DMB

- an overview of the DMB system, and in particular the T-DMB version to be used for mobile video broadcasts.

Digital Multimedia broadcasting, DMB is based on the Eureka 147 Digital Audio Broadcast or DAB system that is widely deployed in the UK and many other countries around the world for audio broadcasting. One of the advantages of using DMB is that it can be rolled out and used without much modification for mobile video applications, simply increasing the level of error correction to cope with the mobile environment.

In view of the different broadcasting platforms that could be used account needs to be taken of this. Eureka 147 allows for broadcasts both from terrestrial transmitters and from satellite based transmitters. For DMB both platforms are possible, but in view of the differing platforms and transmission requirements there would need to be some modifications between the two systems. For terrestrial based transmissions a flavour of the system designated as T-DMB (Terrestrial Digital Multimedia Broadcasting) is used, whereas for satellite broadcasting S-DMB (Satellite Digital Multimedia Broadcasting) is used.

Broadcasting DMB and DAB
Image courtesy RadioScape

RF signal characteristics

Like many other broadcasting systems, DMB and DAB use a form of transmission known as Orthogonal Frequency Division Multiplex (OFDM). This has been adopted because of its high data capacity and suitability for applications such as broadcasting. It also offers a high resilience to interference, can tolerate multi-path effects and is able to offer the possibility of a single frequency network, SFN.

OFDM Spectrum

Note on OFDM: Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. Click on the link for an OFDM tutorial

DMB format

The transmissions for the form of DMB being deployed in many countries occupy approximately 1.5 MHz bandwidth and for the VHF broadcasts the transmission contains 1536 Carriers. However it is possible to use a variety of modes:

• 2K mode     1536 carriers

• 1K mode     768 carriers

• 0.5K mode     384 carriers

• 0.25K mode     192 carriers

Frequency allocations

It would be possible to utilise the DAB transmission system within the UK for DMB, however much of the capacity has been taken up, although some reserve capacity has been retained for future data transmissions of which DMB could be part.

A more likely solution for DMB is to use frequencies within the L-Band DAB allocation (1452 - 1467.5 MHz). This might be possible in some countries where the use of this broadcasting allocation could be used for this purpose with little legislation.

Using a new band it will not only be possible to use smaller antennas, an important element for mobile phones and PDAs, but it will also be possible to tailor the transmission to accommodate the Doppler shifts likely to be encountered by small mobile devices. This can be achieved by reducing the number of carriers. Despite the carrier number reduction, the maximum data rate of 1.152 Mbps is still retained. The drawback of using the L band frequencies is that they would require a much higher density of transmitters to provide the required coverage.

Battery consumption

One of the major requirements for any mobile video system such as DMB is that it shall not place a major load on the battery of the handheld device. With user expectations requiring that battery life shall be several days between recharges, this is a major consideration. While battery technology is improving, and IC technology has enabled current consumption of chips to be reduced, the basic technology can also play its part.

DMB is also ideally suited to the delivery of material to handheld devices. DAB inherently includes a technique known as time slicing by using an effectively using a Time Division Multiplexing delivery method. In this way the receiver is only switched on when it is required, thereby saving battery power.

Summary

It remains to be seen whether DMB or DVB-H will be the major standard that is adopted for mobile video. Some indicate that both schemes may be used in different countries around the world Accordingly many chip manufacturers whoa re addressing this market are catering for both schemes and developing systems that will be able to switch between the variety of bands that will be used around the globe.

In addition to this DMB trials are well advanced, particularly in Korea where it appears DMB will be adopted. For other countries, it remains to be seen what happens.