High-definition (HD) television content is available via satellite but, until now, terrestrial users have not been able to receive HD content because its transmission requires a wider data channel than that used for standard definition.
With the analogue switch-off underway in many countries, spectrum is being freed that will enable new services, such as DVB-T2.
DVB-T2 is the second-generation digital terrestrial broadcast technology. Driven by commercial requirements for at least 30% more payload capacity. It promises efficiencies of 30-50% in its use of terrestrial spectrum compared to DVB-T. It is also expected to be considerably more robust than previous terrestrial broadcast technologies, thanks to several important features.
Like the mobile communications world, the broadcast world has looked to boost throughput and data rate by adopting techniques like orthogonal frequency-division multiplexing (OFDM) and multiple-input and multiple-output (MIMO).
DVB-T and now DVB-T2 join DAB, Wimax, Wireless LAN and LTE in using OFDM, although T2 takes it a step further than other technologies, using much higher order modulation schemes within its OFDM structure. Whereas DVB-T offers three modulation schemes (QPSK, 16QAM and 64QAM), DVB-T2 will boast up to 256QAM modulation, plus a higher number of carriers (up to 32K carriers, versus DVB-T’s 2K or 8K mode) to deliver a much more robust signal.
One implication of higher order modulation is that more powerful error correction is needed. DVB-T2 employs the same coding schemes that were selected for DVB-S2 for satellite broadcasting, namely low-density parity check (LDPC) and Bose-Chaudhuri-Hocquengham (BCH) coding, versus the Reed-Solomon encoder and Convolutional encoders used by DVB-T.
Though offering better performance in the presence of high noise levels and interference, DVB-T2’s coding schemes require greater processing power on the receiver side.
Robustness and efficiency within DVB-T2’s transmission system are further increased by new technologies such as peak to average power ratio (PAPR) reduction, constellation rotation and multiple physical layer pipes (PLP).
DVB-T2 also specifies a transmitter diversity method, known as Alamouti coding, which improves coverage in small-scale single-frequency networks. Importantly, these are all optional features, enabling networks to be adapted to suit different broadcast environments – transmitters will include all these options, with users able to switch them on or off as needed.
Although the variety of different modes offered by DVB-T2 results in a flexible standard, from a test perspective, the number of modes available generates a commensurate increase in test parameters.
For comparison, DVB-T/H offers a selection of guard intervals, code rates, plus three orders of QAM. To this end, those needing to test DVB-T2-capable equipment must add many more guard intervals and code rates, 256QAM and a vast number of additional modes.
Though DVB-T2 will not replace DVB-T in the short to medium term – indeed, it is likely that the two standards will coexist for many years – vendors have begun to develop DVB-T2 equipment, as indicated by the appearance of the first prototypes at the end of 2008. It is expected that manufacturers’ development schedules will be aggressive, encouraged by consumers’ eagerness to benefit from HD-capable TVs.
Darren Tipton and Nick Ward are product managers at Rohde & Schwarz UK