Wireless video multicast enables delivery of popular events (such as a soccer game or headline news) to many wireless users in a bandwidth efficient manner. However, providing good and stable quality video to a large number of users with varying channel conditions remains elusive. We propose to integrate layered video coding with cooperative communication to enable efficient and robust video multicast in infrastructure-based wireless networks (such as WLAN, 3G orWiMAX networks). In a conventional multicast system, the sender chooses the modulation and channel coding schemes that will yield sufficiently low frame error rate for the worst-case receiver in its coverage area. With this design, the receivers with good channel quality unnecessarily suffer and see a lower quality video than they would have if the system were targeted at good receivers. We propose to employ user cooperation to improve the overall system performance.
The basic idea is to let the sender transmission target users with good channel conditions (Group1), and then let these users (or a subset) relay their received information to remaining receivers (Group2). More generally, multiple hops of relay transmissions may be performed. We will investigate cooperation at the physical layer, MAC layer as well as application layer, each with a different performance vs. complexity tradeoff. In the basic system set up, each relay transmits in separate time slots, so that more relays will lead to higher sustainable transmission rates, but also reduced transmission times. To further improve the system performance, we will also consider using directional antennas at relays, and using distributed space-time coding among relays, to enable simultaneous relay transmissions. We further propose to code video into multiple layers and let the sender and relays choose the number of layers to send based on their channel conditions, and allow the relays to forward only a subset of layers that they receive. This way, Group 1 users will receive a high quality video, whereas Group 2 receivers will receive a lower but decent quality video, but not worse than that offered by direct transmission to all receivers. Given that the users are mobile, any given user may experience different channel qualities at different times. This approach will lead to better average quality for every user.
The performance of the proposed system and its advanced options depends on user partition, relay section, and scheduling among sender and relay transmissions, as well as bit allocation among source and channel bits in each transmission. We will develop efficient algorithms for determining these variables that can optimize a given multicast performance criterion. We will also develop protocols for managing and scheduling cooperative transmissions among the sender and individual relays. Finally, we will develop a testbed that implements the proposed system to enable realistic evaluation of the system performance.
This research is partially funded by NSF, WICAT, CATT.