# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 2 Related Work  
Several works can be found in the literature about the challenges of video streaming over VANETs. In [4], the authors enumerate some of the video delivery problems in VANETs (and the solutions proposed by other authors), like link connectivity, error resilience, clustering, and multihop routing. In [5], the authors address the issues of video streaming over urban VANETs. They use Global Mobile System Simulator (GloMoSim) to conduct IEEE 802.11p standard-based vehicular network simulations. They use the H.264/Advanced Video Coding (AVC) codec and some of its error resilience features, such as Flexible Macroblock Ordering and Redundant Frames to protect the encoded bit stream. In our work we use the HEVC video encoder. HEVC is known to outperform H.264/AVC compression efficiency, so by using efficient video encoders, like HEVC, we may save bandwidth or get better video quality at the same bit rate. Some works have evaluated the robustness and suitability of HEVC for video streaming over wireless networks (in the presence of data loss). In [6], the author compares HEVC with H.264/AVC in wireless environments. He uses one video sequence to evaluate HEVC under packet loss rates that range from 0 to 40% (in steps of 10%) using channel conditions borrowed from [7]. He uses Decoded Frame Rate Metric (DFRM), Structural SIMilarity (SSIM), and Peak Signal-to-Noise Ratio (PSNR) metrics to evaluate the transmitted video quality. His main conclusion is that HEVC is more resilient to errors than H.264/AVC under these conditions. In our work, as well as doing evaluations at different packet loss rates, we have used network and traffic simulators which model the network behavior in a realistic way. In [8], the authors have developed a complete framework for testing HEVC under different packet loss rates, bandwidth restrictions, and network delays. This framework first generates a trace file with the bit stream information and then uses this trace file for the streaming simulations. On the receiver side, a log file is created that gathers the transmission results (indicating the missing packets). In the final stage, the framework decodes the complete bit stream (without any loss) and then overrides the areas corresponding to the missing packets (which are tagged in the log file). The authors use this framework to evaluate concurrent multipath transmission in multihomed mobile networks. The above works ([6,8]) \"emulate\" the behavior of the HEVC decoder in the presence of data loss, by decoding the complete bit stream, and after that, removing the video parts that correspond to the missing packets. This is not the real behavior of the HEVC decoder because, by proceeding in this way, the decoder can use all the information present in the original bit stream for the decoding step and this is not accurate because some pieces of data depend on others (which may be missing). On the contrary, in our work we have used the real HEVC reference software for the decoding of damaged/incomplete bit streams. For this task we have modified the decoder to make it robust against packet loss. In [9], the authors use the HEVC codec to encode video sequences in order to evaluate several flooding schemes for soft real-time video transmission in VANETs. The target application could be the timely delivery of video that was recorded by the vehicles involved in an accident. This type of information may be useful for the surrounding vehicles and also for public services, which may be located far from the accident site. They evaluate both the packet arrival ratio and PSNR value of the reconstructed video sequences. In [10], the authors do some real experiments of vehicular video transmission, by using several digital tablets and their WiFi connection. In one of the scenarios, two vehicles exchange video meanwhile they are connected (in coverage), and in the other scenario a person standing in the sidewalk (acting as a road side unit) sends video to a passing by vehicle.