The future of digital terrestrial

Digital terrestrial needs mobile broadcast to succeed.
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The future of digital terrestrial lies in mobile broadcast extending services to portable devices both in the home and on the road. This point has often been lost amid all the anguished debate among the world’s broadcasters over how to defend their spectrum against marauding cellular operators. Not surprisingly, given the failure of most mobile broadcast services so far, the terrestrial transmission community has been reluctant to promote the great advantages it can offer over unicast cellular networks for delivering popular content.

But that is all changing now, as a second generation of mobile broadcast services emerges, spearheaded by the DVB’s Next Generation Handheld (NGH) standard, announced at the end of October 2012. This is the sequel to the DVB’s ill-fated first-generation mobile broadcast standard, DVB-H, which has been abandoned by almost all broadcasters that had, mostly in Europe, deployed it in trials and commercial services.

Yet this failure was largely because DVB-H, while being far from perfect technically, was ahead of its time, before suitable business models had evolved and before there was much support from device makers. User demand failed to materialize, partly because there were too few handsets capable of displaying video in sufficient quality. The fact that DVB-H required new infrastructure also restricted deployment, as did disagreements between operators and broadcasters over which of them should own the service.

Initial failures

These factors did not become apparent until deployments were well under way, and in Europe there was an initial wave of enthusiasm. Among deployments that at first appeared quite successful were KPN’s Mobiel DVB-H service in the Netherlands, which had attracted 40,000 customers by the end of 2008, and a wholesale service from Italy’s largest commercial broadcaster, Mediaset. All these eventually failed, with Mediaset’s DVB-H frequencies being reallocated to terrestrial broadcast services. In the U.S., Qualcomm’s MediaFLO failed for similar reasons early in 2011, becoming the highest-profile mobile broadcast service to close down.

The exceptions to the rule of failure so far have been in Japan, South Korea and Brazil, where linear mobile TV services have been going several years. But, crucially, these services have been delivered over digital or analog terrestrial networks rather than 3G, and this provides an important clue to the future of mobile broadcast and its possible convergence with cellular services.

Even in these three countries, mobile TV has not yet proved very profitable, with ecosystem participants struggling to find sustainable revenue-generating business models. One factor that applies equally in all countries is changing all that: the tablet boom. This has already delivered a large and fast-growing population of wireless connected devices that will increasingly create demand for access to both on-demand and linear TV services outside as well as around the home. This factor alone has changed the game for mobile broadcast, as Qualcomm’s CEO Paul Jacobs acknowledged at the time MediaFLO was closing.

“I still believe strongly in mobile TV,” Jacobs said. “On a tablet it’s pretty compelling. It’s just a question of exactly how it will happen and with which technology.”

Next Generation Handheld

Over in Geneva, the DVB also continued to believe in mobile TV despite the failure of DVB-H to sustain traction, and it argues forcefully that its second standard, DVB-NGH, answers Jacobs’ question about which technology will finally succeed. The DVB says DVB-NGH addresses all the shortcomings of DVB-H, which were as much to do with the assumed business model as any technical deficiencies. It was originally thought that mobile broadcast would just deliver linear TV, while it is now clear it will bring a broad mix of multimedia services, including download to local device memory to cope with variable bandwidth, as well as various caching options.

The mobile service will also have to take account of the user’s behavior, which can vary over time and between devices. The DVB’s executive director, Peter Siebert, insists this has all been taken account of in a much more flexible design for DVB-NGH. It can accommodate different business models, for example, by allowing different levels of protection for audio and video within one service.

But the two killer punches delivered by DVB-H are robust video delivery to moving devices and support for large cells suitable for mobile broadcast. Taken together, these will give digital terrestrial a vital edge over not just 3G, but also emerging Long Term Evolution (LTE)/4G services for mobile broadcast. Although the DVB does not say so directly, DVB-NGH is clearly positioned to help broadcasters defend their spectrum against further invasion from cellular networks and strengthen their hand in negotiations towards future converged services, perhaps involving shared transmission infrastructure. This may appear fanciful at the moment, but the DVB’s stance towards cellular services is conciliatory, although from a position of strength where it holds most of the key cards for mobile broadcast.

MIMO

Considering robustness first, the DVB appeared initially to be aiming for increased capacity instead. The goal for DVB-NGH was actually to increase capacity by 50 percent for a given level of robustness rather than the other way around. The use of twin-antenna multiple-input multiple-output (MIMO) was the main thrust for higher performance in DVB-NGH. This helps overcome the usual multipath fading caused by reflections and obstacles in the transmission environment, which in the case of moving terminals is made even harder to deal with because of the additional Doppler Effect. This is the change in signal frequency resulting from the relative motion of sender and receiver, and although for a car or train this is small compared with the propagation speed of electromagnetic radiation, it is still significant given the short time spaces between the pulses encoding the digital data (known as symbols) in digital radio transmission.

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Figure 1. MIMO uses multiple transmit and receive antennas for multiple transmission paths.

MIMO works by using multiple antennas at either the transmit or receive end or both to spread a signal across multiple paths without increasing either bandwidth or transmit power. (See Figure 1.) There are several variants of MIMO, and for DVB-NGH, cross-polarization is used to create the required signal diversity at the transmit side. Because mobile handsets are small relative to the wavelength of the UHF radiation used in digital terrestrial, the spatial diversity created by simultaneous transmission from adjacent antennae is not sufficient to boost performance much. But cross-polarization works by having one antenna placed so that it generates radio signals oriented with the waves aligned in the vertical direction and the other in the horizontal direction. Then they do not interfere and in effect create two parallel channels of communication subject to independent fading.

MIMO can boost both capacity, in effect through creating parallel channels, and also robustness, by increasing resilience to fading effects resulting from destructive interference between signals taking different paths. An interesting thing about DVB-NGH is that in testing it has done better in terms of robustness than capacity, but that is most important because, for mobile broadcast, bandwidth will not be the commodity in short supply. It is far more essential that the service quality is consistent under varying transmission conditions, ranging from indoor reception by stationary devices to fast-moving devices in, say, a bullet train.

Interleaving

Broadcast also allows use of interleaving, which increases robustness further by spreading packet losses evenly along a transmission stream rather than congregating in clumps as tends to happen otherwise with the effect of overwhelming FEC mechanisms. Interleaving comes at the price of increased latency, which renders it unsuitable for interactive and unicast applications requiring two-way communications, but can be tolerated for broadcast. Interleaving works by separating adjacent units of information to disperse the impact of lost packets and then reassembling them in the correct order at the destination. (See Figure 2.)

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Figure 2. Interleaving disperses packets to mitigate the impact of lost packets.

The second killer punch for DVB-NGH is its support for larger cells than the LTE/3GPP Evolved Multimedia Broadcast Multicast Services (E-MBMS) being adopted for broadcast within 4G/LTE cellular services. This follows simply because DVB-NGH supports larger guard intervals than the 33.33µs limit for LTE/4G. This equates to a maximum cell radius of 10km for LTE/4G, while DVB-NGH allows up to 107km, which enables a single high-powered transmitter to serve the whole area much more cost-effectively than multiple lower-powered ones as in 4G/LTE. Equally, the largest cells work well with single-frequency mode (SFM) digital terrestrial transmission, where adjacent transmitters share the same frequencies, enabling more channels to be delivered within a given spectrum allocation because neighboring transmitters no longer need to send each channel over a different frequency to avoid interference. SFN mode also enables power to be reduced and can increase robustness through multipath transmission from more than one transmitter.

While DVB-NGH also caters well for unicast transmission for on-demand content, here it will not scale well for large volumes. In that event, cell sizes have to be reduced to avoid the number of unicast sessions congesting the available bandwidth within each cell, and this has, of course, happened in the case of cellular services driven by increased user numbers and traffic levels. This points to the potential synergy between cellular and digital terrestrial services, which the DVB is trying to foster, looking perhaps to some future overlay infrastructure where on-demand and interactive mobile applications are served by small cell LTE/4G-type networks, and broadcast via a large cell SFN.

IP multicast

The only real alternative, favored perhaps by cellular operators, is the use of IP multicast down to the cell towers to avoid redundant transmission over the core and backhaul networks, and then maybe some peer-to-peer mechanism to distribute popular traffic within the radio cell. This would avoid swamping the radio bandwidth across the whole cell with multiple unicast streams.

This approach is unlikely to scale well to large numbers of users all receiving a broadcast at once within a single cell, but broadcasters cannot afford to sit back and allow mobile services to progress with hybrid models around IP multicast. The DVB’s NGH should be seized as an opportunity to regain momentum lost in mobile broadcast after the failures of first-generation approaches such as DVB-H and MediaFLO.

Philip Hunter writes the Broadcast Engineering “Beyond the Headlines Europe” e-newsletter.