Handheld screen resolution
Broadcasters initially supported the Quarter VGA (QVGA, 320 X 240 pix- els) standard, while cellular phone carriers supported Quarter Common Intermediate Format (QCIF, 176 X 144 pixels). DVB-H solves the rift between them. When the user watches a program on a mobile phone, there will be two types of content on the mobile phone screen: a broadcast program (such as a sport or drama) by a broadcast service provider, and custom data relevant to the program (such as online shopping information) prepared by a telecom carrier.
DVB-H system properties
The main properties of DVB-H are: time-slicing, IP interfacing, enhanced sig- nalling and in-depth interleaving. In order to save power, a power-saving algo- rithm based on time division has been introduced. The technique, called time slicing, results in a large battery power saving. In order to provide a common platform with Internet services, and for reliable transmission in poor signal- reception conditions, IP interfacing with an enhanced error-protection scheme was developed. This scheme is called multi-protocol encapsulation—forward error correction (MPE-FEC). It employs powerful channel coding on top of the channel coding included in the DVB-T specification and offers a degree of time interleaving. Furthermore, the DVB-H standard features an extra network mode, the 4K mode, offering additional flexibility in designing single-frequency networks (SFNs) which still are well suited for mobile reception, and also pro- vides an enhanced signalling channel for improving access to the various serv- ices. Convergence with Internet services is accomplished by internet protocol (IP) encapsulation of Internet services prior to the transport multiplexing stage.
Time-slicing
A special problem for DVB-H terminals is the limited battery capacity caused by the relatively high power consumption of a DVB-T front end which is in the region of 600–1000 mW. You will recall from Chapter 8 that before any one of the multiplexed elementary streams of the selected pro- grammes can be accessed, the whole data stream has to be decoded first. A large part of the power consumed by the front end is therefore unnec- essary. The power-saving made possible by DVB-H is derived from the fact that essentially only those parts of the transport stream which carry the data of the service currently selected have to be processed.
In order to do this, the data stream needs to be reorganized in a suitable way for that purpose. With DVB-H, several services are multiplexed using pure time division. The data of one particular service are therefore not transmitted continuously but in compact periodical bursts with interrup- tions in between. At the transmitting end, several services with differ- ent bit rates are multiplexed and a continuous, uninterrupted transport stream at a constant bit rate (CBR) is maintained.
To indicate to the receiver when to expect the next burst, the time to the beginning of the next burst is indicated within the burst. Between the bursts, data of the elementary stream is not transmitted, allowing other elementary streams to be transmitted using the remaining bandwidth. Time slicing enables a receiver to stay active only a fraction of the time, while receiving bursts of a requested service.
Bursts entering the receiver have to be buffered and read out of the buffer at the service bit rate. Practically, the duration of one burst is in the range of several hundred milliseconds whereas the power-save time may amount to several seconds. Depending on the ratio of on-time/power- save time, the resulting power saving may be more than 90%.
Time slicing offers another benefit for the terminal architecture. The comparatively long power-save periods may be used to search for chan- nels in neighbouring network cells offering the same service but better reception. This is important as the handheld receiver movement may take the user from one network cell to another. In this way, a channel handover can be performed at the border between two cells which remains imper- ceptible for the user.
IP interfacing
In contrast to other DVB transmission systems which are based on the DVB transport stream adopted from the MPEG-2 standard, the DVB-H system is based on IP. The IP operates at Layer 3 (Network) of the seven- layer OSI model (for details of the 7-layer model, refer to Appendix A5). In the preceding layer (Transport Layer 4), two types of protocols are available: a unicast (one-to-one) Transmission Control protocol (TCP) and multi-cast UDP. TCP is a ‘reliable’ connection-orientated service which ensures that a connection is made and an acknowledgment is received before data is exchanged. In contrast, user data protocol (UDP) is an ‘unreliable’ connectionless service which sends out messages regardless of a connection being established. For the purposes of DVB-H,
UDP is used which is sent datagram packets to Layer 3 for IP encap- sulation. Layer 3 precedes the bottom two layers (Data Link and Physical layers) which incorporate the channel decoder (Figure 24.1). The IP encap- sulated packet from Layer 3 is fed into the channel decoder as just another elementary stream to be multiplexed with other elementary streams from MPEG-2 broadcast services to form the MPEG-2 transport stream.
The IP interface allows the DVB-H system to be combined with other IP-based networks. This combination is one feature of the IP Datacast sys- tem. The manner in which the IP data is embedded into the transport stream is carried out by means of data piping technique known as multi- protocol encapsulation (MPE).