 1). This initial analysis process was closed with a baseline review (BDR), at which a set of algorithms had been selected for more detailed analysis in the following.
In a consecutive task 3 the simulation environment (Topcom++) was adapted to the project and the selected algorithms had been added to the software. A test plan was established and a test readiness review of the tool was held at CDR. In task 4 the TOPCOM++ environment was used to analyse the performance of the algorithms within different mission scenarios. According to the simulation results for scenarios of 2, 3, 4, 5 and 8 carriers, conclusions have been drawn. In an additionally defined task 5, the performance analysis for dual and triple carrier scenarios have been extended to fractional spaced equalization techniques. The results have been summarized in a final report.
Current status

The final review of the last work package was held on March 18, 2014 at ESA/ESTEC. The project will be closed with the delivery of the final documentation.