# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 3 Source Coding Protection
### Intra Refresh Coding Modes  
In the previous tests, two encoding modes have been used: All Intra and Low-delay P modes. AI offers better error resilience properties than LP because an error in one tile does not propagate to other frames. On the other hand, LP mode is very sensitive to lost tiles, because P frames are \"infected\" by erroneous reference frames, and these infected P frames, when used as reference frames, \"spread disease\" until the end of the sequence. There is also an unpredictable reaction in LP mode in the presence of packet losses. For example, when an LP encoded sequence loses the last frame, then the error affects only that frame. However, if the first frame (I) gets lost, then no frame can be correctly reconstructed at all. So, the position of the missing parts is also important when dealing with errors in LP mode. These two encoding modes have been determined as our upper and lower bounds regarding error resiliency. In this work, 7 new encoding modes with different intra-refresh rates have been proposed and evaluated. Intra refresh is an error resilience technique that forces to periodically intra encode (refresh) certain frame areas in P frames, in order to stop the error propagation mentioned before. The 9 encoding methods evaluated are the following ones:  
* AI--Encodes every frame as an I frame. It is considered our upper bound mode. * LP--Encodes the first frame as an I frame and the rest of the frames as P frames. Four previous reference frames are available for temporal estimation and compensation for each P frame. It is considered our lower bound mode. * IPA--Similar to LP mode but every P frame uses only the previous frame as reference frame, instead of having 4 reference frames. * IPAx25 pctCTU--Similar to IPA mode, where 25% of the CTUs are forced to be intra refreshed. * IPAx25 pctTIL--Similar to IPA mode, where 25% of the tiles are forced to be intra refreshed. * IPApattern--Similar to IPA mode, where 25% of the tiles are forced to be refreshed following a specific pattern, which covers all the tiles of a whole frame every 4 frames. * IPPP--Similar to IPA mode, with an I frame inserted every 4 frames. * LPI4--Similar to LP mode, with an I frame inserted every 4 frames. * IPIP--I and P frames are inserted alternatively. Every P frame uses the previous I frame as reference. The 9 encoding modes can be classified into four groups, depending on the whole-frame refreshing rate: (a) LP and IPx modes only have an I frame at the beginning of the sequence, so no refreshing is performed (0%); (b) IPPP, IPx25 pctCTU, IPx25 pctTIL, IPxpattern, and LPI4 modes have an average  
Figure 2: Average BD-rate for different frame partition layouts (slices/tiles). frame-refresh rate of one intra frame every four frames (25%); (c) IPIP mode has an intra frame-refresh rate of one frame out of two (50%); (d) AI mode uses a full rate of intra-refreshed frames (100%). For the following tests, we have selected the BasketballDrill sequence (832 \(\times\) 480 pixels, 25 fps, bbd_25), which exhibits an average coding performance among the 832 \(\times\) 480 sequence set. In Figure 2(a), the resulting rate/distortion curves for the 9 encoding modes (for a very wide range of QP values) are plotted. LP mode results the most efficient mode of all, and AI the least efficient. For the evaluations carried out in Section 3.4 regarding the performance of the 9 encoding modes under packet losses, we have selected an individual QP value for every mode so that they generate a bit stream with a similar bit rate (fair conditions). The QP values used for each encoding mode with the resulting bit rate and PSNR values are listed in Table 2. Figure 2(b) is a zoomed version of Figure 2(a), which shows the curves at the selected QP values. The curves corresponding to encoding modes belonging to group (b) exhibit very similar coding efficiency.