WiBOX - Une passerelle pour une réception robuste de vidéo diffusée via WIMAX et une rediffusion indoor via WIFI

摘要

This PhD study intends to investigate the tools necessary to implement a device (the WiBOX), which can robustly receive video broadcast over WiMAX and then rebroadcast it over WiFi. WiBOX should not only provide WiMAX services access to a WiFi user, but it should also achieve reasonable video quality even with a very weak WiMAX signal, and at the same time for WiFi rebroadcast, it should utilize alternative recovery techniques and avoid delays caused by the conventional retransmissions. This would help to improve WiFi user quality and to remain consistent with the broadcast scenario. To achieve the said objectives one has to consider several robust tools, which are often deployed to solve problems, like packet loss, synchronization failures, high delay, throughput etc., encountered while receiving video through a WiMAX/WiFi-link. These robust tools can be deployed at several protocol layers, among them few notable are, e.g., Joint Source Channel Decoding (JSCD) techniques deployed at the application (APL) layer, iterative decoding techniques deployed at the physical (PHY) layer, and header recovery, estimation, or synchronization tools deployed at various layers. For an efficient performance of these robust tools some cross-layer approach to enable exchange of useful information between the protocol layers and the complete analysis of the protocol stack is required. Some of these tools have requirements that are not compliant with the Standard Protocol Stack (SPS) and require Soft-Permeable Protocol Stack (SPPS), which can allow flow of erroneous packets, containing the soft information, e.g., A Posteriori Probabilities (APP) or likelihood ratios, to the higher layers. More importantly, for performance enhancement these tools should mutually benefit and reinforce each other instead of undoing each other's advantage. To increase the throughput, in both WiMAX and WiFi communication standards, packet aggregation is used; several packets are aggregated at a given layer of the protocol stack in the same burst to be transmitted. One can deploy Frame Synchronization (FS), i.e., to synchronize and recover the aggregated packets, however, when transmission over a noisy channel is considered, FS can cause loss of several error-free or partially errorfree packets, which could otherwise be beneficial for other tools, e.g., JSCD and header recovery tools, functioning at higher layers of the S-PPS. Rebroadcasting video over WiFi can significantly increase packet loss rate as the retransmission is omitted, which can be overcome by the packet-level Forward Error Correction (FEC) techniques. The FS and packet-level FEC decoder for S-PPS should not only allow flow of soft information from the PHY layer but should also mutually benefit from the JSC decoders deployed at the APL layer. In this thesis, we propose several Joint Protocol-Channel Decoding (JPCD) techniques for FS and packet-level FEC decoders operating at S-PPS. In the first part of this thesis, we propose several robust FS methods for S-PPS based on the implicit redundancies present in protocol and the soft information from the soft decoders at PHY layer. First, we propose a trellis-based algorithm that provides the APPs of packet boundaries. The possible successions of packets forming an aggregated packet are described by a trellis. The resulting algorithm is very efficient (optimal in some sense), but requires the knowledge of the whole aggregated packet beforehand, which might not be possible in latency-constrained situations. Thus in a second step, we propose a low-delay and reduced-complexity Sliding Trellis (ST)-based FS technique, where each burst is divided into overlapping windows in which FS is performed. Finally, we propose an on-the-fly three-state (3S) automaton, where packet length is estimated utilizing implicit redundancies and Bayesian hypothesis testing is performed to retrieve the correct FS. These methods are illustrated for the WiMAX Medium Access Control (MAC) layer and do not need any supplementary framing information. Practically, these improvements will result in increasing the amount of packets that can reach the JSC decoders. In the second part, we propose robust packet-level FEC decoder for S-PPS, which in addition to utilizing the introduced redundant FEC packets, uses the soft information (instead of hard bits, i.e., bit-stream of '1's and '0's) provided by the PHY layer along with the protocol redundancies, in order to provide robustness against bit error. Though, it does not impede the flow of soft information as required for S-PPS, it needs support from the header recovery techniques at the lower layers to forward erroneous packets and from the JSC decoders at the APL layer to detect and remove remaining errors. We have investigated the standard RTP-level FEC, and compared the performance of the proposed FEC decoder with alternative approaches. The proposed FS and packet-level FEC techniques would reduce the amount of packets dropped, increase the number of packets relayed to the video decoder functioning at APL layer, and improve the received video quality.