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:
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.
Satellite resources are scarce commodities, especially frequency spectrum available on its payload; spectrum efficiency is considered as the most relevant measure of system performance, since bits/s/Hz drive costs of communications over satellite.
In recent years, satellite bandwidth cost has been fast declining, largely thanks to the development of multi-narrow-spot beam high throughput satellite technology, allowing multiple reuses of available spectrum. Further bandwidth efficiencies would be expected with narrower spot beams and more aggressive frequency reuse schemes. Today’s technology allows a typical spectrum reuse of 4 (every 4th beam).More aggressive frequency reuse could make the system interference limited, where conventional interference suppression schemes would be ineffective. In recent years, some advanced techniques have been developed that exploit the interfering signals to help with recovering degraded desired signals. This project investigated the effectiveness of two of such techniques, one using a conventional non-spreading ‘Overlay’ controlled interference method and the other using the spread spectrum DS-CDMA technique.
The ultimate goal is to cost-effectively provide for various broadband applications, which are continuously demanding larger throughputs and better quality of service (QoS), in remote locations, where terrestrial facilities are insufficient/not available.
Our investigations, through theoretical and simulation studies, have shown promising results for these IM techniques.
This study evaluated interference management capabilities of two potential product concepts, belonging to the multi-user-detection (MUD) category, namely an ‘Overlay’ and a CDM based technique that have been devised to mitigate severe co-channel-interference in interference limited multi-beam 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.
An MMSE-SIC processor (Minimum Mean Squared Error-Successive Interference Canceller) and a suitably modified/newly developed radio resource management module were assumed as the key components at the core of each of these two products. The latter components have been to offer optimal scheduling strategies in resource sharing to improve the offered capacity and its fairness. The SIC module at the receiver is the main building block of our IM techniques, making its high accuracy essential to the successful operation of our IM schemes. In overlay technique, a strict synchronization of the cooperating signals is required for a near error-free operation of the SIC detector.
Our comparative analyses have shown these products to be of sizable benefit to our 1-color (full frequency reuse) 20-beam Scenario 2; however, they would not fare any better for the 2-color 70-beam Scenario 1.
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.
The project plan was as follows:
The project has been completed: