Today, there is a great deal of industry hype surrounding mobile television. Said by some to be the next killer app of the mobile sector and dismissed by others as having no sustainable business model, mobile television is a conjure of possibilities. It lies at the eye of a maelstrom of technologies, network models and frequency bands, waiting for many trials to end and the manifestation of a clue as to the most practical and commercially viable direction.
The notion of delivering TV signals to a moving receiver is not an entirely new concept. For years, several countries have enjoyed live digital television in buses and trains, courtesy of DVB-T technology. Using COFDM modulation, DVB-T was originally designed with mobile applications in mind. The signal can accommodate variations in signal strength, field strength and multiple reflections that typically reach a receiver in motion.
However, despite this foresight in its development, DVB-T is not appropriate for broadcasting to handheld devices. Nor is its U.S. counterpart, the ATSC standard, which uses 8VSB modulation and was never designed for mobile applications.
New mobile technologies
Experiencing television on handheld devices raises a new set of issues that have spawned several new broadcast technology platforms. Attracting the most attention globally is the DVB-H standard, which is derived from DVB-T. The important difference is that DVB-H transmits the signal in bursts in order to conserve handset battery life. It also incorporates greater FEC, essential for boosting handheld reception.
Another difference is the data encapsulation technique. The DVB-H stream is an IP datacast at 200kb/s to 500kb/s per program, yielding up to 50 programs in an 8MHz channel. This resolution is sufficient for the tiny handset screen. In contrast, SD DVB-T uses MPEG-2 (or MPEG-4) encoding at 4Mb/s to 5Mb/s per program, yielding up to five standard-resolution programs per channel.
DVB-H is not the only mobile TV platform finding favor. Korea and China have embraced T-DMB, derived from the Eureka 147 DAB standard. Moreover, the U.S. company QUALCOMM has developed Forward Link Only (FLO) technology for the delivery of multimedia content.
DVB-H, FLO and T-DMB, have each addressed the handset-related issues of battery life, reception and screen resolution, albeit in different ways.
It starts with delivery
The choice of technology platform is just one element of delivery — and delivery just one consideration — in the riddle that is mobile television. Commercial imperatives drive everything and are dependent on such aspects as consumer viewing habits, handset development, content licensing and government regulatory environment. Yet it is with delivery that the whole mobile TV enterprise gets moving, and delivery infrastructure represents a significant proportion of capital outlay. Consequently, the question of which delivery model proves best and most cost-effective is one of high interest.
Speculation is compounded by the existence of several different industry players. On the one hand, there are mobile communications carriers. They have an existing subscriber base and perceive mobile television as a means of extending and differentiating their service. Many have introduced 3G mobile TV services based on the Universal Mobile Telecommunications System (UMTS) in recent months, while at the same time partnering broadcast-based mobile TV trials.
It is generally well accepted that UMTS-based mobile television has limitations. The service is available now, but the unicast (one-to-one) nature of UMTS means that as the viewer base grows, mobile television will not be sustainable on this platform. This is true even as UMTS heads toward 3G long-term evolution or in-band cellular broadcast techniques, such as multimedia broadcast or multicast service. Recent reports have suggested that it makes more sense to use the spectrum for wireless data services that can be charged at a higher rate than television.
Mobile carriers are therefore turning to broadcast models for mobile television. Their quest to use existing base station sites has led to the cellular overlay model for mobile television, where broadcast infrastructure is deployed at mobile base stations to provide mobile TV coverage in a similar way to a cell-based mobile network.
Alternatively, it is relatively straightforward to deploy a mobile TV service using existing free-to-air broadcast transmission sites. However, adjustments to coverage planning need to be made.
Research indicates that the high-power terrestrial broadcast model for mobile television will require more repeater sites than for conventional television. (See Figure 1 on page 15.) Owing to an increase in reflections at ground level, the FEC applied to the signal is increased, resulting in a trade-off in signal strength that needs to be addressed. It has been reported that a receiver at ground level incurs a signal strength penalty of approximately -12dB to -16dB (depending on the frequency band) compared with the average rooftop antenna.
Additionally, consumers have also come to expect their handsets to work indoors and in moving vehicles. These environments each reportedly incur another -8dB to -12dB (or more) of signal impact. The provision of indoor coverage is considered one of the main challenges of mobile TV networks.
Another proposed infrastructure model incorporates satellite blanket coverage supported by low-power terrestrial repeaters. The repeaters would be co-located at mobile base stations to supplement urban coverage and provide indoor coverage.
A unifying element in these network models is the convergence of industries that have been quite separate. Mobile carriers will need to embrace broadcast technology and content. Broadcasters will need to team up with carriers, who already have the subscriber base. In fact, it seems logical for mobile TV systems to be intrinsically linked with mobile phone services, which can provide a one-to-one back-channel for interactivity. This could even prove to be a driver for consumer take-up.
The band debate
From a technical and practical standpoint, the other major delivery option pertains to frequency band. Those under consideration:
- VHF (170MHz to 240MHz);
- UHF (470MHz to 860MHz);
- L band (variable depending on region, but generally falls somewhere between UHF and S band); and
- S band (2170MHz to 2200MHz).
The UHF band is the most popular globally for digital terrestrial television, and it has also seen the most mobile TV activity to date. It has good propagation characteristics and, if deployed using the terrestrial broadcast model, should be capable of providing coverage to a large city using 20 to 50 repeater sites. QUALCOMM uses this model for its commercial MediaFLO service, but, as other trials have shown, it is also ideal for deploying DVB-H.
The UHF band is also suitable for networks deployed using the cellular overlay model, because UHF frequencies are just below the conventional GSM or U.S. cellular code division multiple access (CDMA) frequencies. This type of network is being tested in many countries across Europe.
One of the main challenges associated with the UHF band is the limited availability of spectrum in most parts of the world, especially in Europe. Some governments are considering assigning two or three UHF frequencies for DVB-H mobile TV services, which can be deployed as single-frequency networks (SFN). Although it makes network configuration more complex, an SFN is a highly efficient use of spectrum, and a network of two or three overlapping SFNs could be a promising option.
The VHF Band III has even better RF propagation characteristics than UHF. It is not suitable for the cellular overlay model, because the antennas would be too large for existing base stations, but it is an ideal candidate for the terrestrial broadcast model, where city coverage could be achieved with just a handful of repeaters. From a network deployment perspective, VHF appears to offer the lowest roll-out costs and the best indoor coverage.
Factoring in availability
Korea and China use the VHF Band III for T-DMB mobile TV services. To date, there has been no move to deploy DVB-H in VHF Band III. However, because DVB-T services operate in VHF Band III, there seems little reason why DVB-H would not as well. The main obstacle is spectrum availability. Of the four considered bands, DVB-H has the most limited availability in most countries, coupled perhaps with convention.
The L and S bands are emerging as strong contenders. Although both provide reduced terrestrial propagation and in-building coverage compared with the lower frequency bands, they have the advantage of being more readily available. The L band looks set to support a commercial deployment of DVB-H mobile TV services in the United States. The S band is proposed to support a DVB-H-based hybrid satellite/terrestrial repeater model.
Regardless of which frequency band is selected, the signal polarization must also be considered. The FLO systems use circular polarization, which is a combination of vertical and horizontal components. It has been speculated that a circular polarization signal may facilitate reception at the mobile handset no matter the of orientation. This may, however, be a moot point, because the multiple reflections experienced by horizontal and vertical polarization signals can alter the polarization, effectively producing a mixture of polarization components by the time the signal reaches the handset.
Vertical polarization is favored at present by both DVB-H trials and T-DMB deployments. For T-DMB, this shows it's DAB roots. Radio signals are often vertical polarization to enhance reception by car antennas.
Use of vertical polarization also enhances isolation from horizontal polarization television signals at similar frequencies. Most DVB-H trials use vertical polarization, although at least one uses a horizontal polarization signal. Ultimately, the selection of polarization will depend on the receiver performance when faced with multiple signals from reflections, plus the indoor penetration of the signal.
Which way forward?
The future of mobile television depends on many factors, but if it is proved that consumers want mobile television — and are prepared to pay for it — then half the battle is won. The network model will then be determined by how cost-effectively they can be deployed, and the availability of frequencies and licenses.
Using existing infrastructure will be a key element. It is not difficult to incorporate mobile TV services into existing broadband terrestrial broadcast systems — particularly if the systems were initially designed to accommodate additional services or channels. The most significant capital outlay would come with the deployment of additional repeater stations.
Alternatively, deploying a mobile TV network as a cellular overlay would involve a significant shift in broadcast infrastructure philosophy. The quest to deploy television antennas at existing mobile base stations (hundreds, perhaps thousands, of sites) will encounter the same demands put on mobile phone carriers. These include using low-profile, environmentally friendly antennas; following the mandate for low emissions; participating in site-by-site negotiations; and balancing the trade-off between capital and operational expenditures. It could also promote use of the higher frequency L band, and its inherently more compact infrastructure.
Co-location interference issues also need to be considered when overlaying mobile television and wireless communications services. With UHF frequencies so close to the GSM 900MHz receive band (usually 890MHz to 915MHz) and the CDMA 800MHz receive band (usually 824MHz to 849MHz), careful frequency planning and coordination will be required. Moreover, if the broadcast signal's power is too high, it could cause blocking in the sensitive GSM or CDMA receivers unless RF filtering is deployed. Similar situations arise with the L and S band frequencies, which are in the vicinity of high-band GSM, CDMA and UMTS services.
In addition, it is likely that all mobile TV network topologies will ultimately need to incorporate dedicated wireless indoor solutions to provide coverage inside multilevel buildings, large campuses (such as airports and shopping malls) and underground road tunnels and metro systems. These could be integrated with existing broadband wireless indoor solutions for mobile wireless communications.
Clearly, for mobile television to succeed as a commercial venture, it will involve many players in the wireless sector, including mobile phone carriers, broadcasters, handset manufacturers, content providers, infrastructure groups, base station OEMs, government and licensing bodies. These parties will need to collaborate and form partnerships in order to make mobile television work — both technically and commercially.
The quest to maximize the bottom line will ultimately reveal which network model, technology platform and frequency band combine to form the most viable option for a specific country or market. And it will be dependent on which provides the most attractive and accessible model for consumer uptake. Whatever the outcome, it will represent a true convergence of multiple technologies. From this will materialize the true meaning of mobile television.
Mike Dallimore is vice president broadcast, towers and defense systems for Radio Frequency Systems.
Mobile television at a glance
Major technology platforms
- DVB-H — derived from DVB-T
- T-DMB — derived from DAB
- FLO — developed by QUALCOMM
- Cellular overlay — broadcast network overlaid at mobile communications base stations
- High-power terrestrial broadcast — based on terrestrial broadcast models with an increased number of repeater stations
- Hybrid satellite/terrestrial — uses satellite for blanket coverage, supported by terrestrial repeaters
- VHF Band III (170MHz to 240MHz) — best propagation, including indoor coverage, but limited availability
- UHF television band (470MHz to 860MHz) — good propagation with moderate indoor coverage, but limited availability
- L band terrestrial/satellite (between UHF and S band, depending on region) — lower terrestrial propagation and poor indoor penetration, and availability dependent on country
- S band (2170MHz to 2200MHz) — low terrestrial propagation and indoor penetration, but good availability