PAGE CONTENTS
Objectives
To investigate interference management capabilities of two advanced techniques, belonging to the multi-user-detection (MUD) category, namely an ‘Overlay’ and a ‘spread-spectrum CDM-based’ method, in mitigating severe co-channel-interference in multi-beam interactive star satellite configurations, caused by an aggressive frequency reuse of 2-color and 1-color schemes, used for the purpose of boosting system capacity over a conventional 4-color scheme.
The requirements of the study as relating to its goals are as follows:
- “Maximizing the spectral efficiency satellite network, both in the outbound and inbound directions with minimum signalling requirement.
- Improving the cost effectiveness of the interactive satellite network (minimizing terminal complexity).
- Achieving data delivery latency within normal boundaries of broadband applications.
- Supporting efficiently and flexibly different class of services in the allocated spectrum.
- The ease of operation.”
Challenges
The core activities did not encounter any key challenges, although a number of computational complexities with user terminal were identified, mainly in relation to the chosen MMSE-SIC (Minimum Mean Squared Error-Successive Interference Cancellation) based detectors. MMSE-SIC based detectors have been chosen for the study for performing the best with our IM techniques. However, even the low complexity iterative MMSE-SIC, at the core of the CDM-based solution, would require a capability to perform a relatively large number of mathematical operations for implementing its real-time MMSE filter, which varies from bit to bit. With the Overlay technique, the optimality of detector’s operation depends on high accuracy of clock and phase recovery (synchronisation) as well as SNIR estimation. To meet the requirements for proper operation of our IM techniques the user terminals seem to require significant additional processing power and circuitry, whose costs might not be justifiable for a relatively modest capacity gain of ~25%, under ideal conditions.
Furthermore, it has been shown that aggressive frequency reuse schemes require superior resource sharing and scheduling strategies as well to improve the offered capacity fairness. That is, beyond our physical layer focus, optimal implementation of our IM techniques, each requires a modified/new radio resource management to optimize, respectively, scheduling in overlay coding and bandwidth-on-demand, in our spreading-CDMA technique, potentially using a combined random access and DAMA scheme for the latter.
Finally, it is of importance to be reminded of some key practical issues (outside the scope of this study) that regardless of the planned frequency reuse scheme could seriously hinder actual implementation of large multi-beam systems. These include insufficient Ka-band gateway spectrum, resulting in a large number of gateways and/or use of less explored higher frequencies, such as Q/V-bands, introducing new technical challenges.
System Architecture
The baseline system architecture for the study was set through a practical multi-spot-beam satellite communications payload design and associated end-to-end network parameters for two network configuration scenarios, one comprising 70 narrow spot beams covering Europe (Scenario 1) and the other with 20 narrow spot beams covering a sub-region of Indo China (Scenario 2). The 4-color frequency reuse versions of the two scenarios are not interference limited, i.e., the co-channel interference (CCI) are insignificant and were used as our capacity benchmark configurations, while the 2-color Scenario 1 and 1-color Scenario 2 exhibit significant CCI, making them interference limited and thus good candidates for our comparative capacity performance enhancement evaluation study.
Two interference management techniques, belonging to the general category of multi-user-detection (MUD), were used with the above two interference limited scenarios, namely, a conventional non-spreading ‘Overlay’ controlled interference method and a CDMA-based direct sequence spread spectrum technique. Follow-up analyses of the above system architectures and IM techniques resulted in the establishment of streamlined channel and physical layer simulator models that underwent further assessment in close consultation with ESA for confirmation of implementation feasibility.
Plan
The project plan was as follows:
- Task 1 for setting a common baseline, including reference system architectures and capacity performances, using the state-of-the-art ‘best-in-class’ DVB-S2X-RCS2 ModCods, as benchmarks for our comparative analyses.
- Task 2 for a critical review of IM techniques and optimal process methodologies.
- Task 3 for development of simulation models and software tools, evaluating capacity performance effects.
- Two separate frameworks aligned with two candidate IM techniques in carrying out Tasks 2 and 3
- Task 4 for summary/conclusions, highlighting key outcomes of the study, along with key recommendations for further work and technology roadmap.
Current Status
The project has been completed:
- Task 1’s comparative analysis identified existing performance gaps to be closed by capacity enhancement IM techniques.
- Task 2 consolidated the selected Overlay and Spreading IM techniques, defining optimum methodologies for our theoretical and simulations analyses.
- Task 3 derived capacity performances of the IM techniques that showed there would be capacity enhancements only for the 20-beam Scenario 2, while the results for the larger 70-beam Scenario 1 were rather negative.
- Task 4 consolidated study results and explored potential means of mitigating negative effects of some outstanding issues, along with some recommendations, outlining a way forward.