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Wary Broadcasters may finally see mobile TV on 3GPP horizon

At last there is a version of a cellular video broadcasting standard which has for the first time been created with input from commercial broadcasters, and is not just the pipedream of the cellular community.

There have been so many false dawns for mobile TV that there is inevitable skepticism, when yet another attempt is made to get it off the ground. But there are two big differences this time around, firstly that people are now consuming video on mobile devices in industrial quantities anyway. A lot of that consumption via tablets is over Wi-Fi, but the fastest growth has been through smartphones over cellular by people while on the road. Secondly for the first-time broadcasters have got on board and shaped the development of the latest standard in collaboration with the cellular community.

That gives good reason to believe that this latest standard, promoted within release 14 of the 3GPP mobile specification, will lead to significant commercial deployments where all predecessors, notably DVB-H, Qualcomm’s MediaFlo and even the more recent LTE Broadcast have largely failed.

In the case of LTE Broadcast there was a lack of commercial use cases and insufficient interest from broadcasters or content providers, but that has changed now. Release 14, the final version of 3GPP before 5G starts rolling out, is the only one virtually dedicated to broadcasting. It has been developed in close collaboration with broadcasters led by the EBU (European Broadcasting Union), which in the past has stood out for its opposition to transfer of spectrum from DTT to cellular through the digital dividend of analog switch off. Release 14 therefore represents a coming together of former adversaries and is therefore more significant as a statement of intent than for its actual technical merits.

Nonetheless the technical aspects making mobile networks friendlier for broadcast transmission are important and are after all what release 14 is all about. Each on its own may seem a relatively minor enhancement but collectively they add up to a fairly compelling package for broadcasters.

Some of the features relate to payment models, some to workflow and some to video delivery itself. On the payment front, there is support for FTA (Free-To-Air) reception on top of conventional subscription-based reception modes. There is a receive-only mode enabling reception of selected eMBMS (Evolved Multimedia Broadcast Multicast Services) without the device having to authenticate via the SIM card. This opens the network in theory to delivery of broadcast services to mobile devices without SIMs designed just for video access, although in practice FTA delivery will more often go to conventional SIM-equipped handsets which will then have to be eMBMS enabled.

On the workflow front, standardized interfaces have been introduced at the content ingest level with the xMB interface and at the device access level between clients and receiver applications. The xMB is most significant because it provides all the functionality for content ingest and authentication, supporting the four principle ways of transmitting content over the air, which are streaming, file transfer, applications and transport mode.

The transport-only mode is particularly relevant for broadcasters because it allows distribution of any IP-based data flow over eMBMS, including content formats outside 3GPP such as MPEG-2 Transport Stream. This will allow traditional broadcasters to distribute content in their native format without transcoding via eMBMS to existing radio or TV receivers.

Release 14 is not the last word and does not immediately solve all problems for broadcasters, but has therefore been designed to allow for additions such as better QoS through FEC (Forward Error Correction), which is not yet there. It also caters for addition of audience data gathering and reception monitoring.

On the delivery front a key move is extended timing for the so called cyclic prefix to support large cell sizes, which will help make wide-area LTE eMBMS network coverage more cost effective. The cyclic prefix is a feature of cellular transmission to cater for interference between adjacent symbols resulting from multipath effects within cells caused by reflections off buildings or terrain.

A symbol is simply a carrier unit over a radio network, which is distinct from the actual binary bits it represents. Multiple bits are encoded in the symbol as “constellations” made up of variations in phase or amplitude of the carrier wave with the number dependent on sensitivity of transcribing equipment and transmission noise, reaching 16 bits or 256 distinct states in the case of 256-QAM. The cyclic prefix was an ingenious invention to cater efficiently for interference between adjacent symbols by copying the start of a symbol and shuffling it to the end during transmission. This serves a dual purpose by on the one hand acting as a guard buffer between symbols and secondly enabling the receiving device to detect when it has reached the end of each symbol and reconstitute any missing information by correlating symbols received over different paths. Any confusion can be resolved by having that small start portion of each symbol – the prefix – replicated at the end. It is called cyclic because the process cycles from one symbol to the next.

For larger cells the offset between signals received from multiple paths can be greater and so the length of the cyclic prefix, equating to a longer duration, has been increased under 3GPP Release 14. The stage has been set for mobile video delivery to advance from now on with the cooperation of broadcasters, who have given up fighting the lost cause of defending terrestrial spectrum.

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