The first thing to say about the Advanced Television Systems Committee (ATSC)’s so-called bootstrap for integrating IP services over the air is that it is a fine piece of work that really does create a platform for broadcasters to be competitive in the converging entertainment and mobile world.
The bootstrap has emerged as a key part of the third generation ATSC 3.0 standard, for signaling and discovery of what is contained in each RF channel. It acts as a universal entry point for each frame so that a receiver can accurately decode all the payload data – preceding the preamble and subsequent frames.
This is significant because it will enable broadcasters to multiplex multiple IP streams over a single RF channel, enabling greater capacity and ability to deliver novel services with high levels of integration as required.
On March 28th, the ATSC voted to approve the bootstrap as a key feature of the physical layer for ATSC 3.0. This includes designs developed by One Media, a joint investment between Sinclair Broadcast Group and Coherent Logix set up to build a “Next Generation Broadcast Platform,” enabling broadcasters to become more competitive in the IP era. It is supported by other broadcasters and equipment makers, including the Pearl TV consortium of broadcast companies and also Samsung, – a major player in TVs and mobile devices.
All the furor around ATSC’s bootstrap might come as a surprise to anyone who has not been following standards development around digital terrestrial and hybrid TV for a year or two. All the talk had been of convergence between the world’s major terrestrial standards, especially DVB and ATSC, to enable economies of scale and common global services in the IP era.
We can see why the likes of Samsung as well as the ATSC are so keen on this protocol and willing to expend effort getting it out into the market. As Sinclair Broadcast Group’s Mark Aitken put it, “At the end of the day, what we now have is a wireless, data-agnostic IP pipeline.”
The ATSC had stressed its commitment to that and indicated it would embrace work already done by the DVB with its second generation T2 standards now widely deployed, as well as the European HbbTV hybrid broadcast protocols. To an extent this has happened, with ATSC adopting some of the HbbTV 2.0 work for interactive TV and more fundamentally OFDM (Orthogonal Frequency Division Multiplexing), which had already been adopted in DVB-T2, as well as for that matter by ISDB-T in Japan and much of Latin America as well as the variant of this ISDB-Tb in Brazil. It is also used by Wi-Fi and 4G/LTE.
The implementation of OFDM is complex but the principle is simple enough as a mechanism for encoding a lot of data robustly within a given amount of spectrum over a specified time period. Unlike TDM (Time Division Multiplexing), which gives users all the spectrum but only allocated slices of time, or conventional FDM (Frequency Division Multiplexing), which gives users access all the time but only to portions of the spectrum, OFDM enables access to the whole spectrum all the time.
Instead OFDM separates different users or streams essentially by exploiting the redundancy within bit streams, or the total information available. Data is spread across the whole spectrum and time span in such a way that it is less susceptible either to fading within given portions of the band, or that occur at specific points in time which can affect TDM services.
The big question is why ATSC did not just incorporate the existing OFDM standards from DVB and so be able to exploit silicon already available, saving the trouble of developing further complex enhancements.
The fundamental reason is that since DVB-T2 was developed the challenges facing broadcasting as it converges with IP have evolved and been more clearly identified by, among others, Samsung. Its research teams assessed requirements for ATSC 3.0 and concluded that DVB-T2’s implementation of OFDM had some shortcomings, under the headings of capacity, flexibility and extensibility.
On the capacity front, key attributes wanted included ability to deploy multiple HD programs over a single RF channel and to carry Ultra HD services, which is problematical over DVB-T2 even with HEVC coding. Related to this was the need for even greater resilience against impairments to signal propagation over the air under varying atmospheric conditions.
Under flexibility Samsung identified the need to configure services for both fixed and mobile reception on the same RF channel, as well as the ability to trade-off capacity versus robustness to suit varying demands and use cases.
Then for extensibility an important requirement not conceived for DVB-T2 was the ability to introduce new services targeted at next generation receivers within the same RF channel alongside existing services. ATSC 3.0 bootstrap allows new receivers to be introduced and benefit from emerging services while existing receivers can go on accessing legacy services without any change.
These benefits have been enabled in the first instance within ATSC 3.0 by adopting the latest LDPC FEC (low-density parity-check forward error correction) codes as an improvement over the earlier versions of OFDM. There are also new optimized ways of representing symbols including QPSK (quadrature phase shift keying), in which two bits are modulated at once to double capacity by changing the phase of the carrier wave.
Then the trade-off capability is enabled by allowing different combinations of codes and symbol constellations to be selected, allowing data rates varying from less than 1 Mbps in extremely robust mode suitable perhaps for some mobile services in challenging signal conditions to over 57 Mbps say over a solid SFN DTT infrastructure where the SNR (Signal to Noise Ratio) can be set much higher.
Even more fundamentally, as a result of essentially revamping the physical layer from scratch through the bootstrap, ATSC 3.0 has the ability to swap out the transmission format while continuing to support legacy receivers. This enables new innovations at the transmission level while maintaining backwards compatibility. This is accomplished by incorporating a signal in the bootstrap carrying data identifying the version of the frame following it. This frame could be ATSC 3.0 or a future version of the standard, or even a completely different RF structure. In this way ATSC 3.0 does cater for future convergence with the other variants of DTT.
The way this works is through an intermediate data packet called the preamble as part of each frame between the bootstrap and the frame’s payload. The bootstrap at the start of the frame provides the information required to demodulate the preamble, which in turn holds the data necessary to demodulate the rest of the payload data in the frame. The bootstrap is therefore the most critical part of the signal and so is most robust. Then the preamble will normally be less robust than the bootstrap, but more so than the payload data.
The bootstrap signal then uses a sequence called Zadoff-Chu combined with a PN (pseudo-noise) sequence to create a very robust signal that allows detection and decoding at an extremely low SNR of at most minus 10dB. At this SNR level it is expensive in bandwidth, which is why even the preamble is less robust. But the scheme keeps the bootstrap signal as small as possible.
While conferring the benefits already outlined, the bootstrap is integral to reception under ATSC 3.0, with the additional advantage of greatly reducing the time taken for a complete DTV channel scan down to around 15 seconds typically for about 50 channels. Because the bootstrap is much more robust than the payload data, antenna positioning is not an issue for the scan.
There are also some advantages most relevant for mobile devices, such as transmission of emergency alerts. A tablet or smartphone only has to be on for 2ms to pick up the bootstrap signal and only then switch on the demodulator to display the emergency message.
We have looked closely at the detailed mathematics underlying the bootstrap implementation and can confirm that through clever use of well understood combinatorial theory the ATSC has combined robustness with bandwidth economy, extensibility and backwards compatibility very neatly through one key equation.
As in most digital data communication systems a key unit is the symbol which represents the state of an RF carrier signal at a point in time, defined by the phase, magnitude or frequency of the underlying waveform, or combination of these. As such the symbol can have multiple values and if this is 4 it could encode two bits of data, since each bit can take two values, yielding four combinations in all. Similarly, a symbol with 8 values could carry 3 bits, 16 could carry 4, and so on.
With the ATSC 3.0 bootstrap, the number of bits per symbol can be increased up to a specified maximum as a backward-compatible change. Furthermore, the bootstrap signal duration can be extended to increase the symbol periods, to enable a new symbol to carry additional signaling bits containing more information about the preamble. The ATSC enables this extension to allow new services to be deployed in such a way that an existing receiver system can continue to decode as before and also that the information in the bootstrap is as robust as possible.