New methods for robust audio streaming in a wireless environment

The rapid development of mobile computing is turning mobile terminals into fully equipped entertainment systems capable of reproducing live audio and video. However, wireless access networks still pose significant limitations for the network capacity and the quality of service experienced by the end users, compared to the high standard of service provided by the modern fixed Internet Protocol (IP) networks and access technologies based on digital subscriber lines. This dissertation concentrates in application layer solutions for optimizing the network resource utilization and audio reproduction quality in the audio streaming applications used in wireless environments. Although the major focus is in the audio streaming, many of the proposed approaches are also applicable for other types of streaming data, such as digital video and animation. The first part of this dissertation concentrates in an audio streaming system that is based on shuffling of frequency components and critical blocks within each frame among several transport packets to achieve higher robustness against packet losses. This approach allows efficient co-design with different kinds error recovery schemes, as different level of error protection may be applied to separate frame components, depending on their priorities. We propose several alternatives for error recovery, including Forward Error Correction (FEC), selective retransmissions and a hybrid of these two strategies. Unfortunately, the state-of-the-art audio coding standards, especially Advanced Audio Coding (AAC), do not intrinsically support this kind of fragmentation and data prioritization schemes. This is why we propose also modifications to the baseline AAC bitstream format to support the suggested transport and error recovery strategies better. In the second part of this dissertation, we focus on the characteristics of a wireless link. Several recent studies show that the wireless network resource utilization could be significantly improved if the packets containing bit errors were relayed up to the application instead of using link layer retransmissions or strong FEC included in many wireless standards. In this case, the application must be able to cope with bit errors in the payload. For this purpose, we propose a bit-error robust packetization scheme for AAC streaming. We have also studied the possibility to select adaptively between different error recovery strategies, such as partial retransmissions and application layer FEC, depending on the distribution of bit errors. However, many existing wireless technologies do not allow user to switch off the link layer error recovery mechanisms. Even in this case, a proper packetization scheme at the application layer may be beneficial to optimize the network performance. Especially packet size optimization could significantly improve the application layer quality, the efficiency of the wireless link resource usage and the power efficiency altogether. The proposed new methods and observations have clear potential implications to the future solutions in the field of the wireless multimedia streaming. For example, the concept of prioritized packetization could be highly useful for streaming in the networks with intelligent Quality of Service (QoS) mechanisms, peer-to-peer streaming with link dispersion, and energy efficient streaming relying on bursty transmission mode. On the other hand, the proposed application layer adaptation schemes for bit-error prone environments may be proven beneficial for the cross-layer network system architectures in the future