Hesham El-Adly
Advantech Satellite Networks,
2341 boul. Alfred-Nobel
Saint-Laurent (Montréal), Quebec
Canada
H4S 2A9
European Space Agency
European Space Agency
The activity "Joint Turbo Decoding and Carrier Synchronisation" was carried out by Advantech Satellite Networks and its partners Turbo Concept and Eurecom.
The main objective of this project was to develop and validate new techniques that enable power efficient transmission for satellite receivers. While the algorithm investigations and their performance evaluation were not limited to any specific air interface, the main focus was on DVB-RCS and DVB-S2 compatible receivers. Due to the dominant role of conventional carrier synchronisation techniques in the receiver performance degradation, the work was directed towards improving this aspect; i.e., the frequency and phase synchronisation algorithms.
In the first part of this project, a set of algorithms, providing significant improvement in carrier synchronisation accuracy, was selected. Carrier synchronisation algorithms were developed that can take advantage of the iterative decoding process for turbo codes and LDPC codes in order to enhance the performance, especially at low coding rates and for short bursts and in the presence of severe phase noise.
In the second part, the selected algorithms were implemented and tested in hardware. The final validation included lab testing for a pre-selected set of transmission modes.
Extensive simulations and complexity assessments, later confirmed by implementation and testing in the laboratory, have confirmed the performance improvement and implementation feasibility of such techniques. The "evolutionary" techniques were implemented for a demodulator for the current DVB-RCS air interface and straightforward extensions of this, such as the use of BPSK and 8PSK modulation. To maximise practical utility, the CBC algorithm was successfully implemented using DVB-S2 air interface, jointly with an LDPC decoder.
The main conclusion is that the techniques based on evolution of current techniques bring very worthwhile improvements for DVB-RCS transmissions, in particular for short bursts, at low signal-to-noise ra
The powerful coding schemes employed in DVB-S2 and in the DVB-RCS return link potentially provide transmission performance in terms of power efficiency very close to theoretical baselines. However, in some circumstances the limited accuracy of conventional synchronisation circuits in the receiver prevents the fulfilment of this potential. The use of power efficient transmission schemes is essential to mitigate the impact of the rain fade without introducing large rain margins in the link budget. Such margins increase the cost of the user terminal considerably. The transmissions used during rain fades typically operate at low signal-to-noise ratio and often use low data rates, resulting in increased susceptibility to impairments such as frequency offsets and phase noise. The synchronisation performance limitations affect considerably the use of low coding and data rates as a countermeasure against the rain fade. Overcoming these limitations is therefore one important element of the overall strategy to create robust DVB-S2/DVB-RCS systems that can offer a high quality of service.
Traditionally, demodulation (synchronisation) and decoding are treated as two distinct operations. This is partly due to legacy from the concatenated-coded demodulators, and partly due to limitations of available decoder cores (which do not do any part of the demodulation). At high carrier frequencies (in particular, at Ka-band), RF power amplifiers are major cost drivers for the user terminal. There is therefore a strong interest in developing power-efficient solutions, in order to facilitate the deployment of interactive multimedia systems in the consumer market where the user terminal cost is of paramount importance.
Furthermore, the phase noise characteristics of the low-cost terminals, operating at low baud rate, can be worse than predicted for example in system guidelines. One of the main objectives of this project was to develop robust synchronisation schemes that can tolerat
The initial goals have been achieved and the corresponding activities completed. Extension of the work to consider burst detection and timing synchronisation is expected to start in late 2006 or early 2007.
