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Multimedia services to the handset
Ulrich Reimers, Managing Director, Institute for Communications Technology, Braunschweig Technical University, Germany

In many countries, the decision to select DVB-T for terrestrial television was based on some very specific features of the DVB-T standard.

Theses features enable the use of DVB-T outside the typical living room environment, among them the possibility to also receive broadcast services with portable devices and even in cars and trains traveling at high speed [1].

In consequence, the benefits of such a terrestrial broadcast system attracted the interest of the mobile communications industry. In particular, the ability to reach mobile terminals via a wireless point-to-multipoint link, in connection with a wide geographical coverage and high transmission capacity are features which sparked the interest of this industry. DVB responded to the industry interest by specifying a new transmission standard: DVB-H (Digital Video Broadcasting – Transmission System for Handheld Terminals). DVB-H is the latest development within the set of DVB transmission standards. Work on the technical specification started in autumn 2002 and was finalized in February 2004. The DVB-H set of standards was published by ETSI (European Telecommunications Standards Institute) in November 2004 [2].

DVB-H 

The DVB-H technology is a spin-off of the DVB-T standard. It is to a large extent compatible with DVB-T but takes into account the specific properties of typical terminals, which are expected to be small, lightweight, portable and – very importantly – battery-powered. DVB-H can offer a downstream channel at high data rate which can be used stand-alone or as an enhancement of mobile telecommunications networks which many typical handheld terminals are able to access anyway. DVB-H thus creates a bridge between the classical broadcast systems and the world of cellular radio networks. The broadband, high-capacity down-stream channel provided by DVB-H features a total data rate of several Mbit/s and may be used for audio and video streaming applications, file downloads and for many other kinds of services. The system thereby introduces new ways of distributing services to handheld terminals, offering greatly extended possibilities for content providers and network operators.

Among the various requirements which need to be fulfilled by a broadcast service addressing handheld terminals the following are of special relevance: due to the fact that handheld terminals operate from batteries with a very limited capacity, the DVB-H front-end needs to consume as little power as possible; the need to operate with small, possibly built-in antennas calls for a very robust transmission signal; and reception needs to be possible at very high speeds of travel.

Fig.1: Possible split of the capacity of one DVB-T channel between three TV programnes and an additional eight DVB-H services

Fig. 1 shows the time slicing feature of DVB-H by way of an example. Time slicing enables significant power savings in the receiver relative to the power consumption of a DVB-T receiver. What is shown is the data flow in a DVB-T channel as a function of time. The parameters used to set up this channel are typical for the use of DVB-T in Germany where a DVB-T channel delivers some 13.2 Mbit/s. 

In fig. 1 it is assumed that about three quarters of the DVB-T data rate is allocated to three TV programmes. The remaining 3.2 Mbit/s are used for DVB-H services. In contrast to the TV programmes which require a continuous flow of data, the DVB-H capacity is divided into eight individual services each occupying what is called a ‘time slice’ in the DVB-H terminology. The service data rate available for each of the eight services is easily calculated as 3.2 Mbits/8 = 0.4 Mbit/s. During each time slice 2 Mbit are transported to the receiver which in consequence means that the duration of the time slice is 2/3.2 = 0.625 s. A receiver tuned to service no. 1 can go into power save mode for the period of time during which services no. 2 to 7 are being transmitted. In the example shown this is equivalent to 4.375 s. Taking into account a certain wake-up time required to turn the receiver on again (for example 0.2 s) the ratio of the power consumption of the time slice receiver and that of a receiver turned on continuously can be calculated as (0.2 s + 0.625 s) / (8 * 0.625 s) = 0.17.

In order to enhance the robustness of the signal even beyond the excellent results achieved with DVB-T, the DVB-H system uses an additional level of forward error protection called Multi Protocol Encapsulation-Forward Error Correction (MPE-FEC) which operates as follows. The base band input of a DVB-H modulator accepts Internet Protocol (IP) packets. They are stored in an interleaver matrix column by column. Across the rows of the matrix a Reed-Solomon code is calculated. The resulting content of the interleaver matrix is transported in one time slice. 

DVB-H is a fully proven technical system which was carefully tested by the DVB-H Validation Task Force in the course of 2004. In contrast to other technical systems addressing similar application areas the results of the tests are publicly available.3 Network equipment and receiver equipment interoperability were both explored, in all network configurations (i.e. MFN / SFN, hierarchical / non-hierarchical) and all transmission modes, including the DVB-H 4K mode and the in-depth interleaver provided for 2K and 4K transmissions. The interoperability tests were very successful, confirming a common interpretation of the DVB-H standards portfolio. Interoperability tests revealed the different multiplexing strategies to create the DVB-H service multiplex (i.e. IP encapsulators) which, while remaining fully compliant with the standard, provided different means of flexibility to trade-off power saving (i.e. time slicing parameters) and service access time (i.e. periodicity of the service bursts). The inter-operability tests also demonstrated full compliance and full inter-operability between the modulators and receivers for all possible transmission modes offered by the extended DVB-T physical layer. The most awaited study performed by the Validation Task Force was the performance measurements. The tremendous number of possible combinations of transmission ad service formats, highlighting the incredible flexibility provided by DVB-H, made exhaustive measurements impossible but, focusing on the most probable combinations, the DVB-H Validation Task Force obtained very impressive results in the laboratory tests (confirmed by the field trials), sometimes outperforming the expectations.

The introduction of DVB-H in various parts of the world is underway. Field tests have been carried out in various countries or are still on-going. Those in Helsinki (Finland), Pittsburgh (US) and Berlin (Germany) have received the highest interest in the mobile communications community. Unfortunately, space does not permit to discuss the results of these tests. 

IP Datacast 

The term IP Datacast is used by DVB for the technical elements required to create DVB-H-based services or which are necessary to integrate DVB-H in a hybrid network structure consisting of both a mobile communications network such as GPRS or UMTS and an additional DVB-H downstream. IP Datacast supports various use cases – among them the broadcasting to terminals which do not support an interaction channel.

The IP Datacast system is highly complex due to the wide range of functionalities that have to be addressed in order to support for example the following usage model. The owner of an IP Datacast mobile terminal switches on the device. Immediately, this device identifies the available services via an Electronic Services Guide (ESG). The user orders one of the available services via a secure transaction with the service provider who decides whether a part of the DVB-H downstream will be allocated to provide the service or whether the service will be made available only via the interactive network. In order to take this decision a popularity measure of the requested service will be created which allows optimized allocation of the DVB-H capacity in view of the number of active users in a given coverage area. Audio and video as well other data are transported to the mobile terminal using standardized media formats and transport protocols. Content protection is provided so that only those users who have paid for the service have access to it. A negotiated level of quality of service is guaranteed. If the user moves then the handover both in the interactive network as well as in the DVB-H network is managed seamlessly. Several articles about special aspects of the IP Datacast system can be found in.4 

In September 2005 the DVB Technical Module approved the six documents which form the set of IP Datacast (Phase 1) specifications. It took only one year and one week to create this set which consists of more than 300 pages:

· “IP Datacast over DVB-H: Set of Specifications for Phase 1” is an umbrella for all the other documents and in addition mentions the relevant DVB-H documents. 
· “IP Datacast over DVB-H: Use Cases and Services” describes the variety of use cases from those that require no interactivity and no return channel to, for instance, those that relate to the use of pay content in a prepaid mode. In addition examples of use cases are given: from mobile TV and radio to the broadcast of a common core of services to all terminals, together with a set of services unique to an individual operator.
· “IP Datacast over DVB-H: Architecture” describes the abstract structure of the IPDC system, the reference points and the message flow.

Fig. 2: Architecture of the IP Datacast system

The IP Datacast architecture is shown in fig. 2.

The three remaining documents

· “IP Datacast over DVB-H: PSI/SI”,
· “IP Datacast over DVB-H: Content Delivery Protocols”, and
· “IP Datacast over DVB-H: Electronic Service Guide”

are very comprehensive technical specifications, the most peculiar of which is the one describing the ESG. The ESG contains information about the services available. Through the information in the ESG, the user can select the services and items he/she is interested in and find stored items on the terminal. The document defines the data model, the representation format, the encapsulation and the transport of the ESG of DVB-H. The missing document describing the Service Purchase and Protection (SPP) solution for IP Datacast is expected to be finally approved in November 2005.

Fundamental 

Over more than 13 years DVB has developed a significant number of technical solutions to commercial, scientific and engineering problems. Over time the list of requirements developed significantly and therefore the activities of DVB reflect the changes in the industry since the early 1990s. The first DVB solutions can be considered fundamental enablers of digital broadcasting. The work ongoing in 2005 is much more devoted to finding solutions for ubiquitous access to content wherever it may be and on whatever network it may be available.

The success of DVB is the success of literally hundreds of companies and organizations and of hundreds of people working in the commercial, legal, technical and PR departments of these companies. A meeting of the DVB Technical Module (TM) is typically attended by some 90 engineers. The meetings of the 11 ad-hoc groups of the TM which are active in early 2005 bring together some 200 people. Therefore, it is certainly fair to say that DVB is an example of a truly international organization in which an elite of people works very hard to foster the progress of our industry.

References 

[1] U. Reimers, DVB – The family of international standards for Digital Video Broadcasting. Berlin Heidelberg New York: Springer, 2004. ISBN 3-540-43545-X.
[2] Transmission System for Handheld Terminals (DVB-H). European Norm EN 302 304, European Telecommunications Standards Institute ETSI, November 2004.[3] DVB-H Validation Task Force Report. Telecommunications Report TR 102 401, European Telecommunications Standards Institute ETSI, May 2005.
[4] www.ifn.ing.tu-bs.de/ifa

Ulrich Reimers is the Managing Director of Institut für Nachrichtentechnik (Institute for Communications Technology) at Technische Universitaet Braunschweig (Braunschweig Technical University), Germany. He is one of the founders of the DVB Project and the chairman of the Technical Module of the DVB Project.

pj / nc