Distributed Markov decision process in cooperative peer-to-peer repair for WWAN video broadcast

Error resilient video broadcast over Wireless Wide Area Networks (WWAN) remains difficult due to unavoidable packet losses (a result of the underlying unreliable and time-varying transmission medium) and unavailability of per-packet, per-user retransmissions (stemming from the well-known NAK implosion problem). Previous cooperative solutions for multi-homed devices listening to the same video broadcast call for local recovery via packet sharing: assuming peers are physically located more than one transmission wavelength apart, channels to the streaming source are statistically independent, and peers can exchange different subsets of received packets with neighbors via a secondary network like ad hoc Wireless Local Area Net work (WLAN) to alleviate individual WWAN packet losses. While it is known that using structured network coding (SNC) to encode received packets before peer exchange can further improve packet repair performance, the decisions of who should send repair packets encoded in what SNC types at available transmission opportunities were not optimized in any formal way. In this paper, we propose a distributed decision making strategy based on Markov decision process (MDP), so that each peer can make locally optimal transmission decisions based on observations eavesdropped on the WLAN channel. Our proposed MDP is both computationally scalable and peer-adaptive, so that state transition probabilities in MDP can be appropriately estimated based on observed aggregate behavior of other peers. Experiments show that decisions made using our proposed MDP outperformed decisions made by a random scheme by at least 4dB in PSNR in received video quality.