IST Logo
Home arrow Overview arrow Interoperability
Mobile Television

A one-page summary of the mobile television systems studied within MING-T.

Over the last few years the world has been making the transition from analogue television services to digital television services. Typically, the television services have been consumed in a static environment, i.e. within the home. The terrestrial digital television services have typically been defined on a regional basis. More recently there has been considerable interest in mobile digital television services whereby content can be consumed on a much broader range of devices and under a wider spread of operating conditions. These new conditions have meant that new standards have had to be developed to support these enhanced operating conditions. The resultant mobile digital television standards have come from a range of different sources:
  • Existing digital television standards: DVB-H has been derived from DVB-T and adds extra error protection to mitigate effects of the mobile channel as well as time slicing to reduce the operating power of a DVB-H receiver. ISDB-T is another mobile standard that has been adapted to a mobile environment.

  • Existing digital radio standards: T-DMB, the standard deployed in Korea, has been adapted from the digital radio standard, DAB. DAB already has time-slicing as, at its inception, it was planned to be used in a mobile environment. However, basic DAB’s error protection is insufficient for video services and T-DMB adapts DAB by adding additional error protection.

  • Emerging standards: FLO is a new standard being proposed by Qualcomm through the FLO Forum and is expected to be widely deployed within the USA. DMB-T is a Chinese digital TV broadcast standard that is expected to be deployed within China. Both of these initiatives have the advantage that they can take advantage of the latest advances in signal processing techniques, such as Turbo decoding and LDPC for example, to improve the basic performance of the system. More importantly these new standards are being designed from the outset to deal with mobile and static environments so their terms of reference include all the necessary conditions.

Image

As broadcasters typically operate at a national level, content is likely to follow at this level; worldwide roaming is therefore unlikely to be required across all standards – except for some well-known content brands such as CNN or BBC News which would be available worldwide. The genesis of these different standards has meant that in some regions of the world, as shown in the above figure, multiple standards may exist within a single region, examples of this are: the USA which will support both FLO and DVB-H, China with T-DMB, DVB-H and DTMB, and the UK with T-DMB and DVB-H; Figure 2 represents the current best estimate of the deployment of standards which is likely to evolve over time. These multi-standard regions may adopt different standards on a geographic basis as Germany is doing. In any case, for these multi-standard regions, some degree of interoperability will be required between the different mobile digital television standards within a region to enable consumers to have a consistent experience. The MING-T project will consider physical interoperability between different mobile broadcast standards.

Image

A typical multi-protocol mobile terminal, like the Sagem prototype shown in the above picture, supports GSM and UMTS for mobile communication, includes a megapixel camera for photos or video recording, but also includes a DVB-H receiver module for mobile video.

Within each of the mobile broadcast and network technologies mechanisms have already been defined to support handover between different transmitters of the same standard. The introduction of the multi-standard broadcast technologies within a single region introduces another layer of complexity as handover is now necessary between different broadcast technologies. This handover brings with it a number of different problems that will need to be overcome in order to give consumers a seamless experience:

  • Handing over between different broadcast technologies. The DXB project is looking at the issues associated with providing a common application interface to T-DMB and DVB-H broadcast technologies. The project does not encompass providing handover between the technologies. However handover at the physical layer is a basic requirement.

  • The capacity and protection afforded to content varies by standard. There are two effects that must be considered by content decoders:

    • The channel error properties will differ between the standards and consequently the decoding algorithms may need to adopt different error concealment strategies to assure consistent quality.

    • The channel capacities are different and so the decoders will, as a user moves between broadcast technologies, need to adapt to the differing channel capacities whilst minimizing any quality changes.

The MediaFLO standard developed over the last three years by Qualcomm is a good example of a system which has addressed a number of these issues. With FLO, the basic physical layer has been optimized for use in a mobile environment and implements hierarchical encoding of the data so that, for example, coarse detail is passed through the more robust lower level modulation scheme and fine detail is passed through a higher level modulation scheme which may or may not be used by the decoder depending upon the error rates received. The MediaFLO video decoder itself uses an H.264 base profile for the low-level video information and then uses a proprietary enhancement layer, transmitted in a higher level modulation scheme, to provide the fine detail and higher frame rate information. The MING-T project addresses these handover issues concentrating especially on DVB-H and DTMB.

 

The DTMB Standard

The following items summarize key parameters and features of the recent Chinese DTMB system, whose draft standard appeared in Aug. 2006:

  • The core technique of DTMB is Time Domain Synchronous Orthogonal Frequency Division-Multiplex (TDS-OFDM) modulation, at the same time it also implements better FEC (that is LDPC code) and adopts hierarchical modulation structure for different services. At the physical layer, it

  • provides a very flexible channel coding rate with the combination of different modulation constellations to achieve a wide range of bit rates (5 - 33 Mbit/s).

  • carries the MPEG-2 Transport Streams containing any combination of video, audio and data. In section 3, we will propose our method utilizing the properties of MPEG 2 to transmit a data packet.

  • chooses the PN sequence as the guard interval of the OFDM symbol to achieve much quicker synchronization (time domain processing). This is very important for the packet switching at a high transmission rate. A similar idea can be found in the WLAN system.

  • allows IP multicast and unicast on top of MPEG-2, which lays the foundation of the interactive services. Especially in the IPv6 system, every digital device connected to this network can be assigned an IP address, DMB-T devices can therefore use the IP network for multicasting and unicasting.

  • can work for very high-speed (more than 130km/h) mobile reception with less than 1e-10 BER.

The DTMB system is capable of providing services with both high data rate and high mobility, and the integration of DTMB and communication networks will allow users to access the Internet with a high data rate at high-speed mobility. While most wireless access technologies work bi-directionally, DTMB is uni-directional and can only provide wideband data services in the downlink along with the video services, which is not suitable for the bi-directional IP transparent data transmissions. However, DTMB can work together with other systems as a return signalling channel (uplink) to accommodate the users’ request and acknowledgement. Systems such as cellular mobile radio systems (GSM/GPRS, UMTS), PSTN, ISDN etc. can all serve for this purpose. We may call it Integrated Communication Broadcasting Network (ISBN).

 
< Prev   Next >
© 2017 MING-T   Legal Notice