# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 3 Source Coding Protection  
In this section, we will introduce and evaluate the first of the two complementary approaches proposed in this work, which is based on source coding techniques. The main goal is to increase the robustness of the video streaming by properly configuring some of the features provided by the selected video encoder. So, the robust bit stream will better fight against packet losses resulting in better perceived video quality. First of all, we will evaluate the proposed techniques in the absence of data losses. This is done to measure the compression efficiency of the different alternatives (i.e., which configuration generates a lower bit rate at the same quality level). This allows us to characterize the configurations in ideal conditions or, equivalently, in soft environments where the data losses can be completely neutralized with the FEC mechanisms proposed in Section 4. Therefore, to determine the configuration of the proposed approaches, in Section 3.1, we will analyze the available HEVC frame partitioning schemes (slices and tiles), proposing a novel hybrid approach which we call tileslices, to better fight against video delivery errors. In Section 3.2, we will propose the use of 9 different intra-refresh coding modes which will be in charge of stopping the error propagation effect found in video inter-prediction coding. In Section 3.3, a simple error concealment approach will be introduced in the receiver side to hide the visual artifacts produced by data losses when rendering the video to the user. Finally, in Section 3.4, a performance evaluation of all the above proposals is done, using a simple probabilistic error model.  
Now, we will briefly introduce some concepts about the HEVC video encoding process (for a good overview refer to [16]). HEVC is the most recent video coding standard developed by the Joint Collaborative Team on Video Coding (JCT-VC). Some works confirm that the real efficiency gains over its predecessor (H.264/AVC) range from 30 to 45%. This \"profit margin\" is generally used to encode videos with lower bit rates, but we may use the HEVC bit rate savings to strengthen the encoded video bit stream, producing a much more robust video stream at the same bit rate than H.264/AVC.  
In the encoding process, each frame is divided into small square regions (usually, a 64 \(\times\) 64 pixels area). Let's call them blocks. These blocks can be encoded using one of three modes: (a) without any prediction, (b) using spatial prediction, or (c) using temporal prediction. Spatial prediction exploits redundancy within a single frame. In order to encode a block, it uses previously encoded regions of the same frame to search for similar pixel information to create a prediction. This prediction is subtracted from the current block to obtain the prediction residual. This method is also called intra-frame prediction. Temporal prediction uses previously encoded frames to estimate a block prediction by means of the search of a similar block in other frames (called reference frames), which have been previously encoded, decoded, and stored in a buffer. It exploits temporal redundancy, taking advantage of the fact that nearby frames usually contain blocks that are very similar to the current block and so the residuum of the motion compensation is close to zero. This method is also called inter-frame prediction.