Last week the European Telecommunication Standards Institute (ETSI) ratified a new standard for digital satellite distribution, DVB-S2. The standard provides several key enhancements over the DVB-S standard, which is currently used by many broadcasters for digital satellite distribution. Some of the enhancements include improved forward error correction, a wide range of modulation types to suit different reception environments, and, perhaps most important, adaptive coding and modulation (ACM) which uses a return link from the Earth station to select the appropriate error correction and modulation type on a frame-by-frame basis according to reception conditions (clear sky, rain, etc.).
"This formal ratification of DVB-S2 is good news for satellite broadcasters. We've already seen significant announcements from major broadcasters due to launch HD services this year and there will be more to follow," said Peter MacAvock, executive director of the DVB Project Office.
The DVB announcement of ETSI approval of DVB-S2 noted that DirecTV in the United States and BSkyB in the U.K. are planning services based on DVB-S2. Advantech, AMT, Conexant, EMS Technologies, Pace, Scopus, ST Microelectronics, Tandberg, Thomson, Radyne Comstream, and Zarlink have announced DVB-S2 products.
DVB tested seven "turbo-code" forward error correction systems and selected Low Density Parity Check (LDPC) inner coding concatenated with Bose-Chaudhuri-Hocquenghem (BCH) outer coding for DVB-S2. This combination results in performance as close as 0.7 dB to the Shannon limit. Using QPSK, the coding allows the system to work with C/N as low 2 dB below the noise floor! Using the most complex modulation and least coding, the C/N requirement increases to 16 dB. The DVB-S2 standard includes four modulation types--QPSK and 8PSK for nonlinear transponders operated near saturation and 16APSK and 32APSK for semi-linear transponders. To allow for reduced bandwidth and closer spacing of satellite carriers, DVB-S2 adds roll-offs of "alpha=0.2" and "alpha=0.25" to the traditional roll-off of "alpha=0.35" specified in DVB-S.
The combination of modulation methods and coding allows DVB-S2 to achieve bit-rate capacity gains between 25 and 35 percent for same C/N and symbol rate or increased robustness at the same bit-rate. Spectral efficiencies using DVB-S2 range from 0.5 bit/s/Hz through 4.5 bit/s/Hz. When DVB-S2 is used with new video and audio coding schemes such as H264/AVC or VC9, one 36 MHz transponder can carry between 20-25 SDTV channels or 5-6 HDTV channels.
The DVB-S2 standard includes hierarchical modulation, which allows transmission of backward-compatible channels while allowing simultaneous transmission of a low-priority stream using higher-order modulation incorporating DVB-S2 features. One application of this would be transmission of an HDTV signal receivable only on DVB-S2 receivers along with a lower bit-rate SDTV signal receivable that is receivable on legacy DVB-S receivers.
For uni-cast applications, such as high-speed Internet service and satellite news gathering, the adaptive coding and modulation (ACM) features available in DVB-S2 allow each user to operate with very low C/N margins, maximizing satellite throughput. Rather than selecting modulation and coding based on worst-case receive conditions, ACM allows these parameters to be adjusted in real-time according to propagation conditions (rain, clear sky, beam center, beam edge, etc.). However, for ACM to work, a return channel (terrestrial Internet, DVB-RCS, phone line, etc.) is required.
The DVB-S2 standard is not limited to transport streams. It can also carry generic streams such as Internet protocol (IP) and is flexible enough to accommodate new technology.
For more information on DVB-S2, see the DVB documents New generation DVB system by satellite and the white paper DVB S2 Satellite - Most advanced satellite delivery possible. The DVB announcement of ETSI approval said the ETSI specification (EN 302 307) may be downloaded from http://pda.etsi.org/pda/queryform.asp. While the specification is free, registration is required. Also try the ETSI Work Programme page. Select type "EN" and type 302 307 in the "Number" box.
