# Error Resilient Coding Techniques for Video Delivery over Vehicular Networks
## 3 Source Coding Protection
### Tileslices, a New Frame Partition Proposal  
Slices are regions of an encoded frame that can be independently decoded, regarding other slices of the same frame. Slices are not a new concept in HEVC and were used in previous standards. The purpose of slices is providing error resilience to a frame. If one slice of a frame gets lost, then the rest of the slices of the same frame can be correctly decoded and only the region covered by the missing slice is lost. Slices consist of correlative blocks (Coding Tree Units--CTUs) in raster scan order (see Figure 1a). The main disadvantage of dividing a frame into slices is that the coding efficiency is reduced because of (a) the loss of prediction accuracy, since the information in neighbor slices is not available, and (b) the overhead introduced by slice headers in the bit stream. One of the new features introduced in HEVC is the ability to divide a frame into rectangular regions, called tiles (see Figure 1b). The original aim of using tiles is to take advantage of parallel computing, because they are independently decodable regions of a frame. Tiles are not useful for error resilience purposes, because if one tile of a frame is missing, the whole frame cannot be decoded. As they do not have headers, and due to their rectangular shape, tiles are more efficient than slices. In the search for robust video streaming techniques, while attempting to keep the overhead introduced by slices low, we have devised a new element that we call tileslice. A tileslice is the combination of one tile and one slice. First, one frame is divided into tiles (Figure 1b), and then after encoding all the CTUs of each tile, they are \"inserted\" into the syntax of a slice, including the slice header (as it is shown in Figure 1c). In this way the tileslice keeps its rectangular form (which is more efficient than typical slices), and also it behaves as a slice (so missing tileslices do not prevent the correctly received tileslices from being decoded). This combination is possible in the definition of the HEVC standard, so it can be used in practice. However, although it is a very simple idea, there are no previous works, to the authors' knowledge, that use this particular combination of slices and tiles. As tileslices contain slice headers, overhead introduced by them is not zero. So, is it really worth using tileslices instead of typical slices for error resilience purposes? To answer this question, we have  
Figure 1: Representation of a frame divided into five slices (**a**); a frame divided into four tiles (**b**); and the encoded bit stream for a frame divided into slices, into tiles or into tileslices (**c**). compared the overhead introduced by slices and tileslices. From this point forward we will use the term tile to refer to what we have defined as tileslice, i.e., one slice that contains one rectangular tile. We have compared the compression efficiency of tiles against slices for six different layouts: 1, 2, 4, 6, 8 and 10 slices (or tiles) per frame, using fourteen video sequences, of the HEVC \"common test conditions\" [17]. They are enumerated in Table 1. Seven of them have a resolution of 832 \(\times\) 480 pixels and, the other seven, have a resolution of 416 \(\times\) 240 pixels. Each sequence has a frame rate of 25 or 30 frames per second (FPS). The two encoding modes used for the HEVC evaluation are All Intra (AI) and Low-delay P (LP),which are included in the HEVC reference software [18]. In AI mode, every frame of a video sequence is encoded as an I (intra) frame, so temporal prediction is not used at all. This mode has inherent error resilience properties because errors do not propagate to other frames. The main drawback of this mode is that it produces a bit stream with a high bit rate. In LP mode, the first frame of the sequence is encoded as an I frame, and then P (predictive) frames are generated for the rest of the sequence.