# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 3 Source Coding Protection
### Evaluation of Source Coding Protection  
In this section, first a video quality analysis varying the Tile Loss Ratio (TLR) is carried out, and the reconstructed video quality (PSNR) is measured. In addition, then, the average proportion of packets per tile is presented and experiments varying the Packet Loss Ratio (PLR) are carried out. In these experiments, we measure both how the different configurations produce different TLR for the fixed PLRs, and also the final video quality (by means of the PSNR) provided by each configuration. The obtained results will be analyzed to find out the most appropriate source coding architecture that fulfills our goal. The compression performance of the proposed encoding modes and the overhead introduced by tiles and slices have already been evaluated. Now, we will evaluate the robustness of tiles and encoding modes for different TLR values. The combination of the 9 intra-refresh coding modes with the different tile partition layouts (1, 2, 4, 6, 8 and 10 tiles/frm), under a simple probabilistic data loss model, will be evaluated. Six different TLR values have been fixed for the tests: 1%, 3%, 5%, 7%, 10% and 20%. For every one of these loss rates, 5 different seeds for the random number generator have been used, and the results have been averaged. First of all, we have evaluated the EC methods implemented in the decoder. The frame copy method has been always used in order to obtain a reconstructed video sequence with the same number of frames than the original video sequence. Also, the zero-MV technique has been tested and the PSNR differences when enabling this method are always positive. This means that the EC decoder version provides better PSNR values than the version without EC. The PSNR gains range from 0.14 up to 3.02 dB. However, the LP and IPv encoding modes (no intra-refresh) obtain PSNR values below the quality bounds which are considered the minimum acceptable, even when the EC decoder is enabled. As a product of these observations, from this point forward we will always use the EC decoder version for the reconstruction of video sequences. Regarding the error resilience attributes related to the encoding modes and the tile layouts, in Figure 4 the PSNR values obtained for each encoding mode with a 3% and a 10% TLR are shown. Three of the encoding modes show a non-robust response to tile loss (PSNR values under 29 dB are considered low quality values). For LP and IPv this is the expected result, as they have no intra refreshing strategy. However, the bad performance for IPv25 pctCTU was not expected, because in this mode 25% of the CTUs (one out of four) are forced to be intra-coded. Previously, this encoding mode showed the same coding efficiency as IPv25 pctTIL and IPvpattern (which share the same percentage of intra refreshed areas), but it is clear that a random CTU intra refresh does not provide the expected protection against tile losses. Why intra refresh on the CTU level is not 100% effective? The reason is that intra-frame encoding uses pixel information (surrounding the CTU) to compute a prediction, which is used both in the encoding and decoding processes. If the pixels used to compute that prediction belong to inter-coded CTUs whose reference frames are corrupted, then the pixels used for the intra-frame prediction are not correct, and, even if the intra CTU is correctly received, it will be incorrectly decoded. Inversely, if intra refresh is carried out on tile level, as all the CTUs that belong to a tile do not depend on CTUs from outside that tile, then every correctly received intra CTU will be correctly decoded. Intra refreshing on the CTU level does not provide enough bit stream robustness. One of the consistent results throughout all the tile loss percentages tested is that, for a fixed TLR, when the number of tiles per frame increases (smaller tiles), the quality decreases. The reason is that the loss of small parts in many frames causes a worse effect than the loss of one big part in only one frame, even if the data loss percentage is the same.