1 DVB-H: Technical Overview and Design Requirements for Mobile Television Broadcasting Patrik Hummelbrunner and Werner Robitza, University of Vienna Abstract—Digital Video Broadcasting - Handhelds is emerging amongst them time slicing, the Multi Protocol Encapsulation as a digital broadcasting standard for handheld devices since (MPE) and MPEG-2 specifications for DVB-H. In Section 2006. Its main features are derived from DVB-T, the terrestrial III an overview is given on which requirements have to be version, but especially for handheld receivers new features have been added. Time slicing is introduced in order to allow the met in order for a DVB-H service to be highly accepted receiving device to save power due to a time division multiplex- by the viewers, which of course is an issue for mobile TV ing method, MPE-FEC (Multi Protocol Encapsulation Forward vendors. These include technical and content requirements. Error Correction) provides a robust error correction mechanism Also, interactivity in mobile TV is discussed. In Section IV a based on Reed-Solomon Codes to error-protect the sent multi common approach for DVB analysis is presented. protocol encapsulated data packets in an environment where packet loss is highly common. During the last years, several studies have been made concerning the perceived quality of II. TECHNICAL OVERVIEW mobile broadcasts. Especially in a mobile environment it is The work on the DVB-H standard was started in the year necessary to find out which aspects of the presented audiovisual content are critical for a good acceptance from the viewer. Also, 2002 and released by the ETSI in the year 2004 under the EN certain transmission parameters have an impact on visual quality. 302 304 Standard [2]. It is titled “Digital Video Broadcasting It is also important to summarize the most important criterions (DVB); Transmission System for Handheld Terminals (DVB- and design requirements for mobile television platforms and H)”. applications. Recent studies need to be analyzed and compared in The motivation for DVB-H was to improve the energy order to show what has already been achieved by mobile broad- casting companies and vendors in Austria and other countries. related problems of mobile reception that came along with the earlier introduction of DVB-T. Hence, the physical layer of DVB-H is entirely based on DVB-T [3], but it is enhanced by a new transmission mode which is explained later. I. INTRODUCTION The other layers implement techniques specified in other N the year 2004, DVB-H was specified by the ETSI, the standards: In [4] time slicing and the use of MPE are I European Telecommunication Standards Institute. Today, specified, both methods of DVB data broadcasting. [5] is the it is one of the most common mobile TV technologies of corresponding standard for the MPEG-2 Transport Stream, the whole world, with millions of receivers sold in over one which is used at the data link layer of DVB. In [6] details hundred countries [1]. DVB-H has solved many problems are given about which MPEG-2 information tables are new related to the energy consumption issues of DVB-T and to DVB-H. highly improved the mobile reception of audiovisual content. Behind DVB-H is a consortium of over 270 partners: from the telecommunications industry to the vendors themselves. A. Protocol Layers As mobile television deployments begin to take place all over Europe, more and more customers become test clients for In DVB-H, a protocol stack shall be used according to [7]. a technology that has not yet achieved all goals that may have It is similar to the common ISO/OSI reference model [8], been set in advance. Primarily, the problems lie in the quality although these two models can not be matched perfectly. DVB of the video content and the tune-in and channel switching data broadcasting specifies four methods of delivering content times, which range from half a second to multiple seconds. [4], but only one, MPE, is of interest in this paper. Although receiving devices become smaller and more energy- Generally, in DVB-H with MPE broadcasting, any kind efficient with each generation released, customers are faced of application data is encapsulated in IP packets, optionally with issues like energy saving and perception problems due to being streamed through RTP and UDP beforehand. These the small sized device screens and the service access times. IP packets are then encapsulated again using the MPE This paper is organized as follows: The most important technique at the data link layer. On the same layer the technical features of DVB-H are presented in Section II, content is time sliced. The resulting data is multiplexed into an MPEG-2 transport stream, together with other streaming Patrik Hummelbrunner and Werner Robitza are a students at the Faculty services such as DVB-T content. Therefore, DVB-H and of Computer Science since 2007 and project researchers at the Depart- ment of Distributed and Multimedia Systems since 2009. e-Mail: patrik@ DVB-T content can be distributed simultaneously by a single hummelbrunner.net and [email protected] DVB-T transmitter, using the same physical layer, at which the transport stream is modulated for OFDM (Orthogonal 1 Application Data Table 191 1 RS Table 65 Frequency Division Multiplexing) transmission. 1 gram 2 IP-Datagram 1 B. Application Data ... Padding RS-Data The DVB-H standard itself does not specify the type of IP-Datagram 3 application data that can be used. In fact, any kind of binary bitstream could be possibly sent to the receivers. Although so IP-Data- called data carousels are defined for distributing downloadable Number content over DVB-H [4], they are not used in real implemen- of Cols tations. The most common application data type for DVB-H is video content, though. Figure 1. The MPE-FEC frame As for the video, the MPEG-4 AVC / h.264 codec has to be used, since it is optimized for mobile content shown at small screen resolutions and much more bit-rate efficient than its D. Multi Protocol Encapsulation and FEC predecessor MPEG-2 [1]. There exist technical specifications The data link layer introduces the key technologies to DVB- for DVB-H which define certain profiles and levels for the H [4]: Multi Protocol Encapsulation adds another abstraction video streams [9]: For Standard Definition Television (SDTV) layer by simply encapsulating the IP packets into a so called video, the main profile shall be used (level 3) - for High MPE section. Its header though provides new fields that are Definition Television (HDTV), the high profile is used instead currently allocated for real time transmission parameters that (level 4). It is also implied for HDTV to use a 16:9 aspect are needed by the receiver. Also, MPE works as a translator ratio instead of 4:3, which is currently used for most other between addressable IP data and the underlying DVB structure video content available. [11]. Analyzing the perceived quality of mobile television often At the same layer, a new forward error correction mech- takes place in the application layer, since the effects of anism is used, called MPE-FEC. While the physical layer transmission errors become visual (i.e. in forms of blocking already implements two FEC stages, the third one is especially or frame losses). There are many quality assessment metrics suited for mobile receiving conditions with high packet loss for video, PSNR (Peak Signal to Noise Ratio) being one of probabilities. the most common. However, PSNR values can not perfectly In MPE-FEC, frames are created for allocating the data bits describe whether the quality of a video is acceptable for a (see Figure 1). These frames are divided into 255 columns and human being or not. In the second part of this paper, more a variable number of rows limited to a maximum of 1024. on this topic is explained. Hence, the maximum FEC frame size equals 2 MBit. The frame is also divided into an application data table and a RS- data table. The application data table is filled column-wise C. Streaming with the IP-packets. If one datagram is larger than a single column, its remaining bits are inserted into the next column. In order to distribute the video content to a large number of At the end of the table, padding bytes can be inserted. This is receiving devices, it is streamed via IP broadcasting or multi- the case when an IP packet would not fit into the frame as a casting, the latter meaning the IP addresses of the devices can whole. be limited. As for most of streaming applications today, RTP is Then, a Reed Solomon code (using the input parameters used at the application layer. The RTP packets are encapsulated 255,291) is calculated row-wise and filled into the RS-data by UDP, since in DVB-H, there is no feedback channel table. The redundant RS data allows the decoder to correct that would allow a two-way connection between sender and multiple erroneous bits in the application data table. The MPE- receiver (which is necessary for the TCP handshake). FEC frame is now transmitted row-wise by forming MPE MPEG-4 AVC / h.264 introduces a new technology that sections, which results in an interleaving effect, achieving facilitates mapping video data to RTP packets compared an even stronger error protection against bit errors during to MPEG-2 video: The NAL (Network Abstraction Layer) transmission. generates NAL Units from the VCL (Video Coding Layer) The usage of MPE-FEC is entirely optional in DVB-H, that can be mapped directly to an RTP packet.