CBC TECHNOLOGY REVIEW

Transcription

1 CBC TECHNOLOGY REVIEW Issue 3 January canada.ca DVB S2 THE NEW INTERNATIONAL STANDARD Roman Stasiewicz Broadcast Systems Specialist, New Broadcast Technologies ABSTRACT DVB S2 is the new and advanced satellite transmission standard from DVB that expands and improves upon the earlier DVB S and DVB DSNG standards deployed in broadcast equipment all over the world. The primary driver for the DVB S2 standard was the increase in capacity of satellite digital television transmissions. This development was expedited by the scarcity of Kuband spectrum in the US and the imminent launch of HDTV services requiring more of the scarce resources. This objective has been accomplished with higher order modulation modes and more efficient channel coding BCH (Bose Chaudhuri Hocquenghem) with LDPC (Low Density Parity Check) coding with the designed expectation to increase the spectral capacity by 30% to 35% for given symbol rate and carrier to noise ratio (C/N). The standard specifies four modulation modes of which QPSK and 8PSK are optimized for broadcast applications using non linear satellite transponders driven near saturation. The 16APSK and 32 APSK modulation modes are geared towards more professional applications requiring operation over semi linear transponders in a backed off configuration. DVB S2 supports the transport of single or multiple Standard Definition and/or High Definition input streams. It supports emerging applications comprised of the new H.264/AVC video coding standard resulting in additional spectrum savings for the broadcaster. The DVB S2 standard was designed to support the following range of satellite applications with a variety of modulation modes: 1. Broadcast Services 2. Professional TV Contribution and Satellite News Gathering 3. Interactive Services 4. Backwards Compatibility CBC TECHNOLOGY REVIEW January / 9

2 The Broadcast Services are extensively covered with the DVB S specification and will remain so for a long time to come due to the prevalence of the technology and the significant number of deployed satellite receivers. INTRODUCTION DVB S2 (Digital Video Broadcasting Satellite) is the next generation international standard for satellite broadcasting combining the functionality of DVB S Television and Data broadcasting with the professional DVB DSNG applications for satellite news gathering and contribution services. The primary driver for the DVB S2 standard was to increase the capacity of satellite digital television transmissions. This development was expedited by the scarcity of Ku band spectrum and the imminent launch of HDTV services. The DVB S2 specification was intended to benefit from advances in silicon process technology, low cost digital processing power and the development of powerful decoding and synchronization algorithms to achieve up to 30% increase in spectral efficiency. The intent for it was to be simple and inexpensive, take advantage of commercially available consumer demodulator ASICs and deliver 8PSK payloads to consumer sized antennas and LNBs. To realize these gains, new and more robust higher order modulation modes were added. They addressed the non linear magnitude and phase transfer characteristics of the saturated transponder, Group Delay characteristics of the satellite Input and Output Mux along with oscillator phase noise in up and down conversion equipment and consumer LNBs. The new and more powerful FEC system was based on the concatenation of BCH and LDPC coding. The LDPC inner coding performance is within 1dB of the theoretical maximum performance of Shannon s limit, resulting in 2dB to 3dB threshold improvement over the DVB S standard for given information rate translating to significantly higher system margins or reduced receive antenna sizes. OVERVIEW OF DVB S AND DVB DSNG STANDARDS The DVB S standard only specified QPSK modulation for the broadcast satellite distribution of television and data services. This was a limitation for the professional applications equipped with larger antennas and operating at lower symbol rates in transponder backed off configurations. This professional infrastructure was in place, in need of higher throughput and capable of supporting more advanced modulation schemes with higher thresholds. CBC TECHNOLOGY REVIEW January / 9

3 In support of the higher order modulation schemes for satellite news gathering and contribution services the DVB DSNG standard was created. With the addition of 8PSK and QAM schemes to deliver more capacity, the need for more robust error correction methods with lower Eb/No thresholds became apparent and the successor to the Reed Solomon Viterbi FEC system was being contemplated. DVB S2 SATELLITE APPLICATIONS 1. Broadcast Profile: a. Digital Multi Program SD and HD TV Services 2. Professional Profile: a. Digital Standard Definition Television Contribution SDTV b. More efficient Digital Satellite News Gathering allowing HDTV transmissions at Ku band or performance upgrade to DVB S contribution quality c. Operation with smaller SNG antennas at higher C/Ns d. Emergency Fly Away => DVB S2 + VCM, better accommodates antenna/hpa limitations with FEC & modulation dynamics to improve link performance 3. Interactive Profile: a. Provides interactive data services & Internet access; b. Does not require expensive terrestrial infrastructure; c. Conducive to selective/multi spot coverage with Ka band satellites; d. Uses ACM Adaptive Coding & Modulation mode to selectively deal with propagation conditions by changing FEC & modulation on the fly via return link from STB to improve target service availability; e. Uses VCM Variable Coding & Modulation providing greater throughput at Kaband center where EIRP is higher with lower throughput on the fringes; f. Provides the right combination of adaptive coding and modulation techniques with web acceleration technologies using Ka band space segment to provide, very significant broadband capacity gains. 4. Backwards Compatible Profile: a. Fulfills the requirement for backward compatibility due to the great number of deployed DVB S receivers; b. Provides migration path for existing DVB S & DVB S2 services over same transponder using non uniform 8PSK constellation; c. Supports two (2) High Priority (HP) & Low Priority (LP) streams, DVB S & DVB S2 transport streams with the HP stream delivering DVB S information; d. Old set top boxes (STBs) continue to receive DVB S HP stream program content and the new STBs receive the DVB S2 (LP) stream or additional content; e. The LP stream may support incompatible DVB S2 information with ¼, 1/3,1/2,3/5 code rates; CBC TECHNOLOGY REVIEW January / 9

4 DVB S2 DETAILS f. Larger capacity gains available for LP stream for reduced service availability; g. Link penalties may not make the backward compatible mode viable for broadcast applications. The DVB S2 standard is based on a powerful error correcting mechanism comprised of concatenated LDPC and BCH codes with a wide range of coding rates, four (4) modulation modes and three (3) roll off factors with synchronization tools to tie them together. THE FORWARD ERROR CORRECTION (FEC) SUB SYSTEM The FEC is the key subsystem benefiting from recent developments in channel coding mainly responsible for the significant improvement in satellite link performance in the presence of noise and interference. The DVB S2 group evaluated seven proposals for error correction using Turbo Code families and selected the LDPC inner code concatenated with BCH outer code scheme as their system of choice. This new FEC scheme replaces the existing DVB S convolutional coding with concatenated Reed Solomon (RS) encoding. Comparison of LDPC, TPC & other Coding Schemes The LDPC codes were initially proposed by Gallager in 1963 and have especially good performance for high code rates and long block lengths. They require low decoder complexity and of all the practical error correction methods known to date, turbo codes, together with LDPC codes, come closest to approaching the Shannon limit, the theoretical limit of maximum information transfer rate over a noisy channel. CBC TECHNOLOGY REVIEW January / 9

5 text text The two selected LDPC codes use Normal frame and Short Frame block lengths comprised of 64,000 bits and bits respectively. The short frame block lengths though less efficient are recommended for applications where latency or shorter delay is a requirement. DVB S2 MODULATION MODES There are four DVB S2 modulation modes. The QPSK and 8PSK modulation modes with virtually constant envelope modulations are recommended for broadcast applications operating over non linear satellite transponders near/at saturation. The two new 16APSK and 32APSK higher order modulation schemes replace their DVB S DSNG 16QAM and 32QAM predecessors with increased resiliency and reduced sensitivity to earth station and transponder TWTA AM/AM and AM/PM characteristics. This reduced sensitivity makes APSK more suitable for professional applications operating on satellite transponders in backed off configurations. Their power and bandwidth efficient constellations are better suited for non linear transponders and achieve comparable or better performance over linear channels then QAM modes with their mix of phase and amplitude modulation. Although the APSK schemes are not as power efficient due to their requirement for higher C/N operation, they yield significantly greater spectrum efficiencies. DVB S2 Constellations Q I=MSB Q=LSB Q ρ=1 00 I ρ=1 φ=π/4 000 I QPSK 111 8PSK Q Q R MSB LSB R R R I R I APSK 32APSK CBC TECHNOLOGY REVIEW January / 9

6 The circular concentric rings in the APSK constellations are composed of uniformly spaced points rather than the square grid of points used in the conventional 16QAM schemes. This concentric ring shape minimizes the distortion effects due to high power amplifiers (HPAs). Many studies and simulations have proved the increased resiliency of this modulation scheme to HPA non linearities making the APSK modulation more suitable for non linear satellite transponders while providing the capability for powerful and yet simple predistortion compensation techniques. TWTA Transfer Characteristics Typical TWTA curve with non linear transfer characteristics is presented above. Note the deviation from linear operation near saturation. To maximize the output power of such an amplifier for operation with DVB S2 higher order modulation schemes, uplink pre distortion is recommended for magnitude and phase correction. Pre distortion is a requirement for the APSK modulation schemes. EFFICIENCY GAINS The reason for pursuing the higher order modulation modes is to realize efficiency gains due to increased number of mapped bits unto the constellations. Modulation Efficiencies Modulation Mode Bits per Symbol QPSK 2 8PSK 3 16APSK 4 32APSK 5 This increase in capacity is realized at the expense of higher C/N or Eb/No thresholds. CBC TECHNOLOGY REVIEW January / 9

7 FILTER ROLL OFF & OCCUPIED BANDWIDTH The DVB S2 standard in addition to the single DVB S 35% roll off factor, provides two (2) additional roll off factors to further increase transponder efficiency. DVB S / DVB S2 Filter Roll Off Factors DVB-S DVB-S2 Roll-off α 1+α 35% 35% % % Square Root Raised Cosine with 25% & 35% roll off factors The occupied bandwidth of the modulated signal is the symbol rate multiplied by (1+α) where alpha (α) is the roll off factor and the symbol rate is the 3dB bandwidth. By using lower alpha, carriers can be spaced closer together resulting in increased carrier loading for given transponder or increased symbol rate for a given bandwidth. C Band Contribution Transponder All incremental spectrum efficiencies are welcome. CBC TECHNOLOGY REVIEW January / 9

8 DVB S /DVB S2 PERFORMANCE COMPARISON Table below provides information rate and Eb/No threshold performance for the DVB S, DVB DSNG and DVB S2 modulation and FEC rates. Note the Eb/No threshold reduction of approximately 2.5dB in favour of the DVB S2 standard for given modulation and FEC rates. The values are based on 36MHz C Band transponder utilizing 28.5Mbaud symbol rate. This is CBC Radio Canada s current transponder loading for French and English Distribution services using DVB S/DSNG 8PSK Modulation with 5/6 FEC + RS yielding 65.66Mbps information rate. DVB S / DVB S2 Performance Coding for DVB-S2 (ETSI EN ) Bandwidth Efficiency Coding for DVB-S (ETSI EN ) Bandwidth Efficiency Transmission Rate Information DVB-S2 Threshold DVB-S Threshold Modulation Rate (Mbps) BCH LDPC Total (bps/hz) RS Conv. Total (bps/hz) (Mbps) DVB-S2 DVB-S Es/No* Eb/No* C/No* Es/No Eb/No C/No QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK PSK PSK (DVB-SNG: ETSI EN ) DVB-SNG DVB-SNG Threshold 8PSK PSK PSK PSK PSK APSK QAM (DVB-SNG: ETSI EN APSK APSK APSK QAM APSK APSK APSK APSK APSK APSK APSK * Excludes Demodulator Implementation Margin DVB-S2 Implt'n Margin CONCLUSION The emergence and deployment of the DVB S2 technology will have a significant impact on the satellite communications and broadcast industries. The new standard combines the two earlier DVB S and DSNG specifications and significantly improves on their modulation and coding efficiencies. It fulfills the long awaited need for spectrum efficiency improvements and coupled with the additional gains of the new video compression technologies such as H.264/AVC, allows Direct to Home broadcasters to launch more SD and HDTV broadcast and interactive TV services using available spectrum resources. CBC TECHNOLOGY REVIEW January / 9

9 The new standard does not limit itself to video and audio coding and offers support to any stream format including IP and ATM along with traditional MPEG transport streams. For interactive services such as high speed internet using Ka band space segment with web acceleration and ACM & VCM technologies, more significant capacity gains can be realized resulting in more viable business cases for consumers and small and larger business enterprises. Finally due to the anticipated significant erosion of C band system margins as a result of interference from adjacent satellites at plus/minus two (2) degrees, marginal infrastructures can benefit directly from the 2dB to 2.5dB coding gains with corresponding margin increases. Although the DVB project does not expect DVB S2 to replace DVB S for TV broadcasting in the near future due to the significant investment in the DVB S technology world wide, the effort proved so successful, power and coding gains so impressive and applications all encompassing that a global chant of no need for another system within our lifetime is resonating within the broadcast circles. REFERENCES 1. DVB S2 The second generation DVB system by Satellite Alberto Morello 2. DVB S2: The Second Generation Standard for Satellite Broad band services Alberto Morello and Vittoria Mignone 3. LDPC: Another Key Step Toward Shannon Tony Summers, Comtech AHA Corporation 4. Error Control Coding from Theory to Near Capacity Implementation Pal Orten Nera Research 5. DVB S / DVB S2 Performance Comparison Table Fred Markhauser, Telesat Canada 6. Analysis of the BW Efficiency of DVB S2 in a typical distribution network Dirk Breynaert, Maximilien d Oreye de Lantremange Newtec 7. Digital Video Broadcasting (DVB) ETSI EN Digital Video Broadcasting (DVB) ETSI EN Roman Stasiewicz has been with CBC/Radio Canada s New Broadcast Technologies as a broadcast systems specialist since July He is responsible for evaluation of emerging broadcast and satellite technologies, satellite link budgets and system analyses, with a current focus on adjacent satellite interference implications. Prior to his work with the CBC, he spent more than 17 years as a senior project engineer with Telesat Canada in their Broadcast Engineering Group, participating in major broadcast projects including the conversion of all major Montreal based broadcast customers to DVC technology in early 1990s, and more recently, CBC/Radio Canada s conversion from analog to DVC technology utilizing 8PSK modulation. CBC TECHNOLOGY REVIEW January / 9