Cross-layer Optimization in the Next-generation Broadband Satellite Systems

Next-generation broadband satellite systems will have the capability to provide costeffective universal broadband access for the users. In order to meet users’ requirements on high quality multimedia services, many enhancements have to be made on the existing satellite technologies. One of the promising methods is the introduction of cross-layer design. There are several advantages of a layered approach since modularity, robustness and ease of designs are achieved without difficulty. However the properties of the different layers have substantial interdependencies and a modularised design may therefore be suboptimal with regards to performance and availability in a hybrid satellite and mobile wireless environment. In this paper, we will carry out a review of the cross-layer design in satellite systems. Based on this, a cross-layer architecture for the next-generation broadband satellite system is proposed. The proposed cross-layer architecture has two main components: QoS and resource management and mobility management. In each component, the cross-layer techniques that have been used are described in details. Introduction Cross-layer architectures diverge from the existent network design approaches, where each layer of the protocol stack operates independently and the data between the successive layers is exchanged in a very strict and systematic manner. There are several advantages of a layered approach since modularity, robustness and ease of design are easily achieved. In view of the fact that a layer has to perform certain functions, the design efforts only need to be focused on these functions without concerning the properties and interactions with other layers. The modularity that the layers provide allows for potential arbitrary combination of protocols, and the maintainability is being improved as new versions of a protocol can be inserted without having to alter the rest of the network stack. However the properties of the different layers have substantial interdependencies and a modularised design may be suboptimal with regards to performance especially in hybrid satellite and mobile wireless environments, where the communication channels and traffic patterns are more unpredictable than in wired-line networks. In case of cross-layering, information is allowed to be exchanged between adjacent and non-adjacent layers of the protocol-stack, by means of a broader and much more open data format. In this way the overall system performance can be improved by taking advantage of the available information across different layers. Cross-layer processing adapts the link-, networkand transport-layer parameters to the channel and the applications instantaneous requirements. To achieve these optimization goals, appropriate signalling methods and architectures for cross-layer designs are necessary. Two alternative architectures for cross-layer design are summarized below: • Direct communications architecture where unior bi-directional transfer of information between two adjacent or nonadjacent layers are implemented. This is done by creating new interfaces at the selected layers beyond those normally used between the layers. • Indirect communications architecture where instead of doing communications between specific layers, the cross-layer design utilize a parallel structure that acts as a shared database of the system state, being accessible to whichever layers who choose to utilize it. This paper describes cross-layer design architectures and interaction techniques as implemented and proposed in the EU-funded IST FP6 project Satellite-based communication systems within IPv6 (SATSIX). This includes cross-layer techniques for both direct and indirect communications between layers. Based on existing solutions a cross-layer interaction module (CLIM) adapted to satellite network has been proposed. The main areas for cross-layer design have been identified to be related to QoS and Radio Resource Management and Mobility Management. The QoS and Resource Management component includes: • SIP and MAC cross-layer interactions are used to support the interworking between WiMAX and DVB-RCS, and multimedia QoS-aware application. • Transport layer and MAC cross-layer interaction (i.e. the interaction between TCP PEP and DAMA in the MAC) is designed to optimise the way in which the available resource is used taking into account QoS mapping at the MAC layer and enables data to be sent to the lower layers at the speed at which the MAC layer queues are emptied (flow-control). • IP and MAC scheduling interaction are implemented in a way that can fully take advantage of QoS capabilities offered by the satellite system. • MAC and Physical layer interaction between DRA and DAMA as information in the frame constitution. Details of the CLIM architecture adapted to these areas are described and examples shown how to implement this model and other equivalent interaction models. Cross-layer feedback issues The solutions for cross-layer adaptation seek to enhance the performance of the system by jointly designing the performance of single or multiple cross-layers. The question is to what extent the layered architecture needs to be modified in order to introduce co-operations among protocols belonging to different layers. At one end, solutions based on layer triggers implement interdependencies between protocols maintaining compatibility with strict layering. A full cross-layer design represents the other extreme; introducing stackwide layers’ interdependencies that enable the optimization of each protocol’s performance by exploiting the full knowledge of the network status collected at different layers of the protocol stack:, the status of the wireless links connecting a node to its neighbours, the network topology.