Meeting the Demand for Video Delivery
Mobile video, wherein television programming is delivered to mobile phone handsets or other portable devices, is poised to enter a period of rapid growth, as new equipment becomes available and service offerings proliferate.
Several technologies are competing to meet the demand for this service, which should become available to consumers over the next few years.
Both Long Term Evolution (LTE) and Mobile DTV are candidate technologies for video service delivery that will likely battle for market dominance. Today, it is impossible to determine which will be the winner, but it does make sense to look at the differences between these two technologies to understand how they will compete in the marketplace.
Sometimes called 4G, LTE is a technology specification designed to allow mobile telephone networks to continue to expand the amount of bandwidth that they can deliver, permitting more services to be provided to each subscriber.
It is a successor to 3G services such as Universal Mobile Telephone Service (UMTS) and “3.5G” services like High Speed Download Packet Access (HSDPA). LTE consists of a set of recommendations from the Third Generation Partnership Project (3GPP).
LTE uses advanced modulation techniques (including QAM—Quadrature Amplitude Modulation) and advanced antenna technology (MIMO—Multiple Input Multiple Output) to deliver very high signal bit rates in narrow channel bandwidths. For example, with 64 QAM (allowing 6 bits per symbol) and 2x2 MIMO (which doubles the effective throughput), a total of 12 bits per second/Hz can be achieved. This is very substantial, as it allows a 5 MHz radio channel to carry 42.5 Mbps of user traffic, once the required overheads have been subtracted.
Coupled with the high spectral efficiency of LTE, a significant amount of channel bandwidth has been designated for use by these services. (In fact, some of the LTE spectrum came from TV channels 52 to 69 that were sold to carriers for use after the June 12, 2009 DTV transition in the U.S.).
More than 500 MHz of radio spectrum has been designated for use by LTE carriers over a range of frequencies between 500 MHz and 2400 MHz, with more allocations likely to be made in the future. Assuming full utilization of this radio spectrum, it would be possible to deliver 4.25 Gbps of total traffic to subscribers from a single base station.
Fig. 1: Transmitter coverage for LTE vs. Mobile DTV
Much, if not all, of this data capacity could easily end up being consumed by users who are employing data services to view video content delivered via Web sites or other on-demand sources. For example, some references recommend compressing H.264 video content at a bit rate of 768 kbps for viewing on an iPhone’s 480x320 screen. Adding 82 kbps for audio and overhead, the total becomes 850 kbps, which would mean that each gigabit of channel capacity would be capable of carrying approximately 1,176 video streams.
As has been well explained in other TV Technology articles, mobile DTV technology uses a portion of the bit stream broadcast by a digital television station to deliver a video signal to specially equipped handsets and other devices. With broadcast, each video signal is delivered simultaneously to all devices, so all viewers who are watching the content will see the same images at (roughly) the same time. Viewers make programming choices by choosing one of the available broadcast channels.
A major difference between LTE and mobile DTV is the coverage area of a single transmitter. Mobile DTV signals are designed to occupy a small portion of an ATSC broadcast signal, which are normally engineered to cover an entire metropolitan area, as depicted in Fig. 1(a).
In contrast, LTE signals will be designed to cover limited footprints, allowing the radio spectrum to be re-used several times across a metropolitan area, as shown in Fig. 1(b).
Also, since LTE systems are configured for two-way communication, base stations (cell sites) must be deployed close enough together to permit reception of the low-power signals generated by the mobile handsets. This places natural limits on the size of the coverage areas that can be used for LTE systems.
LTE service providers will probably not install enough base stations to cover an entire metropolitan area when deployment begins. If past patterns hold, installations will focus on areas with the greatest density of users and highest potential revenues. This will mean that video services that depend on LTE will have more limited coverage areas than mobile DTV in the short term, a situation that may persist for years into the future.
With mobile DTV, any bandwidth that is used to deliver mobile content must be subtracted from the fixed total bandwidth allocated to each ATSC broadcast station. As a result, the total number of channels is likely to be fairly limited, due to the bandwidth demands of the other programming channels offered by broadcasters.
From a spectrum utilization standpoint, mobile DTV is quite efficient, because a single bit stream can be received and viewed by tens or hundreds of thousands of users within the coverage profile of a DTV station.
LTE content is delivered to each user individually, allowing each user to choose from a library of programming. Of course, this flexibility comes at a price—with LTE, each user consumes a slice of the available bandwidth, even if they are watching the same content as the person standing next to them. So, for each service area, the number of simultaneous users is constrained by the amount of bandwidth that is available in the local base station’s radio spectrum.
Using the figures calculated earlier in this article, each service area would be able to support 1176 video streams per gigabit of bandwidth, or a total of 5000 streams in the 4.25 gigabits of available bandwidth, if the entire available spectrum of 500 MHz was dedicated to video signal delivery.
Since mobile users will no doubt want to continue to place voice calls, check their e-mail, and surf the Internet, it is probably more realistic to assume that 2,000 or 3,000 viewers might be simultaneously viewing video in a service area before it becomes fully loaded. Increasing the number of users either requires installing more base stations with smaller service areas, or allocating more radio spectrum to mobile LTE services.
PICKING A WINNER
FCC Chairman Julius Genachowski recently spoke about the looming shortage of bandwidth for mobile applications.
“I believe that the biggest threat to the future of mobile in America is the looming spectrum crisis… By some estimates [mobile data usage] will grow from 6 petabytes per month in 2008 to nearly 400 petabytes per month in 2013,” Genachowski said.
Clearly, individualized video services delivered over LTE networks are likely to be major contributors to the bandwidth load. If, instead, mobile DTV services become popular, they could ease some of the bandwidth pressure on LTE networks.
From a cost standpoint, mobile DTV could prove to be more economical to deliver, since the amount of radio spectrum used is quite low. Plus, capital costs of a central transmitter site should be significantly lower than the cost of installing dozens or hundreds of base stations with antenna towers to provide LTE coverage for a metropolitan area.
The cost advantage could become a boon for viewers, who might get ad-supported mobile DTV service for free, in place of a monthly LTE subscription plan.
On the other hand, mobile video delivered by LTE will offer significantly more programming choices than mobile DTV. If home viewing trends apply in the mobile market, viewers will actively seek out providers who offer more choice and flexibility for on-demand content. This could prove to be a decisive factor in the marketplace.
The real battle for viewers might come down to the cost and availability of handsets and other devices. Mobile carriers will have strong incentives to convince their subscribers to buy LTE handsets and the accompanying data service plans. Mobile DTV providers will likely be reliant on the consumer electronics industry to entice customers to purchase devices to receive services.
The battle for viewership between LTE and mobile DTV over the next few years will be very interesting to watch, maybe even on a mobile handset!
Wes Simpson, who would be interested in watching television while taking the train to New York, is the author of the just-published second edition of “IPTV and Internet Video” from Focal Press. He can be reached at firstname.lastname@example.org