## 3 Related Work
### Packet Scheduling for Video  
In [43], the authors combined scheduling at the transmitter with control of playout speed at the receiver, across a time-varying wireless link. This proposal required video-content-dependent decisions with the aim of maximizing video quality at the receiver. Decisions were driven by a Markov process informed by the motion activity of the transmitted video frames. In the scenario of [43], the scheduler discards late packets and catches up with delayed playout during poor wireless channel conditions. The receiver adapts its playout rate (the rate video frames are displayed at), slowing it down when playing video with low motion and, hence, low coding complexity occurs. The aim was to avoid playout interruptions, which are thought to be less disconcerting to the viewer than small slowdowns in the playout rate. The authors concluded their research by identifying the need for practical heuristics that reduce the complexity of their proposed schemes due to the need to inspect the content type, which remains a problem for real-time operation and is the specific problem our simplified scheme seeks to address.  
The optimal scheduling decision is again computed through a Markov process in [44]. However, the authors of [44] are aware that burst errors are a threat and, in particular, may disrupt feedback signals. Therefore, modeling is designed to take into account incomplete information about the receiver's state. The end result is improved video quality but delays to video packet transmission during poor channel states. In this early work, the impact on playout delay was not taken into account and the authors also acknowledge the need to develop heuristic decision-guiding rules.  
In [45], the video content is taken into account to vary the packet scheduling decision at the transmitter. Inspection of the video content does imply an increase in end-to-end delay, as does the need to await protocol acknowledgments. The position of a packet's data within a Group-of-Pictures (GOP) weights its importance to the playout process and, hence, determines its transmission schedule deadline. Similarly, the texture- and motion-complexity governed scheduling decisions when adapting the packet transmission order. Simplified wireless conditions were assumed to test the scheme. However, the authors note that channel coding is insufficient in itself during \"bursty\" error conditions, requiring intelligent ARQs and/or packet scheduling. The result of packet scheduling was improved objective video and a more even distribution of temporal distortion. Data partitioning was also turned on in some of the evaluation tests but with the Moving Picture Experts Group (MPEG)-4 part 2 form of data partitioning, in which each video packet is internally divided between shape/motion data and less important texture data.  
The work in [46] is aimed at IEEE 802.11 Wireless Local Area Networks (WLANs) running the Distributed Coordination Function (DCF) at the MAC layer, _i.e._, retransmitting data packets that are lost. By determining the importance of the video data within a packet, differing re-try limits occur. The codec includes the importance of the video data as side information with the video data. However, to achieve significant gains in video quality requires cross-layer intervention in the wireless protocol stack as well as modification of the video codec. Research in [47] introduced another scheduling scheme that relies on video content side information placed in advance within the bitstream by the encoder. It is designed in a practical fashion for a multi-user, Time Division Multiple Access (TDMA) wireless environment such as High Speed Downlink Packet Access (HSPDA).  
Long Term Evolution (LTE) leaves the choice of packet scheduler to the vendor, resulting in many research proposals [48]. These are system-level schedulers, of which [49] combines the popularproportional fair algorithm with a higher-level scheduler that allocates the amount of data that each real-time flow can send within one time epoch.