About DVB-H: The Mobile TV Broadcasting

K.M.Paul

Former Engineer-In-Chief and Acting Director General (AIR)

 

Abstract: DVB-H, the mobile TV broadcasting is a superset of original DVB-T standard with several powerful supplements. They are –additional level of ‘forward error correction’(MPE-FEC) in the link layer, to protect the signal from the aberrations of mobile propagation channel; a new ‘4K’ mode of operation to take care of the adverse effects of ‘Doppler Frequency Shift’ during reception at high speed and a technique-‘Time Slicing’ which can not only save power in mobile receiver for long use, but also can provide a seamless handover while changing the service cell and ‘Time Diversity’ of the transmitted data. DVB-H uses Internet Protocol Data casting (IPDC). With IPDC, two layers of FEC, ‘Time Diversity’ and ‘Space Diversity’ (due to mobility); DVB-H can effectively download ‘Files’ in addition to ‘streaming audio and video’. Studies and simulated observations made in Spain and Sweden suggest some new techniques to ensure a more robust mobile reception, especially for ‘File’ downloading in mobile receiving condition.                    

                                                          

New Features in DVB-H :

 

DVB-H is the Mobile digital TV Broadcasting standard, developed in Europe as a superset of the original DVB-T standard which was primarily designed for fixed terrestrial digital TV reception with roof-top antenna. Whereas the DVB-H std. uses the same basic ‘Physical Layer’ of the original DVB-T, this new std. for Mobile channel also addresses the issues which are essential for reception in a ‘Mobile Propagation Channel’.

 

            The first issue which is specific to Mobile channel is,fast fading of signal and noise bursts due to mobility and changing shadow effects cast by obstacles. Besides this, being a mobile channel, the reception can be anywhere- inside the building or in any other congested urban areas, like cellular mobile telephone. Therefore the signal needs to be protected by appropriate and adequate ‘Error correction coding’. So that the signal is able to deliver useful and meaningful desired information to the mobile receiver against the entire above mobile channel aberrations. DVB-H standard provides this data protection through the introduction of a second level of ‘Forward Error Correction’ (FEC)in the’ Link Layer’ through a ‘Multi Protocol Encapsulation’(MPE) unit and is known as- MPE-FEC.

 

            The second issue arises owing to the ‘Doppler Frequency Shift’(DFS) of the carriers in the multicarrier COFDM modulated system of DVB-H, when the mobile receiver moves in high speed. The net effect of the DFS is equivalent to adding additional noise to the signal or in other words to degrade the C/N value of the signal. This problem is addressed in DVB-H by adopting a new mode of transmission- ‘4K’ in place of mode ‘8K’ as used in DVB-T standard.

 

            The third issue of concern, specific to mobile channel is the ‘Power saving’ in the mobile handset, so as to enable the user to use the handset receiver for the whole day or for several days before charging the handset by a stationary power source.  DVB-H standard resolved this problem by introducing a power saving technique in the DVB-H handset,  called ‘Time-Slicing’.

 

Contributions of ‘Time Slicing’ :

 

In the ‘Time Slicing’ technique a large number of  the signal data pertaining to a particular programme channel, are bunched together and broadcast in  short duration ‘Bursts’ at regular intervals. Between the two bursts of a particular programme channel, the similar ‘Bursts’ pertainining to other programme channels are broadcast.

 

The receiver front-end after receiving one burst of the desired programme channel, which is tuned in the receiver; goes into ‘Sleep Mode’.  It awakes after a prefixed time to receive the next burst of the same programme channel. In between these two desired bursts, the similar bursts of other programme channels appear at the receiver input. But these bursts are not received, as the receiver front-end remains in ‘Sleep Mode’ during that period. Considering a bouquet of 20 programme channels multiplexed in the DVB-H service, there can be 95% saving of power consumption in the receiver front-end. Other parts of the receiver however remain always active as they do not go to sleep like the front-end. This power saving technique in DVB-H is called the ‘Time Slicing’.

 

            ‘Time-Slicing’ process used in DVB-H not only saves the power in the handset receiver but it  also produces another advantageous by-product viz. ‘Time Diversity’ of the digital stream owing to the time separated bursty nature of transmission. Again where mobility produces signal shadowing, fast fading and noise bursts; it also helps generating advantageous effects like- ‘Space Diversity’ due to the mobility of the receive terminal.  The combined effect of these- ‘Time’ and ‘Space’ Diversities is very positive and helps reception in fast fading mobile channel, reducing error probability.

 

Another contribution of ‘Time-Slicing’- besides power saving and  ‘Time Diversity’ is its capacity to provide the receiver a  smooth and seamless frequency handover when the user leaves one service area in order to enter a new cell (details of this is discussed in the subsequent section).

 

Cotribution of  ‘4K’ Mode:

 

In ‘4K’ mode the no. of carriers being nearly half that of the ‘8K’mode, the frequency spacing between the adjacent carriers of the multicarrier COFDM modulation system, becomes almost double. This enhanced frequency spacing can easily accommodate the DFS upto a much higher speed without the adverse effects like enhancement of noise or degradation of signal C/N. While the use of ‘4K’mode addresses the DFS problem, it reduces the service area owing to eventual reduction in ‘Guard Interval’ (GI) in comparison to the ‘8K’ mode.

 

Contributions of MPE-FEC :

 

Going back to the ‘first issue’ of  mobile channel i.e. how to protect the DVB-H signal from the channel aberrations in mobile receiving condition; it can be stated that the error  protection is basically provided by introducing another level of  ‘Forward Error Correction’ (FEC) in the ‘Link Layer’ through a ‘Multi Protocol Encapsulation(MPE)unit. This FEC known as  MPE-FEC is in addition to the basic physical layer FEC as provided in the DVB-T std. in the form of ‘Convolution Code’.

 

DVB-H std. allows use of various code rates for the MPE-FEC depending on the channel requirements. The lowest limit of this code rate is to have no MPE-FEC at all. Use of MPE-FEC gives a penalty in terms of reduced useful data throughput, due to the overhead introduced by the MPE-FEC section.  However using a weaker DVB-T Convolution Code rate can compensate for this. For example  Convo. code rate 2/3, together with MPE-FEC  code rate 3/4  gives a far better performance than Convo. Code rate ½ without MPE-FEC, even though the useful data throughput is the same in both the cases. Technically, though the use of MPE-FEC in DVB-H is optional, it gives an improvement in C/N performance and Doppler performance in mobile channels and moreover also improves tolerance to impulse interference encountered in mobile receiving environments.

 

IPDC in DVB-H and File Delivery:

 

Internet Protocol Datacasting or IPDC is an important converging technique as adopted in DVB-H standard.The payload of DVB-H can be IP-data grams (Internet Protocol Datagrams) or other network layer datagrams encapsulated into MPE (Multi Protocol Encapsulation ) sections. DVB-H standard however uses IP-datagrams for obvious reasons that the IP or Internet Protocol is a globally used protocol.  Use of  Internet Protocol to carry its data, in so called IP packets, allows DVB-H to rely upon standard components and protocols for content manipulation, storage and transmission and thereby enables  to contribute towards ‘Technology Convergence’.

 

IPDC , in addition to audio and video stream broadcasting, can also broadcast ‘Files’ to mobile users. IPDC through DVB-H can deliver the content in two ways- (i) Streaming Mode- where the user uses the stream directly in a time constrained manner. That is why the signal containing streaming mode data is known as ‘Isochronous Signal’. (ii) File Delivery Mode- where certain amount of data delivered is stored in a buffer and the user takes the data from the buffer to form the complete ‘File’. Types of applications for ‘File Delivery’ are – Video clips, Digital newspaper, Software download etc. Though this buffering technique adopted in ‘File Delivery’ is tolerant to micro delays in the delivery of data stream, the process requires very high degree of error protection for the type of applications it handles viz.- Digital newspaper, Software downloads etc. in mobile receivers through wireless broadcast mode. Technologically it is a very challenging task and requires high degree of  data error immunity.

 

 

IPDC in DVB-H with its two levels of FEC, in conjuction with the above mentioned – ‘Time Diversity’ and ‘Space Diversity’ is capable of handling the ‘File Delivery’. DVB-H standard provides the second level of FEC- called MPE-FEC, at the “Link Layer’.

 

Studies on Enhanced Transmission Robustness

 

The studies and simulated  observations made in Sweden and Spain,  revealed that, if the second level of FEC is added in the ‘Application Layer’(AL-FEC), it enables a better data acquisition probability (less error probability) when compared with the present DVB-H standard.

 

This observation is based on application of ‘Raptor Code’ for AL-FEC.    ‘Raptor Code’ is derived from ‘Fountain Code’. The origin of ‘Fountain Code’ goes with its design as a special class of FEC, meant for very efficient ‘File’ downloading over broadcast channels where there is no feedback channel present. It can be said that the ‘Raptor Code’ is an efficient implementation of ‘Fountain Code’ in terms of computational superiority, as developed by the Digital Fountain Inc. The beauty is that the ‘Raptor Code’ can be implemented in software without need for any dedicated hardware. With this coding, the AL-FEC can provide protection across the entire file, rather than across a single burst as in ‘Link Layer’ FEC (MPE-FEC) in DVB-H.

 

Based on the studies mentioned above, a suggestion has been made for another technique of increasing the transmission robustness in DVB-H. The technique includes transmission of an additional burst containing parity information with a time gap,after every original burst. In case of imperfect reception of the original burst owing to the mobile channel aberrations,when the receiver moves to a different position after a time gap,the additional burst arriving at the receiver, can be received. This additional burst contains enough parity data to recover the data in the original burst. In this system the original burst not only carries the timing of the next original burst, but also the timing of the additional burst; so that the  receiver front-end can get activated from ‘sleep mode’ following that time information and receive both the additional burst as well as the  next original burst. Though this system of ‘additional burst’ containing delayed additional parity bits, trades off ‘system capacity’, ‘power saving’ and adds‘ streaming delay’; it increases user satisfaction by enhancing the transmission robustness which is most vital in mobile broadcasting, particularly in ‘file down loading’ through mobile broadcast channel.

 

 

Frequency Handover for  DVB-H   Mobile Receiver :

 

While moving on highways, the service area goes on changing. Transmitters of different service areas normally have different frequencies. But many of them carry the same ‘Transport Stream’ (TS) i.e. the same programme content. After receiving one burst of the desired programme channel, when the receiver goes through the ‘sleep’ period, it can still scan other frequencies and select the one, which has quality wise the best potential. If the TS of different carrier frequencies are time synchronized, the receiver will receive the next burst, also from the new carrier frequency of the other service cell. This will ensure a seamless handover from one service cell to the other and interestingly the user will not know anything about this silent and efficient handover operation of the DVB-H receiver.

 

 

About the author:

 

The author Mr.K.M.Paul is the former Engineer-In-Chief and Acting Director General of ALL INDIA RADIO (AIR).He served the Public Broadcaster for 35 years for development of broadcasting in India He served- International Telecommunication Union(ITU)-Study Group 6 (for Broadcasting) as its Vice-Chairman; European Broadcasting Union (EBU) as member of DAB ‘Specialists’ Group’; Asia-Pacific Tele-community(APT) as its ‘Mission Expert’ and the Asia-Pacific Broadcasting Union(ABU) in implementing its various projects. Mr. Paul obtained his Bachelor’s Degree & Master’s Degree in Electronics & Telecommunication Engg. from Jadavpur  University, Kolkata in 1967&1969 respectively. He is- the Member PAC of the DST; Member ICA; Sr. Adviser Broadcast Engg. and the Life Fellow of the IETE and the BES(I).