# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 3 Source Coding Protection
### Tileslices, a New Frame Partition Proposal
P frames are mainly formed by inter-coded CTUs, i.e., CTUs that are encoded by using motion estimation and compensation using other frames as reference (temporal prediction). A P frame can also contain intra-coded CTUs (e.g., if the encoder estimates that those CTUs are more efficiently encoded in an intra way). LP mode is much more efficient than AI mode as it generates a much smaller bit stream for the same quality level, but a simple error in a frame is propagated until the end of the sequence, even if the rest of the frames are correctly received. The compression rate of both modes can be selected by means of the Quantization Parameter (QP). This parameter is used by the quantization process, which entails the loss of precision in the transformed coefficients (lossy compression). When high QP values are used, the generated bit stream has a low bit rate and, correspondingly, low visual quality. When low QP values are used, the generated bit stream has a high bit rate with a resulting high visual quality. For the analysis of HEVC, 4 values for the QP have been used (22, 27, 32 and 37) as it is specified in the common test conditions. In Figure 2, Bjorntegaard Delta Rate (BD-Rate) [19] average values of both sets of source video sequences are shown when using 2, 4, 6, 8 and 10 tiles or slices per frame, both for AI and LP modes. BD-Rate is a measurement that shows the relative bit rate increase (or decrease) of one encoding proposal with respect to another one, chosen as reference. For the low resolution source videos (416 \(\times\) 240), dividing a frame into 8 slices is not possible, so, we have decided to use 7 slices per frame instead. We have placed and asterisk (*) in the figure legend to remind this fact. Except in this specific case, the overhead of tile partitions is lower than the one experienced with slice partitions. This is the reason the tile (tileslice) partition will be the favorite frame partition method to protect the video bit streams. The maximum overhead reduction, around 3% less overhead than slices, is obtained with 10 tiles per \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Video Sequence** & **Acronym** & **Resolution** & **FPS** \\ \hline BasketballDrill & bbd\_25 & 832 \(\times\) 480 & 25 \\ BQMall & bqm\_30 & 832 \(\times\) 480 & 30 \\ Flowervasse & fls\_30 & 832 \(\times\) 480 & 30 \\ Keiba & ke8\_30 & 832 \(\times\) 480 & 30 \\ Mobisode2 & mo8\_30 & 832 \(\times\) 480 & 30 \\ PartyScene & psc\_25 & 832 \(\times\) 480 & 25 \\ RaceIforses & rh8\_30 & 832 \(\times\) 480 & 30 \\ \hline BasketballPass & bbp\_25 & 416 \(\times\) 240 & 25 \\ BlowingBubbles & blo\_25 & 416 \(\times\) 240 & 25 \\ BQSquare & bqs\_30 & 416 \(\times\) 240 & 30 \\ Flowervasse & fld\_30 & 416 \(\times\) 240 & 30 \\ Keiba & ke4\_30 & 416 \(\times\) 240 & 30 \\ Mobisode2 & mo4\_30 & 416 \(\times\) 240 & 30 \\ RaceIforses & rh4\_30 & 416 \(\times\) 240 & 30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Video sequences used in the tests. frame in the 832 \(\times\) 480 video sequences for both LP and AI coding modes. As seen, by using tileslices instead of slices we obtain the same level of error resilience and a better compression performance.